Sample records for geochronology

  1. Methods of isotopic geochronology (United States)

    Gorokhov, I. M.; Levchenkov, O. A.

    Papers are presented on such topics as the age of the chemical elements; the age of meteorites, the moon, and the earth; isotopic ages of the most ancient terrestrial formations; and the Archean evolution of Enderby Land in the Antarctic as evidenced by isotopic dating. Consideration is also given to a uranium-lead geochronology technique for investigating Precambrian ore deposits, a Pb-Pb technique of zircon dating, and the potentials and limitations of Sm-Nd geochronology.

  2. Decay constants in geochronology

    Institute of Scientific and Technical Information of China (English)

    IgorM.Villa; PaulR.Renne


    Geologic time is fundamental to the Earth Sciences, and progress in many disciplines depends critically on our ability to measure time with increasing accuracy and precision. Isotopic geochronology makes use of the decay of radioactive nuclides as a help to quantify the histories of rock, minerals, and other materials. Both accuracy and precision of radioisotopic ages are, at present, limited by those of radioactive decay constants. Modem mass spectrometers can measure isotope ratios with a precision of 10-4 or better. On the other hand, the uncertainties associated with direct half-life determinations are, in most cases, still at the percent level. The present short note briefly summarizes progress and problems that have been encountered during the Working Group's activity.

  3. In Situ Planetary Geochronology Technology Project (United States)

    National Aeronautics and Space Administration — Geochronology is the science of determining the age of rocks, fossils, and sediments.  The science of geochronology is the prime tool used to attempt to derive...

  4. Improving Consistency in Laser Ablation Geochronology (United States)

    Horstwood, Matt; Gehrels, George; Bowring, James


    Workshop on Data Handling in LA-ICP-MS U-Th-Pb Geochronology; San Francisco, California, 12-13 December 2009; The use of uranium-thorium-lead (U-Th-Pb) laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) geochronology involves rapid analysis of U-and Th-rich accessory minerals. It routinely achieves 1-2% precision for U-Th-Pb dates constituting detrital mineral age spectra and for dating igneous and metamorphic events. The speed and low setup and analysis cost of LA-ICP-MS U-Th-Pb geochronology has led to a proliferation of active laboratories. Tens of thousands of analyses are produced per month, but there is little agreement on how to transform these data into accurate U-Th-Pb dates. Recent interlaboratory blind comparisons of zircon samples indicate that resolvable biases exist among laboratories and the sources of bias are not fully understood. Common protocols of data reduction and reporting are essential for scientists to be able to compare and interpret these data accurately.

  5. Establishing Denudation Chronology through Weathering Geochronology (United States)

    Riffel, S. B.; Vasconcelos, P. M.; Farley, K. A.; Carmo, I. O.


    Planar landforms - erosion surfaces - are used as temporal markers in denudation chronology. These surfaces are interpreted as the result of long-term weathering and denudation controlled by a specific base level within a given time-interval characterized by long-term tectonic stability. The presence of several planar landforms at distinct elevations is interpreted as evidence for distinct denudation events, separated by periods of tectonic reactivation and crustal uplift. We selected an area in the Paraná-La Plata basin, southern Brazil (25°S lat.) to investigate if the application of weathering geochronology by the 40Ar/39Ar and (U-Th)/He methods could permit differentiating different elevation landsurfaces. We dated supergene Mn oxyhydroxides by 40Ar/39Ar geochronology and coexisting supergene Fe oxyhydroxides by the (U-Th)/He method from one of the three regional landsurfaces - The First, Second, and Third Paraná plateaus - previously identified in this area. Two sites were sampled from the Second Paraná Plateau: a ferricrust at Serra das Almas (7 hand specimens of goethite at 1080 m of altitude) and deeply weathered ferricretes and saprolites at Vila Velha (11 hand specimens of cryptomelane and 14 of goethite at 910 m of altitude). The Serra das Almas sites hosts a stratified weathering profile with ferricrust, and mottle zone. The Vila Velha site results from intense weathering that led to the precipitation of well-crystallized supergene minerals precipitated within fractures in the saprolites. The geochronological results are correlatable between the two sites and the two distinct methods (40Ar/39Ar and (U-Th)/He), and they reveal three generations of weathering and mineral precipitation: Late Eocene-Oligocene, Early Miocene, and Pleistocene. The geochronological results suggested that the Second Paraná Plateau formed by regional erosion during the Oligocene, and that this landsurface has been continuously exposed to weathering and erosion since then

  6. Investigating Age Resolution in Laser Ablation Geochronology (United States)

    Horstwood, Matt; Kosler, Jan; Jackson, Simon; Pearson, Norman; Sylvester, Paul


    Workshop on Data Handling in LA-ICP-MS U-Th-Pb Geochronology; Vancouver, British Columbia, Canada, 12-13 July 2008; Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) uranium-thorium-lead (U-Th-Pb) dating is an increasingly popular approach for determining the age of mineral grains and the timing of geological events. The spatial resolution offered by this technique allows detailed investigations of complex igneous and metamorphic processes, and the speed of data capture allows vast amounts of information to be gathered rapidly. Laser ablation U-Th-Pb dating is therefore becoming an increasingly influential technique to the geochronology community, providing cost-effective and ready access to age data for laboratories and end users worldwide. However, complications in acquiring, processing, and interpreting data can lead to inaccurate age information entering the literature. With the numbers of practitioners expanding rapidly, the need to standardize approaches and resolve difficulties (particularly involving the subjectivity in processing laser ablation U-Th-Pb data) is becoming important.

  7. Detrital geochronology of unroofing magmatic complexes (United States)

    Malusà, Marco Giovanni; Villa, Igor Maria; Vezzoli, Giovanni; Garzanti, Eduardo


    Tectonic reconstructions performed in recent years are increasingly based on petrographic (Dickinson & Suczek, 1979; Garzanti et al., 2007) and geochronological (Brandon et al., 1998; DeCelles et al., 2004) analyses of detrital systems. Detrital age patterns are traditionally interpreted as a result of cooling induced by exhumation (Jäger, 1967; Dodson, 1973). Such an approach can lead to infer extremely high erosion rates (Giger & Hurford 1989) that conflict with compelling geological evidence (Garzanti & Malusà, 2008). This indicates that interpretations solely based on exhumational cooling may not have general validity (Villa, 2006). Here we propose a new detrital geochronology model that takes into account the effects of both crystallization and exhumational cooling on geochronometers, from U-Pb on zircon to fission tracks on apatite. This model, specifically designed for unroofing magmatic complexes, predicts both stationary and moving mineral-age peaks. Because its base is the ordinary interaction between endogenic and exogenic processes, it is applicable to any geological setting. It was tested on the extremely well-studied Bregaglia-Bergell pluton in the Alps, and on the sedimentary succession derived from its erosion. The consistency between predicted and observed age patterns validates the model. Our results demonstrate that volcanoes were active on top of the growing Oligocene Alps, and resolve a long-standing paradox in quantitative erosion-sedimentation modelling, the scarcity of sediment during apparently fast erosion. Dickinson, W. R. & Suczek, C. A. Plate tectonics and sandstone composition. Am. Assoc. Petrol. Geol. Bull. 63, 2164-2172 (1979). Garzanti, E., Doglioni, C., Vezzoli. G. & Andò, S. Orogenic belts and orogenic sediment provenance. J. Geol. 115, 315-334 (2007). Brandon, M. T., Roden-Tice, M. K. & Garver, J. I. Cenozoic exhumation of the Cascadia accretionary wedge in the Olympic Mountains, northwest Washington State. Geol. Soc. Am. Bull

  8. EARTHTIME: Teaching geochronology to high school students (United States)

    Bookhagen, Britta; Buchwaldt, Robert; McLean, Noah; Rioux, Matthew; Bowring, Samuel


    The authors taught an educational module developed as part of the EARTHTIME ( outreach initiative to 215 high school students from a Massachusetts (USA) High School as part of an "out-of-school" field trip. The workshop focuses on uranium-lead (U-Pb) dating of zircons and its application to solving a geological problem. The theme of our 2.5-hour module is the timing of the K-T boundary and a discussion of how geochronology can be used to evaluate the two main hypotheses for the cause of the concurrent extinction—the Chicxlub impact and the massive eruption of the Deccan Traps. Activities are divided into three parts: In the first part, the instructors lead hands-on activities demonstrating how rock samples are processed to isolate minerals by their physical properties. Students use different techniques, such as magnetic separation, density separation using non-toxic heavy liquids, and mineral identification with a microscope. We cover all the steps from sampling an outcrop to determining a final age. Students also discuss geologic features relevant to the K-T boundary problem and get the chance to examine basalts, impact melts and meteorites. In the second part, we use a curriculum developed for and available on the EARTHTIME website ( The curriculum teaches the science behind uranium-lead dating using tables, graphs, and a geochronology kit. In this module, the students start by exploring the concepts of half-life and exponential decay and graphically solving the isotopic decay equation. Manipulating groups of double-sided chips labeled with U and Pb isotopes reinforces the concept that an age determination depends on the Pb/U ratio, not the absolute number of atoms present. Next, the technique's accuracy despite loss of parent and daughter atoms during analysis, as well as the use of isotopic ratios rather than absolute abundances, is explained with an activity on isotope dilution. Here the students

  9. Building Interactive Visualizations for Geochronological Data (United States)

    Zeringue, J.; Bowring, J. F.; McLean, N. M.; Pastor, F.


    Since the early 1990s, Ken Ludwig's Isoplot software has been the tool of choice for visualization and analysis of isotopic data used for geochronology. The software is an add-in to Microsoft Excel that allows users to generate visual representations of data. However, recent changes to Excel have made Isoplot more difficult to use and maintain, and the software is no longer supported. In the last several years, the Cyber Infrastructure Research and Development Lab for the Earth Sciences (CIRDLES), at the College of Charleston, has worked collaboratively with geochronologists to develop U-Pb_Redux, a software product that provides some of Isoplot's functionality for U-Pb geochronology. However, the community needs a full and complete Isoplot replacement that is open source, platform independent, and not dependent on proprietary software. This temporary lapse in tooling also presents a tremendous opportunity for scientific computing in the earth sciences. When Isoplot was written for Excel, it gained much of the platform's flexibility and power but also was burdened with its limitations. For example, Isoplot could not be used outside of Excel, could not be cross-platform (so long as Excel wasn't), could not be embedded in other applications, and only static images could be produced. Nonetheless this software was and still is a powerful tool that has served the community for more than two decades and the trade-offs were more than acceptable. In 2014, we seek to gain flexibility not available with Excel. We propose that the next generation of charting software be reusable, platform-agnostic, and interactive. This new software should allow scientists to easily explore—not just passively view—their data. Beginning in the fall of 2013, researchers at CIRDLES began planning for and prototyping a 21st-century replacement for Isoplot, which we call Topsoil, an anagram of Isoplot. This work is being conducted in the public domain at We

  10. Statistics of large detrital geochronology datasets (United States)

    Saylor, J. E.; Sundell, K. E., II


    Implementation of quantitative metrics for inter-sample comparison of detrital geochronological data sets has lagged the increase in data set size, and ability to identify sub-populations and quantify their relative proportions. Visual comparison or application of some statistical approaches, particularly the Kolmogorov-Smirnov (KS) test, that initially appeared to provide a simple way of comparing detrital data sets, may be inadequate to quantify their similarity. We evaluate several proposed metrics by applying them to four large synthetic datasets drawn randomly from a parent dataset, as well as a recently published large empirical dataset consisting of four separate (n = ~1000 each) analyses of the same rock sample. Visual inspection of the cumulative probability density functions (CDF) and relative probability density functions (PDF) confirms an increasingly close correlation between data sets as the number of analyses increases. However, as data set size increases the KS test yields lower mean p-values implying greater confidence that the samples were not drawn from the same parent population and high standard deviations despite minor decreases in the mean difference between sample CDFs. We attribute this to the increasing sensitivity of the KS test when applied to larger data sets, which in turn limits its use for quantitative inter-sample comparison in detrital geochronology. Proposed alternative metrics, including Similarity, Likeness (complement to Mismatch), and the coefficient of determination (R2) of a cross-plot of PDF quantiles, point to an increasingly close correlation between data sets with increasing size, although they are the most sensitive at different ranges of data set sizes. The Similarity test is most sensitive to variation in data sets with n < 100 and is relatively insensitive to further convergence between larger data sets. The Likeness test reaches 90% of its asymptotic maximum at data set sizes of n = 200. The PDF cross-plot R2 value

  11. Sub-micron geochronology by EPMA (United States)

    Jercinovic, M. J.; Williams, M. L.; Berman, R.


    Actinide-bearing, geochronologically applicable accessory phases can exhibit remarkably complex internal compositional structures, commonly revealing polygenesis and allowing timing constraints to be placed on the structural and metamorphic evolution of multiply tectonized terrains. These fine-scale mineral domains have been revealed by high resolution compositional mapping by EPMA of natural monazite (LREE PO4), and can exist at nearly all spatial scales, including domains substantially below 1 micron in width. Accurate compositional characterization of such domains by any technique is difficult, but hardware and software developments in EPMA offer some opportunities to approach these challenging but important targets accurately. In this regard, we have analyzed Paloeproterozoic (ca. 1.8 Ga) monazite domains which penetrate along cleavage planes in larger, older monazite grains (ca. 2.55 and 2.37 Ga) from the Boothia peninsula, Nanavut Canada. As such, this represents a successful nanogeochronologic analysis. The direct analytical volume, as defined by the convolution of the error functions resulting from primary (beam) electron scattering and characteristic X-ray generation volume dimensions, can be restricted to considerably less than 1 micron with the use of high brightness sources (LaB6, CeB6, or Schottky) and modest beam energy (15kV or less) in monazite (Z ca. 39). However, the analysis is rendered far more complex by the effects of boundary fluorescence and beam damage. For this analysis, we use a high thermal/electrical conductivity double coating of aluminum and carbon to mitigate sample damage at high current density. Although imperfect (there remains a net loss of P relative to REEs and actinides), this analysis results in relatively minor counting errors relative to the concentrations, and accuracy can be further improved by use of time-dependent count acquisition methods. Reduction of the beam energy substantially below 10kV results in extreme damage

  12. Geochronological constraints on the evolution of El Hierro (Canary Islands) (United States)

    Becerril, Laura; Ubide, Teresa; Sudo, Masafumi; Martí, Joan; Galindo, Inés; Galé, Carlos; Morales, Jose María; Yepes, Jorge; Lago, Marceliano


    New age data have been obtained to time constrain the recent Quaternary volcanism of El Hierro (Canary Islands) and to estimate its recurrence rate. We have carried out 40Ar/39Ar geochronology on samples spanning the entire volcanostratigraphic sequence of the island and 14C geochronology on the most recent eruption on the northeast rift of the island: 2280 ± 30 yr BP. We combine the new absolute data with a revision of published ages onshore, some of which were identified through geomorphological criteria (relative data). We present a revised and updated chronology of volcanism for the last 33 ka that we use to estimate the maximum eruptive recurrence of the island. The number of events per year determined is 9.7 × 10-4 for the emerged part of the island, which means that, as a minimum, one eruption has occurred approximately every 1000 years. This highlights the need of more geochronological data to better constrain the eruptive recurrence of El Hierro.

  13. Geochronology of the Rio Formoso estuarine by {sup 210}Pb

    Energy Technology Data Exchange (ETDEWEB)

    Arruda, Gilberto N.; Lyra, Denilson T.; Melo, Julyanne T.B.; Farias, Emerson E.G.; Franca, Elvis J.; Santos, Thiago O., E-mail:, E-mail:, E-mail:, E-mail:, E-mail: [Centro Regional de Ciencias Nucleares do Nordeste (CRCN-NE/CNEN-PE), Recife, PE (Brazil); Souza Neto, Joao A., E-mail: [Universidade Federal de Pernambuco (UFPE), Recife, PE (Brazil)


    Uranium series disequilibrium is useful for dating methods, in which profile sediments can be considered as historical records of anthropogenic events regarding the distribution and impacts of chemical substances on the environment. In this study, 2 deep sediment profiles (about 1 m) were collected, layered at each 3 cm, oven-dried and homogenized. The radiochemical separation of {sup 210}Pb consisted of using hydrobromic acid and an ion exchange resin (DOWEX) for precipitating {sup 210}Pb in the form of lead chromate. After 10 days, the radioactivity was therefore measured by means of the low level gas flow proportional counter, model S5-XLB, from Canberra. Sedimentation rate were obtained by CIC (Constant Initial Concentration) model assumes a constant sedimentation rate throughout the period over which unsupported {sup 210}Pb is measurable. Some sediment profiles were not dated since the percentage of sand was quite high in top layers or a high percentage of organic matter and water in excess were observed in the all sediment samples. {sup 210}Pb geochronology was successfully applied to age nine sediment profiles, in which higher sedimentation rates were observed in the middle portion of the estuarine probably related to shrimp farming impacts. By using geochronology, the detection of human impacts on chemical element distribution could be enhanced in the case of environmental monitoring studies in the Rio Formoso estuarine. (author)

  14. Geochronology and Tectonic Evolution of the Lincang Batholith in Southwestern Yunnan, China


    Hongyuan Zhang; Junlai Liu; Wenbin Wu


    Geochronological research of the Lincang Batholith is one key scientific problem to discuss the tectonic evolution of the Tethys. Two granitic specimens were selected from the Mengku-Douge area in the Lincang Batholith to perform the LA-ICPMS Zircon U-Pb dating based on thorough review of petrological, geochemical, and geochronological data by the former scientists. Rock-forming age data of biotite granite specimen from Kunsai is about 220 Ma, the Norian age. However, the west sample from Men...

  15. Paleogene continental margin truncation in southwestern Mexico: Geochronological evidence (United States)

    Schaaf, Peter; MoráN-Zenteno, Dante; HernáNdez-Bernal, Maria Del Sol; SolíS-Pichardo, Gabriela; Tolson, Gustavo; KöHler, Hermann


    The reasons for, and mechanisms of, continental margin truncation in SW Mexico where Mesozoic-Cenozoic plutons are situated directly on the Pacific coast, are not yet well understood. Large-scale dextral and/or sinistral displacements of the continental margin terranes, now forming parts of Baja California or the Chortis block, have been proposed. The well-defined along-coast NW-SE decreasing granitoid intrusion age trend (˜1.2 cm/yr in the 100 Ma-40 Ma time interval) between Puerto Vallarta and Zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the Chortis block from those latitudes toward the SE. Changes in the dip and velocity of the NNW-SSE trending Cretaceous-Tertiary subduction zone resulted in a landward migration of the magmatic arc. Taking into account certain stratigraphic affinities of Chortis and the Oaxaca and Mixteca terranes, together with the known displacement rates along the North America-Caribbean Plate boundary, the northwesternmost paleoposition of the Chortis block with respect to SW Mexico was near Zihuatanejo. In contrast, between Zihuatanejo and the Isthmus of Tehuantepec, the cessation of the Tertiary magmatism decreased more rapidly (˜7.7 cm/yr), although the trend is not so obvious. Starting in the late Eocene, Chortis moved about 1100 km to the SE along a transform boundary associated with the opening of the Cayman Trough. Based on our geochronological data and structural relationships between mylonite zones and plutons in the Acapulco-Tehuantepec area, we propose an approximately 650 km SE movement of Chortis from about 40-25 Ma, with a velocity of 6.5-4.3 cm/yr. Since this is considerably slower than the decreasing age trend obtained by us using the geochronological data, we consider batholith formation in this segment to predate and postdate the offshore passage of the North America-Farallon-Caribbean triple junction. Geological

  16. Amino acid geochronology of raised beaches in south west Britain (United States)

    Bowen, D. Q.; Sykes, G. A.; Reeves (nee Henry), Alayne; Miller, G. H.; Andrews, J. T.; Brew, J. S.; Hare, P. E.

    Based on (1) the epimerization of L:isoleucine to D:alloisoleucine ( {D}/{L} ratios) in Patella vulgata, Littorina littorea, L. littoralis, L. saxatilis, Littorina species and Nucella lapillus from raised beaches in south west Britain, (2) statistical analysis of the {D}/{L} ratios, and (3) lithostratigraphic and geomorphic evaluation, three ( {D}/{L}) Stages are proposed. The {D}/{L} ratios for all the species measured are converted to a Patella vulgata standard. The three ( {D}/{L}) Stages are: (1) The Minchin Hole ( {D}/{L}) Stage, {D}/{L} ratios 0.175 ± 0.014, defined at a stratotype in Minchin Hole Cave, Gower, Wales. (2) A provisionally defined, but as yet, unamed ( {D}/{L}) Stage, because of the current unavailability of a suitable stratotype, with {D}/{L} ratios of 0.135 ± 0.014 (3) The Pennard ( {D}/{L}) Stage, {D}/{L} ratios 0.105 ± 0.016, defined at a stratotype in Minchin Hole Cave, Gower, Wales. Two geochronological models of the three high sea-level events representing the {D}/{L} Stages are constrained by uranium-series age determinations on stalagmite interbedded with marine beds in Minchin Hole and Bacon Hole Caves, Gower, Wales. A potential 'fixed point' in model evaluation is an age determination which is equivalent to Oxygen Isotope Sub-stage 5e (122 ka). The two models are:

  17. Isotope geochronology of Dapingzhang spilite-keratophyre formation in Yunnan Province and its geological significance

    Institute of Scientific and Technical Information of China (English)

    钟宏; 胡瑞忠; 叶造军; 涂光炽


    On the basis of the geological field investigations and isotope geochronological studies the Sm-Nd isochron age (513 Ma?0 Ma), Rb-Sr isochron age (511 Ma? Ma) and K-Ar age (312-317 Ma) of the Dapingzhang spilite-keratophyre formation in Yunnan Province are presented. From these geochronological data it is evidenced that this suite of volcanic rocks was formed in the Cambrian and the parent magma was derived from a depleted mantle, which was influenced by crustal contamination and/or seawater hydrothermal alteration. During the Late Carboniferous the volcanic rocks experienced relatively strong geological reworking. This study provides geochronological evidence for the occurrence of Cambrian volcanic rocks in the Sanjiang (three-river) area.

  18. Isotope geochronology of Dapingzhang spilite-keratophyre formation in Yunnan Province and its geological significance

    Institute of Scientific and Technical Information of China (English)


    On the basis of the geological field investigations and isotope geochronological studies the Sm-Nd isochron age (513 Ma±40 Ma),Rb-Sr isochron age (511 Ma±8 Ma) and K-Ar age (312-317 Ma) of the Dapingzhang spilite-keratophyre formation in Yunnan Province are presented.From these geochronological data it is evidenced that this suite of volcanic rocks was formed in the Cambrian and the parent magma was derived from a depleted mantle,which was influenced by crustal contamination and/or seawater hydrothermal alteration.During the Late Carboniferous the volcanic rocks experienced relatively strong geological reworking.This study provides geochronological evidence for the occurrence of Cambrian volcanic rocks in the Sanjiang (three-river) area.

  19. Episodic growth of Mt. Shasta, CA, documented by argon geochronology (United States)

    Calvert, A. T.; Christiansen, R. L.


    eruptive focus shifted 1.5 km north (Misery Hill) between 50-35 ka, erupting silicic andesite and mafic dacite onto all sectors of the volcano. Flank vents directly south and north erupted domes and lavas 20-15 ka. At ~11 ka a voluminous episode began with the subplinian Red Banks pumice followed shortly by Shastina andesite/dacite lavas, domes, and pyroclastic flows, and soon after by Black Butte flank dacites. Existing 14C geochronology, and stratigraphic studies of the deposits show no eruptive breaks and constrain the episode to have lasted less than a few hundred years. Subsequent Holocene eruptions all issued from the modern summit (Hotlum cone), producing at least 10 large lava flows directed toward the NE sector, along with pyroclastic and debris flows, and a summit dome. Preliminary argon geochronology in progress dates summit lavas at 8.8, 5.8 and 4.7 ka.

  20. Geology, zircon geochronology, and petrogenesis of Sabalan volcano (northwestern Iran) (United States)

    Ghalamghash, J.; Mousavi, S. Z.; Hassanzadeh, J.; Schmitt, A. K.


    Sabalan Volcano (NW Iran) is an isolated voluminous (4821 m elevation; > 800 km2) composite volcano that is located within the Arabia-Eurasia collision zone. Its edifice was assembled by recurrent eruptions of trachyandesite and dacite magma falling into a relatively restricted compositional range (56-67% SiO2) with high-K calc-alkaline and adakitic trace element (Sr/Y) signatures. Previous K-Ar dating suggested protracted eruptive activity between 5.6 and 1.4 Ma, and a two stage evolution which resulted in the construction of the Paleo- and Neo-Sabalan edifices, respectively. The presence of a topographic moat surrounding Neo-Sabalan and volcanic breccias with locally intense hydrothermal alteration are indicative of intermittent caldera collapse of the central part of Paleo-Sabalan. Volcanic debris-flow and debris-avalanche deposits indicate earlier episodes of volcanic edifice collapse during the Paleo-Sabalan stage. In the Neo-Sabalan stage, three dacitic domes extruded to form the summits of Sabalan (Soltan, Heram, and Kasra). Ignimbrites and minor pumice fall-out deposits are exposed in strongly dissected drainages that in part have breached the caldera depression. Lavas and pyroclastic rocks are varyingly porphyritic with Paleo-Sabalan rocks being trachyandesites carrying abundant phenocrysts (plagioclase + amphibole + pyroxene + biotite). The Neo-Sabalan rocks are slightly more evolved and include dacitic compositions with phenocrysts of plagioclase + amphibole ± alkali-feldspar ± quartz. All Sabalan rock types share a common accessory assemblage (oxides + apatite + zircon). High spatial resolution and sensitivity U-Pb geochronology using Secondary Ionization Mass Spectrometry yielded two clusters of zircon ages which range from 4.5 to 1.3 Ma and 545 to 149 ka, respectively (all ages are averages of multiple determinations per sample). U-Th zircon geochronology for selected Neo-Sabalan rocks agrees with the U-Pb ages, with the youngest zircon rims dating

  1. Geology, geochemistry and geochronology of the Songwe Hill carbonatite, Malawi (United States)

    Broom-Fendley, Sam; Brady, Aoife E.; Horstwood, Matthew S. A.; Woolley, Alan R.; Mtegha, James; Wall, Frances; Dawes, Will; Gunn, Gus


    Songwe Hill, Malawi, is one of the least studied carbonatites but has now become particularly important as it hosts a relatively large rare earth deposit. The results of new mapping, petrography, geochemistry and geochronology indicate that the 0.8 km diameter Songwe Hill is distinct from the other Chilwa Alkaline Province carbonatites in that it intruded the side of the much larger (4 × 6 km) and slightly older (134.6 ± 4.4 Ma) Mauze nepheline syenite and then evolved through three different carbonatite compositions (C1-C3). Early C1 carbonatite is scarce and is composed of medium-coarse-grained calcite carbonatite containing zircons with a U-Pb age of 132.9 ± 6.7 Ma. It is similar to magmatic carbonatite in other carbonatite complexes at Chilwa Island and Tundulu in the Chilwa Alkaline Province and others worldwide. The fine-grained calcite carbonatite (C2) is the most abundant stage at Songwe Hill, followed by a more REE- and Sr-rich ferroan calcite carbonatite (C3). Both stages C2 and C3 display evidence of extensive (carbo)-hydrothermal overprinting that has produced apatite enriched in HREE (<2000 ppm Y) and, in C3, synchysite-(Ce). The final stages comprise HREE-rich apatite fluorite veins and Mn-Fe-rich veins. Widespread brecciation and incorporation of fenite into carbonatite, brittle fracturing, rounded clasts and a fenite carapace at the top of the hill indicate a shallow level of emplacement into the crust. This shallow intrusion level acted as a reservoir for multiple stages of carbonatite-derived fluid and HREE-enriched apatite mineralisation as well as LREE-enriched synchysite-(Ce). The close proximity and similar age of the large Mauze nepheline syenite suggests it may have acted as a heat source driving a hydrothermal system that has differentiated Songwe Hill from other Chilwa carbonatites.

  2. The Violent Early Solar System, as Told by Sample Geochronology (United States)

    Cohen, Barbara


    One of the legacies of the samples collected by the Apollo and Luna missions is the link forged between radiometric ages of rocks and relative ages according to stratigraphic relationships and impact crater size-frequency distributions. Our current understanding of the history of the inner solar system is based on the relative chronology of individual planets, tied to the absolute geochronology of the Moon via these important samples. Sample ages have enabled us to infer that impact-melt breccias from Apollo 14 and 15 record the formation of the Imbrium Basin, those from the highland massifs at Apollo 17 record the age of Serenitatis, those from the KREEP-poor Apollo 16 site record the age of Nectaris, and materials from Luna 24 record the age of Crisium. Ejecta from smaller and younger craters Copernicus and Tycho were sampled at Apollo 12 and 17, respectively, and local craters such as Cone at Apollo 14, and North Ray and South Ray at Apollo 16 were also sampled and ages determined for those events. Much of what we understand about the lunar impact flux is based on these ages. Samples from these nearside locations reveal a preponderance of impact-disturbed or recrystallized ages between 3.75 and 3.95 billion years. Argon and lead loss (and correlated disturbances in the Rb-Sr system) have been attributed to metamorphism of the lunar crust by an enormous number of impacts in a brief pulse of time, called the Lunar Cataclysm or Late Heavy Bombardment. Subsequent high-precision geochronometric analyses of Apollo samples and lunar highlands meteorites show a wider range of ages, but very few older than 4 Ga. The paucity of ancient impact melt rocks has been interpreted to mean that either that most impact basins formed at this time, or that ejecta from the large, near-side, young basins dominates the Apollo samples.

  3. ID-TIMS Geochronology of ca 1 Ma leucogranites from the core of Nanga Parbat (United States)

    Bowring, S. A.; Searle, M. P.; Waters, D. J.; Hodges, K. V.; Schmitz, M. D.; Crowley, J.


    Although isotope-dilution, themal-ioniaztion mass spectrometry (ID-TIMS) is the most precise method available for U-Th-Pb geochronology of accessory minerals, the technique is traditionally not applied to samples youger than ca. 10 Ma due to low concentrations of radiogenic Pb. However, recent analytical advances allow the routine measurement of less than 5-10 picograms of radiogenic Pb, and the reduction of laboratory blanks to extremely low levels. As a consequence, it is now possible to use ID-TIMS for U-Pb geochronology of very young accessory mineral suites to develop new insights regarding the time scales and patterns of inheritance, melt production, and segregation in collisional orogens. The Nanga Parbat syntaxis of the northwestern Himalaya has had a long and complex thermal history, but it is famous for the evidence it provides for extremely young (Nanga Parbat in less than 1 million years.

  4. Provisional zircon and monazite uranium-lead geochronology for selected rocks from Vermont (United States)

    Aleinikoff, John N.; Ratcliffe, Nicholas M.; Walsh, Gregory J.


    This report presents the results of zircon and monazite uranium-lead (U-Pb) geochronologic analyses of 24 rock samples. The samples in this study were collected from mapped exposures identified while conducting either new, detailed (1:24,000-scale) geologic quadrangle mapping or reconnaissance mapping, both of which were used for compilation of the bedrock geologic map of Vermont. All of the collected samples were judged to be igneous rocks (either intrusive or extrusive) on the basis of field relations and geochemistry. The one exception is the Okemo Quartzite on Ludlow Mountain. These geochronologic data were used to supplement regional correlations between igneous suites on the basis of similar geochemistry and geologic mapping.

  5. The Potassium-Argon Laser Experiment (KArLE): In Situ Geochronology for Planetary Robotic Missions (United States)

    Cohen, Barbara


    The Potassium (K) - Argon (Ar) Laser Experiment (KArLE) will make in situ noble-gas geochronology measurements aboard planetary robotic landers and roverss. Laser-Induced Breakdown Spectroscopy (LIBS) is used to measure the K abun-dance in a sample and to release its noble gases; the evolved Ar is measured by mass spectrometry (MS); and rela-tive K content is related to absolute Ar abundance by sample mass, determined by optical measurement of the ablated volume. KArLE measures a whole-rock K-Ar age to 10% or better for rocks 2 Ga or older, sufficient to resolve the absolute age of many planetary samples. The LIBS-MS approach is attractive because the analytical components have been flight proven, do not require further technical development, and provide complementary measurements as well as in situ geochronology.

  6. Geochronology of Cenozoic rocks in the Bodie Hills, California and Nevada (United States)

    Fleck, Robert J.; du Bray, Edward A.; John, David A.; Vikre, Peter G.; Cosca, Michael A.; Snee, Lawrence W.; Box, Stephen E.


    The purpose of this report is to present geochronologic data for unaltered volcanic rocks, hydrothermally altered volcanic rocks, and mineral deposits of the Miocene Bodie Hills and Pliocene to Pleistocene Aurora volcanic fields of east-central California and west-central Nevada. Most of the data presented here were derived from samples collected between 2000–13, but some of the geochronologic data, compiled from a variety of sources, pertain to samples collected during prior investigations. New data presented here (tables 1 and 2; Appendixes 1–3) were acquired in three U.S. Geological Survey (USGS) 40Ar/39Ar labs by three different geochronologists: Robert J. Fleck (Menlo Park, CA), Lawrence W. Snee (Denver, CO), and Michael A. Cosca (Denver, CO). Analytical methods and data derived from each of these labs are presented separately.

  7. Small-Volume U-Pb Zircon Geochronology by Laser Ablation-Multicollector-ICP-MS (United States)


    detrital and metamorphic history of a granulite -facies paragneiss — demonstrate the utility of this technique to a variety of geologic problems and confirm... granulite -facies pelitic gneiss collected inn geochronology by laser ablation-multicollector-ICP-MS, Chemical Fig. 11. A) Concordia plot of all...zircons to unravel the detrital–metamorphic history of a granulite -facies paragneiss from east Greenland. In addition to the small spot diameter, the

  8. The Potassium-Argon Laser Experiment (karle): In Situ Geochronology for Planetary Missions (United States)

    Cohen, B. A.


    Isotopic dating is an essential tool to establish an absolute chronology for geological events. It enables a planet's crystallization history, magmatic evolution, and alteration to be placed into the framework of solar system history. The capability for in situ geochronology will open up the ability for this crucial measurement to be accomplished as part of lander or rover complement. An in situ geochronology package can also complement sample return missions by identifying the most interesting rocks to cache or return to Earth. Appropriate application of in situ dating will enable geochronology on more terrains than can be reached with sample-return missions to the Moon, Mars, asteroids, outer planetary satellites, and other bodies that contain rocky components. The capability of flight instruments to conduct in situ geochronology is called out in the NASA Planetary Science Decadal Survey and the NASA Technology Roadmap as needing development to serve the community's needs. Beagle 2 is the only mission launched to date with the explicit aim to perform in situ K-Ar isotopic dating [1], but it failed to communicate and was lost. The first in situ K-Ar date on Mars, using SAM and APXS measurements on the Cumberland mudstone [2], yielded an age of 4.21 +/- 0.35 Ga and validated the idea of K-Ar dating on other planets, though the Curiosity method is not purpose-built for dating and requires many assumptions that degrade its precision. To get more precise and meaningful ages, multiple groups are developing dedicated in situ dating instruments.

  9. Petrology and geochronology of eclogites from the Variscan Schwarzwald (F.R.G.)


    Kalt, Angelika; Hanel, Michael; Schleicher, Helmut; Kramm, Ulrich


    The Moldanubian basement of the Schwarzwald contains basic to ultrabasic rocks of both crustal and mantle origin which display high-pressure mineral assemblages or relics of such. In order to constrain the P-T-t evolution of the crustal high-pressure rocks, petrological and geochronological studies have been carried out on three eclogite samples. Geothermobarometric estimations indicate minimum metamorphic pressures of 1.6 GPa and equilibration temperatures of 670 750°C. Reaction textures doc...

  10. Zircon geochronology of intrusive rocks from Cap de Creus, eastern Pyrenees



    New petrological and U–Pb zircon geochronological information has been obtained from intrusive plutonic rocks and migmatites from the Cap de Creus massif (Eastern Pyrenees) in order to constrain the timing of the thermal and tectonic evolution of this northeasternmost segment of Iberia during late Palaeozoic time. Zircons from a deformed syntectonic quartz diorite from the northern Cap de Creus Tudela migmatitic complex yield a mean age of 298.8±3.8 Ma. A syntectonic granodiori...

  11. Paleoproterozoic anorogenic granitoids of the Zheltav sialic massif (Southern Kazakhstan): Structural position and geochronology (United States)

    Tretyakov, A. A.; Degtyarev, K. E.; Sal'nikova, E. B.; Shatagin, K. N.; Kotov, A. B.; Ryazantsev, A. V.; Pilitsyna, A. V.; Yakovleva, S. Z.; Tolmacheva, E. V.; Plotkina, Yu. V.


    The basement of the Zheltav sialic massif (Southern Kazakhstan) is composed of different metamorphic rocks united into the Anrakhai Complex. In the southeastern part of the massif, these rocks form a large antiform with the core represented by amphibole and clinopyroxene gneissic granite varieties. By their chemical composition, dominant amphibole (hastingsite) gneissic granites correspond to subalkaline granites, while their petroand geochemical properties make them close to A-type granites. The U-Pb geochronological study of accessory zircons yielded an age of 1841 ± 6 Ma, which corresponds to the crystallization age of melts parental for protoliths of amphibole gneissic granites of the Zheltav Massif. Thus, the structural-geological and geochronological data make it possible to define the Paleoproterozoic (Staterian) stage of anorogenic magmatism in the Precambrian history of the Zheltav Massif. The combined Sm-Nd isotopic—geochronological data and age estimates obtained for detrital zircons indicate the significant role of the Paleoproterozoic tectono-magmatic stage in the formation of the Precambrian continental crust of sialic massifs in Kazakhstan and northern Tien Shan.

  12. Automating U-Pb IDTIMS data reduction and reporting: Cyberinfrastructure meets geochronology (United States)

    Bowring, J. F.; McLean, N.; Walker, J. D.; Ash, J. M.


    We demonstrate the efficacy of an interdisciplinary effort between software engineers and geochemists to produce working cyberinfrastructure for geochronology. This collaboration between CIRDLES, EARTHTIME and EarthChem has produced the software programs Tripoli and U-Pb_Redux as the cyber-backbone for the ID-TIMS community. This initiative incorporates shared isotopic tracers, data-reduction algorithms and the archiving and retrieval of data and results. The resulting system facilitates detailed inter-laboratory comparison and a new generation of cooperative science. The resolving power of geochronological data in the earth sciences is dependent on the precision and accuracy of many isotopic measurements and corrections. Recent advances in U-Pb geochronology have reinvigorated its application to problems such as precise timescale calibration, processes of crustal evolution, and early solar system dynamics. This project provides a heretofore missing common data reduction protocol, thus promoting the interpretation of precise geochronology and enabling inter-laboratory comparison. U-Pb_Redux is an open-source software program that provides end-to-end support for the analysis of uranium-lead geochronological data. The system reduces raw mass spectrometer data to U-Pb dates, allows users to interpret ages from these data, and then provides for the seamless federation of the results, coming from many labs, into a community web-accessible database using standard and open techniques. This EarthChem GeoChron database depends also on keyed references to the SESAR sample database. U-Pb_Redux currently provides interactive concordia and weighted mean plots and uncertainty contribution visualizations; it produces publication-quality concordia and weighted mean plots and customizable data tables. This initiative has achieved the goal of standardizing the data elements of a complete reduction and analysis of uranium-lead data, which are expressed using extensible markup

  13. Advanced Cyberinfrastructure for Geochronology as a Collaborative Endeavor: A Decade of Progress, A Decade of Plans (United States)

    Bowring, J. F.; McLean, N. M.; Walker, J. D.; Gehrels, G. E.; Rubin, K. H.; Dutton, A.; Bowring, S. A.; Rioux, M. E.


    The Cyber Infrastructure Research and Development Lab for the Earth Sciences ( has worked collaboratively for the last decade with geochronologists from EARTHTIME and EarthChem to build cyberinfrastructure geared to ensuring transparency and reproducibility in geoscience workflows and is engaged in refining and extending that work to serve additional geochronology domains during the next decade. ET_Redux (formerly U-Pb_Redux) is a free open-source software system that provides end-to-end support for the analysis of U-Pb geochronological data. The system reduces raw mass spectrometer (TIMS and LA-ICPMS) data to U-Pb dates, allows users to interpret ages from these data, and then facilitates the seamless federation of the results from one or more labs into a community web-accessible database using standard and open techniques. This EarthChem database - - depends on keyed references to the System for Earth Sample Registration (SESAR) database that stores metadata about registered samples. These keys are each a unique International Geo Sample Number (IGSN) assigned to a sample and to its derivatives. ET_Redux provides for interaction with this archive, allowing analysts to store, maintain, retrieve, and share their data and analytical results electronically with whomever they choose. This initiative has created an open standard for the data elements of a complete reduction and analysis of U-Pb data, and is currently working to complete the same for U-series geochronology. We have demonstrated the utility of interdisciplinary collaboration between computer scientists and geoscientists in achieving a working and useful system that provides transparency and supports reproducibility, allowing geochemists to focus on their specialties. The software engineering community also benefits by acquiring research opportunities to improve development process methodologies used in the design, implementation, and sustainability of domain-specific software.

  14. Algorithms and software for U-Pb geochronology by LA-ICPMS (United States)

    McLean, Noah M.; Bowring, James F.; Gehrels, George


    The past 15 years have produced numerous innovations in geochronology, including experimental methods, instrumentation, and software that are revolutionizing the acquisition and application of geochronological data. For example, exciting advances are being driven by Laser-Ablation ICP Mass Spectrometry (LA-ICPMS), which allows for rapid determination of U-Th-Pb ages with 10s of micrometer-scale spatial resolution. This method has become the most commonly applied tool for dating zircons, constraining a host of geological problems. The LA-ICPMS community is now faced with archiving these data with associated analytical results and, more importantly, ensuring that data meet the highest standards for precision and accuracy and that interlaboratory biases are minimized. However, there is little consensus with regard to analytical strategies and data reduction protocols for LA-ICPMS geochronology. The result is systematic interlaboratory bias and both underestimation and overestimation of uncertainties on calculated dates that, in turn, decrease the value of data in repositories such as EarthChem, which archives data and analytical results from participating laboratories. We present free open-source software that implements new algorithms for evaluating and resolving many of these discrepancies. This solution is the result of a collaborative effort to extend the U-Pb_Redux software for the ID-TIMS community to the LA-ICPMS community. Now named ET_Redux, our new software automates the analytical and scientific workflows of data acquisition, statistical filtering, data analysis and interpretation, publication, community-based archiving, and the compilation and comparison of data from different laboratories to support collaborative science.

  15. Discussion: The timing of gold mineralization across the eastern Yilgarn Craton using U-Pb geochronology of hydrothermal phosphate minerals (United States)

    Bateman, Roger; Jones, Sarah


    The presentation of recent geochronological work on orogenic gold deposits in the Eastern Goldfields of the Yilgarn Craton, Western Australia, claims to prove that there is a single broad event of gold mineralization and that structural work demonstrating that there are a number of discrete gold mineralization events is wrong. This new data demonstrates no such thing, as this data, no doubt the best that can currently be produced, shows a very wide and inconsistent range in ages. Geochronology is not yet able to reliably separate these events, which appear to be spread over an interval of perhaps 30 Ma, up to ˜2635 Ma.

  16. Timing is Everything:The Boon and Bane of 14C Geochronology

    Energy Technology Data Exchange (ETDEWEB)

    Guilderson, T; Guilderson, T; Reimer, P J; Brown, T


    There are underappreciated limitations of the conversion of {sup 14}C-ages to the fixed, calendrical time-scale that bear directly upon our understanding of the dynamic climate system, or the relationship between the collapse of one civilization and it's neighbor's. In this paper we present a quantitative assessment of the limits of {sup 14}C-geochronology and calibration onto the absolute calendrical time-scale over the Holocene. We take into account not only the inherent limitations of the {sup 14}C-calendar calibration curve, but also analytical uncertainties.

  17. (U–Th)/He geochronology of goethite and the origin and evolution of cangas


    Monteiro, Hevelyn S.; VASCONCELOS,PAULO M.; Farley, Kenneth A.; Spier, Carlos A.; Mello, Claudio L.


    (U–Th)/He geochronology of 147 grains of goethite cements extracted from ferruginous duricrusts (cangas) developed on banded iron-formations from the Quadrilátero Ferrífero region, Minas Gerais, Brazil, records a history of protracted mineral dissolution–reprecipitation that started at ca. 48.1 ± 4.8 Ma and continues intermittently until the Present. A large majority of the samples (more than 30%) are younger than 2 Ma, revealing active mineral dissolution–reprecipitation in the recent pas...

  18. Applications of dendrochronology and sediment geochronology to establish reference episodes for evaluations of environmental radioactivity

    Energy Technology Data Exchange (ETDEWEB)

    Waugh, W.J.; Carroll, J.; Abraham, J.D.; Landeen, D.S. [Roy F. Weston, Inc., U.S. Department of Energy, Grand Junction Office, 2597 B 3/4 Road, Grand Junction, CO 81503 (United States)


    Dendrochronology and sediment geochronology have been used to demonstrate retrospective monitoring of environmental radioactivity at United States Department of Energy (DOE) sites. {sup 14}C in annual growth rings of sagebrush preserved the temporal and spatial patterns of {sup 14}C resulting from dispersion downwind of a nuclear fuel processing facility at the Hanford Site in Washington State. As far as 10 km downwind of the facility, {sup 14}C concentrations were significantly higher in growth rings formed during a fuel processing episode than in rings produced during preoperational or postoperational episodes. An episode of uranium mill tailings deposition in pond sediments at the Grand Junction Office in Colorado was reconstructed using {sup 210}Pb geochronology constrained by a marker of peak {sup 137}Cs fallout. Uranium concentrations in ponds sediments deposited after the processing episode provide a reasonable cleanup standard. These reference episodes of environmental radioactivity reconstructed from measurements taken within contaminated environments can improve or replace reference area data as baseline information for dose reconstructions, risk assessments, and the establishment of cleanup standards. (Copyright (c) 1998 Elsevier Science B.V., Amsterdam. All rights reserved.)

  19. Chemical Abrasion Applied to LA-ICP-MS U–Pb Zircon Geochronology

    Directory of Open Access Journals (Sweden)

    Quentin G. Crowley


    Full Text Available Zircon (ZrSiO4 is the most commonly used mineral in U–Pb geochronology. Although it has proven to be a robust chronometer, it can suffer from Pb-loss or elevated common Pb, both of which impede precision and accuracy of age determinations. Chemical abrasion of zircon involves thermal annealing followed by relatively low temperature partial dissolution in HF acid. It was specifically developed to minimize or eliminate the effects of Pb-loss prior to analysis using Thermal Ionization Mass Spectrometry (TIMS. Here we test the application of chemical abrasion to Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS by analyzing zircons from both untreated and chemically abraded samples. Rates of ablation for high alpha-dose non-treated zircons are up to 25% faster than chemically abraded equivalents. Ablation of 91500 zircon reference material demonstrates a ca. 3% greater down-hole fractionation of 206Pb/238U for non-treated zircons. These disparities necessitate using chemical abrasion for both primary reference material and unknowns to avoid applying an incorrect laser induced fractionation correction. All treated samples display a marked increase in the degree of concordance and/or lowering of common Pb, thereby illustrating the effectiveness of chemical abrasion to LA-ICP-MS U–Pb zircon geochronology.

  20. Paleontology and geochronology of the Long Beach core sites and monitoring wells, Long Beach, California (United States)

    McDougall, Kristin; Hillhouse, John; Powell, Charles; Mahan, Shannon; Wan, Elmira; Sarna-Wojcicki, Andrei M.


    The U.S. Geological Survey's Focus on Quaternary Stratigraphy in Los Angeles (FOQUS-LA) project was a cooperative coring program between Federal, State, and local agencies. It was designed to provide a better understanding of earthquake potentials and to develop a stratigraphic model of the western Los Angeles Basin in California. The biostratigraphic, geochronologic, and paleoecologic analyses of eight wells drilled during the FOQUS-LA project are presented. These analyses are based on microfossils (benthic and planktic foraminifers), macrofossils, paleomagnetic stratigraphy, optically stimulated luminescence, thermoluminescence, radiocarbon dating, and tephrochronology. A geochronologic framework (incorporating paleomagnetism, luminescence, and tephrochronology) was used to calibrate the sequence stratigraphic units in the FOQUS-LA wells and also was used to calibrate the ages of the microfossil stage and zonal boundaries. The results of this study show that (1) the offshore California margin zones can be used in a nearshore setting, and (2) the California margin zonal scheme refines the chronostratigraphic resolution of the benthic foraminiferal biostratigraphic framework for the Pacific Coast. Benthic foraminiferal stages are modified by the recognition of an early Hallian substage, which is a faunal change recognized throughout the Los Angeles Basin. Although no detailed macrofossil zonations exist for the Quaternary of southern California, several species, whose distribution is regulated by the climatic conditions, are useful as secondary marker species in the shallower water deposits of the Los Angeles Basin.

  1. SHRIMP zircon U-Pb geochronology of Indosinian granites in Hunan Province and its petrogenetic implications

    Institute of Scientific and Technical Information of China (English)

    WANG Yuejun; FAN Weiming; LIANG Xinquan; PENG Touping; SHI Yuruo


    The SHRIMP zircon U-Pb geochronology of three typically Indosinian granitic plutons with peraluminous and potassium-rich affinities (Tangshi ultraunit for Weishan and Baimashan, and Longtan ultraunit for Guandimiao) is presented in Hunan Province, South China. The analyses of zircons from biotite monozonite granites for Weishan, Baimashan and Guandimiao plutons show the single and tight clusters on the concordia, and yield the weighted mean 206Pb/238U ages of 244±4, 243±3 and 239±3 Ma, respectively, representing the crystallized ages of these Indosinian granites. These data suggest that the Indosinian granitic plutons as previously thought formed at a narrow age span. In combination with other data, it is inferred that the Indosinian granites within the South China Block probably distributed in Hunan, Jiangxi, Guangxi and Guangdong provinces as planar shape, and were the derivation of the crustal materials in the intracontinental thickening setting. These precisely geochronological data provide important constraints for better understanding the spatiotemporal pattern of the Indosinian peraluminous granites and early Mesozoic tectonic evolution of the South China Block.

  2. Developement of the Potassium-Argon Laser Experiment (KArLE) for In Situ Geochronology (United States)

    Cohen, Barbara A.


    Absolute dating of planetary samples is an essential tool to establish the chronology of geological events, including crystallization history, magmatic evolution, and alteration. Thus far, radiometric geochronology of planetary samples has only been accomplishable in terrestrial laboratories on samples from dedicated sample return missions and meteorites. In situ instruments to measure rock ages have been proposed, but none have yet reached TRL 6, because isotopic measurements with sufficient resolution are challenging. We have begun work under the NASA Planetary Instrument Definition and Development Program (PIDDP) to develop the Potassium (K) - Argon Laser Experiment (KArLE), a novel combination of several flight-proven components that will enable accurate KAr isochron dating of planetary rocks. KArLE will ablate a rock sample, measure the K in the plasma state using laser-induced breakdown spectroscopy (LIBS), measure the liberated Ar using quadrupole mass spectrometry (QMS), and relate the two by measuring the volume of the abated pit using a optical methods such as a vertical scanning interferometer (VSI). Our preliminary work indicates that the KArLE instrument will be capable of determining the age of several kinds of planetary samples to 100 Myr, sufficient to address a wide range of geochronology problems in planetary science. Additional benefits derive from the fact that each KArLE component achieves analyses common to most planetary surface missions.

  3. Continued Development of in Situ Geochronology for Planetary Using KArLE (Potassium-Argon Laser Experiment) (United States)

    Devismes, D.; Cohen, B. A.


    Geochronology is a fundamental measurement for planetary samples, providing the ability to establish an absolute chronology for geological events, including crystallization history, magmatic evolution, and alteration events, and providing global and solar system context for such events. The capability for in situ geochronology will open up the ability for geochronology to be accomplished as part of lander or rover complement, on multiple samples rather than just those returned. An in situ geochronology package can also complement sample return missions by identifying the most interesting rocks to cache or return to Earth. The K-Ar radiometric dating approach to in situ dating has been validated by the Curiosity rover on Mars as well as several laboratories on Earth. Several independent projects developing in situ rock dating for planetary samples, based on the K-Ar method, are giving promising results. Among them, the Potassium (K)-Argon Laser Experiment (KArLE) at MSFC is based on techniques already in use for in planetary exploration, specifically, Laser-induced Breakdown Spectroscopy (LIBS, used on the Curiosity Chemcam), mass spectroscopy (used on multiple planetary missions, including Curiosity, ExoMars, and Rosetta), and optical imaging (used on most missions).

  4. Scanning ion imaging - a potent tool in SIMS U -Pb zircon geochronology (United States)

    Whitehouse, M. J.; Fedo, C.; Kusiak, M.; Nemchin, A.


    The application of high spatial resolution (BSE) characterisation, has revolutionised geochronology over the past 25 years, re-vealing complexities of crustal evolution from zoned zircons. In addition to ge-ochronology, such studies now commonly form the basis of broader investigations using O- and Hf- isotopes and trace elements obtained from the same growth zone as age, circumventing ambiguities commonly present in bulk-rock isotope studies. The choice of analytical beam diameter is often made to maximise the precision of data obtained from a given area of analysis within an identifiable growth zone. In cases where zircons yield poorly constrained internal structures in SEM, high spatial resolution spot analyses may yield uninterpretable and/or meaningless mixed ages by inadvertent sampling across regions with real age differences. Scanning ion imaging (SII) has the potential to generate accurate and precise geochrono-logical data with a spatial resolution down to ca. 2 μm, much higher than that of a normal spot analysis. SII acquisition utilises a rastered primary beam to image an area of the sample with a spatial resolution dependent on the selected primary beam diameter. On the Cameca ims1270/80 instruments, the primary beam scanning is coupled with the dynamic transfer optical system (DTOS) which deflects the secondary ions back on to the ion optical axis of the instrument regardless of where in the raster illuminated area the ions originated. This feature allows retention of a high field magnification (= high transmission) mode and the ability to operate the mass spectrometer at high mass resolution without any compromise in the quality of the peak shape. Secondary ions may be detected either in a sequential (peak hopping) mono-collection mode or simultaneous multicollection mode using low-noise pulse counting electron multipliers. Regardless of the detection mode, data are acquired over sufficient cycles to generate usable counting statistics from selected

  5. Microstructural and geochronology data of the Coacoyulillo-Intzcuinatoyac sector rocks, Guerrero state, Mexico (United States)

    Espejo Bautista, G.; Hernandez, T.; Elías, M.; Solari, L.


    The south of Mexico is constituted by many conjugated geological events with volcanic, sedimentary, tectonic, plutonic and metamorphic sequences. The exposed evidences establish the pieces of a complex puzzle which were analyzed in this study for geochronological and structural purposes in context with the observed field relationships. According to Campa and Coney (1983), Mexico is constituted by sixteen tectonostratigraphic terranes, which are distinguished by their basement petrology, although they can superficially share an identical geological record. The study area is located in a triple point where the Xolapa, Guerrero and Mixteca terranes converge and covers the adjacent areas to the Intzcuinatoyac and Coacoyulillo villages, southwest of the city of Chilpancingo in the Guerrero state. This study was performed in a region which is integrated by geological events covered by ductile-fragile regional deformation of great magnitude, consisting of mylonitic rocks and less than 60 to 132 Ma magmatic events, associated with a subduction scenario. The analyzed magmatic events are geographically distributed along the El Zapote and El Reparo rivers and can be classified into three different magmatic pulses, determined by field relations and geochronological data. However, the age of the deformations and deformational events were constrained by two or more magmatic and / or tectonic events. The oldest magmatism is represented by felsic intrusive rocks, which were later affected by ductile deformation, developing mylonitic textures. The deformed intrusive forms part of a mylonitic sector associated to a tectonic contact between the Xolapa, Mixteca and Guerrero terranes. This mylonitic sector can be correlated with other mylonitic areas within the state of Guerrero due to its ductile structural features and its geochronology, such as La Venta and the Ocotito areas. The mylonitic areas of Guerrero form a belt that may be associated with the Chacalapa fault in the state of

  6. Relationship between LIBS Ablation and Pit Volume for Geologic Samples: Applications for in situ Absolute Geochronology (United States)

    Devismes, D.; Cohen, Barbara A.


    In planetary sciences, in situ absolute geochronology is a scientific and engineering challenge. Currently, the age of the Martian surface can only be determined by crater density counting. However this method has significant uncertainties and needs to be calibrated with absolute ages. We are developing an instrument to acquire in situ absolute geochronology based on the K-Ar method. The protocol is based on the laser ablation of a rock by hundreds of laser pulses. Laser Induced Breakdown Spectroscopy (LIBS) gives the potassium content of the ablated material and a mass spectrometer (quadrupole or ion trap) measures the quantity of 40Ar released. In order to accurately measure the quantity of released 40Ar in cases where Ar is an atmospheric constituent (e.g., Mars), the sample is first put into a chamber under high vacuum. The 40Arquantity, the concentration of K and the estimation of the ablated mass are the parameters needed to give the age of the rocks. The main uncertainties with this method are directly linked to the measures of the mass (typically some µg) and of the concentration of K by LIBS (up to 10%). Because the ablated mass is small compared to the mass of the sample, and because material is redeposited onto the sample after ablation, it is not possible to directly measure the ablated mass. Our current protocol measures the ablated volume and estimates the sample density to calculate ablated mass. The precision and accuracy of this method may be improved by using knowledge of the sample's geologic properties to predict its response to laser ablation, i.e., understanding whether natural samples have a predictable relationship between laser energy deposited and resultant ablation volume. In contrast to most previous studies of laser ablation, theoretical equations are not highly applicable. The reasons are numerous, but the most important are: a) geologic rocks are complex, polymineralic materials; b) the conditions of ablation are unusual (for example

  7. In Situ Potassium-Argon Geochronology Using Fluxed Fusion and a Double Spike (United States)

    Hurowitz, Joel A.; Hecht, Michael H.; Zimmerman, Wayne F.; Neidholdt, Evan L.; Sinha, Mahadeva P.; Sturhahn, Wolfgang; Coleman, Max; McCleese, Daniel J.; Farley, Kenneth A.; Eiler, John M.; Rossman, George R.; Waltenberg, Kathryn


    A document highlights an Li-based fluxing agent that enables sample fusion and quantitative Ar-release at relatively low temperatures (900-1,000 C), readily achievable with current flight resistance furnace designs. A solid, double spike containing known quantities of Ar-39 and K-41 was developed that, when added in known amounts to a sample, enables the extraction of a Ar-40/K-40 ratio for age estimation without a sample mass measurement. The use of a combination of a flux and a double spike as a means of solving the mechanical hurdles to an in situ K-Ar geochronology measurement has never been proposed before. This methodology and instrument design would provide a capability for assessing the ages of rocks and minerals on the surfaces of planets and other rocky terrestrial bodies in the solar system.

  8. Preliminary K/Ar geochronology of the Crater Basalt volcanic field (CBVF, northern Patagonia

    Directory of Open Access Journals (Sweden)

    Z. Pécskay


    Full Text Available The Crater Basalt volcanic field is one of the Quaternary intraplate basaltic fields in northern Patagonia. A systematic geological, volcanological and geochronological study of CBVF indicates a multistage history of eruptions of basaltic volcanoes. K/Ar dating, using whole rock samples shows that the measured analytical ages are fully consistent with the available stratigraphic control. The radiometric ages fall into three distinct, internally consistent age groups, which give evidence that there were at least three major episodes of volcanic activity, at about 1.0 Ma, 0.6 Ma and 0.3 Ma ago. The age differences appear to be just significant, even although less than 10 % radiogenic argon was found in the isotope analysis of whole rock samples.

  9. Promise and Pitfalls of Lu/Hf-Sm/Nd Garnet Geochronology (United States)

    King, R. L.; Vervoort, J. D.; Kohn, M. J.; Zirakparvar, N. A.; Hart, G. L.; Corrie, S. L.; Cheng, H.


    Our ability to routinely measure Lu-Hf and Sm-Nd isotopes in garnet allows broad new applications in geochronology, petrology, and tectonics. However, applications of these data can be limited by challenges in interpreting the petrologic record and preparing garnets for analysis. Here, we examine petrologic and chemical pitfalls encountered in garnet geochronology. Petrologic factors influencing trace element compositions in garnet include reactions that modify REE availability and partitioning (1,2), kinetically limited transfer of REEs to garnet (3), and bulk compositional heterogeneities (4). Interpreting the effects of these processes on Sm/Nd and Lu/Hf ages requires characterizing REE zonation prior to isotope analysis and age interpretation. Because garnet fractions are traditionally picked from crushed samples without regard to intracrystalline origins or chemistries, isochrons will represent mixtures derived to varying degrees from all periods of garnet growth. While measured zoning might generally indicate what garnet portion dominates the Lu/Hf or Sm/Nd budget, traditional mineral separation will rarely realize the chronologic potential afforded by high precision Hf and Nd isotope measurements. The potential use of alternative techniques, such as microsampling, necessitates selective digestion and/or leaching to eliminate inclusions within garnet. For Sm/Nd geochronology, H2SO4 leaching removes LREE-rich phosphates (e.g. apatite), but not silicates (e.g. epidote), precluding Sm-Nd dating of some rocks. For Lu/Hf geochronology, ubiquitous zircon microinclusions (c. 1 μm) can significantly disrupt age determinations. Microinclusions cannot be detected optically or separated physically, requiring selective chemical digestion. If complete digestion methods, such as bomb digestion, are used for garnet fractions, then "common Hf" from zircon will be contained in final solutions. These mixed analyses are of dubious utility and will fall into one of two

  10. Mercury flux to estuarine sediments, derived from Pb-210 and Cs-137 geochronologies (Guaratuba Bay, Brazil)

    Energy Technology Data Exchange (ETDEWEB)

    Sanders, Christian J. [Departamento de Geoquimica, Universidade Federal Fluminense, 24020-007 Niteroi-RJ (Brazil)]. E-mail; Santos, Isaac R. [Department of Oceanography, Florida State University, Tallahassee, FL, 32306 (United States); Silva-Filho, Emmanoel V. [Departamento de Geoquimica, Universidade Federal Fluminense, 24020-007 Niteroi-RJ (Brazil); Patchineelam, Sambasiva R. [Departamento de Geoquimica, Universidade Federal Fluminense, 24020-007 Niteroi-RJ (Brazil)


    A sediment core from Guaratuba Bay was used to indicate possible Hg modifications to this coastal environment brought about by growing agricultural activity. Sedimentation rates were estimated to be 6.1 mm/year and 5.2 mm/year through {sup 21}Pb and {sup 137}Cs geochronologies, respectively. Mercury concentrations and organic matter ratios in the surface layers are greater than in the older sediments, supporting the hypothesis of anthropogenic enrichment. Results show that the Hg flux has raised more than twofold during the second half of the 20th century. These results point to the need for further studies to substantiate the hypothesis of anthropogenic enrichment and to quantify point sources of Hg to this estuary.

  11. Geochronology of the Palaeoproterozoic Kautokeino Greenstone Belt, Finnmark, Norway, in its Fennoscandian context (United States)

    Bingen, Bernard; Solli, Arne; Viola, Giulio; Sverre Sandstad, Jan; Torgersen, Espen; Whitehouse, Martin J.; Skår, Øyvind; Nasuti, Aziz


    The northeastern part of the Fennoscandian Shield consists of Archaean cratonic blocks alternating with Palaeoproterozoic greenstone belts ranging in age from c. 2500 to 1950 Ma. Traditionally, the greenstones are interpeted as evidence for rifting of the Archaean continent(s) although it remains unclear whether modern-style oceanic lithosphere developed, followed by a Wilson-cycle-type closure during the Svecokarelian orogeny. Existing geological, isotopic and geochronological data show that the exposed basins hosting the greenstones have distinct lithostratigraphies and geological evolutions and are pericontinental rather than oceanic. A diversity of Palaeoproterozoic mafic mantle derived magmatic rocks show a secular increase of Nd value with time, from EpsilonNd =-2 at 2500 Ma (Shalskiy dikes, Onega, Russia) to EpsilonNd =+4.4 at 2090 Ma (Jouttiaapa basalts, Peräpohja, Finland), suggesting that the regional asthenospheric mantle was less depleted than the model MORB-producing depleted mantle before 2090 Ma. In this work, we report new zircon U-Pb geochronological data in 19 samples from Finnmarkvidda, Norway, to constrain the evolution of the Palaeoproterozoic high-strain Kautokeino Greenstone Belt and its relations with the neighbouring felsic Jergul and Ráiseatnu gneiss complexes. The Jergul complex is an Archaean, low heat flow, TTG cratonic bloc of Karelian affinity formed between 2975 ±10 and 2776 ±6 Ma. The Masi formation, at the base of the Kautokeino Greenstone Belt, is a typical Jatulian quartzite unconformably overlying the Archean basement. An albite-magnetite-rich mafic sill, similar to the Haaskalehto intrusion in Finland, provides a minimum age of 2220 ±7 Ma for the deposition of the quartzite. The Likčá and Čáskejas formations represent the main basaltic volcanism. Direct evidence of an oceanic setting or oceanic suture is lacking. A probably synvolcanic gabbro sill gives an age of 2137 ±5 Ma. Published Sm-Nd whole-rock data on

  12. Geochronology of Betula extensions in pollen diagrams of Alpine Late-glacial lake deposits: A case study of the Late-glacial deposits of the Gasserplatz soil archives (Vorarlberg, Austria)

    NARCIS (Netherlands)

    van Mourik, J.M.; Slotboom, R.T.; van der Plicht, J.; Streurman, H.J.; Hoek, W.J.; Kuijper, W.J.; de Graaff, L.W.S.


    Pollen diagrams of Alpine Late-glacial calcareous lake deposits show several extensions of Betula. The geochronology of these extensions cannot be based on radiocarbon dating due to reservoir effects of such lakes on the radiocarbon ages. A robust geochronology can be based on the oxygen isotope str

  13. Geochronology of Betula extensions in pollen diagrams of Alpine Late-glacial lake deposits : A case study of the Late-glacial deposits of the Gasserplatz soil archives (Vorarlberg, Austria)

    NARCIS (Netherlands)

    van Mourik, Jan M.; Slotboom, Ruud T.; van der Plicht, Johannes; Streurman, Harm Jan; Kuijper, Wim J.; Hoek, Wim Z.; de Graaff, Leo W. S.


    Pollen diagrams of Alpine Late-glacial calcareous lake deposits show several extensions of Betula. The geochronology of these extensions cannot be based on radiocarbon dating due to reservoir effects of such lakes on the radiocarbon ages. A robust geochronology can be based on the oxygen isotope str

  14. Geochronology of Betula extensions in pollen diagrams of Alpine Late-glacial lake deposits: A case study of the Late-Glacial deposits of the Gasserplatz soil archives (Vorarlberg, Austria)

    NARCIS (Netherlands)

    van Mourik, J.M.; Slotboom, R.T.; van der Plicht, J.; Streurman, H.J.; Kuijper, W.J.; Hoek, W.Z.; de Graaff, L.W.S.


    Pollen diagrams of Alpine Late-glacial calcareous lake deposits show several extensions of Betula. The geochronology of these extensions cannot be based on radiocarbon dating due to reservoir effects of such lakes on the radiocarbon ages. A robust geochronology can be based on the oxygen isotope str

  15. Community-Based Development of Standards for Geochemical and Geochronological Data (United States)

    Lehnert, K. A.; Walker, D.; Vinay, S.; Djapic, B.; Ash, J.; Falk, B.


    The Geoinformatics for Geochemistry (GfG) Program ( and the EarthChem project ( aim to maximize the application of geochemical data in Geoscience research and education by building a new advanced data infrastructure for geochemistry that facilitates the compilation, communication, serving, and visualization of geochemical data and their integration with the broad Geoscience data set. Building this new data infrastructure poses substantial challenges that are primarily cultural in nature, and require broad community involvement in the development and implementation of standards for data reporting (e.g., metadata for analytical procedures, data quality, and analyzed samples), data publication, and data citation to achieve broad acceptance and use. Working closely with the science community, with professional societies, and with editors and publishers, recommendations for standards for the reporting of geochemical and geochronological data in publications and to data repositories have been established, which are now under consideration for adoption in journal and agency policies. The recommended standards are aligned with the GfG and EarthChem data models as well as the EarthChem XML schema for geochemical data. Through partnerships with other national and international data management efforts in geochemistry and in the broader marine and terrestrial geosciences, GfG and EarthChem seek to integrate their development of geochemical metadata standards, data format, and semantics with relevant existing and emerging standards and ensure compatibility and compliance.

  16. The Potassium-Argon Laser Experiment (KarLE): In Situ Geochronology for Mars and Beyond (United States)

    Cohen, Barbara A.


    The search for life in the solar system depends upon discovering the right moments in planetary evolution: when habitable environments existed, when they declined, and when geologic processes operated to preserve traces of life after death. However, an incomplete knowledge of absolute Martian geochronology limits our ability to understand the timing of Martian evolutionary milestones, major climate changes, and stratigraphic epochs [1, 2]. Absolute dating relates these habitability markers to planetarywide geologic, atmospheric, and climate history places, and ties their occurrence to the history of the solar system, especially the Earth-Moon system and the timescale of evolution of life on Earth. KArLE is being developed to anchor the relative timeline of geological events to an absolute chronology that puts Mars into a wider solar system context. KArLE makes its measurements on rock samples that can be obtained by landers or rovers and inserted into a small, mechanically simple chamber. KArLE interrogates the samples using laser-induced breakdown spectrocopy (LIBS), mass spectrometry, and optical imaging. The KArLE experiment is flexible enough to accommodate any partner providing these instrument components, a creative approach that extends the ability of mission payloads to accomplish an additional highly-desirable science measurement for low cost and risk and minimal extra hardware.

  17. Palaeomagnetism and geochronology of the Proterozoic dolerite dyke from southwest Greenland: indication of low palaeointensity* (United States)

    Miki, Masako; Taniguchi, Aya; Yokoyama, Masahiko; Gouzu, Chitaro; Hyodo, Hironobu; Uno, Koji; Zaman, Haider; Otofuji, Yo-ichiro


    Intensity of the geomagnetic field during the Archaean can potentially be used to study an evolution of the dynamo activity in the Earth's core. In order to investigate this issue, we present new palaeomagnetic and geochronological results from the dolerite dyke, which have been intruded into the Archaean Gneisses of Nuuk area, southwest Greenland. 40Ar/39Ar dating of the pyroxene grains from dolerite yield an age of 2585 +/- 21 Ma. The high temperature component, which has been recognized as a characteristic direction, is identified in 24 dolerite samples. Comparison of the palaeomagnetic directions from dolerite dykes with those from host gneisses suggests a primary origin for this component. Magnetic mineralogical and grain size investigations revealed a pseudo-single domain magnetite as a dominant career of magnetization. Thellier palaeointensity determinations of 14 dolerite specimens yield a mean field value of 13.9 +/- 2.5 μT. Strength of the virtual dipole moment (2.30 +/- 0.42 × 1022 Am2) obtained from this study is about one quarter of the present Earth's filed value. A simulation based numerical model of the Thellier experiments suggests that the presence of multidomain grains can play a role in enhancing the strength of palaeointensity. Even after taking in to account the effects of multidomain grains, an intensity value (13.9 μT) from this study suggests that the strength of geomagnetic field at about 2.6 Ga was much lower than that of the present time.

  18. Analysis of impact crater populations and the geochronology of planetary surfaces in the inner solar system (United States)

    Fassett, Caleb I.


    Analyzing the density of impact craters on planetary surfaces is the only known technique for learning their ages remotely. As a result, crater statistics have been widely analyzed on the terrestrial planets, since the timing and rates of activity are critical to understanding geologic process and history. On the Moon, the samples obtained by the Apollo and Luna missions provide critical calibration points for cratering chronology. On Mercury, Venus, and Mars, there are no similarly firm anchors for cratering rates, but chronology models have been established by extrapolating from the lunar record or by estimating their impactor fluxes in other ways. This review provides a current perspective on crater population measurements and their chronological interpretation. Emphasis is placed on how ages derived from crater statistics may be contingent on assumptions that need to be considered critically. In addition, ages estimated from crater populations are somewhat different than ages from more familiar geochronology tools (e.g., radiometric dating). Resurfacing processes that remove craters from the observed population are particularly challenging to account for, since they can introduce geologic uncertainty into results or destroy information about the formation age of a surface. Regardless of these challenges, crater statistics measurements have resulted in successful predictions later verified by other techniques, including the age of the lunar maria, the existence of a period of heavy bombardment in the Moon's first billion years, and young volcanism on Mars.

  19. Fission Track Geochronology of Xiaonanchuan Pluton and the Morphotectonic Evolution of Eastern Kunlun since Late Miocene

    Institute of Scientific and Technical Information of China (English)


    Apatite fission track (AFT) thermochronology of seven samples from the Xiaonanchuan(小南川) pluton in the Kunlun (昆仑) pass area was carried out, for the purpose of determining the timing of cooling and the relation between the exhumation and the morphotectonic processes. The AFT ages yield low denudation rates of 0.020-0.035 mm/ a during the late Miocene, which correspond to a stable geomorphic and weak tectonic uplifting environment. The low denudation rates can be considered as the approximate tectonic uplifting rates. The AFT geochronology shows paroxysmally rapid cooling since the Pliocene and an apparent material unroofing of more than 3 km in the Xiaonanchuan area. This was not the result of simple denudation. The rapid cooling was coupled with the intensive orogeny since the Pliocene, which was driven by tectonic uplifting. The accelerated relief building was accompanied by a series of faulting, which caused the basin and the valley formation and sinking. The space pattern of the AFT ages also shows differential uplifting, which decreases northwardly. This trend is supported by the regional AFT data, which indicate that the exhumation decreases northwardly in eastern Kunlun. This trend also exists in east-west orientation from the western Kunlun range to the eastern. The uplifting trend is also supported by geomorphic characteristics including the elevation and the relief differences as well as the distribution of the Late Cenozoic volcanism.

  20. Geochronology and Geomorphology of the Pioneer Archaeological Site (10BT676), Upper Snake River Plain, Idaho

    Energy Technology Data Exchange (ETDEWEB)

    Keene, Joshua L. [Idaho National Lab. (INL), Idaho Falls, ID (United States)


    The Pioneer site in southeastern Idaho, an open-air, stratified, multi-component archaeological locality on the upper Snake River Plain, provides an ideal situation for understanding the geomorphic history of the Big Lost River drainage system. We conducted a block excavation with the goal of understanding the geochronological context of both cultural and geomorphological components at the site. The results of this study show a sequence of five soil formation episodes forming three terraces beginning prior to 7200 cal yr BP and lasting until the historic period, preserving one cultural component dated to ~3800 cal yr BP and multiple components dating to the last 800 cal yr BP. In addition, periods of deposition and stability at Pioneer indicate climate fluctuation during the middle Holocene (~7200-3800 cal yr BP), minimal deposition during the late Holocene, and a period of increased deposition potentially linked to the Little Ice Age. In addition, evidence for a high-energy erosion event dated to ~3800 cal yr BP suggest a catastrophic flood event during the middle Holocene that may correlate with volcanic activity at the Craters of the Moon lava fields to the northwest. This study provides a model for the study of alluvial terrace formations in arid environments and their potential to preserve stratified archaeological deposits.

  1. Monazite geochronology, magmatism, and extensional dynamics within the Menderes Massif, western Turkey

    Energy Technology Data Exchange (ETDEWEB)

    Catlos, E J [University of Texas at Austin, Jackson School of Geosciences, Geological Science Department, 1 University Station C1100, Austin, TX 78712-0254 (United States); Baker, C B; Cemen, I [Oklahoma State University, School of Geology, 105 Noble Research Center, Stillwater, OK 74078 (United States); Sorensen, S S [Smithsonian Institution Museum of Natural History, PO Box 37012, MRC 119, Washington, DC, 20013-7012 (United States); Hancer, M [Pamukkale Universitesi, Muhendislik Fakultesi, Jeoloji Muh. Bolmu, Denizli, 20070 Turkey (Turkey)], E-mail:


    Geochemical and geochronological data were collected from S-type, peraluminous granites (Salihli and Turgutlu) that intrude a detachment that bounds the northern edge of the central Menderes Massif core complex (Aegean region, western Turkey). The granites may have been generated due to subduction of the Eastern Mediterranean floor along the Hellenic trench. In situ Th-Pb ion microprobe monazite ages from the rocks range from 21.7{+-}4.5 Ma to 9.6{+-}1.6 Ma ({+-}1{sigma}), which could record their exhumation history. Higher uncertainty in the ages is attributed to monazite common Pb, but the range is consistent with cathodoluminescence (CL) images that document complex textures within the granites. Salihli and Turgutlu granites share many similar characteristics, including multiple generations of plagioclase, plagioclase replacing K-feldspar and the development of myrmekite, evidence for fluid interaction, and multiple generations of microcracks. Ages reported here are similar to dates constraining extension reported elsewhere in the Aegean, but indicate additional complexities when linking movement within the Menderes Massif to large-scale geodynamic processes that created other metamorphic core complexes in the region. Difficulties exist in linking the ages obtained from the granites to specific tectonic events due to the presence of secondary alteration textures, generations of mineral growth and multiple episodes of deformation.

  2. Paragneiss zircon geochronology and trace element geochemistry, North Qaidam HP/UHP terrane, western China (United States)

    Mattinson, C.G.; Wooden, J.L.; Zhang, J.X.; Bird, D.K.


    In the southeastern part of the North Qaidam terrane, near Dulan, paragneiss hosts minor peridotite and UHP eclogite. Zircon geochronology and trace element geochemistry of three paragneiss samples (located within a ???3 km transect) indicates that eclogite-facies metamorphism resulted in variable degrees of zircon growth and recrystallization in the three samples. Inherited zircon core age groups at 1.8 and 2.5 Ga suggest that the protoliths of these rocks may have received sediments from the Yangtze or North China cratons. Mineral inclusions, depletion in HREE, and absence of negative Eu anomalies indicate that zircon U-Pb ages of 431 ?? 5 Ma and 426 ?? 4 Ma reflect eclogite-facies zircon growth in two of the samples. Ti-in-zircon thermometry results are tightly grouped at ???660 and ???600 ??C, respectively. Inclusions of metamorphic minerals, scarcity of inherited cores, and lack of isotopic or trace element inheritance demonstrate that significant new metamorphic zircon growth must have occurred. In contrast, zircon in the third sample is dominated by inherited grains, and rims show isotopic and trace element inheritance, suggesting solid-state recrystallization of detrital zircon with only minor new growth. ?? 2009 Elsevier Ltd.

  3. Half-lives of radionuclides used in nuclear geochronology and cosmochronology (evaluated data)

    CERN Document Server

    Chechev, V P


    Evaluated half-life values are given for the radionuclides sup 2 sup 6 Al, sup 4 sup 0 K, sup 5 sup 3 Mn, sup 6 sup 0 Fe, sup 8 sup 7 Rb, sup 9 sup 3 Zr, sup 9 sup 8 Tc, sup 1 sup 0 sup 7 Pd, sup 1 sup 2 sup 9 I, sup 1 sup 3 sup 5 Cs, sup 1 sup 4 sup 6 Sm, sup 1 sup 7 sup 6 Lu, sup 1 sup 8 sup 2 Hf, sup 1 sup 8 sup 7 Re, sup 2 sup 0 sup 5 Pb, sup 2 sup 3 sup 2 Th, sup 2 sup 3 sup 5 U, sup 2 sup 3 sup 8 U, sup 2 sup 4 sup 4 Pu and sup 2 sup 4 sup 7 Cm. These were obtained from an analysis of published information up to 2001. These half-lives are used in geochronology and cosmochronology to determine different radiometric ages in the history of the earth, solar system and galaxy.

  4. Pleistocene geomorphology and geochronology of eastern Grand Canyon: linkages of landscape components during climate changes (United States)

    Anders, Matt D.; Pederson, Joel L.; Rittenour, Tammy M.; Sharp, Warren D.; Gosse, John C.; Karlstrom, Karl E.; Crossey, Laura J.; Goble, Ronald J.; Stockli, Lisa; Yang, Guang


    We report new mapping, soils, survey, and geochronologic (luminescence, U-series, and cosmogenic-nuclide) data from Pleistocene deposits in the arid setting of eastern Grand Canyon. The result is a stratigraphic framework of inset fill gravels and associated terraces that provide a record of the responses of hillslopes, tributary streams, and the Colorado River to the last ˜400 kyr of glacial-interglacial climate change. The best-preserved last 80 kyr of this record indicates a stratigraphic-chronologic disconnect between both deposition and incision along the Colorado River versus along the trunks of local tributaries. For example, the Colorado River finished aggrading and had already begun incising before the main pulse of aggradation in the trunks of local catchments during Marine Isotope Stage 3, and then tributary incision followed during the millennial-scale fluctuations of the last glacial epoch, potentially concurrent with mainstem aggradation. The mainstem record appears to broadly correlate with regional paleoclimate and upstream geomorphic records and thus may be responding to climatic-hydrologic changes in its mountain headwaters, with aggradation beginning during full-glacial times and continuing into subsequent interglacials. The contrasting lag time in responses of the dryland catchments within Grand Canyon may be largely a function of the weathering-limited nature of hillslope sediment supply.

  5. Geochronological constraints on the metamorphic sole of the Semail ophiolite in the United Arab Emirates

    Directory of Open Access Journals (Sweden)

    Nick M.W. Roberts


    Full Text Available The Semail ophiolite of Oman and the United Arab Emirates (UAE provides the best preserved large slice of oceanic lithosphere exposed on the continental crust, and offers unique opportunities to study processes of ocean crust formation, subduction initiation and obduction. Metamorphic rocks exposed in the eastern UAE have traditionally been interpreted as a metamorphic sole to the Semail ophiolite. However, there has been some debate over the possibility that the exposures contain components of older Arabian continental crust. To help answer this question, presented here are new zircon and rutile U-Pb geochronological data from various units of the metamorphic rocks. Zircon was absent in most samples. Those that yielded zircon and rutile provide dominant single age populations that are 95–93 Ma, partially overlapping with the known age of oceanic crust formation (96.5–94.5 Ma, and partially overlapping with cooling ages of the metamorphic rocks (95–90 Ma. The data are interpreted as dating high-grade metamorphism during subduction burial of the sediments into hot mantle lithosphere, and rapid cooling during their subsequent exhumation. A few discordant zircon ages, interpreted as late Neoproterozoic and younger, represent minor detrital input from the continent. No evidence is found in favour of the existence of older Arabian continental crust within the metamorphic rocks of the UAE.

  6. Instrumentation development for In Situ 40Ar/39Ar planetary geochronology (United States)

    Morgan, Leah; Munk, Madicken; Davidheiser-Kroll, Brett; Warner, Nicholas H.; Gupta, Sanjeev; Slaybaugh, Rachel; Harkness, Patrick; Mark, Darren


    The chronology of the Solar System, particularly the timing of formation of extra-terrestrial bodies and their features, is an outstanding problem in planetary science. Although various chronological methods for in situ geochronology have been proposed (e.g., Rb-Sr, K-Ar), and even applied (K-Ar), the reliability, accuracy, and applicability of the 40Ar/39Ar method makes it by far the most desirable chronometer for dating extra-terrestrial bodies. The method however relies on the neutron irradiation of samples, and thus a neutron source. Herein, we discuss the challenges and feasibility of deploying a passive neutron source to planetary surfaces for the in situ application of the 40Ar/39Ar chronometer. Requirements in generating and shielding neutrons, as well as analysing samples are described, along with an exploration of limitations such as mass, power and cost. Two potential solutions for the in situ extra-terrestrial deployment of the 40Ar/39Ar method are presented. Although this represents a challenging task, developing the technology to apply the 40Ar/39Ar method on planetary surfaces would represent a major advance towards constraining the timescale of solar system formation and evolution.

  7. Extension of 239+240Pu sediment geochronology to coarse-grained marine sediments (United States)

    Kuehl, Steven A.; Ketterer, Michael E.; Miselis, Jennifer L.


    Sediment geochronology of coastal sedimentary environments dominated by sand has been extremely limited because concentrations of natural and bomb-fallout radionuclides are often below the limit of measurement using standard techniques. ICP-MS analyses of 239+240Pu from two sites representative of traditionally challenging (i.e., low concentration) environments provide a "proof of concept" and demonstrate a new application for bomb-fallout radiotracers in the study of sandy shelf-seabed dynamics. A kasten core from the New Zealand shelf in the Southern Hemisphere (low fallout), and a vibracore from the sandy nearshore of North Carolina (low particle surface area) both reveal measurable 239+240Pu activities at depth. In the case of the New Zealand site, independently verified steady-state sedimentation results in a 239+240Pu profile that mimics the expected atmospheric fallout. The depth profile of 239+240Pu in the North Carolina core is more uniform, indicating significant sediment resuspension, which would be expected in this energetic nearshore environment. This study, for the first time, demonstrates the utility of 239+240Pu in the study of sandy environments, significantly extending the application of bomb-fallout isotopes to coarse-grained sediments, which compose the majority of nearshore regions.

  8. Paleozoic accretionary orogenesis in the eastern Beishan orogen: constraints from zircon U-Pb and 40Ar/39Ar geochronology (United States)

    Ao, Songjian; Xiao, Wenjiao; Windley, Brian; Mao, Qigui


    The continental growth mechanism of the Altaids in Central Asia is still in controversy between models of continuous subduction-accretion versus punctuated accretion by closure of multiple oceanic basins. The Beishan orogenic belt, located in the southern Altaids, is a natural laboratory to address this controversy. Key questions that are heavily debated are: the closure time and subduction polarity of former oceans, the emplacement time of ophiolites, and the styles of accretion and collision. This paper reports new structural data, zircon ages and Ar-Ar dates from the eastern Beishan Orogen that provide information on the accretion process and tectonic affiliation of various terranes. Our geochronological and structural results show that the younging direction of accretion was northwards and the subduction zone dipped southwards under the northern margin of the Shuangyingshan micro-continent. This long-lived and continuous accretion process formed the Hanshan accretionary prism. Our field investigations show that the emplacement of the Xiaohuangshan ophiolite was controlled by oceanic crust subduction beneath the forearc accretionary prism of the Shuangyingshan-Mazongshan composite arc to the south. Moreover, we address the age and terrane affiliation of lithologies in the eastern Beishan orogen through detrital zircon geochronology of meta-sedimentary rocks. We provide new information on the ages, subduction polarities, and affiliation of constituent structural units, as well as a new model of tectonic evolution of the eastern Beishan orogen. The accretionary processes and crustal growth of Central Asia were the result of multiple sequences of accretion and collision of manifold terranes. Reference: Ao, S.J., Xiao, W., Windley, B.F., Mao, Q., Han, C., Zhang, J.e., Yang, L., Geng, J., Paleozoic accretionary orogenesis in the eastern Beishan orogen: Constraints from zircon U-Pb and 40Ar/39Ar geochronology. Gondwana Research, doi:

  9. Precambrian tectonic evolution of the Tarim Block, NW China: New geochronological insights from the Quruqtagh domain (United States)

    Shu, L. S.; Deng, X. L.; Zhu, W. B.; Ma, D. S.; Xiao, W. J.


    The Tarim Block is an important tectonic unit to understand the Proterozoic tectonic framework of the Central Asian Orogenic Belt and the supercontinent Rodinia. The granitic, dioritic, gabbroic intrusive rocks and volcanic-volcanoclastic rocks are widely distributed in the Quruqtagh domain of NE-Tarim. The precise ages of these rocks and their tectonic implications in this part of the world are not well understood. This paper reports geochronological data of gabbro, diorite and granitic rocks from Quruqtagh. LA ICPMS U-Pb zircon ages suggest that numerous of gabbroic and granitic rocks were mainly crystallized at ca. 800 Ma. New geochronological data from the magmatic zircons of gabbro, granite and paragneiss can be preliminarily divided into four groups, which are (1) 2469 ± 12 Ma or 2470 ± 24 Ma, (2) 933 ± 11 Ma to 1048 ± 19 Ma, (3) 806 ± 8 Ma, 798 ± 7 Ma, 799 ± 24 Ma, 698 ± 51 Ma (lower intercept age of the paragneiss), and (4) 1930 Ma (upper intercept age of the paragneiss), respectively. These age data are consistent with four tectono-thermal events that took ever place in the Tarim Block. The 93 U-Pb age data (seven for average Concordia age from seven igneous plutons, 86 for xenocrystic and metamorphic ones) from eight samples can be divided into four evolutionary stages: 2360-2550 Ma (peak of 2510 Ma), 1800-2020 Ma (peak of 1870 Ma), 860-1140 Ma (peak of 920 Ma) and 680-840 Ma (peak of 800 Ma), respectively. The age peak of 2500 Ma, consistent with characteristic period of a global building-continent event, indicates that the late Neoarchean-early Paleoproterozoic magmatism had been ever taken place in Tarim. Two peaks at 1870 Ma and 920 Ma, being two assembly periods of the middle Paleoproterozoic Columbia and the Neoproterozoic Rodinia supercontinents, suggest that Tarim had connections with both Columbia and Rodinia, whereas structural evidence of these two events is absent in Tarim. Notable peak of 800 Ma is interpreted as a response to the

  10. U-Pb Geochronology of VMS mineralization in the Iberian Pyrite Belt (United States)

    Barrie, Tucker C.; Amelin, Yuri; Pascual, Emilio


    A geochronology study using U-Pb isotope dilution TIMS analyses of zircon has been conducted to determine the ages of volcanic-associated massive sulfide (VMS) deposits in the Iberian Pyrite Belt (IPB), the world's most prolific VMS province. Ages have been determined for host rocks to four VMS systems that span the IPB: the giant Rio Tinto and Aljustrel districts in the central region, Lagoa Salgada to the west, and Las Cruces to the east. A sample of chloritized quartz porphyritic dacite/rhyolite in the footwall of the San Dionisio massive sulfide deposit of the Rio Tinto district is 349.76±0.90 Ma. This is taken as the best age estimate of the mineralization in the Rio Tinto district, probably the world's largest volcanogenic massive sulfide system. Two xenocrystic zircons from the same sample yielded 207Pb/206Pb ages of 414 and 416 Ma, which provide a minimum estimate for the age of the inherited component. A biotite tonalite from the Campofrio area, 3.5 km north of the center of the Rio Tinto district, is chemically similar to the felsic host rock protolith at Rio Tinto. The Campofrio sample has an age of 346.26±0.81 Ma, slightly younger and outside of the 2σ error for the Rio Tinto age; therefore, this phase of this intrusion was not a heat source for the hydrothermal system that formed the deposits of the Rio Tinto district. The Campofrio sample also has three zircon analyses with 207Pb/206Pb minimum ages of 534, 536, and 985 Ma, indicating inheritance from Ordovician and Neoproterozoic sources. In the Aljustrel VMS district, a U-Pb zircon age of 352.9±1.9 Ma characterizes the altered Green Tuff host rock of the Algares deposit, which is slightly older than the Rio Tinto age. Two zircons with 207Pb/206Pb ages of 531 and 571 Ma from this sample indicate inheritance from a Cambrian or older source. The age of mineralization at Lagoa Salgada is given by essentially identical ages of 356.21±0.73 and 356.4±0.8 Ma, for footwall and hanging wall samples

  11. Geochronology of the Hannan intrusive complex to adjoin the Qinling orogen and its rapid cooling reason

    Institute of Scientific and Technical Information of China (English)


    The geochronological works for the Hannan intrusive complex,cosisting of the gabbro-quartz diorite-plagiogranite bodies, in the north border of the Yangtze craton block, adjoining the Qinling orogen, are reported, because its age and genesis are of great importance to research of the tectonic evalutional history for the Qinling orogen and of the rifted processes for the Late Proterozoic supercontinent Rodinia. A good isochron of t=(837±26) Ma (2σ), corresponding to INd=0.51165(2±2σ), σNd(t)=+1.9, MSWD1.02, was difined by the Nd isotopic analytical results for the 21 whole-rock samples from the varied lithological complex bodies, but for the Rb-Sr isotopic aralytical data there is no isochron. The 40Ar/39Ar plateau age of the biotite, sorted from plagiogranites, is (796±20) Ma(2σ), and yet for the Rb-Sr isotopic data of the plagiogranite whole-rock samples (WR) and the sorted biotite (Bio), plagioclase (Plag) and apatite (Apt) mineral samples from the plagiogranite whole-rock samples, an isochron of t=(824.8±3.8) Ma (2σ) with ISr=0.70393±14(2σ), MSWD2.44 is given. The U-Pb isotopic results for the single zircon, sorted from plagiogranite samples, yielded an upper and a lower intercept ages of ~876 Ma and ~273 Ma. The Nd isotope data of complex indicate that it is probably the products of crystallization differentiation from the magma with (Nd>0 when the rapid uplift in 837—800 Ma took place in the northern border of the Yangtze craton block.

  12. Geochronological (OSL) and geomorphological investigations at the presumed Frankfurt ice marginal position in northeast Germany (United States)

    Hardt, Jacob; Lüthgens, Christopher; Hebenstreit, Robert; Böse, Margot


    The Weichselian Frankfurt ice marginal position in northeast Germany has been critically discussed in the past owing to weak morphological evidence and a lack of clear sedimentological records. This study aims to contribute to this discussion with new geochronological and geomorphological results. Apart from very few cosmogenic exposure ages, the time frame is to date still based on long distance correlation with radiocarbon chronologies. We selected a study site in a key position regarding the classic location of the Frankfurt ice marginal position and the recently described arcuate ridge structures on the Barnim plateau. For the first time we present Optically Stimulated Luminescence (OSL) ages of quartz from glaciofluvial deposits for this Weichselian phase. Our results indicate an advance of the Scandinavian Ice Sheet (SIS) at around 34.1 ± 4.6 ka. This is in agreement with OSL ages from sandur deposits at the Brandenburg ice marginal position located farther south and could also be correlated with the Klintholm advance in Denmark. The subsequent meltdown phase lasted until around 26.3 ± 3.7 ka. During the meltdown phase a minor oscillation of the SIS caused the formation of the recently described arcuate ridges on the Barnim till plain. Recalculated surface exposure ages of glacigenic boulders with an updated global production rate indicate a landscape stabilization phase at around 22.7 ± 1.6 ka, which is in agreement with our ages. A phase of strong aeolian activity has been dated with OSL to 1 ± 0.1 ka; this may have been triggered by human activities that are documented in this region for the medieval period.

  13. Using Zircon Geochronology to Unravel the History of the Naga Hills Ophiolite (United States)

    Roeder, T.; Aitchison, J. C.; Clarke, G. L.; Ireland, T. R.; Ao, A.; Bhowmik, S. K.


    Outcrops of the Naga Hills Ophiolite (NHO), a possible eastern extension of the ophiolitic belt running along the India-Asia suture, in Northeast India include a full suite of ophiolitic rocks. The ophiolite has been dated Upper Jurassic based on radiolarian studies of the unit (Baxter et al., 2011) but details of its emplacement onto the Indian margin have not been the subject of detailed investigation. Conglomerates of the Phokphur Formation unconformably overlie an eroded surface on top of dismembered ophiolite fragments and include sediments sourced from both the ophiolite and the margin of the Indian subcontinent. Notably no Asian margin-derived detritus is recognised (similar to the Liuqu conglomerates of Tibet (Davis et al., 2002)). Thus, a detailed study of the Phokphur sediments can produce valuable details of the NHO history, including constraining the timing of ophiolite emplacement. Studies of detrital sandstone petrography confirm a recycled orogen provenance for the Phokphur Formation and thus serve as validation of the methods of Dickinson and Suczek (1979) and Garzanti et al. (2007). Detrital zircon data provides further insight as to the age of source rocks of Phokphur sediments and help to further constrain the timing of ophiolite emplacement. We present results of sedimentary and detrital zircon geochronology analyses of Phokphur sediments from outcrops near the villages of Salumi and Wazeho as a contribution to furthering research on aspects of the India-Asia collision. Baxter, A.T., et al. 2011. Upper Jurassic radiolarians from the Naga Ophiolite, Nagaland, northeast India. Gondwana Research, 20: 638-644. Davis, A.M., et al. 2002. Paleogene island arc collision-related conglomerates, Yarlung-Tsangpo suture zone, Tibet. Sedimentary Geology, 150: 247-273. Dickinson, W.R. and Suczek, C.A., 1979. Plate tectonics and sandstone compositions. Am. Assoc. Pet. Geol. Bull., 63, 2164-2182, (1979). Garzanti, E., et al., 2007. Orogenic belts and orogenic

  14. Quaternary Geochronology, Paleontology, and Archaeology of the Upper San Pedro River Valley, Sonora, Mexico (United States)

    Gaines, E. P.


    This poster presents the results of multi-disciplinary investigations of the preservation and extent of Quaternary fossil-bearing strata in the San Pedro River Valley in Sonora, Mexico. Geologic deposits in the portions of the San Pedro Valley in southern Arizona contain one of the best late Cenozoic fossil records known in North America and the best record of early humans and extinct mammals on the continent. The basin in the U.S. is one of the type locations for the Blancan Land Mammal Age. Hemiphilian and Irvingtonian fossils are common. Rancholabrean remains are widespread. Strata in the valley adjacent to the international border with Mexico have yielded the densest concentration of archaeological mammoth-kill sites known in the western hemisphere. Despite more than 60 years of research in the U.S., however, and the fact that over one third of the San Pedro River lies south of the international boundary, little has been known about the late Cenozoic geology of the valley in Mexico. The study reported here utilized extensive field survey, archaeological documentation, paleontological excavations, stratigraphic mapping and alluvial geochronology to determine the nature and extent of Quaternary fossil-bearing deposits in the portions of the San Pedro Valley in Sonora, Mexico. The results demonstrate that the Plio-Pleistocene fossil -bearing formations known from the valley in Arizona extend into the uppermost reaches of the valley in Mexico. Several new fossil sites were discovered that yielded the remains of Camelids, Equus, Mammuthus, and other Proboscidean species. Late Pleistocene archaeological remains were found on the surface of the surrounding uplands. AMS radiocarbon dating demonstrates the widespread preservation of middle- to late- Holocene deposits. However, the late Pleistocene deposits that contain the archaeological mammoth-kill sites in Arizona are absent in the valley in Mexico, and are now known to be restricted to relatively small portions of

  15. The Padre Miguel Ignimbrite Suite, central Honduras: Paleomagnetism, geochronology, and tectonic implications (United States)

    Garza, Roberto S. Molina; van Hinsbergen, Douwe J. J.; Rogers, Robert D.; Ganerød, Morgan; Dekkers, Mark J.


    The Padre Miguel Group in western Honduras is a silicic volcanic sequence that forms part of the Central American Miocene volcanic arc built on the Chortis continental fragment. We report new 40Ar/39Ar data of 16.1 ± 0.2 Ma and 14.42 ± 0.08 Ma for the Padre Miguel Group, and present paleomagnetic data for 36 cooling units (mainly ignimbrites) from localities north and northeast of the capital city of Tegucigalpa. These rocks are characterized by univectorial characteristic magnetization carried mainly by low-Ti titanomagnetites, or two component magnetizations with a minor secondary overprint. Dual polarity magnetizations suggest that it is a primary thermo-remanence and the obtained data scatter can be straightforwardly explained by paleosecular variation. The overall mean of 33 selected sites (14.39°N, 87.14°W) defines a paleomagnetic pole at lat = 80.0°N, lon = 142.9°E (K = 24.5, A95 = 5.2°). The overall mean compared with the North America expected direction indicates statistically insignificant rotation and inclination anomaly (R = - 4.1° ± 6.1° and an F = - 8.6° ± 11.3°). Restoring 255 km of sinistral slip in the Polochic-Motagua fault system, as suggested by Cayman Trough reconstructions since 15 Ma, brings the mid-Miocene arcs of southern Mexico (Oaxaca) and the Central America ignimbrite province into alignment; this is consistent with a derivation of the Chortis Block from southern Mexico. Our paleomagnetic and geochronological studies hence support models that explain the offset of the Miocene ignimbrite belt of Central America by post-Middle Miocene displacement of the Chortis Block relative to North America.

  16. Igneous rocks of Arctic Ocean deep sea ridges: new data on petrology, geochemistry and geochronology (United States)

    Petrov, Oleg; Morozov, Andrey; Shokalsky, Sergey; Sobolev, Nikolay; Kashubin, Sergey; Shevchenko, Sergey; Sergeev, Sergey; Belyatsky, Boris; Shatov, Vitaly; Petrov, Eugeny


    The aggregate results of studies of igneous rocks, collected from the central part of the Arctic Ocean during scientific marine expeditions «Arctic-2000, 2005, 2007 and 2012» are presented and discussed in the frame of modern understanding of High Polar Arctic tectonic constraint. Petrological, geochemical and isotope-geochronological studies of more than 500 samples have shown that the sedimentary rocks are of dominated population among the rock fragments dredged from deep-sea bottom, and represented by metamorphosed dolomite and quartz sandstone, limestone, sometimes with the Devonian - Permian fauna. Igneous rocks are 10-15% only (Archean and Paleoproterozoic gneissouse granites and gabbro, Neoproterozoic dolerite) and metamorphic rocks (green shales, metabasites, gneisses). Apparently, these rocks are part of the acoustic basement underlying the Late Mesozoic - Cenozoic layered loose sediments. In addition to the dredged fragments of the ancient mafic rocks, some samples were taken as a core during deep-water drilling in the northern and southern slopes of the Mendeleev Ridge and represented by trachybasalts, marking the border of Late-Cenozoic deposit cover and acoustic basement and quite similar in composition to those of Early-Late Cretaceous basalts form northward of the Chukchi Plateau seamounts, Alpha Ridge, Franz Josef Land, De Long islands and other parts of the large igneous province of the High Arctic (HALIP). Video-filming of Mendeleev Ridge escarps proofs the existing of rock outcrops and supports local origin of most of the rock fragments found in the sampling areas. Thus the continental type of the earth's crust of the Central Arctic Ridges basement is based on all obtained results of our study of sea-bottom excavated rock material.

  17. Preliminary K/Ar geochronology of the Crater Basalt volcanic field (CBVF, northern Patagonia

    Directory of Open Access Journals (Sweden)

    Z. Pécskay


    Full Text Available The Crater Basalt volcanic field is one of the Quaternary intraplate basaltic fields in northern Patagonia. A systematic geological, volcanological and geochronological study of CBVF indicates a multistage history of eruptions of basaltic volcanoes. K/Ar dating, using whole rock samples shows that the measured analytical ages are fully consistent with the available stratigraphic control. The radiometric ages fall into three distinct, internally consistent age groups, which give evidence that there were at least three major episodes of volcanic activity, at about 1.0 Ma, 0.6 Ma and 0.3 Ma ago. The age differences appear to be just significant, even although less than 10 % radiogenic argon was found in the isotope analysis of whole rock samples.El campo volcánico del Basalto Cráter (CVBC constituye uno de los campos basálticos cuaternarios de intraplaca de la Patagonia septentrional. El estudio sistemático de la geología, volcanología y geocronología del CVBC muestra una historia eruptiva multiepisódica de volcanes basálticos. Las dataciones K-Ar realizadas sobre roca total son coherentes con el control estratigráfico. Las edades obtenidas para el Basalto Cráter permiten distinguir tres episodios diferentes, pero individualmente coherentes, de actividad volcánica, ocurridos hace ~1,0 Ma; 0,6 Ma y 0,3 Ma. Las diferencias de edad parecen ser significativas, aún cuando el contenido de argón radiogénico determinado en los análisis de roca total resultó menor al 10 %.

  18. Petrology and Geochronology of High-Grade Metamorphic Rocks from Cedros Island, Baja California, Mexico (United States)

    Gonzales, D.; Leech, M. L.


    High-grade metamorphic rocks exposed on Cedros Island, Baja California, Mexico, record the Mesozoic subduction history of western North America. Blocks of amphibolite, blueschist, and eclogite crop out in a serpentinite-matrix mélange on the southeast and southwestern parts of Cedros Island. Amphibolite blocks contain Amp + Ep + Ab + Chl ± Ms ± Grt ± Ttn ± Qz; blueschist blocks have the assemblage Na-Amp + Ms + Lw + Qz ± Ttn ± Grt ± Jd ± Chl; and eclogite blocks are comprised primarily of Omp + Grt with retrograde Na-Amp + Ms + Lw. Blueschists from Cedros have been dated using 40Ar/39Ar step-heating of white mica and sodic amphiboles that yield ages from 103 ± 4 Ma to 94.9 ± 1.1 Ma, respectively, that represent cooling during exhumation. Apatite fission-track dating gives ages from 32 ± 4 Ma to 22 ± 3 Ma that record exhumation through the upper crust. Related Mesozoic subduction zone rocks of the Franciscan Complex crop out in a serpentinite-matrix mélange along coastal northern California. The Franciscan rocks are older, yielding 40Ar/39Ar step-heating ages of hornblende from amphibolite ranging from 159 to 156 Ma and represent an older part of the subduction history of the oceanic Farallon plate along western North America. I will determine the prograde and peak metamorphic P-T conditions for these high-grade rocks using petrography, mineral chemistries, and isochemical phase diagram modeling with Perple_X to generate complete P-T paths. I will then supplement these data with Sm-Nd and Lu-Hf geochronology for these high-grade Cedros rocks to evaluate their subduction/exhumation history, and develop a tectonic model for these southernmost Franciscan-type rocks. Ultimately, I will compare my results to Franciscan rocks in northern California to better understand the Mesozoic subduction margin of western North America.

  19. Geochronological constraints on 140-85 Ma thermal doming extension in the Dabie orogen, central China

    Institute of Scientific and Technical Information of China (English)


    Regional architecture of geochronology and differential cooling pattern show that the Dabie orogen underwent a thermal doming extension during 140-85 Ma. This extension resulted in widespread re-melting of the Dabie basement, intense volcanic activities in North Huaiyang and the formation of fault-controlled depressions in the Hefei basin. This thermal doming extension can be further divided into two consecutive evolving stages, i.e. the intensifying stage (140-105 Ma) and the declining stage (105-85 Ma). In the first stage (140-105 Ma), the thermal doming mainly was concentrated in the Dabie block, and to a less degree, in the Hongan block. The thermal doming structure of the Dabie block is configured with Macheng-Yuexi thermal axis, Yuexi/Luotian thermal cores and their downslide flanks. The orientation of thermal axis is dominantly parallel to the strike of orogen, and UHP/HP units together with metamorphic rocks of North Huaiyang constitute the downslide flanks. The Yuexi core differs from the Luotian core in both the intensity and the shaping time. To some extent, the Hongan block can be regarded as part of downslide systems of the Dabie doming structure. The doming process is characterized by thermal-center's migration along the Macheng-Yuexi thermal axis; consequently, it is speculated to be attributed to the convective removal of thickened orogenic root, which is a process characterized by intermittance, migration, large-scale and differentiation. During the declining stage (105-85 Ma), the dome- shaped figure still structurally existed in the Dabie orogen, but orogenic units cooled remarkably slow and magmatic activities stagnated gradually. Study on the thermal doming of Dabieshan Mountains can thus provide detailed constraints on the major tectonic problems such as the UHP/HP exhumation model, the boundary between North Dabie and South Dabie, and the orogenesis mechanism.

  20. Cretaceous alkaline intra-plate magmatism in the Ecuadorian Oriente Basin: Geochemical, geochronological and tectonic evidence (United States)

    Barragán, Roberto; Baby, Patrice; Duncan, Robert


    Small volumes of Cretaceous alkaline basaltic magmas have been identified in the sedimentary infill of the Ecuadorian Oriente foreland basin. They are characterized by a restricted range of compositional variation, low LILE/HFSE ratios and Sr-Nd isotope values within the range of oceanic island basalts (OIB). Reflection seismic data show that a pre-existing NNE-SSW Triassic and Jurassic rift controls the location and occurrence of these alkaline eruptive sites. Radiometric ages ( 40Ar- 39Ar, incremental heating method) and the biostratigraphic record of their surrounding sediments indicate a NNE-SSW systematic age variation for the emplacement of this alkaline volcanism: from Albian (110 ± 5.2 Ma) in the northern part of the Oriente Basin, to Campanian (82.2 ± 2.0 Ma) in the west-central part. The geochemical, geochronological and tectonic evidences suggest that asthenospheric mantle has upwelled and migrated to the SSW, into the region underlying the pre-existing Triassic and Jurassic rift (thin-spot?). We propose that subduction was abandoned, subsequent to the accretion of allochthonous terranes onto the Ecuadorian and Colombian margin in the latest Jurassic-earliest Cretaceous, causing the relict slab material, corresponding to the eastwards-directed leading plate, to roll-back. Unmodified asthenospheric mantle migrated into the region previously occupied by the slab. This resulted in partial melting and the release of magmatic material to the surface in the northern part of the Oriente Basin since at least Aptian times. Then, magmatism migrated along the SSW-trending Central Wrench Corridor of the Oriente Basin during the Upper Cretaceous, probably as a consequence of the lateral propagation of the transpressive inversion of the Triassic-Jurassic rift. Eventually, the Late Cretaceous east-dipping Andean subduction system was renewed farther west, and the development of the compressional retro-foreland Oriente Basin system halted the Cretaceous alkaline

  1. The Violent Early Solar System, as Told by Lunar Sample Geochronology (United States)

    Cohen, Barbara


    One of the legacies of the samples collected by the Apollo and Luna missions is the link forged between radiometric ages of rocks and relative ages according to stratigraphic relationships and impact crater size-frequency distributions. Our current understanding of the history of the inner solar system is based on the relative chronology of individual planets, tied to the absolute geochronology of the Moon via these important samples. Samples from these nearside locations reveal a preponderance of impact-disturbed or recrystallized ages between 3.75 and 3.95 billion years. Argon and lead loss (and correlated disturbances in the Rb-Sr system) have been attributed to metamorphism of the lunar crust by an enormous number of impacts in a brief pulse of time, called the Lunar Cataclysm or Late Heavy Bombardment. Subsequent high-precision geochronometric analyses of Apollo samples and lunar highlands meteorites show a wider range of ages, but very few older than 4 Ga. The paucity of ancient impact melt rocks has been interpreted to mean that either that most impact basins formed at this time, or that ejecta from the large, near-side, young basins dominates the Apollo samples. Selenochronology is getting more complicated: new results question meaning of sample ages, crater counts, crater production functions, and the solar system itself. Improved geological mapping of lunar geologic units and boundaries using multiple remote sensing datasets. High-resolution image-based crater counting of discrete geologic units and relating them to location. Improved understanding of the regolith thickness and its global variation (GRAIL). Tying the sampling of impact-melt rocks to the lunar impact flux. Using improved techniques (magnetic fields, diffusion studies, isotopic analysis) on existing samples. New sample return from benchmark craters, particularly SPA, which appears in 2013 Decadal Survey.

  2. Cold seep carbonates along the Norwegian margin, insights into U-Th geochronology and S geochemistry (United States)

    Cremiere, A.; Lepland, A.; Wing, B. A.; Sahy, D.; Condon, D. J.; Chand, S.; Noble, S. R.; Bui, T. H.; Thorsnes, T.; Brunstad, H.


    Cold seep carbonates along the Norwegian margin, insights into U-Th geochronology and S geochemistryAuthigenic carbonate crusts form in shallow subsurface of marine sediments due to the microbial anaerobic oxidation of methane (AOM). As a result they are unique archives of the locus and intensity of past methane seepage that can be dated by using U-daughter decay affording the unique opportunity to constrain the absolute timing of methane release events. Because AOM is mainly driven by the microbial reduction of seawater sulfate, multiple sulfur isotope compositions of paired carbonate-associated sulfate (CAS) and pyrite in seep carbonates taken as proxies for porewater sulfate and sulfide, respectively, have the potential to reconstruct the biogeochemical conditions under which seep carbonates precipitate. Methane-derived carbonate crusts were collected from several seepage sites on the Norwegian continental shelf, including sites in the North Sea, the Norwegian Sea and the Barents Sea. The U-Th dating results constrain the main episode of carbonate crust formation in the Barents and Norwegian seas during the time interval between 14 and 7 ka. Such ages suggest that the methane seepage along the northern Norwegian margin was most active after the collapse of the Scandinavian ice sheet and deglaciation of the area that took place at about 15 ka. The methane flux for the carbonate crust formation was likely provided by the dissociation of methane hydrates that extensively formed in underlying sediments during the last glacial period, but became unstable due to depressuring effects of retreating ice sheet. The precipitation of studied North Sea carbonate crusts occurred more recently, from 6 to 1 ka, suggesting that their formation is unrelated to the glacial history of the area. The paired sulfur stable isotope compositions of pyrite-CAS record a large range of fractionation factors (from 30 to 70 ‰) reflecting change of sulfate-reduction rates possibly controlled

  3. Geochronology and paleothermometry of Neogene sediments from the Vøring Plateau using Sr, C and O isotopes (United States)

    Smalley, P. C.; Nordaa, A.; Råheim, A.


    The Neogene sediments from DSDP site 341 on the Vøring Plateau, Norwegian Sea, contain a thin glauconitic pellet-bearing subunit, which separates underlying pelagic clays from overlying glacial-marine sediments. Oxygen isotope measurements of benthic foraminifera show aδ 18O shift of + 1%. during deposition of this subunit, probably a combined effect of a drop in bottom water temperature and a rise in seawaterδ 18O. The chronology of this sedimentological and O isotope transition is, however, poorly constrained by fossil evidence. Rb sbnd Sr dating of glauconitic pellets indicates that the lower part of the glauconitic subunit was deposited 11.6 ± 0.2 Ma ago. Further geochronological evidence, derived from the Sr and C isotopic compositions of foraminifera compared with known seawater-time variations, indicates that the lower pelagic clays are early to middle Miocene, deposited at a mean rate of ˜ 15 m/Ma. The glauconitic subunit contains part of the middle Miocene and probably all of the late Miocene in a condensed sequence with a very low mean depositional rate (˜ 0.2 m/Ma). The overlying glacial marine sediments are probably Pliocene, with a high mean rate of deposition, ˜ 45 m/Ma. This is the first application of C, O and Sr isotopic stratigraphy combined with Rb sbnd Sr dating of glauconitic minerals, and it illustrates the applications of this integrated approach in geochronology.

  4. Geochemical and geochronological study of the non-granitic pegmatite body "La Panchita", Oaxaca state, Southern Mexico (United States)

    Shchepetilnikova, Valentina; Solé, Jesús; Solari, Luigi; Abdullin, Fanis


    The La Panchita pegmatite body intrudes a >10 m thick pyroxenite dike that in turn is cutting the central portion of ~1Ga Oaxacan Complex rocks, Southern Mexico. The Oaxacan Complex is the largest exposure in Mexico of Neoproterozoic basement rocks metamorphosed up to the granulite facies during the Grenville orogeny. This Complex has multiple intrusions of pegmatite bodies along its extension, some mineralogically simple, some complex. As for the mineralogy, the La Panchita pegmatite is distinct from other pegmatites of the Oaxacan Complex. It contains unusual minerals for a pegmatite, like scapolite and calcite, and it is a non-granitic pegmatite, as suggested before. This work presents preliminary geochemical and geochronological results of this pegmatite body and a discussion about its possible origin. The geochronological study shows that the time of emplacement of this pegmatite is 981.4 ± 7.4 Ma and it is post-tectonic with respect to the granulite facies metamorphic event of the Oaxacan Complex. The geochemical study shows that the pegmatite La Panchita formed during the evolution of an anorogenic magmatic source of carbonatitic-alkaline composition related to a post-Grenvillian rifting event. Medium to low-temperature thermochronometers (K-Ar, fission track and U-Th-He) from this pegmatite are under progress and the results will be given at the meeting.

  5. Re-Os sulfide geochronology of the Red Dog sediment-hosted Zn-Pb-Ag deposit, Brooks Range, Alaska (United States)

    Morelli, R.M.; Creaser, R.A.; Selby, D.; Kelley, K.D.; Leach, D.L.; King, A.R.


    The Red Dog sediment-hosted deposit in the De Long Mountains of northern Alaska is the largest Zn producer in the world. Main stage mineralization is characterized by massive sulfide ore and crosscutting subvertical veins. Although the vein mineralization is clearly younger than the massive ore, the exact temporal relationship between the two is unclear. Re-Os geochronology of pyrite is used to determine the absolute age of main stage ore at Red Dog. A 10-point isochron on both massive and vein pyrite yields an age of 338.3 ?? 5.8 Ma and is interpreted to represent the age of main stage ore. The Re-Os data indicate that both massive and vein ore types are coeval within the resolution of the technique. Formation of the Red Dog deposit was associated with extension along a passive continental margin, and therefore the Re-Os age of main stage ore constrains the timing of rifting as well as the age of the host sedimentary rocks. Sphalerite from both massive and vein ore yields imprecise ages and shows a high degree of scatter compared to pyrite. We suggest that the Re-Os systematics of sphalerite can be disturbed and that this mineral is not reliable for Re-Os geochronology. ?? 2004 by Economic Geology.

  6. What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou basin, Tibet)

    NARCIS (Netherlands)

    Huang, Wentao; Dupont-Nivet, Guillaume|info:eu-repo/dai/nl/313092559; Lippert, Peter C.; Van Hinsbergen, Douwe J J; Dekkers, Mark J.|info:eu-repo/dai/nl/073463744; Waldrip, Ross; Ganerød, Morgan; Li, Xiaochun; Guo, Zhaojie; Kapp, Paul


    The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus, here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong

  7. What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou basin, Tibet)

    NARCIS (Netherlands)

    Huang, Wentao; Dupont-Nivet, Guillaume; Lippert, Peter C.; Van Hinsbergen, Douwe J J; Dekkers, Mark J.; Waldrip, Ross; Ganerød, Morgan; Li, Xiaochun; Guo, Zhaojie; Kapp, Paul


    The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus, here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Gr

  8. Geochronologic evidence for a possible MIS-11 emergent barrier/beach-ridge in southeastern Georgia, USA (United States)

    Markewich, H.W.; Pavich, M.J.; Schultz, A.P.; Mahan, S.A.; Aleman-Gonzalez, W. B.; Bierman, P.R.


    Predominantly clastic, off-lapping, transgressive, near-shore marine sediment packages that are morphologically expressed as subparallel NE-trending barriers, beach ridges, and associated back-barrier areas, characterize the near-surface stratigraphic section between the Savannah and the Ogeechee Rivers in Effingham County, southeastern Georgia. Each barrier/back-barrier (shoreline) complex is lower than and cut into a higher/older complex. Each barrier or shoreline complex overlies Miocene strata. No direct age data are available for these deposits. Previous researchers have disagreed on their age and provenance. Using luminescence and meteoric beryllium-10 (10Be) inventory analyses, we estimated a minimum age for the largest, westernmost, morphologically identifiable, and topographically-highest, barrier/beach-ridge (the Wicomico shoreline barrier) and constrained the age of a suite of younger barrier/beach-ridges that lie adjacent and seaward of the Wicomico shoreline barrier. At the study site, the near-shore marine/estuarine deposits underlying the Wicomico shoreline barrier are overlain by eolian sand and an intervening zone-of-mixing. Optically stimulated luminescence (OSL) data indicate ages of ≤43 ka for the eolian sand and 116 ka for the zone-of-mixing. Meteoric 10Be and pedostratigraphic data indicate minimum residence times of 33.4 ka for the eolian sand, 80.6 ka for the zone-of-mixing, and 247 ka for the paleosol. The combined OSL and 10Be age data indicate that, at this locality, the barrier/beach ridge has a minimum age of about 360 ka. This age for the Wicomico shoreline-barrier deposit is the first for any Pleistocene near-shore marine/estuarine deposit in southeast Georgia that is conclusively older than 80 ka. The 360-ka minimum age is in agreement with other geochronologic data for near-coastline deposits in Georgia and South Carolina. The geomorphic position of this barrier/beach-ridge is similar to deposits in South Carolina considered to be

  9. Geochronology, geochemistry and tectonic evolution of the Western and Central cordilleras of Colombia (United States)

    Villagómez, Diego; Spikings, Richard; Magna, Tomas; Kammer, Andreas; Winkler, Wilfried; Beltrán, Alejandro


    Autochthonous rocks of the pre-Cretaceous continental margin of NW South America (the Tahami Terrane) are juxtaposed against a series of para-autochthonous rock units that assembled during the Early Cretaceous. Allochthonous, oceanic crust of the Caribbean Large Igneous Province collided with and accreted onto the margin during the Late Cretaceous. We present the first regional-scale dataset of zircon U-Pb LA-ICP-MS ages for intrusive and metamorphic rocks of the autochthonous Tahami Terrane, Early Cretaceous igneous para-autochthonous rocks and accreted oceanic crust. The U-Pb zircon data are complemented by multiphase 40Ar/ 39Ar crystallization and cooling ages. The geochronological data are combined with whole rock major oxide, trace element and REE data acquired from the same units to constrain the tectonic origin of the rock units and terranes exposed in the Western Cordillera, Cauca-Patía Valley and the Central Cordillera of Colombia. The Tahami Terrane includes lower Paleozoic orthogneisses (~ 440 Ma) that may have erupted during the active margin stage of the Rheic Ocean. Basement gneisses were intruded by Permian, continental arc granites during the final assembly of Pangea. Triassic sedimentary rocks were subsequently deposited in rift basins and partially melted during high-T metamorphism associated with rifting of western Pangea during 240-220 Ma. Continental arc magmatism during 180-145 Ma is preserved along the whole length of the Central Cordillera and was followed by an Early Cretaceous out-board step of the arc axis and the inception of the Quebradagrande Arc that fringed the continental margin. Back-stepping of the arc axis may have been caused by the collision of buoyant seamounts, which were coeval with plateau rocks exposed in the Nicoya Peninsular of Costa Rica. Rapid westward drift of South America closed the Quebradagrande basin in the late Aptian and caused medium-high P-T metamorphic rocks of the Arquía Complex to exhume and obduct onto

  10. Geochronologic evidence for a possible MIS-11 emergent barrier/beach-ridge in southeastern Georgia, USA (United States)

    Markewich, H. W.; Pavich, M. J.; Schultz, A. P.; Mahan, S. A.; Aleman-Gonzalez, W. B.; Bierman, P. R.


    Predominantly clastic, off-lapping, transgressive, near-shore marine sediment packages that are morphologically expressed as subparallel NE-trending barriers, beach ridges, and associated back-barrier areas, characterize the near-surface stratigraphic section between the Savannah and the Ogeechee Rivers in Effingham County, southeastern Georgia. Each barrier/back-barrier (shoreline) complex is lower than and cut into a higher/older complex. Each barrier or shoreline complex overlies Miocene strata. No direct age data are available for these deposits. Previous researchers have disagreed on their age and provenance. Using luminescence and meteoric beryllium-10 (10Be) inventory analyses, we estimated a minimum age for the largest, westernmost, morphologically identifiable, and topographically-highest, barrier/beach-ridge (the Wicomico shoreline barrier) and constrained the age of a suite of younger barrier/beach-ridges that lie adjacent and seaward of the Wicomico shoreline barrier. At the study site, the near-shore marine/estuarine deposits underlying the Wicomico shoreline barrier are overlain by eolian sand and an intervening zone-of-mixing. Optically stimulated luminescence (OSL) data indicate ages of ≤43 ka for the eolian sand and 116 ka for the zone-of-mixing. Meteoric 10Be and pedostratigraphic data indicate minimum residence times of 33.4 ka for the eolian sand, 80.6 ka for the zone-of-mixing, and 247 ka for the paleosol. The combined OSL and 10Be age data indicate that, at this locality, the barrier/beach ridge has a minimum age of about 360 ka. This age for the Wicomico shoreline-barrier deposit is the first for any Pleistocene near-shore marine/estuarine deposit in southeast Georgia that is conclusively older than 80 ka. The 360-ka minimum age is in agreement with other geochronologic data for near-coastline deposits in Georgia and South Carolina. The geomorphic position of this barrier/beach-ridge is similar to deposits in South Carolina considered to be

  11. Geochronology and Fluid-Rock Interaction Associated with the Nopal I Uranium Deposit, Pena Blanca, Mexico

    Energy Technology Data Exchange (ETDEWEB)

    M. Fayek; P. Goodell; M. Ren; A. Simmons


    The Nopal I uranium (U) deposit, Pena Blanca District, Mexico, largely consists of secondary U{sup 6+} minerals, which occur within a breccia pipe mainly hosted by the 44 Ma Nopal and Colorados volcanic formations. These two units overly the Pozos conglomerate formation and Cretaceous limestone. Three new vertical diamond drill holes (DDHs) were recently drilled at Nopal I. DDH-PB1 with continuous core was drilled through the Nopal I deposit and two additional DDHs were drilled {approx}50 m on either side of the cored hole. These DDHs terminate 20 m below the current water table, thus allowing the detection of possible gradients in radionuclide contents resulting from transport from the overlying uranium deposit. Primary uraninite within the main ore body is rare and fine-grained ({approx}50 micrometers), thus making geochronology of the Nopal I deposit very difficult. Uranium, lead and oxygen isotopes can be used to study fluid-uraninite interaction, provided that the analyses are obtained on the micro-scale. Secondary ionization mass spectrometry (SIMS) permits in situ measurement of isotopic ratios with a spatial resolution on the scale of a few {micro}m. Preliminary U-Pb results show that uraninite from the main ore body gives an age of 32 {+-} 8 Ma, whereas uraninite from the uraniferous Pozos conglomerate that lies nearly 100 m below the main ore body and 25 meters above the water table, gives a U-Pb age that is <1 Ma. Oxygen isotopic analyses show that uraninite from the ore body has a {delta}{sup 18}O = -10.8{per_thousand}, whereas the uraninite within the Pozos conglomerate has a {delta}{sup 18}O = +1.5{per_thousand}. If it is assumed that both uraninites precipitated from meteoric water ({delta}{sup 18}O = -7{per_thousand}), then calculated precipitation temperatures are 55 C for the uraninite from the ore body and 20 C for uraninite hosted by the Pozos conglomerate. These temperatures are consistent with previous studies that calculated precipitation

  12. U-Pb geochronology of basement rocks in central Tibet and paleogeographic implications (United States)

    Guynn, Jerome; Kapp, Paul; Gehrels, George E.; Ding, Lin


    The ages and paleogeographic affinities of basement rocks of Tibetan terranes are poorly known. New U-Pb zircon geochronologic data from orthogneisses of the Amdo basement better resolve Neoproterozoic and Cambro-Ordovician magmatism in central Tibet. The Amdo basement is exposed within the Bangong suture zone between the Lhasa and Qiangtang terranes and is composed of granitic orthogneisses with subordinate paragneisses and metasedimentary rocks. The intermediate-felsic orthogneisses show a bimodal distribution of Neoproterozoic (920-820 Ma) and Cambro-Ordovician (540-460 Ma) crystallization ages. These and other sparse basement ages from Tibetan terranes suggest the plateau is underlain by juvenile crust that is Neoproterozoic or younger; its young age and weaker rheology relative to cratonic blocks bounding the plateau margins likely facilitated the propagation of Indo-Asian deformation far into Asia. The Neoproterozoic ages post-date Rodinia assembly and magmatism of similar ages is documented in the Qaidaim-Kunlun terrane, South China block, the Aravalli-Delhi craton in NW India, the Eastern Ghats of India, and the Prince Charles mountains in Antarctica. The Amdo Neoproterozoic plutons cannot be unambiguously related to one of these regions, but we propose that the Yangtze block of the South China block is the most likely association, with the Amdo basement representing a terrane that possibly rifted from the active Yangtze margin in the middle Neoproterozoic. Cambro-Ordovician granitoids are ubiquitous throughout Gondwana as a product of active margin tectonics following Gondwana assembly and indicate that the Lhasa-Qiangtang terranes were involved in these tectono-magmatic events. U-Pb detrital zircon analysis of two quartzites from the Amdo basement suggest that the protoliths were Carboniferous-Permian continental margin strata widely deposited across the Lhasa and Qiangtang terranes. The detrital zircon age spectra of the upper Paleozoic Tibetan

  13. Spodumene bearing pegmatites in the Austroalpine unit (Eastern Alps): Distribution and new geochronological data (United States)

    Ilickovic, Tanja; Schuster, Ralf; Mali, Heinrich; Petrakakis, Konstantin; Schedl, Albert


    In the Austroalpine unit of the Eastern Alps spodumene bearing pegmatites occur heterogeneously distributed over an E-W distance of more than 400 km. They are usually associated with barren pegmatites which crystallisized in Permian time. There a two schools of thought about the genesis of the spodumene bearing pegmatites: Economic geologists bring forward the argument that these pegmatites only develop by fractionation of granitic parent plutons, whereas metamorphic petrologists consider that the barren pegmatites and even some highly fractionated pegmatites are products of anatexis of metapelitic country rocks. In the first case the virtual absence of co-genetic fertile granites in the Austroalpine units render the model problematic, whereas in the second case the formation of suitable Li-enriched pegmatitic melts is not yet understood. A new understanding of the Austroalpine basement through geological mapping and geochronological and geochemical investigations during the past few years gives the opportunity to reinvestigate this problem: In Permian time the Austroalpine unit was affected by lithospheric extension, causing basaltic underplating, high temperature / low pressure metamorphism and intense magmatic activity. The Permian P-T-t path is characterized by heating at slightly decreasing pressure. In an ongoing project additional spodumene bearing pegmatites have been discovered and some of them show temporal and spatial relations to relatively small leucogranitic bodies. New Sm/Nd data prove a Permian age for spodumene bearing pegmatites and leucogranites supporting a genetic relation with the barren pegmatites. Mapping revealed certain relations of pegmatites and distinct country rocks. Units of migmatitic mica schists with lots of interlayed barren pegmatites represent areas with aborted melt generation. In some places the melts accumulate forming inhomogeneous leucogranitic bodies. Examples are the Martell granite (South Tyrol) as well as leucoganites

  14. Monazite trumps zircon: applying SHRIMP U-Pb geochronology to systematically evaluate emplacement ages of leucocratic, low-temperature granites in a complex Precambrian orogen (United States)

    Piechocka, Agnieszka M.; Gregory, Courtney J.; Zi, Jian-Wei; Sheppard, Stephen; Wingate, Michael T. D.; Rasmussen, Birger


    Although zircon is the most widely used geochronometer to determine the crystallisation ages of granites, it can be unreliable for low-temperature melts because they may not crystallise new zircon. For leucocratic granites U-Pb zircon dates, therefore, may reflect the ages of the source rocks rather than the igneous crystallisation age. In the Proterozoic Capricorn Orogen of Western Australia, leucocratic granites are associated with several pulses of intracontinental magmatism spanning 800 million years. In several instances, SHRIMP U-Pb zircon dating of these leucocratic granites either yielded ages that were inconclusive (e.g., multiple concordant ages) or incompatible with other geochronological data. To overcome this we used SHRIMP U-Th-Pb monazite geochronology to obtain igneous crystallisation ages that are consistent with the geological and geochronological framework of the orogen. The U-Th-Pb monazite geochronology has resolved the time interval over which two granitic supersuites were emplaced; a Paleoproterozoic supersuite thought to span 80 million years was emplaced in less than half that time (1688-1659 Ma) and a small Meso- to Neoproterozoic supersuite considered to have been intruded over 70 million years was instead assembled over 130 million years and outlasted associated regional metamorphism by 100 million years. Both findings have consequences for the duration of associated orogenic events and any estimates for magma generation rates. The monazite geochronology has contributed to a more reliable tectonic history for a complex, long-lived orogen. Our results emphasise the benefit of monazite as a geochronometer for leucocratic granites derived by low-temperature crustal melting and are relevant to other orogens worldwide.

  15. Geochronology and geochemistry of the Heihe mafic pillow lavas in the Qinling Mountains, China

    Institute of Scientific and Technical Information of China (English)

    ZHANG; Zongqing


    [1]Lee, C. Y., A preliminary study of plate tectonics of China, Bull. Chinese Acad. Geol. Sci. (in Chinese), 1980, 2(1): 11-19.[2]Lee, C. Y., Tectonic evolution of Asia, Bull. Chinese Acad. Geol. Sci. (in Chinese), 1984, 10: 3-11.[3]Ren Jishun, Jiang Chunfa, Zhang Zhengkun et al., The Geotectonic Evolution of China (in Chinese), Beijing: Science Press, 1980.[4]Ren Jishun, Chen Tingyu, Niu Baogui et al., Tectonic Evolution of the Continental Hithospheres and Metallogeny in Eastern China and Adjacent Areas (in Chinese), Beijing: Science Press, 1990.[5]Zhang Qiusheng, Zhu Yongzheng, The Paleozoic ophiolite in the east Qinling, J. Changchun College of Geology (in Chi-nese), 1984, 3: 1-13.[6]Xiao Siyun, Zhang Weiji, Song Ziji, The Metamorphic Strata of the Northern Qinling in China (in Chinese), Xi'an: Xi'an Jiaotong University Press, 1988.[7]Xu Zhiqin, Lu Yilun, Tang Yaoqing et al., Formation of the Composite Eastern Qinling Chains (in Chinese), Beijing: China Environmental Science Press, 1988.[8]Zhang Guowei, Formation and evolution of the Qinling orogenic belt (in Chinese), Xi'an: Northwest University Press, 1988.[9]Zhang Guowei, Meng Qingren, Yu Zaiping, Orogenesis and dynamics of the Qinling orogen, Science in China, Ser. D, 1996, 39(3):225-234.[10]Wu Liren, Xu Guizhong, Geological Evolution in the East Qinling-Dabieshan Collision Orogen (in Chinese), Beijing: Science Press, 1988.[11]Sun Yong, Yu Zaiping, A discussion of an ancient ocean and Caledonian in the east Qinling, in A Selection of Papers Pre-sented at the Conference on the Qinling Orogenic Belt (in Chinese), Xi'an: Northwest University Press, 1991, 167-173.[12]Zhang Zongqing, Liu Dunyi,Fu Guomin et al., Geochronology of Metamorphic Strata in the North Qinling Area (in Chi-nese), Beijing: Geological Publishing House, 1994.[13]Cui Zhilin, Sun Yong, Wang Xueren, Discovery of radiolarian fossils within the Danfeng ophiolite in the Qinling and its

  16. Using Detrital Geochronologic and Thermochronologic "Double-Dating" to Constrain Depositional Age, Provenance, and Exhumation Signals in Ancient Forearc Basins (United States)

    Orme, D. A.


    The application of coupled detrital zircon U-Pb geochronology and (U-Th)/He thermochronology to sedimentary basins has the potential for unprecedented details about grain provenance, depositional age and source and basin exhumation signals. Although several studies have implored this technique, it is underutilized and may prove useful in geologic settings that are traditionally difficult to explore. For example, constraining the depositional age of strata in ancient forearc basins is challenging as many horizons are devoid of fossils and post-burial diagenesis of limestone beds limits biostratigraphic age control. This study applies U-Pb detrital zircon geochronology to clastic rocks from the Cretaceous-Eocene Xigaze forearc basin in southern Tibet to (1) to determine the provenance of forearc basin strata and (2) to constrain a maximum depositional age of stratigraphic horizons using the youngest distinct age group from a sample. In addition, (U-Th)/He thermochronology was applied to a subset of the detrital zircons on which U-Pb ages were previously determined in order to determine the timing of exhumation of Xigaze forearc strata and its source region. The use of young populations of zircons is a good method for age control in the Xigaze forearc basin because magmatism in the source area was more-or-less continuous and the lag time between the youngest zircons in a sample and the time of that samples deposition is likely relatively small. A total of 2,330 zircon grains yielded ages with acceptable precision and concordance for geochronologic interpretation. Together with sandstone petrography, the detrital zircons indicate that the primary source of detritus in the basin from ~113 to 54 Ma was the Gangdese magmatic arc. Analysis of the youngest age component of individual samples reveals a decrease in the youngest ages upsection, consistent with maximum depositional ages that are close to the likely true depositional age based on intervening tuff layers. Double

  17. High-Precision U-Pb Geochronology and Correlation: An example Using the Neoproterozic-Cambrian Transition (United States)

    Bowring, S. A.; Grotzinger, J. P.; Amthor, J.; Martin, M. E.


    The precise, global correlation of Precambrian and Paleozoic sedimentary rocks can be achieved using temporally calibrated chemostratigraphic records. This approach is essential for determining rates and causes of environmental and faunal change, including mass extinctions. For example, The Neoproterozoic is marked by major environmental change, including periods of global glaciation, large fluctuations in the sequestration of carbon and major tectonic reorganization followed by the explosive diversification of animals in the earliest Cambrian. The extreme climatic change associated with these glaciations have been implicated as a possible trigger for the Cambrian explosion. The recognition of thin zircon-bearing air-fall ash in Neoproterozoic and Cambrian rocks has allowed the establishment of a high-precision temporal framework for animal evolution and is helping to untangle the history of glaciations. In some cases analytical uncertainties translate to age uncertainties of less than 1 Ma and when integrated with chemostratigraphy, the potential for global correlations at even higher resolution. Progress in the global correlation of Neoproterozoic strata has been achieved through the use of C and Sr isotope chemostratigraphy although it has been hampered by a lack of precise geochronological and faunal control. For example, the period from ca 800-580 Ma is characterized by at least two and perhaps as many as four glacial events that are interpreted by many to be global glaciations on a "Snowball Earth". A lack of precise chronological constraints on the number and duration of glaciations, multiple large excursions in the carbon isotopic record, and an absence of detailed biostratigraphy have complicated global correlation and hindered our understanding of this important period of Earth history. However, the ongoing integration of chemostratigraphic and geochronological data are improving temporal resolution and detailed correlations. These data are critical for

  18. CO2 outburst events in relation to seismicity: Constraints from microscale geochronology, geochemistry of late Quaternary vein carbonates, SW Turkey (United States)

    Ünal-İmer, Ezgi; Uysal, I. Tonguç; Zhao, Jian-Xin; Işık, Veysel; Shulmeister, James; İmer, Ali; Feng, Yue-Xing


    Vein and breccia carbonates precipitated in a highly fractured/faulted carbonate bedrock in SW Turkey were investigated through high-resolution U-series geochronology, microstructural and geochemical studies including C-O-Sr isotope and rare-earth element and yttrium (REY) analyses. Petrographical observations and geochronological data are interpreted as evidence that the calcite veins formed through a crack-seal mechanism, mostly accompanied/initiated by intensive hydraulic fracturing of the host limestone in response to high-pressure fluids, which is manifested by multi-stage breccia deposits. Microscale U-series dates (272.6-20.5 kyr) and geochemical compositions of the vein/breccia samples provide information on the timing and mechanism of the vein formation and identify the source of CO2-bearing fluids responsible for the carbonate precipitation. δ18OVPDB and δ13CVPDB values of the calcite veins range between -5.9 and -1.7‰, and -10.6 and -4.6‰, respectively. The isotopic compositions of the veins show highly fluctuating values as calcite grew successively perpendicular to vein walls, which, in combination with microstructural and geochronological constraints, are interpreted to reflect episodic CO2 degassing events associated with seismic and aseismic deformation. Oxygen and Sr isotope compositions (δ18OVPDB: -5.9 to -1.7‰; 87Sr/86Sr: 0.7082 to 0.7085) together with REY concentrations indicate deep infiltration of meteoric waters with various degrees of interactions mostly with the host limestone and siliciclastic parts of the basement rocks. Oxygen and carbon isotope compositions suggest CO2 degassing through intensive limestone dissolution. While majority of the veins display similar Post-Archaean Australian Shale (PAAS)-normalised REY variations, some of the veins show positive EuPAAS anomalies, which could be indicative of contributions from a deeply derived, heated, and reduced fluid component, giving rise to multiple fluid sources for the

  19. U/Pb geochronology and paleo and proterozoic orthogneisses geochemistry from the Taquaritinga region - Pernambuco State, Brazil; Geocronologia U/Pb e geoquimica de ortognaisses paleo e mesoproterozoicos da regiao de Taquaritinga, PE

    Energy Technology Data Exchange (ETDEWEB)

    Sa, Jaziel Martins [Rio Grande do Norte Univ., Natal, RN (Brazil). Dept. de Geologia; Bertrand, Jean Michel [Savoie Univ., 73 - Chambery (France). Lab. de Geodynamique; Leterrier, Jacques [Centre National de la Recherche Scientifique (CNRS), 54 - Nancy (France). Centre de Recherches Petrographiques et Geochimiques


    The Taquaritinga region is located at the Borborema Province Transversal Zone, northeastern part of Brazil, and not existing geochronological data in this region, this study intended to furnish some information for a geotectonics evolution knowledge. It presents the geological characteristics of this region, describes the various kinds of chemical analysis realized in the gneisses, involving major, minor and rare earth elements, and shows the geochronological data that suggests the orthogneisse age and the epoch of this geotectonic event 8 refs., 2 figs.

  20. 40Ar/39Ar geochronology and exhumation of mylonitized metamorphic complex in Changle-Nanao ductile shear zone

    Institute of Scientific and Technical Information of China (English)

    王志洪; 卢华复


    New 40Ar/39Ar plateau ages from rocks of Changle-Nanao ductile shear zone are 107.9 Ma(Mus), 108.2 Ma(Bi), 107. 1 Ma(Bi), 109. 2 Ma(Hb) and 117. 9 Ma(Bi) respectively, which are concordant with their isochron ages and record the formation age of the ductile shear zone. The similarity and apparent overlap of the cooling ages with respective closure temperatures of 5 minerals document initial rapid uplift during 107-118 Ma following the collision between the Min-Tai microcontinent and the Min-Zhe Mesozoic volcanic arc. The 40Ar/39Ar plateau ages, K-Ar date of K-feldspar and other geochronologic information suggest that the exhumation rate of the ductile shear zone is about 0.18-1.12 mm/a in the range of 107-70 Ma, which is mainly influenced by tectonic extension.

  1. SHRIMP-RG U-Pb zircon geochronology of mesoproterozoic metamorphism and plutonism in the southwesternmost United States (United States)

    Barth, Andrew P.; Wooden, Joseph L.; Coleman, Drew S.


    Mesoproterozoic intrusive and granulite‐grade metamorphic rocks in southern California have been inferred to be exotic to North America on the basis of perceived chronologic incompatibility with autochthonous cratonal rocks. Ion microprobe geochronology indicates that zircons in granulite‐grade gneisses, dated at 1.4 Ga using conventional methods, are composed of 1.68–1.80‐Ga cores and 1.19‐Ga rims. These Early Proterozoic gneisses were metamorphosed at extremely high temperatures and moderate pressures during emplacement of the 1.19‐Ga San Gabriel anorthosite complex. The lack of a 1.4‐Ga metamorphic event suggests that Proterozoic rocks in this region, rather than being exotic to North America, may in fact be a midcrustal window into Mesoproterozoic crustal evolutionary processes in southwestern North America.

  2. Elucidating the magmatic history of the Austurhorn silicic intrusive complex (southeast Iceland) using zircon elemental and isotopic geochemistry and geochronology (United States)

    Padilla, A. J.; Miller, C. F.; Carley, T. L.; Economos, R. C.; Schmitt, A. K.; Coble, M. A.; Wooden, J. L.; Fisher, C. M.; Vervoort, J. D.; Hanchar, J. M.


    The Austurhorn intrusive complex (AIC) in southeast Iceland comprises large bodies of granophyre and gabbro, and a mafic-silicic composite zone (MSCZ) that exemplifies magmatic interactions common in Icelandic silicic systems. Despite being one of Iceland's best-studied intrusions, few studies have included detailed analyses of zircon, a mineral widely recognized as a valuable tracer of the history and evolution of its parental magma(s). In this study, we employ high spatial resolution zircon elemental and isotopic geochemistry and U-Pb geochronology as tools for elucidating the complex construction and magmatic evolution of Austurhorn's MSCZ. The trace element compositions of AIC zircon crystals form a broad but coherent array that partly overlaps with the geochemical signature for zircons from Icelandic silicic volcanic rocks. Typical of Icelandic zircons, Hf concentrations are relatively low (mush-like material and a prolonged lifetime for the complex.

  3. Earth history. U-Pb geochronology of the Deccan Traps and relation to the end-Cretaceous mass extinction. (United States)

    Schoene, Blair; Samperton, Kyle M; Eddy, Michael P; Keller, Gerta; Adatte, Thierry; Bowring, Samuel A; Khadri, Syed F R; Gertsch, Brian


    The Chicxulub asteroid impact (Mexico) and the eruption of the massive Deccan volcanic province (India) are two proposed causes of the end-Cretaceous mass extinction, which includes the demise of nonavian dinosaurs. Despite widespread acceptance of the impact hypothesis, the lack of a high-resolution eruption timeline for the Deccan basalts has prevented full assessment of their relationship to the mass extinction. Here we apply uranium-lead (U-Pb) zircon geochronology to Deccan rocks and show that the main phase of eruptions initiated ~250,000 years before the Cretaceous-Paleogene boundary and that >1.1 million cubic kilometers of basalt erupted in ~750,000 years. Our results are consistent with the hypothesis that the Deccan Traps contributed to the latest Cretaceous environmental change and biologic turnover that culminated in the marine and terrestrial mass extinctions.

  4. SHRIMP U-Pb in zircon geochronology of granitoids from Myanmar: temporal constraints on the tectonic evolution of Southeast Asia (United States)

    Barley, M. E.; Zaw, Khin


    The Mesozoic to Tertiary tectonic evolution of Southeast Asia is the result of the convergence and collision of fragments of Gondwanaland with Eurasia culminating in the collision of India. A rapidly growing geochronological database is placing tight constraints on the timing and duration of magmatic episodes, metallogenic and tectonic events in the Himalayas, Tibet and eastern Indochina. However, there is little comparable geochronology for Myanmar. This SHRIMP U-Pb in zircon geochronology focuses on granitoids from the Mogok Metamorphic Belt (MMB, a belt of high grade metamorphic rocks at the edge of the Shan-Thai Terrane), the Myeik Archipelago (Shan-Thai Terrane) and the west Myanmar Terrane. Strongly deformed granitic orthogneisses in the MMB near Mandalay contain Jurassic (~170 Ma) zircons that have partly recrystallised during ~43 Ma high-grade metamorphism. A hornblende syenite from Mandalay also contains Jurassic zircons with evidence of Eocene metamorphism rimmed by thin zones of 30.9 ±0.7 Ma magmatic zircon. The relative abundance of Jurassic zircons in these rocks is consistent with suggestions that southern Eurasia had an Andean-type margin at that time. Mid-Cretaceous to earliest Eocene (120 to 50 Ma). I-type granitoids in the MMB, Myeik Archipelago and west Myanmar confirm that prior to the collision of India, an up to 200km wide magmatic belt extended along the Eurasian margin. The primitive I-type Khanza Chaung granodiorite in the Wuntho batholith in the west Myanmar terrane hosts porphyry-style mineralisation and has a magmatic age of 94  1 Ma. Triassic (~240 Ma), Jurassic (~170 Ma) and Early Cretaceous xenocryst zircons in this granitoid correspond with peaks of granitoid magmatism in the Shan-Thai terrane and establish that west Myanmar was part of the margin of Eurasia during the Mesozoic. A suite of highly fractionated metaluminous to peraluminous I-type granitoids with associated Sn-W-Ta mineralisation emplaced in the Myeik Archipelago of

  5. Evaluation of the Siltation of River Taquari, Pantanal, Brazil, through 210Pb Geochronology of Floodplain Lake Sediments

    Directory of Open Access Journals (Sweden)

    Godoy José M.


    Full Text Available This work presents the 210Pb geochronology of seven bottom sediment cores, collected in three floodplain lakes located in the area of the middle Taquari River, Pantanal, Brazil. In five of them, a significant increase in the sediment mass deposition rate was observed, reflecting an increase of the sediment input to the Pantanal. Additionally, in order to validate the 210Pb results, the mercury content was determined for two sediment cores, showing that despite a constant concentration, the flux of Hg has increased due to an increase in the mass sedimentation rate. This increase can be attributed to the expansion of agricultural activity in the upper Taquari River during the last 25 years.

  6. The Durulgui rare-metal granite‒pegmatite system in the eastern Transbaikal region: Petrological and geochronological aspects (United States)

    Zagorsky, V. E.


    The Durulgui granite‒pegmatite system unites the Dedova Gora granite massif and pegmatite field with the Chalotskoe beryl deposit. New geochronological data on micas from porphyric biotite granites, fine-grained biotite granites, two-mica granites, and Be-bearing pegmatites are discussed. The plateau age of 128.5(±1.5)-131.2(±1.5) should be considered as indicating the formation time of the granite‒pegmatite system as a whole. The age of the system implies the possibility of its formation owing to several magmatic pulses. This assumption concerns porphyric and fine-grained biotite granites and two-mica and muscovite granites, the contact between which is locally sharp. At the same time, the succession "two-mica granites → muscovite granites → granite‒pegmatites → microcline pegmatites → microcline‒albite pegmatites → albite pegmatites" demonstrates gradual facies transitions between rocks, which indicates their emplacement during a single magmatic pulse.

  7. Geochemistry and geochronology of granitoids in the Kibi-Asamankese area of the Kibi-Winneba volcanic belt, southern Ghana (United States)

    Anum, Solomon; Sakyi, Patrick Asamoah; Su, Ben-Xun; Nude, Prosper M.; Nyame, Frank; Asiedu, Daniel; Kwayisi, Daniel


    In Ghana the West African Craton is represented by Birimian and Tarkwaian rocks with extensive granitoid bodies. Granitoids from Asamankese area of the Kibi-Winneba volcanic belt, southern Ghana were analysed for major and trace element contents and found to be characterised by highly-fractionated REE, enrichments, in LILE, and depletion in Nb, Ta and Sr. The LILE enrichment relative to strong Nb-Ta depression, indicates that these granitoids were emplaced in an active margin. Based on field relations, geochemical composition and geochronological data, the granitoids from the Kibi-Asamankese area can be divided into three types, namely; the Eburnean biotite granodiorite (2133-2127 Ma) and hornblende granodiorite (2147 Ma), and the Pre-Eburnean gneissic biotite granite (2193 Ma). The geochemical data of the studied rocks plot in the tholeiitic field, whereas on the A/CNK-A/NK diagram, they generally fall within the metaluminous field, with A/CNK values between 0.69 and 0.88. U-Pb dating of zircons in the granitoids yielded ages ranging from 2193 to 2127 Ma, which are among the oldest ages obtained from the granitoid plutons in Ghana. Such high-precision geochronological data indicate that magmatism occurred over a time-span of about 70 Ma. This provides further evidence that the period 2.1-2.2 Ga was one of the important stages of Birimian magmatism that led to the generation of the granitoids. From the above-mentioned ages, it is possible to link the geological activities to crustal processes and establish the cyclic geotectonic evolution in the West African Craton over time as part of an arc-back-arc basin system.

  8. Temporal evolution of mineralization events in the Bohemian Massif inferred from the Re-Os geochronology of molybdenite (United States)

    Ackerman, Lukáš; Haluzová, Eva; Creaser, Robert A.; Pašava, Jan; Veselovský, František; Breiter, Karel; Erban, Vojtěch; Drábek, Milan


    Molybdenite is a common mineral accompanying Sn-W, Au, and base metal mineralizations located in different geotectonic units of the Bohemian Massif, but it is also widespread in granitoids and/or related quartz veins/pegmatites forming disseminated Mo mineralization. Thirty Re-Os ages were obtained for molybdenite samples from the Bohemian Massif to provide constraints on the timing and duration of mineralization event(s) within the framework of previously published geochronological data for the host and/or associated rocks. The obtained data for Sn-W-(Li) deposits in the Erzgebirge metallogenetic province indicate the predominance of one and/or multiple short-time mineralization events taking place between ˜319 and 323 Ma, with the exception of the Krupka deposit associated with the Altenberg-Teplice caldera where the data may suggest prolonged activity until ˜315 Ma. The ages of the Pb-Zn-(Au-Mo) Hůrky u Rakovníka and Fe-Cu-As Obří důl mineralizations from the exocontacts of the Čistá pluton and Krkonoše-Jizera Plutonic Complex, respectively, provide evidence for synchronous emplacement of the ore and the associated granitic rocks. In contrast, the Padrť Fe-As-Mo mineralization postdates the age of the associated Padrť granite. Disseminated Mo mineralization in Cadomian and Variscan granitoids and/or related to quartz veins/pegmatites provides Re-Os ages that overlap with the previously published geochronological data for the host rocks, suggesting coeval evolution. Molybdenite samples from the Sázava suite granites of the Central Bohemian Plutonic Complex (CBPC) have resolvable younger ages than their host granites, but similar to the age of spatially related Au mineralization which is associated with the latest evolution of the CBPC.

  9. Provenance and drainage system of the Early Cretaceous volcanic detritus in the Himalaya as constrained by detrital zircon geochronology

    Institute of Scientific and Technical Information of China (English)

    Xiu-Mian Hu; Eduardo Garzanti; Wei An


    The age range of the major intra-plate volcanic event that affected the northern Indian margin in the Early Cretaceous is here deifned precisely by detrital zircon geochronol-ogy. U–Pb ages of Early Cretaceous detrital zircons found in the Cretaceous to the Paleocene sandstones cluster mainly between 142 Ma and 123 Ma in the northern Tethys Himalayan unit, and between 140 Ma and 116 Ma in the southern Tethys Himalayan unit. The youngest and oldest detrital zircons within this group indicate that volcanism in the source areas started in the latest Jurassic and ended by the early Albian. Stratigraphic data indicate that volcaniclastic sedimentation began signiifcantly earlier in southern Tibet (Tithonian) than in Nepal (Valangin-ian), and considerably later in Spiti and Zanskar (Aptian/Albian) to the west. This apparent westward migration of magmatism was explained with progressive westward propagation of extensional/transtensional tectonic activity and development of fractures cutting deeply across the Indian continental margin crust. However, detrital zircon geochronology provides no indi-cation of heterochroneity in magmatic activity in the source areas from east to west, and thus lends little support to such a scenario. Westward migration of volcaniclastic sedimentation may thus relfect instead the westward progradation of major drainage systems supplying vol-canic detritus sourced from the same volcanic centers in the east. Development of multiple radial drainage away from the domal surface uplift associated with magmatic upwelling, as observed for most large igneous provinces around the world, may also explain why U–Pb ages of detrital zircons tend to cluster around 133–132 Ma (the age of the Comei igneous province) in Tethys Himalayan units, but around 118–117 Ma (the age of the Rajmahal igneous province) in Lesser Himalayan units.

  10. The Triassic reworking of the Yunkai massif (South China): EMP monazite and U-Pb zircon geochronologic evidence (United States)

    Chen, Cheng-Hong; Liu, Yung-Hsin; Lee, Chi-Yu; Sano, Yuji; Zhou, Han-Wen; Xiang, Hua; Takahata, Naoto


    Geohistory of the Yunkai massif in South China Block is important in understanding the geodynamics for the build-up of this block during the Phanerozoic orogenies. To investigate this massif, we conduct EMP monazite and U-Pb zircon geochronological determinations on mineral inclusions and separate for seventeen samples in four groups, representing metamorphic rocks from core domain, the Gaozhou Complex (amphibolite facies, NE-striking) and the Yunkai Group (greenschist facies, NW-striking) of this massif and adjacent undeformed granites. Some EMP monazite ages are consistent with the NanoSIMS results. Monazite inclusions, mostly with long axis parallel to the cleavage of platy and elongated hosts, give distinguishable age results for NW- and NE-trending deformations at 244-236 Ma and 236-233 Ma, respectively. They also yield ages of 233-230 Ma for core domain gneissic granites and 232-229 Ma for undefomed granites. Combining U-Pb zircon ages of the same group, 245 Ma and 230 Ma are suggested to constrain the time of two phases of deformation. Aside from ubiquity of Triassic ages in studied rocks, ages of detrital monazite in the meta-sandstone match the major U-Pb zircon age clusters of the metamorphic rock that are largely concentrated at Neoproterozoic (1.0-0.9 Ga) and Early Paleozoic (444-431 Ma). Based on these geochronological data, Triassic is interpreted as representing the time for recrystallization of these host minerals on the Early Paleozoic protolith, and the also popular Neoproterozoic age is probably inherited. With this context, Yunkai massif is regarded as a strongly reactivated Triassic metamorphic terrain on an Early Paleozoic basement which had incorporated sediments with Neoproterozoic provenances. Triassic tectonic evolution of the Yunkai massif is suggested to have been controlled by converging geodynamics of the South China and Indochina Blocks as well as mafic magma emplacement related to the Emeishan large igneous province (E-LIP).

  11. Monazite Geochronology of Al-Fe Granulites Of Amesmessa Area from In-Ouzzal Terrane (Western Hoggar, Algeria) (United States)

    Ahmed, Benbatta; Abderrahmane, Bendaoud; Bénédicte, Cenki-Tok; Zohir, Adjerid; Olivier, Bruguier; Jesus, Garrido Marin Carlos; Safouane, Djemai; Khadija, Ouzegane


    ABSTRACT: The In Ouzzal terrane in western Hoggar (South Algeria) preserves evidence of ultrahigh-temperature (UHT) crustal metamorphism. It consists in Archean crustal units, composed of orthogneissic domes and green stone belts, strongly remobilized during the Paleoproterozoic orogeny (2000 Ma, Peucat et al., 1996). Ouzegane et al. (2003) summarize this UHT metamorphic history as two granulitic stages of high temperature : a prograde evolution with peak conditions around 9-11 kbar and 950-1050°C, leading to the appearance of exceptional paragenèses with corundum-quartz, sapphirine-quartz and sapphirine-spinel-quartz in Al-Mg granulites, Al-Fe granulites and quartzites; followed by retrograde event characterized by a pressure drop to 5-7 kbar. This retrograde event is marked by intrusive carbonatite bodies and the occurrence of leptynites veins. The present study is interested in Al-Fe granulites which outcrop in a still little known region situated in southeastern part of In-Ouzzal terrane. These granulites are mainly composed by quartz, spinel, garnet, sillimanite, cordierite, biotite, perthitic feldspar, ilmenite, ± corundum. The study consist to dating these Al-Fe granulites by monazites (U-Pb - ICP-MS method) combined with their internal structures revealed by BSE imaging. The primary results suggest two major facts: 1- for the first time, the existence of at least one metamorphism older than 2.5 Ga; 2- a long live paleoproterozoic high temperature metamorphism. These geochronological results completed and combined with a detailed phases relationship study of these Al-Fe granulites will are of major importance as for future discussion on the geodynamic context responsible for this regional UHT metamorphism as well as indicating a record the time of the different stages of granulitic metamorphism. Keywords: UHT metamorphism, Granulites, Paleoproterozoïc, Archaean; Southeastern In Ouzzal terrane; Monazite geochronology; Hoggar

  12. The Violent Early Solar System, as Told by Lunar Sample Geochronology (United States)

    Cohen, B. A.


    One of the legacies of the samples collected by the Apollo and Luna missions is the link forged between radiometric ages of rocks and relative ages according to stratigraphic relationships and impact crater size-frequency distributions. Our current understanding of the history of the inner solar system is based on the relative chronology of individual planets, tied to the absolute geochronology of the Moon via these important samples. Sample ages have enabled us to infer that impact-melt breccias from Apollo 14 and 15 record the formation of the Imbrium Basin, those from the highland massifs at Apollo 17 record the age of Serenitatis, those from the KREEP-poor Apollo 16 site record the age of Nectaris, and materials from Luna 24 record the age of Crisium. Ejecta from smaller and younger craters Copernicus and Tycho were sampled at Apollo 12 and 17, respectively, and local craters such as Cone at Apollo 14, and North Ray and South Ray at Apollo 16 were also sampled and ages determined for those events. Much of what we understand about the lunar impact flux is based on these ages. Samples from these nearside locations reveal a preponderance of impact-disturbed or recrystallized ages between 3.75 and 3.95 billion years. Argon and lead loss (and correlated disturbances in the Rb-Sr system) have been attributed to metamorphism of the lunar crust by an enormous number of impacts in a brief pulse of time, called the Lunar Cataclysm or Late Heavy Bombardment. Subsequent high-precision geochronometric analyses of Apollo samples and lunar highlands meteorites show a wider range of ages, but very few older than 4 Ga. The paucity of ancient impact melt rocks has been interpreted to mean that either that most impact basins formed at this time, or that ejecta from the large, near-side, young basins dominates the Apollo samples. The impact history of the Moon has significant implications because the lunar bombardment history mirrors that of the Earth. During the cataclysm, 80% of

  13. An integrated geochronology and paleomagnetic study from Michoacan Block: tectonic implications for the Guadalajara triple junction (United States)

    Rosa-Elguera, J.; Gogichaishvili, A.; Alva-Valdivia, L.; Urrutia-Fucugauchi, J.


    Three alternatives were proposed to expain the Guadalajara triple junction: (a) mantle plume beneath Guadalajara area provoking a regional uplifting, (b) relocation of the East Pacific Rise, and (c) transfer to the SE of rifting-related structure of the California Gulf. However, a different interpretation can be advanced if we consider the tectonics of the northern-Colima and western-Chapala rifts together with counterclockwise rotation of the Michoacan block. We carried out a detailed tectonic and paleomagnetic analysis along the Chapala rift and the Michoacan block. The trend of the measured mesofaults in the Chapala rift displays a dominant E-W direction. These faults represent everywhere the last tectonic event corresponding to Late Miocene to Quaternary times. Western Chapala rift is formed with three normal faults southward dipping which form a "domino-like" fault system. The most southern block of this system is a gently NW-tilted volcanic plateau of Early Pliocene basaltic rocks which belongs to the NW-part of the Michoacan block. Although some faults present a lateral component of motion, the great majority of them have pitches higher than 45° and dip ranging between 45° and 75°. Paleo-stress tensors computed from fault slip data measured at twelve sites in the Chapala rift indicate that average direction of the minimum principal stress is 160°. Paleomagnetic and geochronological sampling were carried out on the Cotija area (nothern part of the of the Michoacan block). Oldest studied rock gave an age of 9.17 +/- 0.92 Ma, whereas youngest one, located above the ancient fault zone, is 3.8 Ma old. A total of 65 oriented samples (12 sites) from the late Miocene lava flows were collected from Cotija area for paleomagnetic analyses. Typically, 5 to 6 samples per flow were subjected to alternating magnetic field treatment. Most sites are characterized by normal polarity and stable single component magnetizations. The Michoacan block yield the mean

  14. El Salvador, Chile porphyry copper deposit revisited: Geologic and geochronologic framework (United States)

    Cornejo, P.; Tosdal, R.M.; Mpodozis, C.; Tomlinson, A.J.; Rivera, O.; Fanning, C.M.


    The Eocene (42 to 41 Ma) El Salvador porphyry copper deposit in the Indio Muerto district, northern Chile (26?? 15??? S Lat.), formerly thought to have formed at the culmination of a 9-m.y. period of episodic magmatism, is shown by new mapping, U-Pb and K-Ar geochronology, and petrologic data to have formed during the younger of two distinct but superposed magmatic events - a Paleocene (???63 to 58 Ma) and an Eocene (44 to 41 Ma) event. In the district, high-K Paleocene volcano-plutonic activity was characterized by a variety of eruptive styles and magmatic compositions, including a collapse caldera associated with explosive rhyolitic magmatism (El Salvador trap-door caldera), a post-collapse rhyolite dome field (Cerro Indio Muerto), and andesitic-trachyandesitic stratovolcanos (Kilo??metro Catorce-Los Amarillos sequence). Precaldera basement faults were reactivated during Paleocene volcanism as part of the collapse margin of the caldera. Beneath Cerro Indio Muerto, where the porphyry Cu deposit subsequently formed, the intersection of two major basement faults and the NNE-striking rotational axis of tilted ignimbrites of the Paleocene El Salvador caldera localized emplacement of post-collapse rhyolite domes and peripheral dikes and sills. Subsequent Eocene rhyolitic and granodioritic-dacitic porphyries intruded ???14 m.y. after cessation of Paleocene magmatism along the same NNE-striking structural belt through Cerro Indio Muerto as did the post-collapse Paleocene rhyolite domes. Eocene plutonism over a 3-m.y. period was contemporaneous with NW-SE-directed shortening associated with regional sinistral transpression along the Sierra Castillo fault, lying ???10 km to the east. Older Eocene rhyolitic porphyries in the Indio Muerto district were emplaced between 44 and 43 Ma, and have a small uneconomic Cu center associated with a porphyry at Old Camp. The oldest granodioritic-dacitic porphyries also were emplaced at ???44 to 43 Ma, but their petrogenetic relation to

  15. Geochronological Constraints on the Exhumation and Emplacement of Subcontinental Lithospheric Mantle Peridotites in the Westernmost Mediterranean (United States)

    Garrido, Carlos J.; Hidas, Károly; Marchesi, Claudio; Varas-Reus, María Isabel; Booth-Rea, Guillermo


    Exhumation of subcontinental mantle peridotite in the Western Mediterranean has been attributed to different tectonic processes including pure extension, transpression, or alternating contractive and extensional processes related with continental subduction followed by extension, before final their contractive intracrustal emplacement. Any model trying to explain the exhumation and emplacement of subcontinental lithospheric mantle peridotites in the westernmost Mediterranean should take into account the available geochronological constraints, as well as the petrological and geochemical processes that lead to internal tectono-magmatic zoning so characteristic of the Betic and Rif orogenic peridotites. Different studies have suggested a Hercynian, Cenozoic-Mesozoic or an Alpine age for the late tectono-magmatic evolution and intra-crustal emplacement of Betic-Rif peridotites. The pervasive presence of Mesozoic U-Pb zircon ages in Ronda UHP and HP garnet pyroxenites does not support a Hercynian age for the intracrustal emplacement of the peridotite. A hyper-extended margin setting for is in good agreement with the Jurassic extensional event that pervasively affected ALKAPECA terrains (i.e. the Alboran, Kabylides, Peloritani, and Calabria domains) in the western Mediterranean due to the opening of the Piemonte-Ligurian Ocean. However, a Jurassic age and a passive margin tectonic setting do not account, among other observations, for the late Miocene thermochronological ages recorded in zircons rims (U-Pb) and garnets (Lu-Hf) in garnet pyroxenites from the Betic-Rif peridotites, the pervasive Miocene resetting of U-Pb zircon and monazite ages in the overlying Jubrique crustal section, the supra-subduction radiogenic signature of late pyroxenite intrusive dikes in the Ronda peridotite, and the arc tholeiitic affinity of late mantle-derived, gabbroic dykes intruding in the Ronda and Ojen plagioclase lherzolites. These data are more consistent with a supra

  16. Using statistics and software to maximize precision and accuracy in U-Pb geochronological measurements (United States)

    McLean, N.; Bowring, J. F.; Bowring, S. A.


    Uncertainty in U-Pb geochronology results from a wide variety of factors, including isotope ratio determinations, common Pb corrections, initial daughter product disequilibria, instrumental mass fractionation, isotopic tracer calibration, and U decay constants and isotopic composition. The relative contribution of each depends on the proportion of radiogenic to common Pb, the measurement technique, and the quality of systematic error determinations. Random and systematic uncertainty contributions may be propagated into individual analyses or for an entire population, and must be propagated correctly to accurately interpret data. Tripoli and U-Pb_Redux comprise a new data reduction and error propagation software package that combines robust cycle measurement statistics with rigorous multivariate data analysis and presents the results graphically and interactively. Maximizing the precision and accuracy of a measurement begins with correct appraisal and codification of the systematic and random errors for each analysis. For instance, a large dataset of total procedural Pb blank analyses defines a multivariate normal distribution, describing the mean of and variation in isotopic composition (IC) that must be subtracted from each analysis. Uncertainty in the size and IC of each Pb blank is related to the (random) uncertainty in ratio measurements and the (systematic) uncertainty involved in tracer subtraction. Other sample and measurement parameters can be quantified in the same way, represented as statistical distributions that describe their uncertainty or variation, and are input into U-Pb_Redux as such before the raw sample isotope ratios are measured. During sample measurement, U-Pb_Redux and Tripoli can relay cycle data in real time, calculating a date and uncertainty for each new cycle or block. The results are presented in U-Pb_Redux as an interactive user interface with multiple visualization tools. One- and two-dimensional plots of each calculated date and

  17. Geochronology of accessory allanite and monazite in the Barrovian metamorphic sequence of the Central Alps, Switzerland (United States)

    Boston, Kate R.; Rubatto, Daniela; Hermann, Jörg; Engi, Martin; Amelin, Yuri


    The formation of accessory allanite, monazite and rutile in amphibolite-facies rocks across the Barrovian sequence of the Central Alps (Switzerland) was investigated with a combination of petrography and geochemistry and related to the known structural and metamorphic evolution of the Lepontine dome. For each of these minerals a specific approach was adopted for geochronology, taking into account internal zoning and U-Th-Pb systematics. In-situ U-Th-Pb dating of allanite and monazite by ion microprobe revealed systematic trends for the ages of main deformation and temperature in the Lepontine dome. Isotope dilution TIMS dating of rutile returns dates in line with this picture, but is complicated by inheritance of pre-Alpine rutile and possible Pb loss during Alpine metamorphism. Allanite is generally a prograde mineral that is aligned along the main foliation of the samples and found also as inclusions in garnet. Prograde allanite formation is further documented by rutile inclusions with formation temperatures significantly lower than the maximum T recorded by the rock mineral assemblage. Allanite ages vary from 31.3 ± 1.1 Ma in orthogneisses in the East to 31.7 ± 1.1 Ma for a Bündnerschiefer and 28.5 ± 1.3 Ma for a metaquartzite in the central area, to 26.8 ± 1.1 Ma in the western part of the Lepontine dome. These ages are interpreted to date the main deformation events (nappe stacking and isoclinal deformation of the nappe stack), close to peak pressure conditions. The timing of the thermal peak in the Lepontine dome is recorded in monazite that grew at the expense of allanite and after a main episode of garnet growth at temperatures of 620 °C. Monazite in the central area yields an age of 22.0 ± 0.3 Ma, which is indistinguishable from the age of 21.7 ± 0.4 Ma from a metapelite in the western part of the Lepontine dome. In the central area some of the classical kyanite-staurolite-garnet schists directly underlying the metamorphosed Mesozoic sediments

  18. Assessing amino acid racemization variability in coral intra-crystalline protein for geochronological applications (United States)

    Hendy, Erica J.; Tomiak, Peter J.; Collins, Matthew J.; Hellstrom, John; Tudhope, Alexander W.; Lough, Janice M.; Penkman, Kirsty E. H.


    a combined dataset to include this natural inter-colony variability THAA Asx D/L, Glx D/L and Ala D/L give a 2σ age uncertainty of ±19, ±38 and ±29 year, for the 20th C respectively; in comparison 2σ age uncertainties from a single colony are ±12, ±12 and ±14 year. This is the first demonstration of FAA D/L for dating coral and following strict protocols 2σ precisions of ±24 years can be achieved across different colonies in samples from the last 150 years, and can be ±10 years within a core from a single colony. Despite these relatively large error estimates, AAR would be a valuable tool in situations where a large number of samples need to be screened rapidly and cheaply (e.g. identifying material from mixed populations in beach or uplift deposits), prior to and complementing the more time-consuming geochronological tools of U/Th or seasonal isotopic timeseries.

  19. Stratigraphy and geochronology of the Comondú Group near Loreto, Baja California sur, Mexico (United States)

    Umhoefer, Paul J.; Dorsey, Rebecca J.; Willsey, Shawn; Mayer, Larry; Renne, Paul


    Upper Oligocene to Middle Miocene volcanic and sedimentary rocks in the Loreto region, Baja California Sur, are widely exposed and make up the Comondú Group as redefined here following McFall. The Comondú Group is part of a volcanic arc and forearc basin that formed along the northwestern margin of Mexico. Regional to detailed scale mapping, stratigraphic analysis, and geochronology in a 10-20-km-wide and 70-km-long belt from the gulf escarpment to the coast near Loreto reveal three main units in the Comondú Group and a composite thickness of ˜1.5-2 km. (1) The lower clastic unit (˜30-19 Ma) contains 200-300 m of fluvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt flows that accumulated in a forearc basin. (2) The middle breccia and lava flow unit (˜19-15 Ma) is up to 750 m thick and consists of massive andesite breccia that was deposited as proximal debris flows interbedded with minor andesite lava flows. (3) The upper lava flow and breccia unit (˜15-12 Ma) consists almost entirely of up to ˜600 meters of andesite lava flows a few kilometers west of Loreto that are part of a composite volcano. North and south of Loreto for up to 25 km, the unit is composed of andesite lava flows and massive andesite breccia that formed from thick debris flows near that and other volcanic centers. The three units of the Comondú Group in the Loreto area correlated with the Salto, Pelones, and Ricasón formations of the Bahı´a Concepción area. The lower clastic unit of this study is time equivalent to the marine San Gregorio, El Cien, and Isidro formations on the western and southern Baja California peninsula. The middle and upper units of the Loreto area are the proximal equivalent to the type section of the Comondú Group near the village of Comondú. During deposition of the lower part of the Comondú Group, the arc lay to the east on mainland Mexico in the Sierra Madre Occidental, and then migrated to the

  20. Geochemical, isotopic and geochronological characterization of listvenite from the Upper Unit on Tinos, Cyclades, Greece (United States)

    Hinsken, Tim; Bröcker, Michael; Strauss, Harald; Bulle, Florian


    We describe a largely unknown listvenite deposit from Tinos, Cyclades, Greece and combine field observations with petrographic, bulk-rock geochemical, isotope (Sr, O, C), and Rb-Sr geochronological data. The volumetrically small listvenite occurrences are associated with metabasic phyllites, talc schists, meta-gabbros, ophicalcites and serpentinites of the Upper Unit. Geochemical characteristics (high Mg#, Cr, Ni), as well as preserved relic Cr-spinel and the typical mesh-texture of serpentinized Mg-silicates, document derivation from ultramafic precursors. Judging from field and textural observations it is very likely that carbonation affected serpentinite and not unaltered meta-peridotite. The direct contact or transition zones to ultramafic rocks are not preserved, but serpentinites that escaped carbonation are closely associated. The listvenites occur near a low-angle normal fault that probably focused fluid infiltration and distribution. The carbonation is associated with the influx of CO2-rich, K-bearing fluids that led to the formation of ferroan magnesite, quartz and Cr-bearing white mica (fuchsite), but otherwise the transformation of serpentinized peridotite into listvenite had been a largely isochemical process. The studied rocks do not contain elevated concentrations of precious metals (Au, Pt, Pd). Field relationships suggest that the listvenite-bearing occurrences most likely represent the same tectonostratigraphic level as Upper Unit rocks that had been thermally overprinted in the contact aureole of Miocene granitoids at ca. 15 Ma. Accordingly the intrusion depth provides a minimum pressure constraint for the somewhat older carbonation. Pressure estimates for thermally overprinted rocks and the granitoids suggest an intrusion depth of ca. 7-10 km that corresponds to a pressure of ca. 2-3 kbar. Chlorite thermometry applied to the Tinos listvenites mostly indicates temperatures of ca. 250 °C during carbonation. Internal Rb-Sr mineral isochrons

  1. In-Situ Geochronology: Extending Larims to Pb-Pb Isocrhons (United States)

    Whitaker, Tom; Anderson, Scott; Levine, Jonathan


    Introduction: We have previously described development of Laser Ablation Resonance Ionization Mass Spectrometry (LARIMS) for in-situ determination of the radiometric age of rocks using isotope ratios of Rb and Sr [1,2]. LARIMS uses laser resonance excitation of the target elements, which provides elemental selectivity, thus eliminating isobaric interferences with little or no sample preparation and allowing thousands of samples to be measured in significantly shorter periods of time than traditional methods. We have recently begun research that aims to extend the Rb-Sr capability to include Pb-Pb measurements. Preliminary measurements of Standard Reference Material 612 (SRM-612) from the National Institute of Standards and Technology (NIST) demonstrate that resonance ionization of Pb can measure samples with as little as 0.12 ppm total Pb. Background: In-situ LARIMS will enable measurements of 1) isotope geochemistry relevant for chronology and igneous evolution, 2) light isotopes relevant for habitability, life, and climate history, as well as 3) elemental abundances relevant to understanding local and regional geology. In particular, the elemental selectivity of LARIMS makes isotopic geochronology measurements possible that heretofore required extensive sample preparation and were thought to be practically impossible for in-situ measurements. For example, we have used Rb-Sr LARIMS to analyze a piece of the Martian meteorite Zagami and the Duluth Gabbro, a lunar analogue. In these measurements, we obtained isochron ages consistent with the published ages within 200 Ma. Pb-Pb geochronology is well-suited for LARIMS analysis. The use of a single element simplifies the laser system and eliminates inter-element fractionation that can be problematic in Rb-Sr analysis or other multi-element LARIMS measurements. In general, there is less interference at masses corresponding to Pb isotopes than at lighter masses. However, there are potential interferences such as Hg and

  2. High-precision U-Pb geochronologic constraints on the Late Cretaceous terrestrial cyclostratigraphy and geomagnetic polarity from the Songliao Basin, Northeast China (United States)

    Wang, Tiantian; Ramezani, Jahandar; Wang, Chengshan; Wu, Huaichun; He, Huaiyu; Bowring, Samuel A.


    The Cretaceous continental sedimentary records are essential to our understanding of how the terrestrial geologic and ecologic systems responded to past climate fluctuations under greenhouse conditions and our ability to forecast climate change in the future. The Songliao Basin of Northeast China preserves a near-complete, predominantly lacustrine, Cretaceous succession, with sedimentary cyclicity that has been tied to Milankocitch forcing of the climate. Over 900 meters of drill-core recovered from the Upper Cretaceous (Turonian to Campanian) of the Songliao Basin has provided a unique opportunity for detailed analyses of its depositional and paleoenvironmental records through integrated and high-resolution cyclostratigraphic, magnetostratigraphic and geochronologic investigations. Here we report high-precision U-Pb zircon dates (CA-ID-TIMS method) from four interbedded bentonites from the drill-core that offer substantial improvements in accuracy, and a ten-fold enhancement in precision, compared to the previous U-Pb SIMS geochronology, and allow a critical evaluation of the Songliao astrochronological time scale. The results indicate appreciable deviations of the astrochronologic model from the absolute radioisotope geochronology, which more likely reflect cyclostratigraphic tuning inaccuracies and omitted cycles due to depositional hiatuses, rather than suspected limitations of astronomical models applied to distant geologic time. Age interpolation based on our new high-resolution geochronologic framework and the calibrated cyclostratigraphy places the end of the Cretaceous Normal Superchon (C34n-C33r chron boundary) in the Songliao Basin at 83.07 ± 0.15 Ma. This date also serves as a new and improved estimate for the global Santonian-Campanian stage boundary.

  3. Large southward motion and clockwise rotation of Indochina throughout the Mesozoic: Paleomagnetic and detrital zircon U-Pb geochronological constraints (United States)

    Yan, Yonggang; Huang, Baochun; Zhao, Jie; Zhang, Donghai; Liu, Xiaohui; Charusiri, Punya; Veeravinantanakul, Apivut


    We report a combined paleomagnetic and U-Pb geochronologic study of sedimentary rocks from the Huai Hin Lat and Nam Phong formations of Mesozoic age in NE Thailand in order to provide independent constraints on the tectonic movement of the Indochina Block during convergence of the major blocks now comprising East Asia. The maximum allowable depositional age of the two formations is estimated to be 227 Ma and 215 Ma, respectively, from detrital zircon U-Pb geochronologic analysis which also indicates a sediment source transition in the Khorat Plateau Basin during the Middle-Late Jurassic. A formation mean paleomagnetic direction of Dg /Ig = 21.4 ° / 38.1 ° (kg = 19.5, α95 = 9.6 °) before and Ds /Is = 43.0 ° / 48.0 ° (ks = 47.4, α95 = 6.1 °, N = 13) after tilt correction is derived from samples with different lithologies, bedding attitudes, magnetic carriers and polarities and yields a positive fold test. Hence, the magnetization is likely primary. The revised Mesozoic APWP of the Indochina Block yields paleolatitudes (for a reference site of 22°N, 102°E) of 33.4 ± 7.2°N during the Norian Late Triassic, 25.9 ± 5.9°N during the Late Triassic to Early Jurassic, 23.9 ± 8°N during the Late Jurassic to Early Cretaceous, 27.5 ± 3.2°N during the Early Cretaceous and 24.5 ± 4.9°N by the Late Cretaceous; corresponding declinations are 45.2 ± 8.6°, 38.0 ± 6.6°, 36.3 ± 8.8°, 29.6 ± 3.6° and 24.9 ± 5.4° respectively. These data indicate a significantly southward displacement accompanied by clockwise rotation during the Mesozoic. A reconstruction of the Indochina Block within the now well-studied merging process of South China and North China indicates that the Indochina Block was located at a higher latitude than the South China Block during the Norian stage of Late Triassic times whilst no significant relative poleward displacement apparently occurred during the Early Jurassic to Early Cretaceous interval. Our study supports a post

  4. Petrology, geochronology, geochemistry and petrogenesis of Bajestan granitoids, North of Ferdows, Khorasan Razvi Province

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    Reyhaneh Ahmadirouhani


    Full Text Available Introduction The investigated area is situated in the south west of the Khorasan Razavi Province along the North West of the Lut Block. Different types of metal ore bodies along with non-metal deposits have already been documented in the Lut Block (Karimpour et al., 2008. Most of the study area is covered with granitoid rocks. Metamorphic rocks with unknown age are present in the north of the area. Skarns are observed in contact with fault zones and intrusive bodies. Eocene volcanic rocks with andesite and andesibasalt composition are located in the east and north east of the area (Ahmadirouhani et al., 2015. The study area that is a part of the Lut Block has a high potentials for Cu, Fe, Au, and Barite mineralization along the observed alteration zones. In the present study, the petrography, petrogenesis, Sr–Nd isotopes, and U–Pb zircon age of acidic granitoids in the east of Bajestan were investigated. Materials and methods In the current study, 400 rock samples were collected from the field and 170 thin sections were prepared for petrography studies. Thirty samples of volcanic rocks, intrusions, and dykes were analyzed using XRF at the Geological Survey of Iran. Twenty-five samples were selected for the elemental analysis using ICP-MS by the Acme Lab Company (Canada, 16 samples of them were related to acidic intrusive bodies and dykes. In addition, zircon crystals from four samples of the granitoids bodies were collected for U–Pb dating. Approximately 50 zircon grains (i.e. euhedral, clear, uncracked crystals with no visible heritage cores and no inclusions were hand-picked from each sample. Through cathodoluminescence imaging, the internal structure and the origin of zircon grains were examined at the Geological Survey of Vienna, Austria. Moreover, zircons were dated using the (LA-ICP-MS method at the Laboratory of Geochronology, the University of Vienna, Austria using the methodology outlined in Klötzli et al., (2009. Sr and Nd

  5. Geochemical and geochronological constraints on the origin and evolution of rocks in the active Woodlark Rift of Papua New Guinea (United States)

    Zirakparvar, Nasser Alexander

    Tectonically active regions provide important natural laboratories to glean information that is applicable to developing a better understanding of the geologic record. One such area of the World is Papua New Guinea, much of which is situated in an active and transient plate boundary zone. The focus of this PhD research is to develop a better understanding of rocks in the active Woodlark Rift, situated in Papua New Guinea's southernmost reaches. In this region, rifting and lithospheric rupture is occurring within a former subduction complex where there is a history of continental subduction and (U)HP metamorphism. The lithostratigraphic units exposed in the Woodlark Rift provide an opportunity to better understand the records of plate boundary processes at many scales from micron-sized domains within individual minerals to regional geological relationships. This thesis is composed of three chapters that are independent of one another but are all related to the overall goal of developing a better understanding of the record of plate boundary processes in the rocks currently exposed in the Woodlark Rift. The first chapter, published in its entirety in Earth and Planetary Science Letters (2011 v. 309, p. 56 - 66), is entitled 'Lu-Hf garnet geochronology applied to plate boundary zones: Insights from the (U)HP terrane exhumed within the Woodlark Rift'. This chapter focuses on the use of the Lu-Hf isotopic system to date garnets in the Woodlark Rift. Major findings of this study are that some of the rocks in the Woodlark Rift preserve a Lu-Hf garnet isotopic record of initial metamorphism and continental subduction occurring in the Late Mesozoic, whereas others only preserve a record of tectonic processes related to lithospheric rupture during the initiation of rifting in the Late Cenozoic. The second chapter is entitled 'Geochemical and geochronological constraints on the origin of rocks in the active Woodlark Rift of Papua New Guinea: Recognizing the dispersed

  6. Sm-Nd isotope geochemistry and U/Pb geochronological data of the Campina Grande complex, Paraiba State, NE Brazil

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    Almeida, C.N. [Pernambuco Univ., Recife, PE (Brazil). Pos-Graduacao em Geociencias; Guimaraes, I.P.; Silva Filho, A.F. da [Pernambuco Univ., Recife, PE (Brazil). Dept. de Geologia; Beurlen, H. [Pernambuco Univ., Recife, PE (Brazil). Dept. de Engenharia de Minas


    The Campina Grande Complex (CCG) constitutes a 250 Km{sup 2} intrusion within the high grade gneiss-migmatitic terrain, in the contact zone between the Mesoproterozoic Pajeu-Paraiba Fold Belt and the Archean Caldas Brandao Massif of the Borborema Province, NE Brazil. The contact of the complex is made by the Guarabira Shear Zone (west) and the Campina Grande Shear Zone in the south. The CCG is composed of coarse grained porphyritic quartz-monzonites, quartz-monzodiorites and medium grained porphyritic granodiorites. Mafic enclaves are common and show composition ranging from diorites to quartz diorites. Field and petrographic features point out to magma mixing and mingling processes involved in the CCG evolution. Basalts of probably Cretaceous age occur as dykes cutting in the CCG. This work presents and discusses the Sm-Nd isotope geochemistry of the CCG and a gabro which occur very close to the south contact of the CCG. This also presents the U/Pb in zircon geochronological data for the CCG. (author) 4 refs., 2 figs.

  7. Trace element signature and U-Pb geochronology of eclogite-facies zircon, Bergen Arcs, Caledonides of W Norway (United States)

    Bingen, Bernard; Austrheim, Håkon; Whitehouse, Martin J.; Davis, William J.

    Secondary-ion mass spectrometry (SIMS) U-Pb and trace element data are reported for zircon to address the controversial geochronology of eclogite-facies metamorphism in the Lindås nappe, Bergen Arcs, Caledonides of W Norway. Caledonian eclogite-facies overprint in the nappe was controlled by fracturing and introduction of fluid in the Proterozoic-Sveconorwegian-granulite-facies meta-anorthosite-norite protolith. Zircon grains in one massive eclogite display a core-rim structure. Sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith, i.e. 0.31<=Th/U<=0.89, heavy rare earth element (HREE) enrichment, and negative Eu anomaly. Weakly-zoned to euhedral oscillatory-zoned Caledonian rims are characterized by Th/U<=0.13, low LREE content (minimum normalized abundance for Pr or Nd), variable enrichment in HREE, and no Eu anomaly. A decrease of REE towards the outermost rim, especially HREE, is documented. This signature reflects co-precipitation of zircon with garnet and clinozoisite in a feldspar-absent assemblage, and consequently links zircon to the eclogite-facies overprint. The rims provide a mean 206Pb/238U crystallization age of 423+/-4 Ma. This age reflects eclogite-forming reactions and fluid-rock interaction. This age indicates that eclogite-facies overprint in the Lindås nappe took place at the onset of the Scandian (Silurian) collision between Laurentia and Baltica.

  8. Archaic Industries of Roussillon (South of France: Geochronological Correlation with the Terraces of the Têt, Tech and Agly Rivers

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    Full Text Available This article studies the lithic industry of the Early Palaeolithic from the terraces of the Roussillon, in the S of France. The geomorphological revision of the flood levels allows these industries to be related to the geochronology of the terraces which were formed in between the late Early Pleistocene and the Middle Pleistocene, and thus allows a regional chrono-cultural framework to be established. By analyzing the technology of the lithic industry derived from the concept of chaîne opératoire and studying the patinas, the technical Modes and the technological aspects of the processes related to configuration and knapping sequences are differentiated. In the fluvial sequence, Mode 1 is documented in the terrace of Mas Ferréol and Mode 2 is identified in the Plane d’en Bourgat and Butte du Four-Llabanère, thus showing a technological continuity in the form of choppers and chopping-tools and poorly organized chains of exploitation (polyhedral. This technological substratum is common to the coeval sites from the NE of the Iberian Peninsula and the S of France, although with a margin of variability between the industries from the different regions.

  9. Metamorphic evolution and U-Pb zircon SHRIMP geochronology of the Belizario ultramafic amphibolite, Encantadas Complex, southernmost Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Hartmann, Leo A.; Porcher, Carla C. [Rio Grande do Sul Univ., Porto Alegre (Brazil). Inst. de Geociencias]. E-mail:; Santos, Joao O.S. [Centro de Pesquisas de Recursos Minerais (CPRM), Porto Alegre, RS (Brazil). Brazilian Geological Survey; Leite, Jayme A.D. [Mato Grosso Univ., Cuiaba (Brazil). Dept. de Recursos Minerais; McNaughton, Neal J. [Western Australia Univ., Nedlands, WA (Australia). Centre for Global Metallogeny


    The integrated investigation of metamorphism and zircon U-Pb SHRIMP geochronology of the Belizario ultramafic amphibolite from southernmost Brazil leads to a better understanding of the processes involved in the generation of the Encantadas Complex. Magmatic evidence of the magnesian basalt or pyroxenite protolith is only preserved in cores of zircon crystals, which are dated at 2257 {+-} 12 Ma. Amphibolite facies metamorphism M{sub 1} formed voluminous hornblende in the investigated rock possibly at 1989 {+-} 21 Ma. This ultramafic rock was re-metamorphosed at 702+- 21 Ma during a greenschist facies event M{sub 2}; the assemblage actinolite + oligoclase + microcline + epidote + titanite + monazite formed by alteration of hornblende. The metamorphic events are probably related to the Encantadas Orogeny (2257 {+-} 12 Ma) and Camboriu Orogeny ({approx}1989 Ma) of the Trans-Amazonian Cycle, followed by an orogenic event (702 {+-} 21 Ma) of the Brasiliano Cycle. The intervening cratonic period (2000-700 Ma) corresponds to the existence of the Supercontinent Atlantica, known regionally as the Rio de la Plata Craton. (author)

  10. 40Ar/39Ar geochronology and geochemical reconnaissance of the Eocene Lowland Creek volcanic field, west-central Montana (United States)

    Dudas, F.O.; Ispolatov, V.O.; Harlan, S.S.; Snee, L.W.


    We report geochronological and geochemical data for the calc-alkalic Lowland Creek volcanic field (LCVF) in westcentral Montana. 40Ar/ 39Ar age determinations show that the LCVF was active from 52.9 to 48.6 Ma, with tuff-forming eruptions at 52.9 ?? 0.14 and 51.8 ?? 0.14 Ma. These dates span the age range of vigorous Eocene igneous activity in the Kamloops-Absaroka-Challis belt. The LCVF evolved upward from basal rhyolites (SiO 2>71 wt%) to dacites and andesites (SiO 2 > 62 wt%). Compositional change parallels a transition from early explosive volcanism to late effusive activity. Four geochemical components can be detected in the rocks. A component with 206Pb/204Pb nd epsilon;Nd near-15 is predominant in anhydrous, two-pyroxene dacites; hydrous rhyolites, rhyodacites, and dacites with epsilon;Nd below-10 are dominated by a second component; hydrous rocks with 206Pb/ 204Pb > 18.3 and epsilon;Nd>-9 contain a third component; and an andesite with low Nd content and epsilon;Nd near-9 probably contains a fourth component. The first three components probably derive from the lower and middle crust, whereas the fourth is probably from the lithospheric mantle. ?? 2010 by The University of Chicago.

  11. The Sm-Nd isotopic method in the geochronology laboratory of the University of Brasília

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    S.M.C.L. GIOIA


    Full Text Available Nd isotopes represent one of the best tools to investigate the processes involved in the evolution of the continental crust and mantle. This is due mainly to the similar geochemical behaviour of Sm and Nd, both light rare earth elements, which inhibits their fractionation during most varied geological processes. In order to carry out crustal evolution studies in central Brazil, the Sm-Nd isotopic method was implanted at the Geochronology Laboratory of the University of Brasília. The Sm-Nd separation methodology is basically that described in Richard et al. (1976, with the addition of some improvements. In this study we describe in detail the methodology used in Brasília. Precision and accuracy were checked with the international standards such as JB-3, BCR-1, BHVO-1 and La Jolla, and the following results were found: JB-3 (Nd=15.74 ppm and Sm=4.28 ppm, BCR-1 (143Nd/144Nd=0.512647±8, Nd=28.73 ppm and Sm=6.66 ppm, BHVO-1 (Nd=24.83 ppm and Sm=6.2 ppm and La Jolla (143Nd/144Nd=0.511835±14.

  12. Geology and geochronology of Cardoso Island, in the southeastern coast of Sao Paulo State; Geologia e geocronologia da Ilha de Cardoso, sudeste do Estado de Sao Paulo

    Energy Technology Data Exchange (ETDEWEB)

    Weber, Werner


    This aim of work is the geological and geochronological study of rocks cropping out on Cardoso Island, on the southeastern coast of Sao Paulo States, close to be boundary with Parana State. The Island with an area of about 151 km{sup 2} is a protected area administered by the Forest Institute of Environment Secretariat of the State of Sao Paulo. It is mountanious, with a peak at 814 m, and is covered by dense Mata Atlantica vegetation. The terrains which compose the island are mainly an igneous complex with light grey leucocratic, inequigranular, medium - to coarse-grained syenites. The predominant Tres Irmaos Syenite (STI), composed of pyroxene, hornblende, and perthitic to mesoperthitic microcline, has a magmatic flow structures, and is cut by the Cambriu alkali-feldspar Granites (GC), which is pinkish grey, leucocratic and medium-grained. Geochemical analysis of STI and GC demonstrate their meta luminous alkaline nature and late-orogenic to an orogenic character. The geochronological results suggest that the bodies were formed between 620 and 570 My according to the U-Pb method in zircons, with cooling between 597 and 531 My (K-Ar in amphiboles). Whole rock Sm-Nd analysis yield T{sub DM} ages in the Meso and Paleoproterozoic (1.200 - 2.200 My). belt of low grade meta sedimentary rocks occurs in the northern part of the island. Quartz schist, quartz-mica schist and mica-quartz schist, often containing andaluzite and cordierite, predominate. The geochemical and geochronological data suggest that the sources of the metasediments were andesites of continental arc whose protolities separated from the mantle during the Paleoproterozoic, between 1.800 and 2.200 My. These metasediments probably continue on the continent in the Taquari region and extend southwards in narrow strips between the granitoids of the Paranagua Domain. Although quaternary deposits are expressive, they were not studied in details since they were not the objectives of this study. (author)

  13. Nature of the Precambrian metamorphic blocks in the eastern segment of Central Tianshan:Constraint from geochronology and Nd isotopic geochemistry

    Institute of Scientific and Technical Information of China (English)

    LIU; Shuwen; GUO; Zhaojie; ZHANG; Zhicheng; LI; Qiugen; ZH


    Granitoid gneisses are widespread in Precambrian metamorphic blocks of eastern segment of the Central Tianshan Tectonic Zone, and they have intrusive contact relationships with their metamorphic sedimentary country rocks of Proterozoic Xingxingxia and Kawabulag groups. Zircon U-Pb ages from a granodioritic gneiss (IW11-1) and a parametamorphic schist (W05-9) are determined at the Weiya area. Euhedral prismatic zircons from the granodioritic gneiss (IW11-1) provide U-Pb isotopic discordia intercept ages of 1218±17 Ma and 426±26 Ma, respectively, and euhedral prismatic zircons from the parametamorphic schist (W05-9) display U-Pb isotopic discordia intercept ages of 1216±74 Ma and 290±15 Ma, respectively. A whole-rock Sm-Nd isotopic isochron is determined in augen granitoid gneiss samples at the Ganggou-Kumishi area and we obtain the isochron age of 1142±120 Ma, and its εNd(t) = -4.3. These geochronological data suggest that these Precambrian metamorphic basement blocks within eastern segment of the Central Tianshan Tectonic Zone can be produced during 1140-1220 Ma, and occur a nearly homochronous metamorphism. Integrated to these geochronological data, Nd depleted mantle model ages (TDM) and epsilon Nd(t) values of these granitoid gneiss samples indicate that they can derive from mixing in various scales both magmas from mantle and crust sources at a late Mesoproterozoic active continental margin tectonic environment. Similarity in geochronology, Sm-Nd isotopic geochemistry between Weiya-Xingxingxia, Pargangtag and Ganggou-Kumishi areas suggests that they could be a bigger uniform metamorphic basement block, which could be formed by the assembly of the supercontinent Rodinia and be separated by late geological processes.

  14. Reconstruction of multiple tectonic events in continental margins by integrated tectonostratigraphic and geochronological analysis: the Mesozoic to Paleogene Caribbean-South American interaction in northeastern Colombia (United States)

    Cardona, Agustin; Montes, Camilo; Bayona, German; Valencia, Victor; Ramirez, Diego; Zapata, Sebastian; Lara, Mario; Lopez-Martinez, Margarita; Thomson, Stuart; Weber, Marion


    Although the older record and successive tectonic scenarios experienced by a continental margin is commonly fragmentary, integrated field, petrological and geochronological analysis can reconstruct the long term tectonic evolution of continental margins and characterized major controls on the orogenic style. We present new geochronological constraints from igneous and low to very low grade metasedimentary rocks from the Caribbean continental margin of northeastern Colombia (Guajira region) in order to reconstruct the different tectonic events recorded by the margin before, during and following the arc-continent collision with the front of the Caribbean plate. Zircon U-Pb LA-ICP-MS geochronology results from leucogranites associated with garnet amphibolites, tonalites and volcanic rocks that made the continental basement of northeastern Colombia reveals and Early to Middle Mesozoic tectonic activity with peaks at ca. 220-230 Ma and 170-180 Ma. This magmatic record is related to a collisional belt link to the final agglutination of Pangea and was followed by an overimposed far field back-arc setting associated to the subduction of the Pacific (Farrallon) plate under the Pangea supercontinent. Muscovite and biotite Ar-Ar geochronology from basement rocks and low grade Mesozoic metasediments also reveals the existence of Middle Jurassic to Early Cretaceous thermal events link to the final opening of the proto-Caribbean ocean. The South American continental margin was subsequently affected by an arc-continent collisional event with the front of the Caribbean plate. This event is recorded by the growth of a Banda-type collisional melange that mixed South American continental margin sediments with mafic and ultramafic blocks of intra-oceanic arc origin, the formation of a coherent metasedimentary belt also made of South American margin sediments, and the mylonitization of the continental basement. Ar-Ar temporal constraints on the low grade metasedimentary rocks and

  15. Lu-Hf geochronology and trace element distribution in garnet: implications for uplift and exhumation of ultra-high pressure granulites in the Sudetes, SW Poland.


    Anczkiewicz, R.; Szczepanski, J.; Mazur, S.; Storey, C.; Crowley, Quentin; Villa, IM; Thirlwall, M; Jeffries, TE


    Combining Lu–Hf garnet geochronology with in situ trace element analyses in garnet allowed us to gain new insight into the metamorphic evolution of UHP–UHT rocks in the Stary Gierałtów region, in the Polish Sudetes. Prograde garnet growth recorded by Rayleigh-type heavy REE (HREE) zoning in the felsic granulites indicates that the obtained 386.6±4.9 Ma Lu–Hf age represents the time of garnet crystallization on a prograde UHP metamorphic path. The surrounding rocks were metamorphosed at the sa...

  16. A chronostratigraphic assessment of the Moenave Formation, USA using C-isotope chemostratigraphy and detrital zircon geochronology: Implications for the terrestrial end Triassic extinction (United States)

    Suarez, Celina A.; Knobbe, Todd K.; Crowley, James L.; Kirkland, James I.; Milner, Andrew R. C.


    The Late Triassic is a period of abrupt climate change associated with a disruption to the global carbon cycle usually ascribed to the emplacement of the Central Atlantic Magmatic Province (CAMP). Geochronologic, paleontologic, and geochemical studies have shown that the CAMP was likely the major factor for the end-Triassic extinction (ETE), however, difficulties correlating and dating terrestrial strata has left the nature of the terrestrial extinction in question. The lacustrine Whitmore Point Member (WPM) of the Moenave Formation is ideal for investigating these details because it is reported to be Late Triassic to Early Jurassic. However, currently there are conflicting age constraints between biostratigraphy and magnetostratigraphy. In this study we attempt to elucidate the ETE by incorporating C-isotope chemostratigraphy and detrital zircon geochronology. Detrital zircon geochronology suggests the upper part of the Dinosaur Canyon Member (DCM) is younger (201.33 ± 0.07/0.12/0.25 Ma) than the ETE (201.564 Ma) suggesting the ETE is in the middle to lower DCM, in agreement with track biostratigraphy (first occurrence of Eubrontes, Anomoepus, and Batrachopus). Meanwhile a distinct negative carbon isotope (NCIE) excursion (-5.5‰) occurs at the base of the WPM at Potter Canyon, AZ with a more subtle NCIE at the base of the WPM at Black Canyon, UT (-2.0‰) that may correlate to the initial NCIE at the ETE. However, the WPM NCIE is correlated to the preservation of organic C (relative %C) suggesting it may be either related to local lake productivity and biases in organic matter preservation or may be a negative CIE in the Jurassic Hettangian stage. With the addition of the detrital zircon data, we suggest the M2r reversal at the base of the WPM is a reversal in the Hettangian (the H24r, H25r, or H26r) and the ETE is within the DCM. Additional C-isotope analysis of the DCM is necessary to determine if the initial NCIE that is the hallmark of the ETE occurs in

  17. Geochronology and structuring of the Ceara State: Borborema Province northwestern part, NE Brazil; Geocronologia e estruturacao do estado do Ceara: NW da provincia Borborema, NE, Brasil

    Energy Technology Data Exchange (ETDEWEB)

    Fetter, A.; Van Schmus, W.R. [Kansas Univ., Lawrence, KS (United States). Dept. of Geology; Santos, Ticiano J. Saraiva dos [UNESP, Rio Claro, SP (Brazil). Inst. de Geociencias e Ciencias Exatas; Arthaud, M.; Nogueira Neto, J. [Ceara Univ., Fortaleza, CE (Brazil). Dept. de Geologia


    The work confirms that the geochronological new data U/Pb in zircon and Samarium/Neodymium from the Ceara State furnished a refined chronology of the geological activity in the NW part of the Borborema Province, indicating an evolutive history since 2,78 Ga and 532 Ma. Furthermore, these data facilitated the different crust domain outlines in the region, putting age maximum limits in the pre-brasilianas supracrusts rocks deposition, and evidencing the epoch and duration of the Brasiliano magmatism and metamorphism in the northwest part of the State

  18. New geochemical and geochronological data of early Cambrian of (SW Iberia): Calc-alkaline magmatism in the transition from active to passive continental margin in North Gondwana


    Sanchez-Garcia, Teresa; Pereira, M. Francisco; Bellido, Felix; Chichorro, Martim; Silva, J. Brandao; Valverde-Vaquero, Pablo; Pin, Christian; Solá, Rita


    The Ossa-Morena Zone (SW Iberia) displays a well-preserved record of the history of the northern Gondwana margin in Late Ediacaran-Early Cambrian times. This period of time is marked by the late-stage evolution of the Cadomian magmatic arc and related back-arc basins (c. 590-545 Ma), and the onset of rifting and widespread magmatism (c. 530-500 Ma) that led to the opening of the Rheic Ocean. Here we present new geochemical and geochronological data on some Cambrian granitoids of Ossa-Morena ...

  19. What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou Basin, Tibet)


    Huang, Wentao; Dupont-Nivet, Guillaume; Lippert, Peter C.; Van Hinsbergen, Douwe J.J.; Dekkers, Mark J.; Waldrip, Ross; Ganerød, Morgan; Li, Xiaochun; Guo, Zhaojie; Kapp, Paul


    International audience; The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Group in the Linzhou Basin. The lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes. We report consistent 40Ar/39Ar and U-P...

  20. Geochemistry and geochronology of HP mélanges from Tinos and Andros, cycladic blueschist belt, Greece (United States)

    Bulle, Florian; Bröcker, Michael; Gärtner, Claudia; Keasling, Alan


    U-Pb zircon geochronology, Sr-Nd isotope and bulk-rock geochemistry have been applied to meta-igneous and meta-sedimentary rocks from high-pressure metamorphic mélanges exposed on the Cycladic islands of Tinos, Syros and Andros. Ion microprobe (SHRIMP) U-Pb zircon dating of 7 samples representing meta-igneous blocks (Tinos), a blackwall zone (Tinos) and chlorite-talc schists from block-matrix contacts (Syros and Tinos) yielded Cretaceous ages of c. 80 Ma. Many of the criteria commonly used to distinguish between magmatic or metamorphic zircon genesis (internal structure, Th/U ratio, REE characteristics, Ti-in zircon thermometry, enclosed mineral phases) do not provide unambiguous constraints for the mode of formation. However, a magmatic origin for Cretaceous zircon of meta-gabbros and eclogites is considered likely. Supporting evidence for a previously suggested metamorphic origin for c. 80 Ma zircon in eclogite has not been found. Zircon of the same age occurring in chlorite-talc schists is presumably related to non-magmatic processes. Well-defined Cretaceous age groups clustering at c. 79 Ma also occur in the detrital zircon populations of 2 quartz mica schists representing the mélange matrix on Tinos, and suggest a much later time for sediment accumulation than previously assumed. The importance of c. 57 Ma zircon ages remains unclear, but may record either HP metamorphic processes or a post-57 Ma depositional age. The youngest age group in a third quartz mica schist from Tinos, collected outside the main mélange occurrences, clusters at c. 226-238 Ma. In all clastic metasediments from Tinos, most data points plot along the concordia between c. 300 and 900 Ma; single data points indicate concordant ages of c. 2.5 Ga, 2.3 Ga and 1 Ga, respectively. The youngest 206Pb/ 238U age group that has been recognized in a felsic paragneiss from Andros indicates an age of 163.1 ± 3.9 Ma, and mostly represents overgrowths around zircon with ages in the range from ˜ 272

  1. Structural and geochronological record of India-Asia suturing in the Lopu Kangri Range, south-central Tibet (United States)

    Laskowski, A. K.; Kapp, P. A.


    Better constraining the geological evolution of the India-Asia suture zone is vital to our comprehension of the Himalayan-Tibetan orogen. This study presents new mapping, igneous U-Pb geochronology, and detrital zircon geochronology from suture zone rocks exposed in the Lopu Kangri Range. Located ~50 km NW of Saga in south-central Tibet, this range exposes a >1.5 km thick panel of suture zone architecture that was exhumed by late Cenozoic orogen-parallel extension across two N-S, moderate-angle normal faults. The Lopu Kangri Range and its surroundings expose all of the major suture-zone assemblages. In the north, Cretaceous--Eocene Gangdese arc granitoids are unconformably overlain by nonmarine, conglomerate-bearing strata of the Eocene (?)--Miocene Kailas Fm. Metaconglomerate exposed in the same structural position within the Range contains garnet porphyroblasts. Gangdese arc and metaconglomerate rocks are confined to the footwall of the north-directed Great Counter Thrust system, which carried Cretaceous-Early Eocene Xigaze forearc, suture zone mélange, and Indian passive margin (Tethyan) strata northwards across three major NE-SW fault strands during the Miocene. Tethyan strata in the Range consist of locally garnet- and sillimanite-bearing calc-gneiss and schist that display N-S stretching lineations and top-N s-c fabrics. These rocks are exposed in the southern half of the range beneath suture zone mélange in the footwall of a moderately N-dipping, normal-sense shear zone. Six detrital zircon samples from the Xigaze forearc, metaconglomerate and meta-Tethyan strata were analyzed to confirm our lithologic correlations and provenance interpretations made in the field. 12 igneous U-Pb samples were analyzed to characterize granitoids that intrude the Range. They reveal a suite of 50-40 Ma granitoids that display a general younging trend from north to south across the suture. Five samples of 35-10 Ma leucogranites contain inherited zircon cores whose age spectra

  2. The Thermo Scientific HELIX-SFT noble gas mass spectrometer: (preliminary) performance for 40Ar/39Ar geochronology (United States)

    Barfod, D. N.; Mark, D. F.; Morgan, L. E.; Tomkinson, T.; Stuart, F.; Imlach, J.; Hamilton, D.


    The Thermo Scientific HELIX-platform Split Flight Tube (HELIX-SFT) noble gas mass spectrometer is specifically designed for simultaneous collection of helium isotopes. The high mass spur houses a switchable 1011 - 1012 Ω resistor Faraday cup and the low mass spur a digital pulse-counting secondary electron multiplier (SEM). We have acquired the HELIX-SFT with the specific intention to measure argon isotopes for 40Ar/39Ar geochronology. This contribution will discuss preliminary performance (resolution, reproducibility, precision etc.) with respect to measuring argon isotope ratios for 40Ar/39Ar dating of geological materials. We anticipate the greatest impact for 40Ar/39Ar dating will be increased accuracy and precision, especially as we approach the techniques younger limit. Working with Thermo Scientific we have subtly modified the source, alpha and collector slits of the HELIX-SFT mass spectrometer to improve its resolution for resolving isobaric interferences at masses 36 to 40. The enhanced performance will allow for accurate and precise measurement of argon isotopes. Preliminary investigations show that we can obtain a valley resolution of >700 and >1300 (compared to standard HELIX-SFT specifications of >400 and >700) for the high and low mass spurs, respectively. The improvement allows for full resolution of hydrocarbons (C3+) at masses 37 - 40 and almost full resolution at mass 36. The HELIX-SFT will collect data in dual collection mode with 40Ar+ ion beams measured using the switchable 1011 - 1012 Ω resistor Faraday cup and 39Ar through 36Ar measured using the SEM. The HELIX-SFT requires Faraday-SEM inter-calibration but negates the necessity to inter-calibrate multiple electron multipliers. We will further present preliminary data from the dating of mineral standards: Alder Creek sanidine, Fish Canyon sanidine and Mount Dromedary biotite (GA1550).

  3. Provenance and U-Pb geochronology of the Upper Cretaceous El Chanate Group, northwest Sonora, Mexico, and its tectonic significance (United States)

    Jacques-Ayala, C.; Barth, A.P.; Wooden, J.L.; Jacobson, C.E.


    The Upper Cretaceous El Chanate Group, northwest Sonora, Mexico, is a 2.8km thick clastic sedimentary sequence deposited in a continental basin closely related to volcanic activity. It consists of three formations: the Pozo Duro (oldest), the Anita, and the Escalante (youngest). Petrographic study, conglomerate pebble counts, and U-Pb geochronology of detrital zircons were performed to determine the source and age of this sequence, and to interpret its tectonic setting. In the sandstones of all three formations, the most abundant grains are those of volcanic composition (Q38F22L 40, Q35F19L46, and Q 31F22L47, respectively). The Pozo Duro Formation includes well-rounded quartz-arenite clast conglomerates, whereas conglomerates of the two upper units have clasts predominantly of andesitic and rhyolitic composition. The most likely source for these sediments was the Jurassic volcanic arc exposed in northern Sonora and southern Arizona. Zircons from five sandstone samples define two main age groups, Proterozoic and Mesozoic. The first ranges mostly from 1000 to 1800Ma, which suggests the influence of a cratonic source. This zircon suite is interpreted to be recycled and derived from the same source area as the quartz-rich sandstone clasts in the basal part of the section. Mesozoic zircons range from Triassic to Late Cretaceous, which confirms the proposed Late Cretaceous age for the sequence, and also corroborates Jurassic felsic source rocks. Another possible source was the Alisitos volcanic arc, exposed along the western margin of the Baja California Peninsula. Of regional significance is the great similarity between the El Chanate Group and the McCoy Mountains Formation of southeastern California and southwestern Arizona. Both are Cretaceous, were deposited in continental environments, and have similar zircon-age patterns. Also, both exhibit intense deformation and locally display penetrative foliation. These features strongly suggest that both units underwent

  4. Petrography, geochemistry and geochronology of granite hosted rhyodacites associated with a disseminated pyrite mineralization (Arnolz, Southern Bohemian Massif, Austria) (United States)

    Göd, Richard; Kurzweil, Johannes; Klötzli, Urs


    The study focuses on a subvolcanic rhyodacite dyke intruding a fine grained biotite granite and paragneisses of the South Bohemian Massif, part of the Variscan Orogenic Belt in Central Europe. The subvertical dyke strikes NNE, displays a thickness of about 30 m and has been traced by boulder mapping for approximately 7 km. The rhyodacites have been affected by two hydrothermal fluids. An older one of oxidizing condition giving rise to a reddish to brownish type of rock (Type I) and a younger fluid of reducing condition causing a greenish variety (Type II). The hydrothermal alteration is associated with the formation of the clay minerals chlorite, sericite, kaolinite and smectite and a disseminated pyrite mineralization. Bulk chemistries of the rhyodacites emphasize the hydrothermal alterations to be isochemical with the exception of sulphur enriched up to a maximum of 0.6 wt%. Trace element composition of the rhyodacites points to a barren geochemical environment in terms of base and precious elements. Sulphur isotope investigations of pyrites from the rhyodacites and the hosting granites respectively yield d34S data ranging from +0.07 to -2.22 ‰, emphasizing a magmatic origin of the sulphur. Geochronological investigations yield in situ U/Pb zircon ages of 312 ± 4 Ma for the biotite granite and of 292 ± 4 Ma for the rhyodacitic dykes indicating a time gap of ≈ 20 Ma between these two intrusive events. A contemporaneous but geochemically specialized granitic intrusion associated with NW striking "felsitic" dykes occurs about 10 to 20 km to the NW of Arnolz. However, the rhyodacites around Arnolz differ significantly from these felsitic dykes in their geochemistry and alteration phenomena which points to a different magmatic source. This coincides with a change in the orientation of the dykes from a NW direction controlling the geochemically specialized intrusions in the NW to a dominating NNE direction mirrored by the studied rhyodacites at Arnolz.

  5. Fluid inclusion characteristics and molybdenite Re-Os geochronology of the Qulong porphyry copper-molybdenum deposit, Tibet (United States)

    Li, Yang; Selby, David; Feely, Martin; Costanzo, Alessandra; Li, Xian-Hua


    The Qulong porphyry copper and molybdenum deposit is located at the southwest margin of the Lhasa Terrane and in the eastern region of the Gangdese magmatic belt. It represents China's largest porphyry copper system, with ˜2200 million tonnes of ore comprising 0.5 % Cu and 0.03 % Mo. The mineralization is associated with Miocene granodiorite, monzogranite and quartz-diorite units, which intruded into Jurassic volcanic units in a post-collisional (Indian-Asian) tectonic setting. Field observations and core logging demonstrate the alteration and mineralization at Qulong are akin to typical porphyry copper systems in subduction settings, which comprise similar magmatic-hydrothermal, potassic, propylitic and phyllic alteration assemblages. Molybdenite Re-Os geochronology confirms the relative timeframe defined by field observations and core logging and indicates that the bulk copper and molybdenum at Qulong were deposited within 350,000 years: between 16.10 ± 0.06 [0.08] (without and with decay constant uncertainty) and 15.88 ± 0.06 [0.08] Ma. This duration for mineralization is in direct contrast to a long-lived intrusive episode associated with mineralization based on previous zircon U-Pb data. Our fluid inclusion study indicates that the ore-forming fluid was oxidized and contained Na, K, Ca, Fe, Cu, Mo, Cl and S. The magmatic-hydrothermal transition occurred at ˜425 °C under lithostatic pressure, while potassic, propylitic and phyllic alteration occurred at hydrostatic pressure with temperature progressively decreasing from 425 to 280 °C. The fluid inclusion data presented here suggests that there has been ˜2.3 km of erosion at Qulong after its formation, and this erosion may be related to regional uplift of the Lhasa Terrane.

  6. The Jurassic of Denmark and Greenland: The Middle Jurassic of western and northern Europe: its subdivisions, geochronology and correlations

    Directory of Open Access Journals (Sweden)

    Callomon, John H.


    Full Text Available The palaeogeographic settings of Denmark and East Greenland during the Middle Jurassic are outlined. They lay in the widespread epicontinental seas that covered much of Europe in the post-Triassic transgression. It was a period of continuing eustatic sea-level rise, with only distant connections to world oceans: to the Pacific, via the narrow Viking Straits between Greenland and Norway and hence the arctic Boreal Sea to the north; and to the subtropical Tethys, via some 1200 km of shelf-seas to the south. The sedimentary history of the region was strongly influenced by two factors: tectonism and climate. Two modes of tectonic movement governed basinal evolution: crustal extension leading to subsidence through rifting, such as in the Viking and Central Grabens of the North Sea; and subcrustal thermal upwelling, leading to domal uplift and the partition of marine basins through emergent physical barriers, as exemplified by the Central North Sea Dome with its associated volcanics. The climatic gradient across the 30º of temperate latitude spanned by the European seas governed biotic diversity and biogeography, finding expression in rock-forming biogenic carbonates that dominate sediments in the south and give way to largely siliciclastic sediments in the north. Geochronology of unrivalled finesse is provided by standard chronostratigraphy based on the biostratigraphy of ammonites. The Middle Jurassic saw the onset of considerable bioprovincial endemisms in these guide-fossils, making it necessary to construct parallel standard zonations for Boreal, Subboreal or NW European and Submediterranean Provinces, of which the NW European zonation provides the primary international standard. The current versions of these zonations are presented and reviewed.

  7. Variscan Collisional Magmatism and Deformation In The Viseu Area (northern Central Portugal) - Constraints From U-pb Geochronology of Granitoids (United States)

    Azevedo, M. R.; Aguado, B. V.; Scaltegger, U.; Nolan, J.; Martins, M. R.; Medina, J.

    The Viseu area is located in the innermost zone of the Iberian Variscan Fold Belt (the Central Iberian Zone). It consists of abundant post-thickening, collision related grani- toids intruded into upper and middle crustal levels. The ascent of granite magmas took place after an extensional tectonic event (D2) and is coeval with D3 dextral and sinis- tral crustal-scale transcurrent shear zones. In the northern part of the area, the presence of a well preserved Upper Carboniferous tectonic basin filled with deformed conti- nental clastic sediments, bounded by contemporaneously exhumed deep crustal rocks and intruded by late-tectonic granites documents an episode of extension involving basin subsidence, uplift and erosion of the basement and granite magmatism in a post- thickening, but syn-convergent scenario. Convergence is manifested by strike-slip tec- tonics and basin inversion. According to structural criteria, the Variscan granitoids can be subdivided into two major groups: (1) syn-D3 granitoids including two dif- ferent petrological associations, highly peraluminous leucogranite and granodiorite- monzogranite intrusions and (2) late-D3 granitoids comprising slightly metaluminous to peraluminous granodiorites and monzogranites. Four plutons representing the syn- D3 leucogranites (Junqueira) and monzogranites (Maceira and Casal Vasco) and the late-D3 biotite granites (Cota) yielded U-Pb zircon + monazite or monazite ages of 310 Ma, 311 Ma, 311 Ma and 306 Ma, respectively. This points to a synchronous emplacement of the different syn-D3 plutons shortly followed by the intrusion of the late-D3 granites and suggests that the Upper Carboniferous plutonism occurred within a short time span of ca. 5 myr. Stratigraphic markers show that the oldest continental sediments in the Carboniferous basin are Westphalian whilst field relationships in- dicate that the deformation occurred prior to the intrusion of the late-D3 granitoids. Precise U-Pb geochronology proves that basin

  8. U-Pb zircon geochronology, Sr-Nd isotope geochemistry, and petrogenesis of oxidant granitoids at Keybarkuh, southwest of Khaf

    Directory of Open Access Journals (Sweden)

    Ehsan Salati


    Full Text Available Keybarkuh area is located 70 km southwest of Khaf, Khorasan Razavi province. The study area is situated in northeastern Lut block. The rock units in the area are Paleozoic metamorphic rocks and Cretaceous to Tertiary subvolcanic intrusions intruded as dike, stock and batholith; their composition varies from granite to diorite. Based on magnetic susceptibility, the intrusive rocks are divided into oxidant and reduced series. In this study, the oxidant intrusions are discussed. These intrusions are mostly high-K to shoshonitic and also meta-aluminous type. Their magma formed in subduction magmatic arc and they belong to I-type granitoid series. Enrichment of Large Ion Lithophile Elements (LILE such as Rb, Cs, K, Ba, and Th relative to High Field Stength Elements (HFSE such as Nb, Zr, and Ti supported the idea. Enrichment of Light Rare Earth Elements (LREE and depletion of Heavy Rare Earth Elements (HREE are also typical of subduction magmatism. Negative anomalies of Eu/Eu* can be attributed to the presence of residual plagioclase in a mantle source and contamination of magma by reduced continental crust. The amount of Nb > 11 ppm, lower ratio of Zr/Nb 0.706, initial 143Nd/144Nd (> 0.512 and εNd (< -3.5 indicate that magma contaminated by reduced continental crust. Hornblende biotite granodiorite porphyry dated using U-Pb zircon geochronology at 43.44 Ma (Middle Eocene. Based on calculated TDM, magma derived from ancient slab with 820 Ma age in the Keybarkuh area, was affected by the highest continental crust contamination during its ascent.

  9. Geochemistry, geochronology, and cathodoluminescence imagery of the Salihli and Turgutlu granites (central Menderes Massif, Western Turkey): Implications for Aegean tectonics (United States)

    Catlos, E. J.; Baker, C.; Sorensen, S. S.; Çemen, I.; Hançer, M.


    The Menderes Massif (western Turkey) is an important metamorphic core complex located in the Aegean region; geochemical and geochronological data from this extensional domain facilitates our understanding of large-scale extension of the Earth's lithosphere. S-type, peraluminous granites (Salihli and Turgutlu) that intrude the Alasehir detachment which bounds the northern edge of the central Menderes Massif may have been generated due to subduction of the Eastern Mediterranean floor along the Hellenic trench. In situ Th-Pb ion microprobe monazite ages from the granites range from 21.7±4.5 Ma to 9.6±1.6 Ma (±1s). The range is consistent with cathodoluminescence (CL) imagery that document complex textures within the samples. Salihli and Turgutlu granites share many similar characteristics, including multiple generations of plagioclase (some with shocked cores consistent with magma mixing), plagioclase replacing K-feldspar and the development of myrmekite, clear evidence for fluid infiltration, and multiple generations of microcracks and microfaults. The granites may have evolved from compositionally distinct magma sources, as Salihli samples in general contain allanite as the major accessory mineral, whereas Turgutlu granites contain monazite. However, the CL imagery document similar alteration textures. Ages reported here are similar to dates constraining extension reported elsewhere in the Aegean, but indicate a level of complexity when linking movement within the Menderes Massif to the large-scale geodynamic processes that created other metamorphic core complexes in the region. Difficulties exist in linking the ages obtained from the granites to specific tectonic events due to the presence of secondary alteration textures, generations of mineral growth, and multiple episodes of deformation.

  10. Geology and geochronology of the Tana Basin, Ethiopia: LIP volcanism, super eruptions and Eocene-Oligocene environmental change (United States)

    Prave, A. R.; Bates, C. R.; Donaldson, C. H.; Toland, H.; Condon, D. J.; Mark, D.; Raub, T. D.


    New geological and geochronological data define four episodes of volcanism for the Lake Tana region in the northern Ethiopian portion of the Afro-Arabian Large Igneous Province (LIP): pre-31 Ma flood basalt that yielded a single 40Ar/39Ar age of 34.05 ± 0.54 / 0.56 Ma; thick and extensive felsic ignimbrites and rhyolites (minimum volume of 2- 3 ×103 km3) erupted between 31.108 ± 0.020 / 0.041 Ma and 30.844 ± 0.027 / 0.046 Ma (U-Pb CA-ID-TIMS zircon ages); mafic volcanism bracketed by 40Ar/39Ar ages of 28.90 ± 0.12 / 0.14 Ma and 23.75 ± 0.02 / 0.04 Ma; and localised scoraceous basalt with an 40Ar/39Ar age of 0.033 ± 0.005 / 0.005 Ma. The felsic volcanism was the product of super eruptions that created a 60-80 km diameter caldera marked by km-scale caldera-collapse fault blocks and a steep-sided basin filled with a minimum of 180 m of sediment and the present-day Lake Tana. These new data enable mapping, with a finer resolution than previously possible, Afro-Arabian LIP volcanism onto the timeline of the Eocene-Oligocene transition and show that neither the mafic nor silicic volcanism coincides directly with perturbations in the geochemical records that span that transition. Our results reinforce the view that it is not the development of a LIP alone but its rate of effusion that contributes to inducing global-scale environmental change.

  11. Petrography, geochemistry and geochronology of granite hosted rhyodacites associated with a disseminated pyrite mineralization (Arnolz, Southern Bohemian Massif, Austria) (United States)

    Göd, Richard; Kurzweil, Johannes; Klötzli, Urs


    The study focuses on a subvolcanic rhyodacite dyke intruding a fine grained biotite granite and paragneisses of the South Bohemian Massif, part of the Variscan Orogenic Belt in Central Europe. The subvertical dyke strikes NNE, displays a thickness of about 30 m and has been traced by boulder mapping for approximately 7 km. The rhyodacites have been affected by two hydrothermal fluids. An older one of oxidizing condition giving rise to a reddish to brownish type of rock (Type I) and a younger fluid of reducing condition causing a greenish variety (Type II). The hydrothermal alteration is associated with the formation of the clay minerals chlorite, sericite, kaolinite and smectite and a disseminated pyrite mineralization. Bulk chemistries of the rhyodacites emphasize the hydrothermal alterations to be isochemical with the exception of sulphur enriched up to a maximum of 0.6 wt%. Trace element composition of the rhyodacites points to a barren geochemical environment in terms of base and precious elements. Sulphur isotope investigations of pyrites from the rhyodacites and the hosting granites respectively yield d34S data ranging from +0.07 to -2.22 ‰, emphasizing a magmatic origin of the sulphur. Geochronological investigations yield in situ U/Pb zircon ages of 312 ± 4 Ma for the biotite granite and of 292 ± 4 Ma for the rhyodacitic dykes indicating a time gap of ≈ 20 Ma between these two intrusive events. A contemporaneous but geochemically specialized granitic intrusion associated with NW striking "felsitic" dykes occurs about 10 to 20 km to the NW of Arnolz. However, the rhyodacites around Arnolz differ significantly from these felsitic dykes in their geochemistry and alteration phenomena which points to a different magmatic source. This coincides with a change in the orientation of the dykes from a NW direction controlling the geochemically specialized intrusions in the NW to a dominating NNE direction mirrored by the studied rhyodacites at Arnolz.

  12. Tectonic evolution of the Dom Feliciano Belt in Southern Brazil: Geological relationships and U-Pb geochronology

    Directory of Open Access Journals (Sweden)

    Ruy Paulo Philipp

    Full Text Available ABSTRACT: The Dom Feliciano Belt is an important Neoproterozoic to Cambrian orogenic complex, extending from eastern Uruguay to southern Brazil. It comprises a collage of oceanic domains and continental fragments developed between 900 and 540 Ma between the Rio de La Plata, Congo and Kalahari cratons. The integration of field and structural data with recent isotopic results has introduced new insights on the sources of the magmatism and sedimentary processes. This paper presents a review of the geochronological results combined with stratigraphic, structural and geochemical data. The evolution of the Dom Feliciano Belt involved three orogenic events known as the Passinho (0.89 - 0.86 Ga, São Gabriel (0.77 - 0.68 Ga and Dom Feliciano (0.65 - 0.54 Ga. The first two events involved the closure of the Charrua Ocean generating an intra-oceanic arc (Passinho and, subsequently, an active continental margin arc (São Gabriel. This ocean separated the continental areas represented by the Rio de la Plata Craton and the Nico Perez continental microplate. Closure of the Adamastor ocean resulted in an important collisional event between the Nico Perez Microplate/Rio de La Plata Craton and Kalahari and Congo cratons between 650 and 620 Ma, involving high T/intermediate P metamorphism. At this time of crustal thickening, the partition of the deformation controled the final evolution of the belt with important escape tectonics, responsible for nucleating crustal-scale transcurrent shear zones. These structures were deep and promoted the rise of mafic magmas, which, associated with high regional thermal gradient, lead to an important event of crustal reworking, responsible for the formation of the Pelotas Batholith. The orogenic collapse is represented by late magmatism of Pelotas Batholith and deposition of upper section of the Camaquã Basin.

  13. Sandstone provenance and detrital zircon geochronology record changes in Plio-Pleistocene tectonism of the Southern Cascadia forearc (United States)

    Garcia, M. J.; Michalak, M.; Hourigan, J. K.


    The Cascadia Subduction Zone (CSZ) in the Pacific NW is defined by subducting oceanic lithosphere (Juan de Fuca-Gorda) beneath the North American Plate, from British Colombia to Northern California. In the Southern CSZ, the forearc is characterized by high topography of the Klamath Mountain Province west of the volcanic arc, the lower lying Northern Coast Ranges, comprising Cretaceous through Miocene Franciscan Formation, and structurally controlled coastal basins of Pliocene-Pleistocene fluvial-marine sedimentary rocks. The southernmost CSZ is affected by northward migration of the Mendocino Triple Junction; high rates of rock uplift and tectonically controlled drainage patterns are evident of its passage spatially and temporally. While provenance sources for early-to-mid Tertiary sedimentary units of the Franciscan are well-described, less is known about provenance sources in Plio-Pleistocene time, and how changes in regional tectonics, such as the migration of the MTJ, and uplift of the Klamath Mountains, may have affected sediment dispersal. Previous work and our preliminary results indicate that the Plio-Pleistocene Wildcat Group of the Eel River basin is characterized by distant sources. Aalto et al., (1998) and Moley (1997) use detrital mica Ar/Ar ages and petrofacies analyses of the Wildcat to hypothesize provenance from the distant Idaho Batholith. We test this hypothesis using detrital zircon U-Pb geochronology, and present eight new detrital sample ages from Plio-Pleistocene sedimentary basins along the coastal margin north of the MTJ. These results are used to inform paleographic reconstructions of Pliocene-Pleistocene regional topography associated with the dynamic tectonics of Southern Cascadia.

  14. Geochronological and geochemical assessment of Cenozoic volcanism from the Terror Rift region of the West Antarctic Rift System (United States)

    Rilling, Sarah E.

    The work presented in this dissertation explains results from three different methods to determine the relation between tectonism and rift-related volcanism in the Terror Rift region of the West Antarctic Rift System (WARS). Alkaline lavas from seven submarine features, Beaufort Island and Franklin Islands, and several locations near Mt Melbourne were dated by 40Ar/39Ar geochronology and analyzed for elemental and isotopic chemical signatures. Each chapter addresses a different aspect of the hypothesis that the presence of volatiles, primarily H2O or CO2, in the magma source has led to anomalously high volumes of magmatism after rift-related decompressional melting rather than requiring an active mantle plume source. Chapter 2 provides the temporal framework, illustrating that the sampled features range in age from 6.7 Ma to 89 ka, post-dating the main Miocene age phase of Terror Rift extension. Chapter 3 illustrates the traditional enriched elemental and isotopic chemical signatures to support the overall homogeneity of these lavas and previously analyzed areas of the WARS. This chapter also provides a new model for the generation of the Pb isotopic signatures consistent with a history of metasomatism in the magma source. Chapter 4 provides an entirely new chemical dataset for the WARS. The first platinum group element (PGE) abundances and extremely unradiogenic Os isotopic signatures of Cenozoic lavas from Antarctica provide the strongest evidence of melting contributions from a lithospheric mantle source. The combined results from these three studies consistently support the original hypothesis of this dissertation. New evidence suggests that WARS related lavas are not related to a mantle plume(s) as previously proposed. Instead, they are generated by passive, decompressional melting of a source, likely a combination of the asthenospheric and lithospheric mantle, which has undergone previous melting events and metasomatism.

  15. Determining age of Pan African metamorphism using Sm-Nd garnet-whole rock geochronology and phase equilibria modeling in the Tasriwine ophiolite, Sirwa, Anti-Atlas Morocco (United States)

    Inglis, Jeremy D.; Hefferan, Kevin; Samson, Scott D.; Admou, Hassan; Saquaque, Ali


    Sm-Nd garnet-whole rock geochronology and phase equilibria modeling have been used to determine the age and conditions of regional metamorphism within the Tasriwine ophiolite complex, Sirwa, Morocco. Pressure and temperature estimates obtained using a NaCaKFMASHT phase diagram (pseudosection) and garnet core and rim compositions predict that garnet growth began at ∼0.72 GPa and ∼615 °C and ended at ∼0.8 GPa and ∼640 °C. A bulk garnet Sm-Nd age of 647.2 ± 1.7 Ma, calculated from a four point isochron that combines whole rock, garnet full dissolution and two successively more aggressive partial dissolutions, provides a precise date for garnet formation and regional metamorphism. The age is over 15 million years younger than a previous age estimate of regional metamorphism of 663 ± 13 Ma based upon a SHRIMP U-Pb date from rims on zircon from the Iriri migmatite. The new data provide further constraints on the age and nature of regional metamorphism in the Anti-Atlas mountains and emphasizes that garnet growth during regional metamorphism may not necessarily coincide with magmatism/anatexis which predominate the signature witnessed by previous U-Pb studies. The ability to couple PT estimates for garnet formation with high precision Sm-Nd geochronology highlights the utility of garnet studies for uncovering the detailed metamorphic history of the Anti-Atlas mountain belt.

  16. Geochronology of recent lake sediments from Longgan Lake,middle reach of the Yangtze River,influenced by disturbance of human activities

    Institute of Scientific and Technical Information of China (English)


    The impacts of human activities in Longgan Lake region on the distribution of 210Pb in lake sediment made it difficult to establish geochronological sequence. However, environmental proxies recorded these human activities and could be used as time marker to check 210Pb dating results. Based on the analysis of 210Pbexc distribution in cores LS-1 and LGL-1, and in combination with the human activities records in grain-size and element concentration, the sedimentation rates in the upper layers of these two cores were determined to be 0.19 and 0.23 cm/a respectively. The MAR (mass accumulation rates) of these two cores demonstrated that natural condition, such as precipitation and runoffs, led to the variation of MAR before human disturbance, and human activities controlled the sedimentary processes afterwards, which obviously reduced the particles influx into the lakes. During 1940s when human activity had not changed the water system within the catchment, 210Pb activity was abnormally strong in lakes along middle to lower reaches of the Yangtze River, which might be attributed to less precipitation and can be used as a time marker for geochronological research.

  17. Amplifying Earth history: Zircon U-Pb geochronology by ID-TIMS at the 0.1 ‰ level using new 1013 ohm resistors (United States)

    Wotzlaw, Jörn-Frederik; von Quadt, Albrecht


    Precise and accurate radiometric dating of volcanic ash beds in sedimentary successions is the backbone of the numerical calibration of Earth history. Uranium-lead geochronology by isotope dilution thermal ionization mass spectrometry (ID-TIMS) is the most precise and accurate dating technique and is applicable to most of Earth history from the Hadean to the Pleistocene. The accessory mineral zircon is the prime target material due to its commonly high U concentration, virtually no initial Pb and high daughter-product retentivity. However, complex crystallization histories as well as magmatic and sedimentary recycling of zircons require the analysis of single crystals resulting in small amounts of radiogenic Pb (Pb* usually dates with uncertainties <0.2 ‰ for high-Pb* zircons and weighted mean ages for populations of closed system zircons with uncertainties <0.1 ‰ . This level of temporal resolution will allow to better quantify the timing and durations of critical intervals in Earth history, evaluate causalities between different events such as flood basalt eruptions and mass extinctions, quantify rates of changes in biodiversity and assess the origin of cyclic patterns in the sedimentary records. [1] von Quadt, A., Wotzlaw, J.F., Buret, Y., Large, S., Peytcheva, I., Trinquier, A., 2016, High-precision zircon U/Pb geochronology by ID-TIMS using new 1013 ohm resistors. J. Anal. At. Spectrom., DOI: 10.1039/C5JA00457H

  18. The Re-Os Isotopic System: Geochemistry and Methodology at the Geochronological Research Center (CPGeo of the University of São Paulo, Brazil

    Directory of Open Access Journals (Sweden)

    Ciro Teixeira Correia


    Full Text Available The Re-Os isotopic system is an important tool for the study of mantle-crust processes, geochronology and the tracingof source reservoirs for metal deposition. Rhenium and osmium differ fundamentally from other lithophile isotopic systemswith regards to their behavior during partial melting processes, coupled with the chalcophile/siderophile nature of bothelements. These differences make the system extremely useful for a number of novel applications not traditionally addressedby lithophile isotopic systems. A low-blank technique for the analysis of Re-Os isotopes in geological materials has beenestablished at the Geochronological Research Center (CPGeo of the Geosciences Institute of the University of São Paulo,Brazil, with the aim of furthering knowledge of regional geology, tectonic evolution, petrology and ore deposition in SouthAmerica. The techniques described here use isotope dilution to simultaneously determine the concentration of Os and Re aswell the Os isotopic composition of geologic materials. Sample digestion and sample-isotopic spike equilibration are achievedin sealed borosilicate glass tubes at high temperature. Osmium is separated and purified by carbon tetrachloride solventextraction and micro-distillation techniques. Rhenium is separated and purified by anion exchange chromatography. Accuracyof the concentration and isotopic determinations is monitored by the analysis of a certified reference material (WPR-1 andthe use of the Department of Terrestrial Magnetism (DTM Os isotopic standard. Measured values and precision of thesestandards is within error and comparable to established Re-Os laboratories.

  19. Geochemistry and zircon U-Pb geochronology of the rhyolitic tuff on Port Island, Hong Kong: Implications for early Cretaceous tectonic setting

    Directory of Open Access Journals (Sweden)

    Longlong Zhao


    Full Text Available Early Cretaceous rhyolitic tuffs, widely distributed on Port Island, provide insights into the volcanism and tectonic setting of Hong Kong. In this paper we present petrological, geochronological and geochemical data of the rhyolitic tuff to constrain the diagenesis age and petrogenesis of the rocks, tectonic setting and early Cretaceous volcanism of Hong Kong. The first geochronological data show that the zircons in the volcanic rocks have U-Pb age of 141.1–139.5 Ma, which reveals that the rhyolitic tuff on Port Island was formed in the early Cretaceous (K1. Geochemically, these acid rocks, which are enriched in large ion lithophile elements (LILEs and light rare earth elements (LREEs, and depleted in high field strength elements (HFSEs, belong to the high K calc-alkaline to shoshonite series with strongly-peraluminous characteristic. The geochemical analyses suggest that the volcanic rocks were derived from deep melting in the continental crust caused by basaltic magma underplating. Based on the geochemical analysis and previous studies, we concluded that the rhyolitic tuffs on Port Island were formed in a back-arc extension setting in response to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.

  20. Zircon U–Pb geochronology and geochemistry of rhyolitic tuff, granite porphyry and syenogranite in the Lengshuikeng ore district, SE China: Implications for a continental arc to intra-arc rift setting

    NARCIS (Netherlands)

    Wang, Changming; Zhang, Da; Wu, Gangguo; Xu, Yigan; Carranza, Emmanuel John M.; Zhang, Yaoyao; Li, Haikun; Geng, Jianzhen


    SE China is well known for its Mesozoic large-scale granitoid plutons and associated ore deposits. Here, zircon U–Pb geochronological and geochemical data have been used to better constrain the petrogenesis of the igneous rocks associated with porphyry Ag–Pb–Zn deposits in the Lengshuikeng ore distr

  1. Zircon U–Pb geochronology and geochemistry of rhyolitic tuff, granite porphyry and syenogranite in the Lengshuikeng ore district, SE China: Implications for a continental arc to intra-arc rift setting

    NARCIS (Netherlands)

    Wang, Changming; Zhang, Da; Wu, Gangguo; Xu, Yigan; Carranza, E.J.M; Zhang, Yaoyao; Li, Haikun; Geng, Jianzhen


    SE China is well known for its Mesozoic large-scale granitoid plutons and associated ore deposits. Here, zircon U–Pb geochronological and geochemical data have been used to better constrain the petrogenesis of the igneous rocks associated with porphyry Ag–Pb–Zn deposits in the Lengshuikeng ore

  2. Geochronology Intermediary Laboratory implantation at the Rio Grande do Norte Federal University: the dating of the Serrinha Granitoid (RN) and the correlate Brasiliana extensional deformation; Implantacao do Laboratorio Intermediario de Geocronologia na UFRN: a datacao do granitoide de Serrinha (RN) e da deformacao extensional brasiliana correlata

    Energy Technology Data Exchange (ETDEWEB)

    Macedo, Maria Helena F.; Sa, Emanuel F. Jardim de; Souza, Zorano S. [Pernambuco Univ., Recife, PE (Brazil). Nucleo de Pesquisa em Geodinamica e Geofisica; Mendes, Franklin S. [Pernambuco Univ., Recife, PE (Brazil). Curso de Quimica; Ramalho, Karlos A.C. [Pernambuco Univ., Recife, PE (Brazil). Curso de Geologia


    The article describes the activities developed by the Geochronology Intermediary Laboratory at the Federal University of the Rio Grande do Norte, a Brazilian university, where there were the preoccupation of establishing strategies for a geochronological development. It relates the Rubidium-Strontium (Rb/Sr) and Samarium-Neodymium (Sm/Nd) methods, describing the analysis realized in these methodologies. Afterward, it presents the geological and petrographic situation of the Granitoide de Serrinha, located at Rio Grande do Norte State, Brazil and its geochronological data 8 refs., 2 figs.

  3. Chemical variations in the Cerro de Mercado (Durango, Mexico) fluorapatite: Assessing the effect of heterogeneity on a geochronologic standard. (United States)

    Boyce, J. W.; Hodges, K. V.


    Despite the lack of an official pronouncement, the fluorapatite of Cerro de Mercado, Durango, Mexico has become the de facto standard for (U-Th)/He geochronology. In addition to being relatively inclusion-free and easily obtained, these crystals are commonly in excess of 5mm in diameter, permitting the removal of the outer skin of the crystal, theoretically eliminating the alpha-ejection correction. However, bulk analyses of the Durango fluorapatite indicate a substantial variation in U and Th concentrations from aliquot to aliquot (167-238 ppm Th; 9.7-12.3 ppm U, [1]). If similar variations were to occur on the sub-grain scale, small fragments of single crystals could contain helium excesses or deficiencies due to alpha-ejection exchange between zones with varying parent element content. We have performed a series of experiments to quantify the intra-grain variation in U and Th, in order to model the effect of this variation on ages determined on Durango fluorapatite. X-ray maps show concentric zonation in U and Th, with similar, but more apparently pronounced zonation in Si and Cl. Preliminary laser-ablation ICPMS data indicate, not surprisingly, that intra-grain variations in U and Th concentrations obtained by analysis of ~35 μ m spots are larger than that which had been previously obtained by bulk analytical techniques (with overall concentrations greater than for bulk analyses). Thus far, analyses yield U concentrations varying from 11 to 16 ppm, and Th concentrations ranging from 220 to 340 ppm. Modeling underway suggests that parent element variations on the order of 50%, such as those observed, and the resulting differential alpha-exchange could lead to several percent error in age, for ~100 μ m fragments. The effect scales inversely with fragment size, with 300 μ m fragments (roughly the size of a large, single grain analysis) having only ~1% error. This may offer an explanation for the previously observed inability to reproduce ages for the Durango

  4. Thermal gradient and geochronology of a Paleozoic high-grade terrane in the northeastern Cathaysia block, South China (United States)

    Zhao, Lei; Zhai, Mingguo; Zhou, Xiwen; Santosh, M.; Ma, Xudong


    The Chencai terrane in the northeastern Cathaysia Block forms part of the northeast extension of the Paleozoic orogeny in South China. Recent field investigations have revealed the occurrence of high grade rocks in this terrane including granulite facies metapelites (khondalites) and orthopyroxene-bearing granulites. However, the thermal gradients and metamorphic history of this region have not been well constrained, thus impeding a better understanding of the geodynamic history of the Paleozoic orogeny in South China. Here we report results from a systematic study on the metamorphic PT conditions and geochronology of the orthopyroxene granulites and metasedimentary rocks (metapelites) from the Chencai area. The orthopyroxene granulites preserve granulite facies peak assemblage of Opx + Grt + Pl + Kfs + Qtz and a retrograde P-T trajectory. Peak PT conditions are 845-860 °C and 0.83-0.86 Gpa, corresponding to a thermal gradient of 30 °C/km which is much hotter than the normal thermal gradient of continental crust. Mineral compositions of the metasedimentary rocks re-equilibrated during retrograde metamorphism so that the peak PT conditions recorded by these rocks are much lower than those obtained from the orthopyroxene granulite. Peak PT conditions are 710-720 °C, 0.57-0.58 Gpa for garnet-biotite gneiss and 760-780 °C, 0.60-0.62 Gpa for biotite garnetite. Zircon U-Pb dating shows that this episode of granulite facies metamorphism occurred at 430-450 Ma and the detrital population shows minor or no contribution from the basement rocks of the Cathaysia Block. The Neoproterozoic rocks occurring along the Jiangnan belt are considered to have provided the sedimentary source as indicated by the two main age peaks of 0.79 Ga and 0.83 Ga from the detrital zircon cores. Lu-Hf isotope data on the zircons indicates extensive alteration and little new zircon growth during the Paleozoic metamorphism. The εHf(t) values and model ages (TDM1) of zircons suggest that the

  5. Geochronological Significance of the Post-Orogenic Mafic-Ultramafic Rocks in the Hongqiling Area of Jilin Province, Northeast China

    Institute of Scientific and Technical Information of China (English)

    Zhang Guangliang; Wu Fuyuan


    Mafic-ultramafic complexes are widespread in Hongqiling in central Jinlin Province, NE China. The Hongqiling complex comprises pyroxenite, olivine websterite, lherzolite, gabbro and leucogabbro. Based on the latest geochronological results, these intrusions yield a zircon U-Pb age of about 216 Ma, implying that they emplaced in the late Triassic period and that the mafic-ultramafic complexes are post-orogenic in origin. These ages are coeval with the emplacement of A-type granites in the area, but slightly younger than syn-orogenic granitic magmatism. The composition of the parent magma during the equilibration of the accumulated olivine crystallizing is basaltic with high MgO, and it comes from depleted or juvenile lithospheric mantle, according to the results of trace elements and Sr-Nd isotopes. Amount of crustal material contaminated the magmatic source, implying that the mafic-ultramafic rocks originate from the mixing product of crust- and mantle-derived magma. Therefore, the magmatic source was contaminated by an amount of crustal material, and the subsequent crystal fractionation resulted in the Cu-Ni mineralization. Dynamic analyses indicate that, after the collision of the North China Craton and Jiamusi Massif, the lithospheric delamination during post-orogeny resulted in the upwelling of asthenosphere and the intruding of the mass and underplating of the mantle-derived magma, which led to the formation of the primary mafic-ultramafic magma due to the mantle-derived magma and partial melting of the lower crust. This result suggests that the mafic-ultramafic complexes belong to a typically important magmatic suite that evolved during post-orogenic processes, and they were exposed as maficultramafic dyke swarms that existed in the post-orogenic extension environment. It also implies that the orogenic process finished ultimately in this regional geological setting. The widespread occurrence of mafic-ultramafic complexes in the Xing'an-Mongolia Orogenic Belt

  6. Pre-Devonian tectonic evolution of the eastern South China Block:Geochronological evidence from detrital zircons

    Institute of Scientific and Technical Information of China (English)


    Using the U-Pb LA-ICP-MS analysis technique we analyzed geochronological features of detrital zircons from Devonian and Ordovician coarse sandstone in southern Jiangxi Province,northern Cathaysia Block.Abundant ancient crustal information was obtained.The 350 groups of U-Pb age center on five ranges:2600-2300 Ma(peak at 2470 Ma),1100-900 Ma(peak at 980 Ma),900-700 Ma(peak at 800 Ma),650-520 Ma(peak at 600 Ma) and 450-400 Ma(peak at 440 Ma).We also found a detrital zircon of ~3.5 Ga.This is the oldest age in northern Cathaysia Block obtained so far.From the analysis we concluded that:(1) the 2600-2300 Ma period,characterized by a global continent-building,records late Neoarchean magmatism that did not occur in the neighboring area of Cathaysia;(2) the marked peak at 1100-900 Ma corresponds with the assembly time of the Neoproterozoic supercontinent,Rodinia,suggesting that the Cathaysia Block was once a part of Rodinia,and numerous euhedral zircons with similar ages likely resulted from the Grenville event;(3) the peak at 900-700 Ma corresponds to the breakup of Rodinia,as evidenced by wide occurrence of Neoproterozoic granite,basic dyke swarms and continental rift-type deposition;(4) the 650-520 Ma period is the typical time of the Pan-African event,but as yet no associated magmatic rock has been reported in this area;and(5) the peak at 450-400 Ma,representing the early Paleozoic orogeny,was recorded in various igneous rocks.Abundant Silurian-Lower Devonian granitic plutons,orthogneisses and their zircon U-Pb dating ages(450-400 Ma) are important evidence of an early Paleozoic orogenic event.Geological data support the interpretation of an Early Paleozoic tectonic heat event in Cathaysia,which was likely to be caused by intracontinental collision.

  7. Hardware and Software Interfacing at New Mexico Geochronology Research Laboratory: Distributed Control Using Pychron and RemoteControlServer.cs (United States)

    McIntosh, W. C.; Ross, J. I.


    We developed a system for interfacing existing hardware and software to two new Thermo Scientific Argus VI mass spectrometers and three Photon Machines Fusions laser systems at New Mexico Geochronology Research Laboratory. NMGRL's upgrade to the new analytical equipment required the design and implementation of a software ecosystem that allows seamless communication between various software and hardware components. Based on past experience and initial testing we choose to pursue a "Fully Distributed Control" model. In this model, hardware is compartmentalized and controlled by customized software running on individual computers. Each computer is connected to a Local Area Network (LAN) facilitating inter-process communication using TCP or UDP Internet Protocols. Two other options for interfacing are 1) Single Control, in which all hardware is controlled by a single application on a single computer and 2), Partial Distributed Control, in which the mass spectrometer is controlled directly by Thermo Scientific's Qtegra and all other hardware is controlled by a separate application. The "Fully Distributed Control" model offers the most efficient use of software resources, leveraging our in-house laboratory software with proprietary third-party applications, such as Qtegra and Mass Spec. Two software products resulted from our efforts. 1) Pychron, a configurable and extensible package for hardware control, data acquisition and preprocessing, and 2) RemoteControlServer.cs, a C# script for Thermo's Qtegra software that implements a TCP/UDP command server. Pychron is written in python and uses standard well-established libraries such as, Numpy, Scipy, and Enthought ETS. Pychron is flexible and extensible, encouraging experimentation and rapid development of new features. A project page for Pychron is located at, featuring an issue tracker and a Version Control System (Mercurial). RemoteControlServer.cs is a simple socket server that listens

  8. Lithology, kinematics and geochronology related to Late Mesozoic basin-mountain evolution in the Nanxiong-Zhuguang area, South China

    Institute of Scientific and Technical Information of China (English)

    SHU; Liangshu; DENG; Ping; WANG; Bin; TAN; Zhengzhong; YU


    Nanxiong basin and evolution of basin-mountain system were controlled both by the Zhuguang granitic-doming and the regional extensional tectonics.Development of the olivine basalt in the basin suggests that tension action was very strong during lava eruption. The magma-type zircon grains of basalt from the Nanxiong basin yielded the SHRIMP age of 96±1Ma, providing reliable geochronological constraint on the tectono-thermal event and basin-mountain evolution in the Nanling region, South China.

  9. U-Pb Zircon geochronology of the Cambro-Ordovician metagranites and metavolcanic rocks of central and NW Iberia (United States)

    Talavera, C.; Montero, P.; Bea, F.; González Lodeiro, F.; Whitehouse, M.


    New U-Pb zircon data from metagranites and metavolcanic rocks of the Schist-Graywacke Complex Domain and the Schistose Domain of Galicia Tras-os-Montes Zone from central and NW Iberia contribute to constrain the timing of the Cambro-Ordovician magmatism from Central Iberian and Galicia Tras-os-Montes Zones which occurred between 498 and 462 Ma. The crystallization ages of the metagranites and metavolcanic rocks from the northern Schist-Graywacke Complex Domain are as follows: (a) in west Salamanca, 489 ± 5 Ma for Vitigudino, 486 ± 6 Ma for Fermoselle and 471 ± 7 Ma for Ledesma; (b) in northern Gredos, 498 ± 4 Ma for Castellanos, 492 ± 4 Ma for San Pelayo and 488 ± 3 Ma for Bercimuelle; (c) in Guadarrama, 490 ± 5 Ma for La Estación I, 489 ± 9 Ma for La Cañada, 484 ± 6 Ma for Vegas de Matute (leucocratic), 483 ± 6 Ma for El Cardoso, 482 ± 8 Ma for La Morcuera, 481 ± 9 Ma for Buitrago de Lozoya, 478 ± 7 Ma for La Hoya, 476 ± 5 Ma for Vegas de Matute (melanocratic), 475 ± 5 Ma for Riaza, 473 ± 8 Ma for La Estación II and 462 ± 11 Ma for La Berzosa; and (d) in Toledo, 489 ± 7 Ma for Mohares and 480 ± 8 Ma for Polán. The crystallization ages of the metagranites from the Schistose Domain of Galicia Tras-os-Montes Zone are 497 ± 6 Ma for Laxe, 486 ± 8 Ma for San Mamede, 482 ± 7 Ma for Bangueses, 481 ± 5 Ma for Noia, 480 ± 10 for Rial de Sabucedo, 476 ± 9 Ma for Vilanova, 475 ± 6 Ma for Pontevedra, 470 ± 6 Ma for Cherpa and 462 ± 8 Ma for Bande. This magmatism is characterized by an average isotopic composition of (87Sr/86Sr)485Ma ≈ 0.712, (ɛNd)485Ma ≈ -4.1 and (TDM) ≈ 1.62 Ga, and a high zircon inheritance, composed of Ediacaran-Early Cambrian (65 %) and, to a lesser extent, Cryogenian, Tonian, Mesoproterozoic, Orosirian and Archean pre-magmatic cores. Combining our geochronological and isotopic data with others of similar rocks from the European Variscan Belt, it may be deduced that Cambro-Ordovician magmas from this belt were mainly

  10. Evolution of Pleistocene to Holocene eruptions in the Lesser Caucasus Mts:Insights from geology, petrology, geochemistry and geochronology (United States)

    Savov, Ivan; Meliksetian, Khachatur; Connor, Charles; Karakhanian, Arkadi; Sugden, Patrick; Navasardyan, Gevorg; Halama, Ralf; Ishizuka, Osamu; Connor, Laura; Karapetian, Sergei


    Both effusive and highly explosive (VEI>5) and often voluminous caldera volcanism has developed atop the collision zone between the Arabian and the Eurasian plates. Currently what is exposed on the Anatolian-Armenian-Iranian active orogenic plateau is post-Mesozoic felsic to intermediate collision-related plutons, and mostly collision or post-collision related Quaternary volcanic structures. We have studied in detail the volcanism, tectonics and geophysics on the territory of E.Turkey and Armenia, where several large stratovolcanoes (Ararat, Lesser Ararat, Aragats, Tsghuk, Ishkhanasar) are surrounded by distinct monogenetic volcanic fields (distributed volcanism). These large in volume stratovolcanoes and the associated low volume monogenetic cones range from normal calk-alkaline to high-K shoshonitic in affinity, with their products ranging from basanites to high K trachytes and rhyolites. Several volcanic provinces, namely Kechut/Javakheti, Aragats, Gegham, Vardenis and Syunik are recognized in Armenia and each of them has > 100 mapped volcanoes. These have distinct geochemical (mineral chemistry, trace element and Sr-Nd-B isotope systematics) and petrological (melt eruption temperatures and volatile contents) fingerprints that may or may not vary over time. Age determinations and volcano-stratigraphy sections for each of the case studies we aim to present shows that the volcanism includes a continuous record from Pleistocene to Holocene, or even historical eruptions. The excellent volcano exposures and the now complete high resolution database (GIS), geological mapping, and new and improved K-Ar and Ar-Ar geochronology, uniquely allows us to evaluate the driving forces behind the volcanism in this continent-continent collision setting that is uniquely associated with long lasting eruption episodes. We shall compare the now well studied historical/Holocene eruptions with those pre-dating them, with the aim to identify possible geochemical or petrological

  11. Timing of the granulite facies metamorphism in the Sanggan area, North China craton: zircon U-Pb geochronology

    Institute of Scientific and Technical Information of China (English)

    GUO; Jinghui


    [1]Lu, L. Z., Jin, S. Q., P-T-t paths and tectonic history of an early Precambrian granulite facies terrane, Jining District, south-eastern Inner Mongolia, China, J. Metamorphic Geol., 1993, 11: 483-498.[2]Liu, F. L., Shen, Q. H., Retrogressive textures and metamorphic reaction features of Al-rich gneisses in the granulite facies belt from northwestern Hebei province, Acta Petrologia Sinica (in Chinese with English abstract), 1999, 15(4): 505-517.[3]Zhai, M. G., Guo, J. H., Yan, Y. H. et al., Discovery of high-pressure basic granulite terrain in North China Archaean craton and preliminary study, Science in China, Ser. B, 1993, 36(11): 1402-1408.[4]Guo, J. H., Zhai, M. G., Zhang, Y. G. et al., Early Precambrian Manjinggou high-pressure granulite melange belt on the south edge of the Huaian complex, North China craton: geological features, petrology and isotopic geochronology, Acta Petrologica Sinica (in Chinese with English abstract), 1993, 9(4): 329-341.[5]Liu, S. W., Shen, Q. H., Geng, Y. S., Metamorphic evolution of two types of garnet-granulites in Northwestern Hebei province and analyses by Gibbs method, Acta Petrologica Sinica (in Chinese with English abstract), 1996, 12(2): 261-275.[6]Wang, R. M., Some evidence of the late Archaean collision zone in the northwestern Hebei Province, in Geological Evolution of the Granulite Terrane in North Part of the North China Craton (eds. Qian, X., Wang, R.), Beijing: Seismolgical Press. 1994, 7-20.[7]Liu, D. Y., Geng, Y. S., Song, B., Late Archean crustal accretion and reworking in northwest Hebei Province: geochronological evidence, Acta Geoscientia Sinica (in Chinese with English abstract), 1997, 18(3): 226-232.[8]Geng, Y. S., Liu, D. Y., Song, B., Chronological framework of the early Precambrian important events of the north-western Hebei granulite terrain, Acta Geologica Sinica (in Chinese with English abstract), 1997, 71:316-327.[9]Guo, J. H., Zhai, M. G., Sm-Nd age dating of high

  12. 40Ar/39Ar geochronology of the Faku tectonites: Implications for the tectonothermal evolution of the Faku block, northern Liaoning

    Institute of Scientific and Technical Information of China (English)

    ZHANG; Xiaohui; WANG; Hui; LI; Tiesheng


    geochronological constraints for re-evaluating the tectonic essence of the Faku Faulted Convex and ascertaining the suturing boundary between the North China Platform and the Xingmeng Fold System.

  13. The Ellsworth terrane, coastal Maine: Geochronology, geochemistry, and Nd-Pb isotopic composition - Implications for the rifting of Ganderia (United States)

    Schulz, K.J.; Stewart, D.B.; Tucker, R.D.; Pollock, J.C.; Ayuso, R.A.


    The Ellsworth terrane is one of a number of fault-bounded blocks that occur along the eastern margin of Ganderia, the western-most of the peri-Gondwanan domains in the northern Appalachians that were accreted to Laurentia in the Paleozoic. Geologic relations, detrital zircon ages, and basalt geochemistry suggest that the Ellsworth terrane is part of Ganderia and not an exotic terrane. In the Penobscot Bay area of coastal Maine, the Ellsworth terrane is dominantly composed of bimodal basalt-rhyolite volcanic sequences of the Ellsworth Schist and unconformably overlying Castine Volcanics. We use new U-Pb zircon geochronology, geochemistry, and Nd and Pb isotopes for these volcanic sequences to constrain the petrogenetic history and paleotectonic setting of the Ellsworth terrane and its relationship with Ganderia. U-Pb zircon geochronology for rhyolites indicates that both the Ellsworth Schist (508.6 ?? 0.8 Ma) and overlying Castine Volcanics (503.5 ?? 2.5 Ma) are Middle Cambrian in age. Two tholefitic basalt types are recognized. Type Tb-1 basalt, present as pillowed and massive lava flows and as sills in both units, has depleted La and Ce ([La/Nd]N = 0.53-0.87) values, flat heavy rare earth element (REE) values, and no positive Th or negative Ta anomalies on primitive mantle-normalized diagrams. In contrast, type Th-2 basalt, present only in the Castine Volcanics, has stightly enriched LREE ([La/Yb]N = 1.42-2.92) values and no Th or Th anomalies. Both basalt types have strongly positive ??Nd (500) values (Th-1 = +7.9-+8.6; Th-2 = +5.6-+7.0) and relatively enriched Pb isotopic compositions (206Ph/204Pb = 18.037-19.784; 207/204Pb = 15.531-15.660; 2088Pb/204Pb = 37.810-38.817). The basalts have compositions transitional between recent normal and enriched mid-ocean-ridge basalt, and they were probably derived by partial melting of compositionatly heterogeneous asthenosphenc mantle. Two types of rhyolite also are present. Type R-1 rhyolite, which mostly occurs as tuffs

  14. Geochemical, isotopic (Sr-Nd-Pb) and geochronological (Ar-Ar and U-Pb) constraints on Quaternary bimodal volcanism of the Nigde Volcanic Complex (Central Anatolia, Turkey) (United States)

    Aydin, F.; Siebel, W.; Uysal, I.; Ersoy, E. Y.; Schmitt, A. K.; Sönmez, M.; Duncan, R.


    The Nigde Volcanic Complex (NVC) is a major Late Neogene-Quaternary volcanic centre within the Cappadocian Volcanic Province of Central Anatolia. The Late Neogene evolution of the NVC generally initiated with the eruption of extensive andesitic-dacitic lavas and pyroclastic flow deposits, and minor basaltic lavas. This stage was followed by a Quaternary bimodal magma suite which forms Na-alkaline/transitional basaltic and high-K calc-alkaline to alkaline silicic volcanic rocks. In this study, we present new geochemical, isotopic (Sr-Nd-Pb) and geochronological (Ar-Ar and U-Pb) data for the bimodal volcanic suite within the NVC. Recent data suggest that the eruption of this suite took place ranges between ~650 and ~220 ka (Middle-Late Pleistocene). Silicic rocks consisting of rhyolite and associated pumice-rich pyroclastic fall out and surge deposits define a narrow range of 143Nd/144Nd isotope ratios (0.5126-0.5127), and show virtually no difference in Pb isotope composition (206Pb/204Pb = 18.84-18.87, 207Pb/204Pb = 15.64-15.67 and 208Pb/204Pb = 38.93-38.99). 87Sr/86Sr isotopic compositions of the silicic (0.704-0.705) and basaltic rocks (0.703-0.705) are rather similar reflecting a common source. The most mafic sample from basaltic rocks related to monogenetic cones is characterized by 87Sr/86Sr = 0.704, 143Nd/144Nd = 0.5127, 206Pb/204Pb = 18.80, 207Pb/204Pb = 15.60 and 208Pb/204Pb = 38.68. These values suggest a moderately depleted signature of the mantle source. The geochronological and geochemical data suggest that NVC silicic and basaltic rocks are genetically closely related to each other. Mantle derived differentiated basaltic melts which experienced low degree of crustal assimilation are suggested to be the parent melt of the rhyolites. Further investigations will focus on the spatial and temporal evolution of Quaternary bimodal magma suite in the NVC and the genetic relation between silicic and basaltic rocks through detailed oxygen isotope analysis and (U

  15. Updates of the KArLE Experiment: New Libs Calibration Under High Vacuum for the Quantification of Potassium in Basalt for In Situ Geochronology (United States)

    Devismes, D.; Cohen, B. A.; Li, Z.-H.; Miller, J. S.


    In planetary exploration, in situ absolute geochronology is one of the main important measurements that needs to be accomplished. Until now, on Mars, the age of the surface is only determined by crater density counting, which gives relative ages. These ages can have a lot of uncertainty as they depend on many parameters. More than that, the curves must be ties to absolute ages. Thus far, only the lost lander Beagle 2 was designed to conduct absolute geochronology measurements, though some recent attempts using MSL Curiosity show that this investigation is feasible and should be strongly encouraged for future flight. Experimental: The Potassium (K)-Argon Laser Experiment (KArLE) is being developed at MSFC through the NASA Planetary Instrument Definition and Development Program (PIDDP). The goal of this experiment is to provide in situ geochronology based on the K-Ar method. A laser ablates a rock under high vacuum, creating a plasma which is sensed by an optical spectrometer to do Laser Induced Breakdown Spectroscopy (LIBS). The ablated material frees gases, including radiogenic 40Ar,which is measured by a mass spectrometer (MS). As the potassium is a content and the 40Ar is a quantity, the ablated mass needed in order to relate them. The mass is given by the product of the ablated volume by the density of this material. So we determine the mineralogy of the ablated material with the LIBS spectra and images and calculate its density. The volume of the pit is measured by using microscopy. LIBS measurement of K under high vacuum: Three independant projects [1, 2, 3] including KArLE, are developing geochronological instruments based on this LA-LIBS-MS method. Despite several differences in their setup, all of them have validated the methods with analyses and ages. However, they all described difficulties with the LIBS measurements of K [3,4]. At ambient pressure, the quantification of K by LIBS on geological materials can be accurate [5]. However the protocol of the LA

  16. Sequence stratigraphy and U/b shrimp geochronology of the active margin deposits of the Cacheuta sub-basin, Cuyo Basin, Northwestern Argentina

    Energy Technology Data Exchange (ETDEWEB)

    Avila, Janaina Nunes [Rio Grande do Sul Univ., Porto Alegre, RS (Brazil). Inst. de Geociencias]. E-mail:; Chemale Junior, Farid [Rio Grande do Sul Univ., Porto Alegre, RS (Brazil). Inst. de Geociencias; Borba, Andre Weissheimer de [Rio Grande do Sul Univ., Porto Alegre, RS (Brazil). Programa de Pos-graduacao em Geociencias; Cingolani, Carlos [La Plata Univ. Nacional (Argentina)


    This study focuses on the stratigraphic and geochronologic analysis of the deposits related to the faulted active margin of the Cacheuta sub-basin, Cuyo Basin (Triassic), in NW-Argentina. This basin was mainly controlled by extensional tectonics along NW-trending structures inherited from Paleozoic sutures. The whole stratigraphic package of this basin is interpreted as a second order depositional sequence. Low stand deposits comprise coarse-grained alluvial fans (sheet flood and debris-flow deposits). Fluvial and minor lacustrine deposits with expressive volcaniclastic contribution make up the transgressive systems tract. The maximum flooding surface was traced on lacustrine black shales of the Cacheuta Formation (source rock for petroleum accumulations). The high stand system tract comprises fluvial meandering facies and more sparse volcanic contribution. U/Pb SHRIMP dating of a pyroclastic rock inter layered with basal alluvial fans yielded a magmatic age of 243 {+-} 4.7 Ma positioned in the Early to Middle Triassic. (author)

  17. Geochemistry, Geochronology and Genesis of Gold Mineralization in Nurt of Northern Altay, Xinjiang:A Case Study on the Aketishikan Gold Deposit

    Institute of Scientific and Technical Information of China (English)

    YUAN Feng; ZHOU Taofa; TAN Lugui; LUO Xianrong; YUE Shucang


    Gold deposits such as the Aketishikan, Togetobie, Tasbig-Kokeydlas, Kums and Hongshanzui gold deposits in the Nurt area in Altay of Xinjiang were found in Member 3 rhyolite tufflava, fragmental lava and ignimbrite of the Carboniferous Hongshanzui Group. Trace and rare earth elements, sulfur, lead, oxygen and hydrogen isotopes, and geochronological studies indicate that the ore-forming material was mostly supplied by the Carboniferous volcanic rocks through water-rock interaction under a low-to-moderate temperature, and the hydrothermal ore-forming fluid came from meteoric water with some magmatic water input evolved from the granitic magmas. Gold deposits in the Nurt area as well as in the northern Altay might form in multiple stages, and the Yanshanian mineralization period should be paid more attention besides the Variscan mineralization period.

  18. Stages of late Paleozoic to early Mesozoic magmatism in the Song Ma belt, NW Vietnam: evidence from zircon U-Pb geochronology and Hf isotope composition (United States)

    Hieu, Pham Trung; Li, Shuang-Qing; Yu, Yang; Thanh, Ngo Xuan; Dung, Le Tien; Tu, Vu Le; Siebel, Wolfgang; Chen, Fukun


    The Song Ma zone in NW Vietnam bears important tectonic implications as a potential subduction corridor between the Indochina and South China blocks. On the basis of U-Pb ages, the Hf isotopic characteristics of zircons and the geochemical composition of granitoids, a two-stage magmatic evolution process of the Song Ma zone at ~290-260 and ~245-230 Ma can be proposed. Isotopic analyses indicate magmatic contributions from Neoproterozoic oceanic island basalt, Proterozoic continental crust, and depleted mantle or juvenile lithosphere. By combining geochronological and geochemical data from the granitoid rocks, we suggest that the staged magmatic processes of Song Ma zone may be related to a long-lasting period of ocean subduction (ca. 290-260 Ma) and subsequent syn-/post-collisional evolution (ca. 245-230 Ma).

  19. Geochronology of source materials from high-pressure and ultrahigh-pressure metamorphic rocks in Jurassic sedimentary rocks of Hefei Basin

    Institute of Scientific and Technical Information of China (English)


    In this report, it is proven through stable isotopic geochronological study that for the Jurassic sedimentary rocks in the Dushan area of Anhui Province, the source materials derived from high-pressure and ultrahigh-pressure metamorphic rocks are the products of subduction and exhumation of the Yangtze plate under the North China plate during the Triassic period. The cobble of high-pressure or ultrahigh-pressure metamorphic rock in the Fenghuangtai Formation of Hefei Basin has Rb-Sr isotopic isochron age of phengite, apatite and whole rock, 232.1±2.1 Ma (MSWD = 0.59). Plateau ages of 40Ar/39Ar age spectra of detrital phengites in the Sanjianpu and Fenghuangtai formations are concordant, and they are 216.5±0.6 Ma and 217.4±1.3 Ma, respectively.

  20. Tracing long term tectonic evolution of accretionary orogens by U-Pb zircon geochronology: Proterozoic to Jurassic tectonics of the Santander Massif, northern Colombia (United States)

    Valencia, V. A.; Cardona, A.; Gehrels, G. E.; Ruiz, J.; Ibañez, M.


    Accurate orogenic models are nedded to reconstruct complex tectonic histories of long lived convergent margins. Integrated zircon U-Pb geochronology on igneous, sedimentary and metasedimentry rocks within single crustal domains is a powerful tool, as it can be used to trace the timing of rock forming events, magmatic style and episodity, and identify crustal recycling. U-Pb detrital zircon and magmatic geochronology was carried on multiple litostratigraphic units of the Santander Massif in the northeastern Andes, in order to reconstruct its long term Late Proterozoic to Early Mesozoic tectonic evolution. Major zircon forming events includ well defined Grenvillian, Late Neoproterozoic to Ordovician, Silurian, Early Permian and Jurassic events. Major peaks of activity at ca. 197 Ma, 440-410 Ma and 470-490 Ma and 950-1052 Ma, support the existence of continental scale tectonic cycles. Older Mesoproterozoic (1.3-1.5 Ga) crustal input in metasediments and magmatic rocks link these units to crustal recycling on the margins of the Amazon Craton, whereas the older 950-1052 Ma peak indicates the link of this crustal segment with other Andean Grenvillian remnant. Previous interpretations of the Paleozoic Silgara Formation seem incorrect, as acquired dates from this study includ different metamorphic units, deposited and formed after the Silurian and Permian during final stages of Pangea's assemblage, probably as Laurentia migrated to its final Alleghanian position. Finally the presence of the NW South America Jurassic arc is also present in the region by granitoid ages. The limited input of this arc signature within the contemporaneous and overlapping Early Cretaceous sedimentary rocks suggest that this arc was developed in a back arc setting.

  1. U-Pb Geochronology, Geochemistry and Kinematic Analyses of Subduction-Related Late Triassic Basins in Northern Chile (24.5º-26ºS). (United States)

    Espinoza, M. E.


    In northern Chile (24.5°-26°S) two Pre-Andean depocenters crop out: the Cifuncho basin in the Coastal Cordillera and the Profeta basin in the Precordillera. These basins have been classically interpreted as a continental rifting unrelated to subduction during the period prior to the Andean orogenic cycle. However, recent petrographic and geochemical data suggest the development of these basins in an active subduction system. In order to test this hypothesis and to establish the geologic evolution of the basins and the strain field during the rifting process, we present preliminary U-Pb geochronological and geochemical data together with structural analyses of synrift structures. The geochronological data along the Cifuncho and Profeta basins, show a main continental sedimentary deposition during the Norian to Raethian. Volcanosedimentary rocks show a main detrital supply of Early Permian age (~297-283 Ma). This input can be associated with the volcanic La Tabla Formation and/or the exhumation of Permian granitoids. A minor supply close to ~478 Ma is related to a source from the Lower Ordovician arc (~480 Ma), suggesting the tectonic exhumation of this source to the east of the Profeta basin during the Late Triassic. On the other hand, structural analysis was carried in third and four order extensional faults (<10 m of slip) along the Profeta basin. Most of the faults show a clear synrift character with the development of fault controlled growing strata. The kinematic analyses evidence a variability in the orientation of the maximum strain axes from a main northwest to a subordinate northeast direction of extension. Thus, the intimate relation between the continental sedimentary deposition and a proximal volcanism of intermediate composition and calk-alkaline affinity, suggests the development of these basins in a supra-subduction setting during the Late Triassic. Structural data probably reflect local variation in the strain field across the basins.

  2. Calibration of the Late Cretaceous to Paleocene geomagnetic polarity and astrochronological time scales: new results from high-precision U-Pb geochronology (United States)

    Ramezani, Jahandar; Clyde, William; Wang, Tiantian; Johnson, Kirk; Bowring, Samuel


    Reversals in the Earth's magnetic polarity are geologically abrupt events of global magnitude that makes them ideal timelines for stratigraphic correlation across a variety of depositional environments, especially where diagnostic marine fossils are absent. Accurate and precise calibration of the Geomagnetic Polarity Timescale (GPTS) is thus essential to the reconstruction of Earth history and to resolving the mode and tempo of biotic and environmental change in deep time. The Late Cretaceous - Paleocene GPTS is of particular interest as it encompasses a critical period of Earth history marked by the Cretaceous greenhouse climate, the peak of dinosaur diversity, the end-Cretaceous mass extinction and its paleoecological aftermaths. Absolute calibration of the GPTS has been traditionally based on sea-floor spreading magnetic anomaly profiles combined with local magnetostratigraphic sequences for which a numerical age model could be established by interpolation between an often limited number of 40Ar/39Ar dates from intercalated volcanic ash deposits. Although the Neogene part of the GPTS has been adequately calibrated using cyclostratigraphy-based, astrochronological schemes, the application of these approaches to pre-Neogene parts of the timescale has been complicated given the uncertainties of the orbital models and the chaotic behavior of the solar system this far back in time. Here we present refined chronostratigraphic frameworks based on high-precision U-Pb geochronology of ash beds from the Western Interior Basin of North America and the Songliao Basin of Northeast China that places tight temporal constraints on the Late Cretaceous to Paleocene GPTS, either directly or by testing their astrochronological underpinnings. Further application of high-precision radioisotope geochronology and calibrated astrochronology promises a complete and robust Cretaceous-Paleogene GPTS, entirely independent of sea-floor magnetic anomaly profiles.

  3. Zircon U-Pb geochronology of the volcanic rocks from Fanchang-Ningwu volcanic basins in the Lower Yangtze region and its geological implications

    Institute of Scientific and Technical Information of China (English)

    YAN Jun; LIU HaiQuan; SONG ChuanZhong; XU XiSheng; AN YaJun; LIU Jia; DAI LiQun


    The latest eruptions in two important Mesozoic volcanic basins of Fanchang and Ningwu located in the middle-lower reaches of the Yangtze River formed the bimodal volcanic rocks of the Kedoushan Formation and ultrapotassic volcanic rocks of the Niangniangshan Formation,respectively.The representative volcanic rocks of the two Formations were selected for LA-ICPMS zircon U-Pb dating.The results indicate that there exist a large amount of magmatic zircons as indicated by high Th/U ratios in these volcanic rocks.The weighted mean age of 21 analyses is 130.7±1.1 Ma for the Kedoushan Formation,and that of 20 analyses is 130.6±1.1 Ma for the Niangniangshan Formation.These U-Pb ages are interpreted to represent the formation times of the volcanic rocks.In combination with other known geochronological data for Mesozoic volcanic rocks from the Lower Yangtze region,it is proposed that the latest volcanic activations in the Jinniu,Luzong,Fanchang and Ningwu volcanic basins probably came to end prior to ca.128 Me.There is no significant time interval between the early and later volcanic activities in the Luzong and Ningwu basins,suggesting e short duration of volcanic activities and thus implying the onset of an extensional tectonic setting at about 130 Ma in the Lower Yangtze region.Integrated studies reveal that the Early Cretaceous magmatic activities and their geochronological framework in the Lower Yangtze region are a response to progressively dynamic deep processes that started with the transformation of tectonic setting from compression to extension,followed by delaminating of the lower part of the thickened lithosphere,lithospheric thinning,asthenosphere upwelling,and crust-mantle interaction.

  4. Ar-Ar and U-Pb geochronology of Late Paleozoic basalts in western Guangxi and its constraints on the eruption age of Emeishan basalt magmatism

    Institute of Scientific and Technical Information of China (English)

    FAN Weiming; WANG Yuejun; PENG Touping; MIAO Laicheng; GUO Feng


    The Late Paleozoic layered or stratoid-layered basalts in western Guangxi have similar elemental and isotopic compositions to Emeishan high-Ti basalts. Whole-rock 40Ar/39Ar and SHRIMP zircon U-Pb dating were carried out for the representative basalt samples in three typical profiles in the area. Three basalts from the upper segment of Yangxu profile and lower segment of Yufeng and Min'an profiles yield the 40Ar/39Ar plateau ages of 253.6±0.4 Ma (20BS-71),255.4±0.4 Ma (20BS-99) and 256.2±0.8 Ma (20BS-119), respectively. Twenty-three analyses on 23 zircons of the basalt from the upper segment of Yangxu profile give a weighted mean 206pb/238U age of 253.7±6.1 Ma with an MSWD = 2.8.These new and published geochronological data for Emeishan large igneous province (LIP) indicate that the Emeishan LIP was initiated at ~260 Ma, voluminously erupted between 253 and 256 Ma, and possibly ended at ~251-253Ma' The age (251-260 Ma) is generally consistent with that of the associated environmental deterioration and mass extinction events at the end-Guadalupian and Permo-Triassic boundary. These precise geochronological data provide important constraints on the dominantly eruptive time of the Emeishan LIP and understanding of the distribution of Emeishan high-Ti basalts and its mantle plume dynamics.

  5. Altered volcanic ash layers of the Late Cretaceous San Felipe Formation, Sierra Madre Oriental (Northeastern Mexico): Usbnd Pb geochronology, provenance and tectonic setting (United States)

    Velasco-Tapia, Fernando; Martínez-Paco, Margarita; Iriondo, Alexander; Ocampo-Díaz, Yam Zul Ernesto; Cruz-Gámez, Esther María; Ramos-Ledezma, Andrés; Andaverde, Jorge Alberto; Ostrooumov, Mikhail; Masuch, Dirk


    A detailed petrographic, geochemical, and Usbnd Pb geochronological study of altered volcanic ash layers, collected in eight outcrops of the Late Cretaceous San Felipe Formation (Sierra Madre Oriental, Northeastern Mexico), has been carried out. The main objectives have been: (1) to establish a deposit period, and (2) to propose a reliable provenance-transport-deposit-diagenetic model. These volcano-sedimentary strata represent the altered remains of vitreous-crystalline ash (main grains: quartz + K-feldspar (sanidine) + Na-plagioclase + zircon + biotite; groundmass: glass + calcite + clinochlore + illite) deposited and preserved in a shallow, relatively large in area, open platform environment. Major and trace element geochemistry indicate that parent volcanism was mainly rhyodacitic to rhyolitic in composition. Discrimination diagrams suggest a link to continental arc transitional to extension tectonic setting. Usbnd Pb geochronology in zircon has revealed that the volcanic ash was released from their sources approximately during the range 84.6 ± 0.8 to 73.7 ± 0.3 Ma, being transported to the depocenters. Burial diagenesis process was marked by: (a) a limited recycling, (b) the partial loss of original components (mainly K-feldspar, plagioclase, biotite and glass), and (c) the addition of quartz, calcite, illite and clinochlore. The location of the source area remains uncertain, although the lack of enrichment in Zr/Sc ratio suggests that ashes were subjected to relatively fast and short-distance transport process. El Peñuelo intrusive complex, at 130-170 km west of the depocenters, is the nearest known zone of active magmatism during the Upper Cretaceous. This intermediate to felsic pluton, characterized by a geochemical affinity to post-orogenic tectonic setting, could be linked to the volcanic sources.

  6. Orogenic development of the Adrar des Iforas (Tuareg Shield, NE Mali): New geochemical and geochronological data and geodynamic implications (United States)

    Bosch, Delphine; Bruguier, Olivier; Caby, Renaud; Buscail, François; Hammor, Dalila


    deposition of sediments intruded by basaltic and dacitic lavas, one of which was dated at 623 ± 6 Ma. East of the IGU, in the Tamaradant domain, metagabbros and meta-anorthosites emplaced within greywackes have geochemical characteristics typical of subduction-related environments (enrichment in LILE and HFSE depletion). A metadiorite intruding the Tamaradant sediments gave an age of 630 ± 6 Ma, which is tentatively taken as evidence for a Pan-African age for the subduction processes that took place east of the IGU. Collision of the Kidal terrane with the eastern margin of the West African Craton is best dated by a syncollisional tonalite, which provides an age of 604 ± 5 Ma. Late kinematic processes shortly followed at 599 ± 4 Ma as exemplified by the emplacement of a monzogranite belonging to the complex Central Batholith. The geochronological and geochemical evidences provided by this study allow proposing that the Kidal terrane, the IGU and Tamaradant domain of the Adrar des Iforas once belonged to a single terrane, which probably extended northward to include the IOGU, and which was later dissected by major lithospheric scale faults during the late Pan-African orogenic phases.

  7. New assemly model of Jiangnan Orogen: insight from detrital zircon geochronology of pre-Cretaceous strata, South China (United States)

    Su, J.; Dong, S.


    , which results in entirely different interpretation of the tectonic background of Jiangnan Orogen. The detrital age spectrums and provenance of basement in SCB are analyzed and compared with each other, which indicate that the depositional processes are totally distinctive and not the product induced by same orogeny along east margin of Yangtze Block. The detrital zircons from Proterozoic and Phanerozoic strata record major Proterozoic populations of 2.0-1.8 Ga, 1.1-0.9Ga and 0.7-0.9 Ga, implying existence of multi-phase orogenies in SCB during Proterozoic. In addition, the SCB has affinity with Australia not only in Columbia supercontinent but also in Rodinia supercontinent. Considering the distribution of Proterozoic orogen in Australia and geochronology of SCB, another amalgamation history between Yangtze and Cathaysia Blocks is proposed. We infer existence of ancient orogen under the western Jiangnan Orogen, which may occur at either Columbia-age or Greenville-age earlier than the Sibao orogeny.


    Langone, A.; Tiepolo, M.; Morishita, T.; Tribuzio, R.


    High-Mg andesites (HMA) are a group of arc products, usually rich in amphibole, whose origin is still debated. High-Mg diorites (HMD) are intrusive counterparts of HMA and are locally found in exhumed arc sections. In places, HMD are closely associated with ultramafic amphibole-rich rocks and may provide further constrains on the role of amphibole in the origin of HMA and in the petrogenesis of arc related-magmas. Small bodies of Cretaceous amph-rich mafic and ultramafic intrusives with similar petrographic features and HMA affinity (e.g. Kamei et al., 2004) are exhumed together with granitoid rocks along the Japan arc. These rocks, originated during the Cretaceous subduction of the Pacific plate under Asia, give the opportunity to investigate modifications in the composition of melts with HMA affinity as a function of the along-margin position and time. Mafic and ultramafic intrusives were sampled in different localities of the Honshu (NE Japan arc) and Kyushu Island (SW Japan arc). In situ U-Pb zircon geochronology by laser ablation ICPMS was carried out to constrain the time relationships among the different intrusive bodies. U-Pb concordia ages show that the HMD were mostly emplaced from ca. 100 Ma up to 118 Ma. In only one case (Honshu Island) U-Pb concordia ages as young as 70 Ma were obtained. High modal proportions of amphibole (40-60 vol%) with pargasite cores (mg# = 0.66-0.77) characterize all studied samples. In ultramafic rocks (Ol-hornblendites and Cpx-hornblendites), pargasitic amphibole is poikilitic on sub-rounded clinopyroxene (mg# up to 88) and olivine (Fo61-78). In mafic rocks (amph-gabbros and amph-diorites), large amphibole grains are dispersed in a fine-grained matrix defining a porphyritic texture. The matrix consists mainly of Mg-hornblende, plagioclase and clinopyroxene. The cores of pargasitic amphibole from mafic rocks have clinopyroxene inclusions with mg# up to 88, similar to what found in ultramafic rocks. Independently from the age

  9. Geochronologic, Isotopic, and Trace Element Constraints on Zircon Recycling in Sierra Crest Intrusive Suites, Sierra Nevada Batholith, USA (United States)

    Miller, J. S.; Lackey, J.; Memeti, V.; Hirt, W. H.; Wooden, J. L.


    Owing to its ubiquity and chemical properties, zircon is the primary tool for obtaining age information from felsic igneous rocks. Numerous geochronologic studies in ancient and recent plutonic and volcanic rocks over the last decade have shown: (1) that assemblages of zircons from single hand samples rarely crystallized at the same time; (2) that zircons from single hand samples may have variable geochemistry and isotopic compositions requiring distinct growth environments. Nonetheless, the conditions under which diverse assemblages of magmatic zircons crystallize, are dispersed and then gathered within such magma systems are less well understood. We have initiated a systematic geochemical (trace element) and isotopic (δ18O, 176Hf/177Hf) investigation of zircons from dated plutons within several of the Cretaceous Sierran Crest zoned intrusive suites of the Sierra Nevada Batholith to better understand melt sources and zircon recycling within these large magma systems. Preliminary O and Hf isotopic results indicate that zircon assemblages between different intrusive suites have variable isotopic compositions with multiple crustal and mantle sources involved. Preliminary trace element analysis of some of the zircon suites also show appreciable variability but important trends have emerged from the data. In particular, there are pronounced differences between trace element patterns and ratios of the youngest, interior, K-feldspar megacryst-bearing granodiorites (e.g. Cathedral Peak, Mono Creek, Whitney) and the older outer units of the intrusive suites, which are generally more mafic granodiorites and tonalites. In contrast to the more mafic outer units, zircons from the inner megacryst-bearing intrusions are overwhelmingly characterized by low Ti-in-zircon (Tzrn,Ti) model temperatures (regardless of uncertainties in aTiO2), low concentrations of MREEs, high Yb/Gd, low Th/U, high Hf, and high Eu/Eu*. These characteristics, and generally low Zr contents of the

  10. Geochronology and correlation of Tertiary volcanic and intrusive rocks in part of the southern Toquima Range, Nye County, Nevada (United States)

    Shawe, Daniel R.; Snee, Lawrence W.; Byers, Frank M.; du Bray, Edward A.


    ago with eruption of the tuff of Round Mountain. The Manhattan caldera south of the Mount Jefferson caldera and northwest of the Big Ten Peak caldera formed in association with eruption of a series of tuffs, principally the Round Rock Formation, mostly ash-flow tuff, about 24.4 million years ago. Extensive 40Ar/39Ar dating of about 60 samples that represent many of the Tertiary extrusive and intrusive rocks in the southern Toquima Range provides precise ages that refine the chronology of previously dated units. New geochronologic data indicate that the petrogenetically related Corcoran Canyon, Ryecroft Canyon, and Mount Jefferson calderas formed during a period of about 560,000 years. Electron microprobe analyses of phenocrysts from 20 samples of six dated units underscore inferred petrogenetic relations among some of these units. In particular, compositions of augite, hornblende, and biotite in tuffs erupted from the Corcoran Canyon, Ryecroft Canyon, and Mount Jefferson calderas are similar, which suggests that magmas represented by these tuffs have similar petrogenetic histories. The unique occurrence of hypersthene in Isom-type tuff confirms its derivation from a source beyond the southern Toquima Range.

  11. Sedimentological and geochronological evidences of anthropogenic impacts on river basins in the Northern Latium coastal area (Italy) (United States)

    Piazzolla, Daniele; Paladini de Mendoza, Francesco; Scanu, Sergio; Marcelli, Marco


    In this work we aimed to compare sedimentological and geochronological data from three sediment core samples (MIG50, MRT50, and GRT50) taken in the Northern Latium (Italy) coastal area, at -50 m depth, to data regarding rainfall, river flows and the land use in the three most important hydrographic basins (Mignone, Marta and Fiora) and in the coastal area. Different trends of sediment mass accumulation rate (MAR) are detected in the three cores: a strongly increasing trend was identified in MIG50 and MRT50 cores while GRT50 doesn't show significant variation. Data from the sedimentological analysis of GRT50 core identify a progressive decrease in the sandy component, which declined from about 30% to the current level of 7% over the last 36 years, while MRT50 and MIG50 cores (mainly composed by pelitic fraction > 95%) showed slight variations of textural ratio between silt and clay. According to the general decrease of pluviometric trend observed in Italy, related to teleconnection pattern tendency (NAO), the statistical analysis of rain identified significative decrease only in the Fiora river basin, whereas in the other two locations the decrease was not as significant. Regarding the Fiora river flow, a significative decreasing trend of average flow is detected, while the flood regime remained unaffected over the past 30 years. The analysis of the land use shows that the human activities are increased of 6-10% over the available time steps (1990 - 2006) in Fiora and Mignone river basins, while the Marta river basin has a strong human impact since 1990 highligting more than 80% of artificial soil covering. The largest variation is observed on the Fiora basin (10%) where the antrhopic activities have expanded to an area of about 85 Km2. Moreover, in the last ten years a large beach nourishment in 2004 (570000 m3) and dredging activities in the early second half of 2000s (1000000 m3 moved) were performed in Marina di Tarquinia beach and in front of the Torrevaldaliga

  12. Origin of paleosubduction-modified mantle for Silurian gabbro in the Cathaysia Block: Geochronological and geochemical evidence (United States)

    Wang, Yuejun; Zhang, Aimei; Fan, Weiming; Zhang, Yanhua; Zhang, Yuzhi


    In the eastern South China Block (SCB), the presence of middle Paleozoic volcanics and mafic rocks is poor in spite of the extensive occurrence of granites in response to the synchronous (Kwangsian) intracontinental orogen event. The nature of the lithosphere and its associated orogenic process are poorly known for such a major orogen. In this paper, a set of new U-Pb zircon geochronological, elemental and Sr-Nd isotopic data are presented for several gabbroic plutons, which were recently identified in the Cathaysia Block. The representative samples of the Longhugang, Xinchuan and Xinsi plutons yielded the weighted mean 206Pb/238U ages of 423 ± 8 Ma, 434 ± 6 Ma and 420 ± 3 Ma, respectively, and their gneissoid country-rocks are dated at 442-466 Ma. This evidences the presence of Silurian mafic rocks in the Cathaysia Block. These gabbroic rocks can be geochemically classified into three groups. Group 1 from the western Longhugang pluton exhibits flat REE patterns represented by (La/Yb)cn = 1.1-1.7, (Gd/Yb)cn = 0.99-1.15, and δEu = 1.29-1.51 with negative Nb-Ta and P and markedly positive Sr anomalies. It shows enrichment in LILEs and 87Sr/86Sr(t) ratios of 0.70412-0.70727 and ɛNd(t) values of - 2.6 to - 0.6. Group 2 from the Xinchuan, Zhouya and Yunlu plutons is characterized by high MgO (11.4-15.8 wt.%), mg-number (66-74) and Cr and Ni contents. It is marked by significantly negative Nb-Ta, Zr-Hf and P-Ti anomalies with high 87Sr/86Sr(t) (0.70920-0.71072) and low ɛNd(t) values (- 3.9 to - 7.9). Group 3 from the Xinsi and eastern Longhugang plutons has ɛNd(t) values from - 2.8 to - 5.7 and shows high Nb content of 7.05-9.89 ppm with pronounced enrichment in LILE and weak depletion in HFSE, resembling Nb-enriched arc basalt in HFSE composition. A synthesis of these geochemical data points to the following petrogenesis: Groups 1 and 2 derived from a spinel-bearing, plagioclase-rich source and a garnet-bearing, orthopyroxene-rich source, respectively

  13. Petrology and geochronology of crustal xenoliths from the Bering Strait region: Linking deep and shallow processes in extending continental crust (United States)

    Akinin, V.V.; Miller, E.L.; Wooden, J.L.


    Petrologic, geochemical, and metamorphic data on gneissic xenoliths derived from the middle and lower crust in the Neogene Bering Sea basalt province, coupled with U-Pb geochronology of their zircons using sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG), yield a detailed comparison between the P-T-t and magmatic history of the lower crust and magmatic, metamorphic, and deformational history of the upper crust. Our results provide unique insights into the nature of lithospheric processes that accompany the extension of continental crust. The gneissic, mostly maficxenoliths (constituting less than two percent of the total xenolith population) from lavas in the Enmelen, RU, St. Lawrence, Nunivak, and Seward Peninsula fields most likely originated through magmatic fractionation processes with continued residence at granulite-facies conditions. Zircon single-grain ages (n ??? 125) are interpreted as both magmatic and metamorphic and are entirely Cretaceous to Paleocene in age (ca. 138-60 Ma). Their age distributions correspond to the main ages of magmatism in two belts of supracrustal volcanic and plutonic rocks in the Bering Sea region. Oscillatory-zoned igneous zircons, Late Cretaceous to Paleocene metamorphic zircons and overgrowths, and lack of any older inheritance in zircons from the xenoliths provide strong evidence for juvenile addition of material to the crust at this time. Surface exposures of Precambrian and Paleozoic rocks locally reached upper amphibolite-facies (sillimanite grade) to granulite-facies conditions within a series of extension-related metamorphic culminations or gneiss domes, which developed within the Cretaceous magmatic belt. Metamorphic gradients and inferred geotherms (??30-50 ??C/km) from both the gneiss domes and xenoliths aretoo high to be explained by crustal thickening alone. Magmatic heat input from the mantle is necessary to explain both the petrology of the magmas and elevated metamorphic temperatures. Deep

  14. From Pangaea to the present: geochronology, thermochronology and isotopic tracking of tectonic processes along the Northern Andes (United States)

    Spikings, R.; Cochrane, R.; Van der Lelij, R.; Villagomez, D.


    Triassic - Tertiary rocks within the Central Cordillera of Colombia and Eastern Cordillera of Ecuador provide a record of the rift-to-drift phase of the western Tethys Wilson Cycle, Jurassic steady-state active margin magmatism, Early Cretaceous attenuation of the margin and the formation of new continental crust, and the accretion of an extensive oceanic plateau and arc sequence at ~75 Ma, which shielded juvenile continental crust from tectonic erosion during the Tertiary. The margin remained active throughout the Tertiary, and exhumed in response to changing oceanic plate kinematics and the collision of heterogeneous oceanic crust. We present geochronological, thermochronological, geochemical and Hf, Nd and O isotopic data that provide a highly-temporally resolved record of the evolution of NW Gondwana from Pangaea to the present. Migmatitic leucosomes and S-type granites were emplaced along the NW South American margin during ~275-225 Ma, and tholeiitic amphibolites intruded during ~240 - 225 Ma. These sequences formed during continental rifting in a back-arc, leading to the formation of ophiolite sequences and oceanic crust by ~216 Ma. The Maya and Oaxaquia terranes of Central America may represent parts of the conjugate margin. The NW South American margin remained passive until ~183 Ma, when subduction gave rise to calc-alkaline, I-type granitoids until ~143 Ma. Earliest Cretaceous roll-back extended and exhumed the margin, causing the arc axis to migrate oceanward while the magmatic rocks became progressively more isotopically juvenile. Arc migration opened Early Cretaceous intra-arc basins that were floored by lavas and filled with arc detritus. The arc axis stabilized at ~130-115 Ma and fringed the continental margin outboard of the Jurassic arc. Compression at 120-110 Ma closed the intra-arc basins, exhumed the buttressing continental margin and obducted variably metamorphosed rocks of the east dipping-subduction channel onto the continental margin during

  15. Uplifting of the Jiamusi Block in the eastern Central Asian Orogenic Belt, NE China: evidence from basin provenance and geochronology (United States)

    Liu, Yongjiang; Wen, Quanbo; Han, Guoqing; Li, Wei


    The main part of Jiamusi Block, named as Huanan-Uplift, is located in the northeastern Heilongjiang, China. The Huanan-Uplift is surrounded by many relatively small Mesozoic-Cenozoic basins, e.g. Sanjiang Basin, Hulin Basin, Boli Basin, Jixi Basin, Shuangyashan Basin and Shuanghua Basin. However previous research works were mainly focused on stratigraphy and palaeontology of the basins, therefore, the coupling relation between the uplift and the surrounding basins have not been clear. Based on the field investigations, conglomerate provenance studies of the Houshigou Formation in Boli Basin, geochronology of the Huanan-Uplift basement, we have been studied the relationships between Huanan-Uplift and the surrounding basins. The regional stratigraphic correlations indicates that the isolated basins in the area experienced the same evolution during the period of the Chengzihe and the Muling Formations (the Early Cretaceous). The paleogeography reconstructions suggest that the area had been a large-scale basin as a whole during the Early Cretaceous. The Huanan-Uplift did not exist. The paleocurrent directions, sandstone and conglomerate provenance analyses show that the Huanan-Uplift started to be the source area of the surrounding basins during the period of Houshigou Formation (early Late Cretaceous), therefore, it suggests that the Jiamusi Block commenced uplift in the early Late Cretaceous. The granitic gneisses in Huanan-Uplift give 494-415 Ma monazite U-Th-total Pb ages, 262-259 Ma biotite and 246-241 Ma K-feldspar 40Ar/39Ar ages. The cooling rates of 1-2 ℃/Ma from 500-260 Ma and 10-11 ℃/Ma from 260-240 Ma have been calculated based on the ages. This suggests that the Jiamusi Block had a rapid exhumation during late Permian, which should be related to the closure of the Paleo-Asian Ocean between the Siberian and North China continents. It is concluded that during the late Paleozoic the Jiamusi Block was stable with a very slow uplifting. With the closure of

  16. National Geochronological Database (United States)

    U.S. Geological Survey, Department of the Interior — Radiometric ages of geological materials by K-Ar, Rb-Sr, Sm-Nd, fission track, and U-Th-Pb dating methods. A revision of DDS-14 correcting locations and providing...

  17. Biomarkers in sediments. The racemization/epiremitation of amino acids like tool in geochronology and paleothermometrics; Estratigrafia biomolecular. La racemizacion/epimerizacion de aminoacidos como herramienta geocronologica y paleotermometrica

    Energy Technology Data Exchange (ETDEWEB)

    Torres, T.; Llamas, F. J.; Canoira, L.; Garcia-Alonso, P.; Ortiz, J. E. [Universidad Politecnica de Madrid (Spain)


    The study of amino acids as biomarkers in sediments has become a necessary methodology and tool for the analysis of palaeoenvironmental conditions and, therefore, of climatic evolution in the past. Research based on the selection and analysis of geological biomarkers, and more specifically activities relating to the racemization/epimerization of amino acids, makes it possible to obtain the geochronological and photoelectrochemical data required to establish different hypotheses for Long-Term Performance Assessment of a repository for high level radioactive wastes. (Author)

  18. Zircon Geochronology (U-Pb, Petrography, Geochemistry and Radioisotopes of Bornaward Metarhyolites (Central Taknar Zone-Northwest of Bardaskan

    Directory of Open Access Journals (Sweden)

    Reza Monazzami Bagherzadeh


    -Pb zircon geochronology Measurement of U-Pb isotopes of the Bornaward metarhyolite zircons of sample BKCh-103, indicates an age of 552.23+4.73,-6.62 Ma (Upper Precambrian. Sr-Nd isotopes The Sr ratios of the metarhyolites (87Sr/86Sr were found to fall in the range of 0.688949 to 0.723435 and the Nd ratios (143Nd/144Ndi were in the range of 0.511701 to 0.511855. These values indicate that the metarhyolites of samples BKCh-12, BKCh-103 and BKCh-177 were affected by hydrothermal alteration since their (87Sr/86SrI ratios are high. The Sr ratios suggest that the more negative Nd anomaly and the more negative ɛNd(552 of the samples BKCh-12, BKCh-103 and BKCh-177 indicate that these lavas originated in an enriched upper mantle source and/or lower continental crust. In contrast, two recent examples (Xua et al., 2005 can be related to sialic continental crust with significant contamination. Petrogenesis The Bornaward metarhyolite- rhyodacites show an enriched pattern for Rb, Th, U, K, Pb, Nd and Y relative to the primitive mantle, while Ba, P, Ti, Sr, Zr and Nb show a reduction as a result of fractional crystallization. Based on isotopic correlations of207Pb/204Pb vs 206Pb/204Pb, the primitive source of the Bornaward metarhyolite- rhyodacites is the lower continental crust. This part of the continental crust is only slightly depleted in Pb. Consequently, it has a low 87Sr/86Sr ratio (Samples BKCh-138 and BKCh-198. In contrast, the samples of BKCh-12, BKCh-103 and BKCh-177 have high 87Sr/86Sr ratios that could be the result of significant contamination to parts of the continental crust with very high 87Sr/86Sr (Karimpour et al., 2011. Results and Conclusions The calc-alkaline compositions of samples BKCh-12, BKCh-103 and BKCh-177, the high K- calc alkaline of samples BKCh-138 and BKCh-198 of the Bornaward metarhyolites and the higher temperature overgrowth of plagioclase on lower temperature microcline phenocrysts can be a reason for entrance lavas with different generations. The

  19. Re-Os geochronology of the El Salvador porphyry Cu-Mo deposit, Chile: Tracking analytical improvements in accuracy and precision over the past decade (United States)

    Zimmerman, Aaron; Stein, Holly J.; Morgan, John W.; Markey, Richard J.; Watanabe, Yasushi


    Sulfide minerals from the El Salvador porphyry Cu-Mo deposit, Chile, were dated by Re-Os geochronology to clarify the timing and duration of mineralization. As these data are collected over the past 10 years, they chronicle the evolution of Re-Os analytical procedures and improvements in spike-sample equilibration, mass spectrometry and data reduction. Included in the data is the first tennantite-pyrite Re-Os isochron along with tennantite-enargite-pyrite Os tracing of ore metals based on initial osmium systematics (187Os/188Osi). Porphyry-associated mineralization follows the traditional early (A), transitional (B), and late (D) stage classification of relative timing of vein formation. Most Cu- and Mo-bearing sulfides were deposited during stages A and B. Field relationships clearly show the B-stage veins are older than the D-stage veins. Samples analyzed with Carius tube digestion and mixed-double spike indicate B-stage molybdenite mineralization between 41.8 and 41.2 Ma. These ages best represent the timing of main-stage mineralization at El Salvador. A five-point 187Re-187Os isochron for pyrite-tennantite from the latest D-stage veins yields an age of 42.37 ± 0.45 Ma. The isochron age, while older, is within error of early analyses of molybdenite by alkaline fusion digestion using single Re and Os spikes (42.4-42.0 Ma). A separate pyrite and tennantite-enargite pair containing significant common Os yields an initial 187Os/188Os ratio of 0.134 ± 0.042, compatible with a predominantly mantle origin. We appreciate the early analyses for their historical significance while emphasize the geological implications of the 41.8-41.2 Ma molybdenite ages. Temperature estimates by sulfur thermometry and fluid inclusions are 390-510 °C for the B-stage molybdenites and 230-240 °C for the latest D-stage pyrite and tennantite. Analyses based on Carius tube digestion and updated spike calibrations suggest a ∼0.6 m.y. duration (∼41.8 to 41.2 Ma) in-line with published

  20. Re-Os geochronology and Os isotope fingerprinting of petroleum sourced from a Type I lacustrine kerogen: insights from the natural Green River petroleum system in the Uinta Basin and hydrous pyrolysis experiments (United States)

    Cumming, Vivien M.; Selby, David; Lillis, Paul G.; Lewan, Michael D.


    Rhenium–osmium (Re–Os) geochronology of marine petroleum systems has allowed the determination of the depositional age of source rocks as well as the timing of petroleum generation. In addition, Os isotopes have been applied as a fingerprinting tool to correlate oil to its source unit. To date, only classic marine petroleum systems have been studied. Here we present Re–Os geochronology and Os isotope fingerprinting of different petroleum phases (oils, tar sands and gilsonite) derived from the lacustrine Green River petroleum system in the Uinta Basin, USA. In addition we use an experimental approach, hydrous pyrolysis experiments, to compare to the Re–Os data of naturally generated petroleum in order to further understand the mechanisms of Re and Os transfer to petroleum. The Re–Os geochronology of petroleum from the lacustrine Green River petroleum system (19 ± 14 Ma – all petroleum phases) broadly agrees with previous petroleum generation basin models (∼25 Ma) suggesting that Re–Os geochronology of variable petroleum phases derived from lacustrine Type I kerogen has similar systematics to Type II kerogen (e.g., Selby and Creaser, 2005a, Selby and Creaser, 2005b and Finlay et al., 2010). However, the large uncertainties (over 100% in some cases) produced for the petroleum Re–Os geochronology are a result of multiple generation events occurring through a ∼3000-m thick source unit that creates a mixture of initial Os isotope compositions in the produced petroleum phases. The 187Os/188Os values for the petroleum and source rocks at the time of oil generation vary from 1.4 to 1.9, with the mode at ∼1.6. Oil-to-source correlation using Os isotopes is consistent with previous correlation studies in the Green River petroleum system, and illustrates the potential utility of Os isotopes to characterize the spatial variations within a petroleum system. Hydrous pyrolysis experiments on the Green River Formation source rocks show that Re and Os transfer

  1. Dating sub-20 micron zircons in granulite-facies mafic dikes from SW Montana: a new approach using automated mineralogy and SIMS U-Pb geochronology (United States)

    Ault, A. K.; Mahan, K. H.; Flowers, R. M.; Chamberlain, K.; Appleby, S. K.; Schmitt, A. K.


    Geochronological data is fundamental to all tectonic studies, but a major limitation for many lithologies is a paucity of sizeable zircons suitable for conventional U-Pb techniques. In particular, mafic dike swarms provide important time markers for tectonometamorphic activity in Precambrian terranes, but commonly yield little or no zircon or baddeleyite sufficient for TIMS or standard ion-probe analysis of crystal separates. We apply a new approach involving in-situ automated mineralogy and high spatial resolution Secondary Ion Mass Spectrometry (SIMS) geochronology to a mafic dike swarm exposed in the Northern Madison Range of SW Montana. The dikes cross-cut early fabrics but are also variably deformed and metamorphosed to P-T conditions as high as 1.2 GPa and 850 C. The swarm emplacement age is inferred to be ca. 2.1 Ga based on similarities to dated dikes in the adjacent Tobacco Root Mountains. Resolving the timing of dike emplacement and high-grade metamorphism in the study area is important for understanding the extent of post-Archean modification to the northwest margin of the Wyoming craton. Identification and textural characterization of zircons were facilitated by in-situ automated mineralogical analysis, in contrast to a standard elemental X-ray mapping approach. Our technique uses an SEM-based platform coupling calibrated BSE data with X-ray data collected by multiple energy dispersive spectrometers to rapidly identify target accessory phases at high spatial resolution. Whole thin section search maps were generated in ~30 minutes at 4 µm pixel resolution. Our dike thin sections commonly contained >300 zircons in a variety of textural settings, with 80% having a short dimension 95% for 206Pb. SIMS data for 22 zircons from a granulite-facies mafic dike thin section define a chord with upper and lower intercepts of 1753.1 ± 9.5 Ma and 63.2 ± 7.9 Ma, respectively (2 sigma error, MSWD = 1.6). A positive correlation between U concentration and degree of

  2. High-precision U-Pb zircon geochronological constraints on the End-Triassic Mass Extinction, the late Triassic Astronomical Time Scale and geochemical evolution of CAMP magmatism (United States)

    Blackburn, T. J.; Olsen, P. E.; Bowring, S. A.; McLean, N. M.; Kent, D. V.; Puffer, J. H.; McHone, G.; Rasbury, T.


    Mass extinction events that punctuate Earth's history have had a large influence on the evolution, diversity and composition of our planet's biosphere. The approximate temporal coincidence between the five major extinction events over the last 542 million years and the eruption of Large Igneous Provinces (LIPs) has led to the speculation that climate and environmental perturbations generated by the emplacement of a large volume of magma in a short period of time triggered each global biologic crisis. Establishing a causal link between extinction and the onset and tempo of LIP eruption has proved difficult because of the geographic separation between LIP volcanic deposits and stratigraphic sequences preserving evidence of the extinction. In most cases, the uncertainties on available radioisotopic dates used to correlate between geographically separated study areas often exceed the duration of both the extinction interval and LIP volcanism by an order of magnitude. The "end-Triassic extinction" (ETE) is one of the "big five" and is characterized by the disappearance of several terrestrial and marine species and dominance of Dinosaurs for the next 134 million years. Speculation on the cause has centered on massive climate perturbations thought to accompany the eruption of flood basalts related to the Central Atlantic Magmatic Province (CAMP), the most aerially extensive and volumetrically one of the largest LIPs on Earth. Despite an approximate temporal coincidence between extinction and volcanism, there lacks evidence placing the eruption of CAMP prior to or at the initiation of the extinction. Estimates of the timing and/or duration of CAMP volcanism provided by astrochronology and Ar-Ar geochronology differ by an order of magnitude, precluding high-precision tests of the relationship between LIP volcanism and the mass extinction, the causes of which are dependent upon the rate of magma eruption. Here we present high precision zircon U-Pb ID-TIMS geochronologic data

  3. New geochronology of the Stalać section at the southern limit of European loess occurrence through pIR50IR290 dating (United States)

    Bösken, Janina; Obreht, Igor; Klasen, Nicole; Zeeden, Christian; Marković, Slobodan B.; Hambach, Ulrich; Lehmkuhl, Frank


    A new geochronology was established for the Stalać loess-paleosol section in central Serbia. The section is located in the Central Balkan, at the transition zone between Atlantic, continental and Mediterranean climate regimes and is therefore potentially extraordinary sensitive to past and present climatic changes. Especially in the context of early human migration, this region is not intensively studied and has potential to contribute valuable information. This is the reason why it is investigated in the frame of the CRC 806 "Our Way to Europe" project. This project focuses on the climatic changes and their influence on the dispersal of anatomically modern human (AMH). For the loess-paleosol sequence exposed in a quarry in Stalać, a multi-proxy approach combining geochemical and sedimentological analysis and luminescence dating is applied. This contribution focusses on the luminescence investigations. The luminescence characteristics of six samples were analyzed using the optically stimulated luminescence signal of quartz by means of the single-aliquot-regenerative-dose (SAR) protocol and the infrared stimulated signal of feldspar (within polymineral samples) by means of the post-infrared-infrared (pIRIR) protocol. While the fine quartz grain (4-11μm) samples show a preheat dependency of equivalent doses that makes them unsuitable for dating, the polymineral fine grain samples measured by pIR50IR290 behave satisfactorily: plateaus within a first IR stimulation temperature test, good dose recovery ratios, low residual doses, high precision and low uncertainty support the established age chronology ranging from 35.2 ± 1.8 ka to 168 ± 9 ka. A tephra within the MIS 6/L2 loess layer was bracket dated, and combining these ages suggests an age between ca. 140 and 175 ka (2σ). Based on the presented results we introduce a new geochronology of the Stalać section, indicating that the previously suggested correlative stratigraphy (Kostić and Protić, 2000) requires

  4. High precision dating of mass extinction events: a combined zircon geochronology, apatite tephrochronology, and Bayesian age modelling approach of the Permian-Triassic boundary extinction (United States)

    Baresel, Björn; Bucher, Hugo; Brosse, Morgane; Bagherpour, Borhan; Schaltegger, Urs


    Chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb dating of single-zircon crystals is preferably applied to tephra beds intercalated in sedimentary sequences. By assuming that the zircon crystallization age closely approximate that of the volcanic eruption and ash deposition, U-Pb zircon geochronology is the preferred approach for dating mass extinction events (such as the Permian-Triassic boundary mass extinction) in the sedimentary record. As tephra from large volcanic eruptions is often transported over long distances, it additionally provide an invaluable tool for stratigraphic correlation across distant geologic sections. Therefore, the combination of high-precision zircon geochronology with apatite chemistry of the same tephra bed (so called apatite tephrochronology) provides a robust fingerprint of one particular volcanic eruption. In addition we provide coherent Bayesian model ages for the Permian-Triassic boundary (PTB) mass extinction, then compare it with PTB model ages at Meishan after Burgess et al. (2014). We will present new high-precision U-Pb zircon dates for a series of volcanic ash beds in deep- and shallow-marine Permian-Triassic sections in the Nanpanjiang Basin, South China. In addition, apatite crystals out of the same ash beds were analysed focusing on their halogen (F, Cl) and trace-element (e.g. Fe, Mg, REE) chemistry. We also show that Bayesian age models produce reproducible results from different geologic sections. On the basis of these data, including litho- and biostratigraphic correlations, we can precisely and accurately constrain the Permian-Triassic boundary in an equatorial marine setting, and correlate tephra beds over different sections and facies in the Nanpanjiang Basin independently from litho-, bio- or chemostratigraphic criteria. The results evidence that data produced in laboratories associated to the global EARTHTIME consortium can provide age information at the 0.05% level of 206

  5. Paleoproterozoic crustal evolution in the East Sarmatian Orogen: Petrology, geochemistry, Sr-Nd isotopes and zircon U-Pb geochronology of andesites from the Voronezh massif, Western Russia (United States)

    Terentiev, R. A.; Savko, K. A.; Santosh, M.


    Andesites and related plutonic rocks are major contributors to continental growth and provide insights into the interaction between the mantle and crust. Paleoproterozoic volcanic rocks are important components of the East Sarmatian Orogen (ESO) belonging to the East European Craton, although their petrogenesis and tectonic setting remain controversial. Here we present petrology, mineral chemistry, bulk chemistry, Sr-Nd isotopes, and zircon U-Pb geochronological data from andesites and related rocks in the Losevo and Vorontsovka blocks of the ESO. Clinopyroxene phenocrysts in the andesites are depleted in LREE, and enriched in HFSE (Th, Nb, Zr, Hf, Ti) and LILE (Ba, Sr). Based on the chemistry of pyroxenes and whole rocks, as well as Fe-Ti oxides, we estimate a temperature range of 1179 to 1262 °C, pressures of 11.3 to 13.0 kbar, H2O content of 1-5 wt.%, and oxygen fu gacity close to the MH buffer for the melts of the Kalach graben (KG) and the Baygora area (BA) andesites. Our zircon U-Pb geochronological data indicate new zircon growth during the middle Paleoproterozoic as displayed by weighted mean 207Pb/206Pb ages of 2047 ± 17 Ma and 2040 ± 16 Ma for andesite and dacite-porphyry of the BA, and 2050 ± 16 Ma from high-Mg basaltic andesite of the KG. The andesites and related rocks of the KG and BA are characterized by high magnesium contents (Mg # up to 0.68). All these volcanic rocks are depleted in LREE and HFSE, and display negative Nb and Ti anomalies relative to primitive mantle. The high-Mg bulk composition, and the presence of clinopyroxene phenocrysts suggests that the parent melts of the KG and BA suite were in equilibrium with the mantle rocks. The rocks show positive εNd(T) values and low initial 87Sr/86Sr, suggesting that the magmas were mostly derived from metasomatized mantle source. The geochemical differences between the two andesite types are attributed to: the predominance of fractional crystallization, and minor role of contamination in the

  6. SHRIMP U-Pb zircon geochronology of Mesozoic granitoids from the Bariloche region (Argentina): Implications for the Middle-Late Jurassic evolution of the North Patagonian batholith. (United States)

    Castro, Antonio; Vujovich, Graciela; Fernández, Carlos; Moreno-Ventas, Iñaki; Martino, Roberto; Corretgé, Guillermo; Díaz-Alvarado, Juan; Heredia, Nemesio; Gallastegui, Gloria


    A detailed U-Pb geochronological study has been carried out on granitoids of the North Patagonian batholith in the region of Bariloche (Argentina), between 40°30' S and 41°45' S. In this region, the calc-alkaline, subduction-related, granitic bodies of the North Patagonian batholith intruded an Early Jurassic volcano-sedimentary sequence contemporary with the intrusion of the Subcordilleran Patagonian batholith (J1 magmatism), and unconformably overlying a metamorphic Gondwanan basement. All these rocks were affected by the Andean compressional phases during the Cenozoic. U-Pb SHRIMP dating of zircon crystals from 11 samples (109 spots) of diorites, tonalites, granodiorites and granites yielded dates ranging from 173 ± 3 Ma to 150 ± 2 Ma (Aalenian to Tithonian). No significant age differences have been identified among the distinct lithological types. Also no spatial trend emerges from these results, although ages tend to be younger westward in the traverse of the Manso River (≈ 41° 35' S). Two peaks appear in the probability density plot of zircon ages. Most of the dated zircons are Bajocian-Bathonian (Middle Jurassic, ≈169 Ma, J2 magmatism), while a secondary peak is observed at the boundary Oxfordian-Kimmeridgian (Late Jurassic, ≈ 156 Ma, J3 magmatism). The J2 magmatic period is coeval to the main stage of effusive activity (V2) in the huge volcanic Chon Aike Province, while J3 coincides with the lesser V3 period of volcanism in Chon Aike. These new geochronological data strongly contribute to the knowledge of the first stages of tectonic evolution of the Andean subduction margin in southern South America. Contrary to previous models, it can be proposed that the subduction-related Mesozoic magmatism started well before the Late Jurassic, and that a continuous supply of calc-alkaline magmas dominated the active margin of South America during at least 190 Ma, from the Early Jurassic to nowadays. Therefore, no dramatic time gap can be observed between

  7. Dating minerals by ID-TIMS geochronology at times of in situ analysis: selected case studies from the CPGeo-IGc-USP laboratory

    Directory of Open Access Journals (Sweden)

    Cláudia R. Passarelli


    Full Text Available Since 1964, the Center for Geochronological Research - CPGeo, one of the interdepartmental centers of the Instituto de Geociências (IG of São Paulo University, has developed studies related to several geological processes associated with different rock types. Thermal Ionization Mass Spectrometry Isotopic Dilution (ID-TIMS has been the technique widely used in the CPGeo U-Pb Laboratory. It provides reliable and accurate results in age determination of superposed events. However, the open-system behavior such as Pb-loss, the inheritance problem and metamictization processes allow and impel us to a much richer understanding of the power and limitations of U-Pb geochronology and thermochronology. In this article, we present the current methodology used at the CPGeo-IGc-USP U-Pb laboratory, the improvements on ID-TIMS method, and report high-precision U-Pb data from zircon, monazite, epidote, titanite, baddeleyite and rutile from different rock types of several domains of the Brazilian south-southeast area, Argentina and Uruguay.O Centro de Pesquisas Geocronológicas (CPGeo, um dos centros interdepartamentais do Instituto de Geociências (IG da Universidade de São Paulo (USP, desde 1964 desenvolve estudos relacionados a diversos processos geológicos que se associam a diferentes tipos de rochas. A técnica amplamente utilizada no Laboratório U-Pb é a diluição isotópica por espectrometria de massa termo ionizada (ID-TIMS. Esta sistemática proporciona resultados bastante confiáveis e precisos na determinação das idades de eventos geológicos superpostos. Entretanto, o comportamento de sistema aberto como perda de Pb, problemas de herança isotópica e processos de metamictização, nos permite o entendimento do poder e limitação da geocronologia e termocronologia U-Pb. Neste artigo apresentamos a metodologia atualmente utilizada no Laboratório U-Pb do CPGeo-IGc-USP, as melhorias atingidas na técnica ID-TIMS e alguns dados obtidos em

  8. Origin and time-space distribution of hydrothermal systems in east-central Australian sedimentary basins: Constraints from illite geochronology and isotope geochemistry. (United States)

    Uysal, I. Tonguç


    Some well-known precious mineral deposits and hydrocarbon resources occur extensively in east-central Australian sedimentary Basins. The metal occurrences are abundant in northwestern and eastern part of Queensland, whereas no significant deposits are known in large areas further south, which may, however, be hidden beneath the Jurassic-Cretaceous sedimentary basins. Important hydrocarbon resources exist within the Jurassic-Cretaceous sedimentary rocks at relatively shallow depths, of which the distribution represent zones of high paleo-geothermal gradients. This study examines the time-space distribution in relation to the regional tectonic history of concealed metal deposits and areas of high paleo-geothermal gradient leading to hydrocarbon maturation. To this end, authigenic illitic clay minerals representing various locations and stratigraphic depths in east-central Australia were investigated, of which the Rb-Sr and Ar-Ar geochronology and stable isotope geochemistry assist in delineating zones of hydrothermal systems responsible for hydro-carbon maturation/migration and potentially ore deposition. The Late Carboniferous - Early Permian crustal extension that affected large areas of eastern Australia and led to the epithermal mineralisations (e.g., the Drummond Basin) is also recorded in northern South Australia and southwest Queensland. A Late Triassic - Early Jurassic tectonic event being responsible for coal maturation and gas generation in the Bowen Basin and the epithermal mineralisation in the North Arm goldfield in SE Queensland likewise affected the areas much further west in Queensland. Some illites from the basement in outback Queensland and fault gouges from the Demon Fault in NE New South Wales yield younger Rb-Sr and Ar-Ar ages indicating the effect of hydrothermal processes as a result of a Middle-Upper Jurassic tectonic event. The majority of illite samples from the crystalline basement rocks, Permian Cooper Basin, and Jurassic

  9. Monazite geochronology and geothermobarometry in polymetamorphic host rocks of volcanic-hosted massive sulphide mineralizations in the Mesoproterozoic Areachap Terrane, South Africa (United States)

    Bachmann, Kai; Schulz, Bernhard; Bailie, Russell; Gutzmer, Jens


    The Areachap Terrane represents the medium-to high-grade metamorphic and deformed remnants of a Mesoproterozoic (ca. 1240-1300 Ma) volcanic arc bound to the margin of the Archean Kaapvaal Craton in the east, and the polydeformed and metamorphosed Proterozoic Namaqua Province in the west. There has been protracted debate as to the exact nature, origin, age and tectonic evolution of this terrane, adjacent to an important Mesoproterozoic crustal suture between the Archean Kaapvaal Craton and the Namaqua Province, which developed during the ∼1200-1000 Ma Namaquan Orogeny. The Areachap Terrane comprises highly deformed bimodal volcanic and volcano sedimentary successions that host a number of massive sulphide base metal orebodies. Samples from three of these orebodies at different locations were analysed to determine the age and P-T conditions of metamorphism along the Areachap Terrane. Metamorphic ages were determined by electron microprobe chemical dating of monazite. Garnet- and amphibole-bearing mineral assemblages were used for geothermobarometry at the Areachap Mine, located in the northern sector of the Areachap Terrane, monazite geochronology yields evidence for two populations of Th-U-Pb-ages at 1432 ± 30 Ma - a possible protolith age - and a metamorphic age of 1153 ± 21 Ma. Kantienpan and Copperton, representing the central and southern sector of the Areachap Terrane respectively, yield monazite ages for a younger metamorphic event with U-Th-Pb-ages of 1108 ± 19 Ma and 1104 ± 17 Ma, respectively. Geothermobarometric data give a differentiated view on the metamorphic evolution of the Areachap Terrane. An arc consistent clockwise P-T evolution path and upper amphibolite-facies peak metamorphic conditions are consistent at the three locations. The Areachap site shows a short prograde development with 8.0 kbar maximum pressure at circa 700 °C maximum temperature and a subsequent retrograde metamorphism. At Kantienpan, on the other hand, maximum metamorphic

  10. U-Pb geochronology of zircon and monazite from Mesoproterozoic granitic gneisses of the northern Blue Ridge, Virginia and Maryland, USA (United States)

    Aleinikoff, J.N.; Burton, W.C.; Lyttle, P.T.; Nelson, A.E.; Southworth, C.S.


    Mesoproterozoic granitic gneisses comprise most of the basement of the northern Blue Ridge geologic province in Virginia and Maryland. Lithology, structure, and U-Pb geochronology have been used to subdivide the gneisses into three groups. The oldest rocks, Group 1, are layered granitic gneiss (1153 ?? 6 Ma), hornblende monzonite gneiss (1149 ?? 19 Ma), porphyroblastic granite gneiss (1144 ?? 2 Ma), coarse-grained metagranite (about 1140 Ma), and charnockite (>1145 Ma?). These gneisses contain three Proterozoic deformational fabrics. Because of complex U-Pb systematics due to extensive overgrowths on magmatic cores, zircons from hornblende monzonite gneiss were dated using the sensitive high-resolution ion microprobe (SHRIMP), whereas all other ages are based on conventional U-Pb geochronology. Group 2 rocks are leucocratic and biotic varieties of Marshall Metagranite, dated at 1112??3 Ma and 1111 ?? 2 Ma respectively. Group 3 rocks are subdivided into two age groups: (1) garnetiferous metagranite (1077 ?? 4 Ma) and quartz-plagioclase gneiss (1077 ?? 4 Ma); (2) white leucocratic metagranite (1060 ?? 2 Ma), pink leucocratic metagranite (1059 ?? 2), biotite granite gneiss (1055 ?? 4 Ma), and megacrystic metagranite (1055 ?? 2 Ma). Groups 2 and 3 gneisses contain only the two younger Proterozoic deformational fabrics. Ages of monazite, seprated from seven samples, indicate growth during both igneous and metamorphic (thermal) events. However, ages obtained from individual grains may be mixtures of different age components, as suggested by backscatter electron (BSE) imaging of complexly zoned grains. Analyses of unzoned monazite (imaged by BSE and thought to contain only one age component) from porphyroblastic granite gneiss yield ages of 1070, 1060, and 1050 Ma. The range of ages of monazite (not reset to a uniform date) indicates that the Grenville granulite event at about 1035 Ma did not exceed about 750??C. Lack of evidence for 1110 Ma growth of monazite in

  11. Geochronological and isotopic records of crustal storage and assimilation in the Wolverine Creek-Conant Creek system, Heise eruptive centre, Snake River Plain (United States)

    Szymanowski, D.; Ellis, B. S.; Wotzlaw, J. F.; Buret, Y.; von Quadt, A.; Peytcheva, I.; Bindeman, I. N.; Bachmann, O.


    Understanding the processes of differentiation of the Yellowstone-Snake River Plain (YSRP) rhyolites is typically impeded by the apparent lack of erupted intermediate compositions as well as the complex nature of their shallow interaction with the surrounding crust responsible for their typically low O isotopic ratios. A pair of normal-δ18O rhyolitic eruptions from the Heise eruptive centre in eastern Idaho, the Wolverine Creek Tuff and the Conant Creek Tuff, represent unique magmatic products of the Yellowstone hotspot preserving abundant vestiges of the intermediate differentiation steps leading to rhyolite generation. We address both shallow and deep processes of magma generation and storage in the two units by combining high-precision ID-TIMS U-Pb zircon geochronology, trace element, O and Hf isotopic studies of zircon, and Sr isotopic analyses of individual high-Mg# pyroxenes inherited from lower- to mid-crustal differentiation stages. The zircon geochronology confirms the derivation of both tuffs from the same rhyolitic magma reservoir erupted at 5.5941 ± 0.0097 Ma, preceded by at least 92 ± 14 ky of continuous or intermittent zircon saturation approximating the length of pre-eruptive magma accumulation in the upper crust. Some low-Mg# pyroxenes enclosing zircons predate the eruption by at least 45 ± 27 ky, illustrating the co-crystallisation of major and accessory phases in the near-liquidus rhyolitic melts of the YSRP over a significant period of time. Coeval zircon crystals are isotopically heterogeneous (two populations at ɛHf -5 and -13), requiring the assembly of isotopically distinct melt pockets directly prior to, or during, the eruption. The primitive Mg# 60-90 pyroxenes are out of isotopic equilibrium with the host rhyolitic melt (87Sr/86Sri = 0.70889), covering a range of 87Sr/86Sri = 0.70705-0.70883 corresponding to ratios typical of the most radiogenic YSRP basalts to the least radiogenic YSRP rhyolites. Together with the low ɛHf in zircon

  12. Growth and collapse of a deeply eroded orogen: Insights from structural, geophysical, and geochronological constraints on the Pan-African evolution of NE Mozambique (United States)

    Viola, G.; Henderson, I. H. C.; Bingen, B.; Thomas, R. J.; Smethurst, M. A.; de Azavedo, S.


    This paper presents results of a large multidiciplinary geological mapping project in NE Mozambique, with a focus on the structural evolution of this part of the East African Orogen (EAO). It integrates field structural studies with geophysical interpretations and presents new geochronological data. The tectonic architecture of NE Mozambique can be subdivided into five megatectonic units on the basis of lithology, structure and geochronology: unit 1, Paleoproterozoic Ponta Messuli Complex in the extreme NW corner of NE Mozambique, which represents the local NW foreland to the EAO; unit 2, a collage of Mesoproterozoic metamorphic complexes, which forms the basement to unit 3, a stack of Neoproterozoic, NW directed imbricate thrust nappes named here the "Cabo Delgado Nappe Complex" (CDNC); unit 4, restricted Neoproterozoic metasedimentary basins; and unit 5, two exotic Neoproterozoic granulite mélange complexes. The units were assembled during a long and complex history of NW directed shortening, which commenced with nappe stacking and emplacement of the CDNC over the Mesoproterozoic basement terranes toward the NW foreland. It is proposed that the CDNC and the Eastern Granulites farther north in Tanzania are remnants of Neoproterozoic volcanic arcs and microcontinents formed "outboard" of the Mesoproterozoic continent after 596 ± 11 Ma. Field and potential field geophysical data show that the nappes were folded by regional-scale NE-SW trending folds that formed in response to a later stage of the same shortening episode and this episode gave rise to the Lurio Belt, a prominent structural feature of northern Mozambique and a key element (often as suture zone) in many Gondwana reconstructions. The Lurio Belt is here interpreted as a structure generated during folding of the CDNC during later stages of the progressive shortening event. It is, however, a repeatedly reactivated shear zone, probably at the site of an older (Mesoproterozoic?) discontinuity, with an

  13. Detrital zircon U-Pb geochronology, Lu-Hf isotopes and REE geochemistry constrains on the provenance and tectonic setting of Indochina Block in the Paleozoic (United States)

    Wang, Ce; Liang, Xinquan; Foster, David A.; Fu, Jiangang; Jiang, Ying; Dong, Chaoge; Zhou, Yun; Wen, Shunv; Van Quynh, Phan


    In situ U-Pb geochronology, Lu-Hf isotopes and REE geochemical analyses of detrital zircons from Cambrian-Devonian sandstones in the Truong Son Belt, central Vietnam, are used to provide the information of provenance and tectonic evolution of the Indochina Block. The combined detrital zircon age spectra of all of the samples ranges from 3699 Ma to 443 Ma and shows with dominant age peaks at ca. 445 Ma and 964 Ma, along with a number of age populations at 618-532 Ma, 1160-1076 Ma, 1454 Ma, 1728 Ma and 2516 Ma. The zircon age populations are similar to those from time equivalent sedimentary sequences in continental blocks disintegrated from the East Gondwana during the Phanerozoic. The younger zircon grains with age peaks at ca. 445 Ma were apparently derived from middle Ordovician-Silurian igneous and metamorphic rocks in Indochina. Zircons with ages older than about 600 Ma were derived from other Gondwana terrains or recycled from the Precambrian basement of the Indochina Block. Similarities in the detrital zircon U-Pb ages suggest that Paleozoic strata in the Indochina, Yangtze, Cathaysia and Tethyan Himalayas has similar provenance. This is consistent with other geological constrains indicating that the Indochina Block was located close to Tethyan Himalaya, northern margin of the India, and northwestern Australia in Gondwana.

  14. Petrology, phase equilibria and monazite geochronology of granulite-facies metapelites from deep drill cores in the Ordos Block of the North China Craton (United States)

    He, Xiao-Fang; Santosh, M.; Bockmann, Kiara; Kelsey, David E.; Hand, Martin; Hu, Jianmin; Wan, Yusheng


    Among the various Precambrian crustal blocks in the North China Craton (NCC), the geology and evolution of the Ordos Block remain largely enigmatic due to paucity of outcrop. Here we investigate granulite-facies metapelites obtained from deep-penetrating drill holes in the Ordos Block and report petrology, calculated phase equilibria and in-situ monazite LA-ICP-MS geochronology. The rocks we studied are two samples of cordierite-bearing garnet-sillimanite-biotite metapelitic gneisses and one graphite-bearing, two-mica granitic gneiss. The peak metamorphic age from LA-ICP-MS dating of monazite in all three samples is in the range of 1930-1940 Ma. The (U + Pb)-Th chemical ages through EPMA dating reveals that monazite occurring as inclusions in garnet are older than those in the matrix. Calculated metamorphic phase diagrams for the cordierite-bearing metapelite suggest peak P-T conditions ca. 7-9 kbar and 775-825 °C, followed by decompression and evolution along a clockwise P-T path. Our petrologic and age data are consistent with those reported from the Khondalite Belt in the Inner Mongolia Suture Zone in the northern part of the Ordos Block, suggesting that these granulite-facies metasediments represent the largest Paleoproterozoic accretionary belt in the NCC.

  15. New insights into the history and origin of the southern Maya block, SE Mexico: U-Pb-SHRIMP zircon geochronology from metamorphic rocks of the Chiapas massif (United States)

    Weber, Bodo; Iriondo, Alexander; Premo, Wayne R.; Hecht, Lutz; Schaaf, Peter


    The histories of the pre-Mesozoic landmasses in southern México and their connections with Laurentia, Gondwana, and among themselves are crucial for the understanding of the Late Paleozoic assembly of Pangea. The Permian igneous and metamorphic rocks from the Chiapas massif as part of the southern Maya block, México, were dated by U–Pb zircon geochronology employing the SHRIMP (sensitive high resolution ion microprobe) facility at Stanford University. The Chiapas massif is composed of deformed granitoids and orthogneisses with inliers of metasedimentary rocks. SHRIMP data from an anatectic orthogneiss demonstrate that the Chiapas massif was part of a Permian (∼ 272 Ma) active continental margin established on the Pacific margin of Gondwana after the Ouachita orogeny. Latest Permian (252–254 Ma) medium- to high-grade metamorphism and deformation affected the entire Chiapas massif, resulting in anatexis and intrusion of syntectonic granitoids. This unique orogenic event is interpreted as the result of compression due to flat subduction and accretionary tectonics. SHRIMP data of zircon cores from a metapelite from the NE Chiapas massif yielded a single Grenvillian source for sediments. The majority of the zircon cores from a para-amphibolite from the SE part of the massif yielded either 1.0–1.2 or 1.4–1.5 Ga sources, indicating provenance from South American Sunsás and Rondonian-San Ignacio provinces.

  16. Petrogenesis and U-Pb and Sm-Nd geochronology of the Taquaral granite: record of an orosirian continental magmatic arc in the region of Corumba - MS

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    Letícia Alexandre Redes

    Full Text Available The Taquaral Granite is located on southern Amazon Craton in the region of Corumbá, westernmost part of the Brazilian state of Mato Grosso do Sul (MS, near Brazil-Bolivia frontier. This intrusion of batholitic dimensions is partially covered by sedimentary rocks of the Urucum, Tamengo Bocaina and Pantanal formations and Alluvial Deposits. The rock types are classified as quartz-monzodiorites, granodiorites, quartz-monzonites, monzo and syenogranites. There are two groups of enclaves genetically and compositionally different: one corresponds to mafic xenoliths and the second is identified as felsic microgranular enclave. Two deformation phases are observed: one ductile (F1 and the other brittle (F2. Geochemical data indicate intermediate to acidic composition for these rocks and a medium to high-K, metaluminous to peraluminous calk-alkaline magmatism, suggesting also their emplacement into magmatic arc settings. SHRIMP zircon U-Pb geochronological data of these granites reveals a crystallization age of 1861 ± 5.3 Ma. Whole rock Sm-Nd analyses provided εNd(1,86 Ga values of -1.48 and -1.28 and TDM model ages of 2.32 and 2.25 Ga, likely indicating a Ryacian crustal source. Here we conclude that Taquaral Granite represents a magmatic episode generated at the end of the Orosirian, as a part of the Amoguija Magmatic Arc.

  17. Quantifying interference of krypton produced from neutron irradiation of inclusion-hosted and lattice-coordinated bromine with 40Ar/39Ar geochronology (United States)

    Rutte, Daniel; Becker, Tim A.; Renne, Paul R.


    Various interfering reactions producing Ar isotopes during neutron irradiation from Cl, Ar, K, and Ca have been previously detailed with the significant ones being routinely corrected for in 40Ar/39Ar geochronology. Interference of double charged 80Kr (80Kr++) with 40Ar has not yet been considered. Significant amounts of 80Kr are produced during neutron irradiation through the reaction 79Br(n,β-)80Kr. While previous workers reported a computed fission spectrum averaged cross section of ∼48 mb-compared to ∼113 mb for 39K(n,p)39Ar-we determined a ∼33-fold higher production rate of 80Kr from Br compared to 39Ar from K in the CLICIT facility of the OSU reactor. Low-K, high-Br phases, e.g., some amphiboles, and fluid or melt inclusion rich samples are prone to 80Kr++ interference biasing 40Ar/39Ar dates in the ‰ to % range. 80Kr++ resembles excess 40Ar in step-heating experiments. The interference can be corrected for by using the parallel reaction 81Br(n,β-)82Kr.

  18. Tectonic evolution of the southern margin of the Amazonian craton in the late Mesoproterozoic based on field relationships and zircon U-Pb geochronology

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    Full Text Available New U-Pb zircon geochronological data integrated with field relationships and an airborne geophysical survey suggest that the Nova Brasilândia and Aguapeí belts are part of the same monocyclic, metaigneous and metasedimentary belt formed in the late Mesoproterozoic (1150 Ma-1110 Ma. This geological history is very similar to the within-plate origin of the Sunsás belt, in eastern Bolivia. Thus, we propose that the Nova Brasilândia, Aguapeí and Sunsás belts represent a unique geotectonic unit (here termed the Western Amazon belt that became amalgamated at the end of the Mesoproterozoic and originated through the reactivation of a paleo-suture (Guaporé suture zone in an intracontinental rift environment. Therefore, its geological history involves a short, complete Wilson cycle of ca. 40 Ma. Globally, this tectonic evolution may be related with the final breakup of the supercontinent Columbia. Mafic rocks and trondhjemites in the northernmost portion of the belt yielded U-Pb zircon ages ca. 1110 Ma, which dates the high-grade metamorphism and the closure of the rift. This indicates that the breakup of supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia at ca. 1.1-1.0 Ga and that the Western Amazon belt was formed during the accretion of the Arequipa-Antofalla basement to the Amazonian craton.

  19. Geochronology and Hf–Fe isotopic geochemistry of the Phanerozoic mafic–ultramafic intrusions in the Damiao area, northern North China Craton: Implications for lithospheric destruction

    Indian Academy of Sciences (India)

    Jiulong Zhou; Zhaohua Luo


    Timing and source of several Fe-mineralized mafic–ultramafic intrusions in the Damiao area are investigated here by coupling new geochronological and Hf–Fe isotopic data with previous results. Although regarded as a Late Paleoproterozoic assemblage previously, two ∼140 Ma intrusions are recognized by zircon U–Pb dating, indicating emplacement of these intrusions from Middle Devonian to Early Cretaceous times. Both Hf and Fe isotopic features lead to the conclusion that distinct mantle components contributed to their magma generation. As the first magmatic phase, the ∼395 Ma intrusions were mainly derived from the slightly-enriched SCLM that was prevalent during the Paleozoic. However, asthenospheric material was strongly involved in the formation of the ∼215 Ma Gaositai intrusion. Therefore, the initiation of lithospheric destruction in the northern NCC is inferred to have occurred in Late Triassic time, triggered by post-orogenic extension following the ∼250 Ma collision between the Siberian Craton and the NCC. The ∼140 Ma intrusions originated from a significantly-enriched mantle component probably resided in the predominant slightly-enriched SCLM. This mantle source would have melted in the Late Mesozoic, when the thin lithosphere enabled enhanced heat transfer from the asthenosphere. In summary, these distinct mantle sources of mafic–ultramafic magmatism provide a record of mantle heterogeneity and the gradual upward migration of the lithosphere–asthenosphere boundary during lithospheric destruction.

  20. Magmatism as a response to exhumation of the Priest River complex, northern Idaho: Constraints from zircon U-Pb geochronology and Hf isotopes (United States)

    Stevens, L. M.; Baldwin, J. A.; Crowley, J. L.; Fisher, C. M.; Vervoort, J. D.


    Zircon and monazite U-Pb geochronology and zircon Hf isotopes place constraints on the temporal and source relationships between crustal anatexis, magmatism, and exhumation of the Priest River metamorphic core complex, northern Idaho. Granitoids that intruded the migmatitic, pelitic Hauser Lake gneiss include the pluton emplacement in the Priest River complex indicates that it was primarily a response to decompression rather than a cause. The mylonitized Silver Point and undeformed Wrencoe plutons bracket the end of a rapid phase of exhumation to c. 50-48 Ma. Zircon εHf(i) values and Lu-Hf isotope evolution indicate that the Silver Point and Wrencoe plutons crystallized from homogeneous magmas sourced from Archean-Proterozoic basement orthogneisses, whereas the Spokane granite and two leucocratic units appear to have been produced by partial melting of the Hauser Lake gneiss. Comparison of the Priest River complex with other deeply exhumed northern Cordilleran complexes indicates variability in the timing and, therefore, relative influences of partial melting and magmatism on the initiation of exhumation, which must be accounted for in numerical models of metamorphic core complex formation and evolution.

  1. Polychronous (Early Cretaceous to Palaeogene) emplacement of the Mundwara alkaline complex, Rajasthan, India: 40Ar/39Ar geochronology, petrochemistry and geodynamics (United States)

    Pande, Kanchan; Cucciniello, Ciro; Sheth, Hetu; Vijayan, Anjali; Sharma, Kamal Kant; Purohit, Ritesh; Jagadeesan, K. C.; Shinde, Sapna


    The Mundwara alkaline plutonic complex (Rajasthan, north-western India) is considered a part of the Late Cretaceous-Palaeogene Deccan Traps flood basalt province, based on geochronological data (mainly 40Ar/39Ar, on whole rocks, biotite and hornblende). We have studied the petrology and mineral chemistry of some Mundwara mafic rocks containing mica and amphibole. Geothermobarometry indicates emplacement of the complex at middle to upper crustal levels. We have obtained new 40Ar/39Ar ages of 80-84 Ma on biotite separates from mafic rocks and 102-110 Ma on whole-rock nepheline syenites. There is no evidence for excess 40Ar. The combined results show that some of the constituent intrusions of the Mundwara complex are of Deccan age, but others are older and unrelated to the Deccan Traps. The Mundwara alkaline complex is thus polychronous and similar to many alkaline complexes around the world that show recurrent magmatism, sometimes over hundreds of millions of years. The primary biotite and amphibole in Mundwara mafic rocks indicate hydrous parental magmas, derived from hydrated mantle peridotite at relatively low temperatures, thus ruling out a mantle plume. This hydration and metasomatism of the Rajasthan lithospheric mantle may have occurred during Jurassic subduction under Gondwanaland, or Precambrian subduction events. Low-degree decompression melting of this old, enriched lithospheric mantle, due to periodic diffuse lithospheric extension, gradually built the Mundwara complex from the Early Cretaceous to Palaeogene time.

  2. Age of the emerald mineralization from the Itabira-Nova Era District, Minas Gerais, Brazil, based on LA-ICP-MS geochronology of cogenetic titanite

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    Hanna Jordt-Evangelista

    Full Text Available ABSTRACT: In the Itabira-Nova Era Emerald District, southeast Brazil, gemological emerald is extracted from underground mines found in schist-type deposits at the contact zone of the Archean Metavolcanosedimentary Sequence of the Guanhães Complex and Paleoproterozoic anorogenic granites of the Borrachudos Suite. Schist-type deposits are commonly generated by reactions enhanced by deformation and heat during regional metamorphism. The age of the mineralization in the region has been a matter of debate for decades: Ages ranging from the Archean to the Neoproterozoic are mentioned in the literature. In the mineralized zone from the Piteiras mine fluorine-aluminum-bearing titanite is found in metamafic rocks. The fluorine content was probably derived from the Borrachudos granites and pegmatites like the beryllium for emerald, thus both minerals could have been generated during the same event. U-Pb titanite geochronology via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS was performed on a thin section of a phlogopite-plagioclase-hornblende schist from the Piteiras mine. The determined age of 576 ± 7 Ma is also the probable age for emerald generation during the Brasiliano cycle, which was the only tectonometamorphic event postdating the intrusion of the granites. This event provided heat and fluids necessary for reactions between the Be- and the Cr-bearing rocks, thus enabling the formation of emeralds.

  3. Constraints on the collision and the pre-collision tectonic configuration between India and Asia from detrital geochronology, thermochronology, and geochemistry studies in the lower Indus basin, Pakistan (United States)

    Zhuang, Guangsheng; Najman, Yani; Guillot, Stephane; Roddaz, Martin; Antoine, Pierre-Olivier; Metais, Gregoire; Carter, Andrew; Marivaux, Laurent; Solangi, Sarfraz


    The timing of India-Asia suturing in the Western Himalaya is complex, with the relative timings of collision between the Indian plate and Asian plate with the Kohistan Island arc and a proposed Tethyan Himalayan microcontinent, debated. Here we present an integrated provenance study of geochronology, thermochronology, and geochemistry on the late Cretaceous-Pleistocene sediments from the lower Indus basin on the Indian plate. The detrital zircon U-Pb and fission track data show a reversal in sediment source from a pure Indian signature to increasing inputs from the suture zone and the Asian plate between the middle Paleocene and early Oligocene. The Nd and Sr isotopes narrow down this change to 50 Ma by revealing input of Asian detritus and the establishment of a Nd & Sr isotopic pattern similar to that of the present-day Indus Fan by 50 Ma, with no significant variations up section, contrary to what might be expected if later major collisions had occurred. Our isotopic data indicate that since 50 Ma, Greater India was occupied by a fluvial-deltaic system, analogous to the present-day Indus and named as the Paleo-Indus, which has been transporting Asian detritus southward across the suture zone and Kohistan-Ladakh arc. This suggests that no other ocean basins were located between India and Asia after this time in this region. Our data require that in the west, the India-Asia collision was accomplished by ˜50 Ma.

  4. Crustal Evolution of a Paleozoic Intra-oceanic Island-Arc-Back-Arc Basin System Constrained by the Geochemistry and Geochronology of the Yakuno Ophiolite, Southwest Japan

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    Yoshimitsu Suda


    Full Text Available The Yakuno ophiolite in southwest Japan is considered to have been obducted by the collision between an intra-oceanic island-arc-back-arc basin (intra-OIA-BAB system and the East Asian continent during the late Paleozoic. New SIMS (SHRIMP zircon U-Pb determinations for amphibolite and metagabbro of BAB origin within the Yakuno ophiolite yield ages of 293.4 ± 9.5 Ma and 288 ± 13 Ma, respectively. These ages are slightly older (however, overlapping within analytical errors than the magmatic age of arc granitoids (ca. 285–282 Ma that intruded into the mafic rocks of BAB origin. Results from geochronological and geochemical data of the Yakuno ophiolite give rise to the following tentative geotectonic model for the Paleozoic intra-OIA-BAB system: the initial stage of BAB rifting (ca. 293–288 Ma formed the BAB crust with island-arc basalt (IAB signatures, which was brought to the OIA setting, and generated the arc granitoids (ca. 285–282 Ma by anatexis of the BAB crust. A later stage of BAB rifting (

  5. U-Pb zircon geochronology and Sr-Nd isotopic composition of the Inchope orthogneiss in Mozambique: Age constraints and petrogenetic implications (United States)

    Manjate, Vicente Albino


    The Inchope orthogneiss comprises a mesoproterozoic group of variously deformed and migmatised orthogneisses in the Chimoio group. This area is well known for its numerous, small pegmatite deposits with cassiterite and columbite. Zircon U-Pb geochronological and whole rock Sr-Nd isotope data are reported for five Inchope orthogneiss samples. The zircon U-Pb data exhibit one period of crystallization between 1065 and 1053 Ma and two metamorphic ages of 956 Ma and 484 Ma. The Inchope orthogneiss displays evolved Nd isotopic compositions with ɛNdi between -11.7 and -13.3, 87Sr/86Sri between 0.7117 and 0.7209 and TDM values of between 2.3 and 2.4 Ga. Therefore, the Inchope orthogneiss crystallized in Mesoproterozoic from the paleoproterozoic metapelites along the eastern margin of the archaen Zimbabwean craton. This was followed by pegmatite veins intrusions and Pan-African tectonometamorphic reworking. These features are typical of S-type and calc-alkaline granites in continental margin arcs.

  6. Isotope Geochronology: Technique and Application%同位素地质年龄测定技术及应用

    Institute of Scientific and Technical Information of China (English)

    陈文; 万渝生; 李华芹; 张宗清; 戴檀谟; 施泽恩; 孙敬博


    了困扰,有时不能给出正确可信的年龄.⑤Re-Os法:是目前能够直接测定金属矿床矿化年龄的唯一成熟方法.但在实验技术和应用方面还存在不少问题:a.近年来发现有些金属矿床辉钼矿的Re-Os年龄高于其赋矿围岩的年龄,原因不明;b.黄铁矿等多数硫化物矿物含Re量很低,并含有一定程度的普通Os,对样品化学制备过程中低本底的要求很高,一般实验室难以达到,普通Os也难以准确扣除;c.后期的热液活动有时可以使Os同位素发生重置,因此,金属硫化物Re-Os同位素体系封闭温度及其影响因素是一个亟待解决的问题.⑥(U-Th)/He法:(U-Th)/He同位素系统的优势是其封闭温度是已有同位素体系中最低的,能够记录地质体经历较低温度范围的时代与温度信息.该方法在矿床年代学研究中也具有可观应用前景.其不足之处是因为封闭温度很低,在用于地质体定年时要特别关注冷却速率和再加热作用的影响.%Isotope geochronology is a newly developed interdisciplinary science which combines geoscience, physics, chemistry and technical sciences, and is a young and energetic branch of earth science. The decay of radioactive isotopes is used to determine the formation age of geological body and the age of geological events, with aiming to study the formation history and evolution of earth and planetary materials. This paper presents brief introduction and summary of relative high-precise isotope dating methods in terms of theory, experiment techniques, application scope, precautions for use, in hope of providing a useful reference to geologist. Main isotope dating methods involved s are U-Pb, Ar-Ar, Rb-Sr, Sm-Nd, Re-Os and (U-Th)/He methods. (l)U-Pb method: It is one of the earliest radioactive methods for determination of geological age as well as the most important isotope dating method in China so far. In the past 10 years, the introduction of zircon U-Pb dating technique

  7. Geochemistry and Sr-Nd isotopes of the subvolcanic sill complex and sandstone geochronology from María Magdalena island, Nayarit, Mexico (United States)

    Villanueva, D.; Schaaf, P. E.; Hernandez, T.; Solis, G.; Weber, B.; Pompa, V.


    María Magdalena island is part of the Islas Marías archipielago, located at the mouth of the Gulf of California. Understanding the nature and origin of the archipelago is very important for reconstructing the paleoposition of Baja California Peninsula prior to the opening of the Gulf of California. We present the first geochemical, isotopic and geochronologic data from María Magdalena, a lithologically different island compared to the rest of the archipelago. María Magdalena island is located southeast of María Madre and northeast of María Cleofas islands and is composed by a sedimentary sequence of sandstones and minor shale, which is intruded by gabbroic sills. The sedimentary sequence dips approximately 20 degrees to the NW. The thickness of sills range from 1 to 3 meters with mineralogical variations of plagioclase +/- orthopyroxene +/- clinopyroxene +/- hornblende and some altered olivine crystals. Textures are mostly porphyritic with plagioclase crystals sizes up to 5 cm and olivines up to 5 mm. The gabbroic sills show SiO2 contents from 42.7 to 47.5 wt. %; TiO2 from 0.8 to 2 wt. %; Fe2O3t from 7.7 to 11.9 wt. %; MgO from 6.2 to 19.8 wt. % and of CaO from 6 to 11.6 wt. %, indicating mafic to ultramafic compositions. A multielement spider diagram as well as REE patterns show compositions very similar to N-MORB or even peridotites, which is confirmed by 87Sr/86Sr values from 0.70273 to 0.70497, and 143Nd/144Nd values from 0.513003 to 0.513100. U-Pb single zircon geochronology of the intruded sandstones display the following age distribution: eight crystals show ages from 80 to 86 Ma, three crystals have ages from 61 to 72 Ma, and three are around 21 Ma which constrains a maximum sandstone deposition age. Consequently, the sills must be younger than 21 Ma. There is not much lithological similarity with neighboring María Madre island to the NW (containing a metamorphic complex, granitoids and acid volcanic roks) and with María Cleofas island to the SE

  8. Combined garnet and zircon geochronology and trace elements studies - constraints of the UHP-(U)HT evolution of Orlica-Śnieżnik Dome (NE Bohemian Massif). (United States)

    Walczak, Katarzyna; Anczkiewicz, Robert; Szczepański, Jacek; Rubatto, Daniela


    The Orlica-Śnieżnik Dome (OSD), located on the NE margin of the Bohemian Massif, is predominantly composed of amphibolite-facies orthogneiss that contain bodies of HP and UHP eclogites and granulites. Numerous geochronological studies have been undertaken to constrain the timing of the ultra-high grade metamorphic event. Despite this, the exact timing of UHP-(U)HT conditions remain dubious (e.g. Brueckner et al., 1991; Anczkiewicz et al., 2007; Bröcker et al., 2009 & 2010). We have utilized garnet and zircon geochronology to provide time constraints on the evolution of the UHT-(U)HP rocks of the OSD. We have combined the ages with trace element analyses in garnet and zircon to better understand the significance of the obtained ages in petrological context. Lu-Hf grt-wr dating of peritectic garnet from two felsic granulites constrained the time of its initial growth at 346.9 ± 1.2 and 348.3 ± 2.0 Ma, recording peak conditions of 2.7 GPa and 950°C (e.g. Ferrero et al., 2015). In situ U-Pb SHRIMP dating of zircon from the same granulite gave a younger age of 342.2 ± 3.4 Ma. HREE partitioning between garnet rim and metamorphic zircon indicate their growth in equilibrium, hence, the U-Pb zircon date constrains the terminal phase of garnet crystallization. Similar ages were obtained from two eclogite bodies from Międzygórze and Nowa Wieś localities; Lu-Hf (grt-cpx-wr) dating provided ages of 346.5 ± 2.4 and 348.1 ± 9.1 Ma for samples from Międzygórze and Nowa Wieś, respectively. The same age (within error) of 346.3 ± 5.2 Ma was reported by Bröcker et al. (2010) for zircon from the Międzygórze eclogite. Comparison of REE concentrations in garnet (this study) and in metamorphic zircon (reported in Bröcker et al., 2010) indicate that garnet and zircon crystallized in equilibrium. Furthermore, M-HREE patterns observed in both garnet and zircon strongly suggest their growth at eclogite facies conditions. Sm-Nd garnet ages obtained for both felsic and mafic

  9. Geochronological, elemental and Sr-Nd-Hf-O isotopic constraints on the petrogenesis of the Triassic post-collisional granitic rocks in NW Thailand and its Paleotethyan implications (United States)

    Wang, Yuejun; He, Huiying; Cawood, Peter A.; Srithai, Boontarika; Feng, Qinglai; Fan, Weiming; Zhang, Yuzhi; Qian, Xin


    New U-Pb geochronological, petrologic, elemental and Sr-Nd-Hf-O isotopic data for the granites from the Inthanon and Sukhothai zones in NW Thailand in conjunction with correlations with SW China are presented to constrain the age and position of the Paleotethys Ocean in this region and the associated assembly of Southeast Asia. The geochronological data show that the granitic rocks in the Inthanon and Sukhothai zones, herein named Group 1 and Group 2 granites, respectively, yield similar crystallization ages of 230-200 Ma. Group 1 samples are characterized by monzogranite and granite with I- and S-type geochemical affinity and Group 2 samples by I-type monzogranite and granodiorite. They have generally similar chondrite-normalized REE and PM-normalized multi-element patterns but distinct Sr-Nd-Hf-O isotopic compositions. Group 1 samples have slightly higher initial 87Sr/86Sr ratios (0.7111-0.7293) but lower εNd(t) values (- 11.1 to - 14.1) than those of Group 2 samples (87Sr/86Sr(i) = 0.7073-0.7278 and εNd(t) = - 8.3 to - 11.0). Group 1 samples show the lower εHf(t) values (- 5.4 to - 18.2), older TDM (1.62-2.40 Ga) and higher δ18O values (+ 7.95 to + 9.94) than those of Group 2 samples (εHf(t) of - 11.1 to + 4.80, TDM of 0.96-1.95 Ga and δ18O of + 4.95 to + 7.98) for the Triassic crystallization zircons. These geochemical signatures are similar to the Kwangsian and Indosinian granites in the South China and Indochina blocks but distinct from those of the Gangdese I-type granite and Sibumasu Paleozoic granite. Our data suggest that Group 1 samples mainly originated from the early Paleozoic supracrustal rocks containing metapelite and metavolcanic components, which had previously experienced the surface weathering. Group 2 samples were derived from a hybridized source of an old metamorphic and a newly underplated mafic component. Synthesis of our data with available regional observations indicates that the Inthanon zone represents the main suture zone of the

  10. Mineral chemistry and shrimp U-Pb Geochronology of mesoproterozoic polycrase-titanite veins in the sullivan Pb-Zn-Ag Deposit, British Columbia (United States)

    Slack, J.F.; Aleinikoff, J.N.; Belkin, H.E.; Fanning, C.M.; Ransom, P.W.


    Small polycrase-titanite veins 0.1-2 mm thick cut the tourmalinite feeder zone in the deep footwall of the Sullivan Pb-Zn-Ag deposit, southeastern British Columbia. Unaltered, euhedral crystals of polycrase and titanite 50-100 ??m in diameter are variably replaced by a finer-grained alteration-induced assemblage composed of anhedral polycrase and titanite with local calcite, albite, epidote, allanite, and thorite or uranothorite (or both). Average compositions of the unaltered and altered polycrase, as determined by electron-microprobe analysis, are (Y0.38 REE0.49 Th0.10 Ca0.04 Pb0.03 Fe0.01U0.01) (Ti1.48 Nb0.54 W0.04 Ta0.02)O6 and (Y0.42 REE0.32 Th0.15 U0.06 Ca0.04 Pb0.01 Fe0.01) (Ti1.57 Nb0.44 W0.04 Ta0.02)O6, respectively. The unaltered titanite has, in some areas, appreciable F (to 0.15 apfu), Y (to 0.40 apfu), and Nb (to 0.13 apfu). SHRIMP U-Pb geochronology of eight grains of unaltered polycrase yields a weighted 207Pb/206Pb age of 1413 ?? 4 Ma (2??) that is interpreted to be the age of vein formation. This age is 50-60 m.y. younger than the ca. 1470 Ma age of synsedimentary Pb-Zn-Ag mineralization in the Sullivan deposit, which is based on combined geological and geochronological data. SHRIMP ages for altered polycrase and titanite suggest later growth of minerals during the ???1370-1320 Ma East Kootenay and ???1150-1050 Ma Grenvillian orogenies. The 1413 ?? 4 Ma age for the unaltered polycrase in the veins records a previously unrecognized post-ore (1370 Ma) mineralizing event in the Sullivan deposit and vicinity. The SHRIMP U-Pb age of the polycrase and high concentrations of REE, Y, Ti, Nb, and Th in the veins, together with elevated F in titanite and the absence of associated sulfides, suggest transport of these high-field-strength elements (HFSE) by F-rich and S-poor hydrothermal fluids unrelated to the fluids that formed the older Fe-Pb-Zn-Ag sulfide ores of the Sullivan deposit. Fluids containing abundant REE, HFSE, and F may have been derived from a

  11. Petrogenesis of the igneous Mucajaí AMG complex, northern Amazonian craton — Geochemical, U-Pb geochronological, and Nd-Hf-O isotopic constraints (United States)

    Heinonen, A. P.; Fraga, L. M.; Rämö, O. T.; Dall'Agnol, R.; Mänttäri, I.; Andersen, T.


    The ca. 1525 Ma igneous Mucajaí anorthosite-monzonite-granite (AMG) complex in northern Brazil is a rare manifestation of Mesoproterozoic intraplate magmatism in the northern Amazonian Craton. The complex comprises a two-phase rapakivi granite batholith with subordinate quartz-fayalite monzonites and syenites and the closely associated Repartimento anorthosite. Zircon U-Pb (ID-TIMS) geochronology reveals that the anorthosite (1526 ± 2 Ma), monzonite (1526 ± 2 Ma), and the main-phase biotite-hornblende granite (1527 ± 2 Ma) of the complex intruded the Paleoproterozoic (~ 1.94 Ga) country rocks simultaneously at ~ 1526 Ma and that the more evolved biotite granite is marginally younger at 1519 ± 2 Ma. Intraplate magmatism in the Mucajaí region was relatively short-lived and lasted 12 million years (1529-1517 Ma) at maximum. The Nd (whole-rock, ID-TIMS; ɛNd from - 1.9 to - 2.8), Hf (zircon, LAM-ICP-MS; ɛHf from - 2.0 to - 3.1), and O (zircon, SIMS; δ18O from 6.1 to 7.0‰) isotopic compositions of the studied rocks are fairly uniform but still reveal a small degree of isotopic heterogeneity in the Paleoproterozoic crust enclosing the complex. The small isotopic differences observed in the two types of rapakivi granites (biotite-hornblende granite and biotite granite) may result either from an isotopically heterogeneous lower crustal source or, more likely, from contamination of the granitic magma derived from a lower crustal source during prolonged residence at upper crustal levels.

  12. Petrology, magnetostratigraphy and geochronology of the Miocene volcaniclastic Tepoztlán Formation: implications for the initiation of the Transmexican Volcanic Belt (Central Mexico) (United States)

    Lenhardt, Nils; Böhnel, Harald; Wemmer, Klaus; Torres-Alvarado, Ignacio S.; Hornung, Jens; Hinderer, Matthias


    The volcaniclastic Tepoztlán Formation (TF) represents an important rock record to unravel the early evolution of the Transmexican Volcanic Belt (TMVB). Here, a depositional model together with a chronostratigraphy of this Formation is presented, based on detailed field observations together with new geochronological, paleomagnetic, and petrological data. The TF consists predominantly of deposits from pyroclastic density currents and extensive epiclastic products such as tuffaceous sandstones, conglomerates and breccias, originating from fluvial and mass flow processes, respectively. Within these sediments fall deposits and lavas are sparsely intercalated. The clastic material is almost exclusively of volcanic origin, ranging in composition from andesite to rhyolite. Thick gravity-driven deposits and large-scale alluvial fan environments document the buildup of steep volcanic edifices. K-Ar and Ar-Ar dates, in addition to eight magnetostratigraphic sections and lithological correlations served to construct a chronostratigraphy for the entire Tepoztlán Formation. Correlation of the 577 m composite magnetostratigraphic section with the Cande and Kent (1995) Geomagnetic Polarity Time Scale (GPTS) suggests that this section represents the time intervall 22.8-18.8 Ma (6Bn.1n-5Er; Aquitanian-Burdigalian, Lower Miocene). This correlation implies a deposition of the TF predating the extensive effusive activity in the TMVB at 12 Ma and is therefore interpreted to represent its initial phase with predominantly explosive activity. Additionally, three subdivisions of the TF were established, according to the dominant mode of deposition: (1) the fluvial dominated Malinalco Member (22.8-22.2 Ma), (2) the volcanic dominated San Andrés Member (22.2-21.3 Ma) and (3) the mass flow dominated Tepozteco Member (21.3-18.8 Ma).

  13. IGSN at Work in the Land Down Under: Exploiting an International Sample Identifier System to Enhance Reproducibility of Australian Geochemcial and Geochronological Data. (United States)

    Bastrakova, I.; Klump, J. F.; McInnes, B.; Wyborn, L. A.; Brown, A.


    The International Geo-Sample Number (IGSN) provides a globally unique identifier for physical samples used to generate analytical data. This unique identifier provides the ability to link each physical sample to any analytical data undertaken on that sample, as well as to any publications derived from any data derived on the sample. IGSN is particularly important for geochemical and geochronological data, where numerous analytical techniques can be undertaken at multiple analytical facilities not only on the parent rock sample itself, but also on derived sample splits and mineral separates. Australia now has three agencies implementing IGSN: Geoscience Australia, CSIRO and Curtin University. All three have now combined into a single project, funded by the Australian Research Data Services program, to better coordinate the implementation of IGSN in Australia, in particular how these agencies allocate IGSN identifiers. The project will register samples from pilot applications in each agency including the CSIRO National Collection of Mineral Spectra database, the Geoscience Australia sample collection, and the Digital Mineral Library of the John De Laeter Centre for Isotope Research at Curtin University. These local agency catalogues will then be aggregated into an Australian portal, which will ultimately be expanded for all geoscience specimens. The development of this portal will also involve developing a common core metadata schema for the description of Australian geoscience specimens, as well as formulating agreed governance models for registering Australian samples. These developments aim to enable a common approach across Australian academic, research organisations and government agencies for the unique identification of geoscience specimens and any analytical data and/or publications derived from them. The emerging pattern of governance and technical collaboration established in Australia may also serve as a blueprint for similar collaborations internationally.

  14. What was the Paleogene latitude of the Lhasa terrane? A reassessment of the geochronology and paleomagnetism of Linzizong volcanic rocks (Linzhou basin, Tibet) (United States)

    Huang, Wentao; Dupont-Nivet, Guillaume; Lippert, Peter C.; van Hinsbergen, Douwe J. J.; Dekkers, Mark J.; Waldrip, Ross; Ganerød, Morgan; Li, Xiaochun; Guo, Zhaojie; Kapp, Paul


    The Paleogene latitude of the Lhasa terrane (southern Tibet) can constrain the age of the onset of the India-Asia collision. Estimates for this latitude, however, vary from 5°N to 30°N, and thus, here, we reassess the geochronology and paleomagnetism of Paleogene volcanic rocks from the Linzizong Group in the Linzhou basin. The lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes. We report consistent 40Ar/39Ar and U-Pb zircon dates of 52 Ma for the upper Linzizong, and 40Ar/39Ar dates ( 51 Ma) from the lower Linzizong are significantly younger than U-Pb zircon dates (64-63 Ma), suggesting that the lower Linzizong was thermally and/or chemically reset. Paleomagnetic results from 24 sites in lower Linzizong confirm a low apparent paleolatitude of 5°N, compared to the upper part ( 20°N) and to underlying Cretaceous strata ( 20°N). Detailed rock magnetic analyses, end-member modeling of magnetic components, and petrography from the lower and upper Linzizong indicate widespread secondary hematite in the lower Linzizong, whereas hematite is rare in upper Linzizong. Volcanic rocks of the lower Linzizong have been hydrothermally chemically remagnetized, whereas the upper Linzizong retains a primary remanence. We suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower Linzizong. We estimate that the Paleogene latitude of Lhasa terrane was 20 ± 4°N, consistent with previous results suggesting that India-Asia collision likely took place by 52 Ma at 20°N.

  15. Structural evolution of the Yeongwol thrust system, northeastern Okcheon fold-thrust belt, Korea: Insights from structural interpretations and SHRIMP U-Pb and K-Ar geochronology (United States)

    Jang, Yirang; Kwon, Sanghoon


    The NE-trending Okcheon Belt is a prominent fold-thrust belt preserved in the Korean Peninsula. In the Yeongwol area, the northeastern Okcheon Belt, the Cambrian-Ordovician (possibly to Silurian) Joseon Supergroup overlies the Carboniferous-Permian (possibly to early Triassic) Pyeongan Supergroup and/or Jurassic Bansong Group by N-S trending thrust faults, having highly connected traces in map view. To understand the structural geometry of these thrust faults and their evolution history, we have conducted structural analyses, together with SHRIMP U-Pb zircon and K-Ar illite age datings. The results show that (1) the thrusts in the Yeongwol area, carrying the lower Paleozoic strata over the upper Paleozoic or Mesozoic strata, are defined as the Yeongwol thrust system. The closed-loops map patterns of this system can further be interpreted by alternative duplex models in terms of a hinterland dipping duplex vs. a combination of major thrusts and connecting splays; (2) newly obtained SHRIMP U-Pb zircon ages from a dike and synorogenic sediments and K-Ar illite ages from fault gouges, together with previously reported evidences form the Yeongwol area, suggest multiple events after Permo-Triassic to early Neogene. The SHRIMP U-Pb detrital zircon ages from the lower Paleozoic rocks of the Yeongwol area can provide tectono-stratigraphic information of this area before the Permian. These further indicate the broader implications in that how detailed structural interpretations supported by the geochronological data can help to understand the tectonic evolution of the Okcheon Belt as well as the fold-thrust belts in general.

  16. Geology, petrology, U-Pb (SHRIMP) geochronology of the Morrinhos granite - Paragua terrane, SW Amazonian craton: implications for the magmatic evolution of the San Ignacio orogeny

    Energy Technology Data Exchange (ETDEWEB)

    Franca, Ohana; Ruiz, Amarildo Salina; Sousa, Maria Zelia Aguiar de, E-mail:, E-mail:, E-mail: [Universidade Federal de Mato Grosso (UFMT), Cuiaba, MT (Brazil). Instituto de Ciencias Exatas e da Terra. Dept. de Geologia Geral; Batata, Maria Elisa Froes, E-mail: [Universidade Federal de Mato Grosso (UFMT), Cuiaba, MT (Brazil). Grupo de Pesquisa em Evolucao Crustal e Tectonica; Lafon, Jean-Michel [Universidade Federal do Para (GEOCIAM/UFPA), Belem, PR (Brazil). Inst. Nacional de Cencia e Tecnologia de Geociencias da Amazonia


    Morrinhos granite is a batholith body that is slightly elongated in the NNW direction and approximately 1,140 km{sup 2} long; it is located in the municipality of Vila Bela da Santissima Trindade of the state of Mato Grosso, Brazil, in the Paragua Terrane, Rondonian-San Ignacio Province, in the SW portion of the Amazonian Craton. This intrusion displays a compositional variation from tonalite to monzogranite, has a medium to coarse inequigranular texture and is locally porphyritic; biotite is the predominant mafic in one of the facies, and hornblende is predominant in the other, with both metamorphosed into the green schist facies. The studied rocks characterize an intermediate to acidic sequence that was formed by a subalkaline magmatism; the series is alkali-calcic to metaluminous to slightly peraluminous, and the rocks evolved through fractioned crystallization mechanisms. The structural data show two deformation phases represented by penetrative foliation (S{sub 1}) and open folds (D{sub 2}), and both phases were most likely related to the San Ignacio Orogeny. The geochronological (U-Pb SHRIMP) and isotopic (Sm-Nd) investigations of these rocks indicated a crystallization age of 1350±12Ma, T{sub DM} of approximately 1.77 Ga and εNd{sub (1.35}) with a negative value of -2.57, suggesting that their generation was related to a partial melting process of a Paleoproterozoic (Statherian) continental crust. The results herein indicate that the Morrinhos granite was generated in a continental magmatic arc in a late- to post-orogenic stage of the San Ignacio Orogeny, and it can be recognized as belonging to the Pensamiento Intrusive Suite. (author)

  17. Structural and Geochronological Evidence for Multiple Episodes of Tertiary Deformation along the Ailaoshan-Red River Shear Zone,Southeastern Asia, Since the Paleocene

    Institute of Scientific and Technical Information of China (English)


    Structural analyses show that the Ailaoshan-Red River shear zone (ASRRSZ) in Ailao Mountain is composed of three different deformational domains. These domains may represent three episodes of left-lateral slip experienced by the ASRRSZ. The first episode of such deformation occurred throughout the eastern high-grade belt of the ASRRSZ under a transtensional regime and produced Ltype tectonites of amphibolite grade. The second episode of left-lateral slip formed high strain zones overprinting the high-grade belt. Its deformational mechanism is similar to simple shear and the deformed rocks are L-S mylonites of greenschist grade. The third episode of left-lateral slip took place chiefly in a western low-grade belt of the ASRRSZ. This deformation occurred in a transpressional regime, formed an overall structure pattern of a sinistral thrust system and produced phyllonites of low-greenschist grade. Geochronological data indicated that the three episodes of left-lateral slip happened before ~58-56 Ma, at least from ~27 Ma to 22 Ma and at ~13-12 Ma respectively. The first episode of slip in the ASRRSZ appeared to correspond to the initial collision of India and Asia at ~60Ma. The second episode took place almost at the same time as the most intensive compression and uplift in Tibet. The latest event might represent a further eastward material flow in Tibet after ~16-13 Ma.Thus, the ASRRSZ of southeastern Asia probably experienced three main episodes of Tertiary leftlateral slip in the course of intracontinental convergence since the India-Asia collision.

  18. U-Pb geochronology and geochemistry of Bibi-Maryam pluton, eastern Iran: Implication for the late stage of the tectonic evolution of the Sistan Ocean (United States)

    Delavari, Morteza; Amini, Sadraddin; Schmitt, Axel K.; McKeegan, Kevin D.; Mark Harrison, T.


    The Bibi-Maryam pluton crops out in the Sistan suture zone, eastern Iran. This pluton is a 1.5 × 2 km stock composed of leucocratic tonalite, granodiorite and granite. U-Pb zircon geochronology of a leucogranite indicates an emplacement age of 58.6 ± 2.1 Ma (95% confidence). The Bibi-Maryam rock suite is sodic with elevated Na2O/K2O (2.9 to 5.5), Sr/Y (15.6-62.2), La/Yb (13.3-22.2), and low MgO (0.86-1.81) abundances. It lacks significant Eu anomalies. Because of these geochemical characteristics, Bibi-Maryam rocks are similar to high-SiO2 adakites. Trace element modeling indicates that the Bibi-Maryam adakitic rocks could be produced by 5-8% non-modal batch partial melting from a source with composition of 95% N-MORB + 5% sediment in the presence of 35-40% amphibole + 5-10% garnet + 55-60% clinopyroxene + 1% apatite + 1% rutile. This source mineralogy is similar to hornblende eclogite or garnet amphibolites. Collectively, these data provide new constraints for the evolution of the Sistan suture zone and suggest that the Bibi-Maryam pluton formed via slab melting in an oceanic arc and pre-plate collision tectonic setting. This implies that the closure of the Sistan Ocean and Lut-Afghan continental blocks collision happened after the Bibi-Maryam emplacement at 58.6 ± 2.1 Ma.

  19. Geochronological, morphometric and geochemical constraints on the Pampas Onduladas long basaltic flow (Payún Matrú Volcanic Field, Mendoza, Argentina) (United States)

    Espanon, Venera R.; Chivas, Allan R.; Phillips, David; Matchan, Erin L.; Dosseto, Anthony


    The Pampas Onduladas flow in southern Mendoza, Argentina, is one of the four longest Quaternary basaltic flows on Earth. Such flows (> 100 km) are relatively rare on Earth as they require special conditions in order to travel long distances and there are no recent analogues. Favourable conditions include: a gentle topographic slope, an insulation process to preserve the melt at high temperature, and a large volume of lava with relatively low viscosity. This study investigates the rheological and geochemical characteristics of the ~ 170 km long Pampas Onduladas flow, assessing conditions that facilitated its exceptional length. The study also reports the first geochronological results for the Pampas Onduladas flow. 40Ar/39Ar step-heating analyses of groundmass reveal an eruption age of 373 ± 10 ka (2σ), making the Pampas Onduladas flow the oldest Quaternary long flow. The methods used to assess the rheological properties include the application of several GIS tools to a digital elevation model (DEM) to determine the length, width, thickness, volume and topographic slope of the flow as well as algorithms to determine its density, viscosity and temperature. The slope of the Pampas Onduladas flow determined from the initial part of the flow on the eastern side of La Carbonilla Fracture to its end point in the province of La Pampa is 0.84% (0.29°), the steepest substrate amongst long Quaternary flows. The rheological properties, such as density viscosity and temperature from the Pampas Onduladas flow are similar to values reported for other long Quaternary flows. However, the minimum volume calculated is relatively low for its length compared with other long Quaternary flows. Therefore, the extension of the Pampas Onduladas flow was probably controlled by a steep slope, combined with an insulating mechanism, which helped in providing optimal conditions for a travel length of almost 170 km.

  20. Paleoproterozoic mojaveprovince in northwestern Mexico? Isotopic and U-Pb zircon geochronologic studies of precambrian and Cambrian crystalline and sedimentary rocks, Caborca, Sonora (United States)

    Lang, Farmer G.; Bowring, S.A.; Matzel, J.; Maldonado, G.E.; Fedo, C.; Wooden, J.


    Whole-rock Nd isotopic data and U-Pb zircon geochronology from Precambrian crystalline rocks in the Caborca area, northern Sonora, reveal that these rocks are most likely a segment of the Paleoproterozoic Mojave province. Supporting this conclusion are the observations that paragneiss from the ??? 1.75 Ga Bamori Complex has a 2.4 Ga Nd model age and contains detrital zircons ranging in age from Paleo- proterozoic (1.75 Ga) to Archean (3.2 Ga). Paragneisses with similar age and isotopic characteristics occur in the Mojave province in southern California. In addition, "A-type" granite exposed at the southern end of Cerro Rajon has ca 2.0 Ga Nd model age and a U-Pb zircon age of 1.71 Ga, which are similar to those of Paleoproterozoic granites in the Mojave province. Unlike the U.S. Mojave province, the Caborcan crust contains ca. 1.1 Ga granite (Aibo Granite), which our new Nd isotopic data suggest is largely the product of anatexis of the local Precambrian basement. Detrital zircons from Neoproterozoic to early Cambrian miogeoclinal arenites at Caborca show dominant populations ca. 1.7 Ga, ca. 1.4 Ga, and ca. 1.1 Ga, with subordinate Early Cambrian and Archean zircons. These zircons were likely derived predominately from North American crust to the east and northeast, and not from the underlying Caborcan basement. The general age and isotopic similarities between Mojave province basement and overlying miogeoclinal sedimentary rocks in Sonora and southern California is necessary, but not sufficient, proof of the hypothesis that Sonoran crust is allochthonous and was transported to its current position during the Mesozoic along the proposed Mojave-Sonora megashear. One viable alternative model is that the Caborcan Precambrian crust is an isolated, autochthonous segment of Mojave province crust that shares a similar, but not identical, Proterozoic geological history with Mojave province crust found in the southwest United States ?? 2005 Geological Society of America.

  1. Fabrics and geochronology of the Wushan ductile shear zone: Tectonic implications for the Shangdan suture zone in the Qinling orogen, Central China (United States)

    Liang, Xiao; Sun, Shengsi; Dong, Yunpeng; Yang, Zhao; Liu, Xiaoming; He, Dengfeng


    The ductile shearing along the Shangdan suture zone during the Paleozoic time is a key to understand the collisional deformation and tectonic regime of amalgamation between the North China Block and the South China Blocks. The Wushan ductile shear zone, a branch of the Shangdan suture, records mylonitic deformation that affected granitic and felsic rocks outcropping in an over 1 km wide belt in the western Qinling Orogenic belt. Shear sense indicators and kinematic vorticity number (0.79-0.99) of the mylonites reveal a dextral shear deformation. The quartz c-axis fabrics indicate activation of combined basal and rhomb slip, prism slip and prism slip. The dynamic recrystallization of quartz is accommodated by combined subgrain rotation and grain boundary migration. These characteristics suggest that the mylonites experienced ductile shear deformation under amphibolite facies conditions at temperatures of ∼500-650 °C. Zircons from granitic mylonite yield a U-Pb age of 910 ± 4.8 Ma, which represents the formation age of the protolith of the mylonite. The ductile shear zone was intruded by a granitic dyke, which yields a zircon U-Pb age of 403 ± 3.5 Ma constraining the minimum age of the ductile shear deformation. Together with regional geology and available geochronological data, these structural characteristics and ages indicate that the Wushan ductile shear zone was formed by dextral shearing following the N-S shortening as a result of collision between the North China and South China blocks along the Shangdan suture.

  2. Temporal evolution of the giant Salobo IOCG deposit, Carajás Province (Brazil): constraints from paragenesis of hydrothermal alteration and U-Pb geochronology (United States)

    deMelo, Gustavo H. C.; Monteiro, Lena V. S.; Xavier, Roberto P.; Moreto, Carolina P. N.; Santiago, Erika S. B.; Dufrane, S. Andrew; Aires, Benevides; Santos, Antonio F. F.


    The giant Salobo copper-gold deposit is located in the Carajás Province, Amazon Craton. Detailed drill core description, petrographical studies, and U-Pb SHRIMP IIe and LA-ICP-MS geochronology unravel its evolution regarding the host rocks, hydrothermal alteration and mineralization. Within the Cinzento Shear Zone, the deposit is hosted by orthogneisses of the Mesoarchean Xingu Complex (2950 ± 25 and 2857 ± 6.7 Ma) and of the Neoarchean Igarapé Gelado suite (2763 ± 4.4 Ma), which are crosscut by the Old Salobo granite. Remnants of the Igarapé Salobo metavolcanic-sedimentary sequence are represented by a quartz mylonite with detrital zircon populations (ca. 3.1-3.0, 2.95, 2.86, and 2.74 Ga). High-temperature calcic-sodic hydrothermal alteration (hastingsite-actinolite) was followed by silicification, iron-enrichment (almandine-grunerite-magnetite), tourmaline formation, potassic alteration with biotite, copper-gold ore formation, and later Fe-rich hydrated silicate alteration. Myrmekitic bornite-chalcocite and magnetite comprise the bulk of copper-gold ore. All these alteration assemblages have been overprinted by post-ore hematite-bearing potassic and propylitic alteration, which is also recognized in the Old Salobo granite. In the central zone of the deposit the mylonitized Igarapé Gelado suite rocks yield an age of 2701 ± 30 Ma. Zircon ages of 2547 ± 5.3 and 2535 ± 8.4 Ma were obtained for the Old Salobo granite and for the high-grade copper ore, respectively. A U-Pb LA-ICP-MS monazite age (2452 ± 14 Ma) from the copper-gold ore indicates hydrothermal activity and overprinting in the Siderian. Therefore, a protracted tectono-thermal event due to the reactivation of the Cinzento Shear Zone is proposed for the evolution of the Salobo deposit.

  3. Geochemistry, geochronology, and tectonic setting of Early Cretaceous volcanic rocks in the northern segment of the Tan-Lu Fault region, northeast China (United States)

    Ling, Yi-Yun; Zhang, Jin-Jiang; Liu, Kai; Ge, Mao-Hui; Wang, Meng; Wang, Jia-Min


    We present new geochemical and geochronological data for volcanic and related rocks in the regions of the Jia-Yi and Dun-Mi faults, in order to constrain the late Mesozoic tectonic evolution of the northern segment of the Tan-Lu Fault. Zircon U-Pb dating shows that rhyolite and intermediate-mafic rocks along the southern part of the Jia-Yi Fault formed at 124 and 113 Ma, respectively, whereas the volcanic rocks along the northern parts of the Jia-Yi and Dun-Mi faults formed at 100 Ma. The rhyolite has an A-type granitoid affinity, with high alkalis, low MgO, Ti, and P contents, high rare earth element (REE) contents and Ga/Al ratios, enrichments in large-ion lithophile (LILEs; e.g., Rb, Th, and U) and high-field-strength element (HFSEs; e.g., Nb, Ta, Zr, and Y), and marked negative Eu anomalies. These features indicate that the rhyolites were derived from partial melting of crustal material in an extensional environment. The basaltic rocks are enriched in light REEs and LILEs (e.g., Rb, K, Th, and U), and depleted in heavy REEs, HFSEs (e.g., Nb, Ta, Ti, and P), and Sr. These geochemical characteristics indicate that these rocks are calc-alkaline basalts that formed in an intraplate extensional tectonic setting. The dacite is a medium- to high-K, calc-alkaline, I-type granite that was derived from a mixed source involving both crustal and mantle components in a magmatic arc. Therefore, the volcanic rocks along the Jia-Yi and Dun-Mi faults were formed in an extensional regime at 124-100 Ma (Early Cretaceous), and these faults were extensional strike-slip faults at this time.

  4. The Bossoroca Complex, São Gabriel Terrane, Dom Feliciano Belt, southernmost Brazil: Usbnd Pb geochronology and tectonic implications for the neoproterozoic São Gabriel Arc (United States)

    Gubert, Mauricio Lemos; Philipp, Ruy Paulo; Stipp Basei, Miguel Angelo


    Usbnd Pb LA-ICPMS geochronological analyses were carried out on zircon grains from metavolcanic rocks of the Bossoroca Complex and for one ash tuff of the Acampamento Velho Formation of the Camaquã Basin, in order to understand the evolution of the Neoproterozoic São Gabriel magmatic arc. A total of 42 analyses of igneous zircon grains were performed in three samples. The results yielded Usbnd Pb ages of 767.2 ± 2.9 Ma for the metavolcanic agglomerate (BOS-02); 765 ± 10 Ma for the metacrystal tuff (BOS-03) and 565.8 ± 4.8 Ma for the ash tuff (BOS-04). The Orogenic Cycle in Brazil is characterized by a set of orogenic belts consisting of petrotectonic associations juxtaposed by two collisional events that occurred at the end of the Neoproterozoic. In southern Brazil this orogeny formed the Dom Feliciano Belt, a unit composed of associations of rocks developed during two major orogenic events called São Gabriel (900-680 Ma) and Dom Feliciano (650-540 Ma). The main São Gabriel associations are tectonically juxtaposed as elongated strips according to the N20-30°E direction, bounded by ductile shear zones. The Bossoroca Complex comprises predominantly metavolcano-sedimentary rocks, characterized by medium-K calc-alkaline association generated in a cordillera-type magmatic arc. The volcanism occurred in sub-aerial environment, developing deposits generated by flow, resurgence and fall, sporadically interrupted by subaqueous epiclastic deposits, suggesting an arc related basin. The São Gabriel Terrane contains the petrotectonic units that represent the closure of the Charrua Ocean associated to the subduction period of the Brasiliano Orogenic Cycle in the Sul-rio-grandense Shield.

  5. Dating slate belts using 40Ar/39Ar geochronology and zircon ages from crosscutting plutons: A case study from east-central Maine, USA (United States)

    Ghanem, Hind; Kunk, Michael J.; Ludman, Allan; Bish, David L.; Wintsch, Robert P.


    We report the ages of cleavage development in a normally intractable lower greenschist facies slate belt, the Central Maine-Aroostook-Matapedia belt in east-central Maine. We have attacked this problem by identifying the minimum ages of muscovite in a regional Acadian cleavage (S1) and in a local ductile fault zone cleavage (S2) using 40Ar/39Ar geochronology and the ages of crosscutting plutons. Our success stems from the regional low-grade metamorphism of the rocks in which each crystallization event preserves a40Ar/39Ar crystallization age and not a cooling age. Evidence for recrystallization via a pressure solution mechanism comes from truncations of detrital, authigenic, and in some rocks S1 muscovite and chlorite grains by new cleavage-forming muscovite and chlorite grains. Low-blank furnace age spectra from meta-arkosic and slaty rocks climb from moderate temperature Devonian age-steps dominated by cleavage-forming muscovite to Ordovician age-steps dominated by a detrital muscovite component. S1- and S2-cleaved rocks were hornfelsed by granitoids of ∼407 and 377 Ma, respectively. The combination of these minimum ages with the maximum metamorphic crystallization ages establishes narrow constraints on the timing of these two cleavage-forming events, ∼410 Ma (S1) and ∼380 Ma (S2). These two events coincide in time with a change in the plate convergence kinematics from the arrival of the Avalon terrane (Acadian orogeny), to a right-lateral transpression arrival of the Meguma terrane in the Neoacadian orogeny.

  6. Zircon U-Pb geochronology, Sr-Nd-Hf isotopic composition and geological significance of the Late Triassic Baijiazhuang and Lvjing granitic plutons in West Qinling Orogen (United States)

    Duan, Meng; Niu, Yaoling; Kong, Juanjuan; Sun, Pu; Hu, Yan; Zhang, Yu; Chen, Shuo; Li, Jiyong


    The Qinling Orogen was a consequence of continental collision of the South China Craton with the North China Craton in the Triassic and caused widespread granitoid magmatism. However, the petrogenesis of these granitoids remains controversial. In this paper, we choose the Baijiazhuang (BJZ) and Lvjing (LJ) plutons in the West Qinling Orogen for a combined study of the zircon U-Pb geochronology, whole-rock major and trace element compositions and Sr-Nd-Hf isotopic characteristics. We obtained zircon crystallization ages of 216 Ma and 212 Ma for the BJZ and the LJ plutons, respectively. The granitoid samples from both plutons have high K2O metaluminous to peraluminous compositions. They are enriched in large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depleted in high field-strength elements (HFSEs) with significant negative Eu anomalies. The BJZ samples have initial Sr isotopic ratios of 0.7032 to 0.7078, εNd(t) of - 10.99 to - 8.54 and εHf (t) of - 10.22 to - 6.41. The LJ granitoids have initial Sr isotopic ratios of 0.7070 to 0.7080, εNd(t) of - 5.37 to - 4.58 and εHf(t) of - 3.64 to - 1.78. The enriched isotopic characteristics of the two plutons are consistent with their source being dominated by ancient continental crust. However, two BJZ samples show depleted Sr isotope compositions, which may infer possible involvement of mantle materials. Mantle-derived melt, which formed from partial melting of mantle wedge peridotite facilitated by dehydration of the subducted/subducting Mianlue ocean crust, provide the required heat for the crustal melting while also contributing to the compositions of these granitoids. That is, the two granitic plutons are magmatic responses to the closure of the Mianlue ocean basin and the continental collision between the Yangtze and South Qinling crustal terranes.

  7. Geochemistry and Re-Os geochronology of the organic-rich sedimentary rocks in the Jingtieshan Fe-Cu deposit, North Qilian Mountains, NW China (United States)

    Yang, Xiuqing; Zhang, Zuoheng; Li, Chao; Duan, Shigang; Jiang, Zongsheng


    The Jingtieshan Group in the North Qilian Mountains, NW China, is dominantly composed of banded iron formations (BIFs), copper deposits and organic-rich sedimentary rocks (ORS, carbonaceous phyllite). X-ray diffraction analysis of the ORS shows the mineral assemblage to be quartz + clay minerals. The total organic carbon contents show a range of 0.44-1.72%. Here we present the results of the geochemistry and Re-Os geochronology of the ORS from the Jingtieshan Group. The high values of Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Alteration (PIA), and Th/U ratio, indicate intense weathering. The Al2O3/TiO2, Zr/Sc, Th/Sc, La/Th ratios, high rare earth elements abundances, light rare earth elements enrichment (normalized to chondrite), and distinctly negative Eu anomalies, suggest that the Jingtieshan Group ORS were derived mainly from felsic volcanic units. The new Re-Os isochron age of 1308 ± 100 Ma (2σ, n = 6, MSWD = 23) broadly overlap with the previous published ages determined using Sm-Nd and U-Pb isotope systems. The new age represents the depositional age of the Jingtieshan Group, as well that of BIF in the Jingtieshan area. Furthermore, the initial 187Os/188Os ratios (0.44 ± 0.07) indicate that the Os in the seawater was dominantly derived from hydrothermal fluids (∼75%). The Ce anomaly (Ce/Ce∗ = 0.95-1.00) and V/(V + Ni) ratios (0.71-0.86), as well as the lack of enrichment in redox-sensitive trace elements such as U, V, Zn, Pb, Cu, Ni, Cr, Co and Mn, together with the presence of overlying BIF, suggest that the Jingtieshan area represents a ferruginous deep-water succession. This, and intense submarine hydrothermal activities contributed to the deposition of the Jingtieshan BIF.

  8. Extensional and Colisional Magmatic Records in the Apiaí Terrane, South-Southeastern Brazil: Integration of Geochronological U-Pb Zircon Ages

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    Oswaldo Siga Junior


    Full Text Available The aim of this work is the presentation of a synthesis of available geochronological data for the basement inliers andmetavolcano-sedimentary sequences which occur in the southern part of the Apiaí Terrane, south-southeastern Brazil.These data, especially those obtained during the last decade, have made substantial modifi cations to the tectonic scenarioof south-southeastern Brazil with the recognition of the presence of extensional basins (continental rifts with magmatismand sedimentation at the late Paleoproterozoic (1790 - 1750 Ma and of the Mesoproterozoic (1600 - 1450 Ma. Theseprocesses started at the late Paleoproterozoic in the Betara, Perau and Apiai Mirim Nuclei, and the basins became widerduring the Mesoproterozoic with the deposition of the Betara, Perau, Votuverava and Água Clara metavolcano-sedimentarysequences. A different pattern is found for the Itaicoca Sequence which occupies the northern part of the Apiaí Terrain.Here two rock associations of different ages are found. The fi rst is a metamorphosed carbonate platform association withsubalkaline, tholeiitic metabasic rocks with minimum deposition ages at the end of the Mesoproterozoic or beginning of theNeoproterozoic (Itaiacoca Sequence; 1030 - 900 Ma, while the second mainly contains metapsammites and metavolcanicrocks including ultrapotassic trachytes deposited during the Cryogenian-Ediacaran transition between 645 and 628 Ma(Abapã Sequence. It is then proposed that the present scenario of the Apiaí Terrane be representative of the distincttectono-sedimentary histories, refl ecting a polycyclic evolution. In the Mesoproterozoic, stable conditions prevailed, withthe deposition of the majority of the sequences (Lajeado, Água Clara, Betara, Perau, Votuverava in a passive margincontext, succeded by a Tonian extensional phase (deposition of the Itaiacoca Sequence. In the Ediacaran the region wastransformed into an unstable active margin, with the predominance of magmatic

  9. Geochronology and Hf Isotope Study of Pegmatite in the Xiaoqinling Area of NW China:Implication for Petrogenesis and Regional Metamorphism

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    Haixiang Zhao; Shaoyong Jiang; Baozhang Dai; Liang Ma; Jianwei Li


    In this study, we carried out petrography, zircon U-Pb geochronology and Hf isotopic analyses on a granitic pegmatite dyke in the Xiaoqinling area in southern margin of the North China Craton (NCC). Our study suggests that the pegmatite dyke likely crystallized from a volatile-rich peg-matitic magma. Different from most other pegmatite elsewhere, zircon from this pegmatite dyke does not contain unusually high U and Th concentrations and suffered no evident radioactive damage, therefore we successfully obtained a zircon U-Pb dating using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), which yields an average 207Pb/206Pb age of 1 814±6 Ma, representing the intrusive age of the granitic pegmatite dyke. ZirconεHf(t) values are between-8.3 and-3.0, corresponding to Hf depleted mantle model ages from 2 649 to 2 991 Ma with an average of 2 881 Ma. These data indicate that this granitic pegmatite dyke may have been derived from partial melting of Meso-Neoarchean metamorphic rocks from the Xiaoqinling basement. Granitic pegmatite magma may have emplaced within the Taihua Group wall rocks during the last stage of the middle to high grade metamorphism. Furthermore, according to the petrographic observation, the 1.81 Ga pegmatite dyke and the 1 800–1 750 Ma Xiong’er Group rocks were not undergone middle to high grade meta-morphism, indicating 1.81 Ga as the termination of the latest regional metamorphism in the southern margin of the NCC.

  10. Applications of detrital geochronology and thermochronology from glacial deposits to the Paleozoic and Mesozoic thermal history of the Ross Embayment, Antarctica (United States)

    Welke, Bethany; Licht, Kathy; Hennessy, Andrea; Hemming, Sidney; Pierce Davis, Elizabeth; Kassab, Christine


    Till from moraines at the heads of six major outlet glaciers in the Transantarctic Mountains (TAM) and from till beneath three West Antarctic ice streams have a ubiquitous zircon U-Pb age population spanning the time of the Ross/Pan-African Orogenies (610-475 Ma). Geochronology and thermochronology of detrital minerals in these Antarctic glacial tills reveal two different thermal histories for the central and southern TAM. Double-dating of the zircons reveals a geographically widespread (U-Th)/He (ZHe) population of 180-130 Ma in most of the till samples. Sandstone outcrops at Shackleton Glacier, and three Beacon Supergroup sandstone clasts from three moraines, have ZHe ages that fall entirely within this range. The similar population and proximity of many of the till samples to Beacon outcrops lead us to suggest that this extensive ZHe population in the tills is derived from Beacon Supergroup rocks and reflects the thermal response of the Beacon Basin to the breakup of Gondwana. A second population of older (>200 Ma) ZHe ages in tills at the head of Byrd, Nimrod, and Reedy Glaciers. For the tills at the head of the Nimrod and Byrd Glaciers, integrating the double-dated zircon results with 40Ar/39Ar of hornblende, muscovite and biotite, and U-Pb and (U-Th-Sm)/He double-dates on apatite yields a typical pattern of early rapid orogenic cooling (˜4-10°C/Myr) 590-475 Ma after the emplacement of the Granite Harbour Intrusives. Low temperature thermochronometers at these sites yield variable but quite old ages (ZHe 480-70 Ma and AHe 200-70 Ma) that require a long history at low temperature.

  11. The 40Ar/39Ar geochronology of escorias in the Pampean loessoid sequences, and their record of meteorite impact events for the southern hemisphere (United States)

    Hames, W.; Schultz, P.; Zarate, M.; Renne, P.; King, J.


    Glass fragments have long been an enigmatic stratigraphic component in loessoid sequences of the Pampean basin, Argentina. Although many mechanisms have been proposed to explain the origin of these 'escorias', it is now clear that certain escorias represent meteorite impact-derived melt materials that have been preserved as a result of characteristics endemic to the Pampean basin. Our work since the early 1990's focused on the origin, age, and stratigraphic context for escorias in type sections. An important example includes the age of 3.3±0.1 Ma for meteorite impact glasses collected in the sea cliffs near Chapadmalal. The challenges presented in dating such material are formidable, as the glasses are commonly highly vesicular (with trapped, extraneous argon), exhibit various stages of alteration, contain varying amounts of xenocrystic material, and they typically occur in sections that lack other stratigraphic markers (e.g., volcanic ash). Through the past several years we have refined our 40Ar/39Ar technique in dating these materials, and combined our geochronologic research with other isotopic and geochemical, paleomagnetic, stratigraphic and paleontologic tools in order to explore other prominent sections of the Pampean loess sequences. Our recent work enables us to define more than five distinct impact-glass horizons, the youngest of which has yielded an age of 6 Ka. The motivation for undertaking such painstaking and integrated study is strong, as the resulting data can provide much needed insights for the Miocene-Recent impact history of the southern hemisphere, the flux rate for meteorite impacts, and chronostratigraphic markers to the rich faunal record of the Pampean loessoid sequences. Such an integrated approach is also absolutely required, because of reworking and similarities in appearance of some escorias, and thus the true radiometric age, geochemical composition, and stratigraphic setting must be established for each escoria occurrence prior to

  12. Integrated Paleomagnetism and U-Pb Geochronology of Mafic Dikes of the Eastern Anabar Shield Region, Siberia: Implications for Mesoproterozoic Paleolatitude of Siberia and Comparison with Laurentia. (United States)

    Ernst; Buchan; Hamilton; Okrugin; Tomshin


    This article reports the first joint paleomagnetic and U-Pb geochronologic study of Precambrian diabase dikes in the Anabar Shield and adjacent Riphean cover of Siberia. It was undertaken to allow comparison with similar published studies in Laurentia and to test Proterozoic reconstructions of Siberia and Laurentia. An east-trending Kuonamka dike yielded a provisional U-Pb baddeleyite emplacement age of 1503+/-5 Ma and a virtual geomagnetic pole at 16 degrees S, 221 degrees E (dm=17&j0;, dp=10&j0;). A paleomagnetic pole at 6 degrees N, 234 degrees E (dm=28&j0;, dp=14&j0;) was obtained from five Kuonamka dikes. An east-southeast-trending Chieress dike yielded a U-Pb baddeleyite emplacement age of 1384+/-2 Ma and a virtual geomagnetic pole at 4 degrees N, 258 degrees E (dm=9&j0;, dp=5&j0;). Kuonamka and Chieress poles are interpreted to be primary but do not average out secular variation. Assuming that the Siberian Plate has remained intact since the Mesoproterozoic, except for mid-Paleozoic opening of the Viljuy Rift, then the above results indicate that the Siberian Plate was in low latitudes at ca. 1503 and 1384 Ma, broadly similar to low latitudes determined for Laurentia from well-dated paleopoles at 1460-1420, 1320-1290, and 1267 Ma. This would allow Laurentia and Siberia to have been attached in the Mesoproterozoic, as suggested in several recent studies based on geological criteria. However, because paleomagnetic results from the Anabar Shield region do not average out secular variation and the ages of poles from Siberia and Laurentia are not well matched, it is not yet possible to distinguish between these reconstructions or to rule out other configurations that also maintain the two cratons at low paleolatitudes.

  13. EPMA monazite geochronology of the basement and supracrustal rocks within the Pur-Banera basin, Rajasthan: Evidence of Columbia breakup in Northwestern India (United States)

    Ozha, Manoj Kumar; Mishra, Biswajit; Hazarika, Pranjit; Jeyagopal, A. V.; Yadav, G. S.


    We deduce the timing of the rift-induced collisional event between two orogenies in the central Aravalli-Delhi Fold Belt using monazite chemical dating and metamorphic P-T estimates from metapelites of the Mangalwar Complex (MC), and the overlying Pur-Banera (PB) supracrustals. The MC rocks preserve evidence of three regional metamorphic events, while the PB rocks record the last event. The M1 metamorphism attained its peak P-T at ∼5.5 kbar and 520-550 °C in the MC rocks at ∼1.82 Ga, followed by the M2 event with peak P-T of ∼7.5 kbar and 580-660 °C at ∼1.35 Ga. The youngest high-pressure M3 metamorphism attained a peak P-T of ∼8.0 kbar and 590-640 °C at ∼0.99 Ga. Thermobarometry coupled with ages of included monazites in chemically zoned garnet from the MC metapelites indicate preservation of ages spanning between ∼1.82 Ga and ∼0.99 Ga from different zones (i.e., core to rim), implying episodic garnet growth during supercontinent cycle. The PB metapelites constitute two prominent ages of ∼1.37 Ga and ∼1.05 Ga. The youngest high-pressure metamorphism (M3) in the PB rocks with maximum P-T of ∼8.0 kbar, and 580-670 °C during the Neoproterozoic has overprinted their earlier metamorphic records. Based on monazite geochronology, we assign the ∼1.82 Ga and ∼1.37-1.35 Ga ages to the amalgamation and breakup of the Columbia supercontinent respectively. The youngest age record of ∼1.05-0.99 Ga indicates evidence of Rodinia formation in and around the Pur-Banera basin.

  14. Coupling thermodynamic modeling and high-resolution in situ LA-ICP-MS monazite geochronology: evidence for Barrovian metamorphism late in the Grenvillian history of southeastern Ontario (United States)

    McCarron, Travis; Gaidies, Fred; McFarlane, Christopher R. M.; Easton, R. Michael; Jones, Peter


    The Flinton Group is a greenschist to upper amphibolite facies package of metasediments in southeastern Ontario that was metamorphosed during the Ottawan Orogeny. Thermodynamic modeling of metapelitic mineral assemblages suggests an increase in peak conditions of metamorphism across the 40 km wide study area from 3.5 to 7.9 kbar and 540 to 715 °C. Garnet isopleth thermobarometry applied to the cores of compositionally zoned porphyroblasts reveals remarkably similar P-T conditions of initial crystallization at approximately 3.7-4.0 kbar and 512-520 °C, corresponding to a relatively high geothermal gradient of ca. 34-45 °C km-1. It is inferred from modeling and reaction textures that metamorphism was along Barrovian P-T paths. Major and trace element zoning in garnet from one sample records a complex growth history as evidenced by major and trace element zoning and the distribution of xenotime, allanite and monazite inclusions. High-resolution (6 μm) LA-ICP-MS U-Pb geochronology performed on monazite in the rock matrix and included in the outer 150 μm of garnet rim-ward of a Y annulus revealed an age of 976 ± 4 Ma. The age is interpreted to reflect monazite growth at the expense of allanite and apatite late in garnet's growth history over the P-T interval 4.5-6.8 kbar and 540-640 °C. This new age estimate for near peak metamorphism fits well into the regional framework but is significantly younger than previously reported ages for Ottawan metamorphism. Based on microstructures this new age suggests that compressional tectonics were operating much later in the history of the Grenville of southeastern Ontario than previously thought.

  15. New models for Paleoproterozoic orogenesis in the Cheyenne belt region: Evidence from the geology and U-Pb geochronology of the Big Creek Gneiss, southeastern Wyoming (United States)

    Jones, D.S.; Snoke, A.W.; Premo, W.R.; Chamberlain, K.R.


    The disputed age of the deep crust of the Colorado Province is central to hypotheses for Paleoproterozoic crustal growth in the region. We studied the high-grade Big Creek Gneiss, southeastern Wyoming, as a potential exposure of pre-1780 Ma basement rocks. New geologic mapping and U-Pb geochronological data indicate that the Big Creek Gneiss exposes a deeper, but coeval, level of the Green Mountain arc relative to the predominantly supracrustal section to the west. The Big Creek Gneiss is composed of: supracrustal rocks; a ca. 1780 Ma Green Mountain arc-correlative, bimodal intrusive suite; a ca. 1763 Ma extensional(?) bimodal intrusive suite; and widespread ca. 1630 Ma pegmatitic leucogranite. The mafic member of the younger bimodal suite is documented here for the first time. U-Pb zircon ages from migmatite leucosomes indicate penetrative deformation of the Big Creek Gneiss at ca. 1750 Ma. We find that the postarc intrusive suite is mantle-involved, implying a second period of crustal growth. Shortening postdates arc magmatism by ~20 m.y., implying that termination of arc magmatism and accretion were separate events. Finally, criteria previously used to constrain the polarity of subduction for the Green Mountain arc are not reliable. We propose two competing models: (1) southward-dipping Green Mountain arc subduction (present coordinates), with slab breakoff-related magmatism following arc accretion; or (2) northward-dipping subduction, with extensional postarc magmatism. In both models, high-temperature deformation coincides with accretion along the Cheyenne belt, and extensional magmatism is an important component of crustal growth. We prefer the northward-dipping subduction model because it can be better integrated with regional tectonic events and published isotopic compositions of the igneous rocks. ?? 2010 Geological Society of America.

  16. Neoproterozoic granitic magmatism along the Ailao Shan-Red River belt: U-Pb zircon geochronology, Lu-Hf isotopes and tectonic implications (United States)

    Chen, Xiaoyu; Liu, Junlai; Qi, Yinchuan; Fan, Wenkui; Burg, Jean-Pierre


    The Neoproterozoic tectonic characteristics of the high grade metamorphic massifs along the Ailao Shan-Red River belt are debated. Controversies are on 1) whether the massifs were parts of the Yangtze block to the northeast or 2) parts of the Indochina block to the southwest and 3) the magmatic rocks represent arc magmatism or rifting linked to break-up of the Rodinia supercontinent. This study presents new and precise LA-ICP-MS U-Pb age dating and geochemical and Hf isotopic analyses of granitic intrusions along the Ailao Shan-Red River belt in an attempt to elucidate the Neoproterozoic magmatic evolution of this belt. In general, zircon U-Pb ages of the studied granitic rocks are between 804 and 724Ma, with a weighted mean of ca. 770 Ma, thus confirming Neoproterozoic magmatism. All samples plot into the peraluminous domain, indicating a major crustal resource. In consistency with these conclusions, most of the Neoproterozoic granitoids show negative ɛHf (t) values near the chondrite line. A few samples possess low positiveɛ Hf (t) values, being signatures of mantle sources. It is therefore concluded that the Neoproterozoic magmatism along the ASRR belt originated from mantle sources with important contributions through anatexis of ancient lower crust. Discrimination diagrams of tectonic settings suggest continental arc magmatism. Neoproterozoic magmatism is widely reported along the margins of the Yangtze block, especially in the northern margin. However, there are fewer reports about Neoproterozoic magmatic activity along the southern and southwestern margins. The geochronology spectrum and geochemisty of the studied Neoproterozoic granitic rocks are similar to those along the western margin of the Yangtze block. The present study, combined with previous results, suggests that oceanic subduction contributed to the generation of the arc magmatisms along the western and southwestern margin of the Yangtze plate and along the ASRR belt (as part of the

  17. Petrology and SHRIMP zircon geochronology of granulites from Vesleknausen, Lützow-Holm Complex, East Antarctica:Neoarchean magmatism and Neoproterozoic high-grade metamorphism

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    Toshiaki Tsunogae; Daniel J. Dunkley; Kenji Horie; Takahiro Endo; Tomoharu Miyamoto; Mutsumi Kato


    We report new petrological data and geochronological measurements of granulites from Vesleknausen in the highest-grade section of the Lützow-Holm Complex, part of the Gondwana-assembling collisional orogen in East Antarctica. The locality is dominated by felsic to intermediate orthogneiss (charnockite and minor biotite gneiss), mafic orthogneiss, and hornblende-pyroxene granulite, with deformed and undeformed dykes of metagranite and felsic pegmatite. Pseudosection analysis of charnockite in the system NCKFMASHTO, supported by geothermometry of mafic orthogneiss, was used to infer peak metamorphic temperatures of 750e850 ?C, approximately 150 ?C lower than those estimated for met-asedimentary gneisses from Rundvågshetta, 6 km to the northeast. SHRIMP U-Pb analysis of zircons from feldspar-pyroxene gneiss, which corresponds to a partially molten patch around mafic orthogneiss, yielded a Concordia upper intercept ages of 2507.9 ? 7.4 Ma, corresponding to the time of formation of the magmatic protolith to the orthogneiss. Partial melting during peak metamorphism probably took place between 591 and 548 Ma, as recorded in rims overgrew around magmatic zircon. Our results suggest that Rundvågshetta-Vesleknausen-Strandnibba region in southwestern Lützow-Holm Bay, where orthogneisses are dominant, consists of a single crustal block, possibly formed by ca. 2.5 Ga arc mag-matism. The Neoarchean magmatic terrane was tectonically mingled with other fragments (such as metasedimentary units in northern Lützow-Holm Bay) by subduction/collision events during the as-sembly of Gondwana supercontinent, and subsequently underwent w850 ?C granulite-facies meta-morphosed during Neoproterozoic to Cambrian final collisional event.

  18. Petrology and SHRIMP zircon geochronology of granulites from Vesleknausen, Lützow-Holm Complex, East Antarctica: Neoarchean magmatism and Neoproterozoic high-grade metamorphism

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    Toshiaki Tsunogae


    Full Text Available We report new petrological data and geochronological measurements of granulites from Vesleknausen in the highest-grade section of the Lützow-Holm Complex, part of the Gondwana-assembling collisional orogen in East Antarctica. The locality is dominated by felsic to intermediate orthogneiss (charnockite and minor biotite gneiss, mafic orthogneiss, and hornblende-pyroxene granulite, with deformed and undeformed dykes of metagranite and felsic pegmatite. Pseudosection analysis of charnockite in the system NCKFMASHTO, supported by geothermometry of mafic orthogneiss, was used to infer peak metamorphic temperatures of 750–850 °C, approximately 150 °C lower than those estimated for metasedimentary gneisses from Rundvågshetta, 6 km to the northeast. SHRIMP U-Pb analysis of zircons from feldspar-pyroxene gneiss, which corresponds to a partially molten patch around mafic orthogneiss, yielded a Concordia upper intercept ages of 2507.9 ± 7.4 Ma, corresponding to the time of formation of the magmatic protolith to the orthogneiss. Partial melting during peak metamorphism probably took place between 591 and 548 Ma, as recorded in rims overgrew around magmatic zircon. Our results suggest that Rundvågshetta-Vesleknausen-Strandnibba region in southwestern Lützow-Holm Bay, where orthogneisses are dominant, consists of a single crustal block, possibly formed by ca. 2.5 Ga arc magmatism. The Neoarchean magmatic terrane was tectonically mingled with other fragments (such as metasedimentary units in northern Lützow-Holm Bay by subduction/collision events during the assembly of Gondwana supercontinent, and subsequently underwent ∼850 °C granulite-facies metamorphosed during Neoproterozoic to Cambrian final collisional event.

  19. Geochronology of the western and central Brooks Range, Alaska: Implications for the geologic evolution of the Anarraaq and Red Dog Zn-Pb-Ag deposits (United States)

    Rombach, C.S.; Layer, P.W.


    A compilation of published geochronology of rocks and minerals from the western and central Brooks Range provides a framework for understanding the complex history of the Brooks Range and northern Alaska. A simplified timeline of events comprises (1) Devonian extension, (2) Mississippian extension and Zn-Pb-Ag mineralization, (3) a passive interval, (4) pre-Brooks Range orogeny rock-formation and thermal event, (5) inception of Brooks Range orogeny, (6) exhumation and the end of main-stage deformation, and (7) subsequent episodic deformation. This compilation is supplemented by new 40Ar/39Ar dates of white mica from the Anarraaq and Red Dog Zn-Pb-Ag (+ barite) deposits from the western Brooks Range. The deposits are hosted in black shale and carbonate rocks of the Late Mississippian-Early Pennsylvanian Kuna Formation. Quartz-pyrite-white mica grains in sedimentary rocks above the Anarraaq deposit yield an age of 195.0 ?? 2.0 Ma, and paragenetically late quartz-pyrite-white mica from the Main orebody at the Red Dog deposit has an age of 126.1 ?? 0.7 Ma. These white micas are much younger than the age of Zn-Pb-Ag mineralization at Red Dog (338 ?? 5.8 Ma Re-Os age of pyrite). The date for white mica from Anarraaq (???195 Ma) appears to be related to a large-scale thermal event in the region immediately before the inception of the Brooks Range orogeny. The white mica from the Red Dog deposit (???126 Ma) correlates with the later stages of the orogeny, a period of blueschist metamorphism, extension, and rapid exhumation, which varied with geographic location. These dates suggest that the Red Dog deposits underwent significant hydrothermal overprinting during multiple episodes of the Brooks Range orogeny. ?? 2004 by Economic Geology.

  20. The Inception of the Colorado Plateau Coring Project: Filling the Triassic Geochronologic Gap and Providing a Continuous Record of Continental Environmental Change in Western Equatorial Pangea (United States)

    Geissman, J. W.; Olsen, P. E.; Kent, D. V.; Irmis, R. B.; Gehrels, G. E.; Mundil, R.; Parker, W.; Bachmann, G. H.; Kurschner, W. M.; Sha, J.


    The Triassic Period was punctuated by two of the largest Phanerozoic mass-extinctions and witnessed the evolution of elements of the modern biota and the advent of the age of dinosaurs. A rich archive of biotic and environmental changes on land for the early Mesozoic is on the Colorado Plateau, which despite over 100 years of study still remains poorly calibrated in time and poorly registered to other global records. Over 15 years ago, a diverse team of scientists began to develop the concept of a multi-phase, long term Colorado Plateau Coring Project (CPCP). Planning involved two major meetings (DOSECC/NSFICDP supported in Fall, 2007, St. George, UT; and International Continental Drilling Program (ICDP) supported in Spring, 2009, Albuquerque, NM). The National Park Service embraced the concept of Phase One drilling at Petrified Forest National Park (PFNP) in northern Arizona, which exposes one of the most famous and best studied successions of the continental Triassic on Earth, and the Phase One target was decided. Most drilling operation costs were secured from ICDP in Summer, 2010. In late 2013, following more recent NSF support, the research team, utilizing Ruen Drilling Inc., drilled a continuous ~530 m core (60o plunge) through the entire section of Triassic strata (Chinle and Moenkopi fms.) in the north end and a ~240 m core (75o plunge) in lower Chinle and all Moenkopi strata at the south end of the PFNP. Our continuous sampling will place this record in a reliable quantitative and exportable time scale, as a reference section in which magnetostratigraphic, geochronologic, environmental, and paleontologic data are registered to a common thickness scale with unambiguous superposition using pristine samples. The cores are being scanned at the High Resolution X-ray Computed Tomography Facility at UT Austin. They will be transported to the LacCore National Lacustrine Core Facility at U Minnesota, where they will be split, imaged, and scanned for several

  1. Contributions to the petrography, geochemistry and geochronology (U-Pb and Sm-Nd) of the Paleoproterozoic effusive rocks from Iricoume Group, Amazonian Craton, Brazil

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    Marques, Suelen Nonata de Souza; Nascimento, Rielva Solimairy Campelo do, E-mail:, E-mail: [Universidade Federal do Amazonas (UFAM), Manaus, AM (Brazil). Inst. de Geociencias; Souza, Valmir da Silva; Dantas, Elton Luiz, E-mail:, E-mail: [Universidade de Brasilia (UnB), DF (Brazil). Inst. de Geociencias; Valerio, Cristovao da Silva, E-mail: [Universidade Federal de Roraima (UFRR), Boa Vista, RR (Brazil). Inst. de Geociencias


    The southernmost region of the Guyana shield, Amazonian craton, hosts large record of Paleoproterozoic effusive rocks of the Iricoume Group. They present remarkably well-preserved igneous textures and structures. The SiO{sub 2} contents reveal a bimodal association marked by a compositional gap between acid (SiO{sub 2} > 67 wt%) and intermediate (SiO{sub 2} < 57.7 wt%) rocks. The acid effusive rocks are rhyolites to rhyodacites with high SiO{sub 2}, alkali, Rb, Zr, Nb + Ta, La + Ce and 104 Ga/Al content and low Fe{sub 2}O{sub 3tot}, TiO{sub 2}, CaO, Sr and Co content. They exhibit subalkaline, metaluminous-to-peraluminous compositions, and geochemically compatible to A-type magmatism emplaced in post-collisional to within-plate tectonic settings. The intermediate rocks are andesitic/basalt to andesite relatively high contents of TiO{sub 2}, Al{sub 2}O{sub 3}, Fe{sub 2}O{sub 3total}, MgO, CaO, Sr and Co; low SiO{sub 2}, K{sub 2}O, Rb, Zr, Nb + Ta, La + Ce. They have subalkaline and metaluminous geochemical composition and plot on within-plate basalt field. The acid rocks crystallized at 1882 ± 11 Ma in U-Pb analyses for LA-MC-ICPMS zircon data. The Sm-Nd isotopic data on all rocks reveal a Nd TDM model ages between 2.59 and 2.16 Ga and ε{sub Nd}(t) values between -5.78 and 0.03, indicate that the magmatic evolution was related to the reworking of older Paleoproterozoic at the Rhyacian-Siderian period, continental crust (Transamazonian crust-forming event) with some mixing with a limited amount mantle-derived magmas or with contamination by Archean crust. The petrographic, geochemical and geochronological data presented in this paper suggest a within-plate to post-collisional tectonic setting for the Iricoume volcanism, involving lower crust uplift and generation of basalt magma in an extensional regime. (author)

  2. An integrative geologic, geochronologic and geochemical study of Gorgona Island, Colombia: Implications for the formation of the Caribbean Large Igneous Province (United States)

    Serrano, Lina; Ferrari, Luca; Martínez, Margarita López; Petrone, Chiara Maria; Jaramillo, Carlos


    The genesis of the Caribbean Large Igneous Province (CLIP) has been associated to the melting of the Galapagos plume head at ~ 90 Ma or to the interaction between the plume and the Caribbean slab window. Gorgona Island, offshore western Colombia, is an accreted fragment of the CLIP and its highly heterogeneous igneous suite, ranging from enriched basalts to depleted komatiites and picrites, was assumed to have formed at ~ 89 Ma from different part of the plume. Here we present new geologic, geochronologic and geochemical data of Gorgona with significant implications for the formation of the CLIP. A new set of 40Ar- 39Ar ages documents a magmatic activity spanning the whole Late Cretaceous (98.7 ± 7.7 to 64.4 ± 5 Ma) followed by a shallower, picritic pyroclastic eruption in the Paleocene. Trace element and isotope geochemistry confirm the existence of an enriched (EDMM: La/Sm N ≥ 1 and ɛNd i of 5.7 to 7.8) and a depleted (DMM: La/Sm N rocks. Higher degree of melting (> 10%) of a mixed DMM + EDMM (40 to 60%) may reproduce the more depleted rocks with temperatures in the range of ambient mantle in absence of plumes. Our results contradict the notion that the CLIP formed by melting of a plume head at ~ 90 Ma. Multiple magmatic pulses over several tens of Ma in small areas like Gorgona, also recognized in other CLIP areas, suggest a long period of diffuse magmatism without a clear pattern of migration. The age span of this magmatism is broadly concurrent with the Caribbean slab window. During this time span the Farallon oceanic lithosphere (later becoming the Caribbean plate) advanced eastward ~ 1500 km, overriding the astenosphere feeding the proto-Caribbean spreading ridge. This hotter mantle flowed westward into, and mixed with, the opening mantle wedge, promoting increasing melting with time. The fortuitous occurrence of a plume passing through the slab gap area cannot be excluded but not required to produce the observed composition and degree of melting.

  3. Floodplain development in engineered and natural settings determined with novel, high resolution 210-Pb geochronology: Insights from sedimentation studies along the lower Sacramento River, California (United States)

    Aalto, R.; Singer, M. B.


    This presentation summarizes results from studies of floodplain sedimentation along the middle and lower Sacramento River that investigate processes using a new, high resolution methodology for 210Pb geochronology of 1-5 m floodplain cores. This approach accounts both for grain-size effects and radon ventilation and can resolve both deposition and erosional events. Therefore, it was possible to assess sedimentation over the past century within a wide array of sedimentary environments throughout the Sacramento Valley, where other techniques are limited. In particular, the Sacramento Valley has naturally low 210Pb activity due to its proximity to the Pacific Ocean, high rates of radon ventilation due to dry, porous floodplain sediment, and deposition of widely varying grain sizes - challenges that we have addressed with our enhanced methodology. The analytical approach affords a new ability to assess and directly compare dates and rates of sedimentation and erosion in disparate sedimentary environments throughout this complex fluvial dispersal system. We compare and contrast sediment deposition in engineered floodplains called bypasses, levied ancestral floodplains which serve as floodways during high flow, with sedimentation occurring in some remaining natural floodplains adjacent to the Sacramento River. We find that bypasses tend to accumulate sand and silt at their entrances, but that rates and textures decline rapidly with distance away from the channel. Essentially, a quasi-natural physical process of levee construction by advective overbank transport and deposition of sediment is operating (Singer and Aalto, ESPL, in press). These engineered floodways tend to siphon sediment out of the active channel, such that relatively low sedimentation rates prevail in floodplains and oxbow lakes within the active meander corridor that is bypassed. However, we document significant accumulation of fine-grained material in sedimentary sinks throughout floodplains upstream

  4. Eruptive History of Volcán Tepetiltic, Mexico: Evidence for Remelting of Silicic Ashflows Revealed by 40Ar/39Ar Geochronology (United States)

    Frey, H. M.; Lange, R. A.; Hall, C. M.; Nelson, S. A.; Granados, H. D.


    Volcán Tepetiltic (VT) is located in the northwestern part of the Trans-Mexican Volcanic Belt and features an elliptical caldera (5 x 2.5 km). Previous detailed 40Ar/39Ar geochronology studies at V. Tequila and V. Ceboruco, showed that cone-building events occur within narrow time intervals (200 kyr between cone building episodes along the southern flank of the volcano. Following construction of the main stratocone (dominated by andesitic effusive activity), there was a clustering of rhyolite eruptions, including the caldera forming event which produced rhyolitic ashflow and airfall deposits. The age of the eruption is constrained by stratigraphic and cross-cutting relations. The eruption must be younger than 231 ± 36 kyr, based on the uppermost andesite dated from the caldera rim. The minimum age of the eruption is constrained by the occurrence of airfall from VT found underneath lava flows from the Amado Nervo shield volcano (220 ± 30 ka). Given these results, the Plinian eruption likely occurred ˜225 ± 40 kyrs ago. The two rhyolite obsidian domes on the eastern flanks of VT were dated at 307 ± 34 ka and 274 ± 52 ka and are thus similar in age or slightly older than the Plinian eruption. The >200 kyr hiatus between the construction of the andesite/dacite stratocone and the rhyolitic activity argues against crystal fractionation of the andesite to form the rhyolite. The upper crustal magma chamber feeding the main edifice ˜524 kyrs ago was likely short-lived (tens of kyrs), and may have crystallized to a granitoid. We suggest that the rhyolites formed ˜230 kyrs ago by partial melting of older shallow basement rhyolitic ashflows prevalent in the area. Evidence for this process is found in an unconsolidated pyroclastic flow from VT with pumiceous rhyolitic blocks and andesitic lithic fragments. Anorthoclase crystals in this deposit yield ages of 4.5 ± 0.34 Ma and 25.9 ± 0.24 Ma. Thus the rhyolites were likely produced by refusing of 4-5 Ma silicic

  5. Brittle tectonothermal evolution in the Forsmark area, central Fennoscandian Shield, recorded by paragenesis, orientation and 40Ar/ 39Ar geochronology of fracture minerals (United States)

    Sandström, Björn; Tullborg, Eva-Lena; Larson, Sven Åke; Page, Laurence


    In this paper, we show how studies of fracture mineral paragenesis, their orientation and 40Ar/ 39Ar geochronology can be applied in order to recognise the brittle tectonothermal evolution in an area. Samples were selected from nearly 18 km of drill cores from the upper 1 km of the Fennoscandian Shield obtained during the site investigation for a repository of spent nuclear fuel in Forsmark, central Sweden. Four major events of fracturing and/or reactivation of fractures associated with fracture mineralisation have been distinguished. The first event was characterised by precipitation of epidote, quartz and chlorite, along preferably sub-horizontal and gently-dipping fractures or steep, WNW-ESE to NW-SE fractures. Precipitation occurred between 1.8 and 1.1 Ga, possibly during the late stage of the Svecokarelian orogeny close to 1.8-1.7 Ga. The second event is associated with precipitation of hematite-stained adularia and albite, prehnite, laumontite, calcite and chlorite, preferably along steep, ENE-WSW to NNE-SSW and NNW-SSE fractures. Precipitation occurred around 1107 ± 7 to 1034 ± 3 Ma, probably due to effects from the Sveconorwegian orogeny. This event was followed by a period with dissolution of fracture minerals and subsequent precipitation of mainly calcite, quartz, pyrite and asphaltite during the Palaeozoic. The formation fluid emanated from an overlying organic-rich sedimentary cover. Precipitation occurred during reactivation of Proterozoic fractures, but formation of new fractures is also inferred, possibly due to far-field effects of the Caledonian orogeny, or elevated hydrostatic pressure due to its foreland basin. The latest event is dominated by clay minerals, chlorite and calcite along hydraulically conductive fractures. These minerals are prominent along sub-horizontal and gently-dipping fractures, but also occur in sets of steeply-dipping fractures. It is inferred that the hydraulically conductive fractures are Proterozoic structures in which

  6. Establishing and validating luminiscence dating procedures for archaoelogical remains in the geochronology laboratory of the University of A Coruña: first results

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    Sanjurjo Sánchez, Jorge


    Full Text Available Thermoluminescence dating (TL of archaeological material is a technique about 50 years old, but it is not much used in Spain. Recent methodological advances have improved the accuracy and precision of the method. The unit of Geochronology of the University Geological Institute “Isidro Parga Pondal” University of A Coruña, has set up recently a Luminescence laboratory. In order to test analytical dating procedures, medieval tiles from an archaeological site next to the Hercules Tower (A Coruña have been dated. The stratigraphical column was previously dated by 14C, so a good chronological control is available. Samples were analysed using two different analytical procedures: a classical one using a multialiquot approach (AD-TL and a recent one using a single aliquot procedure (SAR-TL. Our results show that both methods yield comparable paleodoses, being the SAR-TL the one with smaller error.

    La datación mediante termoluminiscencia (TL de material arqueológico es una técnica con unos 50 años de antigüedad, aunque no muy empleada en España. Avances metodológicos recientes han permitido aumentar la exactitud y precisión del método. La Unidad de Geocronología del Instituto Universitario de Xeoloxía “Isidro Parga Pondal” de la Universidad de A Coruña cuenta en la actualidad con un Laboratorio de Luminiscencia. Para poner a punto los procedimientos analíticos de datación, se han datado tejas medievales de una excavación cercana a la Torre de Hércules (A Coruña en la que la columna estratigráfica fue datada por medio de 14C. Las muestras fueron sometidas a dos procedimientos existentes en la literatura: uno clásico (AD-TL y otro de reciente desarrollo (SAR-TL. Los resultados muestran concordancia entre ambos métodos e incluso mejoras considerables obtenidas con el SAR-TL.

  7. Geochronology, Stratigraphy, and Provenance of the Early Fill of the Magallanes-Austral Basin, Southern Patagonia: Diachronous Initiation of a Retroarc Foreland Basin (United States)

    Malkowski, M. A.; Sharman, G.; Graham, S. A.


    The Magallanes-Austral Basin (MAB) is preserved along a >1000 kilometer north-south trending outcrop belt in the southern Patagonia region of Argentina and Chile. The stratigraphic evolution of the MAB has been well documented in the Chilean sector of the basin, however its along-strike counterpart in Argentina is poorly constrained. We present new stratigraphic and geochronologic data from the early basin fill (Aptian-Turonian) from the Argentine sector (49-51°S) of the MAB to document spatial variability in stratigraphic facies and timing of deposition. The initiation of the MAB is marked by the transition from mudstone to coarse-clastic deposition which is characterized by the consistent presence of thick sandstone beds. These sandy facies are interpreted to represent turbidity current deposits in a submarine fan system. This study documents that such facies are present as far north as El Chalten, Argentina (49°S), indicating that facies-equivalent rocks can be traced along-strike for at least 5 degrees of latitude, based on correlation with strata as far south as the Cordillera Darwin (54°S). Detrital zircon U-Pb ages (477 grains from 6 samples) from sandstone within the Argentine sector reveal similar trends to those documented in the Chilean sector. Age populations primarily consist of Aptian-Campanian (126-75 Ma) arc-related grains as well as Cambrian-Triassic ages (600-200 Ma) derived from the East Andean Metamorphic Complex. Jurassic to earliest Cretaceous ages (200-126) representing rift-phase volcanism and early arc magmatism, typically account for less than 5% of the ages. Finally, 6 new U-Pb ages from ash beds, coupled with maximum depositional ages from detrital zircon populations, reveal a southward younging trend in depositional ages. Interpreted ages range from 115 Ma to 95 Ma in the northern sector, but based on previous studies, are not older than 92 Ma and 89 Ma in the central and southern sectors, respectively. The diachronous delivery of

  8. Caribbean island-arc rifting and back-arc basin development in the Late Cretaceous: Geochemical, isotopic and geochronological evidence from Central Hispaniola (United States)

    Escuder Viruete, J.; Joubert, M.; Urien, P.; Friedman, R.; Weis, D.; Ullrich, T.; Pérez-Estaún, A.


    We present new regional petrologic, geochemical, Sr-Nd isotopic, and U-Pb geochronological data on the Turonian-Campanian mafic igneous rocks of Central Hispaniola that provide important clues on the development of the Caribbean island-arc. Central Hispaniola is made up of three main tectonic blocks—Jicomé, Jarabacoa and Bonao—that include four broad geochemical groups of Late Cretaceous mafic igneous rocks: group I, tholeiitic to calc-alkaline basalts and andesites; group II, low-Ti high-Mg andesites and basalts; group III, tholeiitic basalts and gabbros/dolerites; and group IV, tholeiitic to transitional and alkalic basalts. These igneous rocks show significant differences in time and space, from arc-like to non-arc-like characteristics, suggesting that they were derived from different mantle sources. We interpret these groups as the record of Caribbean arc-rifting and back-arc basin development in the Late Cretaceous. The> 90 Ma group I volcanic rocks and associated cumulate complexes preserved in the Jicomé and Jarabacoa blocks represent the Albian to Cenomanian Caribbean island-arc material. The arc rift stage magmatism in these blocks took place during the deposition of the Restauración Formation from the Turonian-Coniacian transition (~ 90 Ma) to Santonian/Lower Campanian, particularly in its lower part with extrusion at 90-88 Ma of group II low-Ti, high-Mg andesites/basalts. During this time or slightly afterwards adakitic rhyolites erupted in the Jarabacoa block. Group III tholeiitic lavas represent the initiation of Coniacian-Lower Campanian back-arc spreading. In the Bonao block, this stage is represented by back-arc basin-like basalts, gabbros and dolerite/diorite dykes intruded into the Loma Caribe peridotite, as well as the Peralvillo Sur Formation basalts, capped by tuffs, shales and Campanian cherts. This dismembered ophiolitic stratigraphy indicates that the Bonao block is a fragment of an ensimatic back-arc basin. In the Jicomé and

  9. Metamorphic evolution and U-Pb zircon SHRIMP geochronology of the Belizário ultramafic amphibolite, Encantadas Complex, southernmost Brazil

    Directory of Open Access Journals (Sweden)

    Léo A. Hartmann


    Full Text Available The integrated investigation of metamorphism and zircon U-Pb SHRIMP geochronology of the Belizário ultramafic amphibolite from southernmost Brazil leads to a better understanding of the processes involved in the generation of the Encantadas Complex. Magmatic evidence of the magnesian basalt or pyroxenite protolith is only preserved in cores of zircon crystals, which are dated at 2257 ± 12 Ma. Amphibolite facies metamorphism M1 formed voluminous hornblende in the investigated rock possibly at 1989 ± 21 Ma. This ultramafic rock was re-metamorphosed at 702±21 Ma during a greenschist facies eventM2; the assemblage actinolite + oligoclase + microcline + epidote + titanite + monazite formed by alteration of hornblende. The metamorphic events are probably related to the Encantadas Orogeny (2257±12 Ma and Camboriú Orogeny (~ 1989 Ma of the Trans-Amazonian Cycle, followed by an orogenic event (702±21 Ma of the Brasiliano Cycle. The intervening cratonic period (2000-700 Ma corresponds to the existence of the Supercontinent Atlantica, known regionally as the Rio de la Plata Craton.O entendimento dos processos evolutivos do Complexo Encantadas no sul do Brasil foi aperfeiçoado através do estudo integrado do metamorfismo de um anfibolito ultramáfico e da geocronologia U-Pb SHRIMP de zircão. Os núcleos herdados de alguns cristais de zircão tem idades em torno de 2257 ±12 Ma e constituem a única evidência preservada do protólito ígneo, que pode ter sido um basalto magnesiano ou um piroxenito. O metamorfismo M de fácies anfibolito formou abundante hornblenda na amostra investigada, possivelmente há 1989 ±21 Ma. Esta rocha ultramáfica foi re-metamorfizada talvez há cerca de 702 ±21 Ma durante um evento M de fácies xistos verdes do metamorfismo regional. Durante o evento M, a hornblenda foi recristalizada e formou a assembléia actinolita + oligoclásio + microclínio + epidoto + titanita + monazita. Estes eventos foram a manifesta

  10. Migrating Ignimbrite Flares in the Central Andes, Implications for Crustal Evolution Based on Chemical, Isotopic, Geochronological, and GIS-Based Volumetric Data (United States)

    Worner, G.; Brandmeier, M.; Freymuth, H.; Heistek, R. M.


    Temporal and compositional patterns of Neogene ignimbrites in the Central Andes were analysed using GIS and geostatistical modelling based on 203 digitized ignimbrite sheets for which geochronological, geochemical, and Sr-Nd-Pb-isotopic data on pumices as well as Sr-O isotopes on minerals from selected samples were compiled and compared to compositional and isotopic data from andesite lavas. Composition, timing, volumes and sources of erupted ignimbrite deposits are thus constrained and magma volumes through space and time are calculated. The total erupted ignimbrite magma volume of 31,000 km3 (minimum value) in the past 30 Ma indicate an average magmatic addition of 20-30 km3*Ma/km, similar to the basaltic "base"-flux for arc magmatism. Ignimbrite flare-ups are, however, rather punctuated, short-lived events well separated in space and time. There is a clear N-S "younging" of ignimbrite pulses from N to S at 19-24 Ma, 13-14 Ma, 6-10 Ma and 3-6 Ma. Ignimbrite eruptions occurred in the wake of subduction of the Juan-Fernandez ridge on the Nazca Plate passing below the Central Andes from N to S. Low angle subduction caused compression and fluid release is followed by massive inflow and melting of asthenospheric mantle when the slab steepened again after the passing of the ridge. This in turn caused massive melting within the crust aided by advective heat transport. Differences in chemical and isotopic composition of the large-volume ignimbrites are related to changes in crustal thickness, and different "preconditioning" during the Andean orogeny at a given space in time. Isotope data and whole rock compositional data suggest a higher degree of crustal assimilation for the younger Altiplano ignimbrites in the S (c. 50%) compared to the older (22-19 Ma) ignimbrites in the N were the crustal component is significantly less (20%). REE compositions reflect changes in crustal thickness with a "transition" at c. 13-9 Ma that can be related to accelerated crustal shortening

  11. Geochronological and sedimentological evidences of Panyangshan foreland basin for tectonic control on the Late Paleozoic plate marginal orogenic belt along the northern margin of the North China Craton (United States)

    Li, Jialiang; Zhou, Zhiguang; He, Yingfu; Wang, Guosheng; Wu, Chen; Liu, Changfeng; Yao, Guang; Xu, Wentao; Zhao, Xiaoqi; Dai, Pengfei


    There is a wide support that the Inner Mongolia Palaeo-uplift on the northern margin of the North China Craton has undergone an uplifting history. However, when and how did the uplift occurred keeps controversial. Extensive field-based structural, metamorphic, geochemical, geochronological and geophysical investigations on the Inner Mongolia Palaeo-uplift, which suggested that the Inner Mongolia Palaeo-uplift was an uplifted region since the Early Precambrian or range from Late Carboniferous-Early Jurassic. The geochemical characteristics of the Late Paleozoic to Early Mesozoic intrusive rocks indicated that the Inner Mongolia Palaeo-uplift was an Andean-type continental margin that is the extensional tectonic setting. To address the spatial and temporal development of the Inner Mongolia Palaeo-uplift, we have carried out provenance analysis of Permian sedimentary rocks which collected from the Panyangshan basin along the northern margin of the North China Craton. The QFL diagram revealed a dissected arc-recycled orogenic tectonic setting. Moreover, the framework grains are abundant with feldspar (36-50%), indicating the short transport distance and unstable tectonic setting. Detrital zircon U-Pb analysis ascertained possible provenance information: the Precambrian basement ( 2490 and 1840 Ma) and continental arc magmatic action ( 279 and 295 Ma) along the northern margin of the North China Craton. The projection in rose diagrams of the mean palaeocurrent direction, revealing the SSW and SSE palaeoflow direction, also shows the provenance of the Panyangshan basin sources mainly from the Inner Mongolia Palaeo-uplift. The andesite overlying the Naobaogou Formation has yielded U-Pb age of 277.3 ± 1.4 Ma. The additional dioritic porphyry dike intruded the Naobaogou and Laowopu Formations, which has an emplacement age of 236 ± 1 Ma. The above data identify that the basin formed ranges from Early Permian to Middle Triassic (277-236 Ma). Accordingly, the Inner Mongolia

  12. Geochronological, geochemical and Sr-Nd-Hf isotopic constraints on the petrogenesis of Late Cretaceous A-type granites from the Sibumasu Block, Southern Myanmar, SE Asia (United States)

    Jiang, Hai; Li, Wen-Qian; Jiang, Shao-Yong; Wang, He; Wei, Xiao-Peng


    The Late Cretaceous to Paleogene granitoids occur widespread in the Sibumasu block within Myanmar (SE Asia), which show a close association with tin-tungsten mineralization. However, the precise timing, petrogenesis and tectonic significance of these granitoids are poorly constrained so far. In this study, we present a detailed study on geochronology, elemental and Sr-Nd-Hf isotopic geochemistry for the Hermyingyi and Taungphila granites in southern Myanmar, with the aim of determining their petrogenesis and tectonic implications. LA-ICP-MS U-Pb dating of zircon grains from the two granites yield ages of 69-70 Ma, indicating a Late Cretaceous magmatic event. These granitic rocks are weakly peraluminous and belong to the high-K calc-alkaline series. They are both characterized by high SiO2, K2O + Na2O, FeOT/(FeOT + MgO) and Ga/Al ratios and low Al2O3, CaO, MgO, P2O5 and TiO2 contents, enriched in Rb, Th, U and Y, but depleted in Ba, Sr, P, and Eu, suggesting an A-type granite affinity. Moreover, they display prominent tetrad REE patterns and non-CHARAC trace element behavior, which are common in late magmatic differentiates with strong hydrothermal interaction or deuteric alteration. The granites belong to A2-type and probably formed at a high temperature and anhydrous condition. They have zircon εHf(t) values from - 12.4 to - 10.0 and whole-rock εNd(t) values from - 11.3 to - 10.6, with Paleoproterozoic TDM2 ages (1741-1922 Ma) for both Hf and Nd isotopes. Geochemical and isotopic data suggest that these A-type granites were derived from partial melting of the Paleoproterozoic continental crust dominated by metaigneous rocks with tonalitic to granodioritic compositions, without significant input of mantle-derived magma and followed by subsequent fractional crystallization. By integrating all available data for the regional tectonic evolution in SE Asia and adjacent regions, we attribute the formation of the Late Cretaceous A-type granites to a back-arc extension

  13. Geochronology and petrogenesis of Middle Permian S-type granitoid in southeastern Guangxi Province, South China: Implications for closure of the eastern Paleo-Tethys (United States)

    Li, Yan-Jun; Wei, Jun-Hao; Santosh, M.; Tan, Jun; Fu, Le-Bing; Zhao, Shao-Qing


    The Bangxi-Chenxing suture zone is an important region to address the history of closure of the eastern Paleo-Tethys ocean. Among the widespread S-type granite batholiths in the SE Guangxi Province adjacent to this suture is the Nali granodiorite. Here we report zircon U-Pb ages, major and trace element geochemistry, and Sr-Nd-Hf isotope geochemistry of a newly found Middle Permian S-type granite in this region. LA-ICP-MS zircon U-Pb geochronology yields emplacement ages of 265 ± 2 to 262 ± 2 Ma for the Nali granodiorite. Both the inner and outer phases are characterized by variable SiO2 (66.91-71.39 wt.%), high Al2O3 (12.99-14.04 wt.%), K2O + Na2O (4.78-5.98 wt.%), and A/CNK values (1.11-1.50), resembling those of typical S-type granites. The rocks are enriched in Rb, Th, U and light rare earth elements with weak negative Eu anomalies, and show depletion in Nb, Ta, Ti, with typical arc-like affinity. They have relatively high (87Sr/86Sr)i ratios of 0.7228 to 0.7331, low εNd(t) values of - 13.6 to - 11.9, and low zircon εHf(t) values (- 21.9 to - 8.8). High whole-rock Nd isotopic model ages (2.00-2.02 Ga) and zircon Hf isotopic model ages (1.59-2.20 Ga), indicate that they might have been generated by partial melting of ancient lower crust with minor contribution from mantle sources. The granodiorite shows continental arc affinity and likely formed within a subduction setting. By combining previously extant data for Permo-Triassic arc-/collision-related magmatism in the Jinshajiang-Ailaoshan-Song Ma and Bangxi-Chenxing suture zones, we propose that the subduction of the eastern Paleo-Tethys might have started during Middle Permian triggered by the northward subduction of the Indochina Block (ICB) beneath the South China Block (SCB) and closure during Middle Triassic.

  14. Thermal Evolution of Juvenile Subduction Zones ' New Constraints from Lu-Hf Geochronology on HP oceanic rocks (Halilbaǧi, Central Anatolia) (United States)

    Pourteau, Amaury; Scherer, Erik; Schmidt, Alexander; Bast, Rebecca


    The thermal structure of subduction zones plays a key role on mechanical and chemical processes taking place along the slab-mantle interface. Until now, changes through time of this thermal structure have been explored mostly by the means of numerical simulations. However, both "warm" (i.e., epidote-bearing), and "cold" (i.e., lawsonite-bearing) HP oceanic rocks have been reported in some fossil subduction complexes exposed at the Earth's surface (e.g., Franciscan Complex, California; Rio San Juan Complex, Hispañola; Halilbağı Unit, Central Anatolia). These a-priori "incompatible" rocks witness different thermal stages of ancient subduction zones and their study might provide complementary constraints to numerical models. To decipher the meaning of these contrasting metamorphic rocks in the Halilbağı Unit, we are carrying out Lu-Hf geochronology on garnet (grt) and lws from a variety of HP oceanic rocks, as well as the metamorphic sole of the overlying ophiolite. We selected five samples that are representative of the variety of metamorphic evolutions (i.e. peak conditions and P-T paths) encountered in this area. Preliminary analyses yielded 110 Ma (grt-hbl isochron) for a sub-ophiolitic grt amphibolite; 92 Ma (grt-omp) for an eclogite with prograde and retrograde ep; 90 Ma (grt-omp) for an eclogitic metabasite with prograde ep and retrograde ep+lws; 87 Ma (grt-gln) for a lws eclogite with prograde ep; and 86 Ma (grt-gln) for a blueschist with prograde and retrograde lws. These ages are mainly two-point isochrons. Further-refined data will be presented at the EGU General Assembly 2015, in Vienna. The consistent younging trend from "warm" to "cold" metamorphic rocks revealed by these first-order results points to metamorphic-sole formation during the initiation of intra-oceanic subduction at ~110 Ma, and subsequent cooling of the slab-mantle interface between 92 and 86 Ma. Therefore, the contrasting metamorphic evolutions encountered in the Halilbağı Unit

  15. Evolution of the Cretaceous magmatism in the Apuseni-Timok-Srednogorie metallogenic belt and implications for the geodynamic reconstructions: new insight from geochronology, geochemistry and isotope studies (United States)

    von Quadt, A.; Peytcheva, I.; Heinrich, C. A.; Frank, M.; Cvetkovic, V.


    Most major Cu-Au (-PGE) deposits in the Carpathian Balkan orogen are related to a 1500 km long belt of Upper Cretaceous magmatism extending from southern Romania through Yugoslavia to Bulgaria, with a likely continuation southeast of the Black Sea into Turkey, known as the Apuseni Banat Timok Srednogorie (ABTS) belt (Popov et al., 2000). In the frame of ABTS belt a new investigation was started to reveal the relation between Cretaceous magmatism and the Cu-Au-PGE deposits across the belt in East Serbia (Yugoslavia) and the Panagyurishte district (Bulgaria). The Late Cretaceous (Palaeogene?) magmatism of East Serbia developed along the Timok Magmatic Complex (TMC) in the east and the Ridanj Krepoljin Zone (RKZ) in the west. High precision U-Pb single zircon dating and a combination of isotope tracing, geochronological data as well as petrological data were used to provide additional data for the geodynamic evolution. A maximum life span of 2.5 Ma could be calculated for the first phase of volcanic activity in TMC, starting with the Amf-andesites of Veliki Kravelj (86.29 ± 0.32 Ma) and finishing with the Timozites (84.66 ± 0.5 Ma). Ore bearing magmatism in a single deposit (Veliki Kravelj) extended a maximum of 0.6 Ma ("pre-ore": 86.29 ± 0.32 Ma, "post-ore": 86.17 ± 0.15 Ma). Preliminary data for the dacites (70.3 ± 3.5 Ma) outcropping near Krepoljin give evidence for a shifting of the volcanic activity from TMC to RKZ together with changing the ore-deposit type from Cu-Au-PGE (TMC) to Pb-Zn-Cu (RKZ). Isotope tracing give evidence for mantle dominated source with increasing of crustal contamination in the same direction: (87Sr/86Sr ratios: 0.70388 to 0.706050, e-Hf-zircon data: +12 in TMC to +4.5 in RKZ). The Panagyurishte district (Bulgaria) show a duration time of the magmatic activity of 14 Ma, starting in the north at 92 Ma and finishing in the south at 78 Ma. Ore-related magmatism becomes younger in the same direction but finishes with 86 Ma. Multiple short

  16. Geochemical fingerprints and pebbles zircon geochronology: Implications for the provenance and tectonic setting of Lower Cretaceous sediments in the Zhucheng Basin (Jiaodong peninsula, North China)

    Indian Academy of Sciences (India)

    Jin-Long Ni; Jun-Lai Liu; Xiao-Ling Tang; Xiao-Xiao Shi; Hong Zhang; Shuai Han


    This paper conducts a petrogeochemical analysis of the Lower Cretaceous Laiyang Group’s sandstones, compares the results with the Neoproterozoic and Mesozoic intrusive rocks in the southern Sulu Orogen (also called the Jiaonan Orogen), and performs an LA-ICP-MS zircon geochronology analysis of the granitic gneisses in the conglomerates of the Laiyang Group and the intrusive rocks in the JiaonanOrogen. The results show that the major element proportions of the Longwangzhuang Formation (LWZ Fm) and Qugezhuang Formation (QGZ Fm) of the Laiyang Group in the Zhucheng Basin are similar. The values of various indices for the LWZ Fm are similar to the average sandstone content of activecontinental margins, whereas, the values for the QGZ Fm are similar to those of continental island arcs. The comparison shows that the REE characteristics of the LWZ Fm and QGZ Fm of Laiyang Group are similar to those of the Neoproterozoic granitic gneisses in the Jiaonan Orogen but obviously different from those of the Early Cretaceous intrusive rocks. A tectonic setting discrimination diagram revealsthat the provenance of the Laiyang Group includes features of active continental margins and continental island arcs. A number of indicators, e.g., the sandstone type, the Chemical Index of Alteration, the Chemical Index of Weathering, the Plagioclase Index of Alteration and the Index of Chemical Constituent Variation, indicated that the sandstones did not undergo intense weathering and were deposited near the source area. The zircon ages of the granitic gneiss material in the conglomerates at the base of the Laiyang Group are 790±8.4 Ma, close to the ages of the Neoproterozoic granitic gneiss in the Jiaonan Orogen (739–819 Ma), and very different from the ages of the Early Cretaceous intrusive rocks. Combiningwith paleocurrent directions, geochemical character, the Neoproterozoic granitic gneisses in the Jiaonan Orogen may represent the primary provenance of the Laiyang Group in the

  17. Geochronology, structural evolution of the Tierra Colorada area, and tectonic implications for southern Mexico and Chortís block connections (United States)

    Solari, L. A.; Torres de Leon, R.; Hernandez Pineda, G.; Sole', J.; Solis Pichardo, G.; Hernandez Treviño, T.


    The Tierra Colorada area is considered the best exposure of the northern limit between the Xolapa Complex and the Paleozoic-Precambrian Mixteca and Zapotecan terranes of southern Mexico. Geochemistry (major, trace, and REE) and geochronology on deformed granitoids in Tierra Colorada area show evidence of subduction-related magmatism during Mesozoic and Cenozoic. El Pozuelo granite, dated at 129 Ma (concordant U-Pb single-zircon age) is a foliated body, with local porphyric facies. Foliation is penetrative and characterized by oriented micas and stretched quartz. Geochemically it shows a metaluminous character, with arc-related trace and REE patterns. Because it cuts across high-grade gneisses of the Xolapa Complex, its intrusion postdates the second phase of metamorphism in the Xolapa Complex (> 130 Ma), which generated the orthogneisses-migmatites sequence. El Salitre granite (55 Ma, Rb-Sr 4 point isochron) is a foliated peraluminous body with a pronounced negative Zr anomaly. The protomylonitic Las PiÑas granite (54 +/- 6 Ma, U-Pb lower intercept) is characterized by ductile fabric, recognized by cinematic indicators such as mica fish, and indicates a top-to-the NNW sense of shear, dated at 50 +/- 1 Ma and 45.3 +/- 2 Ma (K-Ar on Bt, and Rb-Sr Bt-WR isochron, respectively), ages that indicate the time of ductile deformation along La Venta shear zone, as N-dipping detachment under low greenschist facies conditions. El Salitre and Las PiÑas granites are linked because of their similar HREE patterns. The Tierra Colorada, Xaltianguis and San Juan del Reparo granites, with calcalkaline subduction-related geochemistry, lack of penetrative deformation, and their ages of 30-34 Ma (U-Pb on zircons) constitute the last recognized granitic pulse in this area, post-dating the S-verging thrust of the Morelos limestones on top of sheared granites and volcanics. Together with previously published ages of magmatism in the studied area, we demonstrate here that calcalkaline

  18. Prolonged history of episodic fluid flow in giant hematite ore bodies: Evidence from in situ U Pb geochronology of hydrothermal xenotime (United States)

    Rasmussen, Birger; Fletcher, Ian R.; Muhling, Janet R.; Thorne, Warren S.; Broadbent, Graeme C.


    Absolute ages for hydrothermal mineralization and fluid flow are critical for understanding the geological processes that concentrate metals in the Earth's crust, yet many ore deposits remain undated because suitable mineral chronometers have not been found. The origin of giant hematite ore deposits, which are hosted in Precambrian banded-iron formations (BIFs), remains contentious. Several models have been formulated based on different sources and timing for the mineralizing fluids; supergene-metamorphic, syn-orogenic, late-orogenic extensional collapse and syn-extensional. Precise geochronology of the ore offers a means of discriminating between these models. In this study, two U-Pb chronometers, xenotime and monazite, have been identified in high-grade hematite ore bodies from the Mount Tom Price mine in the Hamersley Province, northwestern Australia. Both phosphate minerals occur as inclusions within the hematite ore and as coarser crystals intergrown with martite (hematite pseudomorph after magnetite) and microplaty hematite, indicating that the xenotime and monazite precipitated during mineralization. In situ U-Pb dating by ion microprobe indicates that both phosphate minerals grew during multiple discrete events. Our results suggest that ore genesis may have commenced as early as ˜ 2.15 Ga, with subsequent hydrothermal remobilization and/or mineralization at ˜ 2.05 Ga, ˜ 1.84 Ga, ˜ 1.67 Ga, ˜ 1.59 Ga, ˜ 1.54 Ga, ˜ 1.48 Ga and ˜ 0.85 Ga. The location of the ore bodies along ancient fault systems, and the coincidence of at least some of the U-Pb phosphate dates with episodes of tectonothermal activity in the adjacent Proterozoic Capricorn Orogen, implies that fluids were channelled through major structures in the southern Pilbara Craton during discrete phases of tectonic compression and extension. Our results show that the hematite ore bodies formed at sites of repeated focussed hydrothermal fluid flow. In contrast to the aforementioned models, our

  19. Zircon U-Pb geochronology, geochemistry, and Sr-Nd-Hf isotopes of granitoids in the Yulekenhalasu copper ore district, northern Junggar, China: Petrogenesis and tectonic implications (United States)

    Yang, Fuquan; Chai, Fengmei; Zhang, Zhixin; Geng, Xinxia; Li, Qiang


    The Yulekenhalasu porphyry copper deposit is located in the Kalaxiange'er metallogenic belt in northern Junggar, China. We present the results from zircon U-Pb geochronology, and geochemical and Sr-Nd-Hf isotope analyses of the granitoids associated with the ore deposits with a view to constrain their petrogenesis and tectonic setting. The granitoids consist of quartz diorite, diorite porphyry, porphyritic monzonite, and quartz porphyry, emplaced at 382, 379, 375-374, and 348 Ma, respectively, which span Late Devonian to early Carboniferous ages. The ore-bearing intrusion is mainly diorite porphyry, with subordinate porphyritic monzonite. The Late Devonian intrusions are characterized by SiO2 contents of 54.5-64.79 wt.%, Na2O contents of 3.82-8.24 wt.%, enrichment in Na, light rare-earth elements (LREEs), and large ion lithophile elements. They also display relative depletion in Y, Ba, P, Nb, Ta, and Ti, and weak negative Eu anomalies (δEu = 0.6-0.87). The early Carboniferous quartz porphyry is characterized by high SiO2 content (72.26-73.35 wt.%), enrichment in LREEs, K, and Sr, and relative depletion in Y (10.82-12.52 ppm) and Yb (1.06-1.15 ppm). The Late Devonian and early Carboniferous granitoids are characterized by positive ɛNd(t) values (5.2-10.1, one sample at - 1.9), positive ɛHf(t) values (7.46-18.45), low (87Sr/86Sr)i values (0.70363-0.70476), and young crustal residence ages. These data indicate that the sources of the granitoids were mainly mantle-derived juvenile rocks. Geochemical and Nd-Sr-Hf isotopic data demonstrate that the Late Devonian granitoids formed in an oceanic island arc, and they were formed from different sources, among which the mineralized diorite porphyry might have originated from a mixed slab-derived and mantle wedge melt source. The early Carboniferous quartz porphyry was likely emplaced in a mature island arc environment, and was probably derived from juvenile crust.

  20. High Spatial Resolution 40Ar/39Ar Geochronology of Impact Melt Breccias from Apollo 17 Boulders at Stations 2, 6, and 7 (United States)

    Mercer, C. M.; Hodges, K. V.; Jolliff, B. L.; Van Soest, M. C.; Wartho, J. A.; Weirich, J. R.


    Several boulders located at the bases of the North and South Massifs were among the primary field targets of the Apollo 17 mission to the Taurus-Littrow Valley on the Moon [1]. Some boulders are polylithologic, including Boulder 1 at Station 2 and the boulders at Stations 6 and 7. These boulders were the subjects of consortium studies [2, 3] that included 40Ar/39Ar geochronology to determine the ages of distinct lithologies within each boulder [e.g., 4-6]. We report new 40Ar/39Ar data for the impact melt breccias 72255, 76315, 77075, and 77135 obtained using the UV laser ablation microprobe (UVLAMP) methods of [7]. For 72255, we obtained a preliminary isochron date ca. 3814 Ma from 22 melt analyses, which is younger than published plateau dates (e.g., 3951-3835 Ma [4, 8]). Fifteen melt analyses of 76315 yield a preliminary isochron date ca. 3850 Ma, younger than the 3900 ± 16 Ma date reported by [8]. Melt analyses of 77075 yield preliminary dates between ca. 3797-3584 Ma, possibly reflecting partial loss of 40Ar. In this case, the oldest date may provide a minimum age for the formation of melt in 77075. Finally, the UVLAMP dates for the 77135 melt range from 3810-3361 Ma and corresponding Ca/K ratios range from ca. 100-6. Electron microprobe analyses of small (ca. 10s of microns wide) pockets of K-rich materials show that both K-rich glass and K-feldspar are present. The UVLAMP dates for 77135 likely reflect spatially variable 40Ar loss, consistent with published step heating results [e.g., 6]. References: [1] Schmitt (1973) Science, 182, 681-690. [2] Ryder (1993). Catalog of Apollo 17 Rocks: Volume 1 - Stations 2 and 3 (South Massif). LPI. [3] Ryder (1993). Catalog of Apollo 17 Rocks: Volume 4 - North Massif. LPI. [4] Leich et al. (1975) The Moon, 14, 407-444. [5] Cadogan & Turner (1976). LPSC, 7, 2267-2285. [6] Stettler et al. (1978). LPSC, 9, 1113-1115. [7] Mercer et al. (2015) Sci. Adv., 1, e1400050. [8] Dalrymple & Ryder (1996). JGR, 101, 26069-26084.

  1. Constraints on the collision and the pre-collision tectonic configuration between India and Asia from detrital geochronology, thermochronology, and geochemistry studies in the lower Indus basin, Pakistan (United States)

    Zhuang, Guangsheng; Najman, Yani; Guillot, Stéphane; Roddaz, Martin; Antoine, Pierre-Olivier; Métais, Grégoire; Carter, Andrew; Marivaux, Laurent; Solangi, Sarfraz H.


    Knowledge of the timing of India-Asia collision is a fundamental prerequisite for understanding the evolution of the Himalayan-Tibetan orogen and its role in global climate, oceanic chemistry, and ecological evolution. Despite much active research, the basic pre-collision tectonic configuration and the timing of terminal India-Asia suturing remain debated. For example, debates regarding when and how the intervening Kohistan-Ladakh arc was sutured with India and Asia still remain elusive; some models propose the arc collided with Asia at about 100 Ma, with India-Asia collision at ca. 55 Ma, whilst a newer model proposed the arc's collision with India at 50 Ma and subsequently with Asia at 40 Ma. Another example is the recent proposition that an oceanic Greater India Basin separated the Tethyan Himalaya microcontinent from the remaining Indian plate until 20- 25 Ma with the consumption of this oceanic basin marking the final collision at this time. These controversies relate to whether the commonly documented 50 Ma contact represents the terminal India-Asia suturing or the amalgamation between various arcs or microcontinents with India or Asia. Here we present an integrated provenance study of geochronology, thermochronology, and geochemistry on the late Cretaceous-Pleistocene sediments from the lower Indus basin on the Indian plate. The detrital zircon U-Pb and fission track data show a reversal in sediment source from a pure Indian signature to increasing inputs from the suture zone and the Asian plate between the middle Paleocene and early Oligocene. The Nd and Sr isotopes narrow down this change to 50 Ma by revealing input of Asian detritus and the establishment of a Nd & Sr isotopic pattern similar to the present-day Indus Fan by 50 Ma, with no significant variations up section, contrary to what might be expected if later major collisions had occurred. Our isotopic data indicate that Greater India was occupied by a fluvial-deltaic system, analogous to the

  2. Revisiting the Baja-British Columbia hypothesis: Argon-argon geochronology and paleomagnetism of the Ecstall, Butedale, and Smith Island plutons, British Columbia, Canada (United States)

    Brownlee, Sarah Jo

    The controversy concerning terrane displacements in western British Columbia, otherwise known as the Baja-BC hypothesis, is revisited using paleomagnetism, rock magnetism, and 40Ar/39Ar geochronology and thermal modeling. We studied three plutons in northwest British Columbia, the Ecstall, Butedale, and Smith Island plutons. The Ecstall pluton is the focus of this research as it has been the subject of considerable debate over the cause of systematic variations in paleomagnetic direction across the ˜25 km wide pluton. Hollister et al. (2004) proposed that magnetization carried by lamellar magnetism in ilmenite-hematite was variably reset during reheating by emplacement of the younger Quottoon plutonic complex on the eastern margin of the Ecstall pluton, thus causing the systematic variations in paleomagnetic direction. To test this hypothesis we conducted a number of studies on samples from the Ecstall pluton. First, a detailed mineralogic study using scanning electron microscopy, and transmission electron microscopy found evidence of microstructural and mineralogic changes to the magnetic oxides of the Ecstall pluton as a function of distance from the Quottoon plutonic complex. Second, rock magnetic experiments on single crystals of Fe-Ti oxides confirm changes to the magnetic properties of single crystals that are related to both microstructure and distance from the Quottoon plutonic complex. Third, spatial trends in 40Ar/39Ar cooling ages from hornblende and biotite were found to be consistent with reheating by the Quottoon plutonic complex. In addition to reheating by the Quottoon plutonic complex, thermal modeling suggests that spatial trends in 40Ar/39Ar cooling ages are most consistent with a thermal history that also includes northeast side up tilt of the Ecstall. The Smith Island and Butedale plutons do not show evidence for tilting in their 40Ar/39Ar cooling age trends, and paleomagnetic directions from these plutons are approximately concordant with the

  3. U-Pb zircon geochronology of Mesoproterozoic postorogenic rocks and implications for post-Ottawan magmatism and metallogenesis, New Jersey Highlands and contiguous areas, USA (United States)

    Volkert, R.A.; Zartman, R.E.; Moore, P.B.


    Postorogenic rocks are widespread in Grenville terranes of the north-central Appalachians where they form small, discordant, largely pegmatitic felsic intrusive bodies, veins, and dikes, and also metasomatic calcic skarns that are unfoliated and postdate the regional 1090 to 1030 Ma upper amphibolite- to granulite-facies metamorphism related to the Grenville (Ottawan) Orogeny. Zircons from magmatic and nonmagmatic rocks from northern New Jersey and southern New York were dated to provide information on the regional tectonomagmatic and metallogenic history following Ottawan orogenesis. We obtained U-Th-Pb zircon ages of 1004 ?? 3 Ma for pegmatite associated with the 1020 ?? 4 Ma Mount Eve Granite near Big Island, New York, 986 ?? 4 Ma for unfoliated, discordant pegmatite that intrudes supracrustal marble at the Buckwheat open cut, Franklin, New Jersey, ???990 Ma for a silicate-borate skarn layer in the Franklin Marble at Rudeville, New Jersey, and 940 ?? 2 Ma for a calc-silicate skarn layer at Lower Twin Lake, New York. This new data, together with previously published ages of 1020 ?? 4 to 965 ?? 10 Ma for postorogenic rocks from New Jersey and southern New York, provide evidence of magmatic activity that lasted for up to 60 Ma past the peak of high-grade metamorphism. Postorogenic magmatism was almost exclusively felsic and involved relatively small volumes of metaluminous to mildly peraluminous melt that fractionated from an A-type granite parent source. Field relationships suggest the melts were emplaced along lithosphere-scale fault zones in the Highlands that were undergoing extension and that emplacement followed orogenic collapse by least 30 Ma. Postorogenic felsic intrusions correspond to the niobium-yttrium-fluorine (NYF) class of pegmatites of C??erny?? (1992a). Geochronologic data provide a temporal constraint on late-stage hydrothermal activity and a metallogenic event in New Jersey at ???990 to 940 Ma that mineralized pegmatites with subeconomic to

  4. Style and age of late Oligocene-early Miocene deformation in the southern Stillwater Range, west central Nevada: Paleomagnetism, geochronology, and field relations (United States)

    Hudson, Mark R.; John, David A.; Conrad, James E.; McKee, Edwin H.


    Paleomagnetic and geochronologic data combined with geologic mapping tightly restrict the timing and character of a late Oligocene to early Miocene episode of large magnitude extension in the southern Stillwater Range and adjacent regions of west central Nevada. The southern Stillwater Range was the site of an Oligocene to early Miocene volcanic center comprising (1) 28.3 to 24.3 Ma intracaldera ash flow tuffs, lava flows, and subjacent plutons associated with three calderas, (2) 24.8 to 20.7 Ma postcaldera silicic dikes and domes, and (3) unconformably overlying 15.3 to 13.0 Ma dacite to basalt lava flows, plugs, and dikes. The caldera-related tuffs, lava flows, and plutons were tilted 60°-70° either west or east during the initial period of Cenozoic deformation that accommodated over 100% extension. Directions of remanent magnetization obtained from these extrusive and intrusive, caldera-related rocks are strongly deflected from an expected Miocene direction in senses appropriate for their tilt. A mean direction for these rocks after tilt correction, however, suggests that they were also affected by a moderate (33.4° ± 11.8°) component of counterclockwise vertical axis rotation. Paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24.8 to 20.7 Ma postcaldera dikes and domes. In detail, an apparent decrease in rotation with decreasing age of individual, isotopically dated bodies of the postcaldera group indicates that most tilting occurred between 24.4 and 24.2 Ma. The onset of tilting immediately following after the final caldera eruptions suggests that the magmatism and deformation were linked. Deformation was not driven by magma buoyancy, however, because tilting equally affected the caldera systems of different ages, including their plutonic roots. It is more likely that regional extension was focused in the southern Stillwater Range due to magmatic warming and reduction of tensile strength of the brittle crust

  5. Sample-scale zircon geochemical and geochronological heterogeneities as indicators of residual liquid infiltration events in the incrementally assembled Caleu Pluton, Central Chile (United States)

    Molina, P. G.; Parada, M. A.; Gutiérrez, F. J.; Ma, C.; Li, J.; Liu, Y.


    The Upper Cretaceous metaluminous Caleu Pluton is emplaced at a depth equivalent of 2kbar and consists of four lithological zones: the Gabbro-Dioritic Zone (GDZ), the Quartz-Monzodioritic Zone (QMDZ), the Granodioritic Zone (GZ) and the Monzogranitic Zone (MGZ). The zones would have been fed from a deeper magma reservoir emplaced at a 4 kbar. U238/Pb206 LA-ICP-MS geochronology of zircon grains of the four lithological zones (82 analyzed spots, 4 samples) indicates a maximum zircon crystallization range of ca. 106-91 Ma for the pluton as a whole. The U-Pb zircon age distribution of the four samples shows three inflection points at about 101, 99 and 96 Ma, separating four zircon crystallization events with the following weighted average ages and 2σ confidence intervals: 103.×1.6 Ma (n=4), 100.3×0.68 Ma (n=14), 97.49×0.49 Ma (n=25) and 94.66×0.44 Ma (n=30). The GDZ sample records the first three events, the GZ and QMDZ samples record the last three events while the MGZ only have zircons formed during the last two events. It is interesting to note that the youngest event of zircon formation coincide with the Ar/Ar cooling ages (95-93 Ma) previously obtained in hornblende, biotite and plagioclase of the four lithological zones, as a consequence of a rapid pluton exhumation. Temperatures of zircon crystallization (Ti-in-Zrn) obtained in each sample are variable and roughly lower than the zircon saturation temperatures. Most of the Ti-in-Zrn temperatures indicate late-stage crystallization conditions, consistent with the calculated melt composition from which zircons would have crystallized and the observed coexistence of zircons with quartz-orthoclase symplectites, hornblende and interstitial anhedral biotite. There are variable and overlapped total incompatible element concentrations in zircons of the four lithological zones regardless its age and Ti-in-Zrn temperatures, indicating that the melts from which zircon crystallized at different moments, were equivalent

  6. Using an independent geochronology based on palaeomagnetic secular variation (PSV) and atmospheric Pb deposition to date Baltic Sea sediments and infer 14C reservoir age (United States)

    Lougheed, Bryan C.; Snowball, Ian; Moros, Matthias; Kabel, Karoline; Muscheler, Raimund; Virtasalo, Joonas J.; Wacker, Lukas


    Dating of sediment cores from the Baltic Sea has proven to be difficult due to uncertainties surrounding the 14C reservoir age and a scarcity of macrofossils suitable for dating. Here we present the results of multiple dating methods carried out on cores in the Gotland Deep area of the Baltic Sea. Particular emphasis is placed on the Littorina stage (8 ka ago to the present) of the Baltic Sea and possible changes in the 14C reservoir age of our dated samples. Three geochronological methods are used. Firstly, palaeomagnetic secular variations (PSV) are reconstructed, whereby ages are transferred to PSV features through comparison with varved lake sediment based PSV records. Secondly, lead (Pb) content and stable isotope analysis are used to identify past peaks in anthropogenic atmospheric Pb pollution. Lastly, 14C determinations were carried out on benthic foraminifera (Elphidium spec.) samples from the brackish Littorina stage of the Baltic Sea. Determinations carried out on smaller samples (as low as 4 μg C) employed an experimental, state-of-the-art method involving the direct measurement of CO2 from samples by a gas ion source without the need for a graphitisation step - the first time this method has been performed on foraminifera in an applied study. The PSV chronology, based on the uppermost Littorina stage sediments, produced ten age constraints between 6.29 and 1.29 cal ka BP, and the Pb depositional analysis produced two age constraints associated with the Medieval pollution peak. Analysis of PSV data shows that adequate directional data can be derived from both the present Littorina saline phase muds and Baltic Ice Lake stage varved glacial sediments. Ferrimagnetic iron sulphides, most likely authigenic greigite (Fe3S4), present in the intermediate Ancylus Lake freshwater stage sediments acquire a gyroremanent magnetisation during static alternating field (AF) demagnetisation, preventing the identification of a primary natural remanent magnetisation for

  7. Geochronology and geochemistry of Early Jurassic volcanic rocks in the Erguna Massif, northeast China: Petrogenesis and implications for the tectonic evolution of the Mongol-Okhotsk suture belt (United States)

    Wang, Wei; Tang, Jie; Xu, Wen-Liang; Wang, Feng


    The Mongol-Okhotsk suture belt played an important role in the tectonic evolution of northeast Asia during the Mesozoic. However, few studies have examined the influence of this tectonic belt on the geological evolution of northeast China. In this paper, we present zircon U-Pb geochronology, major and trace element geochemistry, and zircon Hf-O isotopic data for Early Jurassic volcanic rocks in the Erguna Massif of northeast China, with the aim of constraining the evolution of the Mongol-Okhotsk suture belt and its influence on the tectonic history of China during the Early Jurassic. Zircon U-Pb dating indicates that the trachybasalt and basaltic andesite in the study area were erupted between 193 ± 5 Ma and 181 ± 9 Ma (i.e., in the Early Jurassic). These Early Jurassic volcanic rocks belong to the high-K calc-alkaline series and are enriched in large ion lithophile elements and light rare earth elements, as well as being depleted in heavy rare earth elements and high field strength elements such as Nb and Ta. The rocks show a small negative Eu anomaly. The zircon εHf (182 Ma) values of the volcanic rocks range from - 1.9 to + 5.1, corresponding to TDM1 values of 640-901 Ma and TDM2 values of 901-1345 Ma. Zircons from two volcanic rocks yield δ18O values of 7.2‰ ± 1.5‰ (n = 19) and 6.6‰ ± 0.7‰ (n = 35). Geochemically, these Early Jurassic volcanic rocks are similar to those from active continental margin settings, and their primary magmas could have been derived from the partial melting of a lithospheric mantle wedge modified by fluid from a subducted slab. The discovery of Early Jurassic calc-alkaline volcanic rocks in the Erguna Massif, together with the coeval porphyry Cu-Mo deposits, indicates that an active continental margin existed in the Erguna area during the Early Jurassic. Taken together, we conclude that southward subduction of the Mongol-Okhotsk oceanic plate took place beneath the Erguna Massif during the Early Jurassic.

  8. The timing of eclogite facies metamorphism and migmatization in the Orlica–Śnieżnik complex, Bohemian Massif: Constraints from a multimethod geochronological study (United States)

    Brocker, M.; Klemd, R.; Cosca, M.; Brock, W.; Larionov, A.N.; Rodionov, N.


    The Orlica–Śnieżnik complex (OSC) is a key geological element of the eastern Variscides and mainly consists of amphibolite facies orthogneisses and metasedimentary rocks. Sporadic occurrences of eclogites and granulites record high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic conditions. A multimethod geochronological approach (40Ar–39Ar, Rb–Sr, Sm–Nd, U–Pb) has been used to gain further insights into the polymetamorphic evolution of eclogites and associated country rocks. Special attention was given to the unresolved significance of a 370- to 360 Ma age group that was repeatedly described in previous studies. Efforts to verify the accuracy of c.370 Ma K–Ar phengite and biotite dates reported for an eclogite and associated country-rock gneiss from the location Nowa Wieś suggest that these dates are meaningless, due to contamination with extraneous Ar. Extraneous Ar is also considered to be responsible for a significantly older 40Ar–39Ar phengite date of c. 455 Ma for an eclogite from the location Wojtowka. Attempts to further substantiate the importance of 370–360 Ma zircon dates as an indicator for a melt-forming high-temperature (HT) episode did not provide evidence in support of anatectic processes at this time. Instead, SHRIMP U–Pb zircon dating of leucosomes and leucocratic veins within both orthogneisses and (U)HP granulites revealed two age populations (490–450 and 345–330 Ma respectively) that correspond to protolith ages of the magmatic precursors and late Variscan anatexis. The results of this study further underline the importance of Late Carboniferous metamorphic processes for the evolution of the OSC that comprise the waning stages of HP metamorphism and lower pressure HT overprinting with partial melting. Eclogites and their country rocks provided no chronometric evidence for an UHP and ultrahigh-temperature episode at 387–360 Ma, as recently suggested for granulites from the OSC, based on Lu–Hf garnet

  9. Triassic High-P Metamorphism of the central Qiangtang terrane, Tibet; constraints using mineral equilibria modelling and 40Ar/39Ar geochronology (United States)

    Rajkumar, A.; Hui, L.; Clarke, G. L.; Aitchison, J. C.; Forster, M. A.


    The SE-trending Qiangtang metamorphic belt (QMB) stretches more than 500 km through the Qiangtang terrane in central Tibet and comprises tectonically disrupted blueschist and eclogite in lower-grade garnet-phengite-bearing schist and quartzite. These rocks record the closure of a paleo-Tethyan Triassic ocean that formerly separated Cathaysian and Gondwana components of Asia, now forming the northern and southern Qiangtang blocks. Eclogite is extensively recrystallized to high-P amphibolite and greenschist facies assemblages, formed during water ingression that accompanied terrane uplift. P-T pseudosections constructed in Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O (NCKMASHTO) in the context of petrography and mineral chemistry provides the ability to recover a dynamic PT history for the eclogite facies assemblages. Prograde (S1) assemblages for the Gemu Co eclogite are predicted to have formed at P≈21.5 kbars and T≈505°C and involved garnet, glaucophane, omphacite, rutile, lawsonite and chlorite, based on garnet composition and inferred pseudomorphs after lawsonite. Peak (S2) assemblages of garnet, barroisite, omphacite, rutile, epidote and quartz reflect P≈15 kbars and T≈570°C. Based on textural relations, post-peak stages can be divided into epidote-amphibolite and greenschist facies. The geothermal gradient for the prograde S1 assemblage and the peak S2 assemblage is 7.1 and 11.5°C/km respectively.40Ar/39Ar geochronology of phengitic mica using step heating in recrystallized eclogite components and surrounding garnet-mica schist components both yield maximum ages ranging 230-220 Ma. The congruency in ages of the deeply subducted high-pressure eclogites to the surrounding garnet phengite schists indicate they were the most probable source of fluids to extensively recrystallize most of the high-pressure eclogite components in the high-pressure belt. The P-T history of the high-P rocks of the QMB records the deep subduction of paleo-Tethyan oceanic crust to

  10. Geochemical fingerprints and pebbles zircon geochronology: Implications for the provenance and tectonic setting of Lower Cretaceous sediments in the Zhucheng Basin (Jiaodong peninsula, North China) (United States)

    Ni, Jin-Long; Liu, Jun-Lai; Tang, Xiao-Ling; Shi, Xiao-Xiao; Zhang, Hong; Han, Shuai


    This paper conducts a petrogeochemical analysis of the Lower Cretaceous Laiyang Group's sandstones, compares the results with the Neoproterozoic and Mesozoic intrusive rocks in the southern Sulu Orogen (also called the Jiaonan Orogen), and performs an LA-ICP-MS zircon geochronology analysis of the granitic gneisses in the conglomerates of the Laiyang Group and the intrusive rocks in the Jiaonan Orogen. The results show that the major element proportions of the Longwangzhuang Formation (LWZ Fm) and Qugezhuang Formation (QGZ Fm) of the Laiyang Group in the Zhucheng Basin are similar. The values of various indices for the LWZ Fm are similar to the average sandstone content of active continental margins, whereas, the values for the QGZ Fm are similar to those of continental island arcs. The comparison shows that the REE characteristics of the LWZ Fm and QGZ Fm of Laiyang Group are similar to those of the Neoproterozoic granitic gneisses in the Jiaonan Orogen but obviously different from those of the Early Cretaceous intrusive rocks. A tectonic setting discrimination diagram reveals that the provenance of the Laiyang Group includes features of active continental margins and continental island arcs. A number of indicators, e.g., the sandstone type, the Chemical Index of Alteration, the Chemical Index of Weathering, the Plagioclase Index of Alteration and the Index of Chemical Constituent Variation, indicated that the sandstones did not undergo intense weathering and were deposited near the source area. The zircon ages of the granitic gneiss material in the conglomerates at the base of the Laiyang Group are 790 ± 8.4 Ma, close to the ages of the Neoproterozoic granitic gneiss in the Jiaonan Orogen (739-819 Ma), and very different from the ages of the Early Cretaceous intrusive rocks. Combining with paleocurrent directions, geochemical character, the Neoproterozoic granitic gneisses in the Jiaonan Orogen may represent the primary provenance of the Laiyang Group in the

  11. Constraints on the timing of Quaternary volcanism and duration of magma residence at Ciomadul volcano, east-central Europe, from combined U-Th/He and U-Th zircon geochronology (United States)

    Harangi, S.; Lukács, R.; Schmitt, A. K.; Dunkl, I.; Molnár, K.; Kiss, B.; Seghedi, I.; Novothny, Á.; Molnár, M.


    High-spatial resolution zircon geochronology was applied to constrain the timescales of volcanic eruptions of the youngest, mostly explosive volcanic phase of Ciomadul volcano (Carpathian-Pannonian region, Romania). Combined U-Th and (U-Th)/He zircon dating demonstrates that intermittent volcanic eruptions occurred in a time range of 56-32 ka. The reliability of the eruption dates is supported by concordant ages obtained from different dating techniques, such as zircon geochronology, radiocarbon analysis, and infrared stimulated luminescence dating for the same deposits. The new geochronological data suggest that volcanism at Ciomadul is much younger (volcanic phase occurred after an apparent lull in volcanism that lasted for several 10's of ka, after a period of lava dome extrusion that defines the onset of the known volcanism at Ciomadul. At least four major eruptive episodes can be distinguished within the 56-32 ka period. Among them, relatively large (sub-plinian to plinian) explosive eruptions produced distal tephra covering extended areas mostly southeast from the volcano. The 38.9 ka tephra overlaps the age of the Campanian Ignimbrite eruption and has an overlapping dispersion axis towards the Black Sea region. The wide range of U-Th model ages of the studied zircons indicates prolonged existence of a low-temperature (volcanism. Even the youngest U-Th model ages obtained for the outermost 4 μm rim of individual zircon crystals predate the eruption by several 10's of ka. The zircon age distributions suggest re-heating above zircon saturation temperatures via injection of hot mafic magmas prior to eruption. Intermittent intrusions of fresh magma could play a significant role in keeping the intrusive silicic magmatic reservoir in a partially melted for prolonged period. The previous history of Ciomadul suggests that melt-bearing crystal mush resided beneath the volcano, and was rapidly remobilized after a protracted (several 10's of ka) lull in volcanism to

  12. Petrology and geochronology of ultra-high temperature granulites of the basic unit of Andriamena (north-central Madagascar). Contribution of in-situ U-Th-Pb geochronology to the interpretation of P-T paths; Petrologie et geochronologie des granulites de ultra-hautes temperatures de l'unite basique d'Andriamena (Centre-Nord Madagascar). Apport de la geochronologie in-situ U-Th-Pb a l'interpretation des trajets P-T

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    Goncalves, Ph


    The understanding of orogenesis processes requires the acquisition of data allowing to follow the pressure (P), temperature (T) and deformation (D) variations with time (t). The construction of P-T-D-t paths necessary implies a pluri-disciplinary approach which combines a petrological and structural analysis to the acquisition of geochronological data. Such an approach is made in this work and applied to the ultra-high temperature poly-metamorphic granulites of the Andriamena basic unit of north-central Madagascar. A particular attention is given to the correlation between petrological and geochronological data in order to discuss the signification of the P-T paths and ages obtained in a poly-metamorphic context. The thermomechanical evolution of the Andriamena unit is characterized by the superimposition of at least 4 thermal events ({approx}2.7 Ga, 2.52-2.54 Ga, 790-730 Ma and 530-500 Ma). Because of their refractory character, the ultra-high temperature Mg-granulites have kept several mineralogical textures allowing to redraw a complex and apparently continuous P-T path. Nevertheless, the geochronological data obtained by in-situ electron microprobe dating on monazite show that the petrographic path is discontinuous and the result of the superimposition of two distinct thermal events at 2.5 Ga and 790-730 Ma. It is shown that part of the petrographic path (isothermal decompression) corresponds to an apparent path without tectonic signification. It is suggested that this apparent decompression results from the equilibration of ultra high temperature refractory parageneses (2.5 Ga) at lower pressures, during the middle neo-proterozoic event (790-730 Ma) without reaching the P-T conditions of the observed mineralogical reactions. The distinction between petrographic path and real P-T path shows the importance of determining the absolute age of the different metamorphic minerals and reactions in order to date different parts of the P-T path. This objective has been

  13. Geochemistry and geochronology U-Pb and Sm/Nd data of the orthogneiss from Pontalina region (GO), Brazil; Geoquimica e geocronologia U-Pb e Sm-Nd dos ortognaisses da regiao de Pontalina (GO), Brasil

    Energy Technology Data Exchange (ETDEWEB)

    Navarro, Guillermo Rafael Beltran, E-mail: [Universidade Estadual Paulista Julio de Mesquisa Filho (UNESP), Rio Claro, SP (Brazil). Inst. de Geociencias e Ciencias Exatas. Programa de Pos-Graduacao em Geologia Regional; Zanardo, Antenor, E-mail: [Universidade Estadual Paulista Julio de Mesquisa Filho (UNESP), Rio Claro, SP (Brazil). Dept. de Petrologia e Metalogenia; Conceicao, Fabiano Tomazini da, E-mail: [Universidade Estadual Paulista Julio de Mesquisa Filho (UNESP), Rio Claro, SP (Brazil). Dept. de Planejamento Territorial e Geoprocessamento; Montibeller, Cibele Carolina, E-mail: [Universidade Estadual Paulista Julio de Mesquisa Filho (UNESP), Rio Claro, SP (Brazil). Curso de Geologia


    Orthogneisses of tonalitic to granodioritic composition with calc-alkaline, metaluminous to peraluminous characteristics crop out at the region of Pontalina. The orthogneisses exhibit negative anomalies of Nb, Ta, P and Ti relative to rare earth elements (REE) and litophile elements, besides low contents of Nb, Ta, Y and Yb, similar to the rocks formed in subduction environments. U-Pb geochronological data show a neoproterozoic (681.5 {+-} 7.7 Ma) crystallization for the orthogneisses. The Nd isotope data yielded weakly negative to positive {epsilon}{sub Nd}(T) (+3.2 up to -0.1), with T{sub DM} model ages of {approx} 1.2 Ga. The combined geochemical and isotopic data suggest that the protolith of these orthogneisses are representative of neoproterozoic juvenile magmas, probably developed in an island arc system, generated during a ca.700 and 600 Ma event of crustal accretion in the western edge of the Sao Francisco Craton. (author)

  14. U-Pb zircon geochronology and phase equilibria modelling of a mafic eclogite from the Sumdo complex of south-east Tibet: Insights into prograde zircon growth and the assembly of the Tibetan plateau (United States)

    Weller, O. M.; St-Onge, M. R.; Rayner, N.; Waters, D. J.; Searle, M. P.; Palin, R. M.


    The Sumdo complex is a Permian-Triassic eclogitic metamorphic belt in south-east Tibet, which marks the location of a suture zone that separates the northern and southern Lhasa terranes. An integrated geochronological and petrological study of a mafic eclogite from the complex has constrained its tectonometamorphic history and provides a case study of zircon growth in eclogite as a product of prograde dissolution-precipitation. In situ U-Pb geochronology indicates that the eclogite contains a single population of zircon with a crystallisation age of 273.6 ± 2.8 Ma. The morphology and chemistry of the zircon grains are consistent with growth by dissolution-precipitation of protolith magmatic zircon. The presence of zircon grains as inclusions in the cores of peak phases indicates that zircon dissolution-precipitation occurred during prograde metamorphism, and calculated pressure and temperature conditions over which mineral inclusions in zircon are stable suggest that the zircon most likely precipitated at 15.5-16.5 kbar and 500-560 °C. Subsequent peak metamorphism is calculated to have reached pressure-temperature conditions of 27 ± 1 kbar and 670 ± 50 °C. Previous studies, which have documented a range of peak metamorphic conditions from high- to ultrahigh-pressure at c. 266-230 Ma, indicate that the Sumdo complex is a composite belt that experienced protracted eclogite exhumation. The results of this study are consistent with this interpretation, and extend the age range of high-pressure metamorphism in the complex to over 40 Myr. Analysis of published pressure-temperature-time data indicates two systematic behaviours within this spread. First, peak metamorphic temperatures declined over time. Second, eclogite exhumation occurred in two discrete intervals: soon after formation, and during the demise of the subduction zone. The latter behaviour serves as a reminder that eclogite exhumation is the exception rather than the rule.

  15. Usbnd Pb zircon geochronology constraints on the ages of the Tananao Schist Belt and timing of orogenic events in Taiwan: Implications for a new tectonic evolution of the South China Block during the Mesozoic (United States)

    Chen, Wen-Shan; Huang, Yi-Chang; Liu, Chang-Hao; Feng, Han-Ting; Chung, Sun-Lin; Lee, Yuan-Hsi


    The Tananao Schist Belt is a low-pressure metamorphic complex comprised of three lithological units of marble, schist formation, and granite, in ascending order of proportion. Previous studies have found that the schist formation was formed during the Mesozoic. However, there is a lack of geochronological data to corroborate the schist protolith and metamorphic ages. In this study, we have used Usbnd Pb zircon geochronology to provide a time frame for the creation of the schist formation and metamorphism, as well as a new tectonic model. Twenty-three schist and eleven meta-magmatic samples were used for Usbnd Pb dating by LA-ICP-MS. Results from the youngest peak age of detrital zircon indicate that the schist formed in a new depositional age of 120-110 Ma and, therefore, is different from the previously estimated age of the Paleozoic-Mesozoic. Additionally, the block-in-matrix schist indicates an age that ranges from 270 to 80 Ma and was inferred from the chaotic deposits to be a metamorphic mélange. The Tananao Schist Belt appears to represent an arc-trench system that formed during the Cretaceous in the South China Block margin. Moreover, the youngest Usbnd Pb age of 80 Ma from the leucogranite dike and schist, constrains the upper age limit for a metamorphism that is younger than the previously accepted age of 100-90 Ma. The contact layer between Permian-Triassic marble and the overlying early Cretaceous schists remains a chloritoid bed that is, therefore, considered to be a paleosol at the unconformity, which formed over an extended duration of 60 ± 30 Ma. The stratigraphic contact indicates a long period of erosion during the Jurassic and suggests that a tectonic event occurred. However, we propose that two important metamorphic events took place prior to, and following, the schist formation during the Jurassic and late Cretaceous orogenic events.

  16. Multiple provenance of rift sediments in the composite basin-mountain system: Constraints from detrital zircon U-Pb geochronology and heavy minerals of the early Eocene Jianghan Basin, central China (United States)

    Wu, Lulu; Mei, Lianfu; Liu, Yunsheng; Luo, Jin; Min, Caizheng; Lu, Shengli; Li, Minghua; Guo, Libin


    Zircon U-Pb geochronology and heavy minerals are used in combination to provide valuable insights into the provenance of the early Eocene Jianghan Basin, central China. Five samples for zircon U-Pb dating and eighty-five samples for heavy mineral analysis were collected from drill cores or cuttings of the Xingouzui Formation. Most analyzed zircons are of magmatic origin, with oscillatory zoning. Detrital zircons from sample M96 located on eastern basin have two dominant age groups of 113-158 Ma and 400-500 Ma, and the other samples located on southern basin have three prominent age populations at 113-158 Ma, 400-500 Ma and 700-1000 Ma. Samples on different parts of the basin show distinct differences in heavy mineral compositions and they apparently divide into two groups according to the content of rutile (higher or lower than 4%). The spatial variations of zircon-tourmaline-rutile (ZTR) indices are marked by some noticeable increasing trends from basin margins to the inner part of the basin. Compared with the potential source areas, this study clarifies the multiple source characteristics of the Jianghan basin in the composite basin-mountain system. The majority of clastic material was supplied from the north source area through rift-trough sediment-transport pathways, and the eastern, southern and northwestern source areas also contributed detritus to the basin. This clastic material is broadly dispersed in the basin. The early Eocene paleogeography implies that rift architecture and rifting process had an important influence on sediment dispersal. This study shows that integrated zircon U-Pb geochronology and heavy mineral analysis is a useful and powerful method to identify sediment provenance.

  17. New Insights Into Volcanic Hazards in Western Mexico: Multiple Cone-Building Episodes at Arc Stratovolcanoes Revealed by 40Ar/39Ar Geochronology (United States)

    Frey, H. M.; Lewis-Kenedi, K.; Lange, R. A.; Hall, C. M.; Delgado-Granados, H.


    The detailed eruptive histories of two andesitic stratocones, Volcáns Ceboruco and Tequila, in the western Mexican arc have been documented using 40Ar/39Ar geochronology. The volumes of these volcanoes were obtained with mapping, airphotos, and digital elevation models. The age and volume data constrain the rate and duration of major cone-building events, which bears on the longevity of the underlying upper-crustal magma chambers that fed the eruptions. The results indicate that at each stratovolcano there were two discrete cone-building events, separated by a hiatus. At V. Tequila, six samples from the edifice yielded dates (196 +/- 8, 196 +/- 19, 178 +/- 8, 191 +/- 13, 216 +/- 11, and 198 +/- 11 ka; errors are 1 sigma) with a mean eruption age of 196 +/- 12 ka. Thus the bulk of the main edifice ( ˜31 km3) erupted within 24 kyrs (at the 2 sigma level), leading to a cone-building rate of > 1.3 km3/kyr. After a hiatus of ˜110 kyrs, ˜14 km3 of andesite erupted along the NW and SE flanks of V. Tequila at 90 +/- 19 ka. The last activity at V. Tequila produced a ˜2 km3 parasitic cone at ˜60 ka. Since an eruption has not occurred in the last 60 kyrs, V. Tequila is often considered an extinct volcano. This may be the view held by the > 75,000 inhabitants of the town of Tequila located on the northern flanks. A similar history of two discrete cone-building events is found at V. Ceboruco, ˜75 km to the NW. Seven samples taken from various parts of the edifice, including the inner caldera wall, indicate an initial cone-building event at ˜45 ka in which ˜37 km3 of andesite erupted. After a hiatus of nearly 44 kyrs, a second eruptive period began ˜1000 years ago. The first eruption to occur after the hiatus was Plinian and released 3-4 km3 of dacite. In the last 1 kyr, 9.5 km3 of andesite and dacite erupted effusively, culminating in the historic 1870 flow. The sobering conclusion, in terms of volcanic hazards assessment, is that the only Plinian eruption to occur

  18. Geochronological and geochemical constraints on the petrogenesis of late Cretaceous volcanic rock series from the eastern Sakarya zone, NE Anatolia-Turkey (United States)

    Aydin, Faruk; Oǧuz, Simge; Şen, Cüneyt; Uysal, İbrahim; Başer, Rasim


    New SHRIMP zircon U-Pb ages and whole-rock geochemical data as well as Sr-Nd-Pb and δ18O isotopes of late Cretaceous volcanic rock series from the Giresun and Artvin areas (NE Anatolia, Turkey) in the northern part of the eastern Sakarya zone (ESZ) provide important evidence for northward subduction of the Neo-Tethyan oceanic lithosphere along the southern border of the ESZ. In particular, tectonic setting and petrogenesis of these subduction-related volcanites play a critical role in determining the nature of the lower continental crust and mantle dynamics during late Mesozoic orogenic processes in this region. The late Cretaceous time in the ESZ is represented by intensive volcanic activities that occurred in two different periods, which generally consist of alternation of mafic-intermediate (basaltic to andesitic) and felsic rock series (dacitic to rhyolitic) within each period. Although there is no geochronological data for the lower mafic-intermediate rock series of the first volcanic period, U-Pb zircon dating from the first cycle of felsic rocks yielded ages ranging from 88.6±1.8 to 85.0±1.3 Ma (i.e. Coniacian-Early Santonian). The first volcanic period in the region is generally overlain by reddish biomicrite-rich sedimentary rocks of Santonian-Early Campanian. U-Pb zircon dating for the second cycle of mafic-intermediate and felsic rocks yielded ages varying from 84.9±1.7 to 80.8±1.5Ma (i.e. Early to Middle Campanian). The studied volcanic rocks have mostly transitional geochemical character changing from tholeiitic to calc-alkaline with typical arc signatures. N-MORB-normalised multi-element and chondrite-normalised rare earth element (REE) patterns show that all rocks are enriched in LILEs (e.g. Rb, Ba, Th) and LREEs (e.g. La, Ce) but depleted in Nb and Ti. In particular, the felsic samples are characterised by distinct negative Eu anomalies. The samples are characterized by a wide range of Sr-Nd-Pb isotopic compositions (initial ɛNd values from -7

  19. Geology, Geochemistry and Geochronology of the Upper Cretaceous high-K volcanics in the southern Part of the Eastern Pontides: Implications for Mesozoic Geodynamic Evolution of NE Turkey (United States)

    Eyuboglu, Y.


    The Eastern Pontide Orogenic Belt is one of the most complex geodynamic settings in the Alpine-Mediterranean region. Its geodynamic evolution is very controversial due to lack of systematic, quantitative structural, geochemical and geochronological data. This belt is divided into three subgroups: northern, southern and axial zones, distinguished from north to south by different lithological units, facies changes and tectonic characteristics. Especially, the southern zone is very attractive with its numerous rock associations such as alaskan-type mafic-ultramafic intrusions, shoshonitic and ultrapotassic volcanics, adakitic magmatics, glaucophane-bearing gabbros, metamorphic and ultramafic massifes. This study focuses on the petrology, geotectonic setting and evidence for subduction polarity of the Upper Cretaceous shoshonitic and ultrapotassic volcanics exposed in the most southerly part of the eastern Pontide magmatic arc. Geological, geochemical and isotopic data indicate that there were two distinct cycles of high-K volcanic activity in the southern part of the eastern Pontide magmatic arc during the Late Cretaceous. The first cycle (Early Campanian), represented by shoshonitic trachyandesites and associated pyroclastics, containing high K2O (2.74-4.81 wt %) and Na2O (3.60-5.51 wt %), overlies the Middle-Cretaceous ophiolitic-olistostromal melange formed during the rifting stage of a back-arc basin (Neotethys). The second cycle of high-K volcanism is characterized by potassic or ultrapotassic analcime-bearing volcanics, erupted in a lagoonal environment during the Maastrichtian. Progressive shallowing of the basin indicates that Upper Cretaceous high-K volcanism developed during the final stage of pull-apart basin development in the southern zone of the eastern Pontides. These volcanic rocks, intercalated with continental detritus, are characterized by high Na2O (3.22-7.16 wt %) concentrated in secondary analcime crystals. Their K2O contents also range between 0

  20. Large Igneous Provinces of the Central Asia: data on geochronology, geochemistry and petrology of the Tien Shan and Junggar basaltic complexes (United States)

    Simonov, V.; Mikolaichuk, A.


    During last years Large Igneous Provinces of the Central Asia were an object of steadfast attention of researchers. It was established that on a formation and development of continental earth crust a great influence was rendered by deep magmatic systems of mantle plumes of various age. Undoubtedly that these global processes of basaltic magmatism had in many respects crucial importance for ecology, climate and life development. Our researches of magmatic associations of the Tien Shan and Junggar have allowed to accumulate a considerable volume of new data on geochronology, geochemistry and physico-chemical parameters of petrogenesis of within-plate basaltic complexes of the Central Asia, which area of distribution covers territory over than 285000 km2. Analysis with the help of 40Ar/39Ar method has shown that the basaltic complexes of the Tien Shan have Cretaceous-Paleogene age (61-76 Ma). Basalts of the Southeast Kazakhstan (North Tien Shan) corresponds to Paleozoic age: 305-312 Ma. Rather close values of 40Ar/39Ar data are received for basalts of the Altynemel Ridge (South Junggar) - 282 Ma. Isotope 40Ar/39Ar dating of basalts of the Alakol site (Junggar) has shown Mesozoic age (186-198 Ma). As a whole, the carried out researches testify to formation of Tien Shan and Junggar within-plate basalt complexes as a result of influence of three plumes, operating in various time: Tarim (282-312 Ma), Junggar (186-198 Ma) and Tien Shan (61-76 Ma). Data on petrochemistry, geochemistry of trace and rare-earth elements and mineralogy shows an enriched plume characteristics (close to OIB) of Mesozoic-Cenozoic basalts and presence of group of Paleozoic rocks close to continental and oceanic plateau basalts. As a whole, successive evolution in time of geodynamics of within-plate basalt magmatism of Tien Shan and Junggar is established. Paleozoic - plateau basaltic magmatism like Siberian traps or oceanic plateau basalts of Ontong Java. Mesozoic - development of more local hot

  1. Lu-Hf and Sm-Nd geochronology of garnet gneisses in the central Appalachians, U.S.: Implications for the timing and duration of Grenville Orogeny (United States)

    Vervoort, Jeff; Ramsey, Molly; Mulcahy, Sean; Aleinikoff, John; Southworth, Scott


    The Grenville orogeny is one of the most significant geological events in Earth's history with remnants of this event prominent on virtually every continent. Constraining its timing and duration is important not only for understanding the tectonics of the Grenville itself, but also for understanding supercontinent cycles and other questions of Earth's evolution. In order to provide better constraints on the timing of Grenvillian metamorphism, we analyzed garnet-bearing Mesoproterozoic ortho and paragneisses, collected along a 150 km transect in the northern Blue Ridge Province, using combined Lu-Hf and Sm-Nd geochronology. The orthogneisses have U-Pb zircon crystallization ages of ~1140 and 1100 Ma. The paragneisses have maximum depositional ages ~1050 to 1020 Ma, based on the youngest detrital zircon populations. Zircon overgrowths and monazite ages suggest metamorphic events between ~1050 and 960 Ma. The Lu-Hf and Sm-Nd data for these samples both yield robust garnet ages with large spread of parent/daughter ratios, low age uncertainties, and low MSWD values. Lu-Hf ages define a narrow time span (1043±12 Ma to 1016±4 Ma; wtd. mean, 1024±7 Ma, 2σ). The Sm-Nd ages, determined on the same solutions as Lu-Hf, also define a narrow time range but are systematically younger (974±11 Ma to 932±5 Ma; wtd. mean, 957±10 Ma). The average difference between Lu-Hf and Sm-Nd ages is 67 Ma; the oldest Sm-Nd age is 40 Ma younger than the youngest Lu-Hf age. These large systematic differences in the ages are enigmatic. While Sm-Nd ages younger than Lu-Hf are not uncommon, these differences are typically small. There are, however, potential explanations for these differences. (1) Lu partitions strongly into garnet during growth resulting in high Lu/Hf ratios in the core and yielding ages weighted toward the beginning of growth (e.g., Skora, 2006); no similar partitioning exists in Sm/Nd and these ages reflect mean garnet growth. (2) Lu diffuses much faster than Hf at elevated

  2. Age revision of the Neotethyan arc migration into the southeast Urumieh-Dokhtar belt of Iran: Geochemistry and U-Pb zircon geochronology (United States)

    Hosseini, Mohammad Reza; Hassanzadeh, Jamshid; Alirezaei, Saeed; Sun, Weidong; Li, Cong-Ying


    The Urumieh-Dokhtar magmatic belt of Central Iran runs parallel to the Zagros orogenic belt and has been resulted from Neotethys ocean subduction underneath Eurasia. The Bahr Aseman volcanic-plutonic complex (BAC), covering an area 2000 km2 in the Kerman magmatic belt (KMB) in the southern section of the Urumieh-Dokhtar belt, has long been considered as the earliest manifestation of extensive Cenozoic arc magmatism in KMB. The nature and timing of the magmatism, however, is poorly constrained. An area 1000 km2, in BAC and adjacent Razak volcaniclastic complex and Jebal Barez-type granitoids, was mapped and sampled for geochemistry and geochronology. Andesite and basaltic andesite are the main volcanic components in the study area; plutonic bodies vary from tonalite to quartz diorite, granodiorite and biotite-granite. The rocks in BAC display dominantly normal calc-alkaline character. On spider diagrams, the rocks are characterized by enrichments in LILE relative to HFSE and enrichments in LREE relative to HREE. These features suggest a subduction related setting for the BAC. LaN/YbN ratios for the intrusive and volcanic rocks range from 1.41 to 5.16 and 1.01 to 6.42, respectively. These values are lower than those for other known granitoids in KMB, namely the abyssal, dominantly Oligocene Jebal Barez-type (LaN/YbN = 1.66-9.98), and the shallow, dominantly late Miocene Kuh Panj-type (LaN/YbN = 12.97-36.04) granitoids. This suggests a less evolved magma source for the BAC igneous rocks. In Y vs. Nb and Th/Yb vs. La/Yb discrimination diagrams, an island-arc setting is defined for the BAC rocks. The rocks further plot in primitive island-arc domain in Nb vs. Rb/Zr and Y/Nb vs. TiO2 diagrams. The BAC volcanic and plutonic rocks yielded zircon U-Pb ages of 78.1 to 82.7 Ma and 77.5 to 80.8 Ma, respectively. Zircon U-Pb dating of volcanic rocks and granitoids from the adjacent Razak complex and the Jebal Barez-type granitoids indicated 48.2 Ma and 26.1 Ma ages

  3. The Siderian-Orosirian magmatism in the Gavião Paleoplate, Brazil: U-Pb geochronology, geochemistry and tectonic implications (United States)

    Pereira Cruz, Simone Cerqueira; Figueiredo Barbosa, Johildo Salomão; Pinto, Marilda Santos; Peucat, Jean-Jacques; Paquette, Jean Louis; Santos de Souza, Jailma; de Souza Martins, Violeta; Júnior, Farid Chemale; Carneiro, Mauricio Antonio


    The southern portion of the Gavião Paleoplate is composed by Archean orthogneisses, Archean-Paleoproterozoic metavolcano-sedimentary rocks and Siderian-Rhyacian-Orosirian granitoids. Petrographic, geochemical, U-Pb (Laser Ablation, ICPMS) and Sm-Nd data are presented for five Paleoproterozoic granitoids that were recently mapped: Jussiape II, Lagoa das Almas, Humaitá, Belo Campo and Broco granitoids. These granitoids present U-Pb zircon (LA-ICPMS) ages of 2052 ± 43, 2114 ± 24, 2140 ± 9, 2049 ± 23 and 2038 ± 8 Ma, respectively. In addition to these granitoids, another twenty-five ones were identified and studied by several authors, resulting in a total of twenty-nine plutons. Despite the previous petrography, geochemistry and geochronology studies that have been performed, no model had been proposed to explain the tectonic setting of this extensive granitogenesis. Integration of the new data and the literature has been done and corresponds to the second part of the article. Based on U-Pb dating and geochemical data, Siderian-Rhyacian-Orosirian granitoids of the southern Gavião Paleoplate were classified into five groups, or five suites: 1 (2324 ± 6 to 2091 ± 6.6 Ma), 2a (2054 -6/+8 to 2041 ± 23 Ma), 2b (2066 ± 37 to 2019 ± 32 Ma), 2c (2058 ± 8 to 1852 ± 50 Ma) and 2d (2049 ± 12 to 1929 ± 16 Ma). The granitoids of Group 1 present heterogeneous deformation, while the granitoids of groups 2a to 2d are generally not deformed. Usually the rocks are potassic, but sodic granitic rocks can be found in samples of groups 1, 2c and 2d. Several chemical classification parameters are presented and discussed herein, but it is noteworthy that the granitoids of Group 1 are mainly classified as calcic to calc-alkalic, while the rocks of the second group are mostly classified as alkalic ones. In the remaining groups, the samples vary between calc-alkalic and alkali-calcic. The ɛNd values range between 4.0 and -15.4 and suggest an important and varied share of the

  4. Petrology, geochemistry and U-Pb geochronology of magmatic rocks from the high-sulfidation epithermal Au-Cu Chelopech deposit, Srednogorie zone, Bulgaria (United States)

    Chambefort, Isabelle; Moritz, Robert; von Quadt, Albrecht


    The Chelopech deposit is one of the largest European gold deposits and is located 60 km east of Sofia, within the northern part of the Panagyurishte mineral district. It lies within the Banat-Srednegorie metallogenic belt, which extends from Romania through Serbia to Bulgaria. The magmatic rocks define a typical calc-alkaline suite. The magmatic rocks surrounding the Chelopech deposit have been affected by propylitic, quartz-sericite, and advanced argillic alteration, but the igneous textures have been preserved. Alteration processes have resulted in leaching of Na2O, CaO, P2O5, and Sr and enrichment in K2O and Rb. Trace element variation diagrams are typical of subduction-related volcanism, with negative anomalies in high field strength elements (HFSE) and light element, lithophile elements. HFSE and rare earth elements were relatively immobile during the hydrothermal alteration related to ore formation. Based on immobile element classification diagrams, the magmatic rocks are andesitic to dacitic in compositions. Single zircon grains, from three different magmatic rocks spanning the time of the Chelopech magmatism, were dated by high-precision U-Pb geochronology. Zircons of an altered andesitic body, which has been thrust over the deposit, yield a concordant 206Pb/238U age of 92.21 ± 0.21 Ma. This age is interpreted as the crystallization age and the maximum age for magmatism at Chelopech. Zircon analyses of a dacitic dome-like body, which crops out to the north of the Chelopech deposit, give a mean 206Pb/238U age of 91.95 ± 0.28 Ma. Zircons of the andesitic hypabyssal body hosting the high-sulfidation mineralization and overprinted by hydrothermal alteration give a concordant 206Pb/238U age of 91.45 ± 0.15 Ma. This age is interpreted as the intrusion age of the andesite and as the maximum age of the Chelopech epithermal high-sulfidation deposit. 176Hf/177Hf isotope ratios of zircons from the Chelopech magmatic rocks, together with published data on the

  5. Deciphering Past and Present Tectonics of the Rio Grande Rift in New Mexico Utilizing Apatite Fission Track Thermochronology, Geochronology, Quaternary Faulting, and Cross-Section Restoration (United States)

    Ricketts, J. W.; Karlstrom, K. E.; Kelley, S. A.; Priewisch, A.; Crossey, L. J.; Asmerom, Y.; Polyak, V.; Selmi, M.


    The Rio Grande rift provides an excellent laboratory for understanding styles and processes of extensional tectonics, and their driving forces. We apply apatite fission track (AFT) thermochronology, geochronology, fracture analysis, and cross-section restoration to decipher past and present tectonics of the Rio Grande rift. AFT data has been compiled from rift flank uplifts along the Rio Grande rift in an attempt to recognize long wavelength spatial and temporal patterns. AFT ages record time of cooling of rocks below ~110°C and, when cooling is due to exhumation, age elevation traverses can record upward advection of rocks through paleo 110°C isotherms. The relatively passive sides of half-grabens (e.g. Manzanos and Santa Fe Range) preserve Laramide AFT ages ranging from 45-70 Ma, indicating they were cooled during the Laramide Orogeny and have remained cooler than 110°C since then. Rift flanks on the tectonically active sides of half-grabens, (e.g. Sierra Ladrones, Sandias, Taos Range, and Sierra Blanca) have AFT ages that range from 35 Ma to <10 Ma, and record cooling that initiated with the Oligocene ignimbrite flare-up and continues through the Neogene. Our analysis tracks the approximate elevation of paleo 110°C isotherms in 10 Ma intervals from the Laramide to the present and shows that reconstructed paleoisotherms have been differentially uplifted, warped, and faulted since their time of formation, and hence serve as markers of uplift history and its mechanisms. AFT data at Ladron Peak, an active rift flank along the western margin of the Rio Grande rift in central New Mexico, indicates that it was rapidly unroofed between 20-10 Ma. Preliminary apatite helium data gives a similar age vs. elevation trend, but apatites have highly radiogenically damaged lattices and hence have corrected closure temperatures tens of degrees higher than AFT ages. The style of faulting at Ladron Peak is unusual because it is bounded by the anomalously low-angle (~15°) Jeter

  6. LASS U-Th-Pb monazite and rutile geochronology of felsic high-pressure granulites (Rhodope, N Greece): Effects of fluid, deformation and metamorphic reactions in local subsystems (United States)

    Wawrzenitz, Nicole; Krohe, Alexander; Baziotis, Ioannis; Mposkos, Evripidis; Kylander-Clark, Andrew R. C.; Romer, Rolf L.


    The specific chemical composition of monazite in shear zones is controlled by the syndeformation dissolution-precipitation reactions of the rock-forming minerals. This relation can be used for dating deformation, even when microfabric characteristics like shape preferred orientation or intracrystalline deformation of monazite itself are missing. Monazite contemporaneously formed in and around the shear zones may have different compositions. These depend on the local chemical context rather than reflecting successive crystallization episodes of monazite. This is demonstrated in polymetamorphic, mylonitic high-pressure (HP) garnet-kyanite granulites of the Alpine Sidironero Complex (Rhodope UHP terrain, Northern Greece). The studied mylonitic rocks escaped from regional migmatization at 40-36 Ma and from subsequent shearing through cooling until 36 Ma. In-situ laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) analyses have been carried out on monazite from micro-scale shear zones, from pre-mylonitic microlithons as well as of monazite inclusions in relictic minerals complimented by U-Pb data on rutile and Rb-Sr data of biotite. Two major metamorphic episodes, Mesozoic and Cenozoic, are constrained. Chemical compositions, isotopic characteristics and apparent ages systematically vary among monazite of four different microfabric domains (I-IV). Within three pre-mylonitic domains (inclusions in (I) pre-mylonitic kyanite and (II) garnet porphyroclasts, and (III) in pre-mylonitic microlithons) monazite yields ages of ca. 130-150 Ma for HP-granulite metamorphism, in line with previous geochronological results in the area. Patchy alteration of the pre-mylonitic monazite by intra-grain dissolution-precipitation processes variably increased negative Eu anomaly and reduced the HREE contents. The apparent age of this altered monazite is reduced. Monazite in the syn-mylonitic shear bands (IV) differs in chemical composition from unaltered and

  7. Lu-Hf garnet geochronology applied to plate boundary zones: Insights from the (U)HP terrane exhumed within the Woodlark Rift (United States)

    Zirakparvar, N. A.; Baldwin, S. L.; Vervoort, J. D.


    /early Cenozoic time. In order to relate the (U)HP metamorphic rocks to the larger-scale tectonic evolution of the Woodlark Rift, a third Lu-Hf age of 11.2 ± 2.1 Ma was determined for dynamically recrystallized garnet from the southeastern margin of the rift. While the tectonic implications of the Lu-Hf age from the southeastern-rifted margin are not fully known, this result suggests that the eastern and western parts of the Woodlark Rift might have a different metamorphic history. The garnet Lu-Hf results demonstrate that prior to rifting, the Woodlark Rift underwent a complex history of metamorphism involving garnet growth and recrystallization. The ability of the Lu-Hf system to date Late Miocene garnet growth, and at the same time preserve Latest Cretaceous garnets within Pleistocene shear zones in the Woodlark Rift demonstrate that Lu-Hf garnet geochronology is an essential component of investigations where the primary objective is documenting and understanding the full range of plate boundary processes recorded in metamorphic minerals.

  8. Volatile Outputs From Subduction-Related Magmatism in the Oregon Cascades Estimated From Melt Inclusions, Spring Discharges, Heat Flow Data and Geochronology (United States)

    Wallace, P.; Ruscitto, D.; Rowe, M.; Kent, A.


    Estimates of volatile fluxes provide a primary test for models of magmatism and volatile cycling during subduction in the endmember "hot and dry" Cascadia subduction zone, which is caused by slow convergence (4 cm/a) of the young (~10-12 Ma) Juan de Fuca plate with Western North America. Intra- arc rifting in the Central Oregon segment of the Cascade arc during the past 2 Ma has caused this region to have the highest mafic output along the arc. However, estimates of major volatile (H2O, CO2, S, Cl) fluxes and comparisons with other arcs (e.g. Central America) are not straightforward because there are no passively degassing volcanoes in the area. We estimate volatile outputs for the Central Oregon Cascades by combining data for olivine-hosted melt inclusions with regional heat flow (e.g. Ingebritsen, 1989; Blackwell,1990) and geochronological (Sherrod and Smith, 1990) studies. These flux estimates can be compared with those obtained from spring water studies (e.g. James, 1999; Hurwitz, 2005). This multidisciplinary approach allows us to more accurately constrain volatile fluxes, given that uncertainties in all methods are large and difficult to evaluate. Reported fluxes for Central Oregon springs are 3.4E5 CO2 and 1.5E4 Cl kg/yr/km of arc (James, 1999; Hurwitz, 2005). Melt inclusion data indicate primitive basaltic magmas in the Central Oregon Cascades have 1.0-3.5 wt% H2O, 800-1900 ppm S, and 300-1100 ppm Cl. Assuming global arc magma CO2 contents of ~1 wt% (Wallace, 2005), we estimate H2O/CO2 (1.0-3.5), S/CO2 (0.08-0.19), and Cl/CO2 (0.03-0.11) in magmas, which when combined with spring CO2 estimates, yield an H2O flux of 0.34-1.2E6, a S flux of 2.6-6.5E4, and a Cl flux of 1.0-3.7E4 kg/yr/km of arc. Alternatively, by combining melt inclusion data with magma flux estimates (14-38 km3/Myr/km of arc; Ingebritsen et al. 1989; Sherrod and Smith 1990) we estimate volatile fluxes for H2O: 0.39-5.4E6; S: 0.39-3.9E5; and Cl: 0.16- 2.3E5 kg/yr/km of arc. Given the

  9. Geochronology of the Dahongshan Group

    Institute of Scientific and Technical Information of China (English)

    胡霭琴; 朱炳泉; 等


    The Dahongshan Group is divided into five formations from the bottom to the top:the Laochanghe For-mation;the Manganghe Formation;the Hongshan Formation ,the Feiweihe Formation and the Potou For-mation.As can be seen form the U-Pb Concordia plot for zircon fractions collected from the metamorphic sodic lava of the Hongshan Formation,the upper intersect age with the Concordia is 1665.55-10.86+13.56Ma.Sm-Nd dating of four whole-rock samples and one hornblende from the Manganghe and Hongshan forma-tions yielded an isochron age of 1657±82Ma .This result is in good agreement with the zircon U-Pb age and thus can represent the formation age of the Dahongshan Group.The initial143Nd/144Nd ratio is esti-mated at 0.510646,with END(T)=+3.1±1.8.In addition the Dahongshan Group rocks are character-ized by low REE contents,with δEu>1.All this goes to show that the original rocks of the Dahongshan Group were derived from a depleted mantle.The calculated Sm-Nd model ages range from-1900 to -2000Ma,representing the time of crust/mantle differentiation in the area studied.From the above results,in conjunction with the time-scale scheme for China,the Dahongshan Group should be assigned to the Middle Proterozoic.

  10. Geology, geochemistry, geochronology, and economic potential of Neogene volcanic rocks in the Laguna Pedernal and Salar de Aguas Calientes segments of the Archibarca lineament, northwest Argentina (United States)

    Richards, J. P.; Jourdan, F.; Creaser, R. A.; Maldonado, G.; DuFrane, S. A.


    This study presents new geochemical, geochronological, isotopic, and mineralogical data, combined with new geological mapping for a 2400 km2 area of Neogene volcanic rocks in northwestern Argentina near the border with Chile, between 25°10‧S and 25°45‧S. The area covers the zone of intersection between the main axis of the Cordillera Occidental and a set of NW-SE-trending structures that form part of the transverse Archibarca lineament. This lineament has localized major ore deposits in Chile (e.g., the late Eocene La Escondida porphyry Cu deposit) and large volcanic centers such as the active Llullaillaco and Lastarría volcanoes on the border between Chile and Argentina, and the Neogene Archibarca, Antofalla, and Cerro Galán volcanoes in Argentina. Neogene volcanic rocks in the Laguna Pedernal and Salar de Aguas Calientes areas are mostly high-K calc-alkaline in composition, and range from basaltic andesites, through andesites and dacites, to rhyolites. Magmatic temperatures and oxidation states, estimated from mineral compositions, range from ~ 1000 °C and ∆FMQ ≈ 1.0-1.5 in andesites, to ~ 850 °C and ∆FMQ ≈ 1.5-2.0 in dacites and rhyolites. The oldest rocks consist of early-middle Miocene andesite-dacite plagioclase-pyroxene-phyric lava flows and ignimbrites, with 40Ar/39Ar ages ranging from 17.14 ± 0.10 Ma to 11.76 ± 0.27 Ma. Their major and trace element compositions are typical of the Andean Central Volcanic Zone, and show strong crustal contamination trends for highly incompatible elements such as Cs, Rb, Th, and U. These rocks are geochemically grouped as sub-suite 1. This widespread intermediate composition volcanism was followed in the middle-late Miocene by a period of more focused rhyodacitic flow-dome complex formation. These felsic rocks are characterized by less extreme enrichments in highly incompatible elements, and increasing depletion of heavy rare earth elements. These rocks are geochemically grouped as sub-suite 2. The

  11. High spatial resolution U-Pb geochronology and Pb isotope geochemistry of magnetite-apatite ore from the Pea Ridge iron oxide-apatite deposit, St. Francois Mountains, southeast Missouri, USA (United States)

    Neymark, Leonid; Holm-Denoma, Christopher S.; Pietruszka, Aaron; Aleinikoff, John N.; Fanning, C. Mark; Pillers, Renee M.; Moscati, Richard J.


    The Pea Ridge iron oxide-apatite (IOA) deposit is one of the major rhyolite-hosted magnetite deposits of the St. Francois Mountains terrane, which is located within the Mesoproterozoic (1.5–1.3 Ga) Granite-Rhyolite province in the U.S. Midcontinent. Precise and accurate determination of the timing and duration of oreforming processes in this deposit is crucial for understanding its origin and placing it within a deposit-scale and regional geologic context. Apatite and monazite, well-established U-Pb mineral geochronometers, are abundant in the Pea Ridge orebody. However, the potential presence of multiple generations of dateable minerals, processes of dissolution-reprecipitation, and occurrence of micrometer-sized intergrowths and inclusions complicate measurements and interpretations of the geochronological results. Here, we employ a combination of several techniques, including ID-TIMS and high spatial resolution geochronology of apatite and monazite using LA-SC-ICPMS and SHRIMP, and Pb isotope geochemistry of pyrite and magnetite to obtain the first direct age constraints on the formation and alteration history of the Pea Ridge IOA deposit. The oldest apatite TIMS 207Pb*/206Pb* dates are 1471 ± 1 and 1468 ± 1 Ma, slightly younger than (but within error of) the ~1474 to ~1473 Ma U-Pb zircon ages of the host rhyolites. Dating of apatite and monazite inclusions within apatite provides evidence for at least one younger metasomatic event at ~1.44 Ga, and possibly multiple superimposed metasomatic events between 1.47 and 1.44 Ga. Lead isotop analyses of pyrite show extremely radiogenic 206Pb/204Pb ratios up to ~80 unsupported by in situ U decay. This excess radiogenic Pb in pyrite may have been derived from the spatially associated apatite as apatite recrystallized several tens of million years after its formation. The low initial 206Pb/204Pb ratio of ~16.5 and 207Pb/204Pb ratio of ~15.4 for individual magnetite grains indicate closed U-Pb system behavior in

  12. Monazite ID-TIMS U-Pb geochronology in the LAGIR laboratory, Rio de Janeiro State University: protocols and first applications to the assembly of Gondwana supercontinent in SE-Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Aguair Neto, Carla Cristiane; Valeriano, Claudio M.; Heilbron, Monica; Lobato, Marcela, E-mail: [Universidade do Estado do Rio de Janeiro (UERJ), Rio de Janeiro, RJ (Brazil). Faculdade de Geologia. Lab. de Geocronologia e Isotopos Radiogenicos; Passarelli, Claudia R. [Universidade de Sao Paulo (USP), SP (Brazil). Instituto de Geociencias


    The chemical and spectrometric procedures of the U-Pb geochronology method on monazites, recently installed in the LAGIR laboratory, are described in detail. In addition, preliminary results on monazite samples from the Brasilia and Ribeira belts are reported and discussed in the context of the regional geology. Several experiments for calibration of ion exchange chromatographic columns with the AG-1x8 resin, were performed with HCl, using dissolved natural monazite samples. The Pb blanks of reagents are ∼ 0.5 pg/g in acids and ∼1 pg/g in H{sub 2}O. The total Pb blanks in chemical procedures were below 22 pg. Preliminary results are presented from three case studies related to Brasiliano orogenic belts of SE-Brazil, which correlate very well with previous age determinations from literature: two sub-concordant grains from an Araxa Group quartzite (southern Brasilia belt) define a concordia age of 602.6 ±1.4 Ma; a -0.8% discordant grain from a quartzite of the Sao Fidelis Group (Costeiro Domain, central Ribeira belt) yielded a concordia age of 535.3 ± 2.4 Ma; two 0.4 % and 1.3 % discordant monazite grains from the post-collisional Itaoca Granite (Costeiro Domain, central Ribeira belt) define a concordia age of 476.4 ± 1.8 Ma. (author)

  13. Synthesis of geological, structural, and geochronologic data (phase V, deliverable 53): Chapter A in Second projet de renforcement institutionnel du secteur minier de la République Islamique de Mauritanie (PRISM-II) (United States)

    Bradley, Dwight C.; O'Sullivan, Paul; Cosca, Michael A.; Motts, Holly; Horton, John D.; Taylor, Cliff D.; Beaudoin, Georges; Lee, Gregory K.; Ramezani, Jahan; Bradley, Daniel N.; Jones III, James V.; Bowring, Samuel


    This report is a companion to the new Geologic Map of Mauritania (Bradley and others, 2015; referred to herein as “Deliverable 51”) and the new Structural Geologic Map of Mauritania (Bradley and others, 2015a; referred to herein as “Deliverable 52”). Section 1 contains explanatory information for these two digital maps. Section 2 covers the analytical methods used in obtaining new U-Pb ages from 9 igneous rock samples, new detrital zircon ages from 40 sedimentary or metasedimentary rock samples, and new 40Ar/39Ar ages from 12 samples of metamorphic rocks and veins. Sections 3 through 6 present the new geochronological results, organized by region. In Section 7, we discuss implications of the new ages for the regional geology and discuss problematic results. Finally, in Section 8, we summarize the geology and tectonic evolution of Mauritania in narrative form, drawing on new and published information, in the context of global tectonics. The report is being released in both English and French. In both versions, we use the French-language names for formal stratigraphic units.

  14. Zircon U-Pb geochronology, geochemistry and Sr-Nd-Pb isotopes from the metamorphic basement in the Wuhe Complex: Implications for Neoarchean active continental margin along the southeastern North China Craton and constraints on the petrogenesis of Mesozoic granitoids

    Directory of Open Access Journals (Sweden)

    Andong Wang


    Full Text Available We report zircon U-Pb geochronology, geochemistry and Sr-Nd-Pb isotope data from mafic granulites and garnet amphibolites of the Wuhe Complex in the southeastern margin of the North China Craton (NCC. In combination with previous data, our results demonstrate that these rocks represent fragments of the ancient lower crust, and have features similar to those of the granulite basement in the northern margin of the NCC. A detailed evaluation of the Pb isotope data shows that Pb isotopes cannot effectively distinguish the role of the Yangtze Craton basement from that of the NCC basement with regard to the source and generation of magmas, at least for southeastern NCC. The age data suggest that the protoliths of the granulites or amphibolites in the Wuhe Complex were most likely generated in Neoarchean and that these rocks were subjected to Paleoproterozoic (1.8–1.9 Ga high-pressure granulite facies metamorphism. This study also shows that the Precambrian metamorphic basement in the southeastern margin of the NCC might have formed in a tectonic setting characterized by a late Neoarchean active continental margin.

  15. Zircon U–Pb geochronology and geochemistry of rhyolitic tuff, granite porphyry and syenogranite in the Lengshuikeng ore district, SE China: Implications for a continental arc to intra-arc rift setting

    Indian Academy of Sciences (India)

    Changming Wang; Da Zhang; Ganguo Wu; Yigan Xu; Emmanuel John M Carranza; Yaoyao Zhang; Huaikun Li; Jianzhen Geng


    SE China is well known for its Mesozoic large-scale granitoid plutons and associated ore deposits. Here, zircon U–Pb geochronological and geochemical data have been used to better constrain the petrogenesis of the igneous rocks associated with porphyry Ag–Pb–Zn deposits in the Lengshuikeng ore district, SE China. The Lengshuikeng rhyolitic tuff, granite porphyry and syenogranite yielded zircon U–Pb ages of 161, 155 and 138 Ma, respectively. The Lengshuikeng granite porphyries belong to calc-alkaline series and show fractionated I-type affinities. The rhyolitic tuffs show almost similar characteristics as the granite porphyries. The engshuikeng syenogranites are all alkali-rich and show A-type affinities. The syenogranites have high contents of high field strength elements such as Nb, Ta, Zr, Hf; with Zr + Nb + Ce + Y contents of < 350 ppm. Chondrite-normalized REE patterns show relative enrichment of LREEs and strong negative Eu anomalies. The Lengshuikeng granite porphyries, syenogranites and tuffs were probably derived from partial melting of underlying Proterozoic metasedimentary rocks with minor addition of mantle-derived magmas, accompanied by fractional crystallization. Detailed petrologic and geochemical data for the Jurassic igneous rocks from the Lengshuikeng ore district imply that during the Late Jurassic, SE China on the southeast of the Shi-Hang zone was a continental arc associated with the subduction of the Palaeo-Pacific plate and that since the beginning of the Early Cretaceous an intra-arc rift has been formed along the Shi-Hang zone.

  16. Geochemistry and geochronology from Cretaceous magmatic and sedimentary rocks at 6°35‧ N, western flank of the Central cordillera (Colombian Andes): Magmatic record of arc growth and collision (United States)

    Jaramillo, J. S.; Cardona, A.; León, S.; Valencia, V.; Vinasco, C.


    The spatio-temporal, compositional and deformational record of magmatic arcs are sensible markers of the long-term evolution of convergent margins including collisional events. In this contribution, field relations, U-Pb LA-ICP-MS zircon geochronology from magmatic and sedimentary rocks, and whole-rock geochemistry from volcanic and plutonic rocks are used to reconstruct the Cretaceous arc growth and collision in the awakening of the Northern Andean orogeny in northwestern Colombia. The Quebradagrande Complex that includes a sequence of volcanic rocks intercalated with quartz-rich sediments is a tholeiitic arc characterized by an enrichment in LREE and Nb-Ti anomalies that document crustal thickening in an arc system that was already active by ca. 93 Ma. This arc was built associated with thin continental and newly formed oceanic crust, as suggested by the presence of Triassic and older detrital zircons in the associated sandstones. This fringing arc subsequently experienced deformation and a major switch to and enriched calc-alkaline high-k plutonism between 70 and 73 Ma. The deformation record and changes in composition are related to an opposite double-vergence Molucca-sea type arc-arc collision that ended with the accretion to the continental margin of an allochthonous island arc built on an oceanic plateau associated with the Caribbean plate. The new time-framework suggest that the Late Cretaceous to Paleocene collisional tectonics include various stages before the switching to a subduction-dominated regime in most of the Cenozoic.

  17. Monazite ID-TIMS U-Pb geochronology in the LAGIR laboratory, Rio de Janeiro State University: protocols and first applications to the assembly of Gondwana supercontinent in SE-Brazil

    Directory of Open Access Journals (Sweden)



    Full Text Available The chemical and spectrometric procedures of the U-Pb geochronology method on monazites, recently installed in the LAGIR laboratory, are described in detail. In addition, preliminary results on monazite samples from the Brasília and Ribeira belts are reported and discussed in the context of the regional geology. Several experiments for calibration of ion exchange chromatographic columns with the AG-1x8 resin, were performed with HCl, using dissolved natural monazite samples. The Pb blanks of reagents are ∼0.5 pg/g in acids and ∼1 pg/g in H2O. The total Pb blanks in chemical procedures were below 22 pg. Preliminary results are presented from three case studies related to Brasiliano orogenic belts of SE-Brazil, which correlate very well with previous age determinations from literature: two sub-concordant grains from an Araxá Group quartzite (southern Brasília belt define a concordia age of 602.6 ±1.4 Ma; a -0.8% discordant grain from a quartzite of the São Fidelis Group (Costeiro Domain, central Ribeira belt yielded a concordia age of 535.3 ± 2.4 Ma; two 0.4 % and 1.3 % discordant monazite grains from the post-collisional Itaoca Granite (Costeiro Domain, central Ribeira belt define a concordia age of 476.4 ± 1.8 Ma.

  18. Geochronological, geochemical and mineralogical constraints of emplacement depth of TTG suite from the Sinassi Batholith in the Central African Fold Belt (CAFB) of northern Cameroon: Implications for tectonomagmatic evolution (United States)

    Houketchang Bouyo, M.; Penaye, J.; Njel, U. O.; Moussango, A. P. I.; Sep, J. P. N.; Nyama, B. A.; Wassouo, W. J.; Abaté, J. M. E.; Yaya, F.; Mahamat, A.; Ye, Hao; Wu, Fei


    The Sinassi Batholith in the Central African Fold Belt (CAFB) of northern Cameroon represents the largest volume of plutonic rocks or granitoids massif of the Western Cameroonian Domain. It is made up dominantly of tonalite-trondhjemite-granodiorite (TTG) suite and lesser granite which are locally more or less deformed, and composed of varying proportions of quartz, plagioclase, K-feldspar, biotite, hornblende, sphene, magnetite, apatite and zircon. Major and trace element compositions of fifteen rock samples of granitoids (Djourdé granodiorite, Sinassi quartz diorite and orthogneisses groups) indicate that investigated rocks from the Sinassi Batholith are characterized by medium- to high-K calc-alkaline affinity and metaluminous I-type signature. In addition, their chondrite- and primitive mantle-normalized trace element patterns are strongly fractionated ((La/Yb)N = 2.96-61.40) and show respectively enrichment in LREE relative to HREE and enrichment in LILE compared to HFSE with moderate to slight negative Nb-Ta, Ti and Eu anomalies consistent with a continental magmatic arc setting related to a subduction zone. Geothermobarometric calculations using hornblende-plagioclase thermometry and aluminum-in-hornblende barometry on eleven rock samples indicate that plutons from Sinassi Batholith were emplaced at average temperatures and pressures ranging between 698 and 720 °C and 4.06-5.82 kbar (Djourdé granitoids), 698-728 °C and 4.04-5.34 kbar (Sinassi granitoids) and 667-670 and 4.23-4.76 kbar (orthogneisses group) respectively. The average emplacement depths estimates for the investigated granitoids is constrained at ca 16-18 km, indicating that at least 16 km of crustal rocks of the Sinassi Batholith must have been eroded or uplifted at approximately exhumation rates of 0.08-0.10 mm/year. Regardless of their Th/U ratios, geochronological results highlight three main events characterizing the Neoproterozoic tectonomagmatic evolution within the Sinassi Batholith

  19. SHRIMP Geochronology of Volcanics of the Zhangjiakou and Yixian Formations, Northern Hebei Province, with a Discussion on the Age of the Xing'anling Group of the Great Hinggan Mountains and Volcanic Strata of the Southeastern Coastal Area of China

    Institute of Scientific and Technical Information of China (English)

    NIU Baogui; HE Zhengjun; SONG Biao; REN Jishun; XIAO Liwei


    A zircon U-Pb geochronological study on the volcanic rocks reveals that both of the Zhangjiakou and Yixian Formations, northern Hebei Province, are of the Early Cretaceous, with ages of 135-130 Ma and 129-120 Ma,respectively. It is pointed out that the ages of sedimentary basins and volcanism in the northern Hebei -western Liaoning area become younger from west to east, i. e. the volcanism of the Luanping Basin commenced at c. 135 Ma, the Luotuo Mount area of the Chengde Basin c. 130 Ma, and western Liaoning c. 128 Ma. With a correlation of geochronological stratigraphy and biostratigraphy, we deduce that the Xing'anling Group, which comprises the Great Hinggan Mountains volcanic rock belt in eastern China, is predominantly of the early-middle Early Cretaceous, while the Jiande and Shimaoshan Groups and their equivalents, which form the volcanic rock belt in the southeastern coast area of China, are of the mid-late Early Cretaceous, and both the Jehol and Jiande Biotas are of the Early Cretaceous, not Late Jurassic or Late Jurassic-Early Cretaceous. Combining the characteristics of the volcanic rocks and, in a large area, hiatus in the strata of the Late Jurassic or Late Jurassic-early Early Cretaceous between the formations mentioned above and the underlying sequences, we can make the conclusion that, in the Late Jurassic-early Early Cretaceous, the eastern China region was of high relief or plateau, where widespread post-orogenic volcanic series of the Early Cretaceous obviously became younger from inland in the west to continental margin in the east. This is not the result of an oceanward accretion of the subduction belt between the Paleo-Pacific ocean plate and the Asian continent, but rather reflects the extension feature, i.e. after the closure of the Paleo-Pacific ocean, the Paleo-Pacific ancient continent collided with the Asian continent and reached the peak of orogenesis, and then the compression waned and resulted in the retreating of the post

  20. Linking microstructures, petrology and in situ U-(Th)-Pb geochronology to constrain P-T-t-D evolution of the Greather Himalyan Sequences in Western Nepal (Central Himalaya) (United States)

    Iaccarino, Salvatore; Montomoli, Chiara; Carosi, Rodolfo; Langone, Antonio


    Last advances in forward modelling of metamorphic rocks and into the understanding of accessories minerals behaviour, suitable for geochronology (e.g. zircon and monazite), during metamorphism, bring new insights for understanding the evolution of metamorphic tectonites during orogenic cycles (Williams and Jercinovic, 2012 and reference therein). One of the best exposure of high- to medium grade- metamorphic rocks, is represented by the Greater Himalayan Sequence (GHS) in the Himalayan Belt, one of the most classic example of collisional orogen. Recent field work in Mugu Karnali valley, Western Nepal (Central Himalaya), identified a compressional top to the South ductile shear zone within the core of the GHS, named Magri Shear Zone (MSZ), developed in a high temperature regime as testified by quartz microstructures and syn-kinematic growth of sillimanite. In order to infer the tectono-metamorphic meaning of MSZ, a microstructural study coupled with pseudosection modelling and in situ U-(Th)-Pb monazite geochronology was performed on selected samples from different structural positions. Footwall sample constituted by (Grt + St ± Ky) micaschist shows a prograde garnet growth (cores to inner rims zoning), from ~500°C, ~0.60GPa (close to garnet-in curve) to ~580°C, ~1.2 GPa temporal constrained between 21-18 Ma, by medium Y cores to very low Y mantles monazite micro-chemical/ages domain . In this sample garnet was still growing during decompression and heating at ~640°C, ~0.75 GPa (rims), and later starts to be consumed, in conjunction with staurolite growth at 15-13 Ma, as revealed by high Y rims monazite micro-chemical/ages domain. Hanging-wall mylonitic samples have a porphyroclastic texture, with garnet preserve little memory of prograde path. Garnet near rim isoplets and matrix minerals intersect at ~700°C and ~0.70 GPa. A previous higher P stage, at ~1.10 GPa ~600°C, is testified by cores of larger white mica porhyroclasts. Prograde zoned allanite (Janots

  1. 40Ar/ 39Ar and 14C geochronology of the Albano maar deposits: Implications for defining the age and eruptive style of the most recent explosive activity at Colli Albani Volcanic District, Central Italy (United States)

    Giaccio, B.; Marra, F.; Hajdas, I.; Karner, D. B.; Renne, P. R.; Sposato, A.


    New 40Ar/ 39Ar and 14C ages have been found for the Albano multiple maar pyroclastic units and underlying paleosols to document the most recent explosive activity in the Colli Albani Volcanic District (CAVD) near Rome, Italy, consisting of seven eruptions (Albano 1 = oldest). Both dating methodologies have been applied on several proximal units and on four mid-distal fall/surge deposits, the latter correlated, according to two current different views, to either the Albano or the Campi di Annibale hydromagmatic center. The 40Ar/ 39Ar ages on leucite phenocrysts from the mid-distal units yielded ages of ca. 72 ka, 73 ka, 41 ka and 36 ka BP, which are indistinguishable from the previously determined 40Ar/ 39Ar ages of the proximal Albano units 1, 2, 5 and 7, thus confirming their stratigraphic correspondence. Twenty-one 14C ages of the paleosols beneath Albano units 3, 5, 6 and 7 were found for samples collected from 13 proximal and distal sections, some of which were the same sections sampled for 40Ar/ 39Ar measurements. The 14C ages were found to be stratigraphically inconsistent and highly scattered, and were systematically younger than the 40Ar/ 39Ar ages, ranging from 35 ka to 3 ka. Considering the significant consistence of the 40Ar/ 39Ar chronological framework, we interpret the scattered and contradictory 14C ages to be the result of a variable contamination of the paleosols by younger organic carbon deriving from the superficial soil horizons. These results suggest that multiple isotopic systems anchored to a robust stratigraphic framework may need to be employed to determine accurately the geochronology of the CAVD as well as other volcanic districts.

  2. Geochronology, Nd isotopes and reconnaissance geochemistry of volcanic and metavolcanic rocks of the São Luís Craton, northern Brazil: Implications for tectonic setting and crustal evolution (United States)

    Klein, Evandro L.; Luzardo, Renê; Moura, Candido A. V.; Lobato, Denise C.; Brito, Reinaldo S. C.; Armstrong, Richard


    New field work, in addition to zircon geochronology, Nd isotopes and reconnaissance geochemical data allow the recognition of Paleoproterozoic volcanic and metavolcanic sequences in the São Luís Craton of northern Brazil. These sequences record at least five volcanic pulses occurring probably in three distinct epochs and in different tectonic settings. (1) The Pirocaua Formation of the Aurizona Group comprises early arc-related calc-alkaline metapyroclastic rocks of 2240 ± 5 Ma formed from juvenile protoliths in addition to minor older crustal components. (2) The Matará Formation of the Aurizona Group holds mafic tholeiitic and ultramafic metavolcanic rocks of back arc and/or island arc setting, which are likely coeval to the Pirocaua Formation. (3) The Serra do Jacaré volcanic unit is composed of tholeiitic basalts and predominantly metaluminous, normal- to high- K calc-alkaline andesites of 2164 ± 3 Ma formed in mature arc or active continental margin from juvenile protoliths along with subordinate older (Paleoproterozoic) materials and associated to the main calc-alkaline orogenic stage. (4) The Rio Diamante Formation consists of late-orogenic metaluminous, medium- K, calc-alkaline rhyolite to dacite and tuffs of 2160 ± 8 Ma formed in continental margin setting from reworked Paleoproterozoic crust (island arc) with incipient Archean contribution. (5) The Rosilha volcanic unit is composed of weakly peraluminous, medium- K, calc-alkaline dacite and tuff formed probably at about 2068 Ma from reworked crustal protoliths. As a whole the volcanic and metavolcanic rocks record and characterized better the previously proposed orogenic evolution of the São Luís Craton.

  3. Geochemistry and Geochronology U-Pb SHRIMP of granites from Peixoto de Azevedo: Alta Floresta Gold Province; Geoquimica e geocronologia U-Pb (SHRIMP) de granitos da regiao de Peixoto de Azevedo: Provincia Aurifera Alta Floresta, MT

    Energy Technology Data Exchange (ETDEWEB)

    Silva, Fernanda Rodrigues da; Barros, Marcia Aparecida Sant' Ana; Pierosan, Ronaldo; Pinho, Francisco Edigio Cavalcante; Tavares, Carla; Rocha, Jhonattan, E-mail:, E-mail:, E-mail:, E-mail:, E-mail:, E-mail: [Universidade Federal de Mato Grosso (UFMT), Cuiaba, MT (Brazil). Inst. de Ciencias Exatas e da Terra; Rocha, Mara Luiza Barros Pita; Vasconcelos, Bruno Rodrigo; Dezula, Samantha Evelyn Max, E-mail:, E-mail:, E-mail: [Universidade de Brasilia (UNB), Brasilia, DF (Brazil). Programa de Pos-Graduacao em Geologia


    The analysis of petrographic, geochemical and geochronological data of granites in the Peixoto de Azevedo region, Mato Grosso, Brazil, in the eastern portion of the Alta Floresta Gold Province, led to the recognition of two granitic bodies bounded by regional major faults and shear zones. In the northwestern portion a body with featured as biotite granodiorite, coarse-grained, with porphyritic to inequigranular texture, metaluminous to peraluminous, high-K calc-alkaline and magnesium character. In the southeastern portion of the area, a biotite monzogranite coarse-grained, with equigranular to porphyritic texture, slightly peraluminous, high-K calc-alkaline and dominantly of ferrous character. U- Pb dating (SHRIMP) showed that the biotite monzogranite has an age of 1869 ± 10 Ma, similar to the Matupa Intrusive Suite, while the biotite granodiorite has an age of 1781 ± 10 Ma, that is the age expected to Peixoto Granite. Both units show patterns of rare earth elements with enrichment of light over heavy and negative Eu anomaly (La{sub N}/Yb{sub N} » 7.6 to 17.31 and ratios Eu/EU{sup ⁎} between 0.46 – 0.72 for biotite monzogranite and La{sub N}/Yb{sub N} » 7.13 to 29.09 with ratios Eu/Eu{sup ⁎} between 0.25 - 0.40 for the biotite granodiorite). Trace elements pattern for both present negative anomalies of Ba, P, Ti and Nb indicating an evolution from mineral fractionation and subduction related sources. In this paper, it is suggested that the monzogranite Matupa was developed in mature arc tectonic environment. For the Peixoto Granite, two hypotheses are suggested: (a) it was developed in younger magmatic arc environment associated with the Colider Magmatism or (b) it was generated in extensional tectonic environment during the Columbia Super continent break up. (author)

  4. Structural, petrological and geochronological analysis of the lithotypes from the Pien region (Parana State, Brazil) and adjacences; Analise estrutural, petrologica e geocronologica dos litotipos da Regiao de Pien (PR) e adjacencias

    Energy Technology Data Exchange (ETDEWEB)

    Harara, Ossama Mohamed


    The Pien area presents the major geotectonic domains separated by the Pien Shear Zone (PSZ). The northern one is the Rio Pien Granite-Mylonitic Suite composed by calc-alkaline granitoids of Neoproterozoic age. The southern domain is represented by the Amphibolite-Granulite where high grade metamorphism took place at the end of paleoproterozoic time. Considering the identified lithotypes, their geochemical affinity (particularly the Ti, Cr, Ni and REE content) and the geological context observed in the area, a geotectonical model of active continental margin related to subduction SSZ (Supra-Subduction Zone) is proposed. K-Ar on plagioclase from gabbronorites gave Neoproterozoic ages although Sm-Nd whole rock isochron yielded Paleoproterozoic ages. Based in geochemical data, it is proposed that the biotite gneiss and biotite-amphibole-gneiss which occur near the PSZ have a shoshonitic to high-K calc-alkaline features which are characteristic of active continental margins. K-Ar on biotite extracted from these rocks, gave Neoproterozoic ages. The available radiometric data for the Rio Pien mylonitic granitoids show that between 650-595 Ma the generation, deformation and cooling below the isotherm of 250 deg C occurred. On the other hand, the geochronological data for the Agudos do Sul Massif are in the 590-570 Ma interval showing its younger generation. The Sr{sup 87} / Sr{sup 86} initial ratios for both granitoids suggest more involvement of the continental crust in the origins of Agudos do Sul granitic Massif. The analyses of the entire set of the available data for the Pien area allows the suggestion of a geotectonic scenery related to the evolution of an active continental margin during the collages associated to the Brasiliano Cycle 201 refs., 130 figs., 6 tabs., 2 maps

  5. Sediments of the Lagoa Olho D'Agua: geochronology and accumulation of trace metals; Sedimentos da Lagoa Olho D'Agua: geocronologia e acumulacao de metais-tracos

    Energy Technology Data Exchange (ETDEWEB)

    Honorato, Eliane Valentim


    An assessment of the environmental impact of anthropic activities in the Lagoa Olho D'Agua, located in Jaboatao dos Guararapes County, Brazil, was carried out by assessing the vertical distribution of some trace metals (Cd, Cr, Fe, Hg, Ni, U, Zn and Zr) in dated sediment samples. Sediment cores were collected from thirteen locations at the northern, central and southern sections of the lagoon. The metal content in the samples was determined by Instrumental Neutron Activation Analysis (INAA). Its organic carbon content was also determined and used to characterize the environment as, either oxic or anoxic environments can bias the trace element content of sediment samples. The influence of the hydrodynamic characteristics in the sedimentation process was also studied. The results obtained for the geochronology of sediments showed a pronounced increase in sedimentation rates in the period of 1970 - 1980 and 1980 - 1990 in the sampling stations ST-09, ST-10, ST-11, ST-12 e ST-13 (ca.480% in the 80's to 90's ) compared to ca. 90% increase observed in other sampling stations. This increase can be associated to the demographic growth of ca. 500% that occurred in Jaboatao dos Guararapes County in the 80's to 90's predominantly along the shoreline. The geochemistry analyses of sediment samples, on the other hand, showed that the severe degradation process the occurred in the lagoon in the last 30 years was caused by the release of pollutants from industrial facilities as well as by the discharge of untreated domestic sewage. These domestic sewage favorable the increase of the concentration of organic material in the lagoon, accentuating the adsorption process of the metals on the sediments, mainly Fe and Zr in the suspense particles. The accentuated increase observed in both Fe and Zr concentrations is compatible with period of built industries basic steel works foundries and painting, indicating the anthropogenic origin these metals. (author)

  6. High-precision U-Pb geochronology of the Jurassic Yanliao Biota from Jianchang (western Liaoning Province, China): Age constraints on the rise of feathered dinosaurs and eutherian mammals (United States)

    Chu, Zhuyin; He, Huaiyu; Ramezani, Jahandar; Bowring, Samuel A.; Hu, Dongyu; Zhang, Lijun; Zheng, Shaolin; Wang, Xiaolin; Zhou, Zhonghe; Deng, Chenglong; Guo, Jinghui


    The Yanliao Biota of northeastern China comprises the oldest feathered dinosaurs, transitional pterosaurs, as well as the earliest eutherian mammals, multituberculate mammals, and new euharamiyidan species that are key elements of the Mesozoic biotic record. Recent discovery of the Yanliao Biota in the Daxishan section near the town of Linglongta, Jianchang County in western Liaoning Province have greatly enhanced our knowledge of the transition from dinosaurs to birds, primitive to derived pterosaurs, and the early evolution of mammals. Nevertheless, fundamental questions regarding the correlation of fossil-bearing strata, rates of dinosaur and mammalian evolution, and their relationship to environmental change in deep time remain unresolved due to the paucity of precise and accurate temporal constraints. These limitations underscore the importance of placing the rich fossil record of Jianchang within a high-resolution chronostratigraphic framework that has thus far been hampered by the relatively low precision of in situ radioisotopic dating techniques. Here we present high-precision U-Pb zircon geochronology by the chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) from three interstratified ash beds previously dated by secondary-ion mass spectrometry (SIMS) technique. The results constrain the key fossil horizons of the Daxishan section to an interval spanning 160.89 to 160.25 Ma with 2σ analytical uncertainties that range from ±46 to ±69 kyr. These data place the Yanliao Biota from Jianchang in the Oxfordian Stage of the Late Jurassic, and mark the Daxishan section as the site of Earth's oldest precisely dated feathered dinosaurs and eutherian mammals.

  7. Geology, petrology, U-Pb (shrimp geochronology of the Morrinhos granite -Paraguá terrane, SW Amazonian craton: implications for the magmatic evolution of the San Ignácio orogeny

    Directory of Open Access Journals (Sweden)

    Ohana França

    Full Text Available Morrinhos granite is a batholith body that is slightly elongated in the NNW direction and approximately 1,140 km2 long; it is located in the municipality of Vila Bela da Santíssima Trindade of the state of Mato Grosso, Brazil, in the Paraguá Terrane, Rondonian-San Ignácio Province, in the SW portion of the Amazonian Craton. This intrusion displays a compositional variation from tonalite to monzogranite, has a medium to coarse inequigranular texture and is locally porphyritic; biotite is the predominant mafic in one of the facies, and hornblende is predominant in the other, with both metamorphosed into the greenschist facies. The studied rocks characterize an intermediate to acidic sequence that was formed by a subalkaline magmatism; the series is alkali-calcic to metaluminous to slightly peraluminous, and the rocks evolved through fractioned crystallization mechanisms. The structural data show two deformation phases represented by penetrative foliation (S1 and open folds (D2, and both phases were most likely related to the San Ignácio Orogeny. The geochronological (U-Pb SHRIMP and isotopic (Sm-Nd investigations of these rocks indicated a crystallization age of 1350 ± 12 Ma, TDMof approximately 1.77 Ga and εNd(1.35with a negative value of -2.57, suggesting that their generation was related to a partial melting process of a Paleoproterozoic (Statherian continental crust. The results herein indicate that the Morrinhos granite was generated in a continental magmatic arc in a late- to post-orogenic stage of the San Ignácio Orogeny, and it can be recognized as belonging to the Pensamiento Intrusive Suite.

  8. Mesozoic magmatism and timing of epigenetic Pb-Zn-Ag mineralization in the western Fortymile mining district, east-central Alaska: Zircon U-Pb geochronology, whole-rock geochemistry, and Pb isotopes (United States)

    Dusel-Bacon, Cynthia; Aleinkoff, J.N.; Day, W.C.; Mortensen, J.K.


    The Mesozoic magmatic history of the North American margin records the evolution from a more segmented assemblage of parautochthonous and allochthonous terranes to the more cohesive northern Cordilleran orogenic belt. We characterize the setting of magmatism, tectonism, and epigenetic mineralization in the western Fortymile mining district, east-central Alaska, where parautochthonous and allochthonous Paleozoic tectonic assemblages are juxtaposed, using sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon geochronology, whole-rock geochemistry, and feldspar Pb isotopes of Mesozoic intrusions and spatially associated mineral prospects. New SHRIMP U-Pb zircon ages and published U-Pb and 40Ar/39Ar ages indicate four episodes of plutonism in the western Fortymile district: Late Triassic (216-208 Ma), Early Jurassic (199-181 Ma), mid-Cretaceous (112-94 Ma), and Late Cretaceous (70-66 Ma). All age groups have calc-alkalic arc compositions that became more evolved through time. Pb isotope compositions of feldspars from Late Triassic, Early Jurassic, and Late Cretaceous igneous rocks similarly became more radiogenic with time and are consistent with the magmas being mantle derived but extensively contaminated by upper crustal components with evolving Pb isotopic compositions. Feldspar Pb isotopes from mid-Cretaceous rocks have isotopic ratios that indicate magma derivation from upper crustal sources, probably thickened mid-Paleozoic basement. The origin of the mantle component in Late Cretaceous granitoids suggested by Pb isotopic ratios is uncertain, but we propose that it reflects asthenospheric upwelling following slab breakoff and sinking of an inactive inner subduction zone that delivered the previously accreted Wrangellia composite terrane to the North American continental margin, after the outer Farallon subduction zone was established.

  9. U-Pb geochronology of the Santa Cruz Formation (early Miocene) at the Río Bote and Río Santa Cruz (southernmost Patagonia, Argentina): Implications for the correlation of fossil vertebrate localities (United States)

    Cuitiño, José I.; Fernicola, Juan Carlos; Kohn, Matthew J.; Trayler, Robin; Naipauer, Maximiliano; Bargo, M. Susana; Kay, Richard F.; Vizcaíno, Sergio F.


    The early Miocene Santa Cruz Formation (SCF) in southern Patagonia hosts the Santacrucian South American Land Mammal Age (SALMA), whose age is known mainly from exposures along the Atlantic coast. Zircon U-Pb ages were obtained from intercalated tuffs from four inland sections of the SCF: 17.36 ± 0.63 Ma for the westernmost Río Bote locality, and 17.04 ± 0.55 Ma-16.32 ± 0.62 Ma for central Río Santa Cruz localities. All ages agree with the bounding age of underlying marine units and with equivalent strata in coastal exposures. New ages and available sedimentation rates imply time spans for each section of ∼18.2 to 17.36 Ma for Río Bote and 17.45-15.63 Ma for central Río Santa Cruz (Burdigalian). These estimates support the view that deposition of the SCF began at western localities ∼1 Ma earlier than at eastern localities, and that the central Río Santa Cruz localities expose the youngest SCF in southern Santa Cruz Province. Associated vertebrate faunas are consistent with our geochronologic synthesis, showing older (Notohippidian) taxa in western localities and younger (Santacrucian) taxa in central localities. The Notohippidian fauna (19.0-18.0 Ma) of the western localities is synchronous with Pinturan faunas (19.0-18.0 Ma), but older than Santacrucian faunas of the Río Santa Cruz (17.2-15.6 Ma) and coastal localities (18.0-16.2 Ma). The Santacrucian faunas of the central Río Santa Cruz localities temporally overlap Colloncuran (15.7 Ma), Friasian (16.5 Ma), and eastern Santacrucian faunas.

  10. Middle Neoproterozoic (ca. 705-716 Ma) arc to rift transitional magmatism in the northern margin of the Yangtze Block: Constraints from geochemistry, zircon U-Pb geochronology and Hf isotopes (United States)

    Wang, Ruirui; Xu, Zhiqin; Santosh, M.; Xu, Xianbing; Deng, Qi; Fu, Xuehai


    The South Qinling Belt in Central China is an important window to investigate the Neoproterozoic tectono-magmatic processes along the northern margin of the Yangtze Block. Here we present whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes of a suite of Middle Neoproterozoic intrusion from the Wudang Uplift in South Qinling. Zircon LA-ICP-MS U-Pb ages reveal that these rocks were formed at ca. 705-716 Ma. Geochemical features indicate that the felsic magmatic rocks are I-type granitoids, belong to calcic- to calc-alkaline series, and display marked negative Nb, Ta and Ti anomalies. Moreover, the enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs), combined with depletion of heavy rare earth elements (HREEs) support that these rocks have affinity to typical arc magmatic rocks formed in Andean-type active continental margins. The REE patterns are highly to moderately fractionated, with (La/Yb)N = 5.13-8.10 in meta-granites, and 2.32-2.35 in granodiorite. The granitoids have a wide range of zircon εHf(t) values (-29.91 to 14.76) and zircon Hf two-stage model ages (696-3482 Ma). We suggest that the ca. 705-716 Ma granitoids were sourced from different degrees of magma mixing between partial melting of the overlying mantle wedge triggered by hydrous fluids released from subducted materials and crustal melting. The hybrid magmas were emplaced in the shallow crust accompanied by assimilation and fractional crystallization (AFC). Both isotopic and geochemical data suggest that the ca. 705-716 Ma felsic magmatic rocks were formed along a continental arc. These rocks as well as the contemporary A-type granite may mark a transitional tectonic regime from continental arc to rifting, probably related to slab rollback during the oceanic subduction beneath the northern margin of Yangtze Block.

  11. U-Pb geochronology of modern river sands from the flat-slab segment of the southern central Andes, Argentina, 29-31°S: Implications for Neogene foreland and hinterland basin evolution (United States)

    Capaldi, T.; Horton, B. K.; McKenzie, R.; Stockli, D. F.


    This study investigates how Andean river sediments in the flat-slab segment of western Argentina record active mixing of lithologically and geochemically distinct source regions comprising the Principal Cordillera, Frontal Cordillera, Precordillera fold-thrust belt, Sierras Pampeanas basement uplifts, and recycled Neogene basin fill. Detrital zircon U-Pb geochronological results for modern river sands discriminate variations from hinterland source regions, through river tributaries and main trunks of the Bermejo, Jachal, San Juan, and Mendoza rivers, and their respective fluvial megafans within the active foreland basin. Proportions of proximal zircon populations in the hinterland trunk rivers (with extensive Permian-Triassic and Cenozoic igneous exposures) diminish downstream with progressive contributions from the frontal Precordillera fold-thrust belt (dominantly Paleozoic sedimentary rocks) and Pampean basement uplifts. However, this systematic downstream dilution is perturbed in several catchments by significant recycling of older foreland basin fill. The degree of recycling depends on the position and extent of Oligocene-Pliocene exposures within the catchments. To discern the effects of the variable detrital zircon sources, multiple statistical methods are utilized. Quantitative comparisons suggest that variations in detrital zircon age distributions among the modern sands, and with older foreland basin fill and exposed bedrock, are dependent on spatial and temporal variations in exhumation and drainage network evolution within their respective Andean catchments. The present surface area of competing source regions and the configuration of hinterland tributary rivers largely dictate the degree of downstream dilution and/or recycling. This study provides a modern analogue and baseline for reconstructing Neogene shifts in foreland basin provenance, depositional systems, and drainage configurations during a critical transition to flat-slab subduction.

  12. Petrogenesis and tectonic settings of volcanic rocks of the Ashele Cu-Zn deposit in southern Altay, Xinjiang, Northwest China: Insights from zircon U-Pb geochronology, geochemistry and Sr-Nd isotopes (United States)

    Wu, Yufeng; Yang, Fuquan; Liu, Feng; Geng, Xinxia; Li, Qiang; Zheng, Jiahao


    The Early-Mid-Devonian Ashele Formation of the southern margin of the Chinese Altay hosts the Ashele Cu-Zn volcanogenic massive sulfide (VMS) deposit and consists of intercalated volcanic and sedimentary rocks that have experienced regional greenschist-facies metamorphism. We studied the petrography, zircon U-Pb geochronology, geochemistry, and Sr-Nd isotopes of dacites and basalts in order to understand the petrogenesis of these rocks and the regional tectonic evolution. Two dacites yielded LA-MC-ICP-MS zircon U-Pb ages of 402 ± 6 Ma and 403 ± 2 Ma. The dacites are calc-alkaline, and characterized by high Na2O/K2O ratios (3.6-9.3), and high Mg# values (47-63), enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in Nb, Ta, Ti, and P, and relatively positive εNd(t) values (+3.6 to +7.5), collectively suggesting a sanukitic magma affinity. The variations in the major and trace elements of the dacites indicate that Fe-Ti oxide, plagioclase, and apatite were fractionated during their petrogenesis. The basalts are tholeiitic, and are characterized by high Mg# values (66-73), and negative Nb and Ta anomalies. The geochemical characteristics of the basalts are similar to those of N-MORB. Those characteristics together with the positive εNd(t) values (+6.8 to +9.2) of the basalts, indicate that the precursor magma was derived mainly from an N-MORB-type depleted asthenospheric mantle in an island arc setting. The geochemical similarities between the basalts and dacites indicate that they both originated from a similar depleted mantle source via partial melting under different magmatic conditions in each case, possibly related to ridge subduction.

  13. Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite (United States)

    Aleinikoff, J.N.; Schenck, W.S.; Plank, M.O.; Srogi, L.A.; Fanning, C.M.; Kamo, S.L.; Bosbyshell, H.


    High-grade rocks of the Wilmington Complex, northern Delaware and adjacent Maryland and Pennsylvania, contain morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia. The arc complex has been divided into several members, the protoliths of which include both intrusive and extrusive rocks. Metasedimentary rocks are interlayered with the complex and are believed to be the infrastructure upon which the arc was built. In the Wilmingto n Complex rocks, both igneous and metamorphic zircons occur as elongate and equant forms. Chemical zoning, shown by cathodoluminescence (CL), includes both concentric, oscillatory patterns, indicative of igneous origin, and patchwork and sector patterns, suggestive of metamorphic growth. Metamorphic monazites are chemically homogeneous, or show oscillatory or spotted chemical zoning in backscattered electron images. U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date complexly zoned zircon and monazite. All but one member of the Wilmington Complex crystallized in the Ordovician between ca. 475 and 485 Ma; these rocks were intruded by a suite of gabbro-to-granite plutonic rocks at 434 ?? Ma. Detrital zircons in metavolcanic and metasedimentary units were derived predominantly from 0.9 to 1.4 Ga (Grenvillian) basement, presumably of Laurentian origin. Amphibolite to granulite facies metamorphism of the Wilmington Complex, recorded by ages of metamorphic zircon (428 ?? 4 and 432 ?? 6 Ma) and monazite (429 ?? 2 and 426 ?? 3 Ma), occurred contemporaneously with emplacement of the younger plutonic rocks. On the basis of varying CL zoning patterns and external morphologies, metamorphic zircons formed by different processes (presumably controlled by rock chemistry) at slightly different times and temperatures during prograde metamorphism. In addition, at least three other thermal episodes are

  14. Geochemistry, petrography, and zircon U-Pb geochronology of Paleozoic metaigneous rocks in the Mount Veta area of east-central Alaska: implications for the evolution of the westernmost part of the Yukon-Tanana terrane (United States)

    Dusel-Bacon, Cynthia; Day, Warren C.; Aleinikoff, John N.


    We report the results of new mapping, whole-rock major, minor, and trace-element geochemistry, and petrography for metaigneous rocks from the Mount Veta area in the westernmost part of the allochthonous Yukon–Tanana terrane (YTT) in east-central Alaska. These rocks include tonalitic mylonite gneiss and mafic metaigneous rocks from the Chicken metamorphic complex and the Nasina and Fortymile River assemblages. Whole-rock trace-element data from the tonalitic gneiss, whose igneous protolith was dated by SHRIMP U–Pb zircon geochronology at 332.6 ± 5.6 Ma, indicate derivation from tholeiitic arc basalt. Whole-rock analyses of the mafic rocks suggest that greenschist-facies rocks from the Chicken metamorphic complex, a mafic metavolcanic rock from the Nasina assemblage, and an amphibolite from the Fortymile River assemblage formed as island-arc tholeiite in a back-arc setting; another Nasina assemblage greenschist has MORB geochemical characteristics, and another mafic metaigneous rock from the Fortymile River assemblage has geochemical characteristics of calc-alkaline basalt. Our geochemical results imply derivation in an arc and back-arc spreading region within the allochthonous YTT crustal fragment, as previously proposed for correlative units in other parts of the terrane. We also describe the petrography and geochemistry of a newly discovered tectonic lens of Alpine-type metaharzburgite. The metaharzburgite is interpreted to be a sliver of lithospheric mantle from beneath the Seventymile ocean basin or from sub-continental mantle lithosphere of the allochthonous YTT or the western margin of Laurentia that was tectonically emplaced within crustal rocks during closure of the Seventymile ocean basin and subsequently displaced and fragmented by faults.

  15. Crustal melting and recycling: geochronology and sources of Variscan syn-kinematic anatectic granitoids of the Tormes Dome (Central Iberian Zone). A U-Pb LA-ICP-MS study (United States)

    López-Moro, F. J.; López-Plaza, M.; Gutiérrez-Alonso, G.; Fernández-Suárez, J.; López-Carmona, A.; Hofmann, M.; Romer, R. L.


    In this study, we report U-Pb Laser Ablation ICP-MS zircon and ID-TIMS monazite ages for peraluminous granitoid plutons (biotite ± muscovite ± cordierite ± sillimanite) in the Tormes Dome, one of the gneiss-cored domes located in the Central Iberian Zone of the Variscan belt of northern Spain. Textural domains in zircon, interpreted to represent the magmatic crystallization of the granitoids (and one monazite fraction in the Ledesma pluton) yielded ages around 320 Ma, in agreement with other geochronological studies in the region. This age is interpreted to date the timing of decompression crustal melting driven by the extensional collapse of the orogenic belt in this domain of the Variscan chain of western Europe. In addition, there are several populations of inherited (xenocrystic) zircon: (1) Carboniferous zircon crystals (ca. 345 Ma) as well as one of the monazite fractions in the coarse-grained facies of the Ledesma pluton that also yielded an age of ca. 343 Ma. (2) Devonian-Silurian zircon xenocrysts with scattered ages between ca. 390 and 432 Ma. (3) Middle Cambrian-Ordovician (ca. 450-511 Ma). (4) Ediacaran-Cryogenian zircon ages (ca. 540-840 Ma). (5) Mesoproterozoic to Archaean zircon (900-2700 Ma). The abundance of Carboniferous-inherited zircon shows that crustal recycling/cannibalization may often happen at a fast pace in orogenic scenarios with only short lapses of quiescence. In our case study, it seems plausible that a "crustal layer" of ca. 340 Ma granitoids/migmatites was recycled, partially or totally, only 15-20 My after its emplacement.

  16. U-Pb zircon and Re-Os molybdenite geochronology from La Caridad porphyry copper deposit: insights for the duration of magmatism and mineralization in the Nacozari District, Sonora, Mexico (United States)

    Valencia, V. A.; Ruiz, J.; Barra, F.; Geherls, G.; Ducea, M.; Titley, S. R.; Ochoa-Landin, L.


    Uranium-lead zircon (laser ablation multi-collector ICP-MS spot analysis) ages from La Caridad porphyry copper deposit in the Nacozari District, Northeastern Sonora, Mexico, suggest a short period of magmatism, between 55.5 and 52.6 Ma. Two U-Pb ages from the mineralized quartz monzonite unit, showing different textural characteristics, yielded indistinguishable crystallization ages (~54 Ma), and indicate that the intrusion responsible for the mineralization occurred as a single large complex unit, instead of multiple pulses of magmatism. Some zircons analyzed also show inherited ages in cores recording dates of 112-124 Ma, 141-166 Ma and 1.4 Ga. The Re-Os molybdenite ages from the potassic and phyllic hydrothermal alteration veins yielded identical ages within error, 53.6±0.3 Ma and 53.8±0.3 Ma, respectively (weighted average of 53.7±0.21 Ma), supporting a restricted period for the mineralization. The geochronological data thus indicate a short-lived magmatic and hydrothermal system. The inherited zircons of Precambrian and Late Jurassic-Mid Cretaceous age found in the intrusive rocks of La Caridad deposit, can be explained considering two possible scenarios within the tectonic/magmatic evolution of the area. The first scenario considers the presence of a Precambrian anorogenic granitic basement that is intruded by Mesozoic (Jurassic-Cretaceous) units present beneath the La Caridad deposit. The second scenario suggests that the Mesozoic Glance Conglomerate Formation of Arizona underlies the Paleocene volcanic-igneous pile in the La Caridad area.

  17. Airborne LiDAR analysis and geochronology of faulted glacial moraines in the Tahoe-Sierra frontal fault zone reveal substantial seismic hazards in the Lake Tahoe region, California-Nevada USA (United States)

    Howle, James F.; Bawden, Gerald W.; Schweickert, Richard A.; Finkel, Robert C.; Hunter, Lewis E.; Rose, Ronn S.; von Twistern, Brent


    We integrated high-resolution bare-earth airborne light detection and ranging (LiDAR) imagery with field observations and modern geochronology to characterize the Tahoe-Sierra frontal fault zone, which forms the neotectonic boundary between the Sierra Nevada and the Basin and Range Province west of Lake Tahoe. The LiDAR imagery clearly delineates active normal faults that have displaced late Pleistocene glacial moraines and Holocene alluvium along 30 km of linear, right-stepping range front of the Tahoe-Sierra frontal fault zone. Herein, we illustrate and describe the tectonic geomorphology of faulted lateral moraines. We have developed new, three-dimensional modeling techniques that utilize the high-resolution LiDAR data to determine tectonic displacements of moraine crests and alluvium. The statistically robust displacement models combined with new ages of the displaced Tioga (20.8 ± 1.4 ka) and Tahoe (69.2 ± 4.8 ka; 73.2 ± 8.7 ka) moraines are used to estimate the minimum vertical separation rate at 17 sites along the Tahoe-Sierra frontal fault zone. Near the northern end of the study area, the minimum vertical separation rate is 1.5 ± 0.4 mm/yr, which represents a two- to threefold increase in estimates of seismic moment for the Lake Tahoe basin. From this study, we conclude that potential earthquake moment magnitudes (Mw) range from 6.3 ± 0.25 to 6.9 ± 0.25. A close spatial association of landslides and active faults suggests that landslides have been seismically triggered. Our study underscores that the Tahoe-Sierra frontal fault zone poses substantial seismic and landslide hazards.

  18. New constraints on the subsurface geology of the Mexico City Basin: The San Lorenzo Tezonco deep well, on the basis of 40Ar/39Ar geochronology and whole-rock chemistry (United States)

    Arce, J. L.; Layer, P. W.; Morales-Casique, E.; Benowitz, J. A.; Rangel, E.; Escolero, O.


    The San Lorenzo Tezonco deep well, drilled in 2012 by "Sistema de Aguas de la Ciudad de México" (Water Supply System of Mexico City) in the Mexico Basin to a depth of 2008 m, offers an excellent opportunity to explore the subsurface stratigraphy and general geology of the region. Unfortunately, only chipped samples from this well were recovered, which were examined and analyzed for whole-rock chemistry, 40Ar/39Ar, and U-Pb zircon geochronology, in order to reconstruct the lithology of the well. Contrary to previous deep wells of the Mexico Basin, no basement sedimentary rocks were found in this one. While the upper 70 m are composed of lacustrine sediments associated with Lake Texcoco, volcanic rocks make up the majority of the well and range in age from more than 18 Ma to 0.25 Ma. Andesitic lavas are the most abundant products of the stratigraphic column, followed by acidic products represented by dacitic and rhyolitic lavas and ignimbrite deposits. Less abundant are basaltic andesite lavas appearing in the upper and lower parts of the column. The thickest sequence of the well is represented not only by Miocene volcanic rocks ranging from 5 to 17 Ma, suggesting a period of intense volcanic activity in this area producing mainly andesitic lavas, but also by thick rhyolitic ignimbrite deposits dated at 5 Ma. These deposits suggest the presence of a caldera structure, probably buried by subsequent volcanic products and lacustrine sediments. Trace element concentrations suggest that volcanism is likely produced in a subduction environment with typical negative anomalies of Nb, Ta, Ti, and P and positive anomalies in Pb and Cs. We correlated the well units with units outcropping in mountain ranges in the surrounding area, with the recognition of the following units or formations: Eocene Andesite, San Nicolás Basaltic Andesite, Tepoztlán Formation, Miocene Volcanism, Sierra de las Cruces, and Chichinautzin Volcanic Field products. By correlating the two closest deep

  19. The Indus-Yarlung Zangbo (IYZ) ophiolites from Nanga Parbat to Namche Barwa syntaxes, Southern Tibet: First synthesis of the petrology, geochemistry and geochronology of the IYZ ophiolites, and implications for geodynamic reconstructions of Neo-Tethys (United States)

    Hébert, Réjean; Bezard, Rachel; Guilmette, Carl; Dostal, Jaroslav; Wang, Chengshan; Liu, Zf


    The purpose of this first synthesis is to summarize findings on the Yarlung Zangbo Suture Zone (YZSZ) ophiolites in Southern Tibet, and to discuss some of thel remaining scientific problems. The YZSZ ophiolites have been studied for almost 30 years and constitute the youngest of the sutures recognized on the Tibet Plateau. It is now acknowledged the YZSZ is a complex assemblage of sedimentary, metamorphic and igneous rocks produced during and shortly after the collision between India and Eurasia. The ages of the various lithological units span a time interval from the Jurassic to the Middle Miocene, with some Permian and Devonian exotic blocks in the mélange zone. The YZSZ is characterized by ophiolitic complexes and an ophiolitic mélange. The ophiolites are of two types with complete and incomplete pseudostratigraphies. The complete sections, although tectonically reworked, are observed along the segment from Dazhuqu to Jiding in the Xigaze area and the Spontang ophiolite. The incomplete sequences are found in various locations, including Nidar, Kiogar, Jungbwa, Saga, Sangsang, Xigugabu, Luobusa. The incomplete nature of these ophiolites could be related to intraoceanic or orogenic/collisional origins. The YZSZ ophiolites are also distributed into two groups of ages: the Luobusa, Zedang and Kiogar sequences are Jurassic-Lower Cretaceous in age, whereas all other sequences are of a Lower Cretaceous age. Compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 m.y. from their inital igneous genesis to obduction, which occurred around 70-90 Ma. Although the YZSZ ophiolites differ in terms of their petrological and geochemical characters, they were all generated in a suprasubduction zone setting, and more specifically in arc (few fore-arc) and back-arc environments. Our synthesis of ~500 geochemical analyses show variable mixing of components from N-MORB-type to IAT-CAB and to OIB end-members. The Jurassic

  20. Early carboniferous wrenching, exhumation of high-grade metamorphic rocks and basin instability in SW Iberia: Constraints derived from structural geology and U-Pb and 40Ar-39Ar geochronology (United States)

    Pereira, M. Francisco; Chichorro, Martim; Silva, J. Brandão; Ordóñez-Casado, Berta; Lee, James K. W.; Williams, Ian S.


    New U-Pb and 40Ar-39Ar geochronology and structural data from high- to medium grade metamorphic shear zones of the Ossa-Morena Zone, and structural data from Early Carboniferous basins (Ossa-Morena Zone and South-Portuguese Zone), place additional constraints on the Variscan tectonics in SW Iberia. A zircon U-Pb age of 465 ± 14 Ma (Middle Ordovician) measured on migmatite from the Coimbra-Cordoba shear zone is interpreted as the age of protolith crystallization. This age determination revises the information contained in the geological map of Portugal, in which these rocks were considered to be Proterozoic in age. This paper describes the evolution of Variscan wrench tectonics related to the development of shear zones, exhumation of deep crustal rocks and emplacement of magma in the Ossa-Morena Zone basement. In the Coimbra-Cordoba shear zone (transpressional), migmatites were rapidly exhumed from a depth of 42.5 km to 16.6 km over a period of ca. 10 Ma in the Viséan (ca. 340-330 Ma), indicating oblique slip exhumation rates of 8.5 to 10.6 mm/yr (Campo Maior migmatites) and 3.2 mm/yr (Ouguela gneisses) respectively. In the Évora Massif, the gneisses of the Boa Fé shear zone (transtensional) were exhumed from 18.5 to 7.4 km depth in the period ca. 344-334 Ma (Viséan), with exhumation oblique slip rates of 2.8 to 4.2 mm/yr. At the same time, the Early Carboniferous basins of SW Iberia were filled by turbidites and olistoliths, composed mostly of Devonian rocks. The presence of olistoliths indicates significant tectonic instability during sedimentation with large-scale mass movement, probably in the form of gravity slides. Deformation and metamorphism dated at 356 ± 12 Ma, 321 ± 13 Ma and 322 ± 29 Ma respectively suggests that Variscan wrench movements were active in SW Iberia during the Early Carboniferous for a period of at least 35 Ma.

  1. 40Ar/39Ar Geochronology, Isotope Geochemistry (Sr, Nd, Pb), and petrology of alkaline lavas near Yampa, Colorado: migration of alkaline volcanism and evolution of the northern Rio Grande rift (United States)

    Cosca, Michael A.; Thompson, Ren A.; Lee, John P.; Turner, Kenzie J.; Neymark, Leonid A.; Premo, Wayne R.


    Volcanic rocks near Yampa, Colorado (USA), represent one of several small late Miocene to Quaternary alkaline volcanic fields along the northeast margin of the Colorado Plateau. Basanite, trachybasalt, and basalt collected from six sites within the Yampa volcanic field were investigated to assess correlations with late Cenozoic extension and Rio Grande rifting. In this paper we report major and trace element rock and mineral compositions and Ar, Sr, Nd, and Pb isotope data for these volcanic rocks. High-precision 40Ar/39Ar geochronology indicates westward migration of volcanism within the Yampa volcanic field between 6 and 4.5 Ma, and the Sr, Nd, and Pb isotope values are consistent with a primary source in the Proterozoic subcontinental lithospheric mantle. Relict olivine phenocrysts have Mg- and Ni-rich cores, whereas unmelted clinopyroxene cores are Na and Si enriched with finely banded Ca-, Mg-, Al-, and Ti-enriched rims, thus tracing their crystallization history from a lithospheric mantle source region to one in contact with melt prior to eruption. A regional synthesis of Neogene and younger volcanism within the Rio Grande rift corridor, from northern New Mexico to southern Wyoming, supports a systematic overall southwest migration of alkaline volcanism. We interpret this Neogene to Quaternary migration of volcanism toward the northeast margin of the Colorado Plateau to record passage of melt through subvertical zones within the lithosphere weakened by late Cenozoic extension. If the locus of Quaternary alkaline magmatism defines the current location of the Rio Grande rift, it includes the Leucite Hills, Wyoming. We suggest that alkaline volcanism in the incipient northern Rio Grande rift, north of Leadville, Colorado, represents melting of the subcontinental lithospheric mantle in response to transient infiltration of asthenospheric mantle into deep, subvertical zones of dilational crustal weakness developed during late Cenozoic extension that have been

  2. Geochemistry, U-Pb geochronology, Sm-Nd and O isotopes of ca. 50 Ma long Ediacaran High-K Syn-Collisional Magmatism in the Pernambuco Alagoas Domain, Borborema Province, NE Brazil (United States)

    Francisco da Silva Filho, Adejardo; de Pinho Guimarães, Ignez; Santos, Lucilene; Armstrong, Richard; Van Schmus, William Randall


    The Pernambuco Alagoas (PEAL) domain shows the major occurrence of granitic batholiths of the Borborema Province, NE Brazil, with Archean to Neoproterozoic range of Nd TDM model ages, giving clues on the role of granites during the Brasiliano orogeny. SHRIMP U/Pb zircon geochronological data for seven granitic intrusions of the PEAL domain divide the studied granitoids into three groups: 1) early-to syn-collision granitoids with crystallization ages ca. 635 Ma (Serra do Catú pluton), 2) syn-collision granitoids with crystallization ages 610-618 Ma (Santana do Ipanema, Água Branca, Mata Grande and Correntes plutons) and 3) late-to post-collision granitoids with ages of ca. 590 Ma (Águas Belas, and Cachoeirinha plutons). The intrusions of group 1 and 2, except the Mata Grande and Correntes plutons, show Nd TDM model ages ranging from 1.2 to 1.5 Ga, while the granitoids from group 3, and Mata Grande Pluton and Correntes plutons have Nd TDM model ages ranging from 1.7 to 2.2 Ga. The studied granitoids with ages plutons, together with the available Nd isotopic data suggest that the Brasiliano orogeny strongly reworked older crust, of either Paleoproterozoic or Tonian ages. The studied granitoids are coeval with calc-alkaline granitoids of the Transversal Zone and Sergipano domains and rare high-K calc-alkaline granitoids from the Transversal Zone domain. Such large volumes of high-K granitoids with crystallization ages older than 600 Ma are not recorded in the Transversal Zone domains, suggesting that at least between 600 and 650 Ma, the granitic magmatism of these two areas were distinct. However, the studied granitoids (630-580 Ma) located in the north part of the PEAL domain, north of the Palmares shear zone are coeval with granitoids of similar geochemical compositions in the Transversal Zone domain. It suggests that the southeastern part of the Transversal Zone and the northern part of the PEAL domains belonged to the same crustal block during the Brasiliano

  3. Geochemical and geochronological characteristics of Late Cretaceous to Early Paleocene granitoids in the Tengchong Block, Southwestern China: Implications for crustal anatexis and thickness variations along the eastern Neo-Tethys subduction zone (United States)

    Zhao, Shao-wei; Lai, Shao-cong; Qin, Jiang-feng; Zhu, Ren-Zhi; Wang, Jiang-bo


    The Tengchong Block of Southwestern China is key to tracing the eastward subduction of Neo-Tethys and collision between Indian and Asian continents. The block contains a magmatic belt that represents the southeastward continuation of the Gangdese belt, produced by the eastward subduction of eastern Neo-Tethyan oceanic lithosphere. In this paper we present geochemical and geochronological data of Late Cretaceous to Early Paleocene ( 64 Ma) granitic rocks of the Guyong and Husa batholiths in the Tengchong Block. These can be subdivided into high-silica peraluminous granites and low-silica metaluminous granodiorites, and all belong to the high-K calc-alkaline series, are enriched in LILE, and depleted in HFSE. The Guyong granitoids have high initial Sr ratios of 0.706511-0.711753, negative εNd(t) values of - 9.2 to - 11.6, two-stage model ages of 1.39-1.55 Ga, and Pb isotopic compositions that indicate a crustal affinity. The Husa granodiorites also have high initial Sr ratios of 0.716496, negative εNd(t) value of - 16.5, two-stage model age of 1.89 Ga, variable εHf(t) values of 3.4 to - 18.1 and Pb isotopic compositions similar to lower crustal values. These geochemical and isotopic data indicate that the Guyong granitoids were likely derived from partial melting of ancient crustal metapelite or mixed pelite-greywacke sources, while the Husa granodiorites were derived from the partial melting of lower crustal mixed sources involving metasedimentary and metaigneous rocks. To understand the thermal state and architecture of the Late Cretaceous to Early Paleocene magmatic arc crust, the crust-derived intermediate to acidic igneous rocks of the southern-central Lhasa and Tengchong blocks and eastern Himalayan syntaxis are compared. We infer that partial melting of crust occurred at great depth in the southern Lhasa Block, intermediate depths in the eastern Himalayan syntaxis, and shallow depths in the central Lhasa and Tengchong Block. Sr/Y ratios indicate that the

  4. New geochronological ages (U-Pb/Lu-Hf) from high-pressure rocks of the Escambray terrane and Santa Clara serpentinite mélange, central Cuba. Regional correlations and geodynamic implications (United States)

    Rojas-Agramonte, Y.; García-Casco, A.; Kröner, A.; Herwartz, D.; Ibis Despaigne, A.; Wilde, S.


    Petrological and geochronological data of high pressure (HP) rocks from Cuba and Dominican Republic indicate continuous subduction in the northern edge of the Caribbean since ca. 120 Ma. However, expected correlation of the associated subduction zone towards the west in Guatemala is uncertain, for similar HP rocks in this region have metamorphic ages as old as 150 Ma. In this contribution we present new geochemical and U-Pb/Lu-Hf ages of HP rocks from the Escambray terrane and the central Cuba serpentinite mélange that allow geodynamic correlations between the Greater Antilles and Guatemala during the early and late Cretaceous. The Escambray composite terrane constitutes a metamorphic accretionary complex containing oceanic and platform-derived metasedimentary and metaigneous rocks that were tectonically assembled in the Caribbean subduction environment during the latest Cretaceous. The complex crops out as two domes, named Trinidad to the west and Sancti Spiritus to the east, forming a tectonic window below the arc-related Mabujina amphibolite complex and the allochthonous Cretaceous volcanic arc terrane. The latter overrides the central Cuba mélange, which contains low-pressure ophiolitic blocks and HP blocks of eclogite, garnet amphibolite and blueschist facies rocks within a serpentinitic matrix. This serpentinite mélange and similar melange bodies within the Escambray complex have been interpreted as fragments of the Caribbean subduction channel. The studied HP samples are of basaltic composition, poor in K2O (<0.34 wt %) and Rb (< 3.05 ppm), relatively rich in Ta (0.07-0.77 ppm) and Hf (0.74-4.17 ppm), and rich in Nb (0.68- 13.53 ppm). Most samples show a REE chondrite-normalised patterns characterized by a subtle LREE depletion, except a few samples which are characterized by slight enrichment in LREE. Based on their distinctive trace-element contents, most of the basaltic protholiths of the samples are identified as E- to N-MORB signature. Some samples

  5. New techniques applied to the U-Pb method at the Centre for Geochronological Research of the University of Sao Paulo: chemical digestions, TIMS mass spectrometry and examples of integrated application of SHRIMP; Novas tecnicas aplicadas ao metodo U-Pb no CPGeo-IGc/USP: avancos na digestao quimica, espectrometria de massa (TIMS) e exemplos de aplicacao integrada com SHRIMP

    Energy Technology Data Exchange (ETDEWEB)

    Sato, Kei; Basei, Miguel Angelo Stipp; Siga Junior, Oswaldo; Sproesser, Walter Mauricio; Passarelli, Claudia Regina [Universidade de Sao Paulo (USP), SP (Brazil). Inst. de Geociencias. Centro de Pesquisas Geocronologicas]. E-mails:;;;;


    The present work describes new techniques of U-Pb ID-TIMS developed at the CPGeo-IGc/USP. Techniques of partial and total chemical digestions, U and Pb concentration using anion exchange in a micro column, and the utilization of a {sup 205}Pb spike are discussed. Main geochronological equations used are also discussed. Examples of the application of U-Pb data, obtained by leaching technique using a microwave oven in zircon with isotopic inheritances in the core and border, are presented. The genesis of zircon based on the initial {sup 176}Hf/{sup 177}Hf ratio is discussed. Finally complete listings of the mathematical formulations used in the PBDAT software are given in the appendix. (author)

  6. Geologic evolution of the Paraiba do Sul complex, in the central part of the Ribeira belt, based on the geochemistry and U-Pb geochronology studies; Evolucao geologica do complexo Paraiba do Sul, no segmento central da faixa Ribeira, com base em estudos de geoquimica e geocronologia U-Pb

    Energy Technology Data Exchange (ETDEWEB)

    Valladares, Claudia Sayao


    Aiming the definition of the Complexo Paraiba do Sul evolution, the work presents a geochronological approach by the U-Pb method, considered appropriate to study polideformed terrains. The minerals used to the U-Pb dating (zircon, titanite and monazite) provides information about the ages of rock generation and the successive geological events the rocks were submitted. The U-Pb geochronological approach is pioneer in the investigated area and two questions referent to the Complexo Paraiba do Sul evolution in the central part of the Ribeira belt, were the aim of this approach. The first of them refers to the crystallization epoch and the gneisses metamorphism, interpreted here like orthogneisses, called Unidade Quirino of the Complexo Paraiba do Sul. The second refers to the metamorphism ages and meta sedimentary unit deformation. Looking for the contribution to the Complexo Paraiba do Sul evolution understanding, this work presents the data based on the geologic map of this area, comprising part of the Folha Volta Redonda (1:50.000), including petrographic, structural and microtectonic studies. It also presents and discusses the results of: 17 U-Pb analysis in zircon and titanite of the Complexo Paraiba do Sul orthogneisses collection; 24 geochemical analysis of major, minor, trace elements and ETR of the same collection; an U-Pb analysis in titanite of the Unidade Sao Joao, 5 U-Pb analysis in titanites and/or monazites of granites and granitoids; 12 geochemical analysis including major, minor and trace elements and ETR, in granites and sills of leucogranites. Finally, the data achieved were integrated with the objective to elaborate an evolutive view of the Complexo Paraiba do Sul 157 refs., 54 figs., 17 tabs.

  7. Petrology, geochemistry and zircon U-Pb geochronology of a layered igneous complex from Akarui Point in the Lützow-Holm Complex, East Antarctica: Implications for Antarctica-Sri Lanka correlation (United States)

    Kazami, Sou; Tsunogae, Toshiaki; Santosh, M.; Tsutsumi, Yukiyasu; Takamura, Yusuke


    The Lützow-Holm Complex (LHC) of East Antarctica forms part of a complex subduction-collision orogen related to the amalgamation of the Neoproterozoic supercontinent Gondwana. Here we report new petrological, geochemical, and geochronological data from a metamorphosed and disrupted layered igneous complex from Akarui Point in the LHC which provide new insights into the evolution of the complex. The complex is composed of mafic orthogneiss (edenite/pargasite + plagioclase ± clinopyroxene ± orthopyroxene ± spinel ± sapphirine ± K-feldspar), meta-ultramafic rock (pargasite + olivine + spinel + orthopyroxene), and felsic orthogneiss (plagioclase + quartz + pargasite + biotite ± garnet). The rocks show obvious compositional layering reflecting the chemical variation possibly through magmatic differentiation. The metamorphic conditions of the rocks were estimated using hornblende-plagioclase geothermometry which yielded temperatures of 720-840 °C. The geochemical data of the orthogneisses indicate fractional crystallization possibly related to differentiation within a magma chamber. Most of the mafic-ultramafic samples show enrichment of LILE, negative Nb, Ta, P and Ti anomalies, and constant HFSE contents in primitive-mantle normalized trace element plots suggesting volcanic arc affinity probably related to subduction. The enrichment of LREE and flat HREE patterns in chondrite-normalized REE plot, with the Nb-Zr-Y, Y-La-Nb, and Th/Yb-Nb/Yb plots also suggest volcanic arc affinity. The felsic orthogneiss plotted on Nb/Zr-Zr diagram (low Nb/Zr ratio) and spider diagrams (enrichment of LILE, negative Nb, Ta, P and Ti anomalies) also show magmatic arc origin. The morphology, internal structure, and high Th/U ratio of zircon grains in felsic orthogneiss are consistent with magmatic origin for most of these grains. Zircon U-Pb analyses suggest Early Neoproterozoic (847.4 ± 8.0 Ma) magmatism and protolith formation. Some older grains (1026-882 Ma) are regarded as

  8. Evolution Of An Upper Crustal Plutonic-Volcanic Plumbing System:Insights From High Precision U-Pb Zircon Geochronology Of Intracaldera Tuff And Intrusions In Silver Creek Caldera, Arizona, USA (United States)

    Zhang, T.; Mundil, R.; Miller, C. F.; Miller, J. S.; Paterson, S. R.


    between the different phases of the intrusion is very small and the ages of the intrusion match within errors the age of the PST. This tight time range indicates that the super-eruption and the subsequent reactivation of the caldera by hypabyssal intrusions happened on a much shorter timescale than the evolution of large magma systems that have been described with durations of up to 10 m.y. Additional geochronology in combination with geochemical and AMS analyses are aimed at a more detailed reconstruction of the emplacement and eruption history of this plutonic-volcanic system.

  9. Contribution of Columbia and Gondwana Supercontinent assembly- and growth-related magmatism in the evolution of the Meghalaya Plateau and the Mikir Hills, Northeast India: Constraints from U-Pb SHRIMP zircon geochronology and geochemistry (United States)

    Kumar, Santosh; Rino, Vikoleno; Hayasaka, Yasutaka; Kimura, Kosuke; Raju, Shunmugam; Terada, Kentaro; Pathak, Manjari


    The Meghalaya Plateau and the Mikir Hills constitute a northeastern extension of the Precambrian Indian Shield. They are dominantly composed of Proterozoic basement granite gneisses, granites, migmatites, granulites, the Shillong Group metasedimentary cover sequence, and Mesozoic-Tertiary igneous and sedimentary rocks. Medium to coarse grained, equigranular to porphyritic Cambrian granite plutons intrude the basement granite gneisses and the Shillong Group. U-Pb SHRIMP zircon geochronology and geochemistry of the granite gneisses and granites have been carried out in order to understand the nature and timing of granite magmatism, supercontinent cycles, and crustal growth of the Meghalaya Plateau and Mikir Hills. Zircons from the Rongjeng granite gneiss record the oldest magmatism at 1778 ± 37 Ma. An inherited zircon core has an age of 2566.4 ± 26.9 Ma, indicating the presence of recycled Neoarchaean crust in the basement granite gneisses. Zircons from the Sonsak granite have two ages: 523.4 ± 7.9 Ma and 1620.8 ± 9.2 Ma, which indicate partial assimilation of an older granite gneiss by a younger granite melt. Zircons from the Longavalli granite gneiss of the Mikir Hills has a crystallization age of 1430.4 ± 9.6 Ma and a metamorphic age of 514 ± 18.6 Ma. An inherited core of a zircon from Longavalli granite gneiss has an age of 1617.1 ± 14.5 Ma. Zircons from younger granite plutons have Cambrian mean ages of 528.7 ± 5.5 Ma (Kaziranga), 516 ± 9.0 Ma (South Khasi), 512.5 ± 8.7 Ma (Kyrdem), and 506.7 ± 7.1 Ma and 535 ± 11 Ma (Nongpoh). These plutons are products of the global Pan-African tectonothermal event, and their formation markedly coincides with the later stages of East Gondwana assembly (570-500 Ma, Kuunga orogen). The older inherited zircon cores (2566.4 ± 26.9 Ma, 1758.1 ± 54.3 Ma, 1617.1 ± 14 Ma) imply a significant role for recycled ancient crust in the generation of Cambrian granites. Thus the Meghalaya Plateau and Mikir Hills experienced

  10. New Paleomagnetic Results and Ar-Ar Geochronology From the Vallenar Region (29oS), N. Chile: Implications for the Timing of Rotations in the Andean Forearc Region. (United States)

    Gipson, M. J.; Taylor, G. K.; Grocott, J.


    The Andean forearc in northern Chile comprises from west to east three tectonomorphic zones - the Coastal Cordillera, the Central Valley and the Precordillera/Chilean Altiplano which lie outboard of the High Andes which represent the present-day active magmatic/volcanic arc. In the Vallenar region the Coastal Cordillera passes directly eastward into the Precordillera with no Central Valley present. The Coastal Cordillera is composed of deformed Palaeozoic sediments intruded by Permo-Triassic plutons, which together comprise the basement of the region. This basement is intruded by Jurassic to Upper Cretaceous plutons of the Coastal Batholith, which is in part overlain by and intruded into Lower Cretaceous arc to back-arc sequences of volcanics and marine limestones. This region is separated from the Upper Cretaceous and Lower Tertiary volcanics and intrusives of the Precordillera to the east by a narrow deformational zone which extends for at least 200km along strike and which we believe to have formed as a result of sinistral transpression. New palaeomagnetic results from the westernmost part of the Lower Cretaceous batholith (n = 7/9 sites, dec = 35.5, inc = -45.0, {α }95 = 4.9), dated to between 130-128 Ma using Ar-Ar geochronology, reveal a well constrained clockwise rotation of 47 {+/-} 7o, consistent with previous results from further north in the Coastal Cordillera. Results from the easternmost pluton of the batholith, dated at 91 Ma, yields a characteristic direction (n = 9 sites, dec = 22.6, inc = -51.1, {α }95 = 4.5) and a rotation of 37 {+/-} 7o. The Coastal Cordillera - Precordillera boundary in this region is marked by an interbedded sequence of red sandstones and andesitic flows (the Punte del Cobre Fm.) overlain by limestones (Chanarcillo Gp.) which are here deformed in a relatively narrow (5-10km wide) fold and thrust belt. The characteristic magnetization of these units (n = 9 sites, dec = 31.5, inc = -50.7, α 95 = 4.3) clearly fails the fold

  11. Geochronology and Nd isotope geochemistry of the Gameleira Cu-Au deposit, Serra dos Carajás, Brazil: 1.8-1.7 Ga hydrothermal alteration and mineralization (United States)

    Pimentel, Márcio M.; Lindenmayer, Zara G.; Laux, Jorge H.; Armstrong, Richard; de Araújo, Janice Caldas


    . The geochronological data suggest that the Gameleira Cu-Au mineralization is related to a Paleoproterozoic (ca. 1.83 Ga) episode of hydrothermal activity and is not Archaean. The younger ages of ca. 1.70-1.73 might be interpreted as products of the lower blocking temperatures of biotite in relation to the Ar-Ar and Sm-Nd isotopic systems. Combined with previous geochemistry and stable isotope data, the Nd isotopic data suggest that the mineralizing fluids were derived from, or strongly interacted with, a Paleoproterozoic crustal granite, possibly similar in age and composition to the Pojuca granite.

  12. Petrology and LA-ICP-MS Zircon U-Pb Geochronology of the Qianjinchang Pluton, Southeastern Inner Mongolia%内蒙古前进场岩体岩石学与锆石U-Pb年代学研究

    Institute of Scientific and Technical Information of China (English)

    徐佳佳; 赖勇; 崔栋; 鲁彬


    The Qianjinchang biotite granite from southeastern Inner Mongolia is located in the suture zone between the Siberian and North China Cratons. The Daolundaba Cu-W-Sn polymetallic deposit formed at the contact zone between the north-west margin of the pluton and the Lower Permian Linxi Formation. The biotite granite is high-K calc-alkaline and peraluminous, with Na2O+K2O: 6.12% to 9.31%, Al2O3: 14.42% to 15.59%, A/CNK: 1.09 to 1.22, and a mean 5value of 2.41. The rock samples lie within the S-type granite region in the ACF plot. They are depleted in Sr (89.91 to 192.80 μg/g), Ba, Eu (δEu: 0.46 to 0.81) and δREE (102.65 to 237.92 μg/g), but slightly enriched in Yb (2.17 to 3.46 μg/g) and La, consistent with the characteristics of S-type granites. The rocks further fall in the region of post-collisional granite in the Rb vs. Y+Nb diagram. LA-ICP-MS zircon U-Pb geochronology shows that the pluton emplaced at (273±7.7) to (278 ±4.3) Ma. These Permian granite rocks probably formed in an extensional setting that postdated the collision between the Siberian and North China Cratons.%前进场黑云母花岗岩岩体位于内蒙古东南部,华北板块与西伯利亚板块碰撞的缝合部位。岩体北西边缘与林西组地层的接触带发育道伦达坝中型铜-钨-锡多金属岩浆热液矿床。岩石地球化学研究表明,黑云母花岗岩为高钾钙碱性系列的过铝质花岗岩,Na2O+K2O含量为6.12%-9.31%,Al2O3含量为14.42%-15.59%,A/CNK值为1.09-1.22,万均值为2.41,在花岗岩ACF图解中落人s型花岗岩区。全岩总稀土含量为102.65~237.92μg/g,低Sr(89.91-192.80μg/g)高Yb(2.17~3.46μg/g),δEu值为0.46~0.81,亏损Ba,Sr,Eu,富集La,显示S型花岗岩特征,在Rb-(Y+Nb)图解中显示后碰撞岩浆地球化学性质。利用LA-ICP-MS锆石U-Pb年代学方法,确定岩体形成年代为(273±7.7)-(278±4.3)Ma,在早

  13. La Hornblendita Ushuaia (Tierra del Fuego:: Petrografía, geoquímica y geocronología The Ushuaia Hornblendite (Tierra del Fuego: petrography, geochemistry and geochronology

    Directory of Open Access Journals (Sweden)

    R. D. Acevedo


    Full Text Available Se presentan descripciones petrográficas y datos geoquímicos y geocronológicos sobre la denominada Hornblendita Ushuaia, un complejo de rocas mesocráticas a melanocráticas hidratadas que aflora en las inmediaciones de la ciudad de Ushuaia, Tierra del Fuego. El conjunto litológico está constituido principalmente por hornblenditas, hornblendito-piroxenitas, dioritas, gabros, sienitas y monzodioritas. La composición mineralógica de la muestra más representativa es de hornblenda y diópsido como minerales esenciales, biotita y feldespato potásico en menor proporción, titanita, apatita y minerales opacos (principalmente magnetita, ilmenita, pirita y calcopirita como accesorios. Los fenómenos de alteración consisten en albitización, epidotización, cloritización y carbonatización, los cuales produjeron sustanciales cambios químicos que pudieron haber afectado cualquier clasificación de la roca. Sin embargo, se considera que el magma parental ha sido calcoalcalino, de tipo Andino, relacionado con el Batolito Andino, producido durante el evento subductivo regional. Evidencias geológicas concluyen que estos cuerpos ígneos no pertenecen en modo alguno a una columna ofiolítica. La geoquímica calcoalcalina es atribuida a un proceso de fusión parcial. En realidad, no constituyen fundidos primarios del manto sino fluidos mantélicos evolucionados y contaminados que ascendieron hacia niveles corticales, favoreciendo un proceso de asimilación, en un ambiente de emplazamiento de arco magmático. Los ensayos radimétricos sobre una muestra de hornblendita, usando el método de K/Ar sobre roca total, dieron una edad de 113 ± 5 Ma.Petrographic descriptions and geochemical and geochronological data are given for the so-called Ushuaia Hornblendite, a complex of mesocratic and melanocratic hydrated rocks that crops out in the vicinity of the city of Ushuaia, Tierra del Fuego. The complex consists mainly of hornblendites and hornblendite

  14. Progreso y retos de la geocronología 40Ar/39Ar en Costa Rica y Nicaragua Progress and Challenges Using 40Ar/39Ar Geochronology in Costa Rica and Nicaragua

    Directory of Open Access Journals (Sweden)

    Ian Saginor¹


    Full Text Available Con el principal objetivo de realizar mejores cálculos de la producción volcánica, en el Arco Volcánico de América Central, realizamos 61 dataciones 40Ar/39Ar en tefras y lavas en localidades de frente volcánico de Costa Rica y Nicaragua por medio de varias campañas de campo y análisis de laboratorio (preparación de muestras, envió al reactor nuclear, espectroscopía de masas de las muestras radiactivas, al laboratorio de gases nobles del Departamento de Ciencias de la Tierra y Planetarias de la Universidad Rutgers del 2002-2008. En este artículo presentamos diferentes observaciones que resultaron de este estudio donde se describe la precisión, exactitud y la confiabilidad de las edades 40Ar/39Ar. El primer resultado muestra las limitaciones de esta técnica en muestras baja en K2O con edades To better estimate the extrusive flux of the Central American Arc, from 2002-2008, we obtained sixty one high precision 40Ar/39Ar ages on geographically well-situated lavas and tephra from Costa Rica and Nicaragua. Here, we describe a number of observations encountered during this study using four examples that well document the precision, accuracy and general reliability of the 40Ar/39Ar ages. First, low K2O values, particularly in samples from Nicaragua, is a major limitation in or attempts to obtain reliable dates on samples under 1 My. Second, extensive weathering of samples due to the tropical climate of Central America has resulted in various levels of argon loss even when the hand sample appeared unaltered. Third, our field and geochronological data lead us to conclude that eruptive rates have not been constant over the past 15 to 20 My, but rather appears punctuated by gaps of up to several million years. We attempted to address the temporal gaps in several ways. First, geochemical analyses were used to identify samples that may have erupted during time periods without known volcanism. For example, U/Th values in the active Central

  15. Geochronologic data for the Arabian Shield (United States)

    Aldrich, Lyman Thomas; Brown, G.F.; Hedge, Carl; Marvin, Richard


    The radiometric ages reported below were completed during the period 1957-1936. the rock samples were provided and examined petrographically by Glen F. Brown and his associates at the U.S. Geological Survey. The 25 Rb-Sr ages of biotites and feldspars and the 25 K-Ar ages of biotites, hornblendes, and total rock samples constituted the initial suite of ages for the Arabian Shield. Zircons in the quantity required for analysis were not found in any of the rocks examined.

  16. The applicability of the sediment deposition geochronology with {sup 2'}1{sup '}0{sup P}B as a tool in the sediment accumulation from the Taquari River, Pantanal, MS, Central region, Brazil; Aplicabilidade da geocronologia da deposicao de sedimento com {sup 2{sup }}1{sup {sup }}0{sup P}B como ferramenta na avaliacao do assoreamento do Rio Taquari, Pantanal, MS

    Energy Technology Data Exchange (ETDEWEB)

    Godoy, J.M. [Instituto de Radioprotecao e Dosimetria (IRD), Rio de Janeiro, RJ (Brazil); Padovani, C.R.; Vieira, L.M. [EMBRAPA, Corumba, MS (Brazil). Centro de Pesquisa Agropecuaria do Pantanal; Pereira, J.C.A. [Pontificia Univ. Catolica do Rio de Janeiro, RJ (Brazil). Dept. de Quimica


    This work presents {sup 210} Pb geochronology of five bottom sediment cores from flood plain lakes of middle Taquari River, Pantanal, MS. In two cores of a same lake, an increase of the sediment deposition rate were observed, reflecting an increasing of the sediment input to the Pantanal. The main causes suggested for this increasing are climatic change, and the development of agriculture and cattle-raising in the watershed highlands, for the last 25 years. The results pointed too for the importance of a additional sample in the superior region of the middle Taquari River. (author)

  17. Geochronology, geochemistry and metallogenic implications of the Lamasu intrusion in western Tianshan, NW China%西天山喇嘛苏岩体年代学、地球化学及成矿意义

    Institute of Scientific and Technical Information of China (English)

    解洪晶; 武广; 朱明田; 晋建平; 钟伟; 刘军; 糜梅


    The zircon U-Pb geochronology , major and trace elements and Sr-Nd isotopes determination of the Lamasu intrusion in western Chinese Tianshan were conducted to understand its petrogenesis and geodynamic setting. The Lamasu intrusion is mainly composed of quartz monzodiorite,granodiorite porphyry and tonalite porphyry. Among them the quartz monzodiorite was formed in 394. 8 + 4. 9 Ma, while the granodiorite porphyry and tonalite porphyry were intruded in 380, 9 + 3. 9 Ma. The rocks display the adakitic affinity and show a calc-alkaline and high-K calc-alkaline differentiation trend. They display REE patterns characterized by relative enrichment of LREE ((La/Yb)N: 3. 55 - 15. 52) with slightly negative or positive Eu anomalies (δEu: 0. 53 - 1. 12). They have high Sr ((322 - 808) × 10-6) and low Y contents ((12. 90 - 18. 86) ×10-6) and also enriched in LILE but depleted in HFSE with negative Nb, Ta and Ti anomalies. Initial Sr-Nd isotopic compositions are εNd(t) = -4. 29 to 0. 75 and Isr = 0. 706052 to 0. 708263. The Nd model ages vary from 1. 03 to 1. 46 Ga. The Pb isotopic ratios of granodiorite porphyry and tonalite porphyry are 206Pb/204Pb=18. 500 -19. 044,207Pb/204 Pb= 15. 575 - 15. 626,208Pb/204 Pb= 38. 443 - 38. 864, while the quartz monzodiorite are 206Pb/204Pb=18. 694 - 18. 711,207Pb/204Pb=15. 622 - 15. 630,208Pb/204 Pb = 38. 648 - 38. 660. All these geochemical data demonstrate that the Lamasu intrusion is likely derived from the partial melting of oceanic crust, with the subsequent melts interacted during their uplift with the overlying mantle wedge metasomatized by the subducted sediments and subordinately contaminated by ancient crust. The Lamasu intrusion was emplaced in a continental arc setting during the southward subduction of the Junggar Remnant Ocean Basin in the Late Devonian and are closely related with Cu (Au) mineralization of that region.%文中主要对西天山喇嘛苏岩体进行SHRIMP锆石U-Pb年龄、主微量及Sr-Nd-Pb同位素

  18. Geology and geochronology of the metamorphic suite Colorado and his rock assemblages, southeastern Rondonia, Brazil: implications for the mesoproterozoic evolution of the southwestern Amazon Craton; Geologia e geocronologia da suite metamorfica Colorado e suas encaixantes, SE de Rondonia: implicacoes para evolucao mesoproterozoica do SW do Craton Amazonico

    Energy Technology Data Exchange (ETDEWEB)

    Rizzotto, Gilmar Jose [Centro de Pesquisas de Recursos Minerais (CPRM), Rio de Janeiro, RJ (Brazil). Servico Geologico do Brasil; Bettencourt, Jorge Silva; Teixeira, Wilson; Basei, Miguel Angelo Stipp [Sao Paulo Univ., SP (Brazil). Inst. de Geociencias. Dept. de Mineralogia e Geotectonica; Pacca, Igor Ivory Gil; D' Agrella Filho, Manoel Sousa [Sao Paulo Univ., SP (Brazil). Inst. de Astronomia, Geofisica e Ciencias Atmosfericas; Vasconcelos, Paulo [Queensland Univ., Brisbane, (Australia); Onoe, Artur Takashi [Sao Paulo Univ., SP (Brazil). Inst. de Geociencias. Centro de Pesquisas Geocronologicas; Passarelli, Claudia Regina [Sao Paulo Univ., SP (Brazil). Inst. de Geociencias. Programa de Pos-graduacao em Geoquimica e Geotectonica


    Combined geochronological (U-Pb, 40 Ar/ 39 Ar, and RB-Sr) geological data help define an important tectonomagmatic event in the Colorado do Oeste and Cabixi regions, southeastern Rondonia, Brazil, and characterize the Colorado Metamorphic Suite (CMS), well represented by rock assemblages which experienced deformation and metamorphic recrystallization (upper-amphibolite facies). The suite is composed of: porphyritic monzogranite associated with amphibolite (bimodal magmatism); interlayered clastic and chemical metasedimentary rocks (sillimanite schists and iron formation); muscovite-garnet leucogranite; and mafic-ultramafic intrusive rocks. The CMS mafic rocks occur as underformed isolated bodies of layered coarse-grained metagabbro, still preserving typical cumulate igneous texture. U-Pb zircon isotopic data for three fractions of sample RO-10 define a discordia with an upper intercept (crystallization) age of 1352 + 4/-3 Ma (MSWD = 0,18). The porphyritic orthogneisses (RO-15; monzogranite and amphibolite) yielded a Rb-Sr whole rock isochron age of 1360 {+-} 45 Ma, and 87 Sr/86 Sr, = 0.7040 {+-} 0.0012 (MSWD = 9.2). One sample of aplite that is subparallel to the main regional foliation has furnished an age 1360 {+-}13 Ma. The 40 Ar/39 Ar data for muscovite from a anatectic leucogranite (RO-14) yielded plateau ages of 1312 {+-} 3 Ma (grain 1), and 1303 {+-} 2 Ma/1305 {+-} 2 Ma (grain 2). Grain 3 presented an heterogeneous isotopic spectrum, the integrated age being 1289 {+-} 2 Ma. Hornblende from two amphibolite samples yielded 40 Ar/39 Ar plateau ages of 1313 {+-} 4 Ma; 1313 {+-} 6 Ma and 1312 {+-} 3 Ma (RO-18), and 1325 {+-} 3 Ma; 1326 {+-} 2 Ma; 1330 {+-} 3 Ma (RO-19). The weighted-mean age 1319 {+-} 10 Ma, and is interpreted as the best estimated age for regional metamorphic cooling. These data suggest that southeastern Rondonia was affected by a tectonomagmatic event at ca. 1.36 - 1.32 Ga, predating the evolution of the Nova Brasilandia Terrane (1215 - 1110 Ma

  19. Geology, petrology and U-Pb geochronology of Serra da Rajada Granitic Pluton: implications about ediacaran magmatic evolution in NE portion of the Rio Piranhas-Serido Domain (NE of Borborema Province); Geologia, petrologia e geocronologia U-Pb do Pluton Granitico Serra da Rajada: implicacoes sobre a evolucao magmatica ediacarana na porcao do Dominio Rio Piranhas-Serido (NE da Provincia Borborema)

    Energy Technology Data Exchange (ETDEWEB)

    Costa, Alan Pereira da; Dantas, Alexandre Ranier, E-mail:, E-mail: [Servico Geologico do Brasil (CPRM), Natal, RN (Brazil). Nucleo de Apoio de Natal/Superintendencia Regional de Recife; Nascimento, Marcos Antonio Leite do; Galindo, Antonio Carlos, E-mail:, E-mail: [Universidade Federal do Rio Grande do Norte (UFRN), Natal, RN (Brazil). Departamento de Geologia


    The Ediacaran plutonic activity, related to the Brazilian/Pan-African orogeny, is one of the most important geological features in the Borborema Province, formed by batholiths, stocks and dikes. The Serra da Rajada Granitic Pluton (SRGP), located in the central portion of the Rio Piranhas-Serido Domain, is an example of these bodies. This site is the target of cartographic, petrographic, lithochemical and geochronological studies. Its rocks are described as monzogranites consisting of K-feldspar, plagioclase (oligoclase-An{sub 23-24%}), quartz and biotite (main mafic), having as accessory minerals opaque, titanite, allanite, apatite and zircon. Chlorite, white mica and carbonate are alteration minerals. Lithochemical data from 15 samples show quite evolved rocks (SiO{sub 2} , 69% to 75%), rich in alkalis (Na{sub 2}O + K{sub 2}O ≥ 8.0%), depleted in MgO (≤ 0.45%), CaO (≤ 1.42%) and TiO{sub 2} (≤ 0.36%), and displaying moderate levels of Fe{sub 2}O{sub 3} t (2.16 to 3.53%). These rocks present a transitional nature between metaluminous and peraluminous (predominance of the latter) and have subalkaline/monzonitic affinity (high-K Calc-alkaline). Harker diagrams represent negative correlations in Fe{sub 2}O{sub 3}t, MgO and CaO, indicating fractionation of mafic and plagioclase. The REE spectrum show enrichment of light in relation to heavy REE (LaN/YbN = 23.70 to 10.13), with negative anomaly in Eu (Eu/ Eu* = 0.70 to 10.13) suggesting fractionation or accumulation in the feldspars source (plagioclase). Data integration allows correlating the SRGP rocks with those described as Equigranular high-K Calc-alkaline Suite. The U-Pb geochronology and Sm-Nd isotope dating indicate that the biotite monzogranite have a crystallization age of 557 ± 13 Ma and TDM model age of 2.36 Ga, respectively, and ε{sub Nd} value of - 20.10 for the crystallization age, allowing to infer a crustal source for the magma generated in the Paleoproterozoic age. (author)

  20. Refinement of the time-space evolution of the giant Mio-Pliocene Río Blanco-Los Bronces porphyry Cu-Mo cluster, Central Chile: new U-Pb (SHRIMP II) and Re-Os geochronology and 40Ar/39Ar thermochronology data (United States)

    Deckart, Katja; Clark, Alan H.; Cuadra, Patricio; Fanning, Mark


    Representing one of the largest known (estimated >5 Gt at 1 % Cu and 0.02 % Mo) porphyry system, the Río Blanco-Los Bronces deposit incorporates at least five hypabyssal intrusive and hydrothermal centres, extending for about 5 km from the Río Blanco and Los Bronces mines in the north, through the Don Luis mine, to the Sur Sur mine, La Americana and Los Sulfatos in the south. The new geochronology data, which now include data on different molybdenite vein types, confirm the U-Pb ages of the pre-mineralisation intrusions but slightly increase their age range from 8.8 to 8.2 Ma. The distinct magmatic pulses of the mineralisation-associated porphyritic intrusives (Late Porphyries) indicate an age interval instead of the previously suggested individual ages: the quartz monzonite porphyry ranges from 7.7 to 6.1 Ma (Sur Sur 5.74 ± 0.13 Ma), the feldspar porphyry shows an interval from 5.8 to 5.2 Ma and the Don Luis porphyry from 5.2 to 5.0 Ma. The new Re-Os data on distinct molybdenite vein types confirm the protracted history of Cu(-Mo) mineralisation, inferred previously. The vein development occurred at least from 5.94 to 4.50 Ma, indicating a time-span of 1.5 Ma for the hydrothermal activity. Hydrothermal minerals dated by the 40Ar/39Ar method are generally too young to record the age of early, high-temperature mineralisation. The majority of the 40Ar/39Ar data in the Río Blanco porphyry cluster record reheating by either the youngest member of the Late Porphyry suite or the post-mineralisation dacite or rhyolite plug formations at around 4.9-4.7 Ma.

  1. Geological evolution of the center-southern portion of the Guyana shield based on the geochemical, geochronological and isotopic studies of paleoproterozoic granitoids from southeastern Roraima, Brazil; Evolucao geologica da porcao centro-sul do escudo das Guianas com base no estudo geoquimico, geocronologico e isotopico dos granitoides paleoproterozoicos do sudeste de Roraima, Brasil

    Energy Technology Data Exchange (ETDEWEB)

    Almeida, Marcelo Esteves


    This study focuses the granitoids of center-southern portion of Guyana Shield, southeastern Roraima, Brazil. The region is characterized by two tectonic-stratigraphic domains, named as Central Guyana (GCD) and Uatuma-Anaua (UAD) and located probably in the limits of geochronological provinces (e.g. Ventuari-Tapajos or Tapajos-Parima, Central Amazonian and Maroni-Itacaiunas or Transamazon). The aim this doctoral thesis is to provide new petrological and lithostratigraphic constraints on the granitoid rocks and contribute to a better understanding of the origin and geo dynamic evolution of Guyana Shield. The GCD is only locally studied near to the UAD boundary, and new geological data and two single zircon Pb-evaporation ages in mylonitic biotite granodiorite (1.89 Ga) and foliated hastingsite-biotite granite (1.72 Ga) are presented. These ages of the protholiths contrast with the lithostratigraphic picture in the other areas of Cd (1.96-1.93 Ga). Regional mapping, petrography, geochemistry and zircon geochronology carried out in the Urad have showed widespread Paleoproterozoic calc-alkaline granitic magmatism. These granitoid rocks are distributed into several magmatic associations with different Paleoproterozoic (1.97-1.89 Ga) ages, structural and geochemical affinities. Detailed mapping, petrographic and geochronological studies have distinguished two main sub domains in the UAD. In the northern UAD, the high-K calc-alkaline Martins Pereira (1.97 Ga) and Serra Dourada S-type granites (1.96 Ga) are affected by NE-SW and E-W ductile dextral shear-zones, showing coexistence of magmatic and deformational fabrics related to heterogeneous deformation. Inliers of basement (2.03 Ga) crop out northeast of this area, and are formed by meta volcano-sedimentary sequence (Cauarane Group) and TTG-like calc-alkaline association (Anaua Complex). Xenoliths of meta diorites (Anaua Complex) and para gneisses (Cauarane Group) reinforce the intrusive character of Martins Pereira

  2. Geological evolution of the center-southern portion of the Guyana shield based on the geochemical, geochronological and isotopic studies of paleoproterozoic granitoids from southeastern Roraima, Brazil; Evolucao geologica da porcao centro-sul do escudo das Guianas com base no estudo geoquimico, geocronologico e isotopico dos granitoides paleoproterozoicos do sudeste de Roraima, Brasil

    Energy Technology Data Exchange (ETDEWEB)

    Almeida, Marcelo Esteves


    This study focuses the granitoids of center-southern portion of Guyana Shield, southeastern Roraima, Brazil. The region is characterized by two tectonic-stratigraphic domains, named as Central Guyana (GCD) and Uatuma-Anaua (UAD) and located probably in the limits of geochronological provinces (e.g. Ventuari-Tapajos or Tapajos-Parima, Central Amazonian and Maroni-Itacaiunas or Transamazon). The aim this doctoral thesis is to provide new petrological and lithostratigraphic constraints on the granitoid rocks and contribute to a better understanding of the origin and geo dynamic evolution of Guyana Shield. The GCD is only locally studied near to the UAD boundary, and new geological data and two single zircon Pb-evaporation ages in mylonitic biotite granodiorite (1.89 Ga) and foliated hastingsite-biotite granite (1.72 Ga) are presented. These ages of the protholiths contrast with the lithostratigraphic picture in the other areas of Cd (1.96-1.93 Ga). Regional mapping, petrography, geochemistry and zircon geochronology carried out in the Urad have showed widespread Paleoproterozoic calc-alkaline granitic magmatism. These granitoid rocks are distributed into several magmatic associations with different Paleoproterozoic (1.97-1.89 Ga) ages, structural and geochemical affinities. Detailed mapping, petrographic and geochronological studies have distinguished two main sub domains in the UAD. In the northern UAD, the high-K calc-alkaline Martins Pereira (1.97 Ga) and Serra Dourada S-type granites (1.96 Ga) are affected by NE-SW and E-W ductile dextral shear-zones, showing coexistence of magmatic and deformational fabrics related to heterogeneous deformation. Inliers of basement (2.03 Ga) crop out northeast of this area, and are formed by meta volcano-sedimentary sequence (Cauarane Group) and TTG-like calc-alkaline association (Anaua Complex). Xenoliths of meta diorites (Anaua Complex) and para gneisses (Cauarane Group) reinforce the intrusive character of Martins Pereira

  3. 浙江临安夏色岭钨矿含矿岩体特征及LA-ICP-MS锆石铀-铅年代学研究%Characteristics and LA-ICP-MS Zircon U-Pb Geochronology Study of Metallogenic Intrusion in the Xiaseling Tungsten Deposit in Lin'an,Zhejiang Province

    Institute of Scientific and Technical Information of China (English)

    黄国成; 王登红; 吴小勇


    夏色岭钨矿是浙江省内已知规模最大的石英脉型钨矿,其形成与夏色岭花岗岩体关系密切,前人对该岩体的年代学研究方法比较局限.本研究采集了夏色岭钨矿区的新鲜花岗岩样品,从岩石的矿物学特征及地球化学特征出发,讨论了岩体与成矿作用之间的关系.通过分析精度更高、结果更可靠的激光剥蚀-电感耦合等离子体质谱( LA - ICP - MS)锆石U- Pb测年技术,获得206Pb/238 U加权平均年龄为( 126.9±1.7)Ma,表明为燕山晚期早阶段(早白垩世)的产物,结合区域上的找矿成果,对本地区的地质找矿具有一定指导意义.%The Xiaseling tungsten deposit is the largest discovered quartz-veined tungsten deposit in Zhejiang Province, which is closely related to Xiaseling Granite. The geochronological study on this granite has been very limited in previous studies. Fresh granite samples were collected from the Xiaseling tungsten deposit. The relationship between Xiaseling Granite and mineralization is discussed by mineralogy and geochemical characteristics of the rocks. The in-situ zircon U-Pb dating was performed with Laser Ablation-Multicollector Inductively Coupled Plasma-Mass Spectrometry ( LA-MC-ICPMS) with higher accuracy and greater reliability than previously achieved. Our result indicates that the weighted average 206Pb/238U ages for zircons from the Xiaseling granite is ( 126. 9 ?.7) Ma, suggesting the granite was intruded in the early stage of the late Yenshanian Period of the early Cretaceous era. Together with regional prospecting findings, it is also important for ore exploration work in this area.

  4. Depositional Age of the Pake Formation of Ximeng Group and Its Tectonic Implications:Constraints from Zircon U-Pb Geochronology and Lu-Hf Isotopes%滇西南西盟群帕可组沉积时代厘定及构造意义:锆石U-P b年代学及Lu-Hf同位素证据

    Institute of Scientific and Technical Information of China (English)

    邢晓婉; 张玉芝


    研究滇西南地区出露的西盟群碎屑岩沉积时限及其古地理属性对了解冈瓦纳大陆东缘的构造演化及其古地理重建具有重要意义。本文对西盟群帕可组3个变质砂岩样品中的碎屑锆石进行了 LA-ICP-MS U-Pb测年和原位 Lu-Hf同位素组成分析,获得样品的最小年龄峰值分别为568Ma、541Ma和560Ma,εHf(t)值为-24.0~26.3。结合前人对帕可组内片麻状花岗岩的研究成果,推测西盟群帕可组沉积于早寒武世—中奥陶世(541~460Ma),而非以往所认为的前寒武纪地层。Lu-Hf同位素组成则表明西盟群帕可组物源为古老地壳物质的再循环并混入新生地壳组分,结合其他区域地质资料推断西盟群物源类似东印度板块,与东冈瓦纳大陆具有亲缘性。%Depositional age,provenance and paleogeography characteristics of the Ximeng metasedimentary sequence is key for better understanding the tectonic evolution and paleogeographic reconstruction of East Gondwana Continent.This paper presented the zircon U-Pb geochronology and Lu-Hf isotope for three metamorphic sandstone samples from the Pake Formation of Ximeng Group in SW Yunnan.The group has been traditionally considered as the Precambrian metamorphic basement of the Shan-Thai Block.The detrital zircons from three samples yielded the youngest age peaks at 568Ma, 541Ma and 560Ma,respectively,and the εHf(t)values change from -24.0 to +26.3.In combination with other data,it is inferred that the Pake Formation deposited at 541-460Ma,equivalent to Late Cambrian to early-middle Ordovician pe-riod,different from previous proposed Precambrian strata.Their εHf(t)values indicate that their source rocks might origi-nate from the mixing of recycled ancient continental material with juvenile crustal component.Our results,along with other geological data,suggest that the source rocks of the Ximeng Group have an affinity of Tethys-Himalaya and from

  5. Genesis of the Au-Bi-Cu-As, Cu-Mo ± W, and base-metal Au-Ag mineralization at the Mountain Freegold (Yukon, Canada): constraints from Ar-Ar and Re-Os geochronology and Pb and stable isotope compositions (United States)

    Bineli Betsi, Thierry; Lentz, David; Chiaradia, Massimo; Kyser, Kurt; Creaser, Robert A.


    The genesis of mineralized systems across the Mountain Freegold area, in the Dawson Range Cu-Au ± Mo Belt of the Tintina Au province was constrained using Pb and stable isotope compositions and Ar-Ar and Re-Os geochronology. Pb isotope compositions of sulfides span a wide compositional range (206Pb/204Pb, 18.669-19.861; 208Pb/204Pb, 38.400-39.238) that overlaps the compositions of the spatially associated igneous rocks, thus indicating a magmatic origin for Pb and probably the other metals. Sulfur isotopic compositions of sulfide minerals are broadly similar and their δ34S (Vienna-Canyon Diablo Troilite (V-CDT)) values range from -1.4 to 3.6 ‰ consistent with the magmatic range, with the exception of stibnite from a Au-Sb-quartz vein, which has δ34S values between -8.1 and -3.1 ‰. The δ34S values of sulfates coexisting with sulfide are between 11.2 and 14.2 ‰; whereas, those from the weathering zone range from 3.7 to 4.3 ‰, indicating supergene sulfates derived from oxidation of hypogene sulfides. The δ13C (Vienna Peedee Belemnite (VPDB)) values of carbonate range from -4.9 to 1.1 ‰ and are higher than magmatic values. The δ18O (V-SMOW) values of magmatic quartz phenocrysts and magmatic least-altered rocks vary between 6.2 and 10.1 ‰ and between 5.0 and 10.1 ‰, respectively, whereas altered magmatic rocks and hydrothermal minerals (quartz and magnetite) are relatively 18O-depleted (4.2 to 7.9 ‰ and -6.3 to 1.5 ‰, respectively). Hydrogen isotope compositions of both least-altered and altered igneous rock samples are D-depleted (from -133 to -161 ‰ Vienna-Standard Mean Ocean Water (V-SMOW)), consistent with differential magma degassing and/or post-crystallization exchange between the rocks and meteoric ground water. Zircon from a chlorite-altered dike has a U-Pb crystallization age of 108.7 ± 0.4 Ma; whereas, the same sample yielded a whole-rock Ar-Ar plateau age of 76.25 ± 0.53 Ma. Likewise, molybdenite Re-Os model ages range from 75.8 to

  6. Conflicting Lu-Hf and Sm-Nd ages in the garnet gneisses of the Central Appalachians, Eastern U.S.: Implications for the timing and duration of Grenville Orogeny and a case for using coupled Lu-Hf and Sm-Nd geochronology (United States)

    Vervoort, J. D.; Ramsey, M.; Aleinikoff, J. N.; Southworth, S.


    The Grenville orogeny is one of the most significant geological events in Earth's history with remnants of this event prominent on virtually every continent. Constraining its timing and duration is important not only for understanding the tectonics of the Grenville itself, but also for understanding supercontinent cycles and other questions of Earth's evolution. In order to provide better constraints on the timing of Grenvillian metamorphism, we analyzed garnet-bearing Mesoproterozoic ortho- and paragneisses, collected along a 150 km transect in the northern Blue Ridge Province, using combined Lu-Hf and Sm-Nd geochronology. The orthogneisses have U-Pb zircon crystallization ages of ~1140 and 1100 Ma. The paragneisses have maximum depositional ages ~1050-1020 Ma, based on the youngest detrital zircon populations. Zircon overgrowths and monazite suggest metamorphic events between ~1050 and 960 Ma. The Lu-Hf and Sm-Nd data for these samples both yield robust garnet ages with large spread of parent/daughter ratios, low age uncertainties, and low MSWD values. Lu-Hf ages define a narrow time span (1043×12 Ma to 1016×4 Ma; wtd. mean, 1024×7 Ma, 2σ). The Sm-Nd ages, determined on the same solutions as Lu-Hf, also define a narrow time range but are systematically younger (974×11 Ma to 932×5 Ma; wtd. mean, 957×10 Ma). The average difference between Lu-Hf and Sm-Nd ages is 67 Ma; the oldest Sm-Nd age is 40 Ma younger than the youngest Lu-Hf age. These large systematic differences in the ages are enigmatic. While Sm-Nd ages younger than Lu-Hf are not uncommon, these differences are typically small; nothing of this magnitude has been previously reported. There are, however, potential explanations for these differences. (1) Lu partitions strongly into garnet during growth yielding ages weighted toward the beginning of growth (e.g., Skora, 2006); no similar partitioning exists in Sm-Nd and these ages reflect mean garnet growth. (2) Lu diffuses much faster than Hf at

  7. Geochemistry, geochronology, mineralogy, and geology suggest sources of and controls on mineral systems in the southern Toquima Range, Nye County, Nevada; with geochemistry maps of gold, silver, mercury, arsenic, antimony, zinc, copper, lead, molybdenum, bismuth, iron, titanium, vanadium, cobalt, beryllium, boron, fluorine, and sulfur; and with a section on lead associations, mineralogy and paragenesis, and isotopes (United States)

    Shawe, Daniel R.; Hoffman, James D.; Doe, Bruce R.; Foord, Eugene E.; Stein, Holly J.; Ayuso, Robert A.


    distribution patterns that suggest specific sources and lithologic influences on deposition, as well as multiple episodes of mineralization. Principal episodes of mineralization are Late Cretaceous (molybdenum and tungsten in and near granite; silver at Belmont and Silver Point mines), early Oligocene [tourmaline and base- and precious-metals around the granodiorite of Dry Canyon stock as well as at Manhattan(?)], late Oligocene (gold at Round Mountain and Jefferson), and Miocene (gold at Manhattan). Most likely principal sources of molybdenum, tungsten, silver, and bismuth are Cretaceous granites; of antimony, arsenic, and mercury are intermediate-composition early Oligocene intrusives; and of gold are early and late Oligocene and early Miocene magmas of the volcanic cycle. Lead may have been derived principally from Cretaceous granitic magma and Paleozoic sedimentary rocks. Several areas prospective for undiscovered mineral deposits are suggested by spatial patterns of element distributions related to geologic features. The Manhattan district in the vicinity of the White Caps mine may be underlain by a copper-molybdenum porphyry system related to a buried stock; peripheral high-grade gold veins and skarn deposits may be present below deposits previously mined. The Jefferson district also may be underlain by a copper-molybdenum porphyry system related to a buried stock, it too with peripheral high-grade gold deposits. The Bald Mountain Canyon belt of small gold veins has potential for deeper deposits in buried porous ash-flow tuff similar to the huge Round Mountain low-grade gold-silver deposit. Several other areas have potential for a variety of mineral deposits. Altogether the geochemical, geochronologic, mineralogic, and geologic evidence suggests recurring mineralizing episodes of varied character, from Late Cretaceous to late Tertiary time, related to a long-lived hot spot deep in the crust or in the upper mantle. Granite plutons of Late Cretaceous age were minerali

  8. 藏北羌塘中部果干加年山石炭纪蛇绿岩地球化学特征及LA-ICP-MS锆石U-Pb年龄%Geochemical characteristics and LA-ICP-MS zircon U-Pb geochronology of Guogangjianan Carboniferous ophiolite in central Qiangtang, the Tibetan Plateau

    Institute of Scientific and Technical Information of China (English)

    吴彦旺; 李才; 徐梦婧; 解超明; 王明; 熊盛青; 范正国


    In order to investigate the evolution of the Lungmu Co-Shuanghu plate suture zone and the tectonic history of central Qiangtang, the authors studied Guoganjianian ophiolite in the aspects of petrology, geochemistry and geochronology. The cumulate in Guoganjianian ophiolite was recently discovered during 1∶50000 regional geological survey of the Gemuri area. Geochemical studies show that the cumulate gabbro has low REE content and flatten REE patterns with slight enrichment of LREE, and the degree of differentiation between LREE and HREE is low. These samples are enriched in large ion lithophile elements (LILE) such as Rb and Ba, and depleted in Nb, Ta and Ti, suggesting that they were probably formed in a subduction zone. LA-ICP-MS technique was applied to U-Pb zircon dating of the cumulate gabbro from Guoganjianian ophiolite. The result shows that the ages of cumulate gabbro is 354.8Ma±2.4Ma, suggesting that Lungmu Co-Shuanghu Ocean already began its subduction in Early Carboniferous. The study of Guoganjianian cumulate gabbro provides important basic data for investigating the evolution of Lungmu Co-Shuanghu plate suture zone and the tectonic history of central Qiangtang.%在羌塘地区开展1∶5万戈木日区域地质调查的过程中,为研究龙木错-双湖板块缝合带的演化历史,探讨青藏高原羌塘中部地区的构造演化,对果干加年山地区的蛇绿岩进行了详细研究。石炭纪蛇绿岩变质堆晶辉长岩是填图过程中的新发现。全岩地球化学研究结果显示,岩石ΣREE较低,LREE弱富集,轻、重稀土元素分异程度不高;富集Rb、Bb等大离子亲石元素,明显亏损Nb、Ta、Ti等元素,具有典型的SSZ型蛇绿岩特征,表明其形成于俯冲带之上。蛇绿岩中变质堆晶辉长岩的LA-ICP-MS锆石U-Pb年龄为354.8Ma±2.4Ma,属于早石炭世。最终研究表明,龙木错-双湖板块缝合带所代表的古大洋至少在早石炭世就已经存在俯冲

  9. 新疆东天山觉罗塔格地区自然铜矿化玄武岩的成岩年代及其地质意义%Geochronology and geological indication of the native copper mineralized basalt formation in Jueluotage area, Eastern Tianshan, Xinjiang

    Institute of Scientific and Technical Information of China (English)

    张达玉; 周涛发; 袁峰; 刘帅; LU YongJun; 许超; 宁福泉


    新疆觉罗塔格地区自然铜矿化玄武岩沿着阿齐库都克深大断裂北侧分布,前人对其成岩年代一直存在争议.本文测得玄武岩条带中部的黑尖山玄武岩同岩浆期锆石SHRIMP U-Pb年龄为306.2±4.0Ma;测得东部东尖峰玄武岩的同岩浆期锆石LA-ICPMS U-Pb年龄为308.9±5.8Ma,确定了玄武岩是晚石炭世的岩浆作用,且归属于晚石炭世马头滩组.与觉罗塔格地区著名的晚石炭世-早二叠世铜镍矿化基性-超基性岩成因对比表明,该晚石炭世玄武岩与之具有共同岩浆源区特征,它们都是觉罗塔格地区晚石炭世-早二叠世岩石圈地幔深部受热熔融而形成岩浆作用.马头滩组玄武岩层是自然铜矿化的重要矿源层.%The native copper mineralized basalt ( NCMB) distributes north side along with the Aqikuduke deep fault, Jueluotage area, Eastern Tianshan, Xinjiang. As the basaltic magmatism erupted epoch hasn't been acknowledged yet, this study focuses on the geology and geochronology of the NCMB strata. The authors got the in-situ SHRIMP U-Pb age of basalts sampled from Heijianshan Cu-Fe mineralized occurrence (middle study area) is 306.2 ±4. OMa; and the in-situ La-ICPMS U-Pb age of basalt sampled from Dongjianfeng native copper mineralized occurrence (east study area) is 308. 9 ±5. 8Ma. These two confident and coherent ages can be used for confirming that the basalt bearing in Lower Matoutan Formation is the product of Late Carboniferous magmatism. After compared geochemical characters and genesis with the C2-P1 Cu-Ni sulfide mineralized mafic-ultramafic intrusions in Jueluotage area, It shows that the NCMB shared the same magmatic source with the younger one, and both NCMB and mafic-ultramafic intrusions were the products of the mafic magma, which were derived from the same vulnerable lithosphere mantle through some geodynamic heat abnormalities in Jueluotage area. The basaltic stratum is an important metal source of the Cu mineralized

  10. Continued Development of in Situ Geochronology for Planetary Missions (United States)

    Devismes, D.; Cohen, B. A.


    The instrument 'Potassium (K) Argon Laser Experiment' (KArLE) is developed and designed for in situ absolute dating of rocks on planetary surfaces. It is based on the K-Ar dating method and uses the Laser Induced Breakdown Spectroscopy - Laser Ablation - Quadrupole Mass Spectrometry (LIBSLA- QMS) technique. We use a dedicated interface to combine two instruments similar to SAM of Mars Science Laboratory (for the QMS) and ChemCam (for the LA and LIBS). The prototype has demonstrated that KArLE is a suitable and promising instrument for in situ absolute dating.

  11. Paragenesis and Geochronology of the Nopal I Uranium Deposit, Mexico

    Energy Technology Data Exchange (ETDEWEB)

    M. Fayek; M. Ren


    Uranium deposits can, by analogy, provide important information on the long-term performance of radioactive waste forms and radioactive waste repositories. Their complex mineralogy and variable elemental and isotopic compositions can provide important information, provided that analyses are obtained on the scale of several micrometers. Here, we present a structural model of the Nopal I deposit as well as petrography at the nanoscale coupled with preliminary U-Th-Pb ages and O isotopic compositions of uranium-rich minerals obtained by Secondary Ion Mass Spectrometry (SIMS). This multi-technique approach promises to provide ''natural system'' data on the corrosion rate of uraninite, the natural analogue of spent nuclear fuel.

  12. "Cryptic" diagenesis and its implications for speleothem geochronologies (United States)

    Bajo, Petra; Hellstrom, John; Frisia, Silvia; Drysdale, Russell; Black, Jay; Woodhead, Jon; Borsato, Andrea; Zanchetta, Giovanni; Wallace, Malcolm W.; Regattieri, Eleonora; Haese, Ralf


    Speleothems are usually considered as one of the most amenable palaeoclimate archives for U-series dating. A number of studies in recent years, however, report cases of diagenetic alteration which compromises the use of U-series systematics in speleothems, resulting in inaccurate U-Th ages. Here we present the results of a high-resolution U-Th dating study of a stalagmite (CC26) from Corchia Cave in Italy where we document a number of departures from an otherwise well-defined age-depth model, and explore potential causes for these outliers. Unlike examples illustrated in previous studies, CC26 contains no visible evidence of neomorphism, and appears, at least superficially, ideally suited to dating. Good reproducibility obtained between multi-aliquot U-Th analyses removes any possibility of analytical issues contributing to these outliers. Furthermore, replicate analyses of samples from the same stratigraphic layer yielded ages in stratigraphic sequence, implying very localized open-system behavior. Uranium loss is suggested as a causative mechanism on account of the fact that all the outliers are older than their assumed true age. A limited number of micro-voids were observed under micro-CT analyses, and it is proposed that these were pathways for U loss. Uranium-loss modelling allows us to constrain the possible timing of diagenetic alteration and indicates that the precursor for the outlier with the largest age discrepancy (309%) must have been aragonite. This study indicates that visibly unaltered speleothems may still contain small domains that have experienced post-depositional alteration. Such "cryptic" diagenesis, as recorded in this stalagmite, has implications for the constancy of accuracy of the U-series dating technique, and suggests a need for careful examination of speleothems prior to dating, particularly in low-resolution U-Th studies.

  13. Geochronological review of Sambagawa metamorphic belt in Southwest Japan

    Institute of Scientific and Technical Information of China (English)


    Based on almost all available published age data, the protolith ages, peak metamorphic ages and cooling rate of the Sambagawa metamorphic belt have been discussed and the latest constraints on the ages of the Sambagawa metamorphism and subduction-related accretionary evolutions were summarized. Peak metamorphic conditions attained within the Kuma nappe complex at ca. 145~185 Ma, and uplift through ca. 500℃ at ca. 150 Ma and 350~400℃ at ca. 110~ 115 Ma. The protolith sediments of the Besshi nappe complex were accumulated and subsequently progressively subducted and suffered high P-T prograde metamorphism during the Kuma nappe complex uplifting. The Besshi nappe complex arrived maximum metamorphic conditions at ca. 110 ~ 120 Ma and subsequently started rapid uplift with the cooling rate of ca. 14.2℃/Ma at ca. 75 ~85 Ma, followed with the cooling rate of ca. 6.0 ~8.9℃/Ma. The Oboke nappe complex started subduction later than other tectonic units and arrived the peak metamorphic conditions at ca. 75 Ma, which followed by the uplift with a cooling rate of ca. 8℃/Ma.

  14. New geochronological constraints of the Lassen segment's regional volcanism (United States)

    Germa, A.; Connor, C.; Connor, L.; Malservisi, R.; Tavarez, S.; Charbonnier, S. J.; Clynne, M. A.; Perry, C.; Quidelleur, X.; Ricci, J.


    The Lassen region is the southernmost active volcanic field in the Cascade Range. Since ~3.5 Ma distributed mafic to intermediate calc-alkaline magmas continuously built hundreds of cinder cones, lava flows, and a few small shield volcanoes. A set of 10 new unspiked K-Ar ages obtained on groundmass separates help reveal the timing of this regional volcanism. Although most lavas show a high atmospheric contamination level and a low K content, significant ages were obtained with this technique. Shields were dated at about 2.5 Ma (Clynne and Muffler, 2010) and are commonly dissected by erosion. However, we obtained a K-Ar age of 374 ± 25 ka for the late lava flow at the summit of Crater Mountain, which is less eroded than the other shields. This indicates that this regional volcano's activity overlapped that of the Lassen Volcanic Center (LVC), and raises questions regarding the ages of other shields. The Caribou Volcanic field (CVF), at the eastern boundary of the Lassen region, was also contemporaneous with the LVC. The field's activity initiated ~425 ka ago, but does not appear to have extended into the Holocene (Clynne and Muffler, 2010). We obtained a K-Ar age of 296 ± 13 ka on the basaltic andesite of Eleanor Lake, extending further the activity of the Caribou sequence. Our K-Ar age of 59 ± 3 ka on a basaltic flow confirms that the Bidwell Spring Chain was active between 20 and 70 ka as previously shown. We dated the basaltic andesite tuya of Turnaround Lake at 4 ± 5 ka. However, this conflicts with the timing of Tuya chain (15-18 ka). Finally, to the south-west of LVC, basalts of Cold Creek Butte yield an age of 207 ± 26 ka, and we dated a basaltic flow from Inskip Hill at ~5 ka. The new ages obtained, especially the Holocene ones, are consistent with magnetotelluric soundings and gravity data that show mid-crustal anomalies in the back-arc (Tavarez, 2015). These data demonstrate that the CVF is still active and has the potential for future eruptions.

  15. Bedrock Geologic Map of Vermont - Geochronology Sample Locations (United States)

    Vermont Center for Geographic Information — The bedrock geology was last mapped at a statewide scale 50 years ago at a scale of 1:250,000 (Doll and others, 1961). The 1961 map was compiled from 1:62,500-scale...

  16. Landforms of Romania – The system of geochronologic evolution

    Directory of Open Access Journals (Sweden)



    Full Text Available The corroborationof geomorphological, tectonic and paleoclimatic data led to a geomorphological system with two stages and subdivis ions. The pre-Carpathian stage (Proterozoic-Lower Mesozoic, belonging to Vorland is characterized by: completing the platform units (E, S and the Paleozoic rigid geotectonic system of Northen Dobruja; levelling stages resulting in pe diplains which were subsequently fragmented and fossilized almost entirely (Central and Northen Dobruja sectors. The Carpathian stage is defined by: completing the Carpathian orogenic system (between Jurassic and Quaternary in labile edge sectors of the Eastern European plate in which the evolution of some regional rifts (in Jurassic – Cret aceous and the moving of some rigid blocks (Moessic, Pannonian and Transylvanian created sequ entially tectonic basins where the accumulated materials were folded, faulted and overt hrusted, or mountain systems resulted by lifting (the upper Cretaceous and Miocene; creating tectonic depressions filled with sedimentary formations; reactivating some deep fractures accomp anied by volcanic eruption phases and creating a specific relief; the tectonic movements in the upper Pliocene – Quaternary raised differently all geographic units representing the c urrent orostructural system by Carpathians’s emersion and join to the adjacent regions; the clim ate change from savanna (Paleogene to subtropical (Mio – Pliocene, then temperate and gl acial (late Pleistocene and so on, led to a succession of morphogenetic systems that have gener ated landforms among which the most representative were preserved mainly as steps, erosi on surfaces areas and levels, or terraces from many morphogenetic phases.

  17. Geochronology as a Framework for Planetary History through 2050 (United States)

    Cohen, Barbara; Arevalo, R., Jr.; Bottke, W. F., Jr.; Conrad, P. G.; Farley, K. A.; Fassett, C. I.; Jolliff, B. L.; Lawrence, S. J.; Mahaffy, Paul; Malespin, C.; hide


    Invest this decade in in situ instruments(including sample selection and handling can we choose using VR?) to TRL 6; put them on flight missions in the 2020s and 2030s to relevant destinations where in situ precision can provide meaningful constraints on geologic history.

  18. The Amazcala caldera, Queretaro, Mexico. Geology and geochronology (United States)

    Aguirre-Díaz, Gerardo J.; López-Martínez, Margarita


    The Amazcala caldera is located 30 km NE of Querétaro City, near Amazcala, state of Querétaro. This caldera is the northernmost caldera within the central sector of the Mexican Volcanic Belt (MVB). It has a 11×14 km 2 elliptical shape, and was formed 7.3-6.6 Ma ago. All caldera products are rhyolites. The oldest caldera unit, the Ezequiel Montes pumice (EMP), is a widespread pumice fallout emplaced around 7.3 Ma ago. An isopach map of the EMP shows two dispersal axes, oriented to the SW and SE with respect to the caldera. The EMP is 5 m thick 40 km to the SW and 35 km to the SE of the caldera. An isopleth map of the EMP shows that pumice fragments increase in size toward the caldera, from 1 cm at 40 km to 25 cm near the caldera rim. The EMP is a regional stratigraphic marker. The Colón ignimbrite, dated at 7.3±0.5 Ma, is stratigraphically above the EMP. It consists of several ash-flow units interbedded with minor pumice fall lapilli and ash, with a minimum thickness of about 80 m at Colón. The caldera rim is occupied by several rhyolite lava domes and flows, some of which extend 10 km from the rim. These domes contain parts of fresh, aphyric obsidian. The last caldera event is an intracaldera rhyolitic dome near the NE rim at about 6.6 Ma. The dome is 4×2 km 2 and is elongated in the NE direction. The Amazcala caldera is 480 km from the Middle America Trench and represents the farthest inland caldera in the central sector of the MVB. Its age of 7.3-6.6 Ma indicates that it is the oldest caldera of the MVB so far reported. This confirms the general view that the volcanic activity of the MVB initiated at its northern margin, and then migrated southward in time.

  19. Structural study and geochronology in the Hook Batholith, Central Zambia (United States)

    Naydenov, K.; Lehmann, J.; Saalmann, K.; Milani, L.; Kinnaird, J.; Charlesworth, G.; Frei, D.


    The Pan-African Hook batholith is emplaced N of the Mwembeshi dislocation, a regional scale structure at the contact between Zambezi Belt and Lufilian Arc in Central Zambia. Exposed over 12000 km2 the batholith is composed mainly of fine-grained and coarse-grained porphyritic granites and leucogranites affected by solid-state deformation along high-strain zones. Two main zones of deformation were investigated - the Itezhi-Tezhi Zone (ITZ) in the SW part of the batholith and the Nalusanga Zone (NZ) to the NE. The 2.5 km wide, N-S trending, subvertical ITZ is a medium-grade, pure shear dominated structure, reflecting probably regional scale E-W shortening. In the central part of the zone, augen-gneiss textures developed. Mineral lineations plunging ~40° S are recorded occasionally. The deformed feldspar porphyroclasts show symmetrical tails and rarely sinistral stair-stepping. In the SE part of the Hook batholith the continuation of the ITZ trends E-W. This orientation can be explained by rotation of the original ITZ trend by N-S shortening that also has been recorded in the siliciclastic metasediments S of the contact. S dipping, up to 15 cm wide thrust zones observed in the ITZ area were probably formed during this tectonic event. The 3 km wide NZ is a subvertical to steeply SSW dipping structure, parallel to the NE contact of the batholith, with well-developed foliation and mineral stretching lineations. Field and microstructural analyses defined the NZ as a medium-grade, non-coaxial, sinistral strike-slip shear zone. The transition from weak foliated granite to S-C mylonites and ultramylonites was observed. The sinistral shearing is consistent with E-W shortening in agreement with the tectonic framework of the ITZ. The low grade metasediments to the E of the granite are folded in N to NNW trending structures also implying E-W shortening. Temperature conditions during the deformation in ITZ and NZ inferred from microstructural analyses are about 500°-550°C. The metamorphism in the country rocks E of the batholith is in the lower greenschist facies indicating that deformation along the ITZ and NZ occurred during the cooling of the granite. U-Pb zircon LA-SF-ICP-MS analyses reveal that the coarse-grained and fine-grained granites in the NE part of the batholith have the same age of 549×2 Ma. The age of an undeformed aplite that truncates the NZ's foliation brackets the strike-slip shearing between 549×2 Ma and 541×3 Ma. In the SE margin of the batholith deformed coarse-grained granite is dated at 544×2 Ma and an undeformed granitic vein gave an age of 543×3 Ma, thus relating the fabric formation to the same time interval. To the SW the deformed granite in the ITZ is dated at 533×3 Ma indicating that the E-W shortening was still active at this time. This study reports two deformational stages recorded in the Hook batholith and its country rocks. E-W shortening folded the sediments form the E margin of the granite and formed the solid-state fabric in the batholith. The following N-S shortening cold be related to the final docking of the Zambezi sequence to the Lufilian Arc along the Mwembeshi dislocation.

  20. Lu-Hf and Sm-Nd garnet geochronology

    DEFF Research Database (Denmark)

    Smit, Matthijs Arjen; Scherer, Erik E.; Mezger, Klaus


    are similar among grains having radii larger than 1.3 mm (weighted mean: 2714±6 Ma2714±6 Ma; 2 s.d.) and up to 20 Myr younger for finer fractions. In contrast, Sm–Nd dates are equal to or younger than the 2641–2637 Ma age of peak metamorphism. The roles of concurrently digested inclusions and core–rim Lu......To investigate the systematics of the 176Lu–176Hf and 147Sm–143Nd garnet chronometers, we performed REE and isotope analyses on garnet crystals of different size (0.55–3.1 mm radius) from a single granulite specimen (Archean Pikwitonei Granulite Domain, Manitoba, Canada). The Lu–Hf dates....../Sm fractionation are quantitatively evaluated and excluded as causes for the observed age heterogeneity. Instead, the isotope dates are interpreted to reflect partial loss of radiogenic 176Hf and 143Nd, and are used to constrain the systematics of the garnet chronometers at high temperature. The data constrain...

  1. Geochronology and Geochemistry of Cretaceous Volcanic Rocks from Liuluo Formation in Hainan Island and Their Tectonic Implications%海南白垩纪六罗村组火山岩的年代学、地球化学特征及其大地构造意义

    Institute of Scientific and Technical Information of China (English)

    周云; 梁新权; 梁细荣; 蒋英; 蔡运花; 邹水长; 王策; 付建刚; 董超阁


    A large number of volcanic rocks outcrops have been reported in the southern Hainan Island, but their geochronology, petrogenesis and tectonic implications are still unclear. In this study, we present detailed zircon U-Pb dating results and major and trace elemental compositions of the Liuluocun Formation volcanic rocks in the Liuluo area of Sanya, Hainan Island. LA-ICP-MS zircon U-Pb dating of the rhyolite from the upper Liuluocun Formation yields an age of 102±1 Ma, basaltic andesite from the lower Liuluocun Formation also shows an age of 102±1 Ma, indicating late stage of the Early Cretaceous volcanic activity occurred in southern Hainan Island. The rhyolites are characterized by high contents of silicon (SiO2=73.77%‒74.79%) and potassium (K2O=5.09%‒6.77%) and K2O/Na2O ratios (1.71‒2.80), and low Mg# (22‒28). They show negative Ti, P, Nb and Ta anomalies, obvious depletion of Eu (δEu=0.51‒0.61), enrichment in Rb, Ba and LILE and strong fractionation between LREE and HREE (La/YbCN=15.05‒16.97). The basalts-andesites have SiO2 ranging from 49.82%to 57.93%and lower value of K2O/Na2O (0.32‒1.11) and higher value of Mg# (46‒58) than those of the rhyolites. They show slight Eu anomalies (δEu=0.86~0.94), depletion of Nb and Ta, and enrichment of LILE. They have lower degree fractionation between LREE and HREE (La/YbCN=9.42‒12.24) and weaker negative P, Ti anomalies than those of the rhyolites. Besides, rhyolite and basalt-andesite samples show similar (87Sr/86Sr)i (0.708222‒0.708965 and 0.707532‒0.708401, respectively) andεNd(t) values (‒2.49‒‒2.69 and‒2.35‒‒4.09, respectively), indicating that they may have derived mainly from the same source. Combining with the regional geological data, we propose that these volcanic rocks are most likely generated from continental lithospheric mantle which had been modified by subduction component, and their formation was closely related to the regional lithospheric extension in the South

  2. 江南中段慈化地区新元古代高镁安山岩的厘定及其构造意义%Geochronology and Geochemistry of Cihua Neoproterozoic High-Mg Andesites in Jiangnan Orogen and Their Tectonic Implications

    Institute of Scientific and Technical Information of China (English)

    张玉芝; 王岳军; 郭小飞; 甘成势; 邢晓婉; 宋菁菁


    江南造山带被普遍认为是扬子与华夏陆块在新元古代的拼合带,其拼合机制及精细时代却一直备受争议.在江南造山带中段湘赣交界慈化地区识别出新元古代火山岩,并对其进行了锆石 U-Pb 年代学及主微量地球化学研究.该火山岩发育于冷家溪群地层中,其锆石 LA-ICP-MS 年代学测试得到了832±12 Ma 的206 Pb/238 U 加权平均年龄(n=16,MSWD=0.12),代表其喷发年龄.主量元素结果显示,SiO2和 MgO 含量分别为57.67%~61.33%和3.51%~4.29%,Mg#为52~57,高于正常弧火山岩,属高镁安山岩.微量元素富集轻稀土元素和大离子亲石元素,亏损高场强元素,其 Nb-Ta、Ti 亏损,具“弧型”地球化学特征,可能来源于受板片熔体/流体或者俯冲再循环沉积物交代的难熔地幔源区.上述资料表明,慈化高镁安山岩是江南造山带中段楔形地幔源区受消减组分交代作用的产物,暗示此时江南造山带中段仍在消减,扬子和华夏陆块尚未完全拼合.结合前人研究成果,江南造山带不同区段的闭合时间可能存在差异.%It is generally accepted that the Jiangnan orogenic belt led to the amalgamation of the Yangtze with Cathaysia blocks during the Neoproterozoic period.However,its tectonic evolution and amalgamation timing are still in debate.This paper pres-ents a set of new geochronological and geochemical data for the newly-identified Neoproterozoic high-Mg volcanics from Cihua (Jiangxi).LA-ICP-MS zircon U-Pb dating of the representative sample yields a weighted mean age of 832±12 Ma (n=16, MSWD=0.12),representing the eruption age of the andesite.Geochemical results indicate that they can be classified as high-Mg andesites with SiO2 ranging from 57.67% to 61.33%,MgO from 3.51% to 4.29% and Mg-number from 52 to 57.Their chondrite-normalized REE patterns exhibit a left-sloping pattern with enriched LREEs

  3. Zircon U-Pb geochronology, geochemistry and petrogenesis of the porphyric-like muscovite granite in the Dahutang tungsten deposit, Jiangxi Province%江西大湖塘钨矿床似斑状白云母花岗岩锆石U-Pb年代学、地球化学及成因研究

    Institute of Scientific and Technical Information of China (English)

    黄兰椿; 蒋少涌


    The Dahutang tungsten ( molybdenum, copper, tin) ore fields are located in the middle part of the Jiangnan Orogen which is the junction of three counties of Wuning, Xiushui, Jing'an area in the middle of the northern part of Jiuling Mountains. It is the world's largest tungsten mine. In this paper, we study the mineral chemistry, major elements, trace elements, and Sr-Nd isotopic compositions of the porphyric-like muscovite granite which is closely related to mineralization in the deposit. The results show that LA-ICP-MS zircon U-Pb age of the porphyric-like muscovite granite is 144. 2 ± 1. 3 Ma. The muscovite in the granite belongs to primary magmatic muscovite, and the biotite shows iron-rich characteristic, and its composition indicates a crust origin of the rock. The Fe3 + / Fe composition of biotite indicates a low oxygen fugacity for the magma evolution. The porphyric-like muscovite granite belongs to S-type granite which is characterized by high SiO2(72. 88% to 73. 33% ) , strongly peraluminous, low Nb and Ta, high Rb/Sr, strong LREE/HREE fractionation, and pronounced negative Eu anomalies. The εNd (t) values of the rock varied from - 7. 47 to - 7. 78 with the two-stage model ages tDMc of 1543Ma to 1568Ma. It is suggested that the source region of the granite were from argillaceous sedimentary rocks, likely from the Neoproterozoic Shuangqiaoshan Group. We also suspected that the ore-forming metals such as tungsten in the deposit may have likely derived from the Shuangqiaoshan Group. Based on the geochronological framework of the granite, the Yanshanian magmatic activity took place during the transition of the Jurassic and Cretaceous period in an extensional tectonic environment in Jiuling area.%江西省大湖塘钨(钼、铜、锡)矿集区位于江南造山带中段,九岭山脉中段北部之武宁、修水、靖安三县交界区域,是目前世界最大的钨矿之一.本文对该矿床中与成矿关系密切的似斑状白云母花岗岩进行了

  4. Episodic Growth and Multiple Modification of Precambrian Lower Crust in the Southeastern Margin of North China Craton: Petrologic,Geochronological and Hf-isotopic Evidences%华北克拉通东南缘前寒武纪下地壳的幕式生长与多期改造:岩石学、年代学和Hf同位素证据

    Institute of Scientific and Technical Information of China (English)

    刘贻灿; 王安东


    The early Precambrian is regarded to be a crucial stage for continental crust formation. The Precambrian lower-crustal rocks in the southeastern margin of North China Craton occur as the exposed metamorphic basement (named as the Wuhe metamorphic complex) and xenoliths in the Mesozoic dioritic to monzodioritic porphyry. These rocks provide an excellent natural laboratory to study formation and evolution of the Precambrian lower crust in the region. Integrated investigations on metamorphic petrology, petrologic geochemistry, Hf-isotope and zircon U-Pb geochronology of the rocks suggested that the Precambrian lower crust beneath the studied region experienced an episodic growth and multiple modification history. Intensive tectono-thermo events and metamorphic overprinting mainly occurred at several peaks, such as 2. 5-2. 6, 2. 1, 1. 8-1. 9 Ga and 390, 176 Ma. For the 2. 5-2. 6 Ga lower-crustal rocks, some of them with high radiogenic Pb-isotopic compositions, which were similar to those formed at 2. 1 Ga subduction-related arc setting and underwent 1. 8 Ga high-pressure granulite-facies metamorphism, suffered from high-pressure granulite-facies metamorphism at 2.1 Ga and (or) 1. 8-1. 9 Ga, and subsequent metamorphic overprinting at 390 Ma and 176 Ma; however, others had low radiogenic Pb-isotopic compositions for typical Precambrian lower-crustal rocks, and formed at 2. 55-2. 64 Ga and underwent 2. 48-2. 49 Ga peak metamorphism of granulite-facies without record of post-peak metamorphic overprint at 2. 1 Ga and (or) 1. 8-1. 9 Ga. Therefore, they formed simultaneously, but probably located at different crustal levels and escaped from subsequent metamorphic overprinting, strongly depending on their formation depths. The 2. 7- 2. 8 Ga ages defined by inherited zircons and depleted mantle zircon-Hf model ages could record an earlier crustal growth episode in the area.%早前寒武纪被认为是大陆地壳形成的重要时期.华北克拉通东南缘前寒武纪下地

  5. Análisis geoquímico-geocronológico de rocas granulíticas y calcosilicáticas de las Sierras Pampeanas Noroccidentales Geochemical and geochronological analyses of granulitic and calc-silicate rocks of northwestern Sierras Pampeanas

    Directory of Open Access Journals (Sweden)

    AJ Toselli


    grade and the chemical compositions; they vary from rich in carbonates to rich in silica. They also look very different from the amphibolites. The calc-silicate in the Cumbres Calchaquíes, Aconquija and Ancasti ranges, show mineral associations with quartz, plagioclase, clinozoicite, garnet, hornblende, titanite, diopside, biotite and calcite; while in the sierra de Quilmes, in granulite facies conditions, hypersthene and scarce biotite are present, and both are different to calc-silicate from sierra de Fiambalá. The geochronologic data obtained, allows to correlate to the event of very-low to low metamorphic grade of the Puncoviscana Formation, with ages of 530-568 Ma, with their equivalent ones deeper in the sierra de Quilmes, represented by calc-silicates with ages of 571 Ma, and granulites with 480 Ma, followed by anatexis phenomena, like those to the Cafayate granite with 453 Ma, and the migmatites of Loma Colorada with 446 Ma. This event is reiterated in the El Infiernillo Tonalite of 419 Ma, in the syntectonic-granodiorites of El Portezuelo Formation, with 445 Ma; in the gneiss of La Puntilla with 483 Ma, and in the gneiss of Sierra de Fiambalá with 440 Ma. On the other hand, the deformation zone of the Sierra de Copacabana, with an age of 388 Ma, is related to the granitoids of Zapata with 395 Ma; La Punta with 405 Ma, and Señor de la Peña, of 376 Ma. These values demonstrate that metamorphism, deformation and magmatism of the Famatinian Cycle, are superimposed over the Pampean cycle.

  6. A tentative discussion on zircon U-Pb geochronology and geochemistry of ore-bearing intermediate-acid intrusive rocks in the Bainaimiao copper ore district and the metallogen-ic epoch%内蒙古白乃庙铜矿区含矿中酸性侵入岩LA-ICP-MS锆石U-Pb年龄、地球化学特征及成矿时代

    Institute of Scientific and Technical Information of China (English)

    赵利刚; 李承东; 常青松; 高学生; 许雅雯; 樊建廷; 张阔


    In order to determine the metallogenic epoch and ore-forming environment of the Bainaimiao copper deposit, the au⁃thors carried out research on isotope geochronology and geochemistry of ore-bearing intermediate-acid rocks. LA-MC-ICPMS zir⁃con U-Pb dating shows that the ages of three ore-bearing granodiorites samples are 421.5±1.8Ma, 438.6±3.4Ma and 459.3±3.6Ma respectively. These rocks have peraluminous calc-lkaline characteristics, and are enriched in LREE. There is a weak positive anomaly of Eu. The trace elements are obviously depleted in Ta and Nb. According to comprehensive ages and geochemical characteristics of ore-bearing granodiorites and the regional tectonic background, the authors consider that the Bainaimiao copper deposit was on the whole formed in the early Paleozoic and characterized by multi-period mineralization. In addition, the metallogenesis was related to the subduction event of ancient Mongolia Ocean in the Early Paleozoic.%为了限定白乃庙铜矿的成矿时代和成矿环境,对含矿中酸性岩类进行同位素年龄和岩石地球化学研究。获得的3个含矿花岗闪长岩样品的LA-MC-ICP-MS锆石U-Pb年龄分别为421.5±1.8Ma、438.6±3.4Ma和459.3±3.6Ma。含矿中酸性岩类属于过铝质钙碱性系列岩石,稀土元素曲线显示为轻稀土元素富集型,具有弱正Eu异常,微量元素曲线表现为明显的Ta、Nb亏损。综合含矿中酸性岩类时代、地球化学特征及区域构造背景,认为白乃庙铜矿总体形成于早古生代,具有多期成矿的特点,成矿作用与古蒙古洋早古生代俯冲消减事件有关。

  7. 用于早前寒武纪岩石Ar-Ar法定年的K-Ar法年龄国家二级标准物质——ZMT04白云母的定值结果%The certified results of ZMT04 muscovite reference material utilized for 40Ar/39Ar isotopic geochronological research on Early precambrian rocks in China.

    Institute of Scientific and Technical Information of China (English)

    桑海清; 王非; 龚俊峰; 周晶


    desirable for any relatively old sample.In that occasion,if the neutron flux monitor is much younger than the unknown age sample,the 40 Ar/39Ar ratio in standard may become very small,approaching a smaller limit value (≈3).Therefore,it is quite necessary to prepare an old age standard for the flux monitor of the 40 Ar-39 Ar method,especially for isotopic geochronological research on Early Precambrian rocks and meteorites.ZMT04 muscovite as K-Ar age geostandard of China,were separated from a pegmatite dike which occurs at Tianpishan area,in the southwest of Zhuozi County,Inner Mongolia of China.The pegmatite consists of muscovite,quartz and minor zircon,sphere etc.About 1080g of pure ZMT04 muscovite was obtained at the (10 × 1 × 0.02)mm fraction.The purity is about 100%.Following mixing,the ZMT04 muscovite was divided into 14 jars about 77g each.Each jar sample was further subdivided into14 aliquots about 5.51g each (total 196 aliquots).One aliquot (prefixed C) was retained for control analysis of K and 40Ar*,the other 13 aliquot of the same jar remain to be used fo interlabortory work.40Ar-39 Ar age spectrum of ZMT04 muscovite indicats that the muscovite has remained undisturbed since its crystallisation.The plateau age age is 1823 ± 15Ma.Thirteen stepwise hating Ar isotopic data of the muscovite give a 40Ar/39Ar isochron age of 1824 ± 15Ma and the initial 40Ar/36Ar ration of 288 ± 16,and the MSWD in the 40Ar/36Ar-39Ar/36Ar isochron diagram is 2.10.This age may represent the crystallization time of the muscovite in the pegmatite dike.The homogeneity tests and the certification analyses of 40Ar* and K in ZMT04 muscovite were performed according to the rules from the International Standard Organization (ISO) Guid 35 (1985) and primary referenc material technique of China (1994).During the homogeneity tests the analyses of K and 40Ar* in 77-600 hornblende,Hb3gr hornblende and BSP-1 hornblend were done and the results are almost coincident with their mean value

  8. 华北克拉通孔兹岩带中段大青山-乌拉山变质杂岩立甲子基性麻粒岩年代学及地球化学研究%Geochronological and geochemical study of the Lijiazi mafic granulites from the Daqingshan-Wulashan metamorphic complex, the central Khondalite Belt in the North China Craton

    Institute of Scientific and Technical Information of China (English)

    刘平华; 刘福来; 蔡佳; 刘建辉; 施建荣; 王舫


    The Iijiazi mafic granulites are predominantly composed of Amp Cpx-Opx granulite, Grt-bearing Amp Cpx-Opx granulite and Bt Amp Cpx-Opx granulite, and have been found as irregular lenses and deformed dykes within the Al-rich gneisses and granitic gneisses in the Daqingshan-Wulashan metamorphic complex. The apparent207 Pb/206 Pb ages recorded by the metamorphic zircon domains with the middle-to low-pressure granulite-facies mineral inclusions of Cpx + Amp + Pl, which are consistent to the matrix-type metamorphic assemblages of the mafic granulites, range from 1933 ±39Ma to 1834 ±40Ma with a weighted mean age of 1892 ±7Ma, which are interpreted as the timing of the middle,-to low-pressure granulite-facies metamorphism of the mafic granulites. Mobile elements such as the large ion lithophile elements (K, Na, Sr, Rb) of the mafic granulites changed in significant levels, high field strength elements (such as Nb, Zr, Ti) and rare earth elements basically unchanged and keep stable during the metamorphic process. The Lijiazi mafic granulites from the Daqingshan complex belong to tholeiitic-basaltic rock series, with the concentrations of SiO2 at 45. 58% ~51. 40%, and Mg# value mainly ranging from 41 to 54. The composition of the rare earth elements of the mafic granulites is characterized by the flat, (La/Yb )CN =1. 30 ~ 1.51) with no Eu anomalies (Eu/Eu * = 0. 93 ~ 1. 04) on the chondrite-normalized patterns. All samples of the mafic granulites share obvious negative anomaly in Nb, Zr and Ti of the Phanerozoic island arc basalts. According to the geochemical and geochronological data of the Lijiazi mafic granulites, it is suggested that the protolith ages of the Lijiazi mafic granulites is 2450 ~ 1930Ma, the protoliths of the mafic granulites are undergone middle-to low-pressure granulite-facies metamorphism at ~ 1900Ma, the composition of the major and trace elements of the mafic granulites is characterized by the island arc tholeiitic basaltic rocks, and

  9. The Evolution of Triassic Granites Associated with Mineralization within East Kunlun Orogenic Belt:Evidence from the Petrology,Geochemistry and Zircon U-Pb Geochronology of the Mohexiala Pluton%东昆仑与成矿有关的三叠纪花岗岩演化:基于莫河下拉岩体岩石学、地球化学和锆石 U-Pb 年代学的证据

    Institute of Scientific and Technical Information of China (English)

    张炜; 周汉文; 朱云海; 毛武林; 佟鑫; 马占青; 曹永亮


    The East Kunlun orogenic belt is not only an important intrusive magmatic tectonic belt,but also a very important poly-metallic metallogenic belt for national economy.There are a large number of granites associated with mineralization formed in Triassic in this area,but the connection between them and the relationship about the tectogenesis are still not clear.In this paper,detailed petrography,element geochemical and zircon geochronological studies are presented for the Mohexiala granite-porphyry associated with silver poly-metallic ore;we summarize the characteristics of Triassic granites associated with minerali-zation within the East Kunlun orogenic belt,and discuss their evolution.Results show that:(1)the Triassic granites associat-ed with mineralization within East Kunlun orogenic belt have a clear trend from low-K and mid-K calc-alkaline series to high-K calc-alkaline and shoshonite series during 250-200 Ma,their A/NK ratios decreased from 2.0 to 1.0 during 240-200 Ma;(2) the (87 Sr/86 Sr)i ratios between 0.710 to 0.715,εNd (t)=-0.6-0.0,the value ofεHf (t)are concentrated in -5-1 and the peak ranges between -2 to -1,indicating that the East Kunlun Triassic granites associated with mineralization were derived from the sources of ancient crust and mixed by mantle material;(3)the East Kunlun area got into the post-orogenic stage at 240 Ma and a large number of calc-alkaline granites formed,the granites gradually reduced after 220 Ma and some alkaline A-type granites occurred between 204 Ma and 210 Ma which indicates that the orogenic belt transformed from collision-orogeny to the intraplate break-up stage.%东昆仑造山带在三叠纪不仅是一个重要的构造-岩浆带,也是一个对于国民经济非常重要的多金属成矿带.该区在三叠纪形成了大量与成矿有关的花岗岩,它们之间的联系、与区域构造运动的关系目前尚未明确.在莫河下拉银多金属矿花岗斑岩岩相学、地球化学和锆石年

  10. An integrated zircon geochronological and geochemical investigation into the Miocene plutonic evolution of the Cyclades, Aegean Sea, Greece: Part 1: Geochronology (United States)

    Bolhar, Robert; Ring, Uwe; Allen, Charlotte M.


    We use 369 individual U-Pb zircon ages from 14 granitoid samples collected on five islands in the Cyclades in the Aegean Sea, Greece, for constraining the crystallisation history of I- and S-type plutons above the retreating Hellenic subduction zone. Miocene magmatism in the Cyclades extended over a time span from 17 to 11 Ma. The ages for S-type granites are systematically ~2 million years older than those for I-type granites. Considering plutons individually, the zircon data define age spectra ranging from simple and unimodal to complex and multimodal. Seven of the 14 investigated samples yield more than one distinct zircon crystallisation age, with one I-type granodiorite sample from Mykonos Island representing the most complex case with three resolvable age peaks. Two samples from S-type granites on Ikaria appear to have crystallised zircon over 2-3 million years, whereas for the majority of individual samples with multiple zircon age populations the calculated ages deviate by 1-1.5 million years. We interpret our age data to reflect a protracted history involving initial partial melting at deeper lithospheric levels, followed by crystallisation and cooling at shallower crustal levels. Our study corroborates published research arguing that pluton construction is due to incremental emplacement of multiple magma pulses over a few million years. Assuming that multiple age peaks of our 14 samples can indeed serve to quantify time spans for magmatic emplacement, our data suggest that Aegean plutons were constructed over a few million years. Our tectonic interpretation of the U-Pb ages is that the S-type granites resulted from partial melting and migmatisation of the lower crust, possibly starting at ~23 Ma. The I-type granites and associated mafic melts are interpreted to reflect the magmatic arc stage in the Cyclades starting at ~15 Ma.

  11. Mercury's geochronology revised by applying Model Production Functions to Mariner 10 data: geological implications

    CERN Document Server

    Massironi, M; Marchi, S; Martellato, M; Mottola, M; Wagner, R J


    Model Production Function chronology uses dynamic models of the Main Belt Asteroids (MBAs) and Near Earth Objects (NEOs) to derive the impactor flux to a target body. This is converted into the crater size-frequency-distribution for a specific planetary surface, and calibrated using the radiometric ages of different regions of the Moon's surface. This new approach has been applied to the crater counts on Mariner 10 images of the highlands and of several large impact basins on Mercury. MPF estimates for the plains show younger ages than those of previous chronologies. Assuming a variable uppermost layering of the Hermean crust, the age of the Caloris interior plains may be as young as 3.59 Ga, in agreement with MESSENGER results that imply that long-term volcanism overcame contractional tectonics. The MPF chronology also suggests a variable projectile flux through time, coherent with the MBAs for ancient periods and then gradually comparable also to the NEOs.

  12. Rb-Sr geochronological study on Tatehira granodiorite, Oshima Peninsula, Southwest Hokkaido, Japan

    Energy Technology Data Exchange (ETDEWEB)

    Iizumi, Shigeru; Kobayashi, Hideo; Toyoda, Minoru


    It has been known that Tatehira granodiorite, a pluton which occurs sporadically in the southwest region of Hokkaido, is related with Kanoite formation. The radiometric age of this granodiorite still remains unknown. The Tatehira granodiorite intrudes into late Paleozoic or early Mesozoic Matsumae group, and is overlain by Miocene Usubetsu formation unconformably. It occurs as several separated small plutons. The samples for Rb-Sr analysis were collected from one of the plutons in southern part. The determination of the isochron age of rock-minerals and the initial Sr isotopic ratio of Tatehira granodiorite were carried out. This granodiorite is able to be classified into two phases, namely melanocratic (WRM) and leucocratic (WRL) phases. The samples of WRM and WRL were powdered and subjected to X-ray analysis. It was found that the chemical compositions were similar except slight differences in FeO + Fe2O3, CaO and K2O. The Sr isotopic ratio was determined with a MAT 261 mass spectrometer, and the results were analyzed statistically. The Rb and Sr concentrations and the Sr isotopic ratio of WRM, WRL, and PRF (plagio-clase rich fraction) and BRF (biotite rich fraction) separated from the WRL were determined, and the age of 102.5 +- 0.3 Ma and the initial ratio of 0.70538 +- 0.00001 were obtained. The ratio of Tatehira granodiorite is comparable to that of Okushiri Island. Comparing with the data obtained from other region, the granodiorites of Tatehira and Okushiri Island should be related with the granitoids of Abukuma granite province.

  13. Structural reinterpretation of the Ajo mining district, Pima County, Arizona, based on paleomagnetic and geochronologic studies. (United States)

    Hagstrum, J.T.; Cox, D.P.; Miller, R.J.


    The Ajo mining district of southern Arizona is divided into two main structural blocks by the Gibson Arroyo fault. The eastern Camelback Mountain block contains the Late Cretaceous-early Tertiary porphyry copper deposit which has been previously thought to be associated with the displaced apex of a large intrusion exposed by deeper erosion in the western Cardigan Peak block. However, unpublished U-Pb data support a mid-Tertiary age for the western intrusion. The following sequence of mid-Tertiary events in the district are indicated: 1) emplacement of the western intrusion, 2) movement along the Gibson Arroyo fault, 3) unroofing and perhaps tilting of the pluton approx 70o to the south along with the Camelback Mountain block, 4) syntectonic depositions of the Locomotive Fanglomerate and the Ajo Volcanics, 5) continued uplift and tilting to the south totaling 40o to 60o, 6) intrusion of the youngest dikes with attendant alteration and remagnetization of the host rocks, and 7) minor (?) oblique movement along the Gibson Arroyo fault.-from Authors

  14. Geology and geochronology of the Sub-Antarctic Snares Islands/Tini Heke, New Zealand

    DEFF Research Database (Denmark)

    Scott, JM; Turnbull, IM; Sagar, MW


    The first comprehensive geological map, a summary of lithologies and new radiogenic isotope data (U–Pb, Rb–Sr) are presented for crystalline rocks of the Sub-Antarctic Snares Islands/Tini Heke, 150 km south of Stewart Island. The main lithology is Snares Granite (c. 109 Ma from U–Pb dating of zir...

  15. Geochronology and paleoenvironment of pluvial Harper Lake, Mojave Desert, California, USA (United States)

    Garcia, Anna L.; Knott, Jeffrey R.; Mahan, Shannon; Bright, Jordan


    Accurate reconstruction of the paleo-Mojave River and pluvial lake (Harper, Manix, Cronese, and Mojave) system of southern California is critical to understanding paleoclimate and the North American polar jet stream position over the last 500 ka. Previous studies inferred a polar jet stream south of 35°N at 18 ka and at ~ 40°N at 17–14 ka. Highstand sediments of Harper Lake, the upstream-most pluvial lake along the Mojave River, have yielded uncalibrated radiocarbon ages ranging from 24,000 to > 30,000 14C yr BP. Based on geologic mapping, radiocarbon and optically stimulated luminescence dating, we infer a ~ 45–40 ka age for the Harper Lake highstand sediments. Combining the Harper Lake highstand with other Great Basin pluvial lake/spring and marine climate records, we infer that the North American polar jet stream was south of 35°N about 45–40 ka, but shifted to 40°N by ~ 35 ka. Ostracodes (Limnocythere ceriotuberosa) from Harper Lake highstand sediments are consistent with an alkaline lake environment that received seasonal inflow from the Mojave River, thus confirming the lake was fed by the Mojave River. The ~ 45–40 ka highstand at Harper Lake coincides with a shallowing interval at downstream Lake Manix.

  16. A geochronologic framework for the Ziegler Reservoir fossil site, Snowmass Village, Colorado (United States)

    Mahan, Shannon; Gray, Harrison J.; Pigati, Jeffrey S.; Wilson, Jim; Lifton, Nathaniel A.; Paces, James B.; Blaauw, Maarten


    The Ziegler Reservoir fossil site near Snowmass Village, Colorado, provides a unique opportunity to reconstruct high-altitude paleoenvironmental conditions in the Rocky Mountains during the last interglacial period. We used four different techniques to establish a chronological framework for the site. Radiocarbon dating of lake organics, bone collagen, and shell carbonate, and in situ cosmogenic 10Be and 26Al ages on a boulder on the crest of a moraine that impounded the lake suggest that the ages of the sediments that hosted the fossils are between ~ 140 ka and > 45 ka. Uranium-series ages of vertebrate remains generally fall within these bounds, but extremely low uranium concentrations and evidence of open-system behavior limit their utility. Optically stimulated luminescence (OSL) ages (n = 18) obtained from fine-grained quartz maintain stratigraphic order, were replicable, and provide reliable ages for the lake sediments. Analysis of the equivalent dose (DE) dispersion of the OSL samples showed that the sediments were fully bleached prior to deposition and low scatter suggests that eolian processes were likely the dominant transport mechanism for fine-grained sediments into the lake. The resulting ages show that the fossil-bearing sediments span the latest part of marine isotope stage (MIS) 6, all of MIS 5 and MIS 4, and the earliest part of MIS 3.

  17. Accelerator Mass Spectrometry at Arizona: Geochronology of the Climatic Record and Connections with the Ocean

    Directory of Open Access Journals (Sweden)

    J.T. Jull


    Full Text Available There are many diverse uses of accelerator mass spectrometry (AMS. 14C studies at our laboratory include much research related to paleoclimate, with 14C as a tracer of past changes in environmental conditions as observed in corals, marine sediments, and many terrestrial records. Terrestrial records can also show the influence of oceanic oscillations, whether they are short term, such as ENSO (El Niño/Southern Oscillation, or on the millennial time scale. In tracer applications, we have developed the use of 129I as well as 14C as tracers for nuclear pollution studies around radioactive waste dump sites, in collaboration with IAEA. We discuss some applications carried out in Tucson, AZ, for several of these fields and hope to give some idea of the breadth of these studies.

  18. Accelerator mass spectrometry at Arizona: geochronology of the climate record and connections with the ocean. (United States)

    Jull, A J T; Burr, G S; Beck, J W; Donahue, D J; Biddulph, D; Hatheway, A L; Lange, T E; McHargue, L R


    There are many diverse uses of accelerator mass spectrometry (AMS). Carbon-14 studies at our laboratory include much research related to paleoclimate, both with 14C as a tracer of past changes in environmental conditions as observed in corals, marine sediments and many terrestrial records. Terrestrial records such as forest fires can also show the influence of oceanic oscillations, whether they are short-term such as ENSO, or on the millennial time scale. In tracer applications, we have developed the use of 129I as well as 14C as tracers for nuclear pollution studies around radioactive waste dump sites, in collaboration with IAEA. We discuss some applications carried out in Tucson for several of these fields and hope to give some idea of the breadth of these studies.

  19. Geology and geochronology of type Chasicoan (late Miocene) mammal-bearing deposits of Buenos Aires (Argentina) (United States)

    Zárate, Marcelo A.; Schultz, Peter H.; Blasi, Adriana; Heil, Clifford; King, John; Hames, Willis


    The late Miocene Chasicoan mammal-bearing deposits exposed along the lower reach of Arroyo Chasicó are composed of cross-bedded, very fine sandstones interpreted as a channel-bar deposit (lithofacies association 1) grading upward into sandy siltstones (lithofacies association 2), probably accumulated through relatively high-density flows in a marginal channel and/or floodplain environment. The uppermost levels are dominantly composed of mudstones and sandy siltstones (lithofacies association 3) deposited in generally low-energy conditions of sedimentation in a swampy environment. Several paleosols (lithofacies P) are present, indicating that the succession was the result of episodic fluvial sedimentation. The volcaniclastic composition (primary and reworked pyroclastics) suggests that the fluvial system drained the westward region by the Andean foothills. An impact event dated at 9.23 ± 0.09 Ma and recorded by impact glasses (escorias) during deposition of lithofacies Sp enables the fine tuning of the chronology of the deposits through high-resolution magnetostratigraphic profiles, which indicate that the approximately 9.4 m thick succession recorded by lithofacies association 1 and 2 accumulated between 9.43 and 9.07 Ma. The lithofacial arrangement of the succession does not support the current differentiation of the Arroyo Chasicó Formation into the Vivero and Las Barrancas members. Previous biostratigraphic interpretations contain significant inconsistencies in light of the revised stratigraphy proposed here.

  20. New geochronological, paleoclimatological, and archaeological data from the Narmada Valley hominin locality, central India. (United States)

    Patnaik, Rajeev; Chauhan, Parth R; Rao, M R; Blackwell, B A B; Skinner, A R; Sahni, Ashok; Chauhan, M S; Khan, H S


    The oldest known fossil hominin in southern Asia was recovered from Hathnora in the Narmada Basin, central India in the early 1980's. Its age and taxonomic affinities, however, have remained uncertain. Current estimates place its maximum age at >236ka, but not likely older than the early middle Pleistocene. The calvaria, however, could be considerably younger. We report recent fieldwork at Hathnora and associated Quaternary type-sections that has provided new geological and archaeological insights. The portion of the exposed 'Boulder Conglomerate' within the Surajkund Formation, which forms a relict terrace and has yielded the hominin fossils, contains reworked and stylistically mixed lithic artifacts and temporally mixed fauna. Three mammalian teeth stratigraphically associated with the hominin calvaria were dated by standard electron spin resonance (ESR). Assuming an early uranium uptake (EU) model for the teeth, two samples collected from the reworked surface deposit averaged 49+/-1ka (83+/-2ka, assuming linear uptake [LU]; 196+/-7ka assuming recent uptake [RU]). Another sample recovered from freshly exposed, crossbedded gravels averaged 93+/-5ka (EU), 162+/-8ka (LU) or 407+/-21ka (RU). While linear uptake models usually provide the most accurate ages for this environment and time range, the EU ages represent the minimum possible age for fossils in the deposit. Regardless, the fossils are clearly reworked and temporally mixed. Therefore, the current data constrains the minimum possible age for the calvaria to 49+/-1ka, although it could have been reworked and deposited into the Hathnora deposit any time after 160ka (given the LU uptake ages) or earlier (given the RU ages). At Hathnora, carbonaceous clay, bivalve shells, and a bovid tooth recovered from layers belonging to the overlying Baneta Formation have yielded (14)C ages of 35.66+/-2.54cal ky BP, 24.28+/-0.39cal ky BP, and 13.15+/-0.34ky BP, respectively. Additional surveys yielded numerous lithics and fossils on the surface and within the stratigraphic sequence. At the foot of the Vindhyan Hills 2km from the river, we recovered a typologically Early Acheulean assemblage comprised of asymmetrical bifaces, large cleavers with minimal working, trihedral picks, and flake tools in fresh condition. These tools may be the oldest Acheulean in the Narmada Valley. Several lithics recovered from the Dhansi Formation may represent the first unequivocal evidence for an early Pleistocene hominin presence in India. In situ invertebrate and vertebrate fossils, pollen, and spores indicate a warm, humid climate during the late middle Pleistocene. High uranium concentrations in the mammalian teeth indicate exposure to saline water, suggesting highly evaporative conditions in the past. Late Pleistocene sediment dated between 24.28+/-0.39cal ky BP and 13.15+/-340ky BP has yielded pollen and spores indicating cool, dry climatic conditions corresponding to Oxygen Isotope Stage 2 (OIS 2). An early Holocene palynological assemblage from the type locality at Baneta shows evidence for relatively dry conditions and a deciduous forest within the region. The Dhansi Formation provisionally replaces the Pilikarar Formation as the oldest Quaternary formation within the central Narmada Basin. The Baneta Formation, previously dated at 70ka to 128ka, correlates with the late Pleistocene and early Holocene. Our results highlight the need for further Quaternary geological and paleoanthropological research within the Narmada Basin, especially because dam construction threatens these deposits.

  1. Geochronology and sources of late Neoproterozoic to Cambrian granites of the Saldania Belt (United States)

    Chemale, F.; Scheepers, R.; Gresse, P. G.; van Schmus, W. R.


    The Saldania Belt (SB), located in the southernmost part of South Africa, contains S-, I-, and A-type granites. Whole-rock Sm-Nd data for the Saldania granites indicate the presence of a juvenile as well as inherited crustal signature. The earlier S-type granites have ɛNd( t) values from -4.2 to -3.28 (for t = 550 Ma). In contrast, the intermediate I-type and youngest A-type and highly fractionated I-type granites display ɛNd values ranging from -1.44 to -3.68 (for t = 540 Ma) and from +3.66 to +5.1(for t = 530 Ma), respectively. The U-Pb single zircon data of A-type granites exposed in the Western Branch of the SB yielded dates from 524 ± 8 to 510 ± 4 Ma, whereas an S-type granite, situated in the Southern Branch of the SB and represented by the syn- to late-tectonic Rooiklip Granite, yielded an age of 527 ± 8 Ma. The volcano-sedimentary rocks intruded by these granites display Nd model ages from Ga to 1.67 Ga and ɛNd( t) values from -6.58 to +3.34 (for t = 560 Ma) with isotope signature similar to those of the granites. The S- and I-type granitic magmatism is mostly a product of melting of an earlier crust (Mesoproterozoic to Paleoproterozoic) with different degree of juvenile contribution. The obtained isotope data and field relationship support the hypothesis that the lithological units of the SB were affected by the late Neoproterozoic to Early Cambrian tectonism, related to compressive deformational processes at the southern margin of the Kalahari Plate and probably correlated with the Sierra La Ventana Belt basement.

  2. Petrography, fluid inclusion analysis, and geochronology of the End uranium deposit, Kiggavik, Nunavut, Canada (United States)

    Chi, Guoxiang; Haid, Taylor; Quirt, David; Fayek, Mostafa; Blamey, Nigel; Chu, Haixia


    The End deposit is one of several uranium deposits in the Kiggavik area near the Proterozoic Thelon Basin, which is geologically similar to the Athabasca Basin known for its unconformity-related uranium deposits. The mineralization occurs as uraninite and coffinite in quartz veins and wall rocks (psammopelitic gneisses) in the sub-Thelon basement and is associated with clay- and hematite-altered fault zones. Fluid inclusions were studied in quartz cementing unmineralized breccias formed before mineralization (Q2), quartz veins that were formed before mineralization but spatially associated with uranite (Q4), and calcite veins that were formed after mineralization. Four types of fluid inclusions were recognized, namely liquid-dominated biphase (liquid + vapor), vapor-dominated biphase (vapor + liquid), monophase (vapor-only), and triphase (liquid + vapor + halite) inclusions. The first three types were found in Q2, whereas all four types were found in Q4 and calcite. The coexistence of these different types of inclusions within individual fluid inclusion assemblages is interpreted to indicate fluid immiscibility and heterogeneous trapping. Based on microthermometry, the fluids associated with Q2 are characterized by low salinities (0.4 to 6.6 wt%) and moderate temperatures from 148 to 261 °C, and the fluids associated with calcite show high salinities (26.8 to 29.3 wt%) and relatively low temperatures from 146 to 205 °C, whereas the fluids associated with Q4 have a wide range of salinities from 0.7 to 38.8 wt% and temperatures from 80 to 332 °C. Microthermometric and cryogenic Raman spectroscopic studies indicate that the high-salinity fluids in Q4 and calcite belong to the H2O-NaCl-CaCl2 ± MgCl2 system, with some dominated by NaCl and others by CaCl2. The fluid inclusions in Q2 are interpreted to be unrelated to mineralization, whereas those in Q4 and calcite reflect the mineralizing fluids. The fluid inclusion data are consistent with a genetic link of mineralization with basinal brines derived from the Thelon Basin. However, unlike the conventional deep-burial (>5 km) diagenetic-hydrothermal model proposed for the unconformity-related uranium deposits, the uranium mineralization in the End deposit is inferred to have formed in a shallow environment (probably primary mineralization.

  3. Geologic and Geochronologic Studies of the Early Proterozoic Kanektok Metamorphic Complex of Southwestern Alaska (United States)

    Turner, Donald L.; Forbes, Robert B.; Aleinikoff, John N.; McDougall, Ian; Hedge, Carl E.; Wilson, Frederic H.; Layer, Paul W.; Hults, Chad P.


    The Kanektok complex of southwestern Alaska appears to be a rootless terrane of early Proterozoic sedimentary, volcanic, and intrusive rocks which were metamorphosed to amphibolite and granulite facies and later underwent a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism of overlying sediments. The terrane is structurally complex and exhibits characteristics generally attributed to mantled gneiss domes. U-Th-Pb analyses of zircon and sphene from a core zone granitic orthogneiss indicate that the orthogneiss protolith crystallized about 2.05 b.y. ago and that the protolithic sedimentary, volcanic and granitic intrusive rocks of the core zone were metamorphosed to granulite and amphibolite facies about 1.77 b.y. ago. A Rb-Sr study of 13 whole-rock samples also suggests metamorphism of an early Proterozoic [Paleoproterozoic] protolith at 1.77 Ga, although the data are scattered and difficult to interpret. Seventy-seven conventional 40K/40Ar mineral ages were determined for 58 rocks distributed throughout the outcrop area of the complex. Analysis of the K-Ar data indicate that nearly all of these ages have been totally or partially reset by a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism. Several biotites gave apparent K-Ar ages over 2 Ga. These ages appear to be controlled by excess radiogenic 40Ar produced by the degassing protolith during the 1.77 Ga metamorphism and incorporated by the biotites when they were at temperatures at which Ar could diffuse through the lattice. Five amphibolites yielded apparent Precambrian 40K/40Ar hornblende ages. There is no evidence that these hornblende ages have been increased by excess argon. The oldest 40K/40Ar hornblende age of 1.77 Ga is identical to the sphene 207Pb/206Pb orthogneiss age and to the Rb-Sr 'isochron' age for six of the 13 whole-rock samples. The younger hornblende ages are interpreted as having been partially reset during the late Mesozoic thermal event. 40Ar/39Ar incremental heating experiments suggest metamorphism occurred at least 1.2 b.y. ago but do not exhibit high temperature plateau ages significantly older than the 40Ar/39Ar total fusion ages of these samples. The age spectra are much more uniform than expected from a terrane with such a complex thermal history, perhaps caused by the small grain size of the samples which may possibly be less than the effective Ar diffusion radii of the analyzed hornblendes.

  4. Geochronological database and classification system for age uncertainties in Neotropical pollen records (United States)

    Flantua, S. G. A.; Blaauw, M.; Hooghiemstra, H.


    The newly updated inventory of palaeoecological research in Latin America offers an important overview of sites available for multi-proxy and multi-site purposes. From the collected literature supporting this inventory, we collected all available age model metadata to create a chronological database of 5116 control points (e.g. 14C, tephra, fission track, OSL, 210Pb) from 1097 pollen records. Based on this literature review, we present a summary of chronological dating and reporting in the Neotropics. Difficulties and recommendations for chronology reporting are discussed. Furthermore, for 234 pollen records in northwest South America, a classification system for age uncertainties is implemented based on chronologies generated with updated calibration curves. With these outcomes age models are produced for those sites without an existing chronology, alternative age models are provided for researchers interested in comparing the effects of different calibration curves and age-depth modelling software, and the importance of uncertainty assessments of chronologies is highlighted. Sample resolution and temporal uncertainty of ages are discussed for different time windows, focusing on events relevant for research on centennial- to millennial-scale climate variability. All age models and developed R scripts are publicly available through figshare, including a manual to use the scripts.

  5. Geochronology and paleoclimatic implications of the last deglaciation of the Mauna Kea Ice Cap, Hawaii (United States)

    Anslow, Faron S.; Clark, P.U.; Kurz, M.D.; Hostetler, S.W.


    We present new 3He surface exposure ages on moraines and bedrock near the summit of Mauna Kea, Hawaii, which refine the age of the Mauna Kea Ice Cap during the Local Last Glacial Maximum (LLGM) and identify a subsequent fluctuation of the ice margin. The 3He ages, when combined with those reported previously, indicate that the local ice-cap margin began to retreat from its LLGM extent at 20.5??2.5ka, in agreement with the age of deglaciation determined from LLGM moraines elsewhere in the tropics. The ice-cap margin receded to a position at least 3km upslope for ~4.5-5.0kyr before readvancing nearly to its LLGM extent. The timing of this readvance at ~15.4ka corresponds to a large reduction of the Atlantic meridional overturning circulation (AMOC) following Heinrich Event 1. Subsequent ice-margin retreat began at 14.6??1.9ka, corresponding to a rapid resumption of the AMOC and onset of the B??lling warm interval, with the ice cap melting rapidly to complete deglaciation. Additional 3He ages obtained from a flood deposit date the catastrophic outburst of a moraine-dammed lake roughly coeval with the Younger Dryas cold interval, suggesting a more active hydrological cycle on Mauna Kea at this time. A coupled mass balance and ice dynamics model is used to constrain the climate required to generate ice caps of LLGM and readvance sizes. The depression of the LLGM equilibrium line altitude requires atmospheric cooling of 4.5??1??C, whereas the mass balance modeling indicates an accompanying increase in precipitation of as much as three times that of present. We hypothesize (1) that the LLGM temperature depression was associated with global cooling, (2) that the temperature depression that contributed to the readvance occurred in response to an atmospheric teleconnection to the North Atlantic, and (3) that the precipitation enhancement associated with both events occurred in response to a southward shift in the position of the inter-tropical convergence zone (ITCZ). Such a shift in the ITCZ would have allowed midlatitude cyclones to reach Mauna Kea more frequently which would have increased precipitation at high elevations and caused additional cooling. ?? 2010 Elsevier B.V.

  6. Palaeomagnetism, petrology and geochronology of tertiary magmatic and sedimentary units from Iran (United States)

    Bina, Mohamad Mansour; Bucur, Ileana; Prevot, Michel; Meyerfeld, Yves; Daly, Lucien; Cantagrel, Jean Marie; Mergoil, Jean


    An amount of 160 samples from igneous and sedimentary formations of Palaeocene or Eocene age was collected in Iran, northeast of the Zagros belt. Palaeomagnetic analyses, petrological studies, and K-Ar dating indicate Palaeocene and Eocene palaeomagnetic directions in the Zabol-Baluch region (East Iranian Range) and show that the area remained approximately stable with respect to the Eurasian and African plates from the beginning of the Cainozoic Era. Conversely, large post-Eocene deformations occurred in Central Iran. These large rotations are both clockwise and anti-clockwise. They might have resulted from lateral shears between Arabia and northeast Iran. The Central Elburz underwent a large clockwise rotation which is probably late Oligocene in age. As a whole, our results show that the so-called "Persian Plate" did not exist during the Cainozoic Era.

  7. The evolution of the Cappadocia Geothermal Province, Anatolia (Turkey): geochemical and geochronological evidence (United States)

    Şener, M. Furkan; Şener, Mehmet; Uysal, I. Tonguç


    Cappadocia Geothermal Province (CGP), central Turkey, consists of nine individual geothermal regions controlled by active regional fault systems. This paper examines the age dating of alteration minerals and the geochemistry (trace elements and isotopes) of the alteration minerals and geothermal waters, to assess the evolution of CGP in relation to regional tectonics. Ar-Ar age data of jarosite and alunite show that the host rocks were exposed to oxidizing conditions near the Earth's surface at about 5.30 Ma. Based on the δ18O-δD relationhip, water samples had a high altitude meteoric origin. The δ34S values of jarosite and alunite indicate that water samples from the southern part of the study area reached the surface after circulation through volcanic rocks, while northern samples had traveled to the surface after interacting with evaporates at greater depths. REY (rare earth elements and yttrium) diagrams of alteration minerals (especially illite, jarosite and alunite) from rock samples, taken from the same locations as the water samples, display a similar REY pattern to water samples. This suggests that thermal fluids, which reached the surface along a fault zone and caused the mineral alteration in the past, had similar chemical composition to the current geothermal water. The geothermal conceptual model, which defines a volcanically heated reservoir and cap rocks, suggests there are no structural drawbacks to the use of the CGP geothermal system as a resource. However, fluid is insufficient to drive the geothermal system as a result of scanty supply of meteoric water due to evaporation significantly exceeding rainfall.

  8. Detrital zircon geochronology of the Cretaceous succession from the Iberian Atlantic Margin: palaeogeographic implications (United States)

    Dinis, Pedro A.; Dinis, Jorge; Tassinari, Colombo; Carter, Andy; Callapez, Pedro; Morais, Manuel


    Detrital zircon U-Pb data performed on eight Cretaceous sandstone samples (819 age isotopic results) from the Lusitanian basin (west Portugal) constrain the history of uplift and palaeodrainage of western Iberia following break-up of Pangaea and opening of the North Atlantic Ocean. We examined the links between shifts in provenance and known basinwide unconformities dated to the late Berriasian, Barremian, late Aptian and Cenomanian-Turonian. The detrital zircon record of sedimentary rocks with wider supplying areas is relatively homogenous, being characterized by a clear predominance of late Palaeozoic ages (c. 375-275 Ma) together with variable proportions of ages in the range c. 800-460 Ma. These two groups of ages are diagnostic of sources within the Variscan Iberian Massif. A few samples also reveal significant amounts of middle Palaeozoic (c. 420-385 Ma) and late Mesoproterozoic to early Neoproterozoic (c. 1.2-0.9 Ga) zircon, which are almost absent in the basement to the east of the Lusitanian basin, but are common in terranes with a Laurussia affinity found in NW Iberia and the conjugate margin (Newfoundland). The Barremian unconformity marks a sudden rise in the proportion of c. 375-275 Ma zircon ages accompanied by a decrease in the abundance of the c. 420-385 Ma and c. 1.2-0.9 Ga ages. This shift in the zircon signature, which is contemporaneous with the separation of the Galicia Bank from Flemish Cap, reflects increased denudation of Variscan crystalline rocks and a reduction in source material from NW Iberia and adjoining areas. The late Aptian unconformity, which represents the largest hiatus in the sedimentary record, is reflected by a shift in late Palaeozoic peak ages from c. 330-310 Ma (widespread in Iberia) to c. 310-290 Ma (more frequent in N Iberia). It is considered that this shift in the age spectra resulted from a westward migration of catchment areas following major uplift in northern Iberia and some transport southward from the Bay of Biscay under the influence of a well-established Atlantic circulation.

  9. Biostratigraphy and geochronology of the late Cenozoic of Córdoba Province (central Argentina) (United States)

    Cruz, Laura Edith


    In the last twenty years, several geological and stratigraphical studies have been undertaken in Córdoba Province, and they have provided useful bases for biostratigraphic work in the late Cenozoic. However, paleontological contributions have been limited to preliminary analyses of mammal assemblages, or specific discoveries. The aim of this work is to contribute to biostratigraphic knowledge of Argentina through the study of late Cenozoic mammals from Córdoba Province. Five localities have been analyzed: San Francisco, Miramar, Río Cuarto, Isla Verde, and Valle de Traslasierra. Through biostratigraphic analysis the first records of several taxa were established, and mammal assemblages with the description and correlation of the sedimentary strata were confirmed. Finally, three Assemblage Zones (Biozonas de Asociación) were proposed: 1) Neosclerocalyptus paskoensis-Equus (Amerhippus) assemblage zone with type area and profile based on the San Francisco locality, referred to the Lujanian (late Pleistocene-early Holocene), and comparable to the Equus (Amerhippus) neogeus Biozone of Buenos Aires Province; 2) Neosclerocalyptus ornatus-Catonyx tarijensis assemblage zone with type area and profile based on the San Francisco locality, referred to the Ensenadan (early Pleistocene) and comparable to the Mesotherium cristatum Biozone of Buenos Aires Province, and 3) Nonotherium hennigi-Propanochthus bullifer assemblage zone with type area and profile based on the Los Sauces river, Valle de Traslasierra, referred to the Montehermosan-Chapadmalalan interval (Pliocene), and comparable to the Trigodon gaudryi, Neocavia depressidens and/or Paraglyptodon chapadmalensis Biozones of Buenos Aires Province.

  10. Geochronology of anthropogenic radionuclides in Ribeira Bay sediments, Rio de Janeiro, Brazil

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    Carvalho Gomes, Franciane de [Programa de Engenharia Nuclear, COPPE/UFRJ, Cidade Universitaria, Caixa Postal 68.509, 21945-970 Rio de Janeiro (Brazil); Godoy, Jose Marcus, E-mail: [Instituto de Radioprotecao e Dosimetria (IRD), Caixa Postal 37750, Barra da Tijuca, 22642-970 Rio de Janeiro (Brazil); Departamento de Quimica, Pontificia Universidade Catolica do Rio de Janeiro (PUC-Rio), Rua Marques de Sao Vicente 225, 22453-900 Rio de Janeiro (Brazil); Godoy, Maria Luiza D.P.; Carvalho, Zenildo Lara de [Instituto de Radioprotecao e Dosimetria (IRD), Caixa Postal 37750, Barra da Tijuca, 22642-970 Rio de Janeiro (Brazil); Tadeu Lopes, Ricardo [Programa de Engenharia Nuclear, COPPE/UFRJ, Cidade Universitaria, Caixa Postal 68.509, 21945-970 Rio de Janeiro (Brazil); Sanchez-Cabeza, Joan-Albert; Osvath, Iolanda [IAEA Marine Environment Laboratories, 4 Quai Antoine Premier, MC98000 Principality of Monaco (Monaco); Drude de Lacerda, Luiz [Laboratorio de Ciencias do Mar, Universidade Federal do Ceara, Fortaleza, Ceara (Brazil)


    Ribeira Bay is located approximately 130 km south of the city of Rio de Janeiro and receives discharges of liquid effluent from the Angra dos Reis nuclear power plant (NPP) site, where two pressurized water reactors are located. To test whether the presence of anthropogenic radionuclides in sediments in Ribeira Bay could be correlated to the NPP operations, we sampled seven sediment cores and determined accumulation rates and chronologies. Only one sediment core did not exhibit a superficial mixing layer; this sample was used for dating purposes. Cesium-137 and {sup 207}Bi were observed in this sediment profile, but their presence was associated with atmospheric fall-out rather than the nearby NPP. The exponential decay of {sup 210}Pb concentration with sediment layer depth was verified below a superficial mixing layer for all other sediment cores. Calculated accumulation rates ranged from 1.2 mm y{sup -1} in the inner bay to 6.2 mm y{sup -1} close to its entrance. - Graphical abstract: Display Omitted Highlights: > We have dated seven sediment cores on the region close to the Angra dos Reis nuclear power plant site. > Cs-137 and Bi-207 were detected and were used as a tool to validate the Pb-210 dating results. > The spatial variation of the Pb-210 flux indicates two main sources of sediments: the channel between Ilha Grande (Grande Island), and the continental and soil runoff from the adjacent hills.

  11. Geochronology of anthropogenic radionuclides in Ribeira Bay sediments, Rio de Janeiro, Brazil. (United States)

    de Carvalho Gomes, Franciane; Godoy, José Marcus; Godoy, Maria Luiza D P; de Carvalho, Zenildo Lara; Lopes, Ricardo Tadeu; Sanchez-Cabeza, Joan-Albert; Osvath, Iolanda; de Lacerda, Luiz Drude


    Ribeira Bay is located approximately 130 km south of the city of Rio de Janeiro and receives discharges of liquid effluent from the Angra dos Reis nuclear power plant (NPP) site, where two pressurized water reactors are located. To test whether the presence of anthropogenic radionuclides in sediments in Ribeira Bay could be correlated to the NPP operations, we sampled seven sediment cores and determined accumulation rates and chronologies. Only one sediment core did not exhibit a superficial mixing layer; this sample was used for dating purposes. Cesium-137 and (207)Bi were observed in this sediment profile, but their presence was associated with atmospheric fall-out rather than the nearby NPP. The exponential decay of (210)Pb concentration with sediment layer depth was verified below a superficial mixing layer for all other sediment cores. Calculated accumulation rates ranged from 1.2 mm y(-1) in the inner bay to 6.2 mm y(-1) close to its entrance. Copyright © 2011 Elsevier Ltd. All rights reserved.

  12. Petrology, geochemistry, and geochronology of trondhjemites from the Qori Complex, Neyriz, Iran (United States)

    Fazlnia, Abdolnaser; Schenk, Volker; van der Straaten, François; Mirmohammadi, Mirsaleh


    Metamorphism, magmatism, and thrusting were the result of subduction of Neotethys beneath the continental-margin arc of the Sanandaj-Sirjan shear zone (SSSZ) during the Mesozoic. The Qori metamorphic complex is a part of the southern SSSZ. Leuco-granitic (trondhjemitic) rocks crop out in the Qori metamorphic complex and are rare rock types in the SSSZ. These rocks have intruded into the marbles and garnet amphibolites, the highest grade metamorphic rocks of the Qori metamorphic complex, and in some outcrops, a transitional boundary between the amphibolites and the granitoids can be distinguished. The granitoids are granular in texture and consist of plagioclase (albite-oligoclase), quartz ± K-feldspar ±muscovite and subordinate garnet, spinel, rutile, and apatite which primarily occur as inclusions in the main phases. The peraluminous trondhjemitic rocks are enriched in Na 2O and SiO 2 and depleted in FeO, MgO, and CaO. Similarities with some trondhjemitic liquids produced through partial melting of amphibolites or hydrous basalts (i.e., low-Al 2O 3 content, less than 15 wt.%; low Ba, Sr, TiO 2, and Eu content, all with negative anomalies; moderately enriched LREEs and Y, and flat HREE patterns) suggest that the evolution of the parental magma was controlled by residual plagioclases during partial melting of a garnet amphibolite source. Concentrations of ferromagnesian elements, Mg, Fe, and Mn, are low, suggesting that the granitic rocks were not produced by high degrees of partial melting. Furthermore, they display low amounts of ferromagnesian components from the protolith (garnet amphibolite). This is supported by consideration of compatible elements, especially Cr, Ni and Ti (and the less robust HREE), which respectively show very high and high bulk partition coefficients for relatively small degrees (SHRIMP ages of the trondhjemite show magmatic ages of 147.4 ± 0.76 Ma, Volgian, Late Jurassic, and suggest that Neotethys began to subduct beneath the continental-margin arc of the SSSZ. As a result of this process, an arc-related metamorphism occurred, leading to the development of the garnet amphibolites in the Qori metamorphic complex.

  13. Geochronology of Zircon from Modern Plutons Beneath Two Contrasting Arc Volcanoes (United States)

    Bacon, C. R.


    Ion microprobe (SHRIMP RG) 238U-230Th zircon dating documents recent crystallization of shallow plutons beneath two caldera volcanoes where magmas evolved mainly by crystallization differentiation. Mount Mazama, Oregon, is a medium-K calc-alkaline Cascade arc volcano whose 7.7-ka climactic eruption ejected granodiorite and related plutonic blocks during formation of Crater Lake caldera. Mount Veniaminof, Alaska, is a medium-K tholeiitic Aleutian arc volcano that ejected granodiorite, diorite, and gabbro blocks in its 3.7-ka caldera-enlarging eruption. Zircons in four granodiorite blocks from Mazama crystallized at various times between 20 ka and greater than 300 ka, with concentrations of model ages near 50-70, 110, and 200 ka that correspond to periods of dacitic volcanism dated by K-Ar (Bacon and Lowenstern, 2005, EPSL 233:277-293). Multiple-age zircon populations are common. The youngest zircon model ages in blocks from different locations around the caldera are similar to ages of nearby volcanic vents and may help map the distribution of intrusions within a composite pluton. Mazama zircons typically have many 10's to 100's of ppm U and Th, and grew relatively late in high- crystallinity magmas. U-Th model ages of zircon from a 27-ka rhyodacite, the only eruptive unit known with common zircon, are similar to those from granodiorite. Survival of these recycled crystals in zircon- undersaturated hydrous rhyodacitic magma suggests little time from entrainment to the 27-ka eruption. In contrast, the voluminous 7.7-ka climactic rhyodacite is virtually lacking in zircon, indicating dissolution of any granodioritic debris in the hot, vigorously growing silicic magma body during the intervening period. Veniaminof erupted basaltic through rhyodacitic magmas over the past 250 kyr. Gabbro, diorite, and miarolitic granodiorite blocks from Veniaminof represent cumulate mush and vapor-saturated residual melt segregations (Bacon, Sisson, and Mazdab, 2006, EOS 87:36:U41B-05). SHRIMP RG analyses of zircons define single populations with 238U-230Th isochron ages of 18.4 plus or minus 1.1 ka and 10.2 plus or minus 4.0 ka for granodiorite and diorite, respectively (2-sigma uncertainties). Sparse zircons from two gabbros give 238U- 230Th model ages of 36 plus or minus 10 ka and 25 plus or minus 10 ka. Zircons from granodiorite and diorite, commonly with spongy zones, contain up to 2 wt. pct. U and 5 wt. pct. Th, consistent with crystallization in the presence of late-magmatic aqueous fluid during passive degassing in the shallow subsurface. The zircon ages indicate that crystal cumulates and complementary differentiated melts were freezing beneath Veniaminof at shallow depth as recently as 10-20 kyr B.P. during incremental growth of an intrusive complex that from time-to-time released eruptible volumes of nearly aphyric differentiated melt. Zircons in Veniaminof plutonic blocks record brief unidirectional crystallization episodes that are sample- specific, whereas zircon populations in Mazama blocks include crystals that have been recycled multiple times.

  14. Improved laser ablation U-Pb zircon geochronology through robust downhole fractionation correction

    DEFF Research Database (Denmark)

    Paton, Chad


    models of elemental fractionation such as expo- nential curves and smoothed cubic splines can efficiently correct complex fractionation trends, allowing detection of spatial heterogeneities, while simultaneously maintaining data quality. We present a data reduction module for use with the Iolite software...

  15. Geochronology of the volcanic rocks in the Lu-Zong basin and its significance

    Institute of Scientific and Technical Information of China (English)

    David; COOKE; Sebastien; MEFFRE


    The Lu-Zong (Lujiang-Zongyang) basin is one of the most important volcanic basins in the middle and lower reaches of the Yangtze River area, China. It comprises four shoshonitic volcanic units, which are, in an ascending order, the Longmenyuan, Zhuanqiao, Shuangmiao and Fushan Groups. The LA-ICP MS U-Pb zircon ages of the four units are: 134.8±1.8 Ma for the Longmenyuan Group, 134.1±1.6 Ma for the Zhuanqiao Group, 130.5±0.8 Ma for the Shuangmiao Group, and 127.1±1.2 Ma for the Fushan Group. The results indicate that all volcanic rocks in the Lu-Zong basin were formed in the Early Cretaceous from about 135 Ma to 127 Ma, lasting 8-10 Ma. There were no Jurassic volcanic activities in all the volcanic basins including the Lu-Zong basin in the middle and lower reaches of the Yangtze River area. This work has provided new chronological results for the further study and understanding of the tec- tonic, magmatic and metallogenic processes of eastern China in the Mesozoic.

  16. K—Ar Geochronology and Evolution of Cenozoic Volcanic Rocks in Eastrn China

    Institute of Scientific and Technical Information of China (English)

    王慧芬; 杨学昌; 等


    Cenozoic volcanic rocks widespread in eastern China constitute an important part of the circum-Pacific volcanic belt.This paper presents more than 150K-Ar dates and a great deal of petrochemical analysis data from the Cenozoic volcanic rocks distributed in Tengchong,China's southeast coast,Shandong,Hebei,Nei Monggol and Northeast China.An integrated study shows that ubiquitous but uneven volcanic activities prevailed from the Eogene to the Holocene,characterized as being multi-eqisodic and multicycled.For example,in the Paleocene(67-58Ma),Eocene(57-37.5Ma),Miocene(22-18,16-19Ma),Pliocene(8-3Ma),and Early Pleistocene-Middle Pleistocene(1.2-0.5Ma) there were upsurges of volcanism,while in the Oligocene there was a repose period.In space,the older Eogene volcanic rocks are distributed within the region or in the central part of the NE-NNE-striking fault depression,while the younger Neogene and Quaternary volcanic rocks are distributed in the eastern and western parts.Petrologically,they belong essentially to tholeiite-series and alkali-series basalts,with alkalinity in the rocks increasing from old to youg.The above regularities are controlled by both global plate movement and regional inherent tectonic pattern.

  17. Tectonic events reflected by palaeocurrents, zircon