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Sample records for u-pb zircon geochronology

  1. Archean and proterozoic in the West-European Hercynian chain: isotopic geochemistry (Sr-Nd-Pb) and U-Pb geochronology on zircons

    International Nuclear Information System (INIS)

    Guerrot, C.

    1989-01-01

    The first part of this research thesis reports the study of isotopic (Sr-Nd-Pb) geochemistry and U-Pb geochronology on zircons in the immersed granulites of the Bay of Biscay: U-Pb geochronology on zircons, Nd isotopic geochemistry, Sr isotopic geochemistry, common Pb, Rb-Sr, Sm-Nd and rare earth data on minerals, comparison with other European granulites, comparison with West-Africa, study of Archean and proterozoic in the Hercynian chain. The second part reports the study of the U-Pb geochronology on zircon in the Cadomian, and the third part addresses the Sr-Nd isotopic geochemistry of some Cadomian granitoid, and the crust contamination in different regions [fr

  2. High-precision ID-TIMS zircon U-Pb geochronology using new 1013 Ohm resistors

    Science.gov (United States)

    Von Quadt, A.; Buret, Y.; Large, S.; Peytcheva, I.; Trinquier, A.; Wotzlaw, J. F.

    2015-12-01

    Faraday cups equipped with high gain amplifiers provide a means to measure small ion beams in static mode without the limited linear range of ion counting systems. We tested the application of newly available 1013 Ohm resistors to ID-TIMS zircon U-Pb geochronology using a range of natural and synthetic reference materials. The TritonPlus-RPQ at the Institute of Geochemistry and Petrology, ETH Zurich, is equipped with five new 1013 Ohm resistors and one MasCom secondary electron multiplier, allowing to measure the 202-204-205-206-207-208Pb masses in static mode. U is measured subsequently as U-oxide (265-267-270UO2) during a second step, also in static Faraday mode. The gain calibration of the 1013 Ohm resistors was performed using the procedure of Trinquier (2014), with 144Nd-146Nd being measured using 1011 Ohm resistor and 142-143-145-148-150Nd being measured using 1013 Ohm resitors (Trinquier, 2014; Koornneef et al., 2014). Standard deviations of the noise in all five new 1013 Ohm resistors are lower than 5.0 x 10-6 over a 6 month period, with no shift occurring over this time interval. This new detector set-up was tested by analyzing natural zircon standard materials and synthetic U/Pb solutions (www.earthime.org), ranging in age from ~2 Ma to ~600 Ma. All natural zircon standards were chemically abraded (Mattinson, 2005) and all samples were spiked with the ET2535 tracer solution. U-Pb dates obtained using the static measurement routine are compared to measurements employing dynamic peak jumping routines on the MasCom multiplier. This study illustrates the benefits and current limitations of using high gain amplifiers to measure small ion beams for zircon U-Pb geochronology compared to conventional dynamic ion counting techniques. Mattinson, J.M. (2005) Chemical Geology 220:47-66; Trinquier, A. (2014) Application Note 30281; Koornneef, J. et al (2014) Analytica Chimica Acta 819:49-55.

  3. Tectonic implications of U-Pb (zircon) Geochronology of Chor Granitoids of the Lesser Himalaya, Himachal Pradesh, NW Himalaya

    Science.gov (United States)

    Singh, P.; Bhakuni, S. S.

    2017-12-01

    Granitoids of various ages ranging from Proterozoic to Tertiary occur throughout the Himalayan fold-thrust belt. The occurrence of the Neoproterozoic granitoids are very less in the Himalayan orogen. One of the best example of Neoproterozoic granitoids is Chor granitoids, which are the intrusive granite bodies in the Paleoproterozoic of the Lesser Himalayan Crystallines of the Jutogh Group. In the central part these granites are non-foliated homogeneous that are porphyritic and peraluminous in nature (Singh et al., 2002; Bhargava et al., 2014, 2016), whereas in the peripheral part these are foliated showing south directed shear sense of movement. In this work we present the U-Pb (zircon) geochronology of two different granites samples of the Chor granitoids of Himachal Pradesh, NW Himalaya. The Jutogh Group of rocks is thrust over the Lesser Himalayan Sequence along the Jutogh Thrust or MCT. The geochronology of the Chor Granitoids and Lesser Himalayan Crystallines and their relationship with each other, including with the Indian shield are sparsely obscure. U-Pb zircon geochrnological age populations from these granitoids yield ages between 780 and 980 Ma. One sample gives the prominent age spectra for 206Pb/238U with weighted mean age of 908.3 ± 6.7 Ma (2σ) MSWD = 2.4 (n = 18). Similarly another sample gives the age of crystallization with weight mean age of 917 ± 17 Ma (2σ) MSWD = 3 (n = 11) and Th/U ratios of both samples are >0.1, indicating their magmatic origin. As a result of ductile shearing of granites along the MCT during the Cenozoic Himalayan Orogeny, the age has reduced to 780 Ma. The Neoproterozoic age of Chor granite matches with the Neoproterozoic detrital zircon age (800 to 1000 Ma by Parrish and Hodges 1996, Decelles et al., 2000) of the HHC. On the basis of U-Pb (zircon) geochronological ages, it is revealed that the source of zircons of the Chor granite and HHC rocks was the northern margin of the Pan-African orogen. The Chor granitoids was

  4. The lower Mesozoic record of detrital zircon U-Pb geochronology of Sonora, Mexico, and its paleogeographic implications

    OpenAIRE

    González-León, Carlos M.; Valencia, Víctor A.; Lawton, Timothy F.; Amato, Jeffrey M.; Gehrels, George E.; Leggett, William J.; Montijo-Contreras, Óscar; Fernández, Miguel A.

    2009-01-01

    Detrital zircon U-Pb geochronology from each of the formations of the Triassic-Lower Jurassic Barranca and El Antimonio groups of central and northwestern Sonora and from the Lower Jurassic Basomari and Middle Jurassic Lily formations of northern Sonora indicate they contain distinctive zircon populations. A Proterozoic population has peak ages near 1.8, 1.7, 1.6, 1.4, and 1.1 Ga. A population of Permo-Triassic grains with important peak ages near 269, 254, 245, 234 and 227 Ma. A third popula...

  5. U-Pb Dating of Zircons and Phosphates in Lunar Meteorites, Acapulcoites and Angrites

    Science.gov (United States)

    Zhou, Q.; Zeigler, R. A.; Yin, Q. Z.; Korotev, R. L.; Joliff, B. L.; Amelin, Y.; Marti, K.; Wu, F. Y.; Li, X. H.; Li, Q. L.; hide

    2012-01-01

    Zircon U-Pb geochronology has made a great contribution to the timing of magmatism in the early Solar System [1-3]. Ca phosphates are another group of common accessory minerals in meteorites with great potential for U-Pb geochronology. Compared to zircons, the lower closure temperatures of the U-Pb system for apatite and merrillite (the most common phosphates in achondrites) makes them susceptible to resetting during thermal metamorphism. The different closure temperatures of the U-Pb system for zircon and apatite provide us an opportunity to discover the evolutionary history of meteoritic parent bodies, such as the crystallization ages of magmatism, as well as later impact events and thermal metamorphism. We have developed techniques using the Cameca IMS-1280 ion microprobe to date both zircon and phosphate grains in meteorites. Here we report U-Pb dating results for zircons and phosphates from lunar meteorites Dhofar 1442 and SaU 169. To test and verify the reliability of the newly developed phosphate dating technique, two additional meteorites, Acapulco, obtained from Acapulco consortium, and angrite NWA 4590 were also selected for this study as both have precisely known phosphate U-Pb ages by TIMS [4,5]. Both meteorites are from very fast cooled parent bodies with no sign of resetting [4,5], satisfying a necessity for precise dating.

  6. U-Pb zircon geochronology and evolution of some Adirondack meta-igneous rocks

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    Mclelland, J. M.

    1988-01-01

    An update was presented of the recent U-Pb isotope geochronology and models for evolution of some of the meta-igneous rocks of the Adirondacks, New York. Uranium-lead zircon data from charnockites and mangerites and on baddeleyite from anorthosite suggest that the emplacement of these rocks into a stable crust took place in the range 1160 to 1130 Ma. Granulite facies metamorphism was approximately 1050 Ma as indicated by metamorphic zircon and sphene ages of the anorthosite and by development of magmatitic alaskitic gneiss. The concentric isotherms that are observed in this area are due to later doming. However, an older contact metamorphic aureole associated with anorthosite intrusion is observed where wollastonite develops in metacarbonates. Zenoliths found in the anorthosite indicate a metamorphic event prior to anorthosite emplacement. The most probable mechanism for anorthosite genesis is thought to be ponding of gabbroic magmas at the Moho. The emplacement of the anorogenic anorthosite-mangerite-charnockite suite was apparently bracketed by compressional orogenies.

  7. LA-ICP-MS and SIMS U-Pb and U-Th zircon geochronological data of Late Pleistocene lava domes of the Ciomadul Volcanic Dome Complex (Eastern Carpathians

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    Réka Lukács

    2018-06-01

    Full Text Available This article provides laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS and secondary ionization mass spectrometry (SIMS U-Pb and U-Th zircon dates for crystals separated from Late Pleistocene dacitic lava dome rocks of the Ciomadul Volcanic Dome Complex (Eastern Carpathians, Romania. The analyses were performed on unpolished zircon prism faces (termed rim analyses and on crystal interiors exposed through mechanical grinding an polishing (interior analyses. 206Pb/238U ages are corrected for Th-disequilibrium based on published and calculated distribution coefficients for U and Th using average whole-rock and individually analyzed zircon compositions. The data presented in this article were used for the Th-disequilibrium correction of (U-Th/He zircon geochronology data in the research article entitled “The onset of the volcanism in the Ciomadul Volcanic Dome Complex (Eastern Carpathians: eruption chronology and magma type variation” (Molnár et al., 2018 [1].

  8. U-Th-Pb systematics in zircon and titanite

    International Nuclear Information System (INIS)

    Zartman, R.E.; Kwak, L.M.

    1990-01-01

    U-Th-Pb isotopic analyses of zircon and titanite were made for two core samples of granite from borehole ATK-1 drilled into the Eye-Dashwa Lakes pluton. One of the samples from near the bottom of the hole (990.97 to 996.78 m) yielded zircon and titanite that were slightly to severely disturbed isotopically. Eight fractions of zircon give an upper concordia intercept age of 2625 ± 16 Ma (MSWD = 34), which, based on an evaluation of the more concordant data points and on other geochronological results, is interpreted as being slightly too young. The time of crystallization is probably better approximated by the 207 Pb/ 206 Pb age of 2665 Ma determined on a slightly (∼8 percent) discordant titanite. The other sample from near the surface (3.85 to 9.61 m) generally revealed even more severely disturbed isotopic systematics for both zircon and titanite. The complex nature of the disturbances probably resulted from the penetration of meteoritic water into rock already modified by post-crystallization hydrothermal alteration. Nuclide migration occurred in both minerals -- during the Middle or Late Proterozoic for the zircon and during the modern weathering cycle for the titanite. Material balance calculations are used to demonstrate a recent relative gain of radiogenic Pb and/or loss of Th and U from the freshest-looking, least-altered titanite by exchange with altered, leucoxenite-bearing titanite

  9. U-Pb zircon geochronology in the western part of the Rayner Complex, East Antarctica

    International Nuclear Information System (INIS)

    Horie, Kenji; Hokada, Tomokazu; Motoyoshi, Yoichi; Shiraishi, Kazuyuki; Takehara, Mami; Hiroi, Yoshikuni

    2016-01-01

    U-Pb zircon geochronology was applied to nine metasedimentary samples collected from Mt. Yuzhnaya, Condon Hills, and Mt. Lira in the inland region of the Rayner Complex of western Enderby Land, East Antarctica, in order to define the eastern limits of the western Rayner Complex that underwent the Pan-African metamorphism and to evaluate potential source areas of metasedimentary rocks. Condon Hills and Mt. Lira revealed metamorphic ages of ∼ 894 and ∼ 934 Ma, respectively, which are consistent with previously reported metamorphism in association with Rayner Structural Episode (RSE). Mt. Yuzhnaya samples affected by the RSE contain zircon grains rejuvenated during 590-570 Ma, which indicates that the Pan-African reworking can be extended up to Mt. Yuzhnaya. On the other hand, the Condon Hills samples include Archean detritus, and the age peaks from 3850 to 2491 Ma are the oldest components in the Rayner Complex of western Enderby Land. There is no evidence of reworked Napier Complex rocks in the studied Rayner samples. (author)

  10. Detrital zircon U-Pb geochronological and sedimentological study of the Simao Basin, Yunnan: Implications for the Early Cenozoic evolution of the Red River

    Science.gov (United States)

    Chen, Yi; Yan, Maodu; Fang, Xiaomin; Song, Chunhui; Zhang, Weilin; Zan, Jinbo; Zhang, Zhiguo; Li, Bingshuai; Yang, Yongpeng; Zhang, Dawen

    2017-10-01

    The paleo-Red River is suggested to have been a continental-scale drainage system connecting the Tibetan Plateau to the South China Sea. However, the evolution of the paleo-Red River is still under debate. This study presents new results from sedimentological analyses and detrital zircon U-Pb geochronologic data from fluvial sedimentary rocks of Paleocene to Oligocene age of the Simao Basin to constrain the nature of the paleo-drainage system of the Red River. The detrital zircon U-Pb results reveal multiple age groups at 190-240 Ma, 260-280 Ma, 450-540 Ma, 1700-1900 Ma and 2400-2600 Ma for the Paleocene to late Eocene Denghei Formation (Fm.), but only one conspicuous peak at 220-240 Ma for the late Eocene-Oligocene Mengla Fm. Provenance analyses illustrate that the former likely had source areas that included the Hoh-Xil, Songpan-Ganzi, northern Qiangtang, Yidun and western Yangtze Terranes, which are consistent with the catchments of the Upper and Lower Jinshajiang Segments, whereas the latter mainly transported material from a limited number of sources, such as the Lincang granitic intrusions west of the Simao Basin. Integrated with available detrital zircon U-Pb geochronologic and paleogeographic data, our study suggests the existence of a paleo-Red River during the Paleocene to late Eocene that was truncated and lost its northern sources after approximately 35 Ma, due to left-lateral strike-slip faulting of the Ailao Shan-Red River and clockwise rotation of the Lanping-Simao Terrane.

  11. U-Pb geochronology of zircons form lunar Breccia 73217 using a sensitive high mass-resolution ion microprobe

    International Nuclear Information System (INIS)

    Compston, W.; Williams, I.S.

    1984-01-01

    U-Pb age determinations on four lunar zircons from existing thin-sections of one highland breccia, 73217, using the recently constructed ion microprobe SHRIMP, are reported. The analytical reproducibility of SHRIMP is demonstrated, and procedures for measuring Pb/U, Th/U, and corecting for initial Pb are explained. Electron microprobe analyses for the zircons are also reported. The results show that the four zircons survived the lunar cataclysm without any identifiable effects on their U-Pb systematics. All four indicate a single age of 4356 +23 or -14 m.y. The zircons have experienced small variable amounts of Pb loss since crystallization, from almost zero up to about 10 percent. If this occurred during one later event, then age of the latter is between 1100 and 2300 m.y. 18 references

  12. Zircon U-Pb geochronology and geochemistry of granites in the Zhuguangshan complex, South China: Implications for uranium mineralization

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    Zhang, Long; Chen, Zhenyu; Li, Xiaofeng; Li, Shengrong; Santosh, M.; Huang, Guolong

    2018-05-01

    The Zhuguangshan complex, composed of Caledonian, Indosinian, and Yanshanian granites, and Cretaceous mafic dykes, is one of the most important granite-hosted uranium producers in South China. Here we present LA-ICP-MS zircon U-Pb and hornblende 40Ar/39Ar geochronology and whole-rock and biotite geochemistry for the granites in this complex to evaluate the magmatism and its constraints on uranium mineralization. Samples collected from the Fuxi, Youdong, Longhuashan, Chikeng, Qiling, and Sanjiangkou intrusions yield zircon weighted 206Pb/238U ages of 426.7 ± 5.4 Ma, 226.4 ± 3.5 Ma, 225.0 ± 2.7 Ma, 152.2 ± 3.0 Ma, 153.9 ± 2.1 Ma, and 155.2 ± 2.1 Ma, respectively. A new Ar-Ar dating of the hornblende of the diabase from the Changjiang uranium ore field yields a plateau age of 145.1 ± 1.5 Ma. These results coupled with published geochronological data indicate that six major magmatic events occurred in the study area at 420-435 Ma, 225-240 Ma, 150-165 Ma, 140 Ma, 105 Ma, and 90 Ma. Both U-bearing and barren granites occur in this complex, and they display differences in whole-rock and biotite geochemistry. The barren granites show higher Al2O3, CaO, TFMM, Rb, Zr, Ba, SI, Mg#, (La/Yb)N, and Eu/Eu*, but lower SiO2, ALK, Rb, DI, Rb/Sr, and TiO2/MgO than those of the U-bearing granites. Biotites in the U-bearing granites are close to the Fe-rich siderophyllite-annite end member with Fe/(Fe + Mg) ratios higher than 0.66, whereas those in the barren granites are relatively close to the Mg-rich eastonite-phlogopite end member with Fe/(Fe + Mg) ratios uranium ore potential of the granites in the Zhuguangshan complex. The geochemical variations of U-bearing and barren granites can serve as a potential detector for granite-hosted uranium deposits.

  13. Magmatic tempo of Earth's youngest exposed plutons as revealed by detrital zircon U-Pb geochronology.

    Science.gov (United States)

    Ito, Hisatoshi; Spencer, Christopher J; Danišík, Martin; Hoiland, Carl W

    2017-09-29

    Plutons are formed by protracted crystallization of magma bodies several kilometers deep within the crust. The temporal frequency (i.e. episodicity or 'tempo') of pluton formation is often poorly constrained as timescales of pluton formation are largely variable and may be difficult to resolve by traditional dating methods. The Hida Mountain Range of central Japan hosts the youngest exposed plutons on Earth and provides a unique opportunity to assess the temporal and spatial characteristics of pluton emplacement at high temporal resolution. Here we apply U-Pb geochronology to zircon from the Quaternary Kurobegawa Granite and Takidani Granodiorite in the Hida Mountain Range, and from modern river sediments whose fluvial catchments include these plutons in order to reconstruct their formation. The U-Pb data demonstrate that the Kurobegawa pluton experienced two magmatic pulses at ~2.3 Ma and ~0.9 Ma; whereas, to the south, the Takidani pluton experienced only one magmatic pulse at ~1.6 Ma. These data imply that each of these magmatic systems were both spatially and temporally distinct. The apparent ~0.7 Myr age gap between each of the three magmatic pulses potentially constrains the recharge duration of a single pluton within a larger arc plutonic complex.

  14. Magmatic Longevity Constrained by ID-TIMS U-Pb Dating of Zircon and Titanite

    Science.gov (United States)

    Szymanowski, D.; Wotzlaw, J. F.; Ellis, B. S.; Bachmann, O.; Von Quadt, A.

    2016-12-01

    Clues about the timescales and thermal conditions associated with the growth and evacuation of large silicic magma reservoirs are frequently drawn from radiometric dating, diffusion modelling, or thermomechanical modelling. A growing amount of petrological and geochronological evidence, supported by thermal modelling, suggests that many silicic magma reservoirs may exist for some 104-106 years in the form of high-crystallinity mushes at relatively low temperatures ( 700-750°C; [1-3]). Geochronological studies addressing this issue typically utilise the U-Pb system in zircon capable of recording extended periods of crystallisation, particularly in evolved calc-alkaline systems that spend most of their lifetime zircon-saturated. In this study, we integrate U-Pb dating of zircon and titanite to investigate the longevity of the magma reservoir that produced the Kneeling Nun Tuff, a 35 Ma, >900 km3 crystal-rich rhyolitic super-eruption from the Mogollon-Datil volcanic field in New Mexico (USA). High-precision ID-TIMS U-Pb dates of single crystals of both zircon and titanite independently record a continuous crystallisation history over >400,000 years. We combine the dating of both accessory phases with textural, major, trace element and isotopic studies of single crystals, placing tight constraints on the thermal conditions of magma accumulation and storage while recording differentiation and rejuvenation processes within the magma reservoir. The results suggest a protracted `cool' upper-crustal storage of magma prior to the Kneeling Nun Tuff eruption followed by a melting event which reduced the magma crystallinity and conditioned it for eruption. [1] Bachmann & Bergantz (2004), J. Petrol. 45, 1565-1582. [2] Gelman et al. (2013), Geology 41, 759-762. [3] Cooper & Kent (2014), Nature 506, 480-483.

  15. Zircon U-Pb ages of Guyana greenstone-gneiss terrane

    Energy Technology Data Exchange (ETDEWEB)

    Gibbs, A.K. (Cornell Univ., Ithaca, NY (USA)); Olszewski, W.J. Jr. (New Hampshire Univ., Durham (USA))

    1982-04-01

    Isotopic U-Pb studies of zircons collected from weathered metagreywackes of the Barama-Mazaruni Supergroup of northern Guyana, South America, demonstrate an age of origin of ca. 2250 Ma. This is the best estimate for the age of the associated metavolcanic rocks. Zircons from weathered gneiss of the Bartica complex, adjacent to the volcanic-sedimentary belts, yield a similar age. The contiguous greenstone-gneiss terrane of eastern Venezuela is also of similar age and comparable greenstone-gneiss terranes of eastern Suriname and French Guiana are probably also of this age. Continental crust formation of a style closely comparable to that of the Canadian Archean occurred on a very widespread scale in the Lower Proterozoic of the Guiana shield. The lead losses from the weathered zircons are comparable to those from zircons from fresh rock from the adjacent terrane of Venezuela, and the advantages of field concentration from numerous saprolite exposures warrant use of such material in future geochronological studies of the region.

  16. Geochemistry and zircon U-Pb geochronology of the Pulang complex, Yunnan province, China

    International Nuclear Information System (INIS)

    Pang, Zhenshan; Du, Yangsong; Cao, Yi; Gao, Fuping; Wang, Gongwen; Dong, Qian

    2014-01-01

    The Pulang complex is located tectonically at the southern margin of the Yidun-Zhongdian island arc belt in Yunnan province, China, and is closely related to formation of the Pulang copper deposit, which is the largest copper deposit in Asia. The Pulang complex can be divided into three intrusion stages based on contact relationships and petrological characteristics: (1) a first stage of quartz dioritic porphyry; (2) a second stage of quartz monzonitic porphyry; and (3) a third stage of granodioritic porphyry. The crystallization ages of these intrusion stages were determined by single-zircon U-Pb dating, yielding ages of 221.0 ± 1.0, 211.8 ± 0.5, and 206.3 ± 0.7 Ma for the first, second, and third stages, respectively. These dates, integrated with previous geochronological data and field investigations, indicate that the second-stage quartz monzonitic porphyry has a close spatial and temporal relationship with the large Pulang porphyry copper deposit. These age data, geochemical and Sr-Nd isotopic results suggest that the Pulang complex formed in the Indo-Chinese epoch (257∼ 205 Ma) by multiphase intrusion of a mixture of mantle- and crust-derived magmas. (author)

  17. New zircon (U-Th)/He and U/Pb eruption age for the Rockland tephra, western USA

    Science.gov (United States)

    Coble, Matthew A.; Burgess, Seth D.; Klemetti, Erik W.

    2017-09-01

    Eruption ages of a number of prominent Quaternary volcanic deposits remain inaccurately and/or imprecisely constrained, despite their importance as regional stratigraphic markers in paleo-environment reconstruction and as evidence of climate-altering eruptions. Accurately dating volcanic deposits presents challenging analytical considerations, including poor radiogenic yield, scarcity of datable minerals, and contamination of crystal populations by magma, eruption, and transport processes. One prominent example is the Rockland tephra, which erupted from the Lassen Volcanic Center in the southern Cascade arc. Despite a range in published eruption ages from 0.40 to 0.63 Ma, the Rockland tephra is extensively used as a marker bed across the western United States. To more accurately and precisely constrain the age of the Rockland tephra-producing eruption, we report U/Pb crystallization dates from the outermost ∼2 μm of zircon crystal faces (surfaces) using secondary ion mass spectrometry (SIMS). Our new weighted mean 238U/206Pb age for Rockland tephra zircon surfaces is 0.598 ± 0.013 Ma (2σ) and MSWD = 1.11 (mean square weighted deviation). As an independent test of the accuracy of this age, we obtained new (U-Th)/He dates from individual zircon grains from the Rockland tephra, which yielded a weighted mean age of 0.599 ± 0.012 Ma (2σ, MSWD = 5.13). We also obtained a (U-Th)/He age of 0.628 ± 0.014 Ma (MSWD = 1.19) for the Lava Creek Tuff member B, which was analyzed as a secondary standard to test the accuracy of the (U-Th)/He technique for Quaternary tephras, and to evaluate assumptions made in the model-age calculation. Concordance of new U/Pb and (U-Th)/He zircon ages reinforces the accuracy of our preferred Rockland tephra eruption age, and confirms that zircon surface dates sample zircon growth up to the time of eruption. We demonstrate the broad applicability of coupled U/Pb zircon-surface and single-grain zircon (U-Th)/He geochronology to accurate

  18. Chemical abrasion-SIMS (CA-SIMS) U-Pb dating of zircon from the late Eocene Caetano caldera, Nevada

    Science.gov (United States)

    Watts, Kathryn E.; Coble, Matthew A.; Vazquez, Jorge A.; Henry, Christopher D.; Colgan, Joseph P.; John, David A.

    2016-01-01

    Zircon geochronology is a critical tool for establishing geologic ages and time scales of processes in the Earth's crust. However, for zircons compromised by open system behavior, achieving robust dates can be difficult. Chemical abrasion (CA) is a routine step prior to thermal ionization mass spectrometry (TIMS) dating of zircon to remove radiation-damaged parts of grains that may have experienced open system behavior and loss of radiogenic Pb. While this technique has been shown to improve the accuracy and precision of TIMS dating, its application to high-spatial resolution dating methods, such as secondary ion mass spectrometry (SIMS), is relatively uncommon. In our efforts to U-Pb date zircons from the late Eocene Caetano caldera by SIMS (SHRIMP-RG: sensitive high resolution ion microprobe, reverse geometry), some grains yielded anomalously young U-Pb ages that implicated Pb-loss and motivated us to investigate with a comparative CA and non-CA dating study. We present CA and non-CA 206Pb/238U ages and trace elements determined by SHRIMP-RG for zircons from three Caetano samples (Caetano Tuff, Redrock Canyon porphyry, and a silicic ring-fracture intrusion) and for R33 and TEMORA-2 reference zircons. We find that non-CA Caetano zircons have weighted mean or bimodal U-Pb ages that are 2–4% younger than CA zircons for the same samples. CA Caetano zircons have mean U-Pb ages that are 0.4–0.6 Myr older than the 40Ar/39Ar sanidine eruption age (34.00 ± 0.03 Ma; error-weighted mean, 2σ), whereas non-CA zircons have ages that are 0.7–1.3 Myr younger. U-Pb ages do not correlate with U (~ 100–800 ppm), Th (~ 50–300 ppm) or any other measured zircon trace elements (Y, Hf, REE), and CA and non-CA Caetano zircons define identical trace element ranges. No statistically significant difference in U-Pb age is observed for CA versus non-CA R33 or TEMORA-2 zircons. Optical profiler measurements of ion microprobe pits demonstrate consistent depths of ~ 1.6

  19. Late cenozoic magmatism in the South Patagonian batholith: SHRIMP U-Pb zircon age evidence

    International Nuclear Information System (INIS)

    Fanning, C.M; Herve, F; Pankhurst, R.J; Thomson, S; Faundez, V

    2001-01-01

    The North Patagonian Batholith (NPB) has a zonal age pattern which includes a well defined belt of Miocene and Mio-Pliocene plutons in its central portion (Pankhurst et al., 1999) which are spatially, and probably genetically related to the Liquine-Ofqui Fault Zone. Previous geochronological studies in the Southern Patagonian Batholith (SPB), as summarized by Bruce et al. (1991), have yielded 9 late Cenozoic K-Ar or Ar-Ar ages out of a total of 116 age determinations. None of these young ages correspond to U-Pb determinations on zircons, and some of the young ages correspond to satellite plutons east of the SPB proper, such as the Torres del Paine intrusion. In this paper we present the first late Cenozoic SHRIMP U-Pb zircon ages in the area of the SPB. The morphology of the analysed zircon crystals is described and leads to some inferences on the methodology and on the geological interpretation of the obtained ages (au)

  20. U-Pb ages of detrital zircon from Pegasus Group, Stewart Island

    International Nuclear Information System (INIS)

    Walker, N.; Tulloch, A.J.; Allibone, A.

    1998-01-01

    Stewart Island (New Zealand) is relatively unaffected by Cenozoic deformation related to the present-day plate boundary and has thus been a locale of investigations that focus on the relationship between Mesozoic continental margin magmatic rocks and the Western Province (WP) of NZ. Pegasus Group metasedimentary rocks represent the only candidate for WP equivalents on Stewart Island. We measured U-Pb and 207 Pb / 206 Pb ages of detrital zircons from the Pegasus Group in an effort to validate the correlation with the WP. On SI, Pegasus Group is exposed over a small area in the southern Tin Range where it forms amphibolite facies roof pendants and screens among plutons of mid Paleozoic to mid Cretaceous age. Mica schists predominate, with lesser amounts of psammite and metaquartzite and rare calcareous schist and amphibolite. Zircons were extracted from metaquarzite and 24 grains were chosen on the basis of morphology for geochronologic investigation. Sixteen single crystals were dated by U-Pb TIMS; 8 additional grains were studied by the total evaporation method described by Kober. Despite vigorous air abrasion prior to processing, the 16 grains measured by conventional U-Pb TIMS yielded slightly normally discordant ages that we interpret to reflect minor Pb-loss. The 8 grains studied by the total evaporation method yielded more than four analytically identical /sup 207 Pb / 206 Pb ages within each grain at progressively higher temperatures (1515-1580 degrees C) of evaporation. The results permit the following general conclusions: 1) the age range of 420-2700 Ma is grossly similar to that reported from the Greenland Group/Victoria Paragneiss (Ireland 1992) of the Buller Terrane, WP; 2) the zircon population is dominantly 530-680 Ma but a subsidiary population is 900-1100 Ma; 3) no zircons with ages that match the timing of the Ross Orogen (530-480 Ma) were identified; 4) two grains of 420 Ma and a 453 Ma are probably too young for the paleontogically

  1. Calibrating Late Cretaceous Terrestrial Cyclostratigraphy with High-precision U-Pb Zircon Geochronology: Qingshankou Formation of the Songliao Basin, China

    Science.gov (United States)

    Wang, T.; Ramezani, J.; Wang, C.

    2015-12-01

    A continuous succession of Late Cretaceous lacustrine strata has been recovered from the SK-I south (SK-Is) and SKI north (SK-In) boreholes in the long-lived Cretaceous Songliao Basin in Northeast China. Establishing a high-resolution chronostratigraphic framework is a prerequisite for integrating the Songliao record with the global marine Cretaceous. We present high-precision U-Pb zircon geochronology by the chemical abrasion isotope dilution thermal-ionization mass spectrometry method from multiple bentonite core samples from the Late Cretaceous Qingshankou Formation in order to assess the astrochronological model for the Songliao Basin cyclostratigraphy. Our results from the SK-Is core present major improvements in precision and accuracy over the previously published geochronology and allow a cycle-level calibration of the cyclostratigraphy. The resulting choronostratigraphy suggest a good first-order agreement between the radioisotope geochronology and the established astrochronological time scale over the corresponding interval. The dated bentonite beds near the 1780 m depth straddle a prominent oil shale layer of the Qingshankou Formation, which records a basin-wide lake anoxic event (LAE1), providing a direct age constraint for the LAE1. The latter appears to coincide in time with the Late Cretaceous (Turonian) global sea level change event Tu4 presently constrained at 91.8 Ma.

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

    Directory of Open Access Journals (Sweden)

    Hartmann Léo A.

    2003-01-01

    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.

  3. U-Pb ages in meta-rhyolite zircon from Arai Group and associated granites

    International Nuclear Information System (INIS)

    Pimentel, M.M.; Fuck, R.A.

    1991-01-01

    Geochronology results by V-Pb method in zircon of granite rocks from staniferous province of Goias and rhyolite Arai groups are revealed. Two distinct episode of acid magmatism in Paleo-and Meso proterozoic are presented. V-Pb data in zircon from Sucuri and Soledade granites of the province from Parana River with ages of higher intercepted are also defined. This ages are consider mistakes, similar the V-Pb age of a rhyolite from Arai group. (author)

  4. 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

    2016-07-01

    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

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

    Science.gov (United States)

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

    2005-01-01

    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.

  6. Time-scale calibration by U-Pb geochronology: Examples from the Triassic Period

    Science.gov (United States)

    Mundil, R.

    2009-05-01

    U-Pb zircon geochronology, pioneered by Tom Krogh, is a cornerstone for the calibration of the time scale. Before Krogh's innovations, U-Pb geochronology was essentially limited by laboratory blank Pb (typically hundreds of nanograms) inherent in the then existing zircon dissolution and purification methods. The introduction of high pressure HF dissolution combined with miniature ion exchange columns (1) reduced the blank by orders of magnitude and allowed mass-spectrometric analyses of minute amounts of material (picograms of Pb and U). Krogh also recognized the need for minimizing the effects of Pb loss, and the introduction of the air-abrasion technique was the method of choice for two decades (2), until the development of the combined annealing and chemical abrasion technique resulted in essentially closed system zircons (3). These are the prerequisite for obtaining precise (permil-level) and accurate radio-isotopic ages of individual zircons contained in primary volcanic ash deposits, which are primary targets for the calibration of the time scale if they occur within fossil bearing sediments. A prime example is the calibration of the Triassic time scale which improved significantly using these techniques. The ages for the base and the top of the Triassic are constrained by U-Pb ages to 252.3 (4) and 201.5 Ma (5), respectively. These dates also constrain the ages of major extinction events at the Permian-Triassic and Triassic-Jurassic boundaries, and are statistically indistinguishable from ages obtained for the Siberian Traps and volcanic products from the Central Atlantic Magmatic Province, respectively, suggesting a causal link. Ages for these continental volcanics, however, are mostly from the K-Ar (40Ar/39Ar) system which requires accounting and correcting for a systematic bias of ca 1 % between U-Pb and 40Ar/39Ar isotopic ages (the 40Ar/39Ar ages being younger) (6). Robust U-Pb age constraints also exist for the Induan- Olenekian boundary (251.2 Ma, (7

  7. Petrologic and zircon U-Pb geochronological characteristics of the pelitic granulites from the Badu Complex of the Cathaysia Block, South China

    Science.gov (United States)

    Zhao, Lei; Zhou, Xiwen; Zhai, Mingguo; Liu, Bo; Cui, Xiahong

    2018-06-01

    The recognition of the Indosinian Orogeny in the South China block has been controversial and difficult because of strong weathering and thick cover. High temperature (HT) and high pressure (HP) metamorphic rocks related to this orogeny were considered to be absent from this orogenic belt until the recent discovery of eclogite and granulite facies meta-igneous rocks, occurring as lenses within the meta-sedimentary rocks of the Badu Complex. However, metamorphic state of these meta-sedimentary rocks is still not clear. Besides, there have been no geochronological data of HT pelitic granulites previously reported from the Badu Complex. This paper presents petrographic characteristics and zircon geochronological results on the newly discovered kyanite garnet gneiss, pyroxene garnet gneiss and the HT pelitic granulites (sillimanite garnet gneiss). Mineral assemblages are garnet + sillimanite + ternary feldspar + plagioclase + quartz + biotite for the HT pelitic granulite, kyanite + ternary feldspar + garnet + sillimanite + plagioclase + quartz + biotite for the kyanite garnet gneiss, and garnet + biotite + pyroxene + plagioclase + ternary feldspar + quartz for the pyroxene garnet gneiss, respectively. Decompressional coronas around garnet grains can be observed in all these pelitic rocks. Typical granulite facies mineral assemblages and reaction textures suggest that these rocks experienced HP granulite facies metamorphism and overprinted decompression along a clockwise P-T loop. Results from integrated U-Pb dating and REE analysis indicate the growth of metamorphic zircons from depleted heavy REE sources (100-50 chondrite) compared with detrital zircons derived from granitic sources (typically > 1000 chondrite). Metamorphic zircons in HP granulite exhibit no or subdued negative Eu anomalies, which perhaps indicate zircon overgrowth under eclogite facies conditions. The zircon overgrowth ages range from 250 to 235 Ma, suggesting that HP granulite (eclogite) to

  8. Provenance analysis of the Late Triassic Yichuan Basin: constraints from zircon U-Pb geochronology

    Directory of Open Access Journals (Sweden)

    Meng Xianghong

    2018-03-01

    Full Text Available Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS U-Pb dating has been performed on detrital zircons from the Chunshuyao Formation sandstone of Yichuan Basin. The ages of 85 detrital zircon grains are divided into three groups: 252-290 Ma, 1740-2000 Ma, and 2400-2600 Ma. The lack of Early Paleozoic and Neoproterozoic U-Pb ages indicates that there is no input from the Qinling Orogen, because the Qinling Orogen is characterized by Paleozoic and Neoproterozoic material. In combination with previous research, we suggest that the source of the Chunshuyao Formation is most likely recycled from previous sedimentary rocks from the North China Craton. In the Late Triassic, the Funiu ancient land was uplifted which prevented source material from the Qinling Orogen. Owing to the Indosinian orogeny, the strata to the east of the North China Craton were uplifted and eroded. The Yichuan Basin received detrital material from the North China Craton.

  9. Laser Ablation in situ (U-Th-Sm)/He and U-Pb Double-Dating of Apatite and Zircon: Techniques and Applications

    Science.gov (United States)

    McInnes, B.; Danišík, M.; Evans, N.; McDonald, B.; Becker, T.; Vermeesch, P.

    2015-12-01

    We present a new laser-based technique for rapid, quantitative and automated in situ microanalysis of U, Th, Sm, Pb and He for applications in geochronology, thermochronometry and geochemistry (Evans et al., 2015). This novel capability permits a detailed interrogation of the time-temperature history of rocks containing apatite, zircon and other accessory phases by providing both (U-Th-Sm)/He and U-Pb ages (+trace element analysis) on single crystals. In situ laser microanalysis offers several advantages over conventional bulk crystal methods in terms of safety, cost, productivity and spatial resolution. We developed and integrated a suite of analytical instruments including a 193 nm ArF excimer laser system (RESOlution M-50A-LR), a quadrupole ICP-MS (Agilent 7700s), an Alphachron helium mass spectrometry system and swappable flow-through and ultra-high vacuum analytical chambers. The analytical protocols include the following steps: mounting/polishing in PFA Teflon using methods similar to those adopted for fission track etching; laser He extraction and analysis using a 2 s ablation at 5 Hz and 2-3 J/cm2fluence; He pit volume measurement using atomic force microscopy, and U-Th-Sm-Pb (plus optional trace element) analysis using traditional laser ablation methods. The major analytical challenges for apatite include the low U, Th and He contents relative to zircon and the elevated common Pb content. On the other hand, apatite typically has less extreme and less complex zoning of parent isotopes (primarily U and Th). A freeware application has been developed for determining (U-Th-Sm)/He ages from the raw analytical data and Iolite software was used for U-Pb age and trace element determination. In situ double-dating has successfully replicated conventional U-Pb and (U-Th)/He age variations in xenocrystic zircon from the diamondiferous Ellendale lamproite pipe, Western Australia and increased zircon analytical throughput by a factor of 50 over conventional methods

  10. Morphology and U-Pb ages of zircons from the high-grade metamorphic Precambrian in the Sirdal-Oersdal area, SW Norway

    International Nuclear Information System (INIS)

    Wielens, J.B.W.

    1979-01-01

    The U-Pb systematics of seven suites of zircons and two apatites were investigated in an attempt to obtain data about the pre-Sveconorwegian geochronology. Very little was known about this earlier history, as previous studies have shown that the high-grade metamorphism has obliterated most of the older Rb-Sr record. Three samples were investigated from the augengneisses, two from the pyroxene syenites, one garnetiferous migmatite and one magnetite-ilmenite-zircon veinlet cross-cutting the charnockitic migmatites. (Auth.)

  11. Paleozoic age of high-pressure metamorphic rocks of the Dakh salient, North-Western Caucasus: results of U-Pb-geochronological study

    International Nuclear Information System (INIS)

    Somin, M.L.; Levchenkov, O.A.; Kotov, A.B.; Makeev, A.F.; Komarov, A.N.; Ro, N.I.; Lavrishchev, V.A.; Lebedev, V.A.

    2007-01-01

    U-Pb geochronological studies of an ancient component of the Dakh salient, i.e. metaaplites, which are vein fine-grained rocks made up by albite, microcline, quartz and potash mica, were made. Besides, K-Ar dating of granodiorites breaking through metamorphic rocks was conducted. U-Pb dating of accessory zircons (353 mln. years) defines the lower age boundary of the Dakh salient rock metamorphism. Its upper boundary was identified by K-Ar dating (301 ± 10 mln. years) of hornfels blende of nonmetamophized granodiorites [ru

  12. Zircon U-Pb geochronology and Sr-Nd-Pb-Hf isotopic constraints on the timing and origin of Mesozoic granitoids hosting the Mo deposits in northern Xilamulun district, NE China

    Science.gov (United States)

    Shu, Qihai; Lai, Yong; Zhou, Yitao; Xu, Jiajia; Wu, Huaying

    2015-12-01

    Located in the east section of the Central Asian orogen in northeastern China, the Xilamulun district comprises several newly discovered molybdenum deposits, primarily of porphyry type and Mesozoic ages. This district is divided by the Xilamulun fault into the southern and the northern parts. In this paper, we present new zircon U-Pb dating, trace elements and Hf isotope, and/or whole rock Sr-Nd-Pb isotopic results for the host granitoids from three Mo deposits (Yangchang, Haisugou and Shabutai) in northern Xilamulun. Our aim is to constrain the age and petrogenesis of these intrusions and their implications for Mo mineralization. Zircon U-Pb LA-ICP-MS dating shows that the monzogranites from the Shabutai and Yangchang deposits formed at 138.4 ± 1.5 and 137.4 ± 2.1 Ma, respectively, which is identical to the molybdenite Re-Os ages and coeval well with the other Mo deposits in this region, thereby indicating an Early Cretaceous magmatism and Mo mineralization event. Zircon Ce/Nd ratios from the mineralized intrusions are significantly higher than the barren granites, implying that the mineralization-related magmas are characterized by higher oxygen fugacity. These mineralized intrusions share similar zircon in-situ Hf and whole rock Sr-Nd isotopic compositions, with slightly negative to positive εHf(t) ranging from - 0.8 to + 10.0, restricted εNd(t) values from - 3.7 to + 1.6 but a little variable (87Sr/86Sr)i ratios between 0.7021 and 0.7074, indicative of formation from primary magmas generated from a dominantly juvenile lower crust source derived from depleted mantle, despite diverse consequent processes (e.g., magma mixing, fractional crystallization and crustal contamination) during their evolution. The Pb isotopes (whole rock) also show a narrow range of initial compositions, with (206Pb/204Pb)i = 18.03-18.88, (207Pb/204Pb)i = 15.48-15.58 and (208Pb/204Pb)i = 37.72-38.28, in agreement with Sr-Nd-Hf isotopes reflecting the dominance of a mantle component

  13. Geochronology and Geochemistry of Zircons from the IODP Site U1437 in the Rear of the Izu-Bonin Volcanic Arc

    Science.gov (United States)

    Andrews, G. D.; Schmitt, A. K.; Busby, C. J.; Brown, S. R.

    2015-12-01

    Zircons recovered from International Ocean Discovery Program Expedition 350 Site U1437 (31°47.390'N, 139°01.580'E) in the Izu-Bonin arc were analyzed by SIMS to constrain their age (U/Pb geochronology) and geochemistry (trace elements, δ18O); LA-ICP-MS ɛHf analyses are pending. Seven intervals were dated successfully: six tuffs and lapilli-tuffs between 680.99 and 1722.46 m below sea floor (mbsf) and a single peperitic rhyolitic intrusion at 1388.86 - 1390.07 mbsf. Thirty-two intervals which underwent mineral separation lacked zircon, or yielded zircon much older than age expectations for U1437. Geochronology results from separated zircons confirm and extend the shipboard age model to 1360.77 mbsf where Late Miocene (Tortonian) submarine volcanic rocks (11.3 ±0.7 Ma; n = 17) were sampled. In-situ measurement of zircons associated with magnetite crystals in the rhyolite intrusion yield an age of 13.6 ±1.7 Ma (n = 9). Zircon U contents are low (typically <300 ppm), with trace element ratios characteristic of oceanic lithosphere and near-mantle δ18O values (4-6 ‰). Individual Miocene zircon crystals are difficult to distinguish by age alone from those in the drilling mud (sepiolite) used during Expedition 350; the sepiolite is quarried by IMV Nevada in the Amargosa Valley. Our analysis of thirty-three zircons from the sepiolite finds that they have a broad and varied age distribution (2 - 2033 Ma) with a prominent peak at 12-14 Ma, bimodal δ18O values (peaks at 5-5.5 and 6.5-7.5 ‰), and dominantly continental trace element signatures. Three zircons from U1437 are tentatively identified as sepiolite-derived, but a single Eocene grain (51.7 ±2.4 Ma) recovered from 1722.46 mbsf has an age unlike those in the sepiolite, and potentially is genuinely xenocrystic. The majority of U1437 zircons thus crystallized from evolved melts lacking continental characteristics, although thermal and compositional conditions conducive for zircon crystallization appear to have

  14. U-Pb dating by zircon dissolution method using chemical abrasion

    Energy Technology Data Exchange (ETDEWEB)

    Takehara, Lucy, E-mail: lucytakehara@gmail.com.br [Servico Geologico do Brasil (CPRM), Brasilia, DF (Brazil); Chemale Junior, Farid [Universidade de Brasilia (UnB), Brasilia, DF (Brazil). Inst. de Geociencias. Lab. de Geocronologia; Hartmann, Leo A. [Universidade Federal do Rio Grande do Sul (UFRS), Porto Alegre, RS (Brazil). Inst. de Geociencias; Dussin, Ivo A.; Kawashita, Koji [Universidade de Sao Paulo (USP), SP, (Brazil). Centro de Pesquisa Geocronologicas

    2012-06-15

    Chemical abrasion was carried out on zircons grains of the Temora II standard for U-Pb dating prior to analyses using in situ Laser Ablation-Multi Collector Ion Coupled Plasma Mass Spectrometer (LA-ICPMS) followed by the Isotope Dissolution Thermal Ionization Mass Spectrometer (ID-TIMS) method. The proposed methodology was herein applied in order to reduce primarily the effects of secondary Pb loss, the presence of common lead and/or silicate impurities. Nine Temora II zircon grains were analyzed by the laser ablation method yielding an age of 418.3 +- 4.3 Ma. Zircon grains of a same population were separated for chemical abrasion before dissolution and mass spectrometry analyses. Six fractions of them were separated for isotope dissolution using {sup 235}U-{sup 205}Pb mixed spike after we have checked and assured the laboratory conditions of low blank values for total Pb of less than 2 pg/g. The obtained U-Pb zircon age by the ID-TIMS method was 415.7 +- 1.8 Ma (error 0.43 %) based on four successful determinations. The results are consistent with the published ages for the Temora diorite (Temora I - 416.75 +- 1.3 Ma; Temora II - 416.78 +- 0.33 Ma) and established as 416 +- 0.33 Ma. The technique is thus recommended for high precision U-Pb zircon analyses (error < 1 %), mainly for high resolution stratigraphic studies of Phanerozoic sequences. (author)

  15. Apollo 12 breccia 12013: Impact-induced partial Pb loss in zircon and its implications for lunar geochronology

    Science.gov (United States)

    Thiessen, F.; Nemchin, A. A.; Snape, J. F.; Bellucci, J. J.; Whitehouse, M. J.

    2018-06-01

    Apollo 12 breccia 12013 is composed of two portions, one grey in colour, the other black. The grey portion of the breccia consists mainly of felsite thought to have formed during a single crystallisation event, while the black part is characterized by presence of lithic fragments of noritic rocks and individual plagioclase crystals. In this study, U-Pb analyses of Ca-phosphate and zircon grains were conducted in both portions of the breccia. The zircon grains within the grey portion yielded a large range of ages (4154 ± 7 to 4308 ± 6 Ma, 2σ) and show decreasing U and Th concentrations within the younger grains. Moreover, some grains exhibit recrystallisation features and potentially formation of neoblasts. The latter process requires high temperatures above 1600-1700 °C leading to the decomposition of the primary zircon grain and subsequent formation of new zircon occurring as neoblasts. As a result of the high temperatures, the U-Pb system of the remaining original zircon grains was most likely open for Pb diffusion causing partial resetting and the observed range of 207Pb/206Pb ages. The event that led to the Pb loss in zircon could potentially be dated by the U-Pb system in Ca-phosphates, which have a weighted average 207Pb/206Pb age across both lithologies of 3924 ± 3 Ma (95% conf.). This age is identical within error to the combined average 207Pb/206Pb age of 3926 ± 2 Ma that was previously obtained from Ca-phosphates within Apollo 14 breccias, zircon grains in Apollo 12 impact melt breccias, and the lunar meteorite SaU 169. This age was interpreted to date the Imbrium impact. The zircon grains located within the black portion of the breccia yielded a similar range of ages (4123 ± 13 to 4328 ± 14 Ma, 2σ) to those in the grey portion. Given the brecciated nature of this part of the sample, the interpretation of these ages as representing igneous crystallisation or resetting by impact events remains ambiguous since there is no direct link to their

  16. Decoding a protracted zircon geochronological record in ultrahigh temperature granulite, and persistence of partial melting in the crust, Rogaland, Norway

    Science.gov (United States)

    Laurent, Antonin T.; Bingen, Bernard; Duchene, Stephanie; Whitehouse, Martin J.; Seydoux-Guillaume, Anne-magali; Bosse, Valerie

    2018-04-01

    This contribution evaluates the relation between protracted zircon geochronological signal and protracted crustal melting in the course of polyphase high to ultrahigh temperature (UHT; T > 900 °C) granulite facies metamorphism. New U-Pb, oxygen isotope, trace element, ion imaging and cathodoluminescence (CL) imaging data in zircon are reported from five samples from Rogaland, South Norway. The data reveal that the spread of apparent age captured by zircon, between 1040 and 930 Ma, results both from open-system growth and closed-system post-crystallization disturbance. Post-crystallization disturbance is evidenced by inverse age zoning induced by solid-state recrystallization of metamict cores that received an alpha dose above 35 × 1017 α g-1. Zircon neocrystallization is documented by CL-dark domains displaying O isotope open-system behaviour. In UHT samples, O isotopic ratios are homogenous (δ18O = 8.91 ± 0.08‰), pointing to high-temperature diffusion. Scanning ion imaging of these CL-dark domains did not reveal unsupported radiogenic Pb. The continuous geochronological signal retrieved from the CL-dark zircon in UHT samples is similar to that of monazite for the two recognized metamorphic phases (M1: 1040-990 Ma; M2: 940-930 Ma). A specific zircon-forming event is identified in the orthopyroxene and UHT zone with a probability peak at ca. 975 Ma, lasting until ca. 955 Ma. Coupling U-Pb geochronology and Ti-in-zircon thermometry provides firm evidence of protracted melting lasting up to 110 My (1040-930 Ma) in the UHT zone, 85 My (ca. 1040-955 Ma) in the orthopyroxene zone and some 40 My (ca. 1040-1000 Ma) in the regional basement. These results demonstrate the persistence of melt over long timescales in the crust, punctuated by two UHT incursions.

  17. U-Pb detrital zircon geochronology from the basement of the Central Qilian Terrane: implications for tectonic evolution of northeastern Tibetan Plateau

    Science.gov (United States)

    Liu, Changfeng; Wu, Chen; Zhou, Zhiguang; Yan, Zhu; Jiang, Tian; Song, Zhijie; Liu, Wencan; Yang, Xin; Zhang, Hongyuan

    2018-03-01

    The Tuolai Group dominates the Central Qilian Terrane, and there are different opinions on the age and tectonic attribute of the Tuolai Group. Based on large-scale geologic mapping and zircon dating, the Tuolai Group is divided into four parts: metamorphic supracrustal rocks, Neoproterozoic acid intrusive rocks, early-middle Ordovician acid intrusive rocks and middle Ordovician basic intrusive rocks. The metamorphic supracrustal rocks are the redefined Tuolai complex-group and include gneiss and schist assemblage by faulting contact. Zircon U-Pb LA-MC-ICP-MS dating was conducted on these samples of gneiss and migmatite from the gneiss assemblage, quartzite, two-mica schist and slate from the schist assemblage. The five detrital samples possess similar age spectra; have detrital zircon U-Pb main peak ages of 1.7 Ga with youngest U-Pb ages of 1150 Ma. They are intruded by Neoproterozoic acid intrusive rocks. Therefore, the Tuolai Group belonging to late Mesoproterozoic and early Neoproterozoic. With this caveat in mind, we believe that U-Pb detrital zircon dating, together with the geologic constraints obtained from this study and early work in the neighboring regions. We suggest that the formation age of the entire crystalline basement rocks of metasedimentary sequence from the Central Qilian Terrane should be constrained between the Late Mesoproterozoic and the Late Neoproterozoic, but not the previous Paleoproterozoic. The basement of the Central Qilian Terrane contains the typical Grenville ages, which indicates the Centre Qilian Terrane have been experienced the Grenville orogeny event.

  18. SHRIMP zircon U-Pb age and its significances of volcanic rocks from banshi basin in south jiangxi province

    International Nuclear Information System (INIS)

    Wang Baofeng; Wu Jianhua

    2011-01-01

    SHRIMP zircon U-Pb dating is applied to geochronological study for tuff of original Jilongzhang formation and rhyolite of original Banshi formation in Banshi basin, located in the Sannan (Longnan, Dingnan, Quannan) -Xunwu volcanic rocks belts in the south of Jiangxi. The result shows that zircon SHRIMP U-Pb age of the tuff is (142.5 ± 1.3) Ma and the age of the rhyolite is (131.4 ± 1.3) Ma. According to the latest international stratigraphic chart, the boundary between Jurassic and Cretaceous is (145.4 ± 4.0) Ma. So original Jilongzhang Formation and original Banshi Formation in Banshi basin were formed during early Early Cretaceous. The volcanic series in Banshi basin belongs to only one volcanic cycle. The features of the rock associations consistent with Wuyi group on the Xiajiang-Guangfeng volcanic rocks belts in the north of Jiangxi, so original Jilongzhang formation falls under E'huling formation and original Banshi formation falls under Shixi formation. (authors)

  19. Electron Backscatter Diffraction (EBSD) Analysis and U-Pb Geochronology of the Oldest Lunar Zircon: Constraining Early Lunar Differentiation and Dating Impact-Related Deformation

    Science.gov (United States)

    Timms, Nick; Nemchin, Alexander; Grange, Marion; Reddy, Steve; Pidgeon, Bob; Geisler, Thorsten; Meyer, Chuck

    2009-01-01

    The evolution of the early moon was dominated by two processes (i) crystallization of the Lunar Magma Ocean (LMO) and differentiation of potassium-rare earth element-phosphorous-rich residual magma reservoir referred to as KREEP, and (ii) an intense meteorite bombardment referred to as lunar cataclysm . The exact timing of these processes is disputed, and resolution relies on collection and interpretation of precise age data. This study examines the microstructure and geochronology of zircon from lunar impact breccias collected during the Apollo 17 mission. A large zircon clast within lunar breccia 72215,195 shows sector zoning in optical microscopy, cathodoluminescence (CL) imaging and Raman mapping, and indicates that it was a relict fragment of a much larger magmatic grain. Sensitive high resolution ion microprobe (SHRIMP) U-Pb analysis of the zircon shows that U and Th concentration correlate with sector zoning, with darkest CL domains corresponding with high-U and Th (approx.150 and approx.100 ppm respectively), and the brightest-CL sectors containing approx.30-50 ppm U and approx.10-20 ppm Th. This indicates that variations in optical CL and Raman properties correspond to differential accumulation of alpha-radiation damage in each sector. Electron backscatter diffraction (EBSD) mapping shows that the quality of electron backscatter patterns (band contrast) varies with sector zoning, with the poorest quality patterns obtained from high-U and Th, dark-CL zones. EBSD mapping also reveals a deformation microstructure that is cryptic in optical, CL and Raman imaging. Two orthogonal sets of straight discrete and gradational low-angle boundaries accommodate approx.12 misorientation across the grain. The deformation bands are parallel to the crystallographic {a}-planes of the zircon, have misorientation axes parallel to the c-axis, and are geometrically consistent with formation by dislocation creep associated with {010} slip. The deformation bands are unlike curved

  20. Silicic melt evolution in the early Izu-Bonin arc recorded in detrital zircons: Zircon U-Pb geochronology and trace element geochemistry for Site U1438, Amami Sankaku Basin

    Science.gov (United States)

    Barth, A. P.; Tani, K.; Meffre, S.; Wooden, J. L.; Coble, M. A.

    2016-12-01

    Understanding the petrologic evolution of oceanic arc magmas through time is important because these arcs reveal the processes of formation and the early evolution of juvenile continental crust. The Izu-Bonin (IB) arc system has been targeted because it is one of several western Pacific intraoceanic arcs initiated at 50 Ma and because of its prominent spatial asymmetry, with widespread development of relatively enriched rear arc lavas. We examined Pb/U and trace element compositions in zircons recovered at IODP Site 351-U1438 and compared them to regional and global zircon suites. These new arc zircon data indicate that detrital zircons will yield new insights into the generation of IB silicic melts and form a set of useful geochemical proxies for interpreting ancient arc detrital zircon provenance. Project IBM drilling target IBM1 was explored by Expedition 351 at Site U1438, located in the proximal back-arc of the northern Kyushu-Palau Ridge (KPR) at 27.3°N. A 1.2 km thick section of Paleogene volcaniclastic rocks, increasingly lithified and hydrothermally altered with depth, constitutes a proximal rear arc sedimentary record of IB arc initiation and early arc evolution. The ages and compositions of U1438 zircons are compatible with provenance in one or more edifices of the northern KPR and are incompatible with drilling contamination. Melt zircon saturation temperatures and Ti-in-zircon thermometry suggest a provenance in relatively cool and silicic KPR melts. The abundances of selected trace elements with high native concentrations provide insight into the petrogenesis of U1438 detrital zircon host melts, and may be useful indicators of both short and long-term variations in melt compositions in arc settings. The U1438 zircons are slightly enriched in U and LREE and are depleted in Nb compared to zircons from mid-ocean ridges and the Parece-Vela Basin, as predicted for melts in a primitive oceanic arc setting with magmas derived from a highly depleted mantle

  1. Pb, U, Ti, Hf and Zr distributions in zircons determined by proton microprobe and fission track techniques

    International Nuclear Information System (INIS)

    Clark, G.J.; Gulson, B.L.; Cookson, J.A.

    1979-01-01

    A proton microprobe has been used to determine Pb, Tl, Hf and Zr distributions across four single zircon crystals separated from a 'rapakivi' granite. The Pb and Zr data are quantitative: Pb and Tl concentrations were below the measurable limits for determinations in situ by most other techniques. The distribution of U in the same crystals was determined by the fission track technique. Limits on precision of U allow only a qualitative correlation of U and Pb, whereas the Tl and Pb correlation is more exactly determined. Zircons with distinct cores and overgrowths exhibited uniform Zr and Hf concentrations across the crystals, whereas the high U rims and 'inclusions' (domains) also had high Tl and Pb contents. Since almost all the Pb in these zircons is derived by radioactive decay of U, the Tl substitution has paralleled that of U. The results indicate that the high U domains are 'hot spots' rather than a separate mineral phase. The strong positive correlation of U and Pb indicates that there is little U daughter product migration relative to U, within the crystal. However, for the zircon population investigated here, the data are equivocal on the question of whether U addition to zircon crystals is associated with new zircon growth or not. In either case, the heterogeneous U and Pb distributions complicate any interpretations of U-Pb isotopic analysis for such zircon populations. (author)

  2. 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

    International Nuclear Information System (INIS)

    Valladares, Claudia Sayao

    1996-01-01

    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

  3. Tectonic evolution of the NE section of the Pamir Plateau: New evidence from field observations and zircon U-Pb geochronology

    Science.gov (United States)

    Zhang, Chuan-Lin; Zou, Hai-Bo; Ye, Xian-Tao; Chen, Xiang-Yan

    2018-01-01

    The Pamir Plateau at the western end of the India-Asia collision zone underwent long-term terrane drifting, accretion and collision between early Paleozoic and Mesozoic. However, the detailed evolution of this plateau, in particular, the timing of the Proto- and Palaeo-Tethys ocean subduction and closure, remains enigmatic. Here we report new field observations and zircon U-Pb ages and Hf isotopic compositions of the representative rocks from the so-called Precambrian basement in the northeastern Pamir, i.e., the Bulunkuole Group. The rock associations of the Bulunkuole Group indicate volcano-sedimentary sequences with arc affinities. Geochronological data demonstrate that the deposition age of the Bulunkuole Group in the NE section of the Pamir was Middle to Late Cambrian (530-508 Ma) rather than Paleoproterozoic. The deposition age became progressively younger from south to north. The amphibolite- to granulite facies metamorphism of the Bulunkuole Group took place at ca. 200-180 Ma. Unlike the scenario in the Southern Kunlun terrane (SKT) in the eastern section of the West Kunlun Orogenic Belt (WKOB), early Paleozoic metamorphism (ca. 440 Ma) was absent in this area. Two phases of magmatic intrusions, composed of granites and minor gabbros with arc geochemical signatures, emplaced at 510-480 Ma and 240-200 Ma. The amphibolite (meta mafic sheet? 519 Ma) and the meta-rhyolite (508 Ma) have zircon εHf(t) values of 1.6 to 5.9 and - 1.5 to 1.4, respectively. The 511 Ma gneissic granite sheet and the 486 Ma gabbro have zircon εHf(t) values of - 0.1 to 2.4 and 1.3 to 3.6, respectively. Zircon εHf(t) of the 245 Ma augen gneissic granite sheet varies from - 2.2 to 2.0 whereas the metamorphic zircons from the amphibolite (193 Ma) and high-pressure mafic granulite sample (187 Ma) have negative εHf(t) values of - 5.3 to - 2 and - 15 to - 12, respectively. In line with rock association and the deposition age of the Bulunkuole Group and the Saitula Group in the eastern

  4. Provenance of sediments from Sumatra, Indonesia - Insights from detrital U-Pb zircon geochronology, heavy mineral analyses and Raman spectroscopy

    Science.gov (United States)

    Liebermann, C.; Hall, R.; Gough, A.

    2017-12-01

    The island of Sumatra is situated at the southwestern margin of the Indonesian archipelago. Although it is the sixth largest island in the world, the geology of the Sumatra sedimentary basins and their underlying basement is relatively poorly understood in terms of their provenance. This work is a multi-proxy provenance study utilizing U-Pb detrital zircon dating by LA-ICP-MS combined with optical and Raman spectroscopy-based heavy mineral analysis. It will help to unravel the stratigraphy of Sumatra, contribute to paleogeographic reconstruction of western SE Asia, and aid a wider understanding of Sumatran petroleum plays. Thin section analyses, heavy mineral assemblages, and >3500 concordant U-Pb zircon ages, from samples acquired during two fieldwork seasons indicate a mixed provenance for Cenozoic sedimentary formations, including both local igneous sources and mature basement rocks. Characteristic Precambrian zircon age spectra are found in all analysed Cenozoic sedimentary strata. These can be correlated with zircon age populations found in Sumatran basement rocks; Neoproterozoic and Mesoproterozoic age groups are dominant (c. 500-600 Ma, c. 850-1000 Ma, c. 1050-1200 Ma). Paleoproterozoic to Archaean zircons occur as minor populations. The Phanerozoic age spectra of the Cenozoic formations are characterised by distinct Carboniferous, Permo-Triassic, and Jurassic-Cretaceous zircon populations. Permo-Triassic zircons are interpreted to come from granitoids in the Malay peninsula or Sumatra itself. Eocene to Lower Miocene strata are characterised by ultrastable heavy minerals such as zircon, tourmaline, and rutile, which together with garnet, suggest the principal sources were igneous and metamorphic basement rocks. Cenozoic zircons appear only from the Middle Miocene onwards. This change is interpreted to indicate a new contribution from a local volcanic arc, and is supported by the occurrence of unstable heavy minerals such as apatite and clinopyroxene, and the

  5. Arc-continent collision of the Coastal Range in Taiwan: Geochronological constraints from U-Pb ages of zircons

    Science.gov (United States)

    Geng, Wei; Zhang, Xun-Hua; Huang, Long

    2018-04-01

    The oblique arc-continent collision between the Luzon arc and the southeastern margin of the Eurasian continent caused the uplift of Taiwan. The Coastal Range in eastern Taiwan is the northern section of the Luzon arc in the collision zone and thus records important information about the arc-continent collision. In this paper, we determine and analyze the U-Pb ages of magmatic zircons from the volcanic arc and clastic zircons from the fore-arc basin in the Coastal Range. For the volcanic arc in the Coastal Range, the eruption ages range from 16.8-5 Ma. Given that the initial subduction of the South China Sea oceanic crust (17 Ma) occurred before the Luzon arc formed, we conclude that the volcanic activity of the Coastal Range began at 16.8 ± 1.3 Ma; it was most active from 14 to 8 Ma and continued until approximately 5 Ma. The U-Pb chronology also indicates that the initial stage of arc-continent collision of the Coastal Range started at approximately 5 Ma, when the northern section of the Luzon arc moved away from the magmatic chamber because of the kinematics of the Philippine Sea Plate.

  6. Nd isotopes, U-Pb single grain and SHRIMP zircon ages from basement rocks of the Tocantin Province

    International Nuclear Information System (INIS)

    Fuck, R.A.; Dantas, E.L.; Pimentel, M.M.; Junges, S.L.; Moraes, R

    2001-01-01

    , which were interpreted as minimum ages of the igneous protoliths. Here we present a preliminary Nd isotope survey on basement gneiss and associated rocks sampled along a transect from Arraias to Porto Nacional, Tocantins. Single zircon U-Pb determinations and two SHRIMP U-Pb zircon ages are also presented. This study aims to assess the ages of rocks and sources involved in the continental crust generation of the northern segment of the Tocantins Province. The main goal is to identify different geochronologic provinces or crustal blocks and provide age constraints on the timing of crustal evolution recorded in the basement gneiss. U-Pb single crystal zircon and Sm-Nd whole rock isotopic analyses were carried out at the geochronology laboratory, Universidade de Brasilia. SHRIMP analyses were performed at Australian National University, Canberra (au)

  7. Finding the "true" age: ways to read high-precision U-Pb zircon dates

    Science.gov (United States)

    Schaltegger, U.; Schoene, B.; Ovtcharova, M.; Sell, B. K.; Broderick, C. A.; Wotzlaw, J.

    2011-12-01

    Refined U-Pb dating techniques, applying an empirical chemical abrasion treatment prior to analysis [1], and using a precisely calibrated double isotope Pb, U EARTHTIME tracer solution, have led to an unprecedented two problems: (A) Post-crystallization Pb loss from decay damaged areas is considered to be mitigated by applying chemical abrasion techniques. The success of such treatment can, however, not be assumed a priori. The following examples demonstrate that youngest zircons are not biased by lead loss but represent close-to-youngest zircon growth: (i) coincidence of youngest zircon dates with co-magmatic titanite in tonalite; (ii) coincidence with statistically equivalent clusters of 206Pb/238U dates from zircon in residual melts of cogenetic mafic magmas; (iii) youngest zircons in ash beds of sedimentary sequences do not violate the stratigraphic superposition, whereas conventional statistical interpretation (mean or median values) does; (iv) results of published inter-laboratory cross-calibration tests using chemical abrasion on natural zircon crystals of the same sample arrive at the same 206Pb/238U result within case of single, significantly younger dates (>3 sigma), and are common in many pre-Triassic and hydrothermally altered rocks. (B) Pre-eruptive/pre-intrusive growth is found to be the main reason for scattered zircon ages in igneous rocks. Zircons crystallizing from the final magma batch are called autocrystic [3]. Autocrystic growth will happen in a moving or stagnant magma shortly before or after the rheological lockup by the crystals. Last crystallizing zircons in the interstitial melt may therefore postdate emplacement of the magma. The range of 206Pb/238U ages may yield a time frame for the cooling of a given magma batch, which could be added to quantitative thermal models of magma emplacement and cooling. Hf isotopes and trace elements of the dated zircon are used to trace the nature of the dated grains [4], specifically for identification of

  8. Petrogenesis, detrital zircon SHRIMP U-Pb geochronology, and tectonic implications of the Upper Paleoproterozoic Seosan iron formation, western Gyeonggi Massif, Korea

    Science.gov (United States)

    Kim, Chang Seong; Jang, Yirang; Samuel, Vinod O.; Kwon, Sanghoon; Park, Jung-Woo; Yi, Keewook; Choi, Seon-Gyu

    2018-05-01

    This study involves investigations on the Upper Paleoproterozoic iron formation (viz., Seosan iron formation) from the Seosan Group, Gyeonggi Massif of the southwestern Korean Peninsula. It occurs as thin banded layers within meta-arkosic sandstone, formed by alternating processes of chemical (hydrothermal) and detrital depositions under a shallow marine environment. It mainly consists of alternating layers of iron oxides, mostly hematite, and quartz. Minor amounts of magnetite surrounded by muscovite, clinopyroxene and amphibole indicate hydrothermal alteration since its formation. Meta-arkosic sandstone is composed of recrystallized or porphyroclastic quartz and microcline, with small amounts of hematite and pyrite clusters. The Seosan iron formation has high contents of total Fe2O3 and SiO2 with positive Eu anomalies similar to those of other Precambrian banded iron formations, and its formation is clearly related to hydrothermal alteration since its deposition. Detrital zircon SHRIMP U-Pb geochronology data from a meta-arkosic sandstone (SN-1) and an iron formation (SN-2) show mainly two age groups of ca. 2.5 Ga and ca. 1.9-1.75 Ga. This together with intrusion age of the granite gneiss (ca. 1.70-1.65 Ga) clearly indicate that the iron formations were deposited during the Upper Paleoproterozoic. The dominant Paleoproterozoic detrital zircon bimodal age peaks preserved in the Seosan iron formation compare well with those from the South China Craton sedimentary basins, reflecting global tectonic events related to the Columbia supercontinent in East Asia.

  9. Preliminary results of U/Pb geochronology in Meridional Espinhaco Mountain

    International Nuclear Information System (INIS)

    Machado, N.; Schrank, A.; Abreu, F.R. de; Knauer, L.G.; Abreu, P.A.A.

    1989-01-01

    Some samples of the constituent unities from Meridional Espinhaco Mountain for geochronology determining by U/Pb method are presented. The analytical techniques described by Krogh and the Davis linear regression program are used for treating these samples. (author)

  10. Timing of deformation and rapid subsidence in the northern Altiplano, Peru: Insights from detrital zircon geochronology of the Ayaviri hinterland basin

    Science.gov (United States)

    Horton, B. K.; Perez, N. D.; Saylor, J. E.

    2011-12-01

    Although age constraints on crustal deformation and sediment accumulation prove critical to testing hypotheses of orogenic plateau construction, a common lack of marine facies, volcanic tuffs, and suitable fossils hinders many attempts at chronological reconstructions. A series of elevated retroarc basins along the axis of the Andean orogenic belt provide opportunities to define the timing of deformation and transformation from foreland to hinterland basin configurations. In this study, we present new U-Pb ages of detrital zircons in the Ayaviri intermontane basin of southern Peru (~4 km elevation) in the northern part of the central Andean (Altiplano) plateau. Nearly all sandstone samples show strong unimodal U-Pb age peaks (generally defined by > 5-50 zircons), suggesting these age peaks represent syndepositional volcanism and can be regarded as accurate estimates of true depositional (stratigraphic) age. Integration of these ages with structural and stratigraphic relationships demonstrate the utility of zircon U-Pb geochronology in defining both (1) the timing of basin partitioning and (2) the pace of sediment accumulation. (1) U-Pb ages for several sandstone samples from growth-strata packages associated with two basin-bounding faults reveal structural partitioning of the Ayaviri basin from late Oligocene to Miocene time. In the north, displacement along the southwest-directed Ayaviri thrust fault commenced in late Oligocene time (~28-24 Ma), inducing initial structural partitioning of an upper Eocene-Oligocene, > 5 km thick succession potentially representing an early Andean retroarc foreland basin. In the south, the Ayaviri basin was further disrupted by initial displacement along the northeast-directed Pasani thrust fault in early to middle Miocene time (~18-15 Ma). (2) Additional U-Pb analyses from the Ayaviri basin fill help delimit the long-term rates of sedimentation, suggesting relatively short-lived (< 5 Myr) pulses of accelerated accumulation. Rapid

  11. Pan-African U-Pb zircon age from apatite-magnetite veins of eastern ghats granulite belt, India

    International Nuclear Information System (INIS)

    Kovach, V.P.; Salnikova, E.B.; Kotov, A.B.; Yakovleva, S.Z.; Rao, A.T.

    1997-01-01

    206 Pb/ 238 U ages of zircon correspond to 502±3 Ma and 207 Pb/ 206 Pb ages are equal to 508±14 Ma. U-Pb isotopic study of the Kasipatnam zircon has led to the recognition of pan-African thermal event in the eastern ghats granulite belt, India. (author)

  12. Chemical Th-U-total Pb isochron ages of zircon and monazite from granitic rocks of the Negele area, southern Ethiopia

    International Nuclear Information System (INIS)

    Yihunie, Tadesse

    2003-01-01

    CHIME (Chemical Th-U-total Pb Isochron Method) age determinations were made on zircon and monazite grains from granitoid samples in the Negele area of southern Ethiopia. A 611±32 Ma zircon age was obtained from a syn-tectonic hornblende-biotite granite of the Alghe domain. A 453±29 Ma zircon age and a 459±16 Ma monazite age were also obtained from post-tectonic two-mica granites of the Alghe domain and the Kenticha domain, respectively. Since these ca. 450 Ma zircon and monazite ages coincide well, as significant post-tectonic granitic magmatism appears to have occurred in southern Ethiopia in early Paleozoid time. The new CHIME age results indicate at least two episodes of granitic magmatism in southern Ethiopia in the late Proterozoic to early Paleozoic, with a ca. 150 Ma interval between syn- and post-tectonic granitic emplacements, and have a important geochronological constraint on the evolution of the East African Orogen. (author)

  13. High-Resolution Zircon U-Pb CA-TIMS Dating of the Carboniferous—Permian Successions, Paraná Basin, Brazil

    Science.gov (United States)

    Griffis, N. P.; Mundil, R.; Montanez, I. P.; Isbell, J.; Fedorchuk, N.; Lopes, R.; Vesely, F.; Iannuzzi, R.

    2015-12-01

    The late Paleozoic Ice Age (LPIA) is Earth's only record of a CO2-forced climatic transition from an icehouse to greenhouse state in a vegetated world. Despite a refined framework of Gondwanan ice distribution, questions remain about the timing, volume, and synchronicity of high-latitude continental ice and the subsequent deglaciation. These questions ultimately preclude our understanding of linkages between ice volume, sea level, and high- and low-latitude climate. Poor constraints on the timing and synchronicity of glacial and interglacial transitions reflect a lack of high-resolution radioisotopic dates from high-latitude, ice-proximal Carboniferous-Permian successions. The Rio Bonito Fm in Rio Grande do Sul State of southern Brazil hosts the oldest non-glaciogenic Carboniferous- Permian deposits of the Paraná Basin, thus recording the icehouse-to-greenhouse transition. Despite a widespread effort over the last two decades to constrain these deposits in time by means of U-Pb zircon geochronology, published data sets of the Candiota and Faxinal coals of the Rio Bonito Fm host discrepancies that may reflect post- eruptive open system behavior of zircon and analytical artifacts. These discrepancies have hindered the correlation of the Candiota and Faxinal sediments within the larger Gondwanan framework. Here we present the first U-Pb ages on closed system single zircons using CA-TIMS techniques on Permo-Carboniferous ash deposits of the Paraná Basin. Preliminary results indicate two major and distinct coal-forming periods that are separated by ca 10 Ma. Our results and conclusions are not in agreement with multi- crystal U-Pb TIMS and SIMS ages that suggest coeval deposition of the Candiota and Faxinal coals. CA-TIMS analyses applied to zircons from additional ash deposits are aimed at constructing a robust chronostratigraphic framework for the Carboniferous- Permian succession of the Paraná Basin, which will facilitate a better understanding of the timing and

  14. 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

    1996-12-31

    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.

  15. 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

    1997-12-31

    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.

  16. Air abrasion experiments in U-Pb dating of zircon

    Science.gov (United States)

    Goldich, S.S.; Fischer, L.B.

    1986-01-01

    Air abrasion of zircon grains can remove metamict material that has lost radiogenic Pb and zircon overgrowths that were added during younger events and thereby improve the precision of the age measurements and permit closer estimates of the original age. Age discordance that resulted from a single disturbance of the U-Pb isotopic decay systems, as had been demonstrated by T.E. Krogh, can be considerably reduced, and, under favorable conditions, the ages brought into concordancy. Two or more events complicate the U-Pb systematics, but a series of abrasion experiments can be helpful in deciphering the geologic history and in arriving at a useful interpretation of the probable times of origin and disturbances. In east-central Minnesota, U.S.A., Penokean tonalite gneiss is dated at 1869 ?? 5 Ma, and sheared granite gneiss is shown to have been a high-level granite intrusion at 1982 ?? 5 Ma in the McGrath Gneiss precursor. Tonalite gneiss and a mafic granodiorite in the Rainy Lake area, Ontario, Canada, are dated at 2736 ?? 16 and 2682 ?? 4 Ma, respectively. The tonalitic phase of the Morton Gneiss, southwestern Minnesota, is dated at 3662 ?? 42 Ma. ?? 1986.

  17. 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

    Science.gov (United States)

    Neymark, Leonid; Holm-Denoma, Christopher S.; Pietruszka, Aaron; Aleinikoff, John N.; Fanning, C. Mark; Pillers, Renee M.; Moscati, Richard J.

    2016-01-01

    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

  18. U-Pb Detrital Zircon Geochronologic Constraints on Depositional Age and Sediment Source Terrains of the Late Paleozoic Tepuel-Genoa Basin

    Science.gov (United States)

    Griffis, N. P.; Montanez, I. P.; Isbell, J.; Gulbranson, E. L.; Wimpenny, J.; Yin, Q. Z.; Cúneo, N. R.; Pagani, M. A.; Taboada, A. C.

    2014-12-01

    The late Paleozoic Ice Age (LPIA) is the longest-lived icehouse of the Phanerozoic and the only time a metazoan dominated and vegetated world transitioned from an icehouse climate into a greenhouse. Despite several decades of research, the timing, extent of glaciation and the location of ice centers remain unresolved, which prohibits reconstruction of ice volume. The Permo-Carboniferous sediments in the Tepuel-Genoa Basin, Patagonia contains a near complete record of sedimentation from the lower Carboniferous through lower Permian. Outsized clasts, thin pebble-rich diamictites and slumps represent the last of the late Paleozoic glacially influenced deep-water marine sediments in the Mojón de Hierro Fm. and the Paleozoic of Patagonia. U-Pb analysis of detrital zircons separated from slope sediments reveal groupings (20 myr bins, n≥5 zircons) with peak depositional ages of 420, 540 to 660 and 1040 Ma. Zircon age populations recovered from the Mojón de Hierro Fm. compare well with bedrock ages of the Deseado Massif of SE Patagonia, suggesting this may be a potential source of sediments. The maximum depositional age of the sediments is 306.05 ± 3.7 Ma (2σ) as determined by the median age of the two youngest concordant zircons that overlap in error. The youngest zircon from the analysis yields a 238U/206Pb age of 301.3 ± 4.5 Ma (2σ; MSWD = 2.3). Younger zircons from the analysis compare well with the age of granite bedrock exposed along the basin margin to the E-NE suggesting they may reflect a more proximal source. These data, which indicate a maximum age of late Carboniferous for the Mojón de Hierro Fm, provide the first geochemical constraints for the timing of final deposition of glaciomarine sediments in the Tepuel-Genoa Basin, and contributes to the biostratigraphic correlation of the late Paleozoic succession in Patagonia with other key LPIA basins that has thus far been hindered by faunal provincialism.

  19. Detrital zircon typology and U/Pb geochronology for the Miocene Ladrilleros-Juanchaco sedimentary sequence, Equatorial Pacific (Colombia): New constraints on provenance and paleogeography in northwestern South America

    Energy Technology Data Exchange (ETDEWEB)

    Osorio-Granada, E.; RESTREPO-MORENO, S.A.; MUÑOZ-VALENCIA, J.A.; TREJOS-TAMAYO, R.A.; PARDO-TRUJILLO, A.; BARBOSA-ESPITIA, A.A.

    2017-11-01

    Typology and internal texture analyses were performed on detrital zircons obtained from the Miocene sandstones of the Ladrilleros-Juanchaco sedimentary sequence (Colombia, Equatorial Pacific). This analysis was complemented with zircon U/Pb dating to identify typology-age associations as indicators of sediment provenance. Our results show that zircons with S and P dominant typologies have internal structures/zoning indicative of igneous, and potentially also metamorphic, origins. Morphometric results suggest limited transport from source areas. Both typology and U/Pb data point to the western Cordillera as the principal source of detrital materials for this sedimentary sequence. A paleogeographic reconstruction shows that, during the Late Miocene, significant portions of the western Cordillera were uplifted and actively eroding, thereby forming a fluvio-topographic barrier that prevented sediments from the central Cordillera reaching the Pacific basins. Exhumed Miocene plutons located along the axis of the western Cordillera may also have played a role as geomorphologically active massifs. This study demonstrates that typologic analysis on detrital zircon grains is a useful tool for establishing provenance and paleogeography in complex litho-tectonic areas where overlapping U/Pb signatures can lead to contradictory results.

  20. 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

    2012-10-01

    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.

  1. Petrogenesis of Malaysian tin granites: geochemistry, fractional crystallization, U-Pb zircon geochronology and tectonic setting

    Science.gov (United States)

    Wai-Pan Ng, Samuel; Searle, Mike; Whitehouse, Martin; Chung, Sun-Lin; Ghani, Azman; Robb, Laurence; Sone, Masatoshi; Oliver, Grahame; Gardiner, Nick; Roselee, Mohammad

    2014-05-01

    The Malaysian tin granites forming the backbone of the Thai-Malay Peninsula has been long recognized with two distinct granitic provinces:- 1. Early Permian to Late Triassic Eastern Province with mainly "I-type" (Hbl)-Bt granites with associated Cu-Au deposits, with subordinate Bt granites hosting limited Sn-W deposits, and 2. Late Triassic Main Range Province with mainly "S-type" Bt granites with associated Sn-W deposits, and subordinate (Hbl)-Bt granites. New geochemical data show that Chappell and White's (1974) I-S granite classification adopted in the existing model does not adequately distinguish the granites from one another as previously implied. Trace element geochemistry and Sr-Nd isotopic compositions show that the Malaysian tin granites in both provinces have transitional I-S characteristics. In addition, they inherited within-plate signature from Cambro-Ordovician Gondwana-related source rocks. Previous ages were obtained by whole rock Rb-Sr and biotite K-Ar geochronology in the 70s and 80s, dating methods that may not accurately represent the crystallization age of granites. We re-sampled the entire Malaysian Peninsula and 40 samples were collected for high-precision U-Pb SIMS dating on extracted zircon grains in order to better constrain the magmatic and tectonic evolution of Southeast Asia. The crystallization ages of the Eastern Province granitoids have been constrained ranging from 220 to 290 Ma, while the Main Range (Western) Province granitoids have ages ranging from 200 to 230 Ma. A progressive westward younging trend is apparent across the Eastern Province, but becomes less obvious in the Main Range Province. Our model suggests two east dipping subduction zones. We suggest that subduction roll-back along the Bentong-Raub suture might account for the westward younging trend, in the Eastern province. A second Late Triassic east-dipping subduction zone beneath western Malaysia is proposed in order to explain the "I-type" components to the Main

  2. New SHRIMP U-Pb zircon ages of the metapelitic granulites from NW of Madurai, Southern India

    International Nuclear Information System (INIS)

    Prakash, D.

    2010-01-01

    Zircon cathodoluminescent imaging and SHRIMP U-Pb dating were carried out for metapelitic rocks (sapphirine-hearing granulites and garnet-cordierite gneisses) from the NW of Madurai, Southern India. The cathodoluminescence images reveal the complex, inhomogeneous internal structure having irregular-shaped core and overgrowths. Zircon grains have obliterated oscillatory zoning. SHRIMP U-Pb chronological results yield ages of 550 ± 15 Ma and 530 ± 50 Ma as a time of metamorphic overprint, and the age of 2509 ± 12 Ma and 2509 ± 30 Ma corresponding to a timing of protolith formation for sapphirine-bearing granulites and garnet-cordierite gneisses respectively. Zircon ages reflect that continental crust in the NW of Madurai region resulted from the recycling of Archaean protolith of an igneous origin similar to the preserved crust in the southern part of Dharwar craton. The present SHRIMP U-Pb zircon ages are in close agreement with earlier published Nd isotopic data which suggest an extended precrustal history of their protoliths. The abraded zircon grains indicate multiple recycling and repeated metamorphism that has ultimately resulted in present day continental crust exposed in Madurai region. These SHRIMP U-Pb zircon ages from metapelitic UHT granulites are also significant to understanding the architecture of the SGT during the amalgamation of Gondwana in Neoproterozoic time. (author)

  3. Breaking through the uncertainty ceiling in LA-ICP-MS U-Pb geochronology

    Science.gov (United States)

    Horstwood, M.

    2016-12-01

    Sources of systematic uncertainty associated with session-to-session bias are the dominant contributor to the 2% (2s) uncertainty ceiling that currently limits the accuracy of LA-ICP-MS U-Pb geochronology. Sources include differential downhole fractionation (LIEF), `matrix effects' and ablation volume differences, which result in irreproducibility of the same reference material across sessions. Current mitigation methods include correcting for LIEF mathematically, using matrix-matched reference materials, annealing material to reduce or eliminate radiation damage effects and tuning for robust plasma conditions. Reducing the depth and volume of ablation can also mitigate these problems and should contribute to the reduction of the uncertainty ceiling. Reducing analysed volume leads to increased detection efficiency, reduced matrix-effects, eliminates LIEF, obviates ablation rate differences and reduces the likelihood of intercepting complex growth zones with depth, thereby apparently improving material homogeneity. High detection efficiencies (% level) and low sampling volumes (20um box, 1-2um deep) can now be achieved using MC-ICP-MS such that low volume ablations should be considered part of the toolbox of methods targeted at improving the reproducibility of LA-ICP-MS U-Pb geochronology. In combination with other strategies these improvements should be feasible on any ICP platform. However, reducing the volume of analysis reduces detected counts and requires a change of analytical approach in order to mitigate this. Appropriate strategies may include the use of high efficiency cell and torch technologies and the optimisation of acquisition protocols and data handling techniques such as condensing signal peaks, using log ratios and total signal integration. The tools required to break the 2% (2s) uncertainty ceiling in LA-ICP-MS U-Pb geochronology are likely now known but require a coherent strategy and change of approach to combine their implementation and realise

  4. High-precision U-Pb geochronologic constraints on the Late Cretaceous terrestrial cyclostratigraphy and geomagnetic polarity from the Songliao Basin, Northeast China

    Science.gov (United States)

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

    2016-07-01

    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.

  5. The 230Th correction is the 1st priority for accurate dates of young zircons: U/Th partitioning experiments and measurements

    Science.gov (United States)

    Krawczynski, M.; McLean, N.

    2017-12-01

    One of the most accurate and useful ways of determining the age of rocks that formed more than about 500,000 years ago is uranium-lead (U-Pb) geochronology. Earth scientists use U-Pb geochronology to put together the geologic history of entire regions and of specific events, like the mass extinction of all non-avian dinosaurs about 66 million years ago or the catastrophic eruptions of supervolcanoes like the one currently centered at Yellowstone. The mineral zircon is often utilized because it is abundant, durable, and readily incorporates uranium into its crystal structure. But it excludes thorium, whose isotope 230Th is part of the naturally occurring isotopic decay chain from 238U to 206Pb. Calculating a date from the relative abundances of 206Pb and 238U therefore requires a correction for the missing 230Th. Existing experimental and observational constraints on the way U and Th behave when zircon crystallizes from a melt are not known precisely enough, and thus currently the uncertainty in dates introduced by they `Th correction' is one of the largest sources of systematic error in determining dates. Here we present preliminary results on our study of actinide partitioning between zircon and melt. Experiments have been conducted to grow zircon from melts doped with U and Th that mimic natural magmas at a range of temperatures, and compositions. Synthetic zircons are separated from their coexisting glass and using high precision and high-spatial-resolution techniques, the abundance and distribution of U and Th in each phase is determined. These preliminary experiments are the beginning of a study that will result in precise determination of the zircon/melt uranium and thorium partition coefficients under a wide variety of naturally occurring conditions. This data will be fit to a multidimensional surface using maximum likelihood regression techniques, so that the ratio of partition coefficients can be calculated for any set of known parameters. The results of

  6. Morphology versus U-Pb systematics in zircon: A high-resolution isotopic study of a zircon population from a Variscan dike in the Central Alps

    International Nuclear Information System (INIS)

    Bossart, P.J.; Meier, M.; Oberli, F.; Steiger, R.H.

    1986-01-01

    U/Pb isotopic measurements on individual zircon crystals combined with morphological analyses permit the identification of three distinct components within the zircon population of the Saedelhorn diorite, a Variscan dike from the western Gotthard (Central Alps, Switzerland): (i) 94% of the grains in the zircon population are elongate crystals with pronounced skeletal morphology indicative of rapid growth from a supercooled melt. (ii) 5% of the population consist of turbid, mostly subhedral zircons frequently showing D-type morphology and elevated uranium contents compared to the skeletal variety. Single-crystal and multi-grain U-Pb isotopic data of group (i) and (ii) zircons define an intrusion age of 293 + 5/ -4 m.y. for the dike. (iii) Rare, transparent zircon crystals (< 1% of the zircons population) yield apparent U-Pb ages in the range of 370-490 m.y. and display morphological and isotopic characteristics closely resembling those of a Caledonian orthogneiss intruded by the dike. This implies presence of assimilated wall-rock components in the macroscopically homogeneous dike sample. (orig./WB)

  7. Brittle-ductile deformation effects on zircon crystal-chemistry and U-Pb ages: an example from the Finero Mafic Complex (Ivrea-Verbano Zone, western Alps)

    Science.gov (United States)

    Langone, Antonio; José Alberto, Padrón-Navarta; Zanetti, Alberto; Mazzucchelli, Maurizio; Tiepolo, Massimo; Giovanardi, Tommaso; Bonazzi, Mattia

    2016-04-01

    A detailed structural, geochemical and geochronological survey was performed on zircon grains from a leucocratic dioritic dyke discordantly intruded within meta-diorites/gabbros forming the External Gabbro unit of the Finero Mafic Complex. This latter is nowadays exposed as part of a near complete crustal section spanning from mantle rocks to upper crustal metasediments (Val Cannobina, Ivrea-Verbano Zone, Italy). The leucocratic dyke consists mainly of plagioclase (An18-24Ab79-82Or0.3-0.7) with subordinate amounts of biotite, spinel, zircon and corundum. Both the leucocratic dyke and the surrounding meta-diorites show evidence of ductile deformation occurred under amphibolite-facies conditions. Zircon grains (up to 2 mm in length) occur mainly as euhedral grains surrounded by fine grained plagioclase-dominated matrix and pressure shadows, typically filled by oxides. Fractures and cracks within zircon are common and can be associated with grain displacement or they can be filled by secondary minerals (oxides and chlorite). Cathodoluminescence (CL) images show that zircon grains have internal features typical of magmatic growth, but with local disturbances. However EBSD maps on two selected zircon grains revealed a profuse mosaic texture resulting in an internal misorientation of ca. 10o. The majority of the domains of the mosaic texture are related to parting and fractures, but some domains show no clear relation with brittle features. Rotation angles related to the mosaic texture are not crystallographically controlled. In addition, one of the analysed zircons shows clear evidence of plastic deformation at one of its corners due to indentation. Plastic deformation results in gradual misorientations of up to 12o, which are crystallographically controlled. Trace elements and U-Pb analyses were carried out by LA-ICP-MS directly on petrographic thin sections and designed to cover the entire exposed surface of selected grains. Such investigations revealed a strong

  8. U-Pb ages and Hf isotope compositions of zircons in plutonic rocks from the central Famatinian arc, Argentina

    Science.gov (United States)

    Otamendi, Juan E.; Ducea, Mihai N.; Cristofolini, Eber A.; Tibaldi, Alina M.; Camilletti, Giuliano C.; Bergantz, George W.

    2017-07-01

    The Famatinian arc formed around the South Iapetus rim during the Ordovician, when oceanic lithosphere subducted beneath the West Gondwana margin. We present combined in situ U-Th-Pb and Lu-Hf isotope analyses for zircon to gain insights into the origin and evolution of Famatinian magmatism. Zircon crystals sampled from four intermediate and silicic plutonic rocks confirm previous observations showing that voluminous magmatism took place during a relatively short pulse between the Early and Middle Ordovician (472-465 Ma). The entire zircon population for the four plutonic rocks yields coherent εHf negative values and spreads over several ranges of initial εHf(t) units (-0.3 to -8.0). The range of εHf units in detrital zircons of Famatinian metasedimentary rocks reflects a prolonged history of the cratonic sources during the Proterozoic to the earliest Phanerozoic. Typical tonalites and granodiorites that contain zircons with evolved Hf isotopic compositions formed upon incorporating (meta)sedimentary materials into calc-alkaline metaluminous magmas. The evolved Hf isotope ratios of zircons in the subduction related plutonic rocks strongly reflect the Hf isotopic character of the metasedimentary contaminant, even though the linked differentiation and growth of the Famatinian arc crust was driven by ascending and evolving mantle magmas. Geochronology and Hf isotope systematics in plutonic zircons allow us understanding the petrogenesis of igneous series and the provenance of magma sources. However, these data could be inadequate for computing model ages and supporting models of crustal evolution.

  9. 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

    Science.gov (United States)

    Aleinikoff, J.N.; Schenck, W.S.; Plank, M.O.; Srogi, L.A.; Fanning, C.M.; Kamo, S.L.; Bosbyshell, H.

    2006-01-01

    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

  10. REE, Sm-Nd and U-Pb zircon study of eclogites from the Alpine External Massifs (Western Alps): Evidence for crustal contamination

    International Nuclear Information System (INIS)

    Paquette, J.L.; Eidgenoessische Technische Hochschule, Zurich; Menot, R.P.; Peucat, J.J.

    1989-01-01

    A geochemical and geochronological study of the Alpine External Crystalline Massifs (AECM) of Aiguilles Rouges, Belledonne and Argentera was undertaken in order to constrain the geodynamic evolution of this segment of the Variscan foldbelt. Another aim of the study is to characterize the behaviour of isotopic markers, in particular the U-Pb zircon system, under high-grade metamorphic conditions. The whole-rock geochemistry of eclogites and amphibolites was investigated using major and trace element (including the REE) analytical techniques; isotopic studies were performed by application of the Sm-Nd whole-rock and U-Pb zircon methods. In terms of regional geological history, the early development of metamorphic and magmatic activity in the AECM is typical of the extensional tectonic regime observed throughout the Variscan foldbelt during the Cambro-Ordovician (i.e. basic magmatism dated at 475-450 Ma). The composition of the metabasic rocks is closely similar to tholeiites emplaced into thinned continental crust which are generally associated with the initial stages of oceanic rifting. The source regions for these metabasics are characterized by initial ε Nd values between +6 and +8, suggesting depleted mantle sources influenced by a weak crustal component and/or the existence of a metasomatised lithosphere. The multi-stage eclogite-facies metamorphism is dated at 425-395 Ma (i.e. Silurian). An application of the U-Pb method, associated with the artificial abrasion of zircon grains, has led to the recognition of a weak crustal contamination in the metabasic protoliths. This is implied by the Archaean and Lower Proterozoic upper intercepts on Concordia - devoid of geological significance - which reflect the presence of a pre-existing basement to the AECM. (orig./WL)

  11. Reply to Comment on "Zircon U-Th-Pb dating using LA-ICP-MS: Simultaneous U-Pb and U-Th dating on the 0.1 Ma Toya Tephra, Japan"

    Science.gov (United States)

    Ito, Hisatoshi

    2015-04-01

    Guillong et al. (2015) mentioned that corrections for abundance sensitivity for 232Th and molecular zirconium sesquioxide ions (Zr2O3+) are critical for reliable determination of 230Th abundances in zircon for LA-ICP-MS analyses. There is no denying that more rigorous treatments are necessary to obtain more reliable ages than those in Ito (2014). However, as shown in Fig. 2 in Guillong et al. (2015), the uncorrected (230Th)/(238U) for reference zircons except for Mud Tank are only 5-20% higher than unity. Since U abundance of Toya Tephra zircons that have U-Pb ages Ito (2014) obtained U-Th ages of the Toya Tephra by comparison with Fish Canyon Tuff (FCT) data. Because both the FCT and the Toya Tephra have similar trends of overestimation of 230Th, the effect of overestimation of 230Th to cause overestimation of U-Th age should be cancelled out or negligible. Therefore the pivotal conclusion in Ito (2014) that simultaneous U-Pb and U-Th dating using LA-ICP-MS is possible and useful for Quaternary zircons holds true.

  12. U-Pb zircon geochronology of Late Devonian to Early Carboniferous extension-related silicic volcanism in the northern New England Fold Belt

    International Nuclear Information System (INIS)

    Bryan, S.E.; Holcombe, R.J.; Fielding, C.R.; Allen, C.M.

    2004-01-01

    Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of zircons confirm a Late Devonian to Early Carboniferous age (ca 360-350 Ma) for silicic volcanic rocks of the Campwyn Volcanics and Yarrol terrane of the northern New England Fold Belt (Queensland). These rocks are coeval with silicic volcanism recorded elsewhere in the fold belt at this time (Connors Arch, Drummond Basin). The new U-Pb zircon ages, in combination with those from previous studies, show that silicic magmatism was both widespread across the northern New England Fold Belt (>250 000 km 2 and >500 km inboard of plate margin) and protracted, occurring over a period of -15 million years. Zircon inheritance is commonplace in the Late Devonian - Early Carboniferous volcanics, reflecting anatectic melting and considerable reworking of continental crust. Inherited zircon components range from ca 370 to ca 2050 Ma, with Middle Devonian (385-370 Ma) zircons being common to almost all dated units. Precambrian zircon components record either Precambrian crystalline crust or sedimentary accumulations that were present above or within the zone of magma formation This contrasts with a lack of significant zircon inheritance in younger Permo-Carboniferous igneous rocks intruded through,and emplaced on top of, the Devonian-Carboniferous successions. The inheritance data and location of these volcanic rocks at the eastern margins of the northern New England Fold Belt, coupled with Sr-Nd, Pb isotopic data and depleted mantle model ages for Late Palaeozoic and Mesozoic magmatism, imply that Precambrian mafic and felsic crustal materials (potentially as old as 2050 Ma), or at the very least Lower Palaeozoic rocks derived from the reworking of Precambrian rocks, comprise basement to the eastern parts of the fold belt. This crustal basement architecture may be a relict from the Late Proterozoic breakup of the Rodinian supercontinent. Copyright (2004) Geological Society of Australia

  13. U-Pb geochronology on zircon from the Aouli pluton, Haute Moulouya area, Morocco

    International Nuclear Information System (INIS)

    Oukemeni, D.; Krogh, T.E.

    1995-01-01

    The Aouli Pluton consists of four units: (1) granodiorite; (2) grey granite; (3) pink granite - these three units constituting a spatially continuous massif, and (4) muscovite granite, which is exposed in two small stocks somewhat removed from the other units. U-Pb ages obtained on zircon and titanite are 333 ± 2 Ma and 319 ± 1.5 Ma for the granodiorite and grey granite respectively. The 14 ma age difference between the granodiorite and the grey granite supports the multiple injection hypothesis which was the outcome of geochemical studies (Oukemeni and Bourne, 1993). These two rocks also contain an inherited component which has been dated at 1520 Ma in the granudiorite and at 1245 ma and 1804 Ma in the grey granite. The inherited components suggest the possible presence of Precambian crust below the Haute Moulouya. Since the intrusion is late-to post-tectonic (Oukemeni and Bourne, 1993), the age of the granodiorite (333 ± 2 Ma) indicates that the Hercynian deformation is pre-Visean in the Haute Moulouya area, and, by extension, throughout the eastern meseta region. (authors). 14 refs. 4 figs. 3 tabs

  14. Age of meta-rhyolite of Marata sequence, Araxa Group, Goias: geochronology study by U-Pb in zircon, Rb-Sr and Sm-Nd methods

    International Nuclear Information System (INIS)

    Pimentel, M.M.; Heaman, L.; Fuck, R.A.

    1992-01-01

    U-Pb isotopic analyses in eight zircon fractions separated from a meta-rhyolite sample of the Marata sequence (Araxa Group, Goias) are presented. Two morphologically distinct zircon populations were identified: stubby prismatic crystals (width:length of 1:2 to 1:3) in which core-overgrowth relationships are observed; long prismatic crystals (needles) with width:length ratios of ca. 1:10. Analyses performed on group zircons indicated the presence of a ca. 2.0 Ga. old inherited component. Analyses of group zircons plot very close to the concordia and yield an upper intercepts age of 794 ± 10 Ma for the crystallization of the volcanic protolith. The study was complemented with whole-rock isotopic analyses by the Rb-Sr and Sm-Nd methods. Rb-Sr isochron obtained with samples from two different outcrops gave data of 829 ± 82 Ma (initial 87 Sr/ 86 Sr of 0.7057 ± 0.0157) and 691 ± 30 Ma (initial 87 Sr/ 86 Sr of 0.7337 ± 0.0034). The latter reflects a later Sr-isotopic re-homogenization episode, possibly associated with the intense deformation and metamorphism suffered by these rocks. Sm-Nd isotopic analyses on three samples reveal EN d (T) (T =794 Ma) values in the range of -6.1 to -9.5 which indicate the presence of older (early Proterozoic) crustal Nd. This is consistent with the inheritance pattern shown in the U-Pb isotopic results. (author)

  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

    NARCIS (Netherlands)

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

    2013-01-01

    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

  16. Community-based Approaches to Improving Accuracy, Precision, and Reproducibility in U-Pb and U-Th Geochronology

    Science.gov (United States)

    McLean, N. M.; Condon, D. J.; Bowring, S. A.; Schoene, B.; Dutton, A.; Rubin, K. H.

    2015-12-01

    The last two decades have seen a grassroots effort by the international geochronology community to "calibrate Earth history through teamwork and cooperation," both as part of the EARTHTIME initiative and though several daughter projects with similar goals. Its mission originally challenged laboratories "to produce temporal constraints with uncertainties approaching 0.1% of the radioisotopic ages," but EARTHTIME has since exceeded its charge in many ways. Both the U-Pb and Ar-Ar chronometers first considered for high-precision timescale calibration now regularly produce dates at the sub-per mil level thanks to instrumentation, laboratory, and software advances. At the same time new isotope systems, including U-Th dating of carbonates, have developed comparable precision. But the larger, inter-related scientific challenges envisioned at EARTHTIME's inception remain - for instance, precisely calibrating the global geologic timescale, estimating rates of change around major climatic perturbations, and understanding evolutionary rates through time - and increasingly require that data from multiple geochronometers be combined. To solve these problems, the next two decades of uranium-daughter geochronology will require further advances in accuracy, precision, and reproducibility. The U-Th system has much in common with U-Pb, in that both parent and daughter isotopes are solids that can easily be weighed and dissolved in acid, and have well-characterized reference materials certified for isotopic composition and/or purity. For U-Pb, improving lab-to-lab reproducibility has entailed dissolving precisely weighed U and Pb metals of known purity and isotopic composition together to make gravimetric solutions, then using these to calibrate widely distributed tracers composed of artificial U and Pb isotopes. To mimic laboratory measurements, naturally occurring U and Pb isotopes were also mixed in proportions to mimic samples of three different ages, to be run as internal

  17. Genesis of the Bangbule Pb-Zn-Cu polymetallic deposit in Tibet, western China: Evidence from zircon U-Pb geochronology and S-Pb isotopes

    Science.gov (United States)

    Kan, Tian; Zheng, Youye; Gao, Shunbao

    2016-04-01

    The Banbule Pb-Zn-Cu skarn deposit is located in the Longger-Gongbujiangda volcanic magma arc in the Gangdese-Nyainqentanglha Plate. It is the only lead-zinc polymetallic deposit discovered in the westernmost Nyainqentanglha metallogenic belt. The measured and indicated resources include 0.9 Mt of Pb+Zn (4.77% Pb and 4.74% Zn, respectively), 6499 t of Cu, and 178 t of Ag (18.75g/t Ag). The orebodies mainly occur as lenses, veins and irregular shapes in the contact zone between the quartz-porphyry and limestone of the Upper Permian Xiala Formation, or in the boundaries between limestone and sandstone. Pb-Zn-Cu mineralization in the Banbule deposit is closely associated with skarns. The ore minerals are dominated by galena, sphalerite, chalcopyrite, bornite, and magnetite, with subordinate pyrite, malachite, and azurite. The gangue minerals are mainly garnet, actinolite, diopside, quartz, and calcite. The ore-related quartz-porphyry displays LA-ICP-MS zircon U-Pb age of 77.31±0.74 Ma. The δ34S values of sulfides define a narrow range of -0.8 to 4.7‰ indicating a magmatic source for the ore-forming materials. Lead isotopic systematics yield 206Pb/204Pb of 18.698 to 18.752, 207Pb/204Pb of 15.696 to 15.760, and 208Pb/204Pb of 39.097 to 39.320. The data points are constrained around the growth curves of upper crust and orogenic belt according to the tectonic discrimination diagrams. The calculated Δβ - Δγ values plot within the magmatic field according to the discrimination diagram of Zhu et al. (1995). The S-Pb isotopic data suggest that Bangbule is a typical skarn deposit, and the Pb-Zn-Cu mineralization is genetically related to the quartz-porphyry in the mining district. The discovery of the Bangbule deposit indicates that there is metallogenic potential in the westernmost Nyainqentanglha belt, which is of great importance for the exploration work in this area.

  18. 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

    Science.gov (United States)

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

    2012-12-01

    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

  19. Enhanced provenance interpretation using combined U-Pb and (U-Th)/He double dating of detrital zircon grains from lower Miocene strata, proximal Gulf of Mexico Basin, North America

    Science.gov (United States)

    Xu, Jie; Stockli, Daniel F.; Snedden, John W.

    2017-10-01

    Detrital zircon U-Pb analysis is an effective approach for investigating sediment provenance by relating crystallization age to potential crystalline source terranes. Studies of large passive margin basins, such as the Gulf of Mexico Basin, that have received sediment from multiple terranes with non-unique crystallization ages or sedimentary strata, benefit from additional constraints to better elucidate provenance interpretation. In this study, U-Pb and (U-Th)/He double dating analyses on single zircons from the lower Miocene sandstones in the northern Gulf of Mexico Basin reveal a detailed history of sediment source evolution. U-Pb age data indicate that most zircon originated from five major crystalline provinces, including the Western Cordillera Arc (1800 Ma) terranes. Zircon (U-Th)/He ages record tectonic cooling and exhumation in the U.S. since the Mesoproterozoic related to the Grenville to Laramide Orogenies. The combined crystallization and cooling information from single zircon double dating can differentiate volcanic and plutonic zircons. Importantly, the U-Pb-He double dating approach allows for the differentiation between multiple possible crystallization-age sources on the basis of their subsequent tectonic evolution. In particular, for Grenville zircons that are present in all of lower Miocene samples, four distinct zircon U-Pb-He age combinations are recognizable that can be traced back to four different possible sources. The integrated U-Pb and (U-Th)/He data eliminate some ambiguities and improves the provenance interpretation for the lower Miocene strata in the northern Gulf of Mexico Basin and illustrate the applicability of this approach for other large-scale basins to reconstruct sediment provenance and dispersal patterns.

  20. U-Pb zircon geochronology and Sm-Nd-Pb isotopic constraint for precambrian plutonic rocks in the northeastern part of Ryeongnam massif, Korea

    International Nuclear Information System (INIS)

    Chang, Ho-Wan

    2003-01-01

    The Ryeongnam massif is composed of Precambrian gneisses, Paleozoic and Mesozoic sedimentary rocks and extensive Triassic-Jurassic plutonic rocks of felsic to mafic composition. In the northeast Ryeongnam massif, the oldest rocks belong to the Sobaegsan gneiss complex, which is composed of orthogneisses, paragneisses and mafic plutonic rocks. U-Pb zircon ages for the felsic and mafic intrusive bodies within the Sobaegsan gneiss complex are: the Icheon granite gneiss, 2357±43 and 2342±47 Ma; the Buncheon granite gneiss, 1963±5 Ma; the Pyeonghae granite gneiss, 1936±21 Ma; the Ogbang amphibolite, 1918±10 Ma; the Imwon leucogranite gneiss, 1826±20 Ma. The Hyeondong biotite schist, which is intruded by the Buncheon granite gneiss and the Ogbang amphibolite, yielded an age of 2271±44 Ma. The Nd-Sm-Pb isotopic data indicate that the felsic plutonic rocks are derived from an older Archean crust. The Nd T DM ages are Archean, and the εNd values are negative for the felsic rocks and positive for the amphibolite. Common Pb isotope compositions also indicate a crustal source for the felsic intrusives. The U-Pb ages of Precambrian rocks of the Ryeongnam massifs are similar to those in the Gyeonggi massif, and may have a similar crustal evolutionary history. The Precambrian rocks of South Korea could be related either to the North China block or to the South China block, as the isotope ages and patterns are not unique to either block. Similarly, a geologic correlation with Japan, although possible, is tenuous at present. (author)

  1. Pb-Pb geochronology in zircon of the basement rocks of the southern portion of the Araguaia Belt - Paraiso do Tocantins region, Brazil

    International Nuclear Information System (INIS)

    Arcanjo, Silvia Helena de Souza; Moura, Candido Augusto Veloso

    2000-01-01

    Recent geochronological studies of the basement orthogneisses of the northern region of the Araguaia belt showed the occurrence of both Archean (2.85 Ga) and Early Proterozoic (1.85 Ga) rocks. The oldest gneisses were grouped in the Colmeia Complex, the younger were named the Cantao Gneiss. In the southern portion of the Araguaia belt, the basement sequences include metavolcanic-sedimentary rocks, tonalitic, calc-silicate, and alkaline gneisses, and granitic rocks. All of these sequences have been considered as Archean in age, although no geochronological data are available to test this hypothesis. The metavolcanic-sedimentary rocks are included in the Rio do Coco Group, while the granitic rocks are represented by the Serrote and Matanca granites. The tonalitic and calc-silicate gneisses were grouped with Colmeia and Rio do Mangues Complexes, and the alkaline gneisses in the Monte Santo Suite. These gneisses were investigated geochronologically by single zircon Pb-evaporation methods in order to define their stratigraphic interrelationships, and to contribute to a greater understanding of the geological evolution of this crustal segment. The single zircon ages of the tonalitic and calc-silicate gneisses range between 1.8 and 2.1 Ga. An age of about 1.0 Ga, was obtained for the alkaline gneisses of Serra da Estrela (Monte Santo Suite). These data along with the single zircon age of 1.85 Ga. published for the Serrote Granite, indicate the widespread occurrence of Proterozoic rocks in the basement of the southern part of Araguaia belt. However, the existence of Arquean sequences in this region can not be discarded since the volcanic-sedimentary rocks of the Rio do Coco Group have not as yet been dated. The correlation of tonalitic and calc-silicate gneisses with the Colmeia Complex was not confirmed, so these gneisses have been grouped with the Early proterozoic Rio dos Mangues Complex. Thus, it is suggested that a significant part of the basement of the southern

  2. U-Pb systematics of zircon and titanite from the Gardnos impact structure, Norway: Evidence for impact at 546 Ma?

    Science.gov (United States)

    Kalleson, E.; Corfu, F.; Dypvik, H.

    2009-05-01

    Zircon and titanite were investigated in impactites of the Gardnos structure, a crater formed in Sveconorwegian (ca. 1 Ga) crust, which was then overridden in the Devonian by Caledonian nappes. Observed deformation features in zircons are granular texture, planar microstructures, and likely the incorporation of organic carbon during impact causing black staining of the zircon grains. The grains were studied by scanning electron microscopy (SEM) and cathode luminescence (CL) and dated by U-Pb isotope dilution - thermo-ionization mass spectrometry (ID-TIMS). Zircon grains without impact related features have U-Pb data showing moderate discordance (5-13%) and indicating formation ages mostly in the range of 1600-1000 Ma, except detrital zircon ages as old as >2481 Ma, reflecting the diversity of target rocks in the area. Titanite with concordant ages of 995-999 Ma dates metamorphism during final juxtaposition of the Telemarkia on the Idefjorden terrane to the east. Zircon grains with demonstrated or presumed shock features yield highly discordant (14-40%) U-Pb data, with a majority of them plotting along an array with a lower intercept of about 340 Ma reflecting the influence of the Caledonian orogeny and recent Pb-loss. One zircon grain was totally reset at 379 Ma during late Caledonian metamorphism, which also caused local growth of new titanite. A specific group of zircon grains yields data with relatively high discordance for moderate U contents, and five of these analyses, including that of a grain with proven granular or aggregate texture, fit a discordia line with an upper intercept of 546 ± 5 Ma. These features are interpreted as indicating zircon break-down to an amorphous state during impact, with subsequent recrystallization into microcrystalline aggregates causing extensive to complete Pb loss. We further suggest that their crystallinity prevented Pb loss during the Caledonian orogeny, while the small subgrain size and increasing metamictisation allowed

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

    Science.gov (United States)

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

    2015-01-09

    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. Copyright © 2015, American Association for the Advancement of Science.

  4. Simultaneous determinations of U-Pb age and REE abundances for zircons using AfF excimer laser ablation-ICPMS

    International Nuclear Information System (INIS)

    Iizuka, Tsuyoshi; Hirata, Takafumi

    2004-01-01

    Using a laser-ablation-inductively coupled plasma mass spectrometer (LA-ICPMS), U-Pb age and rare earth element (REE) abundances have been determined simultaneously from a single 20 μm ablation pit of zircon. The laser ablation system utilizing 193 nm wave-length ArF excimer laser produces stable and reproducible signal intensities resulted in good precisions on measurements of element concentrations and isotopic ratios. Because of the higher energy density of the deep ultra-violet laser beam, ablation fractionation between Pb and U were reduced even with the prolonged ablation, and thus accuracy of Pb-U age was improved significantly. A chicane-type ion lens system was applied to a quadrupole-based ICPMS instrument. With the chicane ion lens, higher elemental sensitivity (4 times or light mass range and 3 times for mid to heavy mass range) and lower white background ( 238 U- 206 Pb ages for Nancy standard zircon (Nancy 91500), SHRIMP calibration standard zircon (SL13) and Antarctic zircon (PMA7) obtained in this study were 1064 ± 24 Ma, 569 ± 78 Ma and 2438 ± 101 Ma (2-sigma), respectively. Relative age differences from previous reports were 0.2%, 0.4% and 3.2% respectively, demonstrative of high reliability of the method. The REE abundances in zircon samples were calibrated using a NIST 610 glass standard reference material. The resulting REE abundance data for zircons (Nancy 91500 and SL13) show good agreement with those for literature values within the analytical precision of ∼20%. The matrix effect that may occur between the synthetic glass standard and zircon crystals is obviously smaller than the precision and thus negligible for this precision levels. The data presented here demonstrate clearly that the combination of ArF excimer laser an ICPMS equipped with the chicane ion lens has a potential to become a significant tool for zircon geochemistry. (author)

  5. Precise U-Pb Zircon Dating of the Syenite Phase from the Ditrau Alkaline Igneous Complex

    Directory of Open Access Journals (Sweden)

    Pană Dinu

    2000-04-01

    Full Text Available The Ditrău igneous complex represents the largest alkaline intrusion in the Carpathian-Pannonian region consisting of a plethora of rock types formed by complicated magmatic and metasomatic processes. A detailed U-Pb zircon age study is currently underway and the results for the syenite intrusion phase is reported herein. The U-Pb zircon emplacement age of the syenite of 229.6 +1.7/-1.2 Ma documents the quasi-contemporaneous production and emplacement of the gabbro and syenite magmas. We suggest that the syenite and associated granite formed by crustal melting during the emplacement of the mantle derived gabbroic magma around 230 Ma. The thermal contact aureole produced by the Ditrău alkaline igneous complex constrains the main tectonism recorded by surrounding metamorphic lithotectonic assemblages to be pre-Ladinian.

  6. Geochemical fingerprints and pebbles zircon geochronology

    Indian Academy of Sciences (India)

    Home; Journals; Journal of Earth System Science; Volume 125; Issue 7. 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). Jin-Long Ni Jun-Lai Liu Xiao-Ling Tang ...

  7. Permian A-type rhyolites of the Muráň Nappe, Inner Western Carpathians, Slovakia: in-situ zircon U-Pb SIMS ages and tectonic setting

    Science.gov (United States)

    Ondrejka, Martin; Li, Xian-Hua; Vojtko, Rastislav; Putis, Marian; Uher, Pavel; Sobocký, Tomas

    2018-04-01

    Three representative A-type rhyolitic rock samples from the Muráň Nappe of the inferred Silicic Unit of the Inner Western Carpathians (Slovakia) were dated using the high-precision SIMS U-Pb isotope technique on zircons. The geochronological data presented in this paper is the first in-situ isotopic dating of these volcanic rocks. Oscillatory zoned zircon crystals mostly revealed concordant Permian (Guadalupian) ages: 266.6 ± 2.4 Ma in Tisovec-Rejkovo (TIS-1), 263.3 ± 1.9 Ma in Telgárt-Gregová Hill (TEL-1) and 269.5 ± 1.8 Ma in Veľká Stožka-Dudlavka (SD-2) rhyolites. The results indicate that the formation of A-type rhyolites and their plutonic equivalents are connected to magmatic activity during the Permian extensional tectonics and most likely related to the Pangea supercontinent break-up.

  8. Pb-Pb and U-Pb zircon ages of archean syntetocnic granites of the Carajas metallogenic province, northern Brazil

    International Nuclear Information System (INIS)

    Barros, Carlos Eduardo de Mesquita; Sardinha, Alex Souza; Barbosa, Jaime dos Passos de Oliveira; Krimski, Robert; Macambira, Moacir Jose Buenano

    2001-01-01

    The Carajas Metallogenic Province is located in the southeastern Amazonian Craton. It has been divided in two domains, the southernmost comprises the Rio Maria region and the northernmost corresponds to Caraj region (Souza et al. 1996). The former domain is made up of Archean greenstone sequences (2,97 Ga), TTG (2,9 Ga) and calc-alkaline granitoids (2,87 Ga) (Macambira and Lafon 1995, Leite et al. 1999, Althoff et al. 2000). The Carajas block is constituted of minor mafic granulites (3,00 Ga) and quartzofeldspathic gneisses (2,81 Ga), metavolcanosedimentary sequences (2,76 Ga) and granites (2,76 to 2,56 Ga) (Machado et al. 1991; Huhn et al. 1999, Pidgeon et al. 2000). Widespread anorogenic A-type granites are found in both areas (Docegeo 1988; Dall'Agnol et al. 1994). In the last two decades several authors (Lindenamyer et al. 1994, Barros and Barbey 1998, Huhn et al. 1999 and others) have emphasized the role of the Archean granite magmatism in the tectonicthermal evolution in the Carajas Province. In this paper we discuss the tectonic significance of the Pb- Pb and U-Pb ages obtained in some granitoids from the Carajas region. The Estrela Granite Complex and the granitoids located to the north of Parauapebas were dated by Pb- Pb evaporation zircon method (cf. Kober 1987). Data are presented considering 2σ ∼ . The Pb corrections have been done in the basis of the evolution model of Pb in double stage (cf. Stacey and Kramers 1975). U-Pb zircon method (cf. Krogh 1973, Stacey and Kramers 1975, Parrish 1987, Ludwuig 1999), recently put on routine in the Para-Iso laboratories, was employed to date the granite from the Serra do Rabo area. Analyses were carried on the Finnigan Mat 262 spectrometer (au)

  9. Zircon U-Pb and molybdenite Re-Os geochronology and Sr-Nd-Pb-Hf isotopic constraints on the genesis of the Xuejiping porphyry copper deposit in Zhongdian, Northwest Yunnan, China

    Science.gov (United States)

    Leng, Cheng-Biao; Zhang, Xing-Chun; Hu, Rui-Zhong; Wang, Shou-Xu; Zhong, Hong; Wang, Wai-Quan; Bi, Xian-Wu

    2012-10-01

    The Xuejiping porphyry copper deposit is located in northwestern Yunnan Province, China. Tectonically, it lies in the southern part of the Triassic Yidun island arc. The copper mineralization is mainly hosted in quartz-dioritic and quartz-monzonitic porphyries which intruded into clastic-volcanic rocks of the Late Triassic Tumugou Formation. There are several alteration zones including potassic, strong silicific and phyllic, argillic, and propylitic alteration zones from inner to outer of the mineralized porphyry bodies. The ages of ore-bearing quartz-monzonitic porphyry and its host andesite are obtained by using the zircon SIMS U-Pb dating method, with results of 218.3 ± 1.6 Ma (MSWD = 0.31, N = 15) and 218.5 ± 1.6 Ma (MSWD = 0.91, N = 16), respectively. Meanwhile, the molybdenite Re-Os dating yields a Re-Os isochronal age of 221.4 ± 2.3 Ma (MSWD = 0.54, N = 5) and a weighted mean age of 219.9 ± 0.7 Ma (MSWD = 0.88). They are quite in accordance with the zircon U-Pb ages within errors. Furthermore, all of them are contemporary with the timing of the Garzê-Litang oceanic crust subduction in the Yidun arc. Therefore, the Xuejiping deposit could be formed in a continental margin setting. There are negative ɛNd(t) values ranging from -3.8 to -2.1 and relatively high initial 87Sr/86Sr ratios from 0.7051 to 0.7059 for the Xuejiping porphyries and host andesites. The (206Pb/204Pb)t, (207Pb/204Pb)t and (208Pb/204Pb)t values of the Xuejiping porphyries and host andesites vary from 17.899 to 18.654, from 15.529 to 15.626, and from 37.864 to 38.52, respectively, indicative of high radiogenic Pb isotopic features. In situ Hf isotopic analyses on zircons by using LA-MC-ICP-MS exhibit that there are quite uniform and slightly positive ɛHf(t) values ranging from -0.2 to +3.2 (mostly between 0 and +2), corresponding to relatively young single-stage Hf model ages from 735 Ma to 871 Ma. These isotopic features suggest that the primary magmas of the Xuejiping porphyries and

  10. Combined U-Pb and Lu-Hf isotope analyses by laser ablation MC-ICP-MS: methodology and applications

    Energy Technology Data Exchange (ETDEWEB)

    Matteini, Massimo; Dantas, Elton L.; Pimentel, Marcio M.; Bühn, Bernhard, E-mail: massimo@unb.br [Universidade de Brasilia (UnB), DF (Brazil). Instituto de Geociencias

    2010-06-15

    The Lutetium-Hafnium isotopic system represents one of the most innovative and powerful tools for geochronology and isotopic studies. Combined U-Pb and Lu-Hf in situ analyses on zircon by LA-MC-ICP-MS permit to characterize isotopically the host magma from which it crystallized furnishing significant information for sediment provenance and crustal evolution studies. In this paper e describe the Lu-Hf systematic by LA-MC-ICP-MS developed in the laboratory of Geochronology of the University of Brasilia and report the results obtained by repeated analyses of {sup 176}Hf/{sup 177}Hf isotopic ratio of three zircon standards: GJ-1 = 0.282022 ± 11 (n=56), Temora 2 = 0.282693 ± 14 (n=25) and UQZ = 0.282127 ± 33 (n=11). The {sup 176}Hf/{sup 177}Hf ratio (0.282352 ± 22, n=14) of gem quality zircon used as in-house standard have been also characterized. As a geological application, we analyzed two complex zircons selected from a migmatitic rocks from the Borborema Province, NE Brazil. On the basis of U-Pb and Lu-Hf data, two main crystallization events have been identified in both studied zircons. An older event at ca. 2.05 Ga recognized in the inherited cores represents a well-characterized paleoproterozoic magmatic event that affected the whole Borborema Province. A second crystallization event at ∼ 575 Ma, recognized at the rims, represents a Neoproterozoic (Brazilian) high grade metamorphic-magmatic event. (author)

  11. Significance of zircon U-Pb ages from the Pescadero felsite, west-central California coast ranges

    Science.gov (United States)

    McLaughlin, Robert J.; Moore, Diane E.; ,; Martens, UWE C.; Clark, J.C.

    2011-01-01

    Weathered felsite is associated with the late Campanian–Maastrichtian Pigeon Point Formation near Pescadero, California. Poorly exposed, its age and correlation are uncertain. Is it part of the Pigeon Point section west of the San Gregorio–Hosgri fault? Does it rest on Nacimiento block basement? Is it dextrally offset from the Oligocene Cambria Felsite, ∼185 km to the southeast? Why is a calc-alkaline hypabyssal igneous rock intrusive into the outboard accretionary prism? To address these questions, we analyzed 43 oscillatory-zoned zircon crystals from three incipiently recrystallized pumpellyite ± prehnite ± laumontite-bearing Pescadero felsite samples by sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) techniques. Thirty-three zircons gave late Mesozoic U-Pb ages, with single-grain values ranging from 81 to 167 Ma; ten have pre-Mesozoic, chiefly Proterozoic ages. A group of the four youngest Pescadero zircons yielded an apparent maximum igneous age of ca. 86–90 Ma. Reflecting broad age scatter and presence of partly digested sandstone inclusions, we interpret the rest of the zircons (perhaps all) as xenocrysts. Twenty-three zircons were separated and analyzed from two samples of the similar Cambria Felsite, yielding a unimodal 27 Ma U-Pb age. Clearly, the origin of the Upper Oligocene Cambria Felsite is different from that of the Upper Cretaceous Pescadero felsite; these rocks are not correlated, and do not constrain displacement along the San Gregorio–Hosgri fault. Peak ages differ slightly, but relative probability curves for Mesozoic and pre-Mesozoic Pescadero zircons compare well, for example, with abundant U-Pb age data for detrital zircons from Franciscan metaclastic strata ∼100 km to the east in the Diablo Range–San Francisco Bay area, San Joaquin Great Valley Group turbidites, Upper Cretaceous Nacimiento block Franciscan strata, and Upper

  12. Age revision of the Neotethyan arc migration into the southeast Urumieh-Dokhtar belt of Iran: Geochemistry and U-Pb zircon geochronology

    Science.gov (United States)

    Hosseini, Mohammad Reza; Hassanzadeh, Jamshid; Alirezaei, Saeed; Sun, Weidong; Li, Cong-Ying

    2017-07-01

    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

  13. New U-Pb SHRIMP zircon ages for pre-variscan orthogneisses from Portugal and their bearing on the evolution of the Ossa-Morena tectonic zone

    Directory of Open Access Journals (Sweden)

    Cordani Umberto G.

    2006-01-01

    Full Text Available New SHRIMP U-Pb zircon ages for the Portalegre and Alcáçovas orthogneisses document a complex pre- Variscan history for the Iberian basement in Portugal. The available geochemical and geochronological data for the Alcáçovas orthogneiss (ca. 540 Ma tend to favor its involvement in a Cadomian orogenic event. This is consistent with the development of an active continental margin setting at the end of the Proterozoic and supports a Gondwanan provenance for the Iberian crust. On the other hand, the Ordovician emplacement age obtained for the magmatic precursors of the Portalegre orthogneisses (497?10 Ma provides additional evidence for the occurrence of rift-related magmatic activity during the Lower Paleozoic.

  14. Geotectonic structural interpretation of the basement complex at the eastern border of the Espinhaco ridge, in Guanhaes and Gouveia region, based on an integration of their U/Pb and K/Ar geochronology united

    International Nuclear Information System (INIS)

    Teixeira, W.; Salvador, E.D.; Siga Junior, O.; Sato, K.; Dossin, I.A.; Dossin, T.M.

    1990-01-01

    The basement complex at the eastern border of the Espinhaco ridge is composed of predominantly gneissic rocks which were subjected to migmatization and granitization. Overall the area shows complex tectonic evolution with recurrence of tectonomagmatic and metamorphic events as supported by geological, geochronological and structural studies. The Rb/Sr geochronology carried out on the basement rocks and metavolcanics from the Espinhaco interpreted together with the published U-Pb, Rb-Sr and K-Ar data defines the following scenario for the Precambrian crustal evolution. 1. Primary origin of a sialic crust at 2.97-2.84 Ga. ago as supported by U-Pb zircon ages. 2. Crustal reworking of the Archean crust and subordinate juvenile accretion from upper mantle during the 2.2-2.OGa. period, as suggested by the isochrons. 3. Recurrence of Middle Proterozoic events over the basement rocks (gneisses and charnockites) and metavolcanics of the Espinhaco as showed by isochrons. 4. Development of Late Proterozoic migmatization over the basement rocks (0.75 Ga., R.I.= 0.787) is association with Collisional tectonics and resetting of K-Ar mineral systems. (author)

  15. Mass-spectrometric mining of Hadean zircons by automated SHRIMP multi-collector and single-collector U/Pb zircon age dating: The first 100,000 grains

    Science.gov (United States)

    Holden, Peter; Lanc, Peter; Ireland, Trevor R.; Harrison, T. Mark; Foster, John J.; Bruce, Zane

    2009-09-01

    The identification and retrieval of a large population of ancient zircons (>4 Ga; Hadean) is of utmost priority if models of the early evolution of Earth are to be rigorously tested. We have developed a rapid and accurate U-Pb zircon age determination protocol utilizing a fully automated multi-collector ion microprobe, the ANU SHRIMP II, to screen and date these zircons. Unattended data acquisition relies on the calibration of a digitized sample map to the Sensitive High Resolution Ion MicroProbe (SHRIMP) sample-stage co-ordinate system. High precision positioning of individual grains can be produced through optical image processing of a specified mount location. The focal position of the mount can be optimized through a correlation between secondary-ion steering and the spot position on the target. For the Hadean zircon project, sample mounts are photographed and sample locations (normally grain centers) are determined off-line. The sample is loaded, reference points calibrated, and the target positions are then visited sequentially. In SHRIMP II multiple-collector mode, zircons are initially screened (ca. 5 s data acquisition) through their 204Pb corrected 207Pb/206Pb ratio; suitable candidates are then analyzed in a longer routine to obtain better measurement statistics, U/Pb, and concentration data. In SHRIMP I and SHRIMP RG, we have incorporated the automated analysis protocol to single-collector measurements. These routines have been used to analyze over 100,000 zircons from the Jack Hills quartzite. Of these, ca. 7%, have an age greater than 3.8 Ga, the oldest grain being 4372 +/- 6 Ma (2[sigma]), and this age is part of a group of analyses around 4350 Ma which we interpret as the age when continental crust first began to coalesce in this region. In multi-collector mode, the analytical time taken for a single mount with 400 zircons is approximately 6 h; whereas in single-collector mode, the analytical time is ca. 17 h. With this productivity, we can produce

  16. The White Nile as a source for Nile sediments: Assessment using U-Pb geochronology of detrital rutile and monazite

    Science.gov (United States)

    Be'eri-Shlevin, Yaron; Avigad, Dov; Gerdes, Axel

    2018-04-01

    Basement terranes exposed at the headwaters of the White Nile include Archean-Paleoproterozoic rocks of the Congo Craton, whose northern sectors were severely reworked during Neoproterozoic orogeny. New U-Pb analyses of detrital rutile and monazite from early Quaternary to Recent coastal quartz sands of Israel, at the northeast extension of the Nile sedimentary system, yield mostly late Neoproterozoic ages, with a dominant peak at ca. 600 Ma. While derivation from the reworked sectors of the Craton cannot be negated, the absence of pre-Neoproterozoic rutile and monazite indicates that the detrital contribution from the Congo cratonic nuclei into the main Nile was insignificant. The near absence of White Nile basement-derived heavy minerals from the Nile sands arriving at the Eastern Mediterranean may be explained by a number of factors such as relatively minor erosion of the Cratonic basement nuclei during the Quaternary, late connection of the White Nile to the main Nile system with a possibility that northern segments connected prior to more southerly ones, and a long-term effective sediment blockage mechanism at the mouth of White Nile. Likewise, our previous study demonstrated that Nile sands display a detrital zircon U-Pb-Hf pattern consistent with significant recycling of NE African Paleozoic sediments. It is thus plausible that any detrital contribution from White Nile basement rocks was thoroughly diluted by eroded Paleozoic sediments, or their recycled products, which were likely the greatest sand reservoir in the region. This study adds to previous studies showing the advantage of a multi mineral U-Pb geochronology strategy in constraining sediment provenance patterns.

  17. Phanerozoic polyphase orogenies recorded in the northeastern Okcheon Belt, Korea from SHRIMP U-Pb detrital zircon and K-Ar illite geochronologies

    Science.gov (United States)

    Jang, Yirang; Kwon, Sanghoon; Song, Yungoo; Kim, Sung Won; Kwon, Yi Kyun; Yi, Keewook

    2018-05-01

    We present the SHRIMP U-Pb detrital zircon and K-Ar illite 1Md/1M and 2M1 ages, suggesting new insight into the Phanerozoic polyphase orogenies preserved in the northeastern Okcheon Belt, Korea since the initial basin formation during Neoproterozoic rifting through several successive contractional orogens. The U-Pb detrital zircon ages from the Early Paleozoic strata of the Taebaeksan Zone suggest a Cambrian maximum deposition age, and are supported by trilobite and conodont biostratigraphy. Although the age spectra from two sedimentary groups, the Yeongwol and Taebaek Groups, show similar continuous distributions from the Late Paleoproterozoic to Early Paleozoic ages, a Grenville-age hiatus (1.3-0.9 Ga) in the continuous stratigraphic sequence from the Taebaek Group suggests the existence of different peripheral clastic sources along rifted continental margin(s). In addition, we present the K-Ar illite 1Md/1M ages of the fault gouges, which confirm fault formation/reactivation during the Late Cretaceous to Early Paleogene (ca. 82-62 Ma) and the Early Miocene (ca. 20-18 Ma). The 2M1 illite ages, at least those younger than the host rock ages, provide episodes of deformation, metamorphism and hydrothermal effects related to the tectonic events during the Devonian (ca.410 Ma) and Permo-Triassic (ca. 285-240 Ma). These results indicate that the northeastern Okcheon Belt experienced polyphase orogenic events, namely the Okcheon (Middle Paleozoic), Songrim (Late Paleozoic to Early Mesozoic), Daebo (Middle Mesozoic) and Bulguksa (Late Mesozoic to Early Cenozoic) Orogenies, reflecting the Phanerozoic tectonic evolution of the Korean Peninsula along the East Asian continental margin.

  18. U-Pb geochronology of Gangdese (Transhimalaya) plutonism in the Lhasa-Xigaze region, Tibet

    International Nuclear Information System (INIS)

    Schaerer, U.; Allegre, C.J.; Paris-6 Univ., 75; Paris-7 Univ., 75; Xu, R.H.

    1984-01-01

    A series of different plutons from the Lhasa-Xigaze segment of the Gangdese (Transhimalaya) belt has been studied by high-resolution U-Pb analyses of zircon (using zircon fractions of 5-100 grains each, selected upon specific grain characteristics). For two diorites, located east of Xigaze (Dazhuka), the zircons yield concordant ages of 93.4 +- 1.0 and 94.2 +- 1.0 m.y., respectively. Also concordant ages of 41.1 +- 0.4 and 41.7 +- 0.4 m.y. have been obtained for two granodiorites, collected southwest of Lhasa (Qushui). The precision on the ages of two granites from the Xigaze and Lhasa area, is limited by two factors: the presence of inherited radiogenic lead and the occurrence of subsequent lead loss. However, some concordant zircons, detected in both granites, define approximate ages of about 67 and 53 m.y., respectively. The inherited lead components show the melting of Precambrian material was involved in magma genesis. The U-Pb ages substantiate a magmatic activity lasting from mid-Cretaceous (Cenomanian) to Eocene (Lutetian) time. Such a period of plutonism at the southern margin of Eurasia, as well as the occurrence of magma generation from continental crust, suggest that the Gangdese range results from the subduction of Tethys oceanic lithosphere (Indian plate) underneath Eurasia (Eurasian plate). If this model is true, the collision of India with Eurasia (Along the Lhasa-Xigaze sector) postdates the emplacement of the 41 m.y. old Gangdese granodiorites, i.e. the collision occurred after Lutetian time. (orig.)

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

    Science.gov (United States)

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

    2005-01-01

    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

  20. U-Pb dating of large zircons in low-temperature jadeitite from the Osayama serpentinite melange, southwest Japan: insights into the timing of serpentinization

    Science.gov (United States)

    Tsujimori, T.; Liou, J.G.; Wooden, J.; Miyamoto, T.

    2005-01-01

    Crystals of zircon up to 3 mm in length occur in jadeitite veins in the Osayama serpentinite mélange, Southwest Japan. The zircon porphyroblasts show pronounced zoning, and are characterized by both low Th/U ratios (0.2-0.8) and low Th and U abundances (Th = 1-81 ppm; U = 6-149 ppm). They contain inclusions of high-pressure minerals, including jadeite and rutile; such an occurrence indicates that the zircon crystallized during subduction-zone metamorphism. Phase equilibria and the existing fluid-inclusion data constrain P-T conditions to P > 1.2 GPa at T > 350°C for formation of the jadeitite. Most U/Pb ages obtained by SHRIMP-RG are concordant, with a weighted mean 206Pb/238U age of 472 ± 8.5 Ma (MSWD = 2.7, n = 25). Because zircon porphyroblasts contain inclusions of high-pressure minerals, the SHRIMP U-Pb age represents the timing of jadeitite formation, i.e., the timing of interaction between alkaline fluid and ultramafic rocks in a subduction zone. Although this dating does not provide a direct time constraint for serpentinization, U-Pb ages of zircon in jadeitite associated with serpentinite result in new insights into the timing of fluid-rock interaction of ultramafic rocks at a subduction zone and the minimum age for serpentinization.

  1. Zircon U-Pb age and Hf-O isotopes of felsic rocks from the Atlantis Bank, Southwest Indian Ridge

    Science.gov (United States)

    Liu, C. Z.; Zhang, W. Q.

    2017-12-01

    Hole U1473A was drilled to 790 meters below seafloor on the Atlantis Bank, an oceanic core complex in the Southwest Indian Ridge, where the upper crust has been removed by detachment faulting. The recovered core consists dominantly of olivine gabbro, with subordinate gabbro, gabbro with varying Fe-Ti oxide concentrations. Felsic veins intermittently occur throughout the whole core section. Zircons separated from twenty-four felsic samples have been conducted for U-Pb dating and O isotope analyses on the Cameca 1280 and Lu-Hf isotopes by laser ablation coupled with a MC-ICPMS. The zircons have highly variable contents of U (12-2078 ppm) and Th (5-801 ppm), yielding Th/U ratios of 0.33-0.81. They are typical oceanic zircons as defined by the trace element discrimination plots of Grimes et al. (2015). The weighted mean 206Pb/238U ages of the analyzed zircons vary from 11.29 to 12.57 Ma. Age differences between felsic veins throughout the whole core are not resolved within analytical uncertainty of the SIMS measurements. All felsic samples have similar zircon Hf isotope compositions, with initial 176Hf/177Hf ratios of 0.283126-0.283197 and ɛHf values of 12.76-15.27. Zircons from all felsic samples but one have mantle-like δ18O values of 5.14-5.50‰. Zircons from one sample show partial resorption or total recrystallization; in comparison, they have lower δ18O values of 4.81±0.21‰. Such characteristics provide clear evidence for hydrothermal alteration after magmatic intrusion.

  2. Petrochemistry and zircon U-Pb geochronology of granitic rocks in the Wang Nam Khiao area, Nakhon Ratchasima, Thailand: Implications for petrogenesis and tectonic setting

    Science.gov (United States)

    Fanka, Alongkot; Tsunogae, Toshiaki; Daorerk, Veerote; Tsutsumi, Yukiyasu; Takamura, Yusuke; Sutthirat, Chakkaphan

    2018-05-01

    Carboniferous biotite granite, Late Permian hornblende granite, and Triassic biotite-hornblende granite, all of which belong to the Eastern Granite Belt, expose in the Wang Nam Khiao area, Nakhon Ratchasima, northeastern Thailand. The Carboniferous biotite granite is dominated by quartz, K-feldspar, plagioclase, and biotite. The Late Permian hornblende granite contains dominant assemblages of plagioclase, quartz, K-feldspar, hornblende, and minor amount of biotite, while the Triassic biotite-hornblende granite consists of quartz, plagioclase, K-feldspar with small amounts of biotite, and hornblende. The REE patterns with steep decrease from light to heavy REE together with the LILE (e.g. K, Sr) enrichment and depletion of some particular HFSE (e.g. Nb, Ti) indicate low degree of partial melting. Mineral chemistry of biotite and hornblende in the granites reflects crystallization from hydrous calc-alkaline arc-derived magmas possibly formed by subduction. Amphibole-plagioclase thermometry and Al-in-hornblende barometry indicate that the Late Permian hornblende granite and the Triassic biotite-hornblende granite may have equilibrated at 3.0-5.8 kbar/700-820 °C and 2.0-3.2 kbar/600-750 °C, respectively, in the middle-upper crust (about 10-15 km depth). Zircon U-Pb geochronology of the Carboniferous biotite granite, Late Permian hornblende granite and Triassic biotite-hornblende granite yielded intrusion ages of 314.6-284.9 Ma, 253.4 Ma, and 237.8 Ma, respectively, which implies multiple episodes of arc-magmatism formed by Palaeo-Tethys subduction beneath Indochina Terrane during Late Carboniferous/Early Permian, Late Permian and Middle Triassic.

  3. Timing of the Permian-Triassic biotic crisis: implications from new zircon U/Pb age data (and their limitations)

    Science.gov (United States)

    Mundil, Roland; Metcalfe, Ian; Ludwig, Kenneth R.; Renne, Paul R.; Oberli, Felix; Nicoll, Robert S.

    2001-04-01

    The most profound biotic crisis in the Earth's history, causing the near extinction of both terrestrial and marine life, occurred at the end of the Permian period about 253 Myr ago and marks the Paleozoic-Mesozoic era boundary. The cause of this event is still a matter of vigorous debate, with both brief and catastrophic as well as gradual mechanisms having been proposed. Similar to a recent landmark study, this study uses the U-Pb method on zircons from the uppermost Permian/lowermost Triassic ash fall deposits at Meishan (Zhejiang Province, SE China) in order to examine time and rate constraints for these events. The results of both this study and previous work show that for these ash layers, the effects of Pb loss are combined with varying amounts and sources of inheritance, resulting in an age scatter which prohibits the extraction of a statistically robust age in many cases. Though the effects of Pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid (as opposed to commonly applied air abrasion) prior to analysis, the presence within a single ash layer of multiple generations of older xenocrysts (in many cases only slightly older than the depositional age) has made quantitative interpretation even more difficult. When these combined phenomena bias individual zircon ages by less than a percent, they are extremely difficult to deconvolute, and, if multi-grain analyses are used, can become impossible to recognize (because of the resulting age averaging). Monte Carlo simulations using actual measurements of individual zircon crystals show that age excursions due to Pb loss and xenocrystic contamination for the Meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi-grain zircon samples are compared. Thus this study uses only high-precision analyses of single crystals, whether from our work or that of previous studies. Three main conclusions have emerged. First, our

  4. Cooling rates and the depth of detachment faulting at oceanic core complexes: Evidence from zircon Pb/U and (U-Th)/He ages

    Science.gov (United States)

    Grimes, Craig B.; Cheadle, Michael J.; John, Barbara E.; Reiners, P.W.; Wooden, J.L.

    2011-01-01

    Oceanic detachment faulting represents a distinct mode of seafloor spreading at slow spreading mid-ocean ridges, but many questions persist about the thermal evolution and depth of faulting. We present new Pb/U and (U-Th)/He zircon ages and combine them with magnetic anomaly ages to define the cooling histories of gabbroic crust exposed by oceanic detachment faults at three sites along the Mid-Atlantic Ridge (Ocean Drilling Program (ODP) holes 1270D and 1275D near the 15??20???N Transform, and Atlantis Massif at 30??N). Closure temperatures for the Pb/U (???800??C-850??C) and (U-Th)/He (???210??C) isotopic systems in zircon bracket acquisition of magnetic remanence, collectively providing a temperature-time history during faulting. Results indicate cooling to ???200??C in 0.3-0.5 Myr after zircon crystallization, recording time-averaged cooling rates of ???1000??C- 2000??C/Myr. Assuming the footwalls were denuded along single continuous faults, differences in Pb/U and (U-Th)/He zircon ages together with independently determined slip rates allow the distance between the ???850??C and ???200??C isotherms along the fault plane to be estimated. Calculated distances are 8.4 ?? 4.2 km and 5.0 2.1 km from holes 1275D and 1270D and 8.4 ?? 1.4 km at Atlantis Massif. Estimating an initial subsurface fault dip of 50 and a depth of 1.5 km to the 200??C isotherm leads to the prediction that the ???850??C isotherm lies ???5-7 km below seafloor at the time of faulting. These depth estimates for active fault systems are consistent with depths of microseismicity observed beneath the hypothesized detachment fault at the TAG hydrothermal field and high-temperature fault rocks recovered from many oceanic detachment faults. Copyright 2011 by the American Geophysical Union.

  5. U-Pb zircon in situ dating with LA-MC-ICP-MS using a mixed detector configuration

    Energy Technology Data Exchange (ETDEWEB)

    Chemale Junior, Farid, E-mail: fchemale@unb.br [Universidade de Brasilia (UnB), DF (Brazil). Inst. de Geociencias; Kawashita, Koji; Dussin, Ivo A. [Universidade de Sao Paulo (USP), SP (Brazil). Centro de Pesquisas Geocronologicas; Avila, Janaina N. [Australian National University, Canberra, (Australia). Research School of Earth Sciences; Justino, Dayvisson; Bertotti, Anelise [Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, RS (Brazil). Curso de Pos-Graduacao em Geociencias

    2012-06-15

    The LA-MC-ICP-MS method applied to U-Pb in situ dating is still rapidly evolving due to improvements in both lasers and ICP-MS. To test the validity and reproducibility of the method, 5 different zircon samples, including the standard Temora-2, ranging in age between 2.2 Ga and 246 Ma, were dated using both LA-MC-ICP-MS and SHRIMP. The selected zircons were dated by SHRIMP and, after gentle polishing, the laser spot was driven to the same site or on the same zircon phase with a 213 nm laser microprobe coupled to a multi-collector mixed system. The data were collected with a routine spot size of 25 {mu}m and, in some cases, of 15 and 40 {mu}m. A careful cross-calibration using a diluted U-Th-Pb solution to calculate the Faraday reading to counting rate conversion factors and the highly suitable GJ-1 standard zircon for external calibrations were of paramount importance for obtaining reliable results. All age results were concordant within the experimental errors. The assigned age errors using the LA-MC-ICP-MS technique were, in most cases, higher than those obtained by SHRIMP, but if we are not faced with a high resolution stratigraphy, the laser technique has certain advantages. (author)

  6. Pb-Pb Dating mono zircon from Gloria Norte Massif, Sergipano belt; Datacao de Pb-Pb em monozircao do Macico Gloria Norte, faixa Sergipana

    Energy Technology Data Exchange (ETDEWEB)

    Lisboa, V.A.C.; Rosa, M.L.S.; Conceicao, H., E-mail: viniciuslisboa1@hotmail.com [Pós-Graduacão em Geociências e Análise de Bacias/Universidade Federal de Sergipe, São Cristóvão, SE (Brazil); Macambira, M.J.B.; Galarza, M.A. [Pós-Graduacão em Geociências/Universidade Federal do Pará, Belem, PA (Brazil); Rios, D.C. [Departamento de Geologia/Universidade Federal da Bahia, Salvador, BA (Brazil)

    2014-11-01

    The Gloria Norte Massif (MGN) has about 50 km{sup 2} in area, lies between the cities of Nossa Senhora da Gloria and Monte Alegre, northeastern Sergipe state, is composed of monzonitic rocks, being intrusive in metasediments Domain Macurure, in the Sergipano Belt. In this study Pb-Pb geochronological determinations were performed using the evaporation technique single crystals of zircon in MGN. Two samples were analyzed (03A and 12A) and the results were ages of 646 ± 2.5 Ma and 650 ± 4.5 Ma, place this magmatism in the Cryogenian period. These data may imply that around 645-655 Ma, there was an active magmatic arc in this region of the Sergipano Belt. (author)

  7. The SHRIMP zircon U-Pb dating of felsic volcanic rocks and its geological significance from yutian group in southern jiangxi

    International Nuclear Information System (INIS)

    Ji Chunyu; Wu Jianhua

    2010-01-01

    Past researches have showed that the Rb-Sr isochron ages of felsic end member for r hyolite-basalt b imodal volcanic rocks of Yutian Group in the Changpu and Longnan Basin in Southern Jiangxi Province are 175 ∼ 148 Ma, not only does its amplitude change more significantly, but it does not match with the Rb-Sr isochron ages (179 ∼ 173 Ma) of basic end member. As a result, I choose a method of zircon U-Pb dating with a higher accuracy, to obtain the rhyolite in the bottom of bimodal volcanic rocks in the Changpu Basin and the dacite in the top of of bimodal volcanic rocks in the Longnan Basin, whose zircon SHIRMP U-Pb age are respectively (195.2 ± 2.8) Ma and (191 ± 1.7) Ma. What's more, they are both almost the same in the error limit. It shows that the bimodal volcanic rocks in these both two basins are the product of the same session of magma movement. Simultaneously, it explains they form in a flash during the eruption intervals. According to the the newest International Stratigraphic Chart (Gradsrein et al. , 2004), in terms of geological age, the bimodal volcanic rocks in Changpu Basin and Longnan Basin, belonging to the early Early Jurassic. The zircon SHIRMP U-Pb age are distinctly older than the whole-rock Rb-Sr isochron age, it is probably because of the deviation of the dating method for the wholerock Rb-Sr isochron age. The zircon SHIRMP U-Pb age of bimodal volcanic rocks are 191 ∼ 195 Ma in Southern Jiangxi Province, which indicates that there had been an extensional environment. And after the bimodal volcanic activity, The zircon SHIRMP U-Pb age of felsic volcanic rocks are 145 ∼ 130 Ma. Both of the ages shows a as long as 45 Ma quiet period between 190 Ma and 145 Ma. It is unreasonable possible to interpreted by the single pattern of pacific plate subducting to eurasian plate. (authors)

  8. Contribution of U-Pb geochronology to characterizing the Cadomian magmatism of the southeastern French Massif Central and of the related uranium-ore at Bertholene

    International Nuclear Information System (INIS)

    Leveque, Marie-Helene

    1990-01-01

    U-Pb geochronology and whole-rock geochemistry (major, trace and rare earth elements) were applied to various ortho- and paragneiss formations of the southeastern part of the French Massif Central. The Mendic granite and the Les Palanges granite were dated at 603 ± 9 Ma and 540 ± 18 Ma, respectively. Their micaschist host-rocks show a multi-episodic evolution that matches the U-Pb data obtained on metasediments and leucogranites of the Variscan Orogen. An old zircon source was detected at 1.87 Ga in the Les Palanges micaschist, while, seemingly, this heritage was wiped out of the Mendic micaschist during the Cadomian events. In the Decazeville gneiss, where a crustal component of unknown age has been detected, the major event is the Variscan migmatization. At Bertholene, the geochronology of uranium ores associated to the Les Palanges orthogneiss evidenced two periods of emplacement: -the relicts of the first phase, of Liassic age at 173 ± 9 Ma, are restricted to an uranium oxide; -the second phase, related to coffinites, represents the economical ore emplaced from Oligocene until Recent times, generated through remobilization of the Liassic stock. A comparison between the geochronological data from the ore, and of those from its granite host, suggests a genetic relationship between the Cadomian magmatism and the U-concentrations, spatially linked to the unconformity between the Les Palanges basement and its Permo-Carboniferous cover series. Metamictization of the source minerals combined with tectonic processes would be responsible for the release of uranium. Uranium-rich minerals from Bertholene, from Lodeve and from Les Pierres-Plantees have been subjected to trial dating by the isotopic Xe-Xe method, which confirms the evolution admitted for those occurrences. This technique is demonstrably expeditious and faithful. Various phenomena were investigated, at the scale of the crystalline structure, related to the xenon energy states and to the problem of rare

  9. Effect of 222Rn emanation from crystals on their 206Pb/238U age dating

    International Nuclear Information System (INIS)

    Barretto, Paulo M.C.

    2009-01-01

    The escape of radon from certain minerals with high uranium is of particular interest to those concerned with the determination of ages of rocks, minerals and tectonic events. To the extent that radon escapes, these minerals are not closed systems from the thermodynamic point of view and, more particularly, from the geochronological point of view. This investigation aimed to determine the radon escape from zircon crystals and how this fit into the severe isotopic constraints of the concordia dating model. To evaluate the consequences of radon loss on 238 U/ 206 Pb age dating methods, 20 zircon concentrates were analyzed. The observed range of relative percentage of radon loss was of 0.2-12 % and correlations with weathering of the crystals with natural alpha dose and with U-Pb age discordances were found. These correlations indicate relationships between the amount of lattice damage by radiation, the radon leakage out of the crystal and Pb mobility. Some of the stochastic complexities in specific age determinations are also discussed. (author)

  10. Pre-Alpine evolution of the Seckau Complex (Austroalpine basement/Eastern Alps): Constraints from in-situ LA-ICP-MS Usbnd Pb zircon geochronology

    Science.gov (United States)

    Mandl, Magdalena; Kurz, Walter; Hauzenberger, Christoph; Fritz, Harald; Klötzli, Urs; Schuster, Ralf

    2018-01-01

    The Variscan European Belt is a complex orogen with its southern margin partly obscured by Alpine tectonics and metamorphism. We present a study of one of the units, the Seckau Complex, that constitute the southern part of the Variscan European Belt in the Eastern Alps in order to clarify its origin, age and lithostratigraphy. The magmatic and geochronological evolution of this Complex in the northwestern part of the Seckau Nappe (as part of the Austroalpine Silvretta-Seckau Nappe System) was investigated by zircon Usbnd Pb dating of paragneisses and metagranitoids coupled with petrological and geochemical data. This reveals the distinction of three newly defined lithostratigraphic/lithodemic sub-units: (1) Glaneck Metamorphic Suite, (2) Hochreichart Plutonic Suite and (3) Hintertal Plutonic Suite. The Glaneck Metamorphic Suite is mainly composed of fine-grained paragneisses that yield Usbnd Pb zircon ages in the range between 2.7 Ga and 2.0 Ga, as well as concordia ages from 572 ± 7 Ma to 559 ± 11 Ma. All of these ages are interpreted as detrital zircon ages originating from an igneous source. The paragneisses are the host rock for the large volumes of metagranitoids of the Hochreichart Plutonic Suite and the Hintertal Plutonic Suite. The Hochreichart Plutonic Suite comprises highly fractionated melts with mainly S-type characteristics and late Cambrian to Early Ordovician Usbnd Pb zircon ages (508 ± 9 Ma to 486 ± 9 Ma), interpreted as magmatic protolith ages. The Hintertal Plutonic Suite is composed of metagranitoids with Late Devonian to early Carboniferous (365 ± 11 Ma and 331 ± 10 Ma) protolith ages, that intruded during an early phase of the Variscan tectonometamorphic event. The metagranitoids of the Hintertal Plutonic Suites define a magmatic fractionation trend, seen in variable Rb/Sr ratios. On this base they can be further subdivided into (a) the Griessstein Pluton characterized by S-type metagranitoids and (b) the Pletzen Pluton distinguished by

  11. 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

    Science.gov (United States)

    López-Moro, F. J.; López-Plaza, M.; Gutiérrez-Alonso, G.; Fernández-Suárez, J.; López-Carmona, A.; Hofmann, M.; Romer, R. L.

    2018-04-01

    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.

  12. Two-stage model of development of heterogeneous uranium-lead systems in zircon

    International Nuclear Information System (INIS)

    Mel'nikov, N.N.; Zevchenkov, O.A.

    1985-01-01

    Behaviour of isotope systems of multiphase zircons at their two-stage distortion is considered. The results of calculations testify to the fact that linear correlations on the diagram with concordance can be explained including two-stage discovery of U-Pb systems of cogenetic zircons if zircon is considered physically heterogeneous and losing in its different part different ratios of accumulated radiogenic lead. ''Metamorphism ages'' obtained by these two-stage opening zircons are intermediate, and they not have geochronological significance while ''crystallization ages'' remain rather close to real ones. Two-stage opening zircons in some cases can be diagnosed by discordance of their crystal component

  13. U-Th-Pb zircon geochronology on igneous rocks in the Toija and Salittu Formations, Orijärvi area, southwestern Finland: constraints on the age of volcanism and metamorphism

    Directory of Open Access Journals (Sweden)

    Christopher L. Kirkland

    2008-01-01

    Full Text Available Zircons from a felsic volcanic rock in the Toija Formation and a synvolcanic gabbro intrusion in the Salittu Formation within the Orijärvi area were dated by U-Th-Pb SIMS in order to provide depositional constraints on these formations. Zircon crystals from the felsic rock preserve a two-stage crystallisation history with zoned core domains and homogeneous rim domains. Inner domains yield a 1878±4 Ma concordia age, interpreted to determine the crystallisation of this rock. Rims yield a 1815±3 Ma concordia age interpretedto determine the regional metamorphism. Small rounded zircon grains from the Salittu gabbro, located within the Jyly shear zone, yield a concordia age of 1792±5 Ma. We interpret the grain textures to suggest that they recrystallised from inherited zircon seeds during the heat and fluid flow into the shear zone. Although no direct ages for the Salittu Formation have been recovered, field relationships imply that it was deposited between 1878−1875 Ma.

  14. Extrapolation of tectonic boundaries across the Labrador shelf: U-Pb geochronology of well samples

    International Nuclear Information System (INIS)

    Wasteneys, H.A.; Krogh, T.E.

    1996-01-01

    Near Saglek Fiord, a northerly trending boundary between the early Archean Saglek block and the middle Archean Hopedale block extends between drill sites which, respectively, sampled Uivak amphibolite gneiss with U-Pb zircon intercept ages of 3742 ± 12 and 2752 ± 42 Ma, and migmatitic Lister gneiss with concordant ages of 3213 -4 +21 Ma for restite and 2552 -21 +4 Ma for leucosome. Titanite ages of ca. 2508 Ma are common to both rocks. A nearby metapsammitic gneiss has detrital zircon and monazite ages of 2681 ± 5, 2700 ± 4, ca. 2730, and 2750 ± 2 Ma representing high-grade metamorphism related to the Hopedale-Saglek collision and metamorphic monazite of ca. 2560 Ma age representing metamorphism of the sediment during reactivation of the Saglek-Hopedale suture. Two hundred kilometres southeast, a gneissic granite records a protolith age of 3170 Ma and Late Proterozoic Pb loss. Near the Nain-Makkovik boundary, 1269 ± 4 Ma zircons indicate a significant extension of the Nain Plutonic Suite. South of the Makkovik boundary, a foliated granite yielded an upper intercept age defining intrusion at 1895 ± 8 Ma and concordant 1872 ± 5 Ma titanite ages that date subsequent metamorphism. Discordant U-Pb ages from an alkali-feldspar granite also constrain crystallization to ca. 1890 Ma and together with the gneiss represent the previously defined Iggiuk event in the Kaipokok domain. Wells near the southerly end of the transect record 1801 ± 5, 1813 ± 3, and 1806 ± 8 Ma ages, respectively, that are typical of the synorogenic granitoid suite representing the Cape Harrison domain of southern Makkovik Province. (author). 46 refs., 1 tab., 5 figs

  15. Pb-Pb zircon ages of the Porto Nacional high-grade metamorphic terrain, northern portion of the Goias Massif, central Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Gorayeb, Paulo Sergio de Sousa; Moura, Candido Augusto Veloso [Para Univ., Belem, PA (Brazil). Centro de Geociencias]. E-mail: gorayebp@ufpa.br; c_moura@ufpa.br; Barros, Gisele Ribeiro de [Para Univ., Belem, PA (Brazil). Programa Institucional de Bolsas de Iniciacao Cientifica (PIBIC)]. E-mail: agbarros@amazon.com.br

    2000-03-01

    Single zircon Pb-evaporation ages were determined for a mafic granulite, two enderbites and a kinzigite of the Porto Nacional High-Grade Metamorphic Terrain (PNHGT) in the Goias massif. Zircons from mafic granulites and one of the enderbites of yielded average {sup 207} Pb/{sup 206} Pb ages of 2125{+-}3 Ma and 2153{+-}1 Ma, respectively, being interpreted as minimum ages of the igneous protoliths. The other enderbite, whose zircons presented round terminations, yielded an average {sup 207} Pb/{sup 206} Pb age of 2097 {+-} 2 Ma. Zircons from the kinzigite, displaying cores and rims overgrowth, presented {sup 207} Pb/{sup 206} Pb ages ranging between 2027 and 2115 Ma. However, a very homogeneous zircon crystal without overgrowth yielded age of 2100 {+-} 2 Ma, with a distinctive low Th/U value (0.02). This zircon is interpreted as a metamorphic crystal and its age probably sets the age of the granulite facies metamorphism, indicating that the PNHGT is a result of the Transamazonian thermotectonic event. (author)

  16. Calibrating the Cretaceous normal superchron with high-precision U-Pb zircon geochronology from Songliao Basin, NE China

    Science.gov (United States)

    Wang, T.; Ramezani, J.; Wang, C.

    2017-12-01

    The Cretaceous Normal Superchron (CNS) or C34n is defined as the prolonged period of normal geomagnetic polarity, which lasted for approximately 38 Myr from the Aptian to the beginning of the Campanian. Along with the Kiaman Reverse Superchron (Carboniferous-Permian), they constitute the two longest periods of stability in the Earth's magnetic field. Polarity reversals are geologically abrupt events of global extent that form the basis of the Geomagnetic Polarity Timescale. In addition, a causal relationship between the end of a superchron and global environmental change has been hypothesized by some workers. Thus, the precise timing of the onset and termination of CNS has important implications for the correlation of global tectonic, paleoclimatic and paleobiotic events, and may help us better understand the causes and consequences of superchrons. At present, the exact age and duration of CNS are poorly understood, in part due to the relative scarcity of relevant paleomagnetic and radioisotopic data. The end of CNS or the C34n/C33r chron boundary is also considered a suitable proxy for the Santonian-Campanian stage boundary in the absence of diagnostic fossils of global distribution for the latter. The early Campanian ( 84 Ma to 76 Ma) is characterized by a steady cooling of the (greenhouse) climate, preceded by an abrupt (possibly 5-6°C) drop in the global temperatures at the Santonain-Campanian boundary, based on the oxygen isotope record of benthic foraminifera. The peak of dinosaur diversity throughout vast swaths of the continents was reached during the Campanian, as well. Here we present a new age constraint for the termination of CNS based on ash bed geochronology from a near-continuous, subsurface, Cretaceous lacustrine record recovered from the Songliao Basin in Northeast China. This extraordinary record allows integration of high-precision U-Pb geochronology, magnetostratigraphy and cyclostratigraphy that enables a multi-chronometer approach to the

  17. Geochronology of Zircon in Eclogite Reveals Imbrication of the Ultrahigh-Pressure Western Gneiss Region of Norway.

    Science.gov (United States)

    Young, D. J.; Kylander-Clark, A. R.; Root, D. B.

    2014-12-01

    Eclogite provides the only record of kinematic events at the deepest levels of orogens. Integrating the U-Pb geochronology and trace element chemistry of zircon in eclogite reveals the most complete view of the PTt history, yet low concentrations of uranium and zirconium and drier compositions that hinder zircon growth at peak conditions render it a challenging rocktype for this approach. The iconic Western Gneiss Region (WGR) in Norway is one of the largest terranes of deeply subducted continental rocks in the world, and contains many indicators of ultrahigh-pressure metamorphic conditions (P>2.8 GPa) that developed during the Siluro-Devonian Caledonian Orogeny. A metamorphic transition from amphibolite-facies to ultrahigh-pressure eclogite facies broadly coincides with a km-scale shear zone that underlies the majority of the WGR. A critical unknown is the timing of movement on this feature, which emplaced allochthonous units above the Baltica basement, but might also have accommodated late-orogenic exhumation of the WGR from mantle depths. We carried out laser ablation split-stream ICPMS (LASS) and selected multigrain TIMS analyses of zircons from eleven eclogites across the southern WGR, of which eight are located within or above the shear zone. LASS spots on polished grains mostly yield weakly discordant Proterozoic intrusive ages, and often minimal indication of a Caledonian (U)HP metamorphic overprint. Direct ablation into unpolished zircon reveals thin rims of Caledonian age in some cases. Overall, the dataset shows that all samples began zircon growth at approximately the same time (ca. 430-420 Ma). Eclogite from lower levels of the shear zone does not contain any dates younger than ca. 410 Ma, however, while eclogite from higher levels continued growth until ca. 400 Ma. We interpret this to result from thrusting of the WGR above cooler basement after 410 Ma, terminating new zircon crystallization within the shear zone but allowing limited further growth in

  18. Pacific 187Os/188Os isotope chemistry and U-Pb geochronology: Synchroneity of global Os isotope change across OAE 2

    Science.gov (United States)

    Du Vivier, A. D. C.; Selby, D.; Condon, D. J.; Takashima, R.; Nishi, H.

    2015-10-01

    Studies of OAE 2 sections beyond the Atlantic Ocean, Western Interior Seaway (WIS) and European pelagic shelf are limited. Here, we present initial osmium isotope stratigraphy (187Os/188Os-Osi) from two proto-Pacific sites that span the Cenomanian-Turonian boundary interval (CTBI): the Yezo Group (YG) section, Hokkaido, Japan, and the Great Valley Sequence (GVS), California, USA; to evaluate the 187Os/188Os seawater chemistry of the proto-Pacific. Additionally we combine new 206Pb/238U zircon CA-ID-TIMS geochronology from five volcanic tuff horizons of the Yezo Group section to test and facilitate inter-basinal integration with the WIS using radio-isotopically constrained age-depth models for both sections, and quantitatively constrain the absolute timing and duration of events across the CTBI. The YG shows an almost identical Osi profile to that of the WIS, and very similar to that of other sites of the proto-Atlantic and European pelagic oceans (Turgeon and Creaser, 2008; Du Vivier et al., 2014). The characteristics of the Osi profile are radiogenic and heterogeneous (∼0.55-0.85) prior to the OAE 2, and synchronous with the inferred OAE 2 onset the Osi abruptly become unradiogenic and remain relatively homogeneous (∼0.20-0.30) before showing a gradual return to more radiogenic Osi (∼ 0.70) throughout the middle to late OAE 2. A206Pb/238U zircon age of an interbedded tuff (HK017) in the adjacent horizon to the first unradiogenic Osi value constrains the age of the Osi inflection at 94.44 ± 0.14 Ma. This age, including uncertainty, agrees with the interpolated age of the same point in the Osi profile (94.28 ± 0.25 Ma) in the only other dated OAE 2 section, the WIS; indicating a coeval shift in seawater chemistry associated with volcanism at the OAE 2 onset at the levels of temporal resolution (ca. 0.1 Myr). Further, prior to the onset of OAE 2 an enhanced radiogenic inflection in the Osi profile of the YG is correlative, within uncertainty, with a similar

  19. The ongoing search for the oldest rock on the Danish island of Bornholm: new U-Pb zircon ages for a quartz-rich xenolith and country rock from the Svaneke Granite

    DEFF Research Database (Denmark)

    Waight, Tod Earle; Serre, Simon H.; Næsby, Sebastian H.

    2017-01-01

    Previous geochronological studies on the Danish island of Bornholm have not identified any rocks older than c. 1.46 Ga. New LA-ICP-MS U-Pb zircon ages are presented for a xenolith within, and the country rock gneiss adjacent to, the Svaneke Granite on Bornholm. The xenolith is fine......-grained and quartz-rich and was likely derived from either a quartz-rich sedimentary protolith or a hydrothermally altered felsic volcanic rock. The relatively fine-grained felsic nature of the country rock gneiss and the presence of large zoned feldspars that may represent phenocrysts suggest its protolith may have...... been a felsic volcanic or shallow intrusive rock. A skarn-like inclusion from a nearby locality likely represents an originally carbonate sediment and is consistent with supracrustal rocks being present at least locally. Zircon data from the xenolith define an upper intercept age of 1483 ± 12 Ma (2σ...

  20. Provenance and U-Pb geochronology of the Upper Cretaceous El Chanate Group, northwest Sonora, Mexico, and its tectonic significance

    Science.gov (United States)

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

    2009-01-01

    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

  1. SHRIMP-RG U-Pb isotopic systematics of zircon from the Angel Lake orthogneiss, East Humboldt Range, Nevada: Is this really archean crust?

    Science.gov (United States)

    Premo, Wayne R.; Castineiras, Pedro; Wooden, Joseph L.

    2008-01-01

    New SHRIMP-RG (sensitive high-resolution ion microprobe-reverse geometry) data confirm the existence of Archean components within zircon grains of a sample from the orthogneiss of Angel Lake, Nevada, United States, previously interpreted as a nappe of Archean crust. However, the combined evidence strongly suggests that this orthogneiss is a highly deformed, Late Cretaceous monzogranite derived from melting of a sedimentary source dominated by Archean detritus. Zircon grains from the same sample used previously for isotope dilution-thermal ionization mass spectrometry (ID-TIMS) isotopic work were analyzed using the SHRIMP-RG to better define the age and origin of the orthogneiss. Prior to analysis, imaging revealed a morphological variability and intragrain, polyphase nature of the zircon population. The SHRIMP-RG yielded 207Pb/206Pb ages between ca. 2430 and 2580 Ma (a best-fit mean 207Pb/206Pb age of 2531 ± 19 Ma; 95% confidence) from mostly rounded to subrounded zircons and zircon components (cores). In addition, several analyses from rounded to subrounded cores or grains yielded discordant 207Pb/206Pb ages between ca. 1460 and ca. 2170 Ma, consistent with known regional magmatic events. All cores of Proterozoic to latest Archean age were encased within clear, typically low Th/U (206Pb/238U ages between 72 and 91 Ma, consistent with magmatic ages from Lamoille Canyon to the south. An age of ca. 90 Ma is suggested, the younger 206Pb/238U ages resulting from Pb loss. The Cretaceous and Precambrian zircon components also have distinct trace element characteristics, indicating that these age groups are not related to the same igneous source. These results support recent geophysical interpretations and negate the contention that the Archean-Proterozoic boundary extends into the central Great Basin area. They further suggest that the world-class gold deposits along the Carlin Trend are not underlain by Archean cratonal crust, but rather by the Proterozoic Mojave

  2. U-Pb zircon dating of the Bassies granite (Pyrenees): a syn-tectonic pluton of Westphalian age

    International Nuclear Information System (INIS)

    Paquette, J.L.

    1997-01-01

    A new U-Pb zircon age of 312 ± 2 Ma for the Bassies pluton (Pyrenees) contradicts the previous whole-rock Rb-Sr dating at 276 ± 16 Ma, which was considered as the age of emplacement, therefore regarded as post-tectonic. The new date is in agreement with recent structural studies which suggest a Hercynian syn-tectonic emplacement for the Bassies pluton. These results strengthen the few U-Pb ages already published for the Pyrenean granites and indicate that the Hercynian plutonism of the Pyrenees is essentially Carboniferous in age and syn-tectonic. (authors)

  3. New U-Pb zircon ages and the duration and division of Devonian time

    Science.gov (United States)

    Tucker, R.D.; Bradley, D.C.; Ver Straeten, C.A.; Harris, A.G.; Ebert, J.R.; McCutcheon, S.R.

    1998-01-01

    Newly determined U-Pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the Devonian time-scale. They are: 1) 417.6 ?? 1.0 Ma for an ash within the conodont zone of Icriodus woschmidti/I. w. hesperius Lochkovian); 2) 408.3 ?? 1.9 Ma for an ash of early Emsian age correlated with the conodont zones of Po. dehiscens--Lower Po. inversus; 3) 391.4 ?? 1.8 Ma for an ash within the Po. c. costatus Zone and probably within the upper half of the zone (Eifelian); and 4) 381.1 ?? 1.3 Ma for an ash within the range of the Frasnian conodont Palmatolepis punctata (Pa. punctata Zone to Upper Pa. hassi Zone). U-Pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites-lepidophyta spore zone, are dated at 363.8 ?? 2.2 Ma and 363 ?? 2.2 Ma and 363.4 ?? 1.8 Ma, respectively, suggesting an age of ~363 Ma for a level within the late Famennian Pa. g. expansa Zone. These data, together with other published zircon ages, suggest that the base and top of the Devonian lie close to 418 Ma and 362 Ma, respectively, thus lengthening the period of ~20% over current estimates. We suggest that the duration of the Middle Devonian (Eifelian and Givitian) is rather brief, perhaps no longer than 11.5 Myr (394 Ma-382.5 Ma), and that the Emsian and Famennian are the longest stages in the period with estimated durations of ~15.5 Myr and 14.5 Myr, respectively.

  4. U-Pb La-ICP-ms geochronology and regional correlation of middle Jurassic intrusive rocks from the Garzon Massif, Upper Magdalena Valley and central cordillera, southern Colombia

    International Nuclear Information System (INIS)

    Bustamante, Camilo; Cardona, Agustin; Bayona, German; Mora, Andres; Valencia, Victor; Gehrels, George; Vervoort, Jeff

    2010-01-01

    New U-Pb zircon geochronology from four granitic units sampled along a southeast-northwest transect between the Garzon massif and the Serrania de las Minas (central cordillera), records a middle Jurassic magmatic activity with two different spatio-temporal domains at ca. 189 ma and 180-173 ma. Reconnaissance data suggest that the four granitoids are characterized by mineralogical and geochemical characteristics akin to a continental magmatic arc setting. The new results suggest that the southern Colombian continental margin includes remnants of tectonomagmatic elements formed by the subduction of the Farallon plate under the South American continental margin. This middle Jurassic arc magmatism is part of the broader Andean scale arc province, and is significant for understanding the tectonic and paleogeographic scenario that characterized the Mesozoic tectonic evolution of the northern Andes.

  5. SHRIMP U-Pb dating of detrital zircons in metamorphic rocks from northern Kyushu, western Japan

    International Nuclear Information System (INIS)

    Tsutsumi, Yukiyasu; Yokoyama, Kazumi; Terada, Kentaro; Sano, Yuji

    2003-01-01

    Radiometric ages of detrital zircons in psammitic schists from the Nagasaki, Kurume, Konoha and Kiyama areas, northern Kyushu, were obtained from 238 U/ 206 Pb ratio and isotopic compositions of Pb using a Sensitive High Resolution Ion Microprobe (SHRIMP II). Zircons from the Nagasaki, Kurume and Konoha areas show bimodal age distribution with peaks at ca. 1900 Ma and 250 Ma. It is suggested from this study that the older zircons were derived from Proterozoic landmass and the Korean Peninsula. Zircons from the Kiyama metamorphic rock show a different pattern with ages concentrated at 380-590 Ma. Such zircons are rare in rock samples from the Nagasaki, Kurume and Konoha areas, indicating that Kiyama rocks and a different origin than those from the other three areas. The youngest zircons from the Kiyama, Nagasaki, Kurume and Konoha areas show ages of 382±23 Ma, 238±13 Ma, 249±13 Ma, and 175±4 Ma, respectively, These data mark the upper age limit of their deposition. Since a continuous igneous activity occurred during the period from 300 to 170 Ma in Far East Asia, and the metamorphic age has been close to the zircon age of each area, these youngest ages for the Nagasaki, Kurume and Konoha areas are considered nearly contemporary to the depositional ages. An evaluation of the nature of metamorphism and available ages suggest the possibility that the Nagasaki metamorphic rocks as well as the schist from the Kurume area belong to the Suo zone of the Sangun belt, whereas the metamorphic rocks in the Konoha area may belong to the Ryoke belt or Suo zone of the Sangun belt. (author)

  6. From opening to subduction of an oceanic domain constrained by LA-ICP-MS U-Pb zircon dating (Variscan belt, Southern Armorican Massif, France)

    Science.gov (United States)

    Paquette, J.-L.; Ballèvre, M.; Peucat, J.-J.; Cornen, G.

    2017-12-01

    In the Variscan belt of Western Europe, the lifetime and evolution of the oceanic domain is poorly constrained by sparse, outdated and unreliable multigrain ID-TIMS U-Pb zircon dating. In this article, we present a complete in situ LA-ICP-MS dataset of about 300 U-Pb zircon analyses obtained on most of the ophiolitic and eclogitic outcrops of Southern Brittany, comprising new dating of previously published zircon populations and newly discovered rock samples. In situ dating and cathodo-luminescence imaging of each zircon grain yields new absolute time-constraints on the evolution of the Galicia-Moldanubian Ocean. The new results confirm that the opening of this oceanic domain is well defined at about 490 Ma. In contrast, the generally-quoted 400-410 Ma-age for the high-pressure event related to the subduction of the oceanic crust is definitely not recorded in the zircons of the eclogites. In light of these new data, we propose that the obduction of oceanic rocks occurred at about 370-380 Ma while the high-pressure event is recorded at 355 Ma in only a few zircon grains of some eclogite samples. Additionally, this large scale dating project demonstrates that the zircons from eclogites do not systematically recrystallise during the high pressure event and consequently their U-Pb systems do not record that metamorphism systematically. These zircons rather preserve the isotopic memory of the magmatic crystallization of their igneous protolith. Another example of an eclogite sample from the French Massif Central illustrates the frequent mistake in the interpretation of the ages of the early hydrothermal alteration of zircons in the oceanic crust versus partial or complete recrystallization during eclogite facies metamorphism.

  7. New Sm/Nd and U/Pb geochronological constraints of the Archean to neoproterozoic evolution of the Amparo basement complex of the Central Ribeira Belt, Southeastern Brazil

    International Nuclear Information System (INIS)

    Fetter, A.H.; Hackspacher, P.C.; Ebbert, H.D; Dantas, E.L; Costa, A.C.D. da

    2001-01-01

    The Amparo Basement Complex is a distinctive collage of migmatitic tronjhemitetonalite- granodiorite (TTG) orthogneisses that represents the older basement exposures within the Central Ribeira Belt, a Late Neoproterozoic (ca. 600 Ma) collisional belt in southeastern Brazil. These basement gneisses are overlain by Mesoproterozoic to Neoproterozoic supracrustal sequences, and intruded by Neoproterozoic collisional granitoids. Pioneering Rb/Sr, Pb/Pb and K/Ar geochronological studies of the Amparo Complex, e.g. (Wernick et al., 1981; Wernick and Oliveira, 1986; Arthur, 1988; Tassinari, 1988; Campos Neto, 1991) provided some initial insights into the antiquity and geologic evolution of the complex, but little about the crustal evolution of the constituent gneisses. Furthermore, the susceptibility of these systems to partial isotopic resetting, left some doubt about the timing and true number of geologic events recorded by these polydeformed rocks. Recent Sm/Nd whole rock (Dantas et al., 2000) and new U/Pb single crystal zircon and monazite data obtained from the Amparo Complex, however, now furnish information on the crustal growth history of the basement and provide precise age constraints on the timing of events related to the geologic evolution of the complex. Based on these new data, it appears that the oldest rocks within the complex are polymigmatized tronjhemitic gneisses located near the town of Amparo. The oldest phase of this migmatite yields a U/Pb zircon age of 3,024 +/- 9 Ma. Sm/Nd data from this locale yields a Nd T(DM) model age of 3.28 Ga suggesting that the genesis of this crustal unit involved some input from yet older crust. Data from banded tonalitic gneisses collected ca. 50 km south of Amparo indicate that subsequent Archean crustal growth around the older core occurred around 2.77 Ga (U/Pb zircon age of 2,772 +/- 26 Ma. A Nd T(DM) model age of 3.02 Ga obtained from these tonalites also indicate enrichment from older crustal sources during their

  8. Constraints on the timing of multiple thermal events and re-equilibration recorded by high-U zircon and xenotime: Case study of pegmatite from Piława Górna (Góry Sowie Block, SW Poland)

    Science.gov (United States)

    Budzyń, Bartosz; Sláma, Jiří; Kozub-Budzyń, Gabriela A.; Konečný, Patrik; Holický, Ivan; Rzepa, Grzegorz; Jastrzębski, Mirosław

    2018-06-01

    The application of zircon and xenotime geochronometers requires knowledge of their potential and limitations related to possible disturbance of the age record. The alteration of the intergrown zircon and xenotime in pegmatite from the Góry Sowie Block (SW Poland) was studied using the electron microprobe analysis, X-ray WDS compositional mapping, micro-Raman analysis, and LA-ICP-MS U-Pb dating of zircon and xenotime, as well as the U-Th-total Pb dating of uraninite. These microanalytical techniques were applied to understand the formation mechanisms of the secondary textures related to post-magmatic processes in the zircon and xenotime intergrowth, and to constrain their timing. Textural and compositional features combined with U-Pb data indicate that the pegmatite-related crystallization of the zircon and xenotime intergrowth occurred ca. 2.09 Ga (2086 ± 35 Ma for zircon and 2093 ± 52 Ma for xenotime), followed by the re-equilibration of zircon and xenotime ca. 370 Ma (373 ± 18 Ma and 368 ± 6 Ma, respectively) during the formation of the younger pegmatite. The zircon and xenotime were most likely derived from Precambrian basement rocks and emplaced in the pegmatite as a restite. The zircon preserved textures related to diffusion-reaction processes that affected its high-U core (up to ca. 9.6 wt% UO2), which underwent further metamictization and amorphization due to self-radiation damage. The zircon rim and xenotime were affected by coupled dissolution-reprecipitation processes that resulted in patchy zoning, age disturbance and sponge-like textures. Xenotime was also partially replaced by fluorapatite or hingganite-(Y) and Y-enriched allanite-(Ce). The termination of the low-temperature alteration was constrained by the U-Th-total Pb age of the uraninite inclusions that crystallized in zircon at 281 ± 2 Ma, which is consistent with the age of 278 ± 15 Ma obtained from the youngest cluster of U-Pb ages in the re-equilibrated high-U zircon domains. This study

  9. Sr-Nd isotopic characteristic, U-Pb zircon geochronology, and petrogenesis of Najmabad Granodiorite batholith, Eastern Iran

    Directory of Open Access Journals (Sweden)

    Mehrab Moradi

    2011-11-01

    Full Text Available The study area is located south of Ghonabad in Eastern Iran. This area is situated between two major faults, Darouneh to the north and Dashtbyaz to the south. The movements of these faults cased major dislocation of this block. Najmabad granodiorite to granite batholith is elongated trending east-west 2×8 Km2. Mineralization is associated with granite and monzonite. Alteration zones are: propylitic, sericitic, argillic and silicification. Chemically, granite-granodiorite is met-aluminous,, spider diagram normalized to lower continental crust show enrichment of LILE such as Rb, Cs, K and LREE (La, Ce , depletion of Ti, Sr, Ta, Nb, Ba, Cs. Based on low values of magnetic susceptibility [(5 to 11 × 10-5 SI], therefore it is classified as belonging to the ilmenite-series (reduced type. The result of U-Pb zircon age dating of granodiorite is 161.85 Ma (Middle Jurassic, Callovian time. Based on Initial εNd isotope values for granodiorite is -6.51, initial 87Sr/86Sr and 143Nd/144Nd ratios for granodiorite is (0.70913 and 0.512095, the magma for granite and granodiorite originated from the continental crust. During Middle Jurassic (Callovian due to continental collision, Upper Triassic to Lower Jurassic rocks is regionally metamorphosed. During the continental collision, Middle Jurassic (164-162Ma reduced type granitoid magma (Ilmenite series, continental crust melting formed and intruded these regional metamorphosed rock in Najmabad, Shah Kuh and Sorkh Kuh area.

  10. U-Pb detrital zircon ages of the upper metasedimentary sequences in the region of the Guarinos Greenstone Belt, Goiás, Brazil; Idades U-Pb de zircão detrítico da sequência metassedimentar superior na região do Greenstone Belt de Guarinos, Goiás

    Energy Technology Data Exchange (ETDEWEB)

    Silva, Alex Joaquim Choupina Andrade; Simões, Luiz Sérgio Amarante, E-mail: jqchoupina@hotmail.com, E-mail: lsimoes@rc.unesp.br [Universidade Estadual Paulista Julio de Mesquita Filho (UNESP), Rio Claro, SP (Brazil). Programa de Pós-Graduação em Geociências e Meio Ambiente

    2017-07-15

    At the north portion of the Archean Block of Goiás occur two greenstone belts (Guarinos and Pilar de Goiás) covered by a younger meta sedimentary sequence, attributed by different authors to the Araxá Group and by other to the Serra da Mesa/Serra Dourada and, by other writers, simply as Mesoproterozoic Sequence. Although there are lithostratigraphic similarities between the Araxá and Serra da Mesa groups, most recent studies have shown that the evolution of basins and their sedimentation ages have peculiarities that differentiate one from the other. This work presents geochronological data of U-Pb on detrital zircon grains obtained by laser ablation method (LA-MC-ICP-MS) in quartzite samples. The youngest zircon has the age of 1405 ± 10 Ma, marking the maximum depositional age and indicating source area. The samples have a higher frequency of Paleoproterozoic zircons, between 1796 and 2472 Ma and the oldest populations of mesoneoarchean age between 2672 and 3112 Ma. Whereas the Paleoproterozoic to Archean zircons can be derived from the rocks that form the Archean Block of Goiás, the mesoproterozoic source is still uncertain, perhaps this source can be related to the Goiás Tin Province rocks. (author)

  11. The 3.5 Ga granulites of the Bug polymetamorphic complex, Ukraine (U-Pb SHRIMP-II zircon data)

    Science.gov (United States)

    Lobach-Zhuchenko, Svetlana; Kaulina, Tatiana; Baltybaev, Shauket; Yurchenko, Anastasija; Balagansky, Victor; Skublov, Sergei; Sukach, Vitaliji

    2014-05-01

    and Cpx). Zircon U-Pb isotopic analysis was carried out using SHRIMP II ion microprobe technique at the Isotopic Centre of VSEGEI, St.-Petersburg. Six transparent grains of the (1) group form a discordia line with Concordia intercepts at 3499+/-33 Ma (and 2638+/-240 Ma (MSWD=2.3). According to internal textures and chemical composition of zircons their formation is associated with granulite metamorphism. The 207 Pb/ 206 Pb data for 11 grains from (3) group are highly variable in age from 3330+/-5 to 2356+/-7 Ma indicating isotopic disturbance. They do not form an isochrone, thus reliable determination of their age is not yet possible. Thus, the oldest granulitic event at 3499 ± 33 Ma has been identified and justified for rocks of the Bug polymetamorphic granulite complex. Recognition of this oldest granulite metamorphism proved possible due to preserved isotopic and geochemical features of zircon. The work was financially supported by program ONZ - 6.

  12. U-Pb and K-Ar geochronology from the Cerro Empexa Formation, 1st and 2nd Regions, Precordillera, northern Chile

    International Nuclear Information System (INIS)

    Tomlinson, A.J; Martin, M.W; Blanco, N.; Perez de Arce, C

    2001-01-01

    The Cerro Empexa Formation (Galli, 1957) is a regionally distributed andesitic volcanic and continental sedimentary unit exposed in the Precordillera of the 1st and 2nd Regions of northern Chile. The formation has generally been considered to lie within the Lower or 'mid' Cretaceous, however, this assignment is based on scant, unreliable geochronologic data. Furthermore, there are conflicting interpretations as to whether the unit predates or postdates the first major Mesozoic shortening event affecting northern Chile. Because of the formation's presumed mid-Cretaceous age and its stratigraphic position over older back-arc sedimentary successions, the unit has been interpreted to represent products of the first eastward jump in the Andean magmatic arc from the arc's initial position in the Cordillera de la Costa (Scheuber and Reutter, 1992). In this paper we present the results of mapping and field observations that indicate exposures previously assigned to the Cerro Empexa Formation include two andesitic volcanic units separated by a major unconformity. The Cerro Empexa Formation proper lies above this unconformity. We also present U-Pb zircon and K-Ar geochronology that indicate the Cerro Empexa Formation is latest Cretaceous in its lower levels, and integrate our data with previously reported 40 Ar/ 39 Ar and fission-track data in the Cerros de Montecristo area (Maksaev, 1990; Maksaev and Zentilli, 1999) to show that 1800±600 m of rocks were deposited within ca. 2.5 m.y (au)

  13. Zircon U-Pb geochronology, Sm-Nd and Pb-Pb isotope systematics of Ediacaran post-collisional high-silica Acampamento Velho volcanism at the Tupanci area, NW of the Sul-Rio-Grandense Shield, Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Sommer, Carlos Augusto; Leitzke, Felipe Padilha; Lima, Evandro Fernandes de; Barreto, Carla Joana Santos; Matté, Vinicius; Philipp, Ruy Paulo; Conceição, Rommulo Vieira, E-mail: casommer@sinos.net, E-mail: eflgeologo@gmail.com, E-mail: ruy.philipp@ufrgs.br, E-mail: rommulo.conceicao@ufrgs.br [Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, RS (Brazil). Instituto de Geociências; Lafon, Jean Michel, E-mail: lafonjm@ufpa.br [Universidade Federal do Pará (UFPA), Belém, PA (Brazil). Laboratório de Geologia Isotópica; Basei, Miguel Ângelo Stipp, E-mail: baseimas@usp.br [Universidade de São Paulo (CPGeo/IGc/USP), São Paulo, SP (Brazil)

    2017-10-15

    We present new U-Pb zircon ages and Sm-Nd-Pb isotopic data for volcanic and hypabyssal acid rocks from the northernmost exposure of the Acampamento Velho Formation in the NW portion of the Sul-Rio-Grandense Shield, Brazil. The first volcanic episode, grouped in the high-Ti rhyolites from the Tupanci hill, shows age of 579 ± 5.6 Ma, which is in agreement with the post-collisional Acampamento Velho Formation volcanism in the Bom Jardim Group of the Camaquã Basin. A poorly constrained age of 558+/- 39Ma was obtained for rhyolites from the low-Ti group at the Picados Hill, which may indicate a younger acid volcanism, or a greater time span for the volcanism of the Acampamento Velho Formation in southernmost Brazil. Regarding magmatic sources, Sm/Nd isotopic data coupled to Pb isotopes and a review of trace element geochemistry indicate different amounts of Paleoproterozoic (Dom Feliciano, Pinheiro Machado Suite) to Neoproterozoic (Rio Vacacaí terrane) lower crust melting. Our data, coupled with literature data, contribute to a better understanding of the stratigraphic evolution for the Neoproterozoic post-collisional volcanic successions of the Camaquã Basin in the Sul-Rio-Grandense Shield. (author)

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

    Science.gov (United States)

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

    2013-12-01

    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

  15. Permian U-Pb (CA-TIMS) zircon ages from Australia and China: Constraining the time scale of environmental and biotic change

    Science.gov (United States)

    Denyszyn, S. W.; Mundil, R.; Metcalfe, I.; He, B.

    2010-12-01

    In eastern Australia, the interconnected Bowen and Sydney Basins are filled with terrestrial sediments of late Paleozoic to early Mesozoic age. These sedimentary units record significant evolutionary events of eastern Gondwana during the time interval between two major mass extinctions (end Middle Permian and Permian-Triassic), and also provide lithological evidence for the Carboniferous-Permian Late Paleozoic Ice Age of southern Pangea, considered to be divisible into up to seven discrete glaciation events in Australia [e.g., 1]. These glaciations are currently assigned ages that indicate that the last of the glaciations predate the end Middle Permian mass extinction at ca. 260 Ma. However, the estimates for the time and durations are largely based on biostratigraphy and lithostratigraphy that, in the absence of robust and precise radioisotopic ages, are unacceptably fragile for providing an accurate high-resolution framework. Interbedded with the sediments are numerous tuff layers that contain zircon, many of which are associated with extensive coal measures in the Sydney and Bowen Basins. Published SHRIMP U-Pb zircon ages [2, 3] have been shown to be less precise and inaccurate when compared to ages applying the CA-TIMS method to the same horizons. Also within the late Middle Permian, the eruption of the Emeishan flood basalts in SW China has been proposed to have caused the end Middle Permian mass extinction [e.g., 4], though a causal link between these events demands a rigorous test that can only be provided by high-resolution geochronology. We present new U-Pb (CA-TIMS) zircon ages on tuff layers from the Sydney and Bowen Basins, with the purpose of generating a timescale for the Upper Permian of Australia to allow correlation with different parts of the world. Initial results, with permil precision, date a tuff layer within the uppermost Bandanna Fm. to ca. 252 Ma, a tuff within the Moranbah Coal Measures to ca. 256 Ma, and a tuff within the Ingelara Fm. to

  16. Relict zircon U-Pb age and O isotope evidence for reworking of Neoproterozoic crustal rocks in the origin of Triassic S-type granites in South China

    Science.gov (United States)

    Gao, Peng; Zheng, Yong-Fei; Chen, Yi-Xiang; Zhao, Zi-Fu; Xia, Xiao-Ping

    2018-02-01

    Granites derived from partial melting of sedimentary rocks are generally characterized by high δ18O values and abundant relict zircons. Such relict zircons are valuable in tracing the source rocks of granites and the history of crustal anatexis. Here we report in-situ U-Pb ages, O isotopes and trace elements in zircons from Triassic granites in the Zhuguangshan and Jiuzhou regions, which are located in the Nanling Range and the Darongshan area, respectively, in South China. Zircon U-Pb dating yields magma crystallization ages of 236 ± 2 Ma for the Zhuguangshan granites and 246 ± 2 Ma to 252 ± 3 Ma for the Jiuzhou granites. The Triassic syn-magmatic zircons are characterized by high δ18O values of 10.1-11.9‰ in Zhuguangshan and 8.5-13.5‰ in Jiuzhou. The relict zircons show a wide range of U-Pb ages from 315 to 2185 Ma in Zhuguangshan and from 304 to 3121 Ma in Jiuzhou. Nevertheless, a dominant age peak of 700-1000 Ma is prominent in both occurrences, demonstrating that their source rocks were dominated by detrital sediments weathered from Neoproterozoic magmatic rocks. Taking previous results for regional granites together, Neoproterozoic relict zircons show δ18O values in a small range from 5 to 8‰ for the Nanling granites but a large range from 5 to 11‰ for the Darongshan granites. In addition, relict zircons of Paleozoic U-Pb age occur in the two granitic plutons. They exhibit consistently high δ18O values similar to the Triassic syn-magmatic zircons in the host granites. These Paleozoic relict zircons are interpreted as the peritectic product during transient melting of the metasedimentary rocks in response to the intracontinental orogenesis in South China. Therefore, the relict zircons of Neoproterozoic age are directly inherited from the source rocks of S-type granites, and those of Paleozoic age record the transient melting of metasedimentary rocks before intensive melting for granitic magmatism in the Triassic.

  17. The 40Ar/39Ar and U/Pb dating of young rhyolites in the Kos-Nisyros volcanic complex, Eastern Aegean Arc, Greece: Age discordance due to excess 40Ar in biotite

    Science.gov (United States)

    Bachmann, O.; Schoene, B.; Schnyder, C.; Spikings, R.

    2010-08-01

    High-precision dating of Quaternary silicic magmas in the active Kos-Nisyros volcanic center (Aegean Arc, Greece) by both 40Ar/39Ar on biotite and U/Pb on zircon reveals a complex geochronological story. U/Pb ID-TIMS multi and single-grain zircon analyses from 3 different units (Agios Mammas and Zini domes, Kefalos Serie pyroclasts) range in age from 0.3 to 0.5 to 10-20 Ma. The youngest dates provide the maximum eruption age, while the oldest zircons indicate inheritance from local continental crust (Miocene and older). Step-heating 40Ar/39Ar experiments on 1-3 crystals of fresh biotite yielded highly disturbed Ar-release patterns with plateau ages typically older than most U/Pb ages. These old plateau ages are probably not a consequence of inheritance from xenocrystic biotites because Ar diffuses extremely fast at magmatic temperatures and ratios are reset within a few days. On the basis of (1) elevated and/or imprecise 40Ar/36Ar ratios, (2) shapes of the Ar release spectra, and (3) a high mantle 3He flux in the Kos-Nisyros area, we suggest that biotite crystals retained some mantle 40Ar that led to the observed, anomalously old ages. In contrast, sanidine crystals from the only sanidine-bearing unit in the Kos-Nisyros volcanic center (the caldera-forming Kos Plateau Tuff) do not appear to store any excess 40Ar relative to atmospheric composition. The eastern edge of the Aegean Arc is tectonically complex, undergoing rapid extension and located close to a major structural boundary. In such regions, which are characterized by high fluxes of mantle volatiles, 40Ar/39Ar geochronology on biotite can lead to erroneous results due to the presence of excess 40Ar and should be checked either against 40Ar/39Ar sanidine or U/Pb zircon ages.

  18. U-Pb zircon and monazite geochronology of the Hercynian two-mica granite composite pluton of Cabeceiras de Basto (Northern Portugal); Geochronologie U-Pb sur zircon et monazite du massif composite de granite a deux micas hercynien de Cabeceiras de Basto (Nord-Portugal)

    Energy Technology Data Exchange (ETDEWEB)

    Almeida, A.; Noronha, F. [Universidade do Porto (Portugal). Centro de Geologia; Leterrier, J. [Centre de Recherches Petrographiques et Geochimiques, 54 - Vandoeuvre-les-Nancy (France); Bertrand, J.M. [Universite de Savoie, 73 - Le-Bourget-du-Lac (France). Laboratoire de geodynamique des Chaines alpines

    1998-06-01

    The zircon and monazite U-Pb minimum age of the Hercynian peraluminous two-mica composite granite pluton of Cabeceiras de Basto (Northern Portugal) is 311 {+-} 1 Ma (2 {sigma}). This age, well constrained by a concordant monazite of 311{+-}4 Ma (2{sigma}), is in good agreement with the age suggested for the syn-kinematic, syn-D3, Iberian granites of the Hercynian orogeny (315-305; Pinto et al., 1987). The ({sup 87}Sr/{sup 86}Sr){sub i} and {xi}{sub Nd} isotopic ratios, calculated for 311 Ma, suggest that the three granite units which constitute the massif may derive from an heterogeneous crustal source dated (upper intercept of the discordance) around 1200 Ma. (authors) 21 refs.

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

    Science.gov (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.

    2017-06-01

    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.

  20. Outline of Upper Precambrian and Lower Paleozoic evolution of the Iberian Peninsula according to U-Pb dating of zircons

    International Nuclear Information System (INIS)

    Lancelot, J.R.; Allegret, A.; Iglesias Ponce de Leon, M.

    1985-01-01

    In the northwestern part of the Iberian Peninsula, recent geological studies have defined the following stratigraphic sequence from bottom to top: basal orthogneiss of Miranda do Douro is overlain by a complex of schist and greywacke which have been assigned to the Cambrian or late Precambrian; these are followed by the Ollo de Sapo gneisses of volcanic-sedimentary origin which in turn are overlain by the fossiliferous layers of the lower Ordovician. The schist and greywacke complex and the lower part of the Ollo de Sapo Formation are transected by granitic dikes (Viana del Bollo orthogneiss). All these rocks were folded, faulted, metamorphosed, and intruded by granite plutons during Variscan times. The emplacement of the Miranda do Douro and Viana del Bollo orthogneisses have been respectively dated at 618+-9 Ma and 465+-10 Ma by the U-Pb method on zircons. These data yield lower and upper limits for the deposition of the Ollo de Sapo Formation and demonstrate the occurrence of a Precambrian basement (''Pan-African'' or ''Cadomian'' orogeny) in the Variscan belt in the northwestern part of the Iberian Peninsula. The emplacement of Viana del Bollo orthogneiss could be genetically related to the main phase of crustal thinning (470-520 Ma) characteristic of the Prevariscan evolution of western Europe. Typology and U-Pb data of the Ollo de Sapo zircons indicate the occurrence of inherited old Precambrian zircons and yield an age of 325+-3 Ma for the Variscan main thermal event in the area considered. Four possible evolutionary models for the U-Pb system of the Ollo de Sapo zircons are discussed. (orig.)

  1. Cenomanian-? early Turonian minimum age of the Chubut Group, Argentina: SHRIMP U-Pb geochronology

    Science.gov (United States)

    Suárez, Manuel; Márquez, Marcelo; De La Cruz, Rita; Navarrete, César; Fanning, Mark

    2014-03-01

    Four new SHRIMP U-Pb zircon ages older than 93 Ma from samples of the two uppermost formations accumulated in two different depocenters (Golfo de San Jorge and Cañadón Asfalto basins) of the Chubut Group in central Argentinean Patagonia, establish a pre-late Cenomanian-? early Turonian age for the group. It also confirms a coeval and comparable evolution of the two depocenters, where distal pyroclastic material was deposited together with fluvial and lacustrine facies.

  2. Zircon U-Pb geochronology and emplacement history of intrusive rocks in the Ardestan section, central Iran

    Energy Technology Data Exchange (ETDEWEB)

    Sarjoughian, F.; Kananian, A.

    2017-11-01

    The Urumieh-Dokhtar Magmatic Arc (UDMA) is part of the Alpine–Himalayan orogenic belt and interpreted to be a subduction-related Andean-type magmatic arc. Along this belt, Eocene volcanics and some gabbroic to granitic bodies crop out. The main rock types of the studied intrusion are granite, granodiorite, and diorite. They have geochemical features typical of magnesian, calc-alkaline, metaluminous to slightly peraluminous granites and I-type intrusive rock that have a strong enrichment in Large-Ion Lithophile (LIL) elements (e.g. Rb, Ba, Sr), and a depletion in High Field Strength (HFS) elements (e.g. Nb, Ti, P), typical of subduction-related magmas. Zircon U-Pb dating was applied to determine the emplacement ages of the different intrusions in the Ardestan area. Among them the Kuh-e Dom diorite is 53.9±0.4Ma old; the Kuh-e Dom granodiorite is 51.10±0.4Ma old; the Mehrabad granodiorite is 36.8±0.5Ma old, the Nasrand granodiorite is 36.5±0.5Ma old, the Zafarghand granodiorite is 24.6±1.0Ma old, and the Feshark granodiorite is 20.5±0.8Ma old. These results delineate more accurately the magmatic evolution related to the Neotethyan subduction from the Lower Eocene to Lower Miocene, and the subsequent Zagros orogeny that resulted from the Arabia-Eurasia collision. The emplacement of these intrusive rocks inside the UDMA, which has a close relationship with the collisional orogeny, is transitional from a subduction-related setting to post-collisional setting in the Ardestan area.

  3. Zircon U-Pb geochronology and emplacement history of intrusive rocks in the Ardestan section, central Iran

    International Nuclear Information System (INIS)

    Sarjoughian, F.; Kananian, A.

    2017-01-01

    The Urumieh-Dokhtar Magmatic Arc (UDMA) is part of the Alpine–Himalayan orogenic belt and interpreted to be a subduction-related Andean-type magmatic arc. Along this belt, Eocene volcanics and some gabbroic to granitic bodies crop out. The main rock types of the studied intrusion are granite, granodiorite, and diorite. They have geochemical features typical of magnesian, calc-alkaline, metaluminous to slightly peraluminous granites and I-type intrusive rock that have a strong enrichment in Large-Ion Lithophile (LIL) elements (e.g. Rb, Ba, Sr), and a depletion in High Field Strength (HFS) elements (e.g. Nb, Ti, P), typical of subduction-related magmas. Zircon U-Pb dating was applied to determine the emplacement ages of the different intrusions in the Ardestan area. Among them the Kuh-e Dom diorite is 53.9±0.4Ma old; the Kuh-e Dom granodiorite is 51.10±0.4Ma old; the Mehrabad granodiorite is 36.8±0.5Ma old, the Nasrand granodiorite is 36.5±0.5Ma old, the Zafarghand granodiorite is 24.6±1.0Ma old, and the Feshark granodiorite is 20.5±0.8Ma old. These results delineate more accurately the magmatic evolution related to the Neotethyan subduction from the Lower Eocene to Lower Miocene, and the subsequent Zagros orogeny that resulted from the Arabia-Eurasia collision. The emplacement of these intrusive rocks inside the UDMA, which has a close relationship with the collisional orogeny, is transitional from a subduction-related setting to post-collisional setting in the Ardestan area.

  4. U-Pb zircon and biostratigraphic data of high-pressure/low-temperature metamorphic rocks of the Talea Ori: tracking the Paleotethys suture in central Crete, Greece

    Science.gov (United States)

    Zulauf, G.; Dörr, W.; Krahl, J.; Lahaye, Y.; Chatzaras, V.; Xypolias, P.

    2016-10-01

    Inherited deformation microfabrics of detrital quartz grains and U-Pb (Laser ablation (LA)-ICPMS and ID TIMS) ages of detrital zircons separated from the Phyllite-Quartzite Unit s.l. of the Talea Ori, central Crete, suggest strikingly different source rocks. Albite gneiss of the lower Rogdia Beds includes Cambrian and Neoproterozoic rounded zircons with main U-Pb age peaks at 628 and 988 Ma. These and minor Paleoproterozoic and Archean peaks, together with the lack of Variscan-aged and Mesoproterozoic zircons, are similar to the age spectra obtained from the Phyllite-Quartzite Unit s.str. of the Peloponnesus and eastern Crete and from the Taurides. All of these zircons should be derived from the northeastern passive margin of Gondwana (Cimmeria). Metatuffites of the uppermost Rogdia Beds and metasandstone of Bali beach, on the other hand, include euhedral detrital zircons displaying a Variscan U-Pb age spectra at ca. 300 Ma with concordia ages at 291 ± 3, 300 ± 1 Ma (Rogdia) and 286 ± 3, 300 ± 3, 313 ± 2 Ma (Bali). Both types of metasediments and their zircons are similar to those of the pre-Alpine basement and overlying Tyros Beds of eastern Crete, revealing a provenance at the southern active margin of Laurasia. Thus, in central Crete the Paleotethys suture should be situated inside the Rogdia Beds. Magmatic zircons separated from a rhyolite boulder of the lower Achlada Beds yielded a concordant U-Pb zircon age at 242 ± 2 Ma placing a maximum age for the deposition of the (meta)conglomerate from which the boulder was collected. This age is compatible with an Olenekian-early Anisian age of the underlying Vasilikon marble suggested by new findings of the foraminifera Meandrospira aff. pusilla. Both the Achlada Beds and the Vasilikon marble can be attributed to the lower Tyros Beds of eastern Crete. The Alpine deformation led to a pervasive mylonitic foliation, which is affecting most of the studied rocks. This foliation results from D2 top

  5. U-Pb (SHRIMP II) Age of zircons from ash tuffs of the upper vendian Chernyi Kamen formation (Sylvitsa group, Middle Urals)

    International Nuclear Information System (INIS)

    Ronkin, Yu.L.; Grazhdankin, D.V.; Maslov, A.V.; Mizens, G.A.; Matukov, D.I.; Krupenin, M.T.; Petrov, G.A.; Lepikhina, O.P.; Kornilova, A.Yu.

    2006-01-01

    To make more precise the model of correlation of the Middle Urals western slope upper vendian layers with the White Sea remote layers one carried out the SHRIMP-II procedure base U-Pb-dating of the volcanogenic zircons from the ash tuffs and of the volcanogenic zircon enclosing argillites of the Middle Urals Chernyi Kamen formation. The obtained age value of the studied zircons equal to 557+-13 million years is in line with the geological data [ru

  6. Zircon U-Pb age of the Pescadero felsite: A late Cretaceous igneous event in the forearc, west-central California Coast Ranges

    Science.gov (United States)

    Ernst, W.G.; Martens, U.C.; McLaughlin, R.J.; Clark, J.C.; Moore, Diane E.

    2011-01-01

    Weathered felsite is associated with the late Campanian-Maastrichtian Pigeon Point Formation near Pescadero, California. Poorly exposed, its age and correlation are uncertain. Is it part of the Pigeon Point section west of the San Gregorio-Hosgri fault? Does it rest on Nacimiento block basement? Is it dextrally offset from the Oligocene Cambria Felsite, ~185 km to the southeast? Why is a calc-alkaline hypabyssal igneous rock intrusive into the outboard accretionary prism? To address these questions, we analyzed 43 oscillatory-zoned zircon crystals from three incipiently recrystallized pumpellyite ?? prehnite ?? laumontite-bearing Pescadero felsite samples by sensitive high-resolution ion microprobe-reverse geometry (SHRIMPRG) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques. Thirty-three zircons gave late Mesozoic U-Pb ages, with single-grain values ranging from 81 to 167 Ma; ten have pre-Mesozoic, chiefl y Proterozoic ages. A group of the four youngest Pescadero zircons yielded an apparent maximum igneous age of ca. 86-90 Ma. Refl ecting broad age scatter and presence of partly digested sandstone inclusions, we interpret the rest of the zircons (perhaps all) as xenocrysts. Twenty-three zircons were separated and analyzed from two samples of the similar Cambria Felsite, yielding a unimodal 27 Ma U-Pb age. Clearly, the origin of the Upper Oligocene Cambria Felsite is different from that of the Upper Cretaceous Pescadero felsite; these rocks are not correlated, and do not constrain displacement along the San Gregorio-Hosgri fault. Peak ages differ slightly, but relative probability curves for Mesozoic and pre-Mesozoic Pescadero zircons compare well, for example, with abundant U-Pb age data for detrital zircons from Franciscan metaclastic strata ~100 km to the east in the Diablo Range- San Francisco Bay area, San Joaquin Great Valley Group turbidites, Upper Cretaceous Nacimiento block Franciscan strata, and Upper Cretaceous

  7. Tithonian age of dinosaur fossils in central Patagonian, Chile: U-Pb SHRIMP geochronology

    Science.gov (United States)

    Suárez, Manuel; De La Cruz, Rita; Fanning, Mark; Novas, Fernando; Salgado, Leonardo

    2016-11-01

    Three Tithonian concordant U-Pb SHRIMP zircon ages of 148.7 ± 1.4, 147.9 ± 1.5 and 147.0 ± 1.0 from tuffs intercalated in a clastic sedimentary succession with exceptional dinosaur bones including the new taxon Chilesaurus diegosuarezi gen. et sp. nov. exposed in central Chilean Patagonia (ca. 46°30'S) are reported herein. The fossiliferous beds accumulated in a synvolcanic fan delta reaching a shallow marine basin as indicated by glauconite present in some of the beds, and coeval with the beginning of the transgression of the Aysén Basin.

  8. Geochronology of the Thompson Creek Mo Deposit: Evidence for the Formation of Arc-related Mo Deposits

    Science.gov (United States)

    Lawrence, C. D.; Coleman, D. S.; Stein, H. J.

    2016-12-01

    The Thompson Creek Mo deposit in central ID, has been categorized as an arc-related Mo deposit due to the location, grade of Mo, and relative lack of enrichments in F, Rb, and Nb, compared to the Climax-type Mo deposits. Geochronology from this arc-related deposit provides an opportunity to compare and contrast magmatism, and mineralization to that in Climax-type deposits. Distinct pulses of magmatism were required to form the Thompson Creek Mo deposit, which is consistent with recent geochronology from Climax-type deposits. Molybdenite Re-Os geochronology from five veins requires at least three pulses of magmatism and mineralization between 89.39 +/- 0.37 and 88.47 +/- 0.16 Ma. Zircon U-Pb ages from these mineralized samples overlap with molybdenite mineralization, but show a much wider range (91.01 +/- 0.37 to 87.27 +/- 0.69). Previous work from Climax-type Mo deposits suggest a correlation between a super eruption, and the subsequent rapid (<1 Ma) onset, and completion of Mo mineralizing intrusions. The longer life (3-4 Ma) for the Thompson Creek Mo deposit suggests that the mineralizing intrusions for arc-related Mo deposits may not need to have as high [Mo] as the Climax-type deposits. This study also finds a shift in the source of magmatism from the pre- to syn-mineralizing intrusions. Zircons from pre-mineralizing intrusions have much higher (15-60 pg) concentrations of radiogenic Pb than zircons from mineralized intrusions, which all have less than 15 pg, though whole rock [U] are similar.

  9. Zircon U-Pb Ages Chronicle 3 Myr of Episodic Crystallization in the Composite Miocene Tatoosh Pluton, Mount Rainier National Park, Washington Cascades

    Science.gov (United States)

    Bacon, C. R.; Du Bray, E. A.; Wooden, J. L.; Mazdab, F. K.

    2007-12-01

    Zircon geochronology of upper crustal plutons can constrain longevities of intermediate to silicic magmatic systems. As part of a larger study of the geochemistry and metallogeny of Tertiary Cascades magmatic arc rocks, we used the USGS-Stanford SHRIMP RG to determine 20 to 28 238U-206Pb ages for zircons from each of 6 quartz monzodiorite (qmd), quartz monzonite (qm), or granodiorite (grd) samples representative of the Tatoosh pluton, and one grd from the nearby Carbon River stock. The 7x12 km composite Tatoosh pluton, discontinuously exposed on the south flank of Mount Rainier, consists of at least 4 petrographic/compositional phases, here termed Pyramid Peak, Nisqually, Reflection Lake, and Tatoosh. These collectively intrude gently folded and weakly metamorphosed basaltic andesite flows and volcaniclastic rocks of the Eocene Ohanapecosh Formation, silicic ignimbrites and sedimentary rocks of the Oligocene Stevens Ridge Formation, and basaltic to intermediate volcanic rocks of the Miocene Fifes Peak Formation. Histograms and relative probability plots of U- Pb ages indicate 2 to 4 age populations within each sample. The weighted mean age of each of the youngest populations (all ±2σ) is interpreted as the time of final solidification: Pyramid Peak qmd (58.5% SiO2) 17.4±0.2 Ma, Nisqually grd (in Paradise Valley; 65.4% SiO2) 16.7±0.2 Ma, Nisqually grd (at Christine Falls; 66.4% SiO2) 17.3±0.2 Ma, Reflection Lake qm (along Pinnacle Peak trail; 66.6% SiO2) 17.1±0.2 Ma, Tatoosh grd (in Stevens Canyon; 67.8% SiO2) 18.2±0.2 Ma, Tatoosh grd (south of Louise Lake; 69.3% SiO2) 19.3±0.1 Ma, and Carbon River grd (68.0% SiO2) 17.4±0.3 Ma. The older Nisqually grd age is indistinguishable from a TIMS zircon age of 17.5±0.1 Ma reported by Mattinson (GSA Bulletin 88:1509-1514, 1977) for grd from a nearby locality. None of the 164 SHRIMP-RG U-Pb ages, including cores, is older than 21 Ma. The relatively small, high-level pluton likely was emplaced and solidified in pulses

  10. Zircon U-Pb ages and geochemistry of migmatites and granites in the Foping dome: Evidence for Late Triassic crustal evolution in South Qinling, China

    Science.gov (United States)

    Zhang, He; Li, Shuang-Qing; Fang, Bo-Wen; He, Jian-Feng; Xue, Ying-Yu; Siebel, Wolfgang; Chen, Fukun

    2018-01-01

    Migmatites provide a record of melt formation and crustal rheology. In this study we present zircon U-Pb ages and geochemical composition of migmatites from the Foping dome and granites from the Wulong pluton. U-Pb results from migmatite zircons indicate two episodes of partial melting. Rim domains from a leucosome in the Longcaoping area yield an age of ca. 209 Ma. Migmatites collected from the Foping dome yield U-Pb zircon ages of 2910 to 190 Ma, suggesting the involvement of meta-sedimentary source components. Rim domains of the zircons with low Th/U ratios (pluton can be subdivided into high Sr/Y and low Sr/Y granites. U-Pb zircon ages vary from 219 to 214 Ma for the high Sr/Y granites and from 214 to 192 Ma for the low Sr/Y granites. High Sr/Y granites have higher Na2O and Sr contents than the low Sr/Y granites. They also lack negative Eu anomalies and are depleted in HREE compared to the low Sr/Y granites. Initial 87Sr/86Sr ratios and εNd values of all the samples roughly overlap with those of Neoproterozoic basement rocks exposed in South Qinling. Including previous studies, we propose that the high and low Sr/Y granites formed by melting of thickened and normal crust, respectively. Close temporal-spatial relationship of the high and low Sr/Y granites with the two-stage migmatization events implies variation of crustal thickness and thermal overprints of the orogenic crust in post-collisional collapse. Following the collision of South Qinling and the Yangtze block prior to 219 Ma, partial melting of the deep crust occurred. The melts migrated upwards to form the high Sr/Y granites. This process occurred rapidly and caused collapse of the thickened crust and carried heat upwards, leading to further partial melting within the shallower crust and formation of the low Sr/Y granites.

  11. Neoproterozoic-Early Paleozoic Peri-Pacific Accretionary Evolution of the Mongolian Collage System: Insights From Geochemical and U-Pb Zircon Data From the Ordovician Sedimentary Wedge in the Mongolian Altai

    Science.gov (United States)

    Jiang, Y. D.; Schulmann, K.; Kröner, A.; Sun, M.; Lexa, O.; Janoušek, V.; Buriánek, D.; Yuan, C.; Hanžl, P.

    2017-11-01

    Neoproterozoic to early Paleozoic accretionary processes of the Central Asian Orogenic Belt have been evaluated so far mainly using the geology of ophiolites and/or magmatic arcs. Thus, the knowledge of the nature and evolution of associated sedimentary prisms remains fragmentary. We carried out an integrated geological, geochemical, and zircon U-Pb geochronological study on a giant Ordovician metasedimentary succession of the Mongolian Altai Mountains. This succession is characterized by dominant terrigenous components mixed with volcanogenic material. It is chemically immature, compositionally analogous to graywacke, and marked by significant input of felsic to intermediate arc components, pointing to an active continental margin depositional setting. Detrital zircon U-Pb ages suggest a source dominated by products of early Paleozoic magmatism prevailing during the Cambrian-Ordovician and culminating at circa 500 Ma. We propose that the Ordovician succession forms an "Altai sedimentary wedge," the evolution of which can be linked to the geodynamics of the margins of the Mongolian Precambrian Zavhan-Baydrag blocks. This involved subduction reversal from southward subduction of a passive continental margin (Early Cambrian) to the development of the "Ikh-Mongol Magmatic Arc System" and the giant Altai sedimentary wedge above a north dipping subduction zone (Late Cambrian-Ordovician). Such a dynamic process resembles the tectonic evolution of the peri-Pacific accretionary Terra Australis Orogen. A new model reconciling the Baikalian metamorphic belt along the southern Siberian Craton with peri-Pacific Altai accretionary systems fringing the Mongolian microcontinents is proposed to explain the Cambro-Ordovician geodynamic evolution of the Mongolian collage system.

  12. Archean crustal evolution of the Narryer Gneiss Terrane, Western Australia, as revealed by the U-Pb age and Hf-isotope compositions of zircon from the granitic gneisses

    Science.gov (United States)

    Sylvester, P.; Souders, K.; Crowley, J. L.; Myers, J.

    2011-12-01

    The Narryer Gneiss Terrane of the Yilgarn Craton, Western Australia, is an important area for studies of early crustal evolution because of the preservation of (1) detrital zircons of Hadean to Archean age in the Jack Hills and Mt. Narryer metasedimentary belts, and (2) several widespread units of granitic gneisses emplaced between ca. 3.7 and 2.6 Ga. We have analyzed the U-Pb geochronology and Hf-isotope geochemistry of magmatic zircons from 38 samples of the granitic gneisses using laser ablation - (multicollector) - ICPMS. The sample suite is dominated by the Meeberrie gneiss, a banded quartz-microcline-oligoclase-biotite gneiss of monzogranite to granodiorite composition, and the Dugel gneiss, a leucocratic, pegmatite-layered syenogranite gneiss, but gneisses of dioritic to tonalitic composition, as well as less deformed granite sheets, are also represented. Magmatic zircons were identified on the basis of the preservation of oscillatory zoning in BSE and CL images, igneous Th/U ratios (>0.2), and concordant U-Pb isotopic systematics with low common Pb contents. The results indicate many of the gneisses are composed of the products of multiple magmatic events, as has been reported previously for samples of the Meeberrie gneiss (Kinny & Nutman, 1996, Precambrian Res. 78, 165-178). Major ages of magmatism preserved in the gneisses occurred at ca. 3685-3665 Ma, 3620-3565 Ma, 3495-3440 Ma, 3375-3330 Ma, and 3300-3260 Ma. The late granite sheets crystallized at 2710-2645 Ma. Hf-isotope compositions of the zircons trend to less radiogenic values with decreasing age, with ɛHf values of ca. 0 to -5 for 3.7-3.4 Ga gneisses, ca. -1 to -9 for 3.4-3.2 Ga gneisses and ca. -5 to -20 for the late granite sheets. The array of the Hf isotopic compositions with time for the entire sample set are fit well by a regression indicating a source reservoir with a 176Lu/177Hf of 0.022 extracted from the depleted mantle at 3.9 Ga. This suggests that the Narryer gneisses and late granite

  13. Zircon U-Pb and molybdenite Re-Os geochronology and geological significance of the Baoshan porphyry Cu polymetallic deposit in Jiangxi province

    Science.gov (United States)

    Jia, Liqiong; Wang, Liang

    2017-10-01

    Baoshan porphyry Cu polymetallic deposit belongs to Jiujiang-Ruichang Cu-Au ore field, which is a component part of the Middle-Lower Yangtze River Cu-Au metallogenic belt. The U-Pb LA-MC-TCP MS dating of the zircons from Baoshan granodiorite porphyry yields an age of 147.81±0.48Ma (MSWD=1.07). Six molybdenite samples separated from Baoshan deposit are used for Re-Os dating and obtained the weighted average age of 147.42±0.84Ma and an isochron age of 147.7±1.2Ma. These ages suggest that the mineralization in the Baoshan deposit is genetically associated to the granodiorite porphyry, and the process of rock-and ore-forming is continuous. These data indicate that ages of intrusion and ore-body from Baoshan deposit are almost identical to other typical magmatic intrusion and deposits in Jiujiang-Ruichang metallogenic district. Tt is inferred that the Baoshan deposit was formed in the transition from EW-striking Tndosinian tectonic domain to NE-striking Paleo-Pacific tectonic domain.

  14. The complex systematics of zircons in migmatitic gneisses: An example from an Archean migmatite along the Patos Shear Zone, Borborema Province, NE Brazil

    International Nuclear Information System (INIS)

    Costa, A.C.D; Hackspacher, P.C; Dantas, E.L; Fetter, A.H.

    2001-01-01

    The Northem Tectonic Domain Borborema Province, in Northeast of Brazil records a complex history of tectonic activity ranging from 3.4 Ga to 0.6 Ga (Brito Neves, 1995 and Dantas, 1996). U-Pb systematics of zircons from a migmatitic gneiss just north of the Patos Shear Zone provide an excellent example of the difficulties encountered using conventional single-grain U/Pb zircon geochronology in polydeformed gneiss terranes. Our conventional single grain zircon analyses of a migmatite yielded Archean ages between ca. 3.3 at 2.8 a, as well as some highly discordant Paleoproterozoic ages. Subsequent cathodoluminescence images of these zircon grains showed complex internal structures that possibly record up to 4 separate stages of zircon growth. With such internal complexity, is impossible resolve primary crystallization ages as well as the ages of subsequent overgrowth events using conventional single grain analyses. Such resolution will require analyses of the individual grain domains using the SHRIMP method (au)

  15. SHRIMP U-Pb zircon dating of Pedras Grandes Suite, southern Santa Catarina State, Brazil

    Directory of Open Access Journals (Sweden)

    Andréa R. Jelinek

    2005-03-01

    Full Text Available Two major magmatic pulses of the granitic Florianópolis Batholith in Santa Catarina State, southern Brazil, occurred between 613±5Ma and 595±5 Ma, during the Neoproterozoic Brasiliano Cycle. These ages were obtained by U-Pb isotopic determinations with the sensitive high mass-resolution ion microprobe on igneous zircons from Pedras Grandes Suite in Santa Catarina State. Euhedral zircons remained unaltered close to a fluorite vein deposited at 180?C or more. These ages suggest a northern limit for the Pedras Grandes Suite, explaining the spatial relationship between the fluorite veins and the source rock.Dois dos principais pulsos da atividade granítica no Batólito Florianópolis em Santa Catarina ocorreram entre 613 ±5 Ma e 595 ±5 Ma, durante o Neoproterozóico do Ciclo Brasiliano. Estas idades foram obtidas a partir dedeterminações isotópicas U-Pb em cristais de zircão da Suite Pedras Grandes por "Sensitive high-resolution íon microprobe" - SHRIMP II. Os cristais de zircão permaneceram inalterados mesmo mediante condições hidrotermais com temperaturas iguais e, até mesmo, superiores a 180ºC. Estas idades sugerem a delimitação norte do Maciço Pedras Grandes, explicando a relação espacial existente entre os filões de fluorita e a rocha fonte destas mineralizações.

  16. Geochronology and Genesis of the Xitian W-Sn Polymetallic Deposit in Eastern Hunan Province, South China: Evidence from Zircon U-Pb and Muscovite Ar-Ar Dating, Petrochemistry, and Wolframite Sr-Nd-Pb Isotopes

    Directory of Open Access Journals (Sweden)

    Jingya Cao

    2018-03-01

    Full Text Available The recently explored Xitian tungsten-tin (W-Sn polymetallic ore field, located in Hunan province, South China, is one of the largest ore fields in the Nanling Range (NLR. Two major metallogenic types appeared in this ore field, skarn- and quartz vein-type. They are distributed within Longshang, Heshuxia, Shaiheling, Hejiangkou, Goudalan, and so on. Hydrothermal zircons from two altered granites yielded U-Pb ages of 152.8 ± 1.1 Ma, and 226.0 ± 2.8 Ma, respectively. Two muscovite samples from ore-bearing quartz vein yielded 40Ar/39Ar plateau ages of 156.6 ± 0.7 Ma, 149.5 ± 0.8 Ma, respectively. Combined with the geological evidence, two metallogenic events are proposed in the Xitian ore field, with skarn-type W-Sn mineralization in Late Triassic (Indosinian and quartz vein/greisen type W-Sn mineralization in Late Jurassic (Yanshanian. The relatively low Ce/Ce* ratios and high Y/Ho ratios in zircons from two altered granites indicate that the hydrothermal fluids of two metallogenic events are characterized by low oxygen fugacities and enrichment in F. The similar chondrite-normalized patterns between the skarn and Xitian Indosinian granites and Sr-Nd-Pb isotopic compositions of wolframite suggest that the metal sources for both types W-Sn mineralization are derived from a crustal source.

  17. From Crustal Anatexis to Pluton Emplacement: High-Precision Zircon Geochronology Reveals the Thermal History of the Larderello-Travale Geothermal System (Italy)

    Science.gov (United States)

    Farina, F.; Dini, A.; Ovtcharova, M.; Davies, J.; Bouvier, A. S.; Baumgartner, L. P.; Caricchi, L.; Schaltegger, U.

    2017-12-01

    Late Miocene to recent post-collisional extension in Tuscany (Italy) led to the emplacement of shallow-level granitic plutons and to the eruption of small rhyolitic bodies. The intrusion of peraluminous two-mica and tourmaline-bearing granites triggered the formation of the steam-dominated Larderello-Travale geothermal system. In this study, zircon crystals from granite samples obtained from drill holes at 3.0-4.5 km depth were investigated by combining in-situ oxygen isotopes analysis and high-precision CA-ID-TIMS U-Pb age determinations to gain insight into the nature of the magmatic heat source fuelling the geothermal field. Magmatic zircon crystals display δ18O values ranging from 8.6 to 13.5‰ and crystals from individual samples exhibit inter- and intra-grain oxygen isotope variability exceeding 3‰. The geochronological data indicates the existence of three magmatic pulses with ages between 3.637 ± 0.008 and 1.671 ± 0.004 Ma. More importantly, zircon crystals from individual samples exhibit an age spread as large as 200-400 ky. This age dispersion, which is more than one order of magnitude greater than the uncertainty on a single date, suggest that most of the zircon did not crystallize at the emplacement level, but within isolated and isotopically distinct magma batches before large-scale homogenization in a magmatic reservoir at depth. The rate of assembly and final volume of this reservoir is estimated using the distribution of precise U-Pb zircon dates following the approach of Caricchi et al. (2014). Thermal modelling indicates that the heat flow at the surface in the geothermal field cannot be sustained by the inferred reservoir or by heat advection from the mantle. Our data suggest the existence of a younger shallow-level intrusion, whose occurrence also accounts for the existence of confined magmatic fluids at the top of the Larderello-Travale intrusion. We conclude that a multi-disciplinary approach, integrating high-precision zircon dating, in

  18. The Ellsworth terrane, coastal Maine: Geochronology, geochemistry, and Nd-Pb isotopic composition - Implications for the rifting of Ganderia

    Science.gov (United States)

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

    2008-01-01

    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

  19. Detrital zircon U-Pb geochronology of Cambrian to Triassic miogeoclinal and eugeoclinal strata of Sonora, Mexico

    Science.gov (United States)

    Gehrels, G.E.; Stewart, John H.

    1998-01-01

    One hundred and eighty two individual detrital zircon grains from Cambrian through Permian miogeoclinal strata, Ordovician eugeoclinal rocks, and Triassic post-orogenic sediments in northwestern Sonora have been analyzed. During Cambrian, Devonian, Permian, and Triassic time, most zircons accumulating along this part of the Cordilleran margin were shed from 1.40-1.45 and 1.62-1.78 Ga igneous rocks that are widespread in the southwestern United States and northwestern Mexico. Zircons with ages of approximately 1.11 Ga are common in Cambrian strata and were apparently shed from granite bodies near the sample site. The sources of 225-280 Ma zircons in our Triassic sample are more problematic, as few igneous rocks of these ages are recognized in northwestern Mexico. Such sources may be present but unrecognized, or the grains could have been derived from igneous rocks of the appropriate ages to the northwest in the Mojave Desert region, to the east in Chihuahua and Coahuila, or to the south in accreted(?) arc-type terranes. Because the zircon grains in our Cambrian and Devonian to Triassic samples could have accumulated in proximity to basement rocks near their present position or in the Death Valley region of southern California, our data do not support or refute the existence of the Mojave-Sonora megashear. Ordovician strata of both miogeoclinal and eugeoclinal affinity are dominated by >1.77 Ga detrital zircons, which are considerably older than most basement rocks in the region. Zircon grains in the miogeoclinal sample were apparently derived from the Peace River arch area of northwestern Canada and transported southward by longshore currents. The eugeoclinal grains may also have come from the Peace River arch region, with southward transport by either sedimentary or tectonic processes, or they may have been shed from off-shelf slivers of continents (perhaps Antarctica?) removed from the Cordilleran margin during Neoproterozoic rifting. It is also possible that the

  20. U-Pb and Lu-Hf zircon geochronology of the Cañadón Asfalto Basin, Chubut, Argentina: Implications for the magmatic evolution in central Patagonia

    Science.gov (United States)

    Hauser, N.; Cabaleri, N. G.; Gallego, O. F.; Monferran, M. D.; Silva Nieto, D.; Armella, C.; Matteini, M.; Aparicio González, P. A.; Pimentel, M. M.; Volkheimer, W.; Reimold, W. U.

    2017-10-01

    The Cañadón Asfalto basin, central Chubut, Argentina, comprises a volcano-sedimentary sequence related to the opening of the Atlantic Ocean during Mesozoic times. The Lonco Trapial, Cañadón Asfalto and Cañadón Calcáreo formations are the main units related to the evolution of this basin. The Las Chacritas and Puesto Almada members are distinguished in the Cañadón Asfalto Formation. LA-HR-ICP-MS U-Pb and Lu-Hf data on zircon were obtained on these units. The Lonco Trapial Formation gave a weighted average age of 172.3 ± 1.8 Ma. A pyroclastic level from the Las Chacritas Member gave a weighted average age of 168.2 ± 2.2 Ma. Two U-Pb concordant ages of 160.3 ± 1.7 Ma on a laminated tuffite and 158.3 ± 1.3 Ma on a pyroclastic level were obtained for the Puesto Almada Member. Two maximum depositional ages constrain the sedimentary provenance areas for the basin: 1) A sample from the Sierra de la Manea range, where a controversial unit related either to the Cañadón Asfalto or to the Cañadón Calcáreo formation occurs, gave an age of 176.6 ± 1.0 Ma. Two younger zircon crystals indicate that this unit may be related to the Cañadón Calcáreo Formation. 2) A sandstone with cross-stratification from the Puesto Almada Member gave a maximum depositional age of 173.6 ± 6.4 Ma. In terms of U-Pb and Lu-Hf isotopes, two magmatic events are identified in central Patagonia: the Mamil Choique magmatic event characterized by negative εHf values around -5.0 and representing recycling during Permian times of Mesoproterozoic crust (TDM of ∼1.5 Ga), and the Cañadón Asfalto magmatic event with negative (-8.2) to positive (+4) εHf values and Meso- to Neoproterozoic TDM between 1.5 and 0.8 Ga. The younger event is characterized by three main cycles: C1 related to the Lonco Trapial magmatism, C2 to the Las Chacritas volcanism, and C3 to the Puesto Almada volcanism. These cycles are related with Marifil, Chon Aike and El Quemado formations volcanics events of

  1. Dating the Indo-Asia collision in NW Himalaya: constraints from Sr-Nd isotopes and detrital zircon (U-Pb) and Hf isotopes of Paleogene-Neogene rocks in the Katawaz basin, NW Pakistan

    Science.gov (United States)

    Zhuang, Guangsheng; Najman, Yani; Millar, Ian; Chauvel, Catherine; Guillot, Stephane; Carter, Andrew

    2015-04-01

    The time of collision between the Indian and Asian plates is key for understanding the convergence history and the impact on climatic systems and marine geochemistry. Despite much active research, the fundamental questions still remain elusive regarding when and where the Indian plate collided with the Asian plate. Especially in the west Himalaya, the questions become more complex due to disputes on the amalgamation history of interoceanic Kohistan-Ladakh arcs (KLA) with Karakoram of the Asian plate and the Indian plate. Here, we present a result of multiple-isotopic geochemistry and geochronology study in the Katawaz Basin in NW Pakistan, a remnant oceanic basin on the western Indian plate which was the repository for the sediments eroded from the west Himalaya ( Qayyum et al., 1996, 1997a, 1997b, 2001; Carter et al., 2010), to evaluate the time and character of collision in this region. In this study, we analyzed 22 bulk mudstone samples for Sr-Nd isotopes and 11 medium-grained sandstones for detrital zircon (U-Pb) geochronology and Hf isotopes. We constructed the Cenozoic chronology in the Katawaz Basin based on our newly collected detrital zircon U-Pb ages and fission track ages. We present the first record of Katawaz chronology that constrained the Khojak Formation to be current study revealed that the Katawaz sedimentary sequence ranges in age from Eocene to the earliest Miocene. The samples from the Nisai Formation show the 87Sr/86Sr - ɛNd values overlapping those of the end member of the Karakoram of Asian origin, revealing the arrival of Asian detritus on the Indian plate prior to 50 Ma. There are two parallel lines of evidence supporting this conclusion: (1) young zircon grains (Journal of the Geological Society 154, 753-756. Qayyum, M., Lawrence, R.D., Niem, A.R., 1997b. Molasse-Delta-flysch continuum of the Himalayan orogeny and closure of the Paleogene Katawaz Remnant Ocean, Pakistan. International geology review 39, 861-875. Qayyum, M., Niem, A

  2. Conventional U-Pb dating versus SHRIMP of the Santa Barbara Granite Massif, Rondonia, Brazil

    Science.gov (United States)

    Sparrenberger, I.; Bettencourt, Jorge S.; Tosdal, R.M.; Wooden, J.L.

    2002-01-01

    The Santa Ba??rbara Granite Massif is part of the Younger Granites of Rondo??nia (998 - 974 Ma) and is included in the Rondo??nia Tin Province (SW Amazonian Craton). It comprises three highly fractionated metaluminous to peraluminous within-plate A-type granite units emplaced in older medium-grade metamorphic rocks. Sn-mineralization is closely associated with the late-stage unit. U-Pb monazite conventional dating of the early-stage Serra do Cicero facies and late-stage Serra Azul facies yielded ages of 993 ?? 5 Ma and 989 ?? 13 Ma, respectively. Conventional multigrain U-Pb isotope analyses of zircon demonstrate isotopic disturbance (discordance) and the preservation of inherited older zircons of several different ages and thus yield little about the ages of Sn-granite magmatism. SHRIMP U-Pb ages for the Santa Ba??rbara facies association yielded a 207Pb/206Pb weighted-mean age of 978 ?? 13 Ma. The textural complexity of the zircon crystals of the Santa Ba??rbara facies association, the variable concentrations of U, Th and Pb, as well as the mixed inheritance of zircon populations are major obstacles to using conventional multigrain U-Pb isotopic analyses. Sm-Nd model ages and ??Nd (T) values reveal anomalous isotopic data, attesting to the complex isotopic behaviour within these highly fractionated granites. Thus, SHRIMP U-Pb zircon and conventional U-Pb monazite dating methods are the most appropriate to constrain the crystallization age of the Sn-bearing granite systems in the Rondo??nia Tin Province.

  3. Characterization and zircon SHRIMP U-Pb geochronology of the subvolcanic rocks from Yarumalito Porphyry System, Marmato District, Colombia; Caracterizacao e geocronologia SHRIMP U-Pb em zircao das rochas subvulcanicas do sistema porfiro Yarumalito, Distrito de Marmato, Colombia

    Energy Technology Data Exchange (ETDEWEB)

    Henrichs, Isadora A.; Frantz, Jose Carlos; Marques, Juliana C.; Castoldi, Marco S., E-mail: isahenrichs@gmail.com, E-mail: jose.frantz@ufrgs.br, E-mail: juliana.marques@ufrgs.br [Universidade Federal do Rio Grande do Sul (UFRS), Porto Alegre, RS (Brazil). Instituto de Geociencias; Ordonez-Carmona, Oswaldo, E-mail: oswaldo.geologo@gmail.com [Universidad Nacional de Colombia, Medellin (Colombia). Facultad de Minas; Sato, Kei, E-mail: keisato@usp.br [Universidade de Sao Paulo (USP), Sao Paulo, SP (Brazil). Centro de Pesquisas Geocronologicas

    2014-09-15

    The mining District of Marmato, located in the Central Cordillera, is considered one of the oldest gold districts in Colombia and its exploration dates back to the Inca’s times, being exploited regularly for more than a thousand years. Inserted in this context lies the Yarumalito porphyry system (YPS), characterized to concentrate ore in structure related veins and stockworks. The YPS is related to the Miocene magmatism of the Combia Formation. In this paper, the subvolcanic rocks directly associated with the mineralized zones were described in order to obtain U-Pb ages in zircon to the intrusions. Selected samples from two fertile intrusions, one andesitic (more abundant in the area) and other dioritic (more restricted), were carefully described and dated by SHRIMP. The results points to a very restricted interval for the ages, with weighted average {sup 206}Pb/{sup 238}U varying from 7,00 ± 0,15 Ma for the andesitic porphyry and 6.95 ± 0.16 Ma for the dioritic porphyry. These results constrain the Yarumalito system to the final stages of the Combia magmatism and suggest a brief period for the crystallization of the mineralized subvolcanic rocks in the area and in the Marmato District. (author)

  4. Matrix-Matched Iron-Oxide Laser Ablation ICP-MS U–Pb Geochronology Using Mixed Solution Standards

    Directory of Open Access Journals (Sweden)

    Liam Courtney-Davies

    2016-08-01

    Full Text Available U–Pb dating of the common iron-oxide hematite (α-Fe2O3, using laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS, provides unparalleled insight into the timing and processes of mineral deposit formation. Until now, the full potential of this method has been negatively impacted by the lack of suitable matrix-matched standards. To achieve matrix-matching, we report an approach in which a U–Pb solution and ablated material from 99.99% synthetic hematite are simultaneously mixed in a nebulizer chamber and introduced to the ICP-MS. The standard solution contains fixed U- and Pb-isotope ratios, calibrated independently, and aspiration of the isotopically homogeneous solution negates the need for a matrix-matched, isotopically homogenous natural iron-oxide standard. An additional advantage of using the solution is that the individual U–Pb concentrations and isotope ratios can be adjusted to approximate that in the unknown, making the method efficient for dating hematite containing low (~10 ppm to high (>1 wt % U concentrations. The above-mentioned advantage to this solution method results in reliable datasets, with arguably-better accuracy in measuring U–Pb ratios than using GJ-1 Zircon as the primary standard, which cannot be employed for such low U concentrations. Statistical overlaps between 207Pb/206Pb weighted average ages (using GJ-1 Zircon and U–Pb upper intercept ages (using the U–Pb mixed solution method of two samples from iron-oxide copper-gold (IOCG deposits in South Australia demonstrate that, although fractionation associated with a non-matrix matched standard does occur when using GJ-1 Zircon as the primary standard, it does not impact the 207Pb/206Pb or upper intercept age. Thus, GJ-1 Zircon can be considered reliable for dating hematite using LA-ICP-MS. Downhole fractionation of 206Pb/238U is observed to occur in spot analyses of hematite. The use of rasters in future studies will hopefully minimize

  5. Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix effect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards

    Science.gov (United States)

    Black, L.P.; Kamo, S.L.; Allen, C.M.; Davis, D.W.; Aleinikoff, J.N.; Valley, J.W.; Mundil, R.; Campbell, I.H.; Korsch, R.J.; Williams, I.S.; Foudoulis, C.

    2004-01-01

    Precise isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) documentation is given for two new Palaeozoic zircon standards (TEMORA 2 and R33). These data, in combination with results for previously documented standards (AS3, SL13, QGNG and TEMORA 1), provide the basis for a detailed investigation of inconsistencies in 206Pb/238U ages measured by microprobe. Although these ages are normally consistent between any two standards, their relative age offsets are often different from those established by ID-TIMS. This is true for both sensitive high-resolution ion-microprobe (SHRIMP) and excimer laser ablation-inductively coupled plasma-mass spectrometry (ELA-ICP-MS) dating, although the age offsets are in the opposite sense for the two techniques. Various factors have been investigated for possible correlations with age bias, in an attempt to resolve why the accuracy of the method is worse than the indicated precision. Crystallographic orientation, position on the grain-mount and oxygen isotopic composition are unrelated to the bias. There are, however, striking correlations between the 206Pb/238U age offsets and P, Sm and, most particularly, Nd abundances in the zircons. Although these are not believed to be the primary cause of this apparent matrix effect, they indicate that ionisation of 206Pb/238U is influenced, at least in part, by a combination of trace elements. Nd is sufficiently representative of the controlling trace elements that it provides a quantitative means of correcting for the microprobe age bias. This approach has the potential to reduce age biases associated with different techniques, different instrumentation and different standards within and between laboratories. Crown Copyright ?? 2004 Published by Elsevier B.V. All rights reserved.

  6. Subduction Initiation Existed Along the Ancient Continent Margins? Evidence of U-Pb ages of zircons from the Bonin Trench, Japan

    Science.gov (United States)

    Li, Y. B.; Pearce, J. A.; Ryan, J. G.; Li, X. H.; Haraguchi, S.; Iizuka, T.; Kon, Y.; Yamamoto, S.; Sawaki, Y.; Ishii, T.; Maruyama, S.

    2017-12-01

    Although it is not cleanly known when and where the subduction initiation began on the Paleo-Izu-Bonin-Mariana (IBM) Trench, Jurassic and Cretaceous plutonic rocks, such as gabbroic, granitic and metamorphic rocks had been sampled from the Amami Plateau-Daito Ridge-Okidaito Ridge (ADO) in the Philippine Sea Plate. Furthermore, Mesozonic to Paleozonic ages zircons were obtained from volcaniclastic sandstones collected from northern Izu-Bonin forarc (Tani et al., 2012). We present U-Pb ages, Hf-O isotopes and trace element compositions of zircon grains separated from sediment, volcanic rock, dolerite and gabbro, collected from Chichijima Island and Bonin forearc seafloor (KH03-3, KT04-28 cruise of the University of Tokyo, IODP Leg 352). In the zircon age histogram, several age groups were identified. The age peaks are 0-3 Ma and 13 Ma (Hahajima Seamount: soft mud and volcanic tuff); 38 Ma (Oomachi Seamount: sandstone); 45 Ma (Chichijima Island: volcanic rock); 40 Ma, 48 Ma and 52 Ma (Hahajima Seamount: dolerite and gabbro); 45 Ma and 164-165 Ma (IODP Leg 352: volcanic rock), respectively. Zircon U-Pb ages ranging 0-52 Ma correspond well to the multi-stages of magmatism in the IBM. However, 164-165 Ma maybe represent the ages of zircon xenocryst including in forearc volcanic rock , which pre-existing in ancient continent crustal materials (SE China Continent Crust?) as the basement of Paleo-IBM. It seems reasonable to suppose that the subduction initiation of IBM existed along the ancient SE China Continent margins. The initiation of subduction zone is a consequence of lateral compositional buoyancy contrast within the lithosphere, that advocated by Niu et al. (2003, 2016).

  7. Lifetime of an ocean island volcano feeder zone: constraints from U-Pb dating on coexisting zircon and baddeleyite, and 40/39Ar age determinations, Fuerteventura, Canary Islands

    Science.gov (United States)

    Allibon, James; Ovtcharova, Maria; Bussy, Francois; Cosca, Michael; Schaltegger, Urs; Bussien, Denise; Lewin, Eric

    2011-01-01

    High-precision isotope dilution - thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 ± 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. 40Ar/39Ar amphibole dating yielded ages from 21.9 ± 0.6 to 21.8 ± 0.3, identical within errors to the U-Pb ages, despite the expected 1% theoretical bias between 40Ar/39Ar and U-Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i.e., less than the 0.3 to 0.6 Ma 40Ar/39Ar age uncertainties) from closure temperatures (Tc) of zircon (699-988 °C) to amphibole (500-600 °C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the 40Ar/39Ar and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole Tc is 0.8 Ma, suggesting PX1 lifetime of 520,000 to 800,000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 ± 0.07/0.08/0.15 Ma and 21.58 ± 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope δ18O values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO2-rich carbonatite-derived fluids released during contact metamorphism.

  8. Cenozoic foreland basins of Central Andes: a preliminary provenance U-Pb zircon analysis of sedimentary sequences of Calchaqui Valley

    International Nuclear Information System (INIS)

    Oliveira, Alisson Lopes; Hauser, Natalia; Pimentel, Marcio Martins; Matteini, Massimo; Coira, Beatriz; Alonso, Ricardo; Barrientos, Andrea

    2015-01-01

    The Eocene of northwestern Argentina records complex basin and structural evolution, including continental sedimentation of the post-rift Salta Basin and the beginning of the Andean uplift and foreland system evolution. This illuminates a significant period of evolutionary history of this and surrounding basins in northwestern Argentina. U-Pb zircon analyses by LA-ICP-MS for three formations representing post-rift to foreland stages allowed interpretation about provenance terrains. The Lumbrera Formation, representing the post-rift stage, shows bimodal sources with a main zircon population around 462 Ma, and a second population around 1023 Ma. The Los Colorados and Angastaco Formations representing the sedimentation in a foreland basin, show a unimodal source around 490 Ma, and 517 Ma respectively. Zircons younger than 50 Ma were not identified during this study. (author)

  9. Cenozoic foreland basins of Central Andes: a preliminary provenance U-Pb zircon analysis of sedimentary sequences of Calchaqui Valley

    Energy Technology Data Exchange (ETDEWEB)

    Oliveira, Alisson Lopes; Hauser, Natalia; Pimentel, Marcio Martins; Matteini, Massimo, E-mail: alisson_oliveira@hotmail.com [Universidade de Brasilia (UnB), DF (Brazil). Laboratorio de Geocronologia; Galli, Claudia Ines [Faculdad de Ingenieria, Universidad Nacional de Jujuy (Argentina); Coira, Beatriz [CIT Jujuy, CONICET. Instituto de Geologia y Mineria (Argentina); Alonso, Ricardo; Barrientos, Andrea [Instituto CEGA, CONICET. Universidad Nacional de Salta (Argentina)

    2015-07-01

    The Eocene of northwestern Argentina records complex basin and structural evolution, including continental sedimentation of the post-rift Salta Basin and the beginning of the Andean uplift and foreland system evolution. This illuminates a significant period of evolutionary history of this and surrounding basins in northwestern Argentina. U-Pb zircon analyses by LA-ICP-MS for three formations representing post-rift to foreland stages allowed interpretation about provenance terrains. The Lumbrera Formation, representing the post-rift stage, shows bimodal sources with a main zircon population around 462 Ma, and a second population around 1023 Ma. The Los Colorados and Angastaco Formations representing the sedimentation in a foreland basin, show a unimodal source around 490 Ma, and 517 Ma respectively. Zircons younger than 50 Ma were not identified during this study. (author)

  10. Possible zircon U-Pb evidence for Pan-African granulite-facies metamorphism in the Mozambique belt of southern Tanzania

    International Nuclear Information System (INIS)

    Coolen, J.J.M.M.M.

    1982-01-01

    Four zircon fractions of garnet-bearing two-pyroxene granulite, from the Furua granulite complex of southern Tanzania, plot very close to concordia. A discordia yields a lower intercept at 652 +- 10 Ma, an age slightly higher than the Rb-Sr whole-rock and mineral ages reported from the surrounding amphibolite-facies rocks. The U-Pb systematics indicate the presence of a very small amount of older (2-3 Ga) radiogenic lead. The zircon data may be interpreted as indicating an event of granulite-facies metamorphism during the Pan-African thermotectonic episode. This interpretation is at variance with current models postulating that the granulite complexes in the Mozambique belt are relicts of older, possibly Archaean events of metamorphism. (Auth.)

  11. Devonian alkaline magmatism in the northern North China Craton: Geochemistry, SHRIMP zircon U-Pb geochronology and Sr-Nd-Hf isotopes

    Directory of Open Access Journals (Sweden)

    Dingling Huang

    2017-01-01

    Full Text Available The Wulanhada pluton is among the rare suite of Devonian alkaline plutons occurring along the northern margin of the North China Craton (NCC. The intrusion is mainly composed of quartz-monzonite. Here we report zircon SHRIMP U-Pb data from this intrusion which shows emplacement age of ca. 381.5 Ma. The rock is metaluminous with high (Na2O + K2O values ranging from 8.46 to 9.66 wt.%. The REE patterns of the rocks do not show any Eu anomaly whereas the primitive-mantle-normalized spider diagram shows strong positive Sr and Ba anomalies. The Wulanhada rocks exhibit high initial values of (87Sr/86Srt = 0.70762–0.70809, low ɛNd(t = −12.76 to −12.15 values and negative values of ɛHf(t = −23.49 to −17.02 with small variations in (176Hf/177Hft (0.281873–0.282049. These geochemical features and quantitative isotopic modeling results suggest that the rocks might have been formed through the partial melting of Neoarchean basic rocks in the lower crust of the NCC. The Wulanhada rocks, together with the Devonian alkaline rocks and mafic-ultramafic complex from neighboring regions, constitute a post-collisional magmatic belt along the northern NCC.

  12. Developing Zircon as a Probe of Planetary Impact History

    Science.gov (United States)

    Wielicki, Matthew

    2014-12-01

    cratering and future diffusion studies may provide better constraints on this problem. Although little disturbance was identified in Pb* of target zircon, other low temperature geochronometers, zircon (U-Th)/He dating in this case, have been shown to be completely 'reset' and accurately date impacts. Zircon (U-Th)/He ages isolated from the target rock below ~850 m of well-dated impact melt at Morokweng yield ages consistent with the impact melt sheet and provide an alternative tool to dating events where such melts no longer exists. This geochronometer was also applied to impactites from Popigai, Russia and results in an age that is significantly younger than that reported in the literature and coincident with the Eocene-Oligocene boundary mass extinction event however the lack of any impact signatures at this boundary is puzzling. Constraining the impact flux to the Earth-Moon system not only allows for a better understanding into early Earth evolution and the formation of a habitable planet but also provides constraints on the modern impactor flux, important criteria for estimating the likelihood of future impact events. Zircon geochronology offers an exciting new tool by which to date impact events and has the potential to assist understanding of complex impactite samples from terrestrial craters and future sample return missions.

  13. Using Detrital Zircon Geochronology to Constrain Paleogene Provenance and Its Relationship to Rifting in the Zhu 1 Depression, Pearl River Mouth Basin, South China Sea

    Science.gov (United States)

    Wang, Wei; Ye, Jiaren; Bidgoli, Tandis; Yang, Xianghua; Shi, Hesheng; Shu, Yu

    2017-11-01

    Paleogene syn-rift successions in the South China Sea are poorly understood and systematic provenance analysis, which could provide clues to their history, is lacking. Here we report 409 new concordant U-Pb ages from detrital zircons separated from the Paleogene Wenchang, Enping, and Zhuhai formations in the Zhu 1 depression, Pearl River Mouth Basin. The new data, combined with the published age data from the region, document changes in the provenance of syn-rift successions. Detrital zircons from the Eocene Wenchang Formation are unimodal, with Jurassic-Cretaceous (180-80 Ma) ages making up >80% of grains. The ages are consistent with the geochronology of intrabasinal highs, dominated by igneous rocks emplaced during the Yanshanian orogeny, and suggest local provenance. By contrast, detrital zircons from the upper Eocene to lower Oligocene Enping Formation form three well-recognized age-clusters, with peaks at 150, 254, and 438 Ma that match documented tectonomagmatism in South China Block (SCB). Combined with increasing numbers of Precambrian zircons, the data suggest increasing influence of regional provenance of the SCB. Similar age peaks are also recognized from the limited number of zircons analyzed from the upper Oligocene Zhuhai Formation and comparability with modern shelf and river sediment indicates the unit was mainly sourced from the SCB and likely transported by a paleo-Pearl River. We infer that the change in provenance, from local uplifts within the Zhu 1 to the SCB, is related to distinct phases of PRMB rift development; however, later changes are best explained by SCB drainage evolution.

  14. U-Pb zircon age for a volcanic suite in the Rankin Inlet Group, Rankin Inlet map area, District of Keewatin, Northwest Territories

    International Nuclear Information System (INIS)

    Tella, S.; Roddick, J.C.; VanBreemen, O.

    1996-01-01

    U-Pb zircon analyses from a felsic band within dominantly mafic volcanics of the Rankin Inlet Group yields a U-Pb upper concordia intercept age of 2663 ± 3 Ma. These supracrustals at Rankin Inlet appear to be 15-20 Ma younger than volcanics of the Kaminak Group in the Tavani area, 70 km to the southwest. The 2.68-2.66 Ga volcanism in the Tavani and Rankin Inlet areas coincided with the last stage of the main phase of magmatism in the Slave Structural Province. (author). 16 refs., 1 tab., 3 figs

  15. U-Pb zircon age for a volcanic suite in the Rankin Inlet Group, Rankin Inlet map area, District of Keewatin, Northwest Territories

    Energy Technology Data Exchange (ETDEWEB)

    Tella, S; Roddick, J C; VanBreemen, O [Geological Survey of Canada, Ottawa, ON (Canada)

    1997-12-31

    U-Pb zircon analyses from a felsic band within dominantly mafic volcanics of the Rankin Inlet Group yields a U-Pb upper concordia intercept age of 2663 {+-} 3 Ma. These supracrustals at Rankin Inlet appear to be 15-20 Ma younger than volcanics of the Kaminak Group in the Tavani area, 70 km to the southwest. The 2.68-2.66 Ga volcanism in the Tavani and Rankin Inlet areas coincided with the last stage of the main phase of magmatism in the Slave Structural Province. (author). 16 refs., 1 tab., 3 figs.

  16. SHRIMP U-Pb zircon dating of Archean core complex formatio and pancratonic strike-slip deformation in the East Pilbara Granite-Greenstone Terrain

    NARCIS (Netherlands)

    Zegers, T.E.; Nelson, D.R.; Wijbrans, J.R.; White, S.H.

    2001-01-01

    Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of zircons from granitic rocks in the East Pilbara Granite-Greenstone Terrain has provided time constraints for main tectonic events in the Shaw Granitoid Complex and has shown that deformation was intricately related to granitoid

  17. Age intercalibration of 40Ar/39Ar sanidine and chemically distinct U/Pb zircon populations from the Alder Creek Rhyolite Quaternary geochronology standard

    DEFF Research Database (Denmark)

    Rivera, Tiffany; Storey, Michael; Schmitz, M. D.

    2013-01-01

    , and have less pronounced positive Ce and negative Eu anomalies, lower incompatible trace element contents, higher TiZR temperatures that range up to 840 °C, and significantly younger dates. The youngest Group A dates yield a weighted mean of 1.1978 ± 0.0046 Ma (2σ, including systematic uncertainties...... with the astronomical age estimate for the Cobb Mountain subchron determined by correlating the oxygen isotope record of the giant piston core MD972143 to the La93(1,1) orbital solution. On the basis of independent radio-isotopic and orbital forcing results, we propose the refined age of 1.1850 ± 0.0016 Ma (2σ external......We report results from a 40Ar/39Ar sanidine and CA-TIMS 238U/206Pb zircon dating study of eruption and crystal residence timescales of the Alder Creek Rhyolite (ACR), California, extruded during the Cobb Mountain normal-polarity subchron (C1r.2n). A 40Ar/39Ar ACR sanidine date of 1.1850 ± 0.0016 Ma...

  18. Development of CA-ID-TIMS zircon U–Pb dating technique at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences

    Directory of Open Access Journals (Sweden)

    Yu-Ting Zhong

    2017-06-01

    Pretreatment protocols for zircon analysis, and community-wide innovations spurred on by EARTHTIME tends to be at the leading edge of technological development. Also, more and more Chinese researchers perform lots of laboratory work in improving the technique in China. Here we present the running condition, procedural blanks and zircon standards measurement results in the clean laboratory of the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS. The results show the total procedural blanks (Pbc are below ∼1 pg mostly, suitable for the sample pretreatment and operation. The 206Pb/238U dates of R33 and 500 Ma solution are 419.88 ± 0.94 Ma (n = 7; MSWD = 6.8 and 500.50 ± 0.19 Ma (n = 9; MSWD = 0.84, respectively. These results demonstrate that the CA-ID-TIMS procedure is well established at GIGCAS. Our efforts are beneficial to the development of EARTHTIME-CN program. In particular, when this technique is combined with the SIMS technique, it provides an efficient way to solve a number of geochronological problems encountered in China and worldwide.

  19. A U-Pb zircon age constraint on the oldest-recorded air-breathing land animal.

    Directory of Open Access Journals (Sweden)

    Stephanie E Suarez

    Full Text Available The oldest-known air-breathing land animal is the millipede Pneumodesmus newmani, found in the Cowie Harbour Fish Bed at Stonehaven, Scotland. Here we report the youngest, most concordant 238U-206Pb zircon age from ash below the fish bed of 413.7±4.4 Ma (±2σ, whereas the youngest age from a tuffaceous sandstone above the fish bed is statistically indistinguishable at 414.3±7.1 Ma. The Cowie Harbour Fish Bed thus appears to be lowermost Devonian (Lochkovian, contrary to the previously accepted mid-Silurian age based on palynomorphs from adjacent exposures. This has implications for the evolutionary timetable of land colonization, as the Cowie ages overlap late Lochkovian zircon ages reported elsewhere for andesite below the nearby (~50 mi Rhynie Chert, which has more advanced terrestrial biota. The results postdate the possible late Silurian Ludford Lane locality in Shropshire, England. Pneumodesmus newmani is thus not the earliest air-breathing land animal, unless the Ludford Lane locality is younger than presently assigned.

  20. U-Pb ages in zircon of some classic granites from the Borborema province

    International Nuclear Information System (INIS)

    Neves, Benjamin Bley de Brito; Basei, Miguel Angelo Stipp; Passarelli, Claudia Regina; Santos, Edilton Jose dos

    2003-01-01

    The Neoproterozoic evolution of the Borborema Province in the east-northeast part of the South American platform exhibits remarkable, much studied granitic plutonism. In 1967 F. F. Almeida and co-workers discriminated a series of granitic typologies as 'Conceicao type', 'Itaporanga type' etc, mostly based upon general geological features and petrographic grounds. This preliminary classification has somehow been maintained in all subsequent papers up to now, despite all the newly observed plutons and the newly generated tectonic, geochemical and isotopic data since then. Samples of these classical types of granites from their type-localities were analyzed geo chronologically using the U-Pb method in zircons in response to the felt needs of the geological community, because previous age-determinations involved less accurate methods. The new data have been added to all other preexisting U-Pb data (from other granitic bodies) obtained over the last decade, making it possible to recognize three main stages of granitic magmatism for the Borborema Province: 650-625 Ma, 580-570 Ma and 545-520 Ma. The last group of dates is closely association with the escape tectonics of the Brasiliano-Pan African collage, which preceded the assembly of the super continental landmass of Western Gondwana. (author)

  1. Detrital U-Pb zircon analysis of an Eocene McMurdo Erratic sandstone, McMurdo Sound, Antarctica

    International Nuclear Information System (INIS)

    Paulsen, T.; Encarnacion, J.; Valencia, V.A.; Roti Roti, J.M.; Rasoazanamparany, C.

    2011-01-01

    Some of the most important Cenozoic sedimentary rocks in Antarctica are glacial erratics of fossiliferous Palaeogene to Neogene siliclastic rocks known as the McMurdo Erratics. Detrital zircon U-Pb isotopic data reported herein provide new information on the provenance of these siliciclastic rocks. The U-Pb age data from a sandstone glacial erratic that is likely of Eocene age show a dominant age cluster that ranges from 488 to 635 Ma, accompanied by subsidiary Neoproterozoic-Archaean peaks. The dominant Neoproterozoic-Ordovician age cluster, in conjunction with the arkosic lithologies of some erratics and the local presence of granitic and metasedimentary clasts, indicates that the Neoproterozoic-Ordovician basement complex of the Transantarctic Mountains likely served as a major source for the Eocene siliciclastic sediments. This is consistent with derivation of the McMurdo Erratics from locations proximal to the Transantarctic Mountains such as the Discovery Deep and/or major mountain outlet glacier areas. (author). 19 refs., 3 figs., 1 tab.

  2. High precision time calibration of the Permo-Triassic boundary mass extinction by U-Pb geochronology

    Science.gov (United States)

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

    2014-05-01

    U-Pb dating using Chemical Abrasion, Isotope Dilution Thermal Ionization Mass Spectrometry (CA-ID-TIMS) is the analytical method of choice for geochronologists, who are seeking highest temporal resolution and a high degree of accuracy for single grains of zircon. The use of double-isotope tracer solutions, cross-calibrated and assessed in different EARTHTIME labs, coinciding with the reassessment of the uranium decay constants and further improvements in ion counting technology led to unprecedented precision better than 0.1% for single grain, and 0.05% for population ages, respectively. These analytical innovations now allow calibrating magmatic and biological timescales at resolution adequate for both groups of processes. To construct a revised and high resolution calibrated time scale for the Permian-Triassic boundary (PTB) we use (i) high-precision U-Pb zircon age determinations of a unique succession of volcanic ash beds interbedded with shallow to deep water fossiliferous sediments in the Nanpanjiang Basin (South China) combined with (ii) accurate quantitative biochronology based on ammonoids and conodonts and (iii) carbon isotope excursions across the PTB. Using these alignments allows (i) positioning the PTB in different depositional environments and (ii) solving age/stratigraphic contradictions generated by the index, water depth-controlled conodont Hindeodus parvus, whose diachronous first occurrences are arbitrarily used for placing the base of the Triassic. This new age framework provides the basis for a combined calibration of chemostratigraphic records with high-resolution biochronozones of the Late Permian and Early Triassic. Besides the general improvement of the radio-isotopic calibration of the PTB at the ±100 ka level, this will also lead to a better understanding of cause and effect relations involved in this mass extinction.

  3. U-Pb and Lu-Hf systematics of zircons from Sargur metasediments, Dharwar Craton, Southern India: new insights on the provenance and crustal evolution

    International Nuclear Information System (INIS)

    Maibam, Bidyananda; Gerdes, Axel; Srinivasan, R.; Goswami, J.N.

    2017-01-01

    A study of U-Pb and Lu-Hf-Yb isotope data in zircons from metamorphosed psammopelite and quartzite from the type area of Archaean Sargur Group, Dharwar Craton, India is carried out. Two age populations are observed: an older population with concordant U-Pb ages between 2.7 and 2.8 Ga, and a younger population with ages in the 2.4 - 2.6 Ga age range. The εHf values of 0 to + 2.0 for the older zircon population suggest that they were derived from juvenile crust formed at 2.7- 2.8 Ga. Sub-chondritic εHf values for the younger population indicate metamorphism and/or crustal reworking at ∼2.5 Ga. Meta-sedimentary enclaves in the Sargur type area are therefore part of the gneiss-supracrustal complex of different antiquities and may not have an independent stratigraphic status. (author)

  4. 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

    Science.gov (United States)

    Capaldi, T.; Horton, B. K.; McKenzie, R.; Stockli, D. F.

    2015-12-01

    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.

  5. U-Pb ages of detrital zircon from Cenozoic sediments in the southwestern Tarim Basin, NW China: Implications for Eocene-Pliocene source-to-sink relations and new insights into Cretaceous-Paleogene magmatic sources

    Science.gov (United States)

    Yang, Wei; Fu, Ling; Wu, Chaodong; Song, Yan; Jiang, Zhenxue; Luo, Qun; Zhang, Ziya; Zhang, Chen; Zhu, Bei

    2018-05-01

    A detailed investigation of potential provenance is still lacking in the southwestern Tarim Basin, which restricts our complete understanding of Cenozoic source-to-sink relations between the basin interior and the Pamir salient - western Kunlun Mountain Range. Debate also exists concerning the potential sources of the Paleogene and Cretaceous igneous detritus present in the Cenozoic sedimentary sequences. Here, we present U-Pb (LA-ICP-MS) ages of detrital zircons from the continuous Eocene-Pliocene sediment series in the well-exposed Aertashi section to investigate changes in sediment provenance through time. The U-Pb detrital zircon ages range widely from 45 to 3204 Ma and can be divided into seven main groups: 45-65 Ma (sub-peak at 49 Ma), 67-103 Ma (sub-peak at 95 Ma), 196-251 Ma (sub-peak at 208 Ma), 252-416 Ma (sub-peak at 296 Ma), 417-540 Ma (sub-peak at 446 Ma), 550-1429 Ma (sub-peaks at 614 Ma, 828 Ma and 942 Ma) and 1345-3204 Ma (sub-peaks at 1773 Ma and 2480 Ma). These zircons were mainly derived from the western Kunlun Mountain Range and northern Pamir salient to the west and south. The evolution of the provenance and source-to-sink relationship patterns in the southwestern Tarim Basin can be divided into three stages: (1) The Middle Eocene to Lower Oligocene sediments display a wide variety of detrital zircon ages, suggesting that the source area was extensive. (2) A major change in provenance occurred during the Late Oligocene to Early Miocene and was characterized by an abrupt increase in the proportion of Triassic and Lower Paleozoic igneous components, implying a significant adjustment in topography induced by the initial uplift and exhumation of the western Kunlun Mountain Range and northern Pamir salient. (3) In the Late Miocene, the source-to-sink system transformed again, and contributions of Triassic to Lower Paleozoic material weakened substantially due to the sufficient indentation of the Pamir salient. Our integrated analyses of zircon

  6. Geochronology and structuring of the Ceara State: Borborema Province northwestern part, NE Brazil

    International Nuclear Information System (INIS)

    Fetter, A.; Van Schmus, W.R.; Santos, Ticiano J. Saraiva dos; Arthaud, M.; Nogueira Neto, J.

    1997-01-01

    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

  7. Timing of mafic magmatism in the Tapajós Province (Brazil) and implications for the evolution of the Amazon Craton: evidence from baddeleyite and zircon U Pb SHRIMP geochronology

    Science.gov (United States)

    Santos, João Orestes Schneider; Hartmann, Léo Afraneo; McNaughton, Neal Jesse; Fletcher, Ian Robert

    2002-09-01

    The precise timing and possible sources of the mafic rocks in the Amazon craton are critical for reconstruction of the Atlantica supercontinent and correlation of mafic magmatism worldwide. New SHRIMP U-Pb baddeleyite and zircon ages and the reinterpretation of 207 existing dates indicate one orogenic (Ingarana) and four postorogenic (Crepori, Cachoeira Seca, Piranhas, and Periquito) basaltic events in the Tapajós Province, south central Amazon craton. Orogenic gabbro dikes that host gold mineralization are 1893 Ma and interpreted as associated with the Ingarana gabbro intrusions of the bimodal calk-alkalic Parauari intrusive suite. The age of 1893 Ma can be used as a guide to discriminate older and mineralized orogenic dikes from younger and nonmineralized Crepori- and Cachoeira Seca-related mafic dikes. The baddeleyite U-Pb age of the postorogenic Crepori dolerite (gabbro-dolerite sills and dikes) is 1780±9 Ma, ˜150 my older than the ages provided by K-Ar. This value correlates well with the Avanavero tholeiitic intrusions in the Roraima group, in the northern part of the craton in Guyana, Venezuela, and Roraima in Brazil. Early Statherian tholeiitic magmatism was widespread not only in the Amazon craton, but also in the La Plata craton of southern South America, where it is known as the giant Piedra Alta swarm of Uruguay and the post-Trans-Amazonian dikes of Tandil in Argentina. The Cachoeira Seca troctolite represents laccoliths, Feixes, and São Domingos, whose baddeleyite U-Pb age is 1186±12 Ma, 120-150 my older than the known K-Ar ages. This age is comparable to other Stenian gabbroic rocks with alkalic affinity in the craton, such as the Seringa Formation in NE Amazonas and the basaltic flows of the Nova Floresta formation in Rondônia. Dolerite from the giant Piranhas dike swarm in the western Tapajós Province has a Middle Cambrian age (507±4 Ma, baddeleyite) and inherited zircons in the 2238-1229 Ma range. The Piranhas dikes fill extensional NNE and

  8. U-Pb zircon geochronology of intrusive and basement rocks in the Jacurici Valley region, Sao Francisco Craton, BA, Brazil

    International Nuclear Information System (INIS)

    Silveira, Carlos Jose Sobrinho da; Frantz, Jose Carlos; Marques, Juliana Charao; Roos, Siegbert; Peixoto, Vinicius Medina

    2015-01-01

    The Jacurici Complex, located in the NE of the Sao Francisco Craton, is constituted by several Cr-mineralized mafic-ultramafic N-S bodies, possible fragments of a single sill disrupted during deformation. Some works suggest it is intruded on the Serrinha Block while others consider it in the Salvador-Curaca Belt. The basement on this region is informally divided into paragneisses and orthogneisses; the latter is supposed to be younger considering it is less deformed. Petrography revealed that some of the paragneisses are alkali-feldspar granite strongly milonitized. The orthogneisses occur at the north and consist, at least in part, of monzogranites with heterogeneous deformation, locally of low temperature. U-Pb zircon dating were performed for five representative samples. Just three provided good concordia ages. A mafic rock produced a 2102 ± 5 Ma age and it is petrographically similar to the metanorites described in the Jacurici Complex, being interpreted as the record of the first pulses of the mafic magmatism. A monzogranite yielded a 2995 ± 15 Ma age, older than expected, related to the Serrinha Block. The alkali-feldspar granite yielded a 2081 ± 3 Ma age. The Itiuba Syenite and the pegmatites that crosscut the Jacurici Complex have similar ages. Considering the lack of information about the supracrustal sequence that hosts the intrusive alkaline and mafic-ultramafic rocks at the Ipueira and the Medrado areas, it is possible that part of the terrain belongs to the Salvador-Curaca Belt. We suggest that the Jacurici Complex could be intruded after the tectonic amalgamation between the Serrinha Block and the older part of the Salvador-Curaca Belt and, therefore, could be hosted by both terrains. (author)

  9. Hydrothermal titanite from the Chengchao iron skarn deposit: temporal constraints on iron mineralization, and its potential as a reference material for titanite U-Pb dating

    Science.gov (United States)

    Hu, Hao; Li, Jian-Wei; McFarlane, Christopher R. M.

    2017-09-01

    Uranium-lead isotopes and trace elements of titanite from the Chengchao iron skarn deposit (Daye district, Eastern China), located along the contact zones between Triassic marine carbonates and an early Cretaceous intrusive complex consisting of granite and quartz diorite, were analyzed using laser ablation inductively coupled plasma mass spectrometry to provide temporal constraints on iron mineralization and to evaluate its potential as a reference material for titanite U-Pb geochronology. Titanite grains from mineralized endoskarn have simple growth zoning patterns, exhibit intergrowth with magnetite, diopside, K-feldspar, albite and actinolite, and typically contain abundant primary two-phase fluid inclusions. These paragenetic and textural features suggest that these titanite grains are of hydrothermal origin. Hydrothermal titanite is distinct from the magmatic variety from the ore-related granitic intrusion in that it contains unusually high concentrations of U (up to 2995 ppm), low levels of Th (12.5-453 ppm), and virtually no common Pb. The REE concentrations are much lower, as are the Th/U and Lu/Hf ratios. The hydrothermal titanite grains yield reproducible uncorrected U-Pb ages ranging from 129.7 ± 0.7 to 132.1 ± 2.7 Ma (2σ), with a weighted mean of 131.2 ± 0.2 Ma [mean standard weighted deviation (MSWD) = 1.7] that is interpreted as the timing of iron skarn mineralization. This age closely corresponds to the zircon U-Pb age of 130.9 ± 0.7 Ma (MSWD = 0.7) determined for the quartz diorite, and the U-Pb ages for zircon and titanite (130.1 ± 1.0 Ma and 131.3 ± 0.3 Ma) in the granite, confirming a close temporal and likely genetic relationship between granitic magmatism and iron mineralization. Different hydrothermal titanite grains have virtually identical uncorrected U-Pb ratios suggestive of negligible common Pb in the mineral. The homogeneous textures and U-Pb characteristics of Chengchao hydrothermal titanite suggest that the mineral might be a

  10. Zircon U-Pb and Hf isotopic constraints on the magmatic evolution of the Northern Luzon Arc

    Directory of Open Access Journals (Sweden)

    Yu-Ming Lai

    2018-01-01

    Full Text Available The complete volcanic sequences restored in the Coastal Range of Taiwan are key archives for better understanding the magmatic and tectonic evolution of the Northern Luzon Arc. This paper reports (1 new zircon U-Pb ages and Hf isotopic data of fourteen volcanic samples from different sequences of four major volcanoes in the Coastal Range, (2 Hf isotopic data of dated magmatic and detrital zircons from two offshore volcanic islands, Lutao and Lanyu. These data indicate that the arc magmatism in the Coastal Range started at ~15 Ma, most active at ~9 Ma, and ceased at ~4.2 Ma. Magmatic zircons from the arc rocks show a significant variation in Hf isotopic composition, with εHf(T values varying from +24.9 to +4.8. As pointed out by our previous studies, old continental zircons that show Cathaysian-type ages and Hf isotope features are common in samples from the Yuemei, Chimei, and Lanyu volcanoes, supporting the notion for the influence of the existence of an accreted micro-continent or continental fragment plays a role in the petrogenesis. Such inherited zircons are not observed in the Chengkuang’ao and Tuluanshan volcanoes and uncommon in Lutao, implying the discontinuity or a limited extent of the accreted continental fragment. The εHf(T values are high and positive from ~15 - 8 Ma (+25 to +15; ±5ε-unit variation, and became lower from ~6 to 4.2 Ma (+20 to +8; ±6ε units and the lowest from ~1.3 Ma (+19 to +5; ±7ε units. Such a temporal variation in zircon Hf isotopic ratios can be also identified in whole-rock Hf and Nd isotopic compositions, which decrease from ~6 Ma when the Northern Luzon Arc may have started colliding with the Eurasian continental margin.

  11. U-Pb SHRIMP ages of detrital zircons from Hiriyur formation in Chitradurga Greenstone belt and its implication to the Neoarchean evolution of Dharwar craton, South India

    International Nuclear Information System (INIS)

    Nasheeth, A.; Okudaira, T.; Horie, K.; Hokada, T.; Satish Kumar, M.

    2016-01-01

    We report newly obtained U-Pb SHRIMP ages of detrital zircons from metagreywackes in the Hiriyur Formation (Chitradurga Group, Dharwar Supergroup) from the central eastern part of the Chitradurga greenstone belt. U-Pb analyses yield three major Neoarchean age populations ranging from 2.70 - 2.54 Ga with some minor age population of Mesoarchean. The maximum age of deposition is constrained by the youngest detrital zircon population at 2546 Ma. This is the first report of the occurrence of supracrustal rocks less than 2.58 Ga in the central part of Chitradurga greenstone belt. Close evaluation of detrital ages with the published ages of surrounding igneous rocks suggest that the youngest detrital zircons might be derived from rocks of the Eastern Dharwar craton and the inferred docking of the western and eastern Dharwar cratons happened prior to the deposition of the Hiriyur Formation. The Chitradurga shear zone, dividing the Dharwar craton into western and eastern blocks, probably developed after the deposition. Furthermore, the lower intercept is interpreted as evidence for the Pan-African overprints in the study area. (author)

  12. Integrated Laser Ablation U/Pb and (U-Th)/He Dating of Detrital Accessory Minerals from the Naryani River, Central Nepal

    Science.gov (United States)

    Horne, A.; Hodges, K. V.; Van Soest, M. C.

    2015-12-01

    The newly developed 'laser ablation double dating' (LADD) technique, an integrated laser microprobe U/Pb and (U-Th)/He dating method, could be an exceptionally valuable tool in detrital thermochronology for identifying sedimentary provenance and evaluating the exhumation history of a source region. A recent proof-of-concept study has used LADD to successfully date both zircon and titanite crystals from the well-characterized Fish Canyon tuff, but we also believe that another accessory mineral, rutile, could be amenable to dating via the LADD technique. To continue the development of the method, we present an application of LADD to detrital zircon, titanite, and rutile from a sample collected on the lower Naryani River of central Nepal. Preliminary analyses of the sample have yielded zircon U/Pb dates ranging from 31.4 to 2405 Ma; zircon (U-Th)/He from 1.8 to 15.4 Ma; titanite U/Pb between 18 and 110 Ma; titanite (U-Th)/He between 1 and 16 Ma; rutile U/Pb from 6 to 45 Ma; and rutile (U-Th)/He from 2 to 25 Ma. In addition to the initial data, we can use Ti-in-zircon, Zr-in-titanite, and Zr-in-rutile thermometers to determine the range of possible long-term cooling rates from grains with U/Pb ages younger than collision. Thus far our results from zircon analyses imply a cooling rate of approximately 15°C/Myr; titanite analyses imply between 10 and 67°C/Myr; and rutile between 9 and 267°C/Myr. This spread in potential cooling rates, especially in the order of magnitude differences of cooling rates calculated from the rutile grains, suggests that the hinterland source regions of the Naryani river experienced dramatically different exhumation histories during Himalayan orogenisis. Ongoing analyses will expand the dataset such that we can more adequately characterize the range of possibilities represented in the sample.

  13. U-Pb (LA-ICPMS) zircon ages and Nd isotopes for granitoids of the Tamboril-Santa Quiteria Complex, Ceara Central Domain: implication for neoproterozoic syncollisional magmatism in north Borborema Province, Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Costa, Felipe Grandjean da; Araujo, Carlos Eduardo Ganade de; Vasconcelos, Antonio Maurilio, E-mail: felipe.costa@cprm.gov.br, E-mail: caegeo@gmail.com, E-mail: maurilio.vasconcelos@cprm.gov.br [Servico Geologico do Brasil (CPRM), Fortaleza, CE (Brazil); Amaral, Wagner da Silva, E-mail: wamaral@ufc.br [Universidade Federal do Ceara (UFC), Fortaleza, CE (Brazil). Departamento de Geologia; Rodrigues, Joseneusa Brilhante, E-mail: joseneusa.rodrigues@cprm.gov.br [Servico Geologico do Brasil (CPRM), Brasilia, DF (Brazil)

    2013-06-15

    The Tamboril-Santa Quiteria Complex (TSQC) is one of the largest Neoproterozoic plutonic manifestations in the north Borborema Province (NE Brazil). It represents an anatectic/igneous association characterized by a number of magmatic pulses that occurred in the 650-610 Ma interval. In this paper, we present U-Pb (LA-MC-ICP-MS) zircon ages and Nd isotopes for quartz monzonite and quartz diorites of the southern part of TSQC. The quartz monzonite belong to a hybrid granitoid association, including monzonite, syenites and quartz syenites, all with abundant mafic magmatic enclaves. A quartz monzonite sample yielded a U-Pb zircon age of 634 {+-} 10 Ma and a TDM age of 2.69 Ga. The quartz diorites are much more homogeneous in composition and yielded a U-Pb zircon age of 618 {+-} 23 Ma and a TDM age of 2.19 Ga. The presence of coeval mantle-derived magmatism and diatexites (crustal anatexis) post-dating high-pressure metamorphism (ca. 650 Ma), and together with high-temperature metamorphism (ca. 630-610 Ma), suggests that this large magmatic manifestation evolved in a collisional setting, probably related to slab break off during the Western Gondwana amalgamation. (author)

  14. U-Pb Detrital Zircon Ages from Sarawak: Changes in Provenance Reflecting the Tectonic Evolution of Southeast Asia

    Science.gov (United States)

    Breitfeld, H. T.; Galin, T.; Hall, R.

    2014-12-01

    Sarawak is located on the northern edge of Sundaland in NW Borneo. Five sedimentary basins are distinguished with ages from Triassic to Cenozoic. New light mineral, heavy mineral and U-Pb detrital zircon ages show differences in provenance reflecting the tectonic evolution of the region. The oldest clastic sediments are Triassic of the Sadong-Kuching Basin and were sourced by a Carnian to Norian volcanic arc and erosion of Cathaysian rocks containing zircons of Paleoproterozoic age. Sandstones of the Upper Jurassic to Cretaceous Bau-Pedawan Basin have distinctive zircon populations indicating a major change of tectonic setting, including initiation of subduction below present-day West Sarawak in the Late Jurassic. A wide range of inherited zircon ages indicates various Cathaysian fragments as major source areas and the arrival of the SW Borneo Block following subduction beneath the Schwaner Mountains in the early Late Cretaceous. After collision of the SW Borneo Block and the microcontinental fragments with Sundaland in the early Late Cretaceous, deep marine sedimentation (Pedawan Formation) ceased, and there was uplift forming the regional Pedawan-Kayan unconformity. Two episodes of extension were responsible for basin development on land from the latest Cretaceous onwards, probably in a strike-slip setting. The first episode formed the Kayan Basin in the Latest Cretaceous (Maastrichtian) to Early Paleocene, and the second formed the Ketungau Basin and the Penrissen Sandstone in the Middle to Late Eocene. Zircons indicate nearby volcanic activity throughout the Early Cenozoic in NW Borneo. Inherited zircon ages indicate an alternation between Borneo and Tin Belt source rocks. A large deep marine basin, the Rajang Basin, formed north of the Lupar Line fault. Zircons from sediments of the Rajang Basin indicate they are of similar age and provenance as the contemporaneous terrestrial sediments to the south suggesting a narrow steep continental Sundaland margin at the

  15. Zircon U-Pb chronology, geochemistry and Sr-Nd-Pb isotopic compositions of the Volcanic Rocks in the Elashan area, NW China: petrogenesis and tectonic implications

    Science.gov (United States)

    Zhou, H.; Wei, J.; Shi, W.; Li, P.; Chen, M.; Zhao, X.

    2017-12-01

    Elashan area is located in the intersection of the East Kunlun Orogenic Belt (EKOB) and the West Qinling Orogenic (WQOB). We present petrology, zircon U-Pb ages, whole-rock geochemistry and Sr-Nd-Pb isotopic compositions from the andesite and felsic volcanic rocks (rhyolite and rhyolitic tuffs) in Elashan group volcanic rock. The LA-ICP-MS zircon U-Pb age data indicate that the volcanic rocks are emplaced at 250 247 Ma. The volcanic rocks have high -K and aluminum - peraluminous characteristics, A/CNK = 1.07 1.82, δ ranges from 1.56 2.95, the main body is calc-alkaline rock. They are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted in some high field strength elements (HFSEs, e.g., Nb, Ta, P and Ti), while having a flat heavy REE (HREEs) pattern. The ∑REE values of 178.68 to 298.11 ppm, average 230.50 ppm. The LREE/HREE values of 4.39 to 11.78 ppm, average 6.77 ppm. REE fractionation is obvious, REE distribution curve was right smooth, and have slightly negative Eu anomalies (Eu/Eu*=0.44-0.80, average 0.60), which as similar to the island arc volcanic rocks. The volcanic rocks have initial 87Sr/86Sr ratios of 0.71028-0.71232, ɛNd(t) values of -6.7 to -7.6, with T2DM-Nd ranging from 1561 to 1640 Ma. Pb isotopic composition (206 Pb / 204 Pb)t = 18.055 18.330, (207 Pb / 204 Pb)t = 15.586 15.618, (208 Pb / 204 Pb)t = 37.677 38.332. Geochemical and Sr-Nd-Pb isotopes indicates that Elashan group volcanic magma derived mainly from the lower crust. Elashan group volcanic rocks is the productive East Kunlun block and West Qinling block collision, which makes the thicken crust caused partial melting in the study area. The source rocks is probably from metamorphic sandstone of Bayankala. But with Y-Nb and Rb-(Y+Nb), R1-R2 and Rb/10-Hf-Ta*3 diagrams showing that intermediate-acid rocks mainly formed in volcanic arc-collision environment, probably the collision event is short , therefore rocks retain the original island

  16. The U-Pb System in Schorlomite from Calcite-Amphobole-Pyroxene Pegmatite of the Afrikanda Complex (Kola Peninsula)

    Science.gov (United States)

    Salnikova, E. B.; Stifeeva, M. V.; Chakhmouradian, A. R.; Glebovitsky, V. A.; Reguir, E. P.

    2018-02-01

    The geochronological U-Pb study of shorlomite from igneous rocks of the alkali-ultramafic Afrikanda massif (Kola Peninsula) was performed. The results demonstrate the reliability of calcium garnet as a mineral for the U-Pb geochronology of a wide range of igneous rocks, i.e., carbonatite, syenite, foidolite, foidite, melilitolite, melilitite, lamprophyres, micaceous kimberlites, etc., and associated rare earth and trace elements (REE, Nb, Zr) mineralization.

  17. Comment on "Zircon U-Th-Pb dating using LA-ICP-MS: Simultaneous U-Pb and U-Th dating on 0.1 Ma Toya Tephra, Japan" by Hisatoshi Ito

    Science.gov (United States)

    Guillong, M.; Schmitt, A. K.; Bachmann, O.

    2015-04-01

    Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of eight zircon reference materials and synthetic zircon-hafnon end-members indicate that corrections for abundance sensitivity and molecular zirconium sesquioxide ions (Zr2O3+) are critical for reliable determination of 230Th abundances in zircon. Other polyatomic interferences in the mass range 223-233 amu are insignificant. When corrected for abundance sensitivity and interferences, activity ratios of (230Th)/(238U) for the zircon reference materials we used average 1.001 ± 0.010 (1σ error; mean square of weighted deviates MSWD = 1.45; n = 8). This includes the 91500 and Plešovice zircons, which were deemed unsuitable for calibration of (230Th)/(238U) by Ito (2014). Uranium series zircon ages generated by LA-ICP-MS without mitigating (e.g., by high mass resolution) or correcting for abundance sensitivity and molecular interferences on 230Th such as those presented by Ito (2014) are potentially unreliable.

  18. Zircon (U-Th)/He Thermochronometric Constraints on Himalayan Thrust Belt Exhumation, Bedrock Weathering, and Cenozoic Seawater Chemistry

    Science.gov (United States)

    Colleps, Cody L.; McKenzie, N. Ryan; Stockli, Daniel F.; Hughes, Nigel C.; Singh, Birendra P.; Webb, A. Alexander G.; Myrow, Paul M.; Planavsky, Noah J.; Horton, Brian K.

    2018-01-01

    Shifts in global seawater 187Os/188Os and 87Sr/86Sr are often utilized as proxies to track global weathering processes responsible for CO2 fluctuations in Earth history, particularly climatic cooling during the Cenozoic. It has been proposed, however, that these isotopic records instead reflect the weathering of chemically distinctive Himalayan lithologies exposed at the surface. We present new zircon (U-Th)/He thermochronometric and detrital zircon U-Pb geochronologic evidence from the Himalaya of northwest India to explore these contrasting interpretations concerning the driving mechanisms responsible for these seawater records. Our data demonstrate in-sequence southward thrust propagation with rapid exhumation of Lesser Himalayan strata enriched in labile 187Os and relatively less in radiogenic 87Sr at ˜16 Ma, which directly corresponds with coeval shifts in seawater 187Os/188Os and 87Sr/86Sr. Results presented here provide substantial evidence that the onset of exhumation of 187Os-enriched Lesser Himalayan strata could have significantly impacted the marine 187Os/188Os record at 16 Ma. These results support the hypothesis that regional weathering of isotopically unique source rocks can drive seawater records independently from shifts in global-scale weathering rates, hindering the utility of these records as reliable proxies to track global weathering processes and climate in deep geologic time.

  19. The Comparison of Detrital Zircon Ages to Point Count Provenance Analysis for the Pottsville Sandstone in the Northern Appalachian Foreland Basin Venango County, Pennsylvania

    Science.gov (United States)

    Loveday, S.; Harris, D. B.; Schiappa, T.; Pecha, M.

    2017-12-01

    The specific sources of sediments deposited in the Appalachian basin prior to and immediately following the Alleghenian orogeny has long been a topic of debate. Recent advances in U-Pb dating of detrital zircons have greatly helped to determine some of the sources of these sediments. For this study, sandstone samples were collected from the Pottsville Formation in the northern Appalachian Foreland Basin, Venango County, Pennsylvania to provide supplementary data for previous work that sought to describe the provenance of the same sediments by point counts of thin sections of the same units. Results of this previous work established that the provenance for these units was transitional recycled orogenic, including multiple recycled sediments, and that a cratonic contribution was not able to be determined clearly. The previous results suggested that the paleoenvironment was a fluvial dominated delta prograding in the northern direction. However, no geochronologic data was found during this study to confirm this interpretation. We sought to verify these results by U-Pb analysis of detrital zircons. Samples were collected from the areas where the previous research took place. U-Pb ages were found from sample at the highest elevation and lowest elevation. In the first sample, sample 17SL01 (younger sample stratigraphically), the zircons yield U-Pb age range peaks at 442-468 ma and 1037-1081 ma. The probability density plot for this specific sample displays a complete age gap from 500 ma to 811 ma. In the second sample, sample 17SL03 (older rock stratigraphically), the zircons yield U-Pb ages range peaks of 424-616 ma and 975-1057 ma. This sample doesn't show any ages younger than 424 ma and it doesn't display the sample age gap as sample 17SL01 does. The ages of zircons are consistent with thin section point counting provenance results from previous research suggesting zircon transport from the northern direction.

  20. 3-D Characterization of Detrital Zircon Grains and its Implications for Fluvial Transport, Mixing, and Preservation Bias

    Science.gov (United States)

    Markwitz, V.; Kirkland, C. L.; Mehnert, A.; Gessner, K.; Shaw, J.

    2017-12-01

    Detrital zircon studies can suffer from selective loss of provenance information due to U-Pb age discordance, metamictization, metamorphic overprinting and fluviatile transport processes. The relationship between isotopic composition and zircon grain shape, and how grain shape is modified during transport, is largely unknown. We combine X-ray tomography with U-Pb geochronology to quantify how fluvial transport affects 3-D zircon shape, detrital age signature, and grain density along the Murchison River, whose catchment comprises Eoarchean to Early Paleozoic source rocks in Western Australia. We acquired tomographic volumes and isotopic data from 373 detrital zircons to document changes in size, shape and density in transport direction, and explore how grain shape, age spectra and the proportion of discordant material vary along the channel. Results show that shape characteristics are sensitive to transport distance, stream gradient, proximity to source material, and whether the source consists of primary or recycled zircons. With increasing transport distance, grain lengths decrease more than their widths. Furthermore, the loss of metamict grains occurs at a near constant rate, resulting in a linear increase of mean calculated zircon density by ca. 0.03 g/cm3 per 100 km transport distance. 3-D grain shape is therefore strongly linked to detrital age signature, and mean grain density is a function of the absolute transport distance. 3-D shape characteristics provide valuable information on detrital zircon populations, including the interaction between source materials with fluvial transport processes, which significantly affects preservation bias and, by inference, the representativeness of the sampled data.

  1. In situ isotopic analyses of U and Pb in zircon by remotely operated SHRIMP II, and Hf by LA-ICP-MS: an example of dating and genetic evolution of zircon by {sup 176}Hf/{sup 177}Hf from the Ita Quarry in the Atuba Complex, SE, Brazil; Analises in situ de U e Pb em zircao por SRIMP II por controle remoto e de Hf por LA-ICP-MS: um exemplo de datacao e da evolucao genetica de zircao atraves da razao {sup 176}Hf/{sup 177} em amostra da Pedreira Ita no Complexo Atuba, SE, Brasil

    Energy Technology Data Exchange (ETDEWEB)

    Sato, K.; Siga Junior, Oswaldo; McReath, Ian; Sproesser, Walter; Basei, Miguel Angelo Stipp [Universidade de Sao Paulo (USP), SP (Brazil). Inst. de Geociencias. Centro de Pesquisas Geocronologicas], e-mail: keisato@usp.br, e-mail: osigajr@usp.br, e-mail: ianmcr@usp.br, e-mail: wmspres@usp.br, e-mail: baseimas@usp.br; Silva, Josiane Aline da [Universidade de Sao Paulo (USP), SP (Brazil). Programa de Pos-graduacao em Geoquimica e Geotectonica; Dunyi, Liu [Institute of Geology, Beijing (China); Iizuka, Takafumi; Rino, Shuji; Hirata, Takafumi [Tokyo Institute of Technology, Tokyo (Japan)

    2009-10-15

    Remotely-operated SHRIMP dating of zircon is an interesting alternative for dating of zircon crystals. Although it does not represent any technical progress of the geochronological method using the U-Pb system in zircon it is a very useful and cheap facility. The procedure was first used for mass spectrometric analyses involving two international laboratories in Sao Paulo, Brazil and Beijing, China. It was applied to samples of three gneiss-migmatitic rocks from the Ita quarry in the Atuba Complex (located between the Luis Alves and the Apiai Domain) to test previous controversial hypotheses about its evolution. The presence of important archaean and paleo proterozoic components in the complex is confirmed by analyses of zircon found in probably neo proterozoic leucosomes. Diorite intrusion also occurred during the neo proterozoic, associated with the 0.6Ga continental collisions involved in the assembly of Gondwana. The determination of Hf isotope ratios by LA-ICP/MS represents a new option for checking the relative importance of mantle ({epsilon}{sub Hf} > 0) and crustal contributions (({epsilon}{sub Hf} < 0) during the growth of the zircon crystals. While the archaean component in the complex was derived from the mantle ({epsilon}{sub Hf} + 1.5 to + 8.7) the paleo proterozoic component had a crustal contribution ({epsilon}{sub Hf} - 9.1 to -10.1). (author)

  2. Precambrian-Cambrian provenance of Matinde Formation, Karoo Supergroup, northwestern Mozambique, constrained from detrital zircon U-Pb age and Lu-Hf isotope data

    Science.gov (United States)

    Bicca, Marcos Müller; Jelinek, Andrea Ritter; Philipp, Ruy Paulo; de Carvalho Lana, Cristiano; Alkmim, Ana Ramalho

    2018-02-01

    The Permian-Triassic time interval was a period of high sedimentation rates in the intracontinental Karoo rift basin of northwestern Mozambique, reflecting high exhumation rates in the surrounding high ground Precambrian-Cambrian basement and juxtaposed nappes. U-Pb LA-MC-ICPMS dating and Lu-Hf isotopic analysis of detrital zircons from the Late Permian-Early Triassic Matinde Formation of the Karoo Supergroup is used as a reliable proxy to map denudation patterns of source regions. Data allow discrimination of U-Pb age populations of ca. 1250-900 Ma, a secondary population between ca. 900-700 and a major contribution of ages around ca. 700-490 Ma. Zircon grains of the Mesoproterozoic age population present Mesoproterozoic (1000-1500 Ma) to Paleoproterozoic (1800-2300 Ma) Hf TDM ages, with positive (0 to +11) and negative εHf values (-3 to -15), respectively. The younger U-Pb age population also presents two different groups of zircon grains according to Lu-Hf isotopes. The first group comprise Paleoproterozoic (1800-2300 Ma) ages, with highly negative εHf values, between -10 and -22, and the second group exhibits Mesoproterozoic ages (1200-1500 Ma), with increased juvenile εHf values (ca. 0 to -5). These Hf isotopes reinforce the presence of unexposed ancient crust in this region. The oldest U-Pb age population resembles the late stages of Grenville Orogeny and the Rodinia Supercontinent geotectonic activity mostly represented by magmatic rocks, which are widely present in the basement of northern Mozambique. The juvenile Hf-isotope signature with an older age component is associated to rocks generated from subduction processes with crust assimilation by continental arcs, which we correlate to rocks of the Nampula Complex, south and east of the Moatize-Minjova Basin. The U-Pb ages between 900 and 700 Ma were correlated to the calc-alkaline magmatism registered in the Guro Suite, related to the breakup phase of Rodinia, and mark the western limit of the Moatize

  3. Single-zircon dating by stepwise Pb-evaporation constrains the Archean history of detrital zircons from the Jack Hills, Western Australia

    International Nuclear Information System (INIS)

    Kober, B.; Lippolt, H.J.; Pidgeon, R.T.

    1989-01-01

    Pb isotope analyses have been carried out on 42 zircon grains from a Western Australian metaconglomerate using stepwise Pb-evaporation directly in the ion source of a thermal ionization mass spectrometer. The metaconglomerate is from the Archean Jack Hills Metasedimentary Belt, and is known from ion microprobe (''SHRIMP'') analyses to contain a complex zircon population with ages between 4.2 Ga and 3.1 Ga. The same complex pattern of ages is found by the Pb evaporation studies. Five grains yielded minimum crystallization ages from 4.17 Ga to 4.07 Ga. The main population appears significantly younger, having been generated at about 3.55-3.3 Ga. The agreement between the two analytical approaches confirms the SHRIMP results and demonstrates the value of the stepwise-evaporation technique in determining the age patterns of mixing zircon populations. In many of the evaporative Pb isotope records the 207/206 ratios remained constant for all evaporation steps, which we interpret as evaporation from concordant zircon phases. However, for the majority of zircons 207/206 ratios increased with increasing evaporation temperature, and usually approached constant values during evaporation at the highest temperatures. This can be attributed to mixing of different radiogenic Pb components from either crystalline zircon phases of different ages or from domains of isotopically disturbed metamict zircon. Present results confirm > 4 Ga zircon ages in the metaconglomerate from the Hack Hills and substantiate formation of crust at a very early stage in the evolution of the earth. Results also confirm a major crust-forming event 3.55-3.3 Ga ago. (orig.)

  4. U Pb and Lu Hf isotope record of detrital zircon grains from the Limpopo Belt Evidence for crustal recycling at the Hadean to early-Archean transition

    Science.gov (United States)

    Zeh, Armin; Gerdes, Axel; Klemd, Reiner; Barton, J. M., Jr.

    2008-11-01

    Detrital zircon grains from Beit Bridge Group quartzite from the Central Zone of the Limpopo Belt near Musina yield mostly ages of 3.35-3.15 Ga, minor 3.15-2.51 Ga components, and numerous older grains grouped at approximately 3.4, 3.5 and 3.6 Ga. Two grains yielded concordant Late Hadean U-Pb ages of 3881 ± 11 Ma and 3909 ± 26 Ma, which are the oldest zircon grains so far found in Africa. The combined U-Pb and Lu-Hf datasets and field relationships provide evidence that the sedimentary protolith of the Beit Bridge Group quartzite was deposited after the emplacement of the Sand River Gneisses (3.35-3.15 Ga), but prior to the Neoarchean magmatic-metamorphic events at 2.65-2.60 Ga. The finding of abundant magmatic zircon detritus with concordant U-Pb ages of 3.35-3.15 Ga, and 176Hf/ 177Hf of 0.28066 ± 0.00004 indicate that the Sand River Gneiss-type rocks were a predominant source. In contrast, detrital zircon grains older than approximately 3.35 Ga were derived from the hinterland of the Limpopo Belt; either from a so far unknown crustal source in southern Africa, possibly from the Zimbabwe Craton and/or a source, which was similar but not necessarily identical to the one that supplied the Hadean zircons to Jack Hills, Western Australia. The Beit Bridge Group zircon population at >3.35 Ga shows a general ɛHf t increase with decreasing age from ɛHf 3.9Ga = -6.3 to ɛHf 3.3-3.1Ga = -0.2, indicating that Hadean crust older than 4.0 Ga ( TDM = 4.45-4.36 Ga) was rejuvenated during magmatic events between >3.9 and 3.1 Ga, due to a successive mixing of crustal rocks with mantle derived magmas. The existence of a depleted mantle reservoir in the Limpopo's hinterland is reflected by the ˜3.6 Ga zircon population, which shows ɛHf 3.6Ga between -4.6 and +3.2. In a global context, our data suggest that a long-lived, mafic Hadean protocrust with some tonalite-trondhjemite-granodiorite constituents was destroyed and partly recycled at the Hadean/Archean transition, perhaps

  5. Reassessment of the geologic evolution of selected precambrian terranes in Brazil, based on SHRIMP U-Pb data, part 2: mineiro and Aracuai orogens and Southern Sao Francisco craton; Reavaliacao da evolucao geologica em terrenos pre-cambrianos brasileiros com base em novos dados U-Pb SHRIMP, parte 2: orogeno Aracuai, cinturao mineiro e craton Sao Francisco Meridional

    Energy Technology Data Exchange (ETDEWEB)

    Silva, Luiz Carlos da; Pimentel, Marcio [Brasilia Univ., DF (Brazil). Inst. de Geociencias]. E-mail: luizcarlos@aneel.gov.br; Leite, Carlos Augusto; Vieira, Valter Salino; Silva, Marcio Antonio da; Paes, Vinicius Jose de Castro; Cardoso Filho, Joao Moraes [Companhia de Pesquisas de Recursos Minerais (CPRM), Belo Horizonte, MG (Brazil); Armstrong, Richard [Australian National Univ., Canberra (Australia). Research School of Earth Sciences; Noce, Carlos Mauricio; Pedrosa-Soares, Antonio Carlos [Minas Gerais Univ., Belo Horizonte (Brazil). Inst. de Geociencias. Centro de Pesquisa Manuel Teixeira da Costa; Carneiro, Mauricio Antonio [Ouro Preto Univ., MG (Brazil). Dept. de Geologia

    2002-12-15

    This paper discusses new zircon SHRIMP (Sensitive High Resolution Ion Microprobe) U-Pb geochronological data for 19 key-exposures of several geological units exposed at the eastern border of the Southern Sao Francisco Craton and at the adjacent Proterozoic Mineiro and Aracuai orogens. Samples were collected along several E-W tran sects, aiming at tracing the precise limit of the Sao Francisco Craton Archean basement, as well as assessing the extension of the successive proterozoic orogenic collages. Due to the complex geologic history and/or high grade metamorphism which most of the rock units investigated have undergone, zircon morphology and the U-Pb analytical data exhibit very complex patterns. These are characterized by a combination of inheritance, partial resetting and new zircon growth during high-grade metamorphism. As a consequence, very careful and detailed analyses of cathodoluminescence imagery were required to allow distinction between inheritance, newly melt-precipitated zircon and partially reset zircons, as well as between the ages of magmatic and metamorphic events. In the southeastern border of the craton 5 units yielded Archean crystallization ages ranging from ca. 3000-2700 Ma, with poorly constrained metamorphic ages ranging from ca. 2850 to 550 Ma. The TTG gneissic complex exposed to the east and south of the Quadrilatero Ferrifero, formerly ascribed to the Archean basement, have crystallization ages from ca. 2210 Ma to 2050 Ma, and can now be interpreted as representing pre- to syn-collisional magmatic phases of the Mineiro Belt. Metamorphic ages of ca. 2100 Ma and 560 Ma are also well constrained in zircon populations from these gneisses. The crystallization age of ca 1740 Ma observed for an alkaline granite of the Borrachudos Suite (intrusive into the Archean basement east of the Southern Espinhaco Range) confirmed previous conventional U-Pb data for this Paleoproterozoic rift-related magmatism. One of the major basement inliers within the

  6. U-Pb geochronology documents out-of-sequence emplacement of ultramafic layers in the Bushveld Igneous Complex of South Africa.

    Science.gov (United States)

    Mungall, James E; Kamo, Sandra L; McQuade, Stewart

    2016-11-14

    Layered intrusions represent part of the plumbing systems that deliver vast quantities of magma through the Earth's crust during the formation of large igneous provinces, which disrupt global ecosystems and host most of the Earth's endowment of Pt, Ni and Cr deposits. The Rustenburg Layered Suite of the enormous Bushveld Igneous Complex of South Africa has been presumed to have formed by deposition of crystals at the floor of a subterranean sea of magma several km deep and hundreds of km wide called a magma chamber. Here we show, using U-Pb isotopic dating of zircon and baddeleyite, that individual chromitite layers of the Rustenburg Layered Suite formed within a stack of discrete sheet-like intrusions emplaced and solidified as separate bodies beneath older layers. Our U-Pb ages and modelling necessitate reassessment of the genesis of layered intrusions and their ore deposits, and challenge even the venerable concept of the magma chamber itself.

  7. Early cretaceous zircon SHRIMP U-Pb age of the trachyte and its significances of the Gan-Hang belt

    International Nuclear Information System (INIS)

    Liu Feiyu; Wu Jianhua; Liu Shuai

    2009-01-01

    The Shixi basin was located at Gan-Hang tectonic volcanic uranium deposit of rock-magma belt which belong to a part of the Mesozoic volcanic rocks in the northeastern of China. The appearance of the trachyte in Shixi basin have the majoy elements characteristic of the shoshonite series volcanic rocks. To determine the geological age of trachyte have very important significance on the geodynamics research and the study on the cause of uranium mineralization. The zircons of the trachyte have clear ring and high Th/U ratio which belong to the typical magmatic zircons. The zircon SHRIMP U-Pb dating resules show that 14points' age range is very smaller is 132-144 Ma and the weighted average age is (137.00±0.94)Ma which represents the diagenetic age of volcanic rocks. Accronding to the latest International Stratigraphic Chart the boundary of Jurassic and Cretaceous is (145.4±4.0)Ma. So the trachyte of Shixi Group belong to early Cretaceous. The large-scale acidic volcanic activity occurred in the Early Cretaceous in Southeastern China, and the Volcanic uranium deposit of Gan-Hang tectonic belt relate to Alkali metasomatism Uranium mineralization also occurred in the Early Cretaceous (120-130 Ma). The determined of trachyte in Shixi Group in the Early Cretaceous show that the acidic volcanic activity have connection with magma activity and the early Uranium mineralization consistent with the Alkali magma activity. (authors)

  8. Rb-Sr, Pb-Pb, U-Pb dating in the Bandja plutonic series of Western Cameroon. Donnees geochronologiques (Rb-Sr, Pb-Pb, U-Pb) sur le complexe plutonique de Bandja (Centre-Ouest Cameroun)

    Energy Technology Data Exchange (ETDEWEB)

    Tchankam, C N [Nancy-1 Univ., 54 (France); Vialette, Y [Clermont-Ferrand-2 Univ., 63 - Aubiere (France)

    1994-08-01

    The results of U-Pb zircon and Pb-Pb on minerals and whole rocks are reported on a charnockite syn-D1 from the Bandja series in the western Cameroon. Data are interpreted as representing a plutonic emplacement at 640 Ma. A syn- to post-tectonic pluton is dated at 557 [+-] 8 Ma (Rb-Sr whole rocks isochron). These results confirm the Pan-African age of the charnockitic intrusive body. Initial isotopic [sup 87]Sr/[sup 86]Sr ratios of charnockite (0.709) and granite (0.7089) show the importance of crustal imprint in the magma genesis. (authors).

  9. U-Pb ages for two tonalitic gneisses, pegmatitic granites, and K-feldspar porphyries, Olkiluoto study site, Eurajoki, SW Finland

    International Nuclear Information System (INIS)

    Maenttaeri, I.; Lindberg, A.; Aaltonen, I.

    2007-08-01

    Secondary ion microprobe zircon U-Pb ages have been determined for two tonalitic gneisses, two pegmatitic granites, and two potassium feldspar porphyry samples from the Olkiluoto study site, Eurajoki, S-W Finland. Moreover, monazites from the Kfeldspar porphyries were dated using TIMS U-Pb method. The tonalitic gneiss A1879 TTG 1 reveals bimodal zircon population and for A1880 TTG it is homogeneous. The samples yield similar overlapping concordia ages of 1851 ± 5 Ma and 1856 ± 5 Ma, respectively. The pegmatitic granite samples A1881 PGR 1 and A1883 PGR 2 have mostly zircons resembling those of the TTG's. The supposed pegmatitic zircons with high U and low Th are strongly altered. The zircon U-Pb data of A1881 PGR 1 plot roughly in two separate lines on a concordia diagram. The apparently younger ∼1.79 Ga data are all from the high U and low Th/U zircons and therefore certainly set the minimum age for the A1881 PGR 1. It is suggested, that the ∼1.85 Ga data comprise analyses from inherited zircons as it include both lower and higher Th/U zircons and 1.85 Ga coevals with age of the tonalitic gneisses. Thus, the apparent age for the A1881 PGR 1 is ∼1.79 Ga. The U-Pb data of sample A1883 PGR 2 also divide into two groups. The higher Th/U, inherited zircons determine an age of 1852 ± 9 Ma which is the same as that of the TTG's. The low Th/U zircon data scatter and the age of 1.83 Ga for A1883 PGR 2 is only poorly determined. The both potassium feldspar porphyry samples A1882 KFP 1 and A1884 KFP 2 reveal heterogeneous zircon populations. The A1882 KFP 1 zircons showing magmatic zoning in BSE images conceivably determine a concordia age of 1842± 6 Ma for the rock. In addition to that a few ∼1.9 Ga inherited zircon and metamorphic low Th/U rims with ages between 1.88 Ga and 1.83 Ga were detected. The age for the youngest metamorphic zircon rims overlaps with that of the magmatic zircons. The zircons in the other KFP sample A1884 show a wide range of ages

  10. Cryogenian alkaline magmatism in the Southern Granulite Terrane, India: Petrology, geochemistry, zircon U-Pb ages and Lu-Hf isotopes

    Science.gov (United States)

    Santosh, M.; Yang, Qiong-Yan; Ram Mohan, M.; Tsunogae, T.; Shaji, E.; Satyanarayanan, M.

    2014-11-01

    The Southern Granulite Terrane (SGT) in India preserves the records of the formation and recycling of continental crust from Mesoarchean through Paleoproterozoic to Neoproterozoic and Cambrian, involving multiple subduction-accretion-collision associated with major orogenic cycles. A chain of unmetamorphosed and undeformed alkaline magmatic intrusions occurs along the northern margin of the SGT aligned along paleo-suture zones. Here we investigate two representative plutons from this suite, the Angadimogar syenite (AM) and the Peralimala alkali granite (PM) through field, petrological, geochemical, zircon U-Pb and Lu-Hf studies. Magma mixing and mingling textures and mineral assemblages typical of alkaline rocks are displayed by these plutons. The whole-rock major and trace element data characterize their alkaline nature. In trace element discrimination diagrams, the AM rocks straddle between the VAG (volcanic-arc granites) and WPG (within plate granites) fields with most of the samples confined to the VAG field, whereas the PM rocks are essentially confined to the WPG field. The diversity in some of the geochemical features between the two plutons is interpreted to be the reflection of source heterogeneities. Most zircon grains from the AM and PM plutons display oscillatory zoning typical of magmatic crystallization although some grains, particularly those from the PM pluton, show core-rim structures with dark patchy zoned cores surrounded by irregular thin rims resulting from fluid alteration. The weighted mean 206Pb/238U ages of the magmatic zircons from three samples of the AM syenite are in the range of 781.8 ± 3.8 Ma to 798 ± 3.6 Ma and those from two samples of the PM alkali granite yield ages of 797.5 ± 3.7 Ma and 799 ± 6.2 Ma. A mafic magmatic enclave from the AM pluton shows weighted mean 206Pb/238U age of 795 ± 3.3 Ma. The AM and PM plutons also carry rare xeneocrystic zircons which define upper intercept concordia ages of 3293 ± 13 Ma and 2530

  11. U-Pb SHRIMP-RG zircon ages and Nd signature of lower Paleozoic rifting-related magmatism in the Variscan basement of the Eastern Pyrenees

    Science.gov (United States)

    Martinez, F.J.; Iriondo, A.; Dietsch, C.; Aleinikoff, J.N.; Peucat, J.J.; Cires, J.; Reche, J.; Capdevila, R.

    2011-01-01

    The ages of orthogneisses exposed in massifs of the Variscan chain can determine whether they are part of a pre-Neoproterozoic basement, a Neoproterozoic, Panafrican arc, or are, in fact, lower Paleozoic, and their isotopic compositions can be used to probe the nature of their source rocks, adding to the understanding of the types, distribution, and tectonic evolution of peri-Gondwanan crystalline basement. Using SHRIMP U-Pb zircon geochronology and Nd isotopic analysis, pre-Variscan metaigneous rocks from the N??ria massif in the Eastern Pyrenean axial zone and the Guilleries massif, 70km to the south, have been dated and their Nd signatures characterized. All dated orthogneisses from the N??ria massif have the same age within error, ~457Ma, including the Ribes granophyre, interpreted as a subvolcanic unit within Caradocian sediments contemporaneous with granitic magmas intruded into Cambro-Ordovician sediments at deeper levels. Orthogneisses in the Guilleries massif record essentially continuous magmatic activity during the Ordovician, beginning at the Cambro-Ordovician boundary (488??3Ma) and reaching a peak in the volume of magma in the early Late Ordovician (~460Ma). Metavolcanic rocks in the Guilleries massif were extruded at 452??4Ma and appear to have their intrusive equivalent in thin, deformed veins of granitic gneiss (451??7Ma) within metasedimentary rocks. In orthogneisses from both massifs, the cores of some zircons yield Neoproterozoic ages between ~520 and 900Ma. The age of deposition of a pre-Late Ordovician metapelite in the Guilleries massif is bracketed by the weighted average age of the youngest detrital zircon population, 582??11Ma, and the age of cross-cutting granitic veins, 451??7Ma. Older detrital zircons populations in this metapelite include Neoproterozoic (749-610Ma; n=10), Neo- to Mesoproterozoic (1.04-0.86Ga; n=7), Paleoproterozoic (2.02-1.59Ga; n=5), and Neoarchean (2.74-2.58Ga; n=3). Nd isotopic analyses of the N??ria and Guilleries

  12. High-spatial resolution SIMS U-Pb zircon dating on Malay tin granites: New insights to crustal evolution of the Malaysian Peninsula

    Science.gov (United States)

    Ng, S.; Searle, M. P.; Whitehouse, M. J.; Chung, S.; Robb, L. J.; Ghani, A. A.; Sone, M.

    2012-12-01

    The Southeast Asian tin granite province has previously been recognized as comprising three distinct granitic belt (Cobbing et al., 1986): (1) Middle Permian - Late Triassic Eastern Province with mainly subduction-related I-type granites associated with Cu-Au deposits, (2) Late Permian - Early Jurassic Main Range Province with mainly S-type granites associated with Sn-W deposits, and (3) Late Triassic - Cretaceous Western Province with mixed I- and S-type granites associated with Sn-W deposits. These ages were obtained mainly using whole rock Rb-Sr and K-Ar geochronology in the 70s and 80s, methods that are no longer considered suitable for obtaining crystallization ages of granites due to unstable behaviour of these isotopes in hydrothermal systems. During 2011 we collected over 50 samples of granites across all three zones in Northern Malaysia for U-Pb dating using high-spatial resolution precision SIMS at the Swedish Museum of Natural History. The results reveal a westward younging trend across the Malay Peninsula. The Eastern Province hornblende- and biotite-granites in Eastern Malaysia formed by subduction-related processes have ages lying between 220-285 Ma. The Main Range Province dominantly S-type granites in west and central Malaysia have ages lying between 206-226 Ma. These include dominantly biotite granites with less common tourmaline+biotite leucocratic granites related to crustal thickening processes following collision of Sibumasu with Indochina and closing of the Palaeo-Tethyan Bentong-Raub suture zone. Some enigmatic young Cretaceous zircon rim ages (c. 81-80 Ma) may indicate a young hydrothermal - metasomatic origin for some tin mineralization from both the Eastern Province (Tioman Island) and the Western province (Phuket, Thailand) (Searle et al., 2012). Granites in Malaysia cannot be simply categorized as I-, S- or A-type as most of them failed to show distinctive minerals for classification and there is considerable overlap in geochemical

  13. Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops

    Science.gov (United States)

    Peterman, Emily M.; Reddy, Steven M.; Saxey, David W.; Snoeyenbos, David R.; Rickard, William D. A.; Fougerouse, Denis; Kylander-Clark, Andrew R. C.

    2016-01-01

    Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 μm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe microscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The 207Pb/206Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages. PMID:27617295

  14. Reassessment of the geologic evolution of selected precambrian terranes in Brazil, based on SHRIMP U-Pb data, part 2: mineiro and Aracuai orogens and Southern Sao Francisco craton

    International Nuclear Information System (INIS)

    Silva, Luiz Carlos da; Pimentel, Marcio; Armstrong, Richard; Noce, Carlos Mauricio; Pedrosa-Soares, Antonio Carlos; Carneiro, Mauricio Antonio

    2002-01-01

    This paper discusses new zircon SHRIMP (Sensitive High Resolution Ion Microprobe) U-Pb geochronological data for 19 key-exposures of several geological units exposed at the eastern border of the Southern Sao Francisco Craton and at the adjacent Proterozoic Mineiro and Aracuai orogens. Samples were collected along several E-W tran sects, aiming at tracing the precise limit of the Sao Francisco Craton Archean basement, as well as assessing the extension of the successive proterozoic orogenic collages. Due to the complex geologic history and/or high grade metamorphism which most of the rock units investigated have undergone, zircon morphology and the U-Pb analytical data exhibit very complex patterns. These are characterized by a combination of inheritance, partial resetting and new zircon growth during high-grade metamorphism. As a consequence, very careful and detailed analyses of cathodoluminescence imagery were required to allow distinction between inheritance, newly melt-precipitated zircon and partially reset zircons, as well as between the ages of magmatic and metamorphic events. In the southeastern border of the craton 5 units yielded Archean crystallization ages ranging from ca. 3000-2700 Ma, with poorly constrained metamorphic ages ranging from ca. 2850 to 550 Ma. The TTG gneissic complex exposed to the east and south of the Quadrilatero Ferrifero, formerly ascribed to the Archean basement, have crystallization ages from ca. 2210 Ma to 2050 Ma, and can now be interpreted as representing pre- to syn-collisional magmatic phases of the Mineiro Belt. Metamorphic ages of ca. 2100 Ma and 560 Ma are also well constrained in zircon populations from these gneisses. The crystallization age of ca 1740 Ma observed for an alkaline granite of the Borrachudos Suite (intrusive into the Archean basement east of the Southern Espinhaco Range) confirmed previous conventional U-Pb data for this Paleoproterozoic rift-related magmatism. One of the major basement inliers within the

  15. Geochemistry and U/Pb geochronology of the Neuvy-Bouin pluton (Vendean Haut-Bocage): an example of a multistage granite pluton

    International Nuclear Information System (INIS)

    Cuney, M.; Brouand, M.; Dautel, D.; Michard, A.; Stussi, J.M.; Poncet, D.; Bouton, P.; Colchen, M.; Vervialle, J.P.

    1993-01-01

    New geological, structural and geochemical data show the presence of at least four magmatic units in the Neuvy-Bouin massif. U/Pb zircon age (340±4 Ma) obtained on the Largeasse granites indicates that in the Neuvy-Bouin pluton the magmatic activity began during Visean times. The emplacement of the Largeasse granites is followed by the aluminopotassic two-mica granites of Pougne-Herisson and by two mica leucogranites. The small high-K calc-alkaline granite body discovered in the Neuvy-Bouin pluton represents the first occurrence of this type of magmatism in the area

  16. Advantages of conducting in-situ U-Pb age dating of multiple U-bearing minerals from a single complex: Case in point - the Oka Carbonatite Complex

    Science.gov (United States)

    Chen, W.; Simonetti, A.

    2012-12-01

    A detailed radiometric investigation is currently underway focusing on U-bearing accessory minerals apatite, perovskite, and niocalite from the Oka Carbonatite Complex (Canada). One of the main objectives is to obtain a comparative chronology of melt crystallization for the complex. Unlike other commonly adopted U-bearing minerals (e.g., zircon, monazite) for in-situ dating investigations, apatite, perovskite, and niocalite contain relatively high contents of common Pb. Hence, careful assessment of the proportion and composition of the common Pb, and usage of appropriate matrix-matched external standards are imperative. The Madagascar apatite was utilized as the external standard for apatite dating, and the Emerald Lake and Durango apatites were adopted as secondary standards; the latter yield ages of 92.6 ±1.8 and 32.2 ±1.1 Ma, respectively, and these are identical to their accepted ages. Pb/U ages for apatite from Oka were obtained for different rock types, including 8 carbonatites, 4 okaites, 3 ijolites and 3 alnoites, and these define a range of ages between ~105 and ~135 Ma; this result suggests a protracted crystallization history. In total, 266 individual analyses define two peaks at ~115 and ~125Ma. For perovskite dating, the Ice River perovskite standard was utilized as the external standard. The perovskites from one okaite sample yield an age of 112.2 ±1.9 Ma, and is much younger than the previously reported U-Pb perovskite age of 131 ±7 Ma. Hence, the combined U-Pb perovskite ages also suggest a rather prolonged time of melt crystallization. Niocalite is a rare, accessory silicate mineral that occurs within the carbonatites at Oka. The international zircon standard BR266 was selected for use as the external standard and rastering was employed to minimize the Pb-U fractionation. Two niocalite samples give young ages at 110.6 ±1.2 and 115.0 ±1.9 Ma, and are identical to their respective apatite ages (given associated uncertainties) from the same

  17. U/Pb zircon dating and Sr and Nd isotope characteristics of Permian volcanism in the Western Pyrenees: the Ossau and Anayet Massif. Datation U/Pb sur zircon et geochimie isotopique Sr et Nd du volcanisme permien des Pyrenees Occidentales (Ossau et Anayet)

    Energy Technology Data Exchange (ETDEWEB)

    Briqueu, L [Montpellier-2 Univ., 34 (France); Innocent, C [Aix-Marseille-3 Univ., 13 - Marseille (France)

    1993-03-01

    Several zircon populations have been extracted from a peraluminous rhyolite and from a dacite which are the rock types characteristic of the first two volcanic phases of the Pic du Midi d'Ossau (Eastern Pyrenees). Using a Concordia Diagram, their U/Pb isotopic study confirms that volcanism started in the Autunian age (272 to 278 My). These two volcanic cycles display initial Sr and Nd isotopic signatures which are comparable, and show a strong crustal imprint. Volcanism subsequently evolves towards an increasingly alkaline composition and the corresponding isotopic characteristics are compatible with a depleted asthenospheric mantle source.

  18. In situ detrital zircon (U-Th)/He thermochronology

    Science.gov (United States)

    Tripathy, A.; Monteleone, B. D.; van Soest, M. C.; Hodges, K.; Hourigan, J. K.

    2010-12-01

    Detrital studies of both sand and rock are relevant to many problems, ranging from the climate and tectonics feedback debate to the long-term record of orogenic evolution. When applying the conventional (U-Th)/He technique to such studies, two important issues arise. Often, only euhedral grains are permissible for analysis in order to make simple geometric corrections for α-recoil. In detrital samples, this is problematic because euhedral grains can be scarce due to mechanical abrasion during transport, and potentially introduce bias in favour of more proximally sourced grains. Second, inherent to detrital studies is the need to date many grains (>100) per sample to ensure a representative sampling of the sediment source region, thus making robust conventional detrital studies both expensive and time-consuming. UV laser microprobes can improve this by permitting careful targeting of the grain interior away from the α-ejection zone, rendering the α-recoil correction unnecessary, thus eliminating bias toward euhedral grains. In the Noble Gas, Geochemistry, and Geochronology Laboratory at ASU, apatite and zircon have been successfully dated using in situ methods. For this study, the conventional and in situ techniques are compared by dating zircons from a modern river sand that drains a small catchment in the Mesozoic-Cenozoic Ladakh Batholith in NW India. This sample has a simple provenance, which allows us to demonstrate the robustness of the in situ method. Moreover, different microbeam techniques will be explored to establish the most efficient approach to obtain accurate and precise U-Th concentrations using synrock, which is our powdered, homogenized, and reconstituted zircon-rock standard. Without this, such in situ U-Th data would be difficult to obtain. 117 zircons were dated using the conventional (U-Th)/He method, revealing dates ranging from 9.70±0.35 to 106.6±3.5 Ma (2σ) with the major mode at 26 Ma. For comparison, 44 grains were dated using the in

  19. U-Pb isotopic dating of zircon from damaogou granite stock of Guangshigou uranium deposit in Danfeng area and it's significance

    International Nuclear Information System (INIS)

    Zuo Wenqian; Sha Yazhou; Chen Bing; Luo Zhongcheng; Zhang Zhanshi

    2010-01-01

    Danfeng area is an important pegmatite-type uranium production area in China, Guangshigou deposit is one of the biggest and the most important uranium deposit in this areas, Damaogou granite stock has a very close relationship with Guangshigou uranium deposit. Therefore the accurate formation dating and its evolution history of this stock is crucial for understanding the mineralization of this district. LA-ICP-MS U-Pb dating of zircon has been applied and yield the formation age of 418.3 ± 8.8 Ma of Damaogou stock, which is corresponding to the uranium mineralization age in Guangshigou deposit. The age of 1980.5 ± 19.47 Ma revealed by residual zircon might infer that Damaogou stock was derived from Qinling group, while the age of 465 Ma for the inner core of zircon grain E might represent the formation age of Huichizi gneissoid monzonitic granite. There was 30 Ma time gap between the formation of Huichizi pluton and Damaogou stock. (authors)

  20. 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

    Directory of Open Access Journals (Sweden)

    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.

  1. Stratigraphy, tectonics and detrital zircon U-Pb (LA-ICP-MS) geochronology of the Rio Preto Belt and northern Paramirim corridor, NE, Brazil

    Energy Technology Data Exchange (ETDEWEB)

    Alcantara, Davi da Costa Bezerra Gobira de; Uhlein, Alexandre; Caxito, Fabricio de Andrade; Pedrosa-Soares, Antonio Carlos, E-mail: davibgobira@hotmail.com, E-mail: auhlein@gmail.com, E-mail: facaxito@yahoo.com.br, E-mail: pedrosasoares@gmail.com [Universidade Federal de Minas Gerais (CPMTC/UFMG), Belo Horizonte, MG (Brazil). Instituto de Geociencias. Centro de Pesquisa Professor Manoel Teixeira da Costa; Dussin, Ivo, E-mail: ivodusin@yahoo.com.br [Universidade Estadual do Rio de Janeiro (UERJ), Rio de Janeiro, RJ (Brazil). Faculdade de Geologia

    2017-04-15

    Two important Proterozoic metasedimentary sequences, the Rio Preto and Santo Onofre Groups, crop out along the northwestern margin of the Sao Francisco craton and in northern Paramirim corridor, respectively. The Rio Preto Group, involved in the eponymous fold-thrust belt along the northwestern cratonic boundary, comprises the Formosa (garnet schist, quartz schist, quartzite, chlorite-sericite schist and ferriferrous quartz schist) and Canabravinha (quartzite, micaceous quartzite, metarhytmite, phylite, schist and metaturbidite) formations. The Santo Onofre Group occurs exclusively in the Paramirim corridor, and is composed of quartzite and minor carbonaceous or Mn-rich phylite. These units record sedimentation in shallow to deep-water marine settings related to rift basins, and were deformed and metamorphosed under green schist facies conditions during the Brasiliano orogeny. Here we present 427 new detrital zircon U-Pb ages, which constrain the maximum depositional ages of ca. 971 Ma for the Santo Onofre Group, ca. 912 Ma for the Canabravinha Formation, and ca. 65 Ma for the Formosa Formation of the Rio Preto Group. Our data suggests that the Santo Onofre and the Rio Preto Groups accumulated in two distinct basin settings. The latter, composed mostly of sandy rocks, would represent a relatively stable, shallow-marine shelf environment. The Rio Preto Group, with metadiamictite, quartzite, pelitic and rhythmitic rocks, represents a shallow to deep marine environment influenced by gravity flows. Both groups were probably deposited in the Late Tonian, and are potential correlatives of the lower (pre-glacial) units of the Macaubas Group of the Aracuai belt. (author)

  2. Zircon growth in a granitic pluton with specific mechanisms, crystallization temperatures and U-Pb ages. Implication to the 'spatiotemporal' formation process of the Toki granite, central Japan

    International Nuclear Information System (INIS)

    Yuguchi, Takashi; Ishibashi, Masayuki; Sasao, Eiji; Iwano, Hideki; Danhara, Tohru; Kato, Takenori; Sakata, Shuhei; Hattori, Kentaro; Hirata, Takafumi; Sueoka, Shigeru; Nishiyama, Tadao

    2016-01-01

    Zircons collected from a granitic pluton provide evidence of serial growth events with specific mechanisms, crystallization temperatures and U-Pb ages, revealing details of the sequential formation process from intrusion through emplacement to crystallization/solidification. The events have been identified by: 1) the study of the internal structure of zircon using cathodoluminescence, 2) deriving crystallization temperatures using Ti-in-zircon thermometry of the internal structure and 3) U-Pb age dating of the internal structure. The magmatic zircons from the Toki granite, central Japan, show two kinds in their internal structure: a low luminescence core (LLC) and oscillatory zonation (OZ). The LLC was produced by interfacial reaction-controlled growth in the granitic magma with cooling from about 910 to 760°C. The formation of OZ occurred by diffusion-controlled growth in a cooling magma chamber from about 850 to 690°C. The U-Pb ages derived from the LLC ranges from 74.7 ± 4.2 to 70.5 ± 1.3 Ma, indicating the incipient intrusion timing of the magma into the shallow crust. The OZ ages distribute from 72.7 ± 0.6 to 70.4 ± 1.7 Ma, which mean the timing from emplacement to crystallization/solidification of the granite pluton. Thus, the serial processes from intrusion through emplacement to crystallization/solidification occurred within a few million years. The old LLC and OZ ages are recognized in the western margins of the Toki granite, implying that the magma forming the western margins was the first to intrude, emplace and crystallize/solidify. The western margins with initial intrusion may accompany the crustal assimilation in order to create sufficient magma reservoir space, which is consistent with larger SrI and ASI values found in the western margins of the granite. (author)

  3. Crustal evolution of granitoids and gneisses from the Cambaizinho belt, southern Brazil: Review zircon Pb-Pb evaporation ages and Pb-Nd-Sr isotopes

    International Nuclear Information System (INIS)

    Remus, M.V.D; Macambira, M.B; Hartmann, L.A.; Beilfuss, M

    2001-01-01

    Deformed granitoids and gneisses from the Cambai Complex (900-700 Ma) along Cambaizinho Creek and in the Vila Nova do Sul region, state of Rio Grande do Sul, Brazil, were formed in a remarkably short time, about 10 m.y., between 704±13 and 697±3 Ma. The data base of this work includes eighteen zircon Pb/Pb evaporation analyses, five Pb isotope in feldspar and whole rock. The oldest known rocks in the region are polydeformed dioritic gneisses dated by conventional U-Pb zircon at 704±13 Ma. New Pb-Pb zircon evaporation data on the late transcurrent, less deformed and more evolved granitoids (Sanga do Jobim Granitoids) yield a 697± Ma age and indicates that the evolution of the plutonic magmatism in the area was nearly contemporaneous. These data contrast with previous interpretations based on Rb-Sr data which considered that these rock associations were formed during a longer time period (700-640 Ma). All these granitoids intruded the supracrustal sequence. These granitoids yield a minimum age of about 700 Ma for the formation of the supracrustal sequence and its regional dynamothermal metamorphism. Lead isotope composition of K-feldspar from Sanga do Jobim Granitoids plot close to, but slightly below the lead isotope evolution curve of orogeny in the Zartmann and Doe model (1981). This indicates that the setting for these granitoids was that of a juvenile magmatic arc. These new data plus previous data in the region also corroborate that the crustal evolution involved juvenile crust accreted between 760-700 Ma. In contrast, the Cacapava and Sao Sepe Granites intruded the supracrustal sequences along the eastern side of the Sao Gabriel Block at 562 Ma and 550 Ma, respectively, and show Pb and Nd isotope signatures from an old basement. This evidence suggests that the juvenile terrane was thrusted over the older basement situated along the eastern part of the shield during the Dom Feliciano collisional orogeny at about 620-590 Ma (au)

  4. 40Ar/39Ar dating and zircon chronochemistry for the Izu-Bonin rear arc, IODP site U1437

    Science.gov (United States)

    Schmitt, A. K.; Konrad, K.; Andrews, G. D.; Horie, K.; Brown, S. R.; Koppers, A. A. P.; Busby, C.; Tamura, Y.

    2016-12-01

    The scientific objective of IODP Expedition 350 drilling at Site U1437 (31°47.390'N, 139°01.580'E) was to reveal the "missing half of the subduction factory": the rear arc of a long-lived intraoceanic subduction zone. Site U1437 lies in a 50 km long and 20 km wide volcano-bounded basin, 90 km west of the Izu arc front, and is the only IODP site drilled in the rear arc. The Izu rear arc is dominated by Miocene basaltic to dacitic seamount chains, which strike at a high angle to the arc front. Radiometric dating targeted a single igneous unit (1390 mbsf), and fine to coarse volcaniclastic units for which we present zircon and 40Ar/39Ar (hornblende, plagioclase, and groundmass) age determinations. All zircons analyzed as grain separates were screened for contamination from drill-mud (Andrews et al., 2016) by analyzing trace elements and, where material was available, O and Hf isotope compositions. Igneous Unit 1 is a rhyolite sheet and yielded concordant in-situ and crystal separate U-Pb zircon ages (13.7±0.3 Ma; MSWD = 1.3; n = 40 spots), whereas the 40Ar/39Ar hornblende plateau age (12.9±0.3; MSWD = 1.1; n = 9 steps) is slightly younger, possibly reflecting pre-eruptive zircon crystallization, or alteration of hornblende. U-Pb zircon and 40Ar/39Ar plateau ages from samples above igneous Unit 1 are concordant with biostratigraphic and paleomagnetic ages (available to 1300 mbsf), but plagioclase and groundmass samples below 1300 m become younger with depth, hinting at post-depositional alteration. A single zircon from 1600 mbsf yielded a U-Pb age of 15.4±1.8 Ma; its trace element composition resembles other igneous zircons from U1437, and is tentatively interpreted as a Middle Miocene age for the lowermost lithostratigraphic unit VII. Oxygen and Hf isotopic values of igneous zircon indicate mantle origins, with some influence of assimilation of hydrothermally altered oceanic crust evident in sub-mantle oxygen isotopic compositions. Lessons from site U1437 are

  5. Detrital zircon geochronology and provenance of the Chubut Group in the northeast of Patagonia, Argentina

    Science.gov (United States)

    Navarro, Edgardo L.; Astini, Ricardo A.; Belousova, Elena; Guler, M. Verónica; Gehrels, George

    2015-11-01

    The Chubut Group constitutes the most widespread sedimentary unit in NE Patagonia, characterized by variable-energy fluvial deposits. U-Pb analysis of detrital zircons from two sections of the Chubut Group constraint the age of the oldest sedimentary rocks in the northeast of the Somuncurá - Cañadón Asfalto Basin. In the Cañadón Williams area, at San Jorge section, 20 km NW of Telsen locality, dating of 56 detrital zircons from a medium to coarse sandstone indicated a maximum depositional age of 109 ± 1 Ma (n = 4). These sandstones were interpreted to represent shallow channels, associated with a lacustrine system. In the Telsen locality, a laser ablation analysis of 115 detrital zircons from a medium to coarse-grained sandstone, from fluvial channel facies, yielded a maximum depositional age of ca. 106 ± 1 Ma (n = 8). Both ages are consistent with volcanic events of the Barremian to Albian age in the central Patagonian Andes Region. Cathodoluminescence images of zircons from the San Jorge sample suggest an igneous origin, which is further supported by Th/U values above 0.5 in most of the grains. The distribution of the statistical modes of the main age populations of detrital zircons for the two samples [182, 185 and 189 Ma for Telsen sample (T2S) and 181 ± 1 Ma for San Jorge sample (SJS)] matches the age of the volcanic Marifil Formation. The rocks of the Marifil Formation of these ages are exposed NE to SE of the study area. The abundance of zircons of similar Jurassic ages (n = 52 for SJS and n = 105 for T2S) and the external morphology of the zircons in the sample SJS, implies a close proximity of the source area. Suggestion that the Marifil Formation was the main provenance source is also supported by northeast-southeasterly paleocurrents measured at the San Jorge and Telsen sections.

  6. U-Pb, Re-Os, and Ar/Ar geochronology of rare earth element (REE)-rich breccia pipes and associated host rocks from the Mesoproterozoic Pea Ridge Fe-REE-Au deposit, St. Francois Mountains, Missouri

    Science.gov (United States)

    Aleinikoff, John N.; Selby, David; Slack, John F.; Day, Warren C.; Pillers, Renee M.; Cosca, Michael A.; Seeger, Cheryl; Fanning, C. Mark; Samson, Iain

    2016-01-01

    Rare earth element (REE)-rich breccia pipes (600,000 t @ 12% rare earth oxides) are preserved along the margins of the 136-million metric ton (Mt) Pea Ridge magnetite-apatite deposit, within Mesoproterozoic (~1.47 Ga) volcanic-plutonic rocks of the St. Francois Mountains terrane in southeastern Missouri, United States. The breccia pipes cut the rhyolite-hosted magnetite deposit and contain clasts of nearly all local bedrock and mineralized lithologies.Grains of monazite and xenotime were extracted from breccia pipe samples for SHRIMP U-Pb geochronology; both minerals were also dated in one polished thin section. Monazite forms two morphologies: (1) matrix granular grains composed of numerous small (minerals includes Re-Os on fine-grained molybdenite and 40Ar/39Ar on muscovite, biotite, and K-feldspar.Ages (±2σ errors) obtained by SHRIMP U-Pb analysis are as follows: (1) zircon from the two host rhyolite samples have ages of 1473.6 ± 8.0 and 1472.7 ± 5.6 Ma; most zircon in late felsic dikes is interpreted as xenocrystic (age range ca. 1522–1455 Ma); a population of rare spongy zircon is likely of igneous origin and yields an age of 1441 ± 9 Ma; (2) pale-yellow granular monazite—1464.9 ± 3.3 Ma (no dated xenotime); (3) reddish matrix granular monazite—1462.0 ± 3.5 Ma and associated xenotime—1453 ± 11 Ma; (4) coarse glassy-yellow monazite—1464.8 ± 2.1, 1461.7 ± 3.7 Ma, with rims at 1447.2 ± 4.7 Ma; and (5) matrix monazite (in situ)—1464.1 ± 3.6 and 1454.6 ± 9.6 Ma, and matrix xenotime (in situ)—1468.0 ± 8.0 Ma. Two slightly older ages of cores are about 1478 Ma. The young age of rims on the coarse glassy monazite coincides with an Re-Os age of 1440.6 ± 9.2 Ma determined in this study for molybdenite intergrown with quartz and allanite, and with the age of monazite inclusions in apatite from the magnetite ore (Neymark et al., 2016). A 40Ar/39Ar age of 1473 ± 1 Ma was obtained for muscovite from a breccia pipe sample.Geochronology and

  7. Geochronology, petrogenesis and tectonic settings of pre- and syn-ore granites from the W-Mo deposits (East Kounrad, Zhanet and Akshatau), Central Kazakhstan

    Science.gov (United States)

    Li, GuangMing; Cao, MingJian; Qin, KeZhang; Evans, Noreen J.; Hollings, Pete; Seitmuratova, Eleonora Yusupovha

    2016-05-01

    There is significant debate regarding the mineralization ages of the East Kounrad, Zhanet and Akshatau W-Mo deposits of Central Kazakhstan, and the petrogenesis and tectono-magmatic evolution of the granites associated with these deposits. To address these issues, we present molybdenite Re-Os dating, zircon U-Pb dating, whole rock geochemistry as well as Sr-Nd-Pb and zircon O-Hf isotopic analyses on the pre-mineralization and ore-forming granites. U-Pb dating of zircons from pre-mineralization granitic rocks yield Late Carboniferous ages of 320-309 Ma, whereas ore-forming granites have Early Permian ages of 298-285 Ma. Molybdenite Re-Os isotopic data indicate a mineralization age of 296 Ma at East Kounrad, 294 Ma at Akshatau and 285 Ma at Zhanet. The pre-ore and ore-forming granites are high-K calc-alkaline, metaluminous to slightly peraluminous I-type granites. The pre-mineralization granites are relatively unfractionated, whereas the ore-forming granites are highly fractionated. The fractionating mineral phases are probably K-feldspar, apatite, Ti-bearing phases and minor plagioclase. The pre-mineralization and ore-forming rocks are characterized by similar Sr-Nd-Pb-Hf-O isotopic compositions ((87Sr/86Sr)i = 0.70308-0.70501, εNd (t) = - 0.5 to + 2.8, 207Pb/204Pb = 15.60-15.82, zircon εHf (t) = + 1.2 to + 15.6 and δ18O = + 4.6 to + 10.3‰), whole rock TDMC (Nd) (840-1120 Ma) and zircon TDMC (Hf) (320-1240 Ma). The isotopic characteristics are consistent with a hybrid magma source caused by 10-30% assimilation of ancient crust by juvenile lower crust. The geochronology and geochemistry of these granites show that the Late Carboniferous pre-mineralization granitic rocks formed during subduction, whereas the Early Permian ore-forming, highly fractionated granite probably underwent significant fractionation with a restite assemblage of K-feldspar, apatite, Ti-bearing phases and minor plagioclase and developed during collision between the Yili and Kazakhstan

  8. EARTHTIME: Teaching geochronology to high school students

    Science.gov (United States)

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

    2010-05-01

    The authors taught an educational module developed as part of the EARTHTIME (www.earth-time.org) 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 (http://www.earth-time.org/Lesson_Plan.pdf). 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. U-Pb (SHRIMP) and Sm-Nd geochronology of basaltic green schists of the Aracuai orogen: implications for the age of the Macaubas group; Geocronologia U-Pb (SHRIMP) e Sm-Nd de xistos verdes basalticos do orogeno Aracuai: implicacoes para a idade do grupo Macaubas

    Energy Technology Data Exchange (ETDEWEB)

    Babinski, Marly [Universidade de Sao Paulo, SP (Brazil). Inst. de Geociencias. Centro de Pesquisas Geocronologicas]. E-mail: babinski@usp.br; Gradim, Rafael Jaude; Alkmim, Fernando Flecha de [Universidade Federal de Ouro Preto, MG (Brazil). Escola de Minas. Dept. de Geologia]. E-mails: rafaelgradim@gmail.com; alkmim@degeo.ufop.br; Pedrosa-Soares, Antonio Carlos; Noce, Carlos Mauricio [Minas Gerais Univ., Belo Horizonte (Brazil). Inst. de Geociencias; Liu, Dunyi [Chinese Academy of Geological Sciences, Beijing (China). Beijing SHRIMP Lab.

    2005-12-15

    The age and stratigraphic position of the basaltic green schists of the Rio Preto valley, located in the western part of the Aracuai Belt (ca. 60 km north of Diamantina, Minas Gerais) were a matter of controversy in the geologic literature, because several authors correlated them to the Neo proterozoic Macaubas Group but others to the lower Espinhaco Supergroup (ca. 1.7 Ga). However, detailed studies demonstrate that these green schists represent an interplay of submarine basaltic volcanism, volcanoclastic sedimentation and fire fountaining, and that they belong to the Chapada Acaua Formation of the Macaubas Group (Gradim et al., 2005). Geochemical studies indicate that the green schist protoliths evolved in a continental intra plate environment. Zircon crystals were separated from a green schist sample and analyzed by the U-Pb SHRIMP method. A Sm-Nd whole-rock isotopic analysis was obtained from the same sample. The Sm- Nd model age of ca. 1.52 Ga suggests that the green schist protoliths are younger than the magmatism of the Espinhaco rift. Most analyzed zircon crystals show features of detrital grains. The older ages indicate zircon grains inherited from the Archean- Paleoproterozoic basement and from magmatic rocks of the Espinhaco rift. The younger U-Pb values constrain the maximum age of the green schist protoliths at ca. 1,16 Ga. (author)

  10. Geology and U-Pb geochronology of the Banabuiu granite, Northeastern Ceara, Brazil; Geologia y geocronologia U-Pb del granito de Banabuiu, Noreste de Ceara, Brasil

    Energy Technology Data Exchange (ETDEWEB)

    Lima, M. N.; Nogueira Neto, J. A.; Azevedo, M. R.; Valle Aguado,

    2010-07-01

    The Banabuiu massif crops out in the Central Ceara Domain (DCC) of the Borborema Province (NE Brasil), as an N-S elongate granite intrusion, concordant with the regional structures. It was emplaced into basement rocks of Paleoproterozoic age, extensively transformed into gneisses and migmatites during the Brasilian orogeny ({approx}600 Ma). Using U-Pb zircon dating, the crystallization age of the Banabuiu syn-kinematic two-mica granite was estimated at 578.6 {+-} 6.5 Ma. The granite is strongly peraluminous (A/CNK 1,098 - 1,134) and shows a typical S-type geochemical signature. The {epsilon}Nd{sub 5}80 values are strongly negative ({epsilon}Nd{sub 5}80 = -19 a - 23) and partially overlap with those of the Paleoproterozoic gneissmigmatite complex ({epsilon}Nd{sub 5}80 = -12 to -26), suggesting that the parental magmas of the Banabuiu granite could have been produced by partial melting of similar crustal materials. (Author) 21 refs.

  11. U-Pb geochronology of Martín García, Sola, and Dos Hermanas Islands (Argentina and Uruguay): Unveiling Rhyacian, Statherian, Ectasian, and Stenian of a forgotten area of the Río de la Plata Craton

    Science.gov (United States)

    Santos, João O. S.; Chernicoff, Carlos J.; Zappettini, Eduardo O.; McNaughton, Neal J.; Greau, Y.

    2017-12-01

    The Río de la Plata Craton is one of the three major cratons of South America. The craton is largely covered by sedimentary basins where its most exposed area is Buenos Aires-Piedra Alta Province (Chernicoff et al., 2014). This province includes the Martín García Island in the confluence of Uruguay River and the Río de la Plata estuary. Despite to be a reference area for the craton the Martín García Island lacks modern geological investigation. We present U-Pb SIMS (secondary ion mass spectrometer) geochronological data on zircon and titanite, as well as Hf isotope determinations on zircon, from rocks of Martín García Island (Argentina), Sola and Dos Hermanas Islands (Uruguay) and from Paso Severino Formation (Uruguay). We investigated: 1) Rhyacian intermediate-to acidic plutonic, arc-type rocks of the Florida Belt dated between 2090 Ma and 2115 Ma, derived from juvenile Neoarchean crust (TDMHf: 2.52 Ga; average εHf: +3.62); 2) Rhyacian metadacite (San José metamorphic belt) of 2127 Ma; 3) Statherian metagabbros of 1724-1734 Ma, with Transplatense inheritance; 4) Early Ectasian metagabbro of 1392 Ma, with Rhyacian inheritance; and 5) Stenian metagabbros of 1193 Ma (TDMHf: 2.00 Ga, εHf: 0.1). Most of the dated orthogneisses show Brasilian-age (from 778 to 550 Ma) Pb loss in the zircons, interpreted to be caused by shearing or uplifting during Neoproterozoic. The results show that the area is not exclusively Rhyacian in age but encompasses Statherian, Ectasian, and Stenian gabbros. The last two are interpreted as distal intrusions associated to the Sunsás Orogen. All post-Transplatense metagabbros have geochemical signature of island arc basalt derived from primitive mantle with enrichment of LILE and depletion of HFSE. These characteristics point to the recurrence of magma chambers intermittently active during the Rhyacian, Statherian, Ectasian, and Stenian, all with a similar source. The term "Transplatense" is used to replace "Trans-Amazonian" events

  12. Molybdenite Re-Os, zircon U-Pb dating and Lu-Hf isotopic analysis of the Xiaerchulu Au deposit, Inner Mongolia Province, China

    Science.gov (United States)

    Wang, Jia-xin; Nie, Feng-Jun; Zhang, Xue-ni; Jiang, Si-hong

    2016-09-01

    The Xiaerchulu Au deposit, located in the Southern Orogenic Belt (SOB) of Western Inner Mongolia (WIM), is hosted in an Early Permian (271-261 Ma) volcanic-plutonic sequence. Mineralization took place in silicified biotite granites or along the contact zone between the Neoproterozoic Baiyinbaolage Group and the biotite granite. In order to constrain the timing of the Xiaerchulu mineralization and discuss the petrogenesis of the hosting granites, molybdenite Re-Os, and zircon U-Pb and, Lu-Hf, and REE, geochemical, and Sr-Nd isotopic studies were completed in this study. We measured Re-Os isotopes of six molybdenite samples from the main ore body, which yielded a weighted average model age of 261.7 ± 1.5 Ma with a MSWD of 0.55, indicating that the time of mineralization was at ca. 262 Ma. High precision U-Pb dating for the studied granites yields Permian 206Pb/238U ages ranging from 271 to 269 Ma. These age data confirm that both the intrusion and related mineralization were initiated in Early Permian period. These granites are strongly peraluminous with A/CNK = 1.11-1.12, high SiO2-K2O contents, as well as containing biotite and muscovite, indicating a petrogenesis of typical S-type granites, the above consideration is also consistent with the result of discrimination diagrams. The Re contents of molybdenite, εNd(t), and zircon εHf(t), as well as the 176Hf/177Hf values of the granites, fall into the ranges from 1.153 to 2.740 μg/g, - 11.1 to - 9.3, - 8.8 to - 0.9, and 0.282358 to 0.282688, respectively. All of this evidence suggests that the metals were derived from a predominantly crustal source, the granites originated from crust in an extensional setting, and the rejuvenation of the continent may have play an important role during the ore-forming processes of the Early Permian epoch.

  13. Constraints on the timing of Co-Cu ± Au mineralization in the Blackbird district, Idaho, using SHRIMP U-Pb ages of monazite and xenotime plus zircon ages of related Mesoproterozoic orthogneisses and metasedimentary rocks

    Science.gov (United States)

    Aleinikoff, John N.; Slack, John F.; Lund, Karen; Evans, Karl V.; Fanning, C. Mark; Mazdab, Frank K.; Wooden, Joseph L.; Pillers, Renee M.

    2012-01-01

    The Blackbird district, east-central Idaho, contains the largest known Co reserves in the United States. The origin of strata-hosted Co-Cu ± Au mineralization at Blackbird has been a matter of controversy for decades. In order to differentiate among possible genetic models for the deposits, including various combinations of volcanic, sedimentary, magmatic, and metamorphic processes, we used U-Pb geochronology of xenotime, monazite, and zircon to establish time constraints for ore formation. New age data reported here were obtained using sensitive high resolution ion microprobe (SHRIMP) microanalysis of (1) detrital zircons from a sample of Mesoproterozoic siliciclastic metasedimentary country rock in the Blackbird district, (2) igneous zircons from Mesoproterozoic intrusions, and (3) xenotime and monazite from the Merle and Sunshine prospects at Blackbird. Detrital zircon from metasandstone of the biotite phyllite-schist unit has ages mostly in the range of 1900 to 1600 Ma, plus a few Neoarchean and Paleoproterozoic grains. Age data for the six youngest grains form a coherent group at 1409 ± 10 Ma, regarded as the maximum age of deposition of metasedimentary country rocks of the central structural domain. Igneous zircons from nine samples of megacrystic granite, granite augen gneiss, and granodiorite augen gneiss that crop out north and east of the Blackbird district yield ages between 1383 ± 4 and 1359 ± 7 Ma. Emplacement of the Big Deer Creek megacrystic granite (1377 ± 4 Ma), structurally juxtaposed with host rocks in the Late Cretaceous ca. 5 km north of Blackbird, may have been involved in initial deposition of rare earth elements (REE) minerals and, possibly, sulfides. In situ SHRIMP ages of xenotime and monazite in Co-rich samples from the Merle and Sunshine prospects, plus backscattered electron imagery and SHRIMP analyses of trace elements, indicate a complex sequence of Mesoproterozoic and Cretaceous events. On the basis of textural relationships

  14. Geochronology of the Rio Formoso estuarine by 210Pb

    International Nuclear Information System (INIS)

    Arruda, Gilberto N.; Lyra, Denilson T.; Melo, Julyanne T.B.; Farias, Emerson E.G.; Franca, Elvis J.; Santos, Thiago O.; Souza Neto, Joao A.

    2015-01-01

    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 210 Pb consisted of using hydrobromic acid and an ion exchange resin (DOWEX) for precipitating 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 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. 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)

  15. U-Pb geochronologic constraints on Paleoproterozoic orogenesis in the northwestern Makkovik Province, Labrador, Canada

    International Nuclear Information System (INIS)

    Ketchum, J.W.F.; Dunning, G.R.; Dunning, G.R.

    1997-01-01

    A 45 km wide, shear-zone-bounded segment of the northwestern Makkovik Province, Labrador, is underlain by Archean gneisses derived from the adjacent Nain craton. This lithotectonic block (Kaipokok domain) was reworked at high metamorphic grade, overthrust by supracrustal sequences (Lower Aillik and Moran Lake groups), and intruded by granitoid plutons during the Paleoproterozoic. Initial amphibolite-facies reworking of the Kaipokok domain at 1896 ± 6 Ma is indicated by U-Pb ages of metamorphic zircon from a foliated Kikkertavak metadiabase dyke. This is one of the oldest Paleoproterozoic tectonic events dated thus far in northeast Laurentia and may be linked with ca. 1890 Ma plutonism documented elsewhere in the Kaipokok domain. Intrusion of granitoid plutons at 1882 -6 +10 , 1877 ± 5, and 1871 -3 +4 Ma in the Kaipokok Bay area postdates early thick- and thin-skinned thrusting (possibly east to northeast directed) that involved Lower Aillik Group strata. U-Pb titanite ages of 1866 - 1847 Ma in part record a metamorphic event that followed this plutonic-tectonic activity. These early events are temporally and kinematically difficult to reconcile with accretion of juvenile Makkovikian terranes in the southeast and may instead be related to early stages of the ca. 1.91 - 1.72 Ga Torngat orogeny along the western margin of the Nain craton. In contrast, high-grade metamorphism, dextral shearing, and northwestward thrusting between 1841 and 1784 Ma, including crystallization of an Iggiuk granitic vein at 1811 ± 8 Ma, are in accord with accretion of Makkovikian terranes in a dextral transpressional regime (Makkovikian orogeny sensu stricto). Coeval sinistral transpression in the Torngat orogen suggests that both orogenic belts accommodated relative northward tectonic escape of the Nain craton during this interval. (author)

  16. Molybdenum mineralization at Alpeiner Scharte, Tyrol (Austria): results of in-situ U-Pb zircon and Re-Os molybdenite dating

    International Nuclear Information System (INIS)

    Langthaler, K.J.; Raith, J.G.; Cornell, D.H.; Stein, H.J.; Melcher, F.

    2004-01-01

    Vein-type Mo mineralization at Alpeiner Scharte occurs in the Penninic units of the western Tauern Window in the Eastern Alps. Three types of previously undated metagranitoids (central gneisses) are distinguished and preserve intrusive contacts with pre-Alpine metamorphosed supracrustal rocks. The granitic protoliths represent fractionated late to post-orogenic, calc-alkaline, I-type magmas with minor S-type components. The Mo veins are restricted to a biotite and alkali feldspar-rich gneiss variety and occur in E-W trending normally sub-vertical quartz veins with adjacent thin discontinuous garnet- and biotite-rich zones; the latter are interpreted as metamorphosed vein selvages. Prior to this work the age of the intrusive host rocks as well as the age of Mo mineralization were unknown. The pre-Alpine Mo deposit and its host rocks were affected by four Alpine deformation events (D 1 -D 4 ) and Young-Alpine regional metamorphism. The P-T conditions of this metamorphic event were ∼ 550 o C and ∼ 8 kbar and are in agreement with results of previous regional studies. Zircon grains from two orthogneiss samples were dated with the U-Pb method using ion probe techniques. Zircons from the metagranitic host rock of the Mo-veins yielded an emplacement age of 306.8 ± 3.8 Ma (2σ). A second sample from a more leucocratic gneiss lacking Mo-veins gave 305.0 ± 6.6 Ma (2 σ). Re-Os dating of molybdenite from the veins yielded an age of 306.8 ± 3.1 Ma in good agreement with the U-Pb zircon ages. This study confirms one of two alternative hypotheses discussed in the literature. lt supports the idea that vein-type Mo-mineralization in the western Tauern Window is genetically related to Late Carboniferous (Westphalian) granitoids that were emplaced during the late to post-orogenic stage of the Variscan orogeny. They do not constitute an Alpine metamorphic-hydrothermal deposit. This study further confirms the strength of the Re-Os molybdenite chronometer, in that it was

  17. Development of a petrochronology laboratory in Russia

    Science.gov (United States)

    Yavryan, M.; Powerman, V.; Nourgaliev, D. K.

    2017-12-01

    Each year more and more U-Pb detrital zircon studies are coming from Russia. However, the latest developments in zircon studies, namely combining U-Pb age with the measurements of REE concentrations and Lu-Hf isotopy on the same grain, have been unavailable in Russia. Kazan Federal U. has undertaken certain efforts in order to develop Russia's first petrochronology laboratory. The following equipment was recently purchased, installed and put into operation: Neptune Plus HR MC ICPMS, with jet-interface and RPQ filter. iCAP-Qc quadruple ICPMS. 2 x NWR213 laser ablation systems; one of them equipped with the TwoVol2 ablation chamber. Helium gas is used to flush the ablation chamber; 5 ml/min. of nitrogen is mixed downstream before plasma in order to increase the signal. The following methods have been set up at KFU on the Neptune Plus ICPMS: (1) U-Pb geochronology on zircons. A set of standards have been dated (R33, 91500, Plesovice, Mudtank, GJ-1, AS-3); (2) U-Pb geochronology on perovskites. We have collected, separated and analyzed Baikal Tazheran 463 Ma perovskite, previously dated on TIMS (e.g., Ireland et al., 1990; Li et al., 2010), using 91500 zircon as a primary standard. We intend to start using Tazheran perovskite as an in-house standard for dating perovskites. (3) Lu-Hf isotopy on a set of standard zircons. All results will be presented during the poster session. Our next steps will include setting the (a) U-Pb geochronology and (b) REE measurements on the quadruple MS; (c) splitting the ablation stream into two and directing them to Neptune for Lu-Hf measurements and to iCAP for either U-Pb, or REE, or both types of measurements; (d) experimenting with pre-ablation in order to diminish common lead, with (e) rastering during ablation to diminish downhole fractionation, with (f) squid signal smoothing device.

  18. Evidence of early Archean crust in northwest Gondwana, from U-Pb and Hf isotope analysis of detrital zircon, in Ediacaran surpacrustal rocks of northern Spain

    Science.gov (United States)

    Naidoo, Thanusha; Zimmermann, Udo; Vervoort, Jeff; Tait, Jenny

    2018-03-01

    The Mora Formation (Narcea Group) is one of the oldest Precambrian supracrustal successions in northern Spain. Here, we use U-Pb and in situ Hf isotope analysis on detrital zircon to determine its age and provenance. The youngest U-Pb dates constrain the maximum depositional age of the Mora Formation at 565 ± 11 Ma. Results indicate: (1) a dominant Ediacaran zircon population (33%; 565-633 Ma, Cadomian) within a spectrum of Neoproterozoic ages (40%; 636-996 Ma); and (2) smaller Mesoproterozoic (5%; 1004-1240 Ma), Palaeoproterozoic (11%; 1890-2476 Ma) and Archean (11%; 2519-3550 Ma) populations. Results here do not point to one specific cratonic source area; instead, detritus may have been derived from the West African craton and Amazonia, or even the concealed Iberian basement. The lack of 1.3-1.8 Ga grains suggests exclusion of the Sahara Craton as a major source, but this is not certain. This mixed composition favours a complex source history with reworking of detritus across terrane/craton boundaries. Hafnium isotope compositions indicate a range of crustal and juvenile sources, with initial ɛHf values between -15.8 and 11.1, and Hf model ages from 0.8 to 3.7 Ga. For Neoproterozoic zircons (80%), juvenile components (ɛHf(i) +10) may be related to Rodinia fragmentation and the onset of an active margin setting leading to the Cadomian orogeny. Palaeoproterozoic to Paleoarchean grains (20%) all have negative ɛHf values and Meso- to Eoarchean Hf model ages. This indicates an early (Archean) sialic crustal component for northwestern Gondwana.

  19. U-series in zircon and 40Ar/39Ar geochronology reveal the most recent stage of a supervolcanic cycle in the Altiplano-Puna Volcanic Complex, Central Andes

    Science.gov (United States)

    Tierney, C.; de Silva, S. L.; Schmitt, A. K.; Jicha, B.; Singer, B. S.

    2010-12-01

    The ignimbrite flare up that produced the Altiplano-Puna Volcanic Complex of the Central Andes is characterized by episodic supervolcanism over a ~10 Ma time-span that climaxed about 4Ma. Since peak activity, the temporal and spatial record of volcanism suggests a waning of the system with only one other supervolcanic eruption at 2.6Ma. The most recent phase of volcanism from the APVC comprises a series of late Pleistocene domes that share a general petrochemical resemblance to the ignimbrites. New U-series data on zircons and high precision 40Ar/39Ar age determinations reveal that these effusive eruptions represent a temporally coherent magmatic episode. The five largest domes (Chao, Chillahuita, Chanka, Chascon-Runtu Jarita, and Tocopuri) have a combined volume >40 km3, and are distributed over an elliptical area of over 3000km2 centered at 22°S 68°W. They are crystal rich (>50%) dacites to rhyolites. New 40Ar/39Ar age determinations on biotite for the domes range range from 108±6 to 190±50 ka. However, 40Ar/39Ar ages from sanidine for some of the domes are more precise and span from 87±4 to 97±2 ka. We therefore interpret the eruption age of all these domes to be ~90 - 100 ka. This is consistent with SIMS U-series crystallization ages from the rims of 66 zircon crystals from four of the domes that reveal a fairly continuous spread of ages from ~90 ka to >300 ka with potentially common peaks in zircon ages at 100 ka and ~200 ka. U-Pb dating on the interiors of some of these zircon crystals indicates crystallization ages of up to 1.5 Ma. The common peaks of zircon crystallization between domes suggest that magma that fed these domes shared a larger regional source. Furthermore, the large volume of this potential source and the crystal-rich nature of the lava imply that this source was likely a large body of crystal-mush. The continuous nature of the zircon rim age population indicates that the residence time of this magma body was likely >200kyr. Potential

  20. U-Th-Pb dating of the Brossasco ultrahigh-pressure metagranite, Dora-Maira massif, western Alps

    Energy Technology Data Exchange (ETDEWEB)

    Paquette, J.L.; Montel, J.M. [Centre National de la Recherche Scientifique, 63 - Clermont-Ferrand (France); Chopin, C. [Ecole Normale Superieure, 75 - Paris (France). Lab. de Geologie

    1999-01-01

    The Brossasco metagranite is part of the coherent ultrahigh-pressure metamorphic terrane in the Dora-Maira massif and, although it has reached Alpine peak metamorphic conditions of about 725 C/30 kbar, still locally preserves an undeformed magmatic texture. In order to obtain new chronological constraints on this key area of the western Alps, we studied the behaviour of the U-Th-Pb system in zircons and monazites from such an undeformed facies. Only zircon fractions containing very small needle-shaped inclusions are concordant at 304{+-}3 Ma. This result is interpreted as the emplacement age of the granite. All other zircons show an old inherited memory and the more U-rich of these crystals were affected by an episodic Pb loss with a poorly defined lower intercept at 60{+-}45 Ma. Both TIMS (thermo-ionisation mass spectrometry) and electron-microprobe analysis of monazites show that this last event has also disturbed their U-Pb and Th-Pb isotopic systems, without resetting them completely. Comparison with cofacial rock-types which have been completely reworked under these extreme P-T conditions shows that deformation and mass transfer as well as the nature of the protoliths and the characteristics of their zircons have a much more pronounced influence on the behaviour of isotopic systems than the temperature and/or pressure parameters. This evidence is an obvious limitation to the use of the blocking-temperature concept. (orig.)

  1. U-Th-Pb dating of the Brossasco ultrahigh-pressure metagranite, Dora-Maira massif, western Alps

    International Nuclear Information System (INIS)

    Paquette, J.L.; Montel, J.M.; Chopin, C.

    1999-01-01

    The Brossasco metagranite is part of the coherent ultrahigh-pressure metamorphic terrane in the Dora-Maira massif and, although it has reached Alpine peak metamorphic conditions of about 725 C/30 kbar, still locally preserves an undeformed magmatic texture. In order to obtain new chronological constraints on this key area of the western Alps, we studied the behaviour of the U-Th-Pb system in zircons and monazites from such an undeformed facies. Only zircon fractions containing very small needle-shaped inclusions are concordant at 304±3 Ma. This result is interpreted as the emplacement age of the granite. All other zircons show an old inherited memory and the more U-rich of these crystals were affected by an episodic Pb loss with a poorly defined lower intercept at 60±45 Ma. Both TIMS (thermo-ionisation mass spectrometry) and electron-microprobe analysis of monazites show that this last event has also disturbed their U-Pb and Th-Pb isotopic systems, without resetting them completely. Comparison with cofacial rock-types which have been completely reworked under these extreme P-T conditions shows that deformation and mass transfer as well as the nature of the protoliths and the characteristics of their zircons have a much more pronounced influence on the behaviour of isotopic systems than the temperature and/or pressure parameters. This evidence is an obvious limitation to the use of the blocking-temperature concept. (orig.)

  2. Episodic growth and homogenization of plutonic roots in arc volcanoes from combined U-Th and (U-Th)/He zircon dating

    Science.gov (United States)

    Schmitt, Axel K.; Stockli, Daniel F.; Lindsay, Jan M.; Robertson, Richard; Lovera, Oscar M.; Kislitsyn, Roman

    2010-06-01

    Tracing the fate of unerupted magma is challenging because plutonic roots of young volcanoes are largely inaccessible. Here we develop the use of zircon age spectra to determine crystal provenance and source rocks for volcanic products, in analogy to detrital crystals in sediments. U-Th zircon crystallization ages for the Soufrière Volcanic Complex, Saint Lucia (Lesser Antilles) frequently predate their eruption as determined from combined U-Th and (U-Th)/He zircon dating. The oldest dated eruptions are 273 ± 15 ka and 264 ± 8 ka (1σ uncertainty) for Morne Bonin dacite and Bellevue pumice deposit, respectively. The most recent eruptions formed morphologically pristine domes in the center of the Qualibou depression (Belfond: 13.6 ± 0.4 ka; Terre Blanche: 15.3 ± 0.4 ka). U-Th (U-Pb) zircon crystallization ages determined for crystal rims and interiors range between near-eruption ages to ∼ 600 ka. Older xenocrysts are absent. Zircon crystallization age distributions are complex, yet systematic: crystal rim ages in the most recently erupted volcanic rocks match those of co-erupted plutonic inclusions, whereas crystal interiors are equivalent to the cumulative distribution of zircon ages from older eruptions. This is evidence that silicic lava domes and pyroclastic flows share a common source that is located underneath the Qualibou depression, where the intrusive roots of this long-lived arc volcanic system became homogenized through thermal and mechanical reprocessing of individual batches of unerupted magma from earlier volcanic episodes within timescales of < 100 ka.

  3. Detrital zircon and igneous protolith ages of high-grade metamorphic rocks in the Highland and Wanni Complexes, Sri Lanka: Their geochronological correlation with southern India and East Antarctica

    Science.gov (United States)

    Kitano, Ippei; Osanai, Yasuhito; Nakano, Nobuhiko; Adachi, Tatsuro; Fitzsimons, Ian C. W.

    2018-05-01

    The high-grade metamorphic rocks of Sri Lanka place valuable constraints on the assembly of central parts of the Gondwana supercontinent. They are subdivided into the Wanni Complex (WC), Highland Complex (HC) and Vijayan Complex (VC), but their correlation with neighbouring Gondwana terranes is hindered by a poor understanding of the contact between the HC and WC. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of remnant zircon cores from 45 high-grade metamorphic rocks in Sri Lanka reveals two domains with different age characteristics that correlate with the HC and WC and which help constrain the location of the boundary between them. The HC is dominated by detrital zircon ages of ca. 3500-1500 Ma from garnet-biotite gneiss, garnet-cordierite-biotite gneiss, some samples of garnet-orthopyroxene-biotite gneiss and siliceous gneiss (interpreted as paragneisses) and igneous protolith ages of ca. 2000-1800 Ma from garnet-hornblende-biotite gneiss, other samples of garnet-orthopyroxene-biotite gneiss, garnet-two-pyroxene granulite, two-pyroxene granulite and charnockite (interpreted as orthogneisses). In contrast, the WC is dominated by detrital zircon ages of ca. 1100-700 Ma from paragneisses and igneous protolith ages of ca. 1100-800 Ma from orthogneisses. This clearly suggests the HC and WC have different origins, but some of our results and previous data indicate their spatial distribution does not correspond exactly to the unit boundary proposed in earlier studies using Nd model ages. Detrital zircon and igneous protolith ages in the HC suggest that sedimentary protoliths were eroded from local 2000-1800 Ma igneous rocks and an older Paleoproterozoic to Archean craton. In contrast, the WC sedimentary protoliths were mainly eroded from local late Mesoproterozoic to Neoproterozoic igneous rocks with very minor components from an older 2500-1500 Ma craton, and in the case of the WC precursor sediments there was possibly

  4. New zircon U-Pb LA-ICP-MS ages and Hf isotope data from the Central Pontides (Turkey): Geological and geodynamic constraints

    Science.gov (United States)

    Çimen, Okay; Göncüoğlu, M. Cemal; Simonetti, Antonio; Sayit, Kaan

    2018-05-01

    The Central Pontides in northern Anatolia is located on the accretionary complex formed by the closure of Neotethyan Intra-Pontide Ocean between the southern Eurasian margin (Istanbul-Zonguldak Terrane) and the Cimmerian Sakarya Composite Terrane. Among other components of the oceanic lithosphere, it comprises not yet well-dated felsic igneous rocks formed in arc-basin as well as continent margin settings. In-situ U-Pb age results for zircons from the arc-basin system (öangaldağ Metamorphic Complex) and the continental arc (Devrekani Metadiorite and Granitoid) yield ages of 176 ± 6 Ma, 163 ± 9 Ma and 165 ± 3 Ma, respectively. Corresponding in-situ average (initial) 176Hf/177Hf initial ratios are 0.28261 ± 0.00003, 0.28267 ± 0.00002 and 0.28290 ± 0.00004 for these units and indicative of a subduction-modified mantle source. The new U-Pb ages and Hf isotope data from these oceanic and continental arc units together with regional geological constraints support the presence of a multiple subduction system within the Intra-Pontide Ocean during the Middle Jurassic.

  5. Isotope, scanning electron microscope, and energy dispersive spectrometer studies of heterogeneous zircons from radioactive granites in the Grenville structural province, Quebec and Ontario

    International Nuclear Information System (INIS)

    Rimsaite, J.

    1981-01-01

    Heterogeneous zircons yielded discordant Pb-U, Pb-Th, and 207 Pb- 206 Pb isotopic ages. Most data points fall below the concordia curve, implying losses of daughter elements, bqt they define a discordia line that intersects the concordia at approximately 90 Ma and 1020 Ma. To obtain evidence for mobilization of U and radiogenic Pb, zircon grains were studied using a scanning electron microscope coupled with an energy dispersive spectrometer. High magnification backscattered and secondary electron images of the zircon revealed narrow fractures, zoning and diverse mineral inclusions. Three groups of mineral inclusions observed were: 1) those predating zoned zircon and apparently serving as a nucleus; 2) uraninite, feldspar, and apatite associated with the growth and zoning of the host zircon; and 3) fracture-fillings that postdate crystallization of the host zircon. The U- and Pb-rich inclusions incorporated into the zircon grains during and after its crystallization markedly affect isotopic ages of the host zircon. Migration of Pb and U have occurred along fractures in zircon. Zircon, uraninite, and other associated minerals have decomposed and complex reactions have taken place between the liberated Zr, U, Th and other elements to produce overgrowths on mineral grains and unidentified Zr-bearing material in fractures

  6. U-Pb GEOCHRONOLOGY OF METASEDIMENTARY SCHISTS IN ...

    African Journals Online (AJOL)

    PROF EKWUEME

    The geology of Akwnaga area of northcentral Nigeria is dominated by schists, metaigneous gneisses and weakly to non- foliated granites. The Schist shows grades up to the sillimanite zone of the Barrovian type regional metamorphism and occurs as xenoliths in other rocks of the area. The zircons separated from a sample ...

  7. Geology, geochemistry, and geochronology (U-Pb) of the Rio Fortuna Gneiss - Serra do Bau intrusive Suite - Paragua Terrane SW Amazonian Craton

    Energy Technology Data Exchange (ETDEWEB)

    Faria, Debora Almeida; Ruiz, Amarildo Salina; Matos, Joao Batista; Sousa, Maria Zelia Aguiar de; Lima, Gabrielle Aparecida de [Research Group on Crustal and Tectonic Evolution, Guapore, RS (Brazil); Universidade Federal de Mato Grosso (ICET/UFMT), Cuiaba, MT (Brazil). Instituto de Ciencias Exatas e da Terra; Inst. Nacional de Ciencia e Tecnologia de Geociencias da Amazonia (GEOCIAM), Belem, PA (Brazil); Moacir Jose Buenano Macambira, E-mail: defaal.debora@gmail.com, E-mail: gabilimagel@gmail.com, E-mail: asruiz@gmail.com, E-mail: jmatos@ufmt.br, E-mail: prof.mzaguiar@gmail.com, E-mail: moamac@ufpa.br [Research Group on Crustal and Tectonic Evolution, Guapore, RS (Brazil); Universidade Federal do Para (UFPA), Belem, PA (Brazil)

    2014-07-01

    The Rio Fortuna Gneiss crops out in the Serra Santa Barbara, near the Fortuna military headquarters, on the Brazil-Bolivia border. These orthogneisses are located in a portion of the Paragua terrain affected by the Sunsas Orogeny (1.0-0.9 Ga.). They are classified as monzo to granodiorite orthogneisses and underwent at least three episodes of deformation. The U-Pb zircon age of 1,711 ± 13 Ma obtained by laser ablation MC-ICP-MS is interpreted as the crystallization age of this orthogneiss. Geochemically, these rocks form a sequence comprising acidic subalkaline magmatism, calc-alkalic-type high-K, and metaluminous to peraluminous. (author)

  8. LA-ICP-MS Pb-U Dating of Young Zircons from the Kos-Nisyros Volcanic Centre, SE Aegean Arc (Greece)

    Science.gov (United States)

    Guillong, M.; Von Quadt, A.; Peytcheva, I.; Bachmann, O.

    2014-12-01

    Zircon Pb-U dating has become a key technique for answering many important questions in geosciences. This paper describes a new LA-ICP-MS approach. We show, using previously dated samples of a large quaternary rhyolitic eruption in the Kos-Nisyros volcanic centre (the 161 ka Kos Plateau Tuff), that the precision of our LA-ICP-MS method is as good as via SHRIMP, while ID-TIMS measurements confirm the accuracy. Gradational age distribution over >140 ka of the Kos zircons and the near-absence of inherited cores indicate near-continuous crystallisation in a growing magma reservoir with little input from wall rocks. Previously undated silicic eruptions from Nisyros volcano (Lower Pumice, Nikia Flow, Upper Pumice), which are stratigraphically constrained to have happened after the Kos Plateau Tuff, are dated to be younger than respectively 124 ± 35 ka, 111 ± 42 ka and 70 ± 24 ka. Samples younger than 1 Ma were corrected for initial thorium disequilibrium using a new formula that also accounts for disequilibrium in 230Th decay. Guillong, M. et al., 2014, JAAS, 29, p. 963-967; doi: 10.1039/c4ja00009a.

  9. Depositional history of a condensed shallow marine reservoir succession: stratigraphy and detrital zircon geochronology of the Jurassic Sto Formation, Barents Sea

    Czech Academy of Sciences Publication Activity Database

    Klausen, T. G.; Müller, R.; Sláma, Jiří; Olaussen, S.; Rismyhr, B.; Helland-Hansen, W.

    2018-01-01

    Roč. 175, č. 1 (2018), s. 130-145 ISSN 0016-7649 Institutional support: RVO:67985831 Keywords : binary alloys * deposition * erosion * geochronology * lead * lead alloys * sea level * silicate minerals * uranium alloys * zircon Subject RIV: DB - Geology ; Mineralogy OBOR OECD: Geology Impact factor: 3.037, year: 2016

  10. 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: gnarruda@cnen.gov.br, E-mail: denilsonengseg@gmail.com, E-mail: julyanne.melo@ufpe.br, E-mail: emersonemiliano@yahoo.com.br, E-mail: ejfranca@cnen.gov.br [Centro Regional de Ciencias Nucleares do Nordeste (CRCN-NE/CNEN-PE), Recife, PE (Brazil); Souza Neto, Joao A., E-mail: adauto@ufpe.br [Universidade Federal de Pernambuco (UFPE), Recife, PE (Brazil)

    2015-07-01

    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)

  11. Polycyclic evolution of the Quadrilatero Ferrifero: an analysis based on the actual knowledge of the U-Pb geochronology and Sm-Nd isotopic geochemistry

    International Nuclear Information System (INIS)

    Carneiro, Mauricio Antonio; Noce, Carlos Mauricio; Teixeira, Wilson

    1995-01-01

    The tectonic evolution of the Quadrilatero Ferrifero region, based on geochronological U-Pb, Pb-Pb, Sm-Nd, Rb-Sr e K-Ar data, is characterized by several processes of crustal growth, which began in Middle Archean. The Lower Archean geological evolution of the Quadrilatero Ferrifero was finished by the Rio de Velhas tectono-thermal event around 2,78 Ga. After this, during the proterozoic era, three tectono-sedimentary cycles took place on this Lower Archean crustal fragment, whose products are represented by the meta sedimentary sequences of the Minas Supergroup, Itacolomi Group and Espinhaco Supergroup. The Transamazonico Event (ca. 2,0 Ga) ended the geological evolution of Minas Supergroup, but its geological records did not have the same size imprints throughout region. Many places, such as the Bonfim Metamorphic Complex, were not affected by this tectonic event. Later tectonic event. Later tectonic events (e.g. Brasiliano) had even more discrete geological occurred during the Proterozoic era, when many sedimentary basins developed (e.g. Espinhaco and Sao Francisco basins). (author)

  12. Chemical evolution of Himalayan leucogranites based on an O, U-Pb and Hf study of zircon

    Science.gov (United States)

    Hopkinson, Thomas N.; Warren, Clare J.; Harris, Nigel B. W.; Hammond, Samantha J.; Parrish, Randall R.

    2015-04-01

    Crustal melting is a characteristic process at convergent plate margins, where crustal rocks are heated and deformed. Miocene leucogranite sheets and plutons are found intruded into the high-grade metasedimentary core (the Greater Himalayan Sequence, GHS) across the Himalayan orogen. Previously-published Himalayan whole-rock data suggest that these leucogranites formed from a purely meta-sedimentary source, isotopically similar to those into which they now intrude. Bulk rock analyses carry inherent uncertainties, however: they may hide contributions from different contributing sources, and post-crystallization processes such as fluid interaction may significantly alter the original chemistry. In contrast, zircon is more able to retain precise information of the contributing sources of the melt from which it crystallises whilst its resistant nature is impervious to post-magmatic processes. This multi-isotope study of Oligocene-Miocene leucogranite zircons from the Bhutan Himalaya, seeks to differentiate between various geochemical processes that contribute to granite formation. Hf and O isotopes are used to detect discrete changes in melt source while U-Pb isotopes provide the timing of zircon crystallisation. Our data show that zircon rims of Himalayan age yield Hf-O signatures that lie within the previously reported whole-rock GHS field, confirming the absence of a discernible mantle contribution to the leucogranite source. Importantly, we document a decrease in the minimum ɛHf values during Himalayan orogenesis through time, correlating to a change in Hf model age from 1.4 Ga to 2.4 Ga. Nd model ages for the older Lesser Himalayan metasediments (LHS) that underthrust the GHS are significantly older than those for the GHS (2.4-2.9 Ga compared with 1.4-2.2 Ga), and as such even minor contributions of LHS material incorporated into a melt would significantly increase the resulting Hf model age. Hence our leucogranite data suggest either a change of source within

  13. Brief review on and prospect of the development of geochronology

    International Nuclear Information System (INIS)

    Chen Xuanhua; Dong Shuwen; Liu Dunyi; Ding Tiping; Shi Jing

    2003-01-01

    Geochronology is an important fundamental branch of modern geology, which was coming into being in the early 20 century and got rapid development in 1950s and 1960s. The common used and newly developed geochronological methods, such as U-Pb, Sm-Nd, Rb-Sr, Lu-Hf, 40 Ar/ 39 Ar, Fission Track, (U-Th)/He, TIMS U-series, cosmogenetic nuclides (including 14 C, etc.) and CFC and 3 H/ 3 He for young groundwater dating, are generally discussed in this paper on their some aspects of application and thermochronology. Through the statistics of geology and geochronological documents based on GeoRef and GDS data-base, we prospect that the geochronology in the international geologic society will get more and more important status in the near future, and suggest that the geologists in China may not neglect this kind of tendency. According to the using of geochronological methods in three GSA journals, we consider the U-Pb, 40 Ar/ 39 Ar and 14 C as the mostly used and reliable geochronometers in recent years. Also, it is recognized in this paper that thermochronology may have good future for low temperature history during mountain range exhumation and geomorphic formation, and young geological body such as ground water. (authors)

  14. Petrogenesis and zircon U-Pb dating of skarnified pyroxene-bearing dioritic rocks in Bisheh area, south of Birjand, eastern Iran

    Directory of Open Access Journals (Sweden)

    Malihe Nakhaei

    2014-10-01

    Full Text Available Introduction The study area is located 196 km south of Birjand in eastern border of the Lut block Berberian and King, 1981 in eastern Iran between 59°05′35" and 59°09′12" E longitude and 31°42′29" and 31°44′13" N latitude. The magmatic activity in the Lut block began in the middle Jurassic such as Kalateh Ahani, Shah Kuh and Surkh Kuh granitoids that are among the oldest rocks exposed within the Lut block (Esmaeily et al., 2005; Tarkian et al., 1983; Moradi Noghondar et al., 2011-2012. Eastern Iran, and particularly the Lut block, has great potential for different types of mineralization as skarnification in Bisheh area which has been studied in this paper. The goal of this study is to highlight the geochronology, geochemistry of major and trace elements, Rb-Sr, Sm-Nd isotopes for skarnified pyroxene-bearing diorites. Materials and methods Major element compositions of thirteen samples were determined by wavelength-dispersive X-ray fluorescence (XRF spectrometry, using fused discs and the Phillips PW 1410 XRF spectrometer at Ferdowsi University, Mashhad, Iran. These samples were analysed for trace elements using inductively coupled plasma-mass spectrometry (ICP-MS in the Acme Analytical Laboratories, Vancouver, British Columbia, Canada. Zircon grains were separated from pyroxene diorite porphyrys using heavy liquid and magnetic techniques at the Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan. Zircon U-Pb dating was performed by laser ablation-inductively-coupled plasma-mass spectrometry (LA-ICP-MS, using an Agilent 7500 s machine and a New Wave UP213 laser ablation system, equipped at the Dr Shen-Su Sun memorial laboratory in the Department of Geosciences, National Taiwan University, Taiwan. Strontium and Nd isotopic analyses were performed on a six-collector Finnigan MAT 261 thermal-ionization mass spectrometer at the University of Colorado, Boulder, Colorado, United States. 87Sr/86Sr ratios were determined using four

  15. Allogenic and Autogenic Signals in the Detrital Zircon U-Pb Record of the Deep-Sea Bengal Fan

    Science.gov (United States)

    Blum, M. D.; Rogers, K. G.; Gleason, J. D.; Najman, Y.

    2017-12-01

    The Himalayan-sourced Ganges-Brahmaputra river system and the deep-sea Bengal Fan represent Earth's largest sediment-dispersal system. This presentation summarizes a new detrital zircon U-Pb (DZ) provenance record from the Bengal Fan from cores collected during IODP Expedition 354, with coring sites located 1350 km downdip from the shelf margin. Each of our 15 samples were collected from medium- to fine-grained turbidite sand and, based on shipboard biostratigraphic analyses, our samples are late Miocene to late Pleistocene in age. Each sample was analyzed by LA-ICPMS at the Arizona Laserchron facility, with an average of n=270 concordant U-Pb ages per sample. Our goals are to use these data to evaluate the influence of allogenic controls vs. autogenic processes on signal propagation from source-to-sink. At the first order, large-scale sediment transfer to the Bengal Fan clearly records the strong tectonic and climatic forcing associated with the Himalayas and Ganges-Brahmaputra system: after up to 2500 km of river transport, and 1350 km of transport in turbidity currents, the DZ record faithfully represents Himalayan source terrains. The sand-rich turbidite part of the record is nevertheless biased towards glacial periods when rivers extended across the shelf in response to climate-forced sea-level fall, and discharged directly to slope canyons. However, only part of the Bengal Fan DZ record represents either the Ganges or the Brahmaputra, with most samples representing varying degrees of mixing of sediments from the two systems: this mixing, or the lack thereof, represents the signal of autogenic avulsions on the delta plain that result in the two river systems delivering sediment separately to the shelf margin, or together as they do today. Within the allogenic framework established by tectonic processes, the climatic system, and global climate-forced sea-level change, the DZ U-Pb record of sediment mixing or the lack thereof provides a fingerprint of autogenic

  16. New Zircon U-Pb Age Constrain of the Origin of Devil's River Uplift (SW Texas) and Insights into the Late Proterozoic and Paleozoic Evolution of the Southern Margin of Laurentia

    Science.gov (United States)

    Rodriguez, E.; Dickerson, P. W.; Stockli, D. F.

    2017-12-01

    The Devils River Uplift (DRU) in SW Texas records the evolution of the southern Laurentian margin from Grenvillian orogenesis and assembly of Rodinia, to its fragmentation by rifting, and to the amalgamation of Pangaea. It was cored by a well (Shell No. 1 Stewart), penetrating Precambrian gneisses and Cambrian metasediments and sandstones. New zircon LA-ICP-MS data from a total of 10 samples elucidate the crystallization and depositional ages, as well as the detrital provenance, of Precambrian and Cambrian rocks from the DRU. Zircons from five Precambrian crystalline basement samples (6000-9693') yield uniform U-Pb crystallization ages of 1230 Ma that are similar to ages for young gneisses of the Valley Spring Domain (Llano uplift) in central Texas, where they mark the cessation of arc magmatism within the Grenville orogenic belt. The 1230 Ma igneous basement is overlain by L.-M. Cambrian metasedimentary rocks ( 4000-6000') with maximum depositional ages of 533-545 Ma. Detrital zircons from Cambrian strata are dominated by a 1070-1080 Ma population, likely derived from basement units exposed in Texas (Llano uplift, Franklin Mts.), with minor contributions from local 1230 Ma Precambrian basement and the 1380-1500 Ma Granite Rhyolite Province. The L.-M. Cambrian interval is dominated (>80%) by Neoproterozoic detrital magmatic zircons with two major distinct age clusters at 570-700 Ma and 780-820 Ma, supporting a two-stage Rodinia rift model and providing strong evidence for major Cryogenian-Eocambrian intraplate magmatism along the southern margin of Rodinia. Moreover, detrital zircon signatures for L.-M. and U. Cambrian strata strongly correlate with those from the Cuyania terrane of W. Argentina - notably the W. Sierras Pampeanas (Sa. Pie de Palo, Sa. de Maz): 1230 Ma from metasandstones (PdP); 1081-1038 Ma from metasiliciclastics (PdP, SdM); Cryogenian-Eocambrian [774 & 570 Ma] plutons (SdM, PdP). In summary, these new zircon U-Pb data from DRU in SW Texas show

  17. 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

    Science.gov (United States)

    Dusel-Bacon, Cynthia; Day, Warren C.; Aleinikoff, John N.

    2013-01-01

    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.

  18. Petrotectonic characteristics, geochemistry, and U-Pb geochronology of Jurassic plutons in the Upper Magdalena Valley-Colombia: Implications on the evolution of magmatic arcs in the NW Andes

    Science.gov (United States)

    Rodríguez, G.; Arango, M. I.; Zapata, G.; Bermúdez, J. G.

    2018-01-01

    Field, petrographic, and geochemical characterization along with U-Pb zircon geochronology of the Jurassic plutons exposed in the Upper Magdalena Valley (Colombia) allowed recognizing distinct western and eastern suites formed in at least three magmatic pulses. The western plutons crop out between the eastern flank of the Central Cordillera and the Las Minas range, being limited by the Avirama and the Betania-El Agrado faults. The western suite comprises a quartz monzonite - quartz monzodiorite - quartz diorite series and subordinate monzogranites. Chemically, the rocks are high-K calc-alkaline I-type granitoids (some reaching the shoshonitic series) with metaluminous of magnesium affinity. Trace-element tectonic discrimination is consistent with magmatism in a continental arc environment. Most rocks of this suite crystallized between 195 and 186 Ma (Early Jurassic, Pliensbachian), but locally some plutons yielded younger ages between 182 and 179 Ma (Early Jurassic, Toarcian). The eastern suite crops out in the eastern margin of the Upper Magdalena Valley, east of the Betania - El Agrado fault. Plutons of this unit belong to the monzogranite series with rock types ranging between syenogranites and granodiorites. They are high-K calc-alkaline continental granitoids, some metaluminous and some peraluminous, related to I-type granites generated in a volcanic arc. Crystallization of the suite was between 173 and 169 Ma (Middle Jurassic, Aalenian-Bajocian), but locally these rocks contain zircon with earlier inherited ages related to the magmatic pulse of the western suite between 182 and 179 Ma (Early Jurassic, Toarcian). The evolution of the Jurassic plutons in the Upper Magdalena Valley is best explained by onset or increase in subduction erosion of the accretionary prism. This explains the eastward migration of the arc away from the trench. Subduction of prism sediments increased the water flux from the subducting slab, decreasing solidus temperatures, therefore

  19. Detrital zircon U-Pb and (U-Th)/He double-dating of Upper Cretaceous-Cenozoic Zagros foreland basin strata in the Kurdistan Region of northern Iraq

    Science.gov (United States)

    Barber, D. E.; Stockli, D. F.; Koshnaw, R. I.; Horton, B. K.; Tamar-Agha, M. Y.; Kendall, J. J.

    2014-12-01

    The NW Zagros orogen is the result of the multistage collisional history associated with Late Cretaceous-Cenozoic convergence of the Arabian and Eurasian continents and final closure of Neotethys. Siliciclastic strata preserved within a ~400 km segment of the NW Zagros fold-thrust belt and foreland basin in the Iraqi Kurdistan Region (IKR) provide a widespread record of exhumation and sedimentation. As a means of assessing NW Zagros foreland basin evolution and chronostratigraphy, we present coupled detrital zircon (DZ) U-Pb and (U-Th)/He geo-thermochronometric data of Upper Cretaceous to Pliocene siliciclastic strata from the Duhok, Erbil, and Suleimaniyah provinces of IKR. LA-ICP-MS U-Pb age analyses reveal that the foreland basin fill in IKR in general was dominantly derived from Pan-African/Arabian-Nubian, Peri-Gondwandan, Eurasian, and Cretaceous volcanic arc terrenes. However, the provenance of these strata varies systematically along strike and through time, with an overall increase in complexity upsection. DZ age distribution of Paleocene-Eocene strata is dominated by a ~95 Ma grain age population, likely sourced from the Late Cretaceous Hassanbag-Bitlis volcanic arc complex along the northern margin of Arabia. In contrast, DZ U-Pb age distributions of Neogene strata show a major contribution derived from various Eurasian (e.g., Iranian, Tauride, Pontide; ~45, 150, 300 Ma) and Pan-African (~550, 950 Ma) sources. The introduction of Eurasian DZ ages at the Paleogene-Neogene transition likely records the onset of Arabian-Eurasian collision. Along strike to the southeast, the DZ U-Pb spectra of Neogene strata show a decreased percentage of Pan-African, Peri-Gondwandan, Tauride, and Ordovician ages, coupled with a dramatic increase in 40-50 Ma DZ ages that correspond to Urumieh-Dokhtar magmatic rocks in Iran. Combined with paleocurrent data, this suggests that Neogene sediments were transported longitudinally southeastward through an unbroken foreland basin

  20. High-Precision U-Pb Geochronology and Correlation: An example Using the Neoproterozic-Cambrian Transition

    Science.gov (United States)

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

    2001-05-01

    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

  1. Evidence From Detrital Zircon U-Pb Analysis for Suturing of Pre-Mississippian Terranes in Arctic Alaska

    Science.gov (United States)

    Moore, T. E.; Potter, C. J.; O'Sullivan, P. B.; Aleinikoff, J. N.

    2007-12-01

    Detrital zircon U-Pb ages of pre-Mississippian sandstones were determined using SHRIMP and LA-ICPMS techniques for four key geographic parts of the Arctic Alaska terrane, northern Alaska. In the northeastern Brooks Range, a sample of quartz-rich turbidites from the Proterozoic Neroukpuk Quartzite yielded zircon ages ranging from 980 Ma to 2.9 Ga with clusters at 980-1100 Ma, 1680-1850 Ma and 2220-2660 Ma. Quartz and chert-bearing sandstone in the Tulageak well from Ordovician-Silurian argillite in basement beneath the North Slope yielded a broad spectrum of ages between 1.0 to 2.1 Ga and 2.8 Ga, including peaks at 1.0-1.2 and 1.5-1.7 Ga. Paleozoic zircons cluster at 390 and 440 Ma in this sample, indicating it is Devonian. Lithic sandstone from the Silurian Iviagik Group at Cape Dyer on the Lisburne Peninsula yielded a variety of ages from 450 to 1600 Ma, with a large peak at 475-600 Ma and several grains between 1.9 and 2.5 Ga. In contrast to the broad distributions of the latter two samples, zircons in metamorphosed Proterozoic-Cambrian(?) lithic sandstone from the an unnamed metagraywacke unit near Mt. Snowden on the Dalton Highway in the southern Brooks Range are largely 600-650 Ma with lesser clusters at 1050-1200 Ma and 1600-1900 Ga. Samples of quartz-rich Mississippian sandstone at the base of the unconformably overlying Mississippian to Triassic Ellesmerian sequence near three of the pre-Mississippian sample locations were also analyzed. Mississippian sandstones from the West Dease well (near the Tulageak well) and at Cape Dyer on the Lisburne Peninsula display zircon distributions similar to those found in the underlying pre-Mississippian samples, indicating the Mississippian clastic strata are locally derived and that the observed zircon distributions are representative of a broad area. However, the Mississippian Kekiktuk Conglomerate, which rests on the Neroukpuk Quartzite in the northeastern Brooks Range, also contains a variety of ages between 560 and

  2. Testing the reliability of information extracted from ancient zircon

    Science.gov (United States)

    Kielman, Ross; Whitehouse, Martin; Nemchin, Alexander

    2015-04-01

    Studies combining zircon U-Pb chronology, trace element distribution as well as O and Hf isotope systematics are a powerful way to gain understanding of the processes shaping Earth's evolution, especially in detrital populations where constraints from the original host are missing. Such studies of the Hadean detrital zircon population abundant in sedimentary rocks in Western Australia have involved analysis of an unusually large number of individual grains, but also highlighted potential problems with the approach, only apparent when multiple analyses are obtained from individual grains. A common feature of the Hadean as well as many early Archaean zircon populations is their apparent inhomogeneity, which reduces confidence in conclusions based on studies combining chemistry and isotopic characteristics of zircon. In order to test the reliability of information extracted from early Earth zircon, we report results from one of the first in-depth multi-method study of zircon from a relatively simple early Archean magmatic rock, used as an analogue to ancient detrital zircon. The approach involves making multiple SIMS analyses in individual grains in order to be comparable to the most advanced studies of detrital zircon populations. The investigated sample is a relatively undeformed, non-migmatitic ca. 3.8 Ga tonalite collected a few kms south of the Isua Greenstone Belt, southwest Greenland. Extracted zircon grains can be combined into three different groups based on the behavior of their U-Pb systems: (i) grains that show internally consistent and concordant ages and define an average age of 3805±15 Ma, taken to be the age of the rock, (ii) grains that are distributed close to the concordia line, but with significant variability between multiple analyses, suggesting an ancient Pb loss and (iii) grains that have multiple analyses distributed along a discordia pointing towards a zero intercept, indicating geologically recent Pb-loss. This overall behavior has

  3. Two mineralization events in the Baiyinnuoer Zn-Pb deposit in Inner Mongolia, China: Evidence from field observations, S-Pb isotopic compositions and U-Pb zircon ages

    Science.gov (United States)

    Jiang, Si-Hong; Chen, Chun-Liang; Bagas, Leon; Liu, Yuan; Han, Ning; Kang, Huan; Wang, Ze-Hai

    2017-08-01

    The Xing-Mong Orogenic Belt (XMOB) is located in the eastern part of the Central Asian Orogenic Belt (CAOB) and has experienced multiple tectonic events. The Baiyinnuoer Pb-Zn deposit may be a rare case that documents two periods of mineralization in the tectonically complex XMOB. There are two types of Pb-Zn mineralization in the deposit: (1) skarn-type ore, hosted by the skarn in the contact zone between marble and granodiorite and within the marble and (2) vein-type ore, hosted by crystal tuff and feldspar porphyry. This study revealed that the host rocks, mineral assemblages, mineralization occurrences, S-Pb isotopes, and ages between the two types of ore are notably different. Zircon U-Pb dating indicates that the granodiorite was emplaced in the Early Triassic (244 ± 1 to 242 ± 1 Ma), the crystal tuff was deposited in the Early Cretaceous (140 ± 1 to 136 ± 1 Ma), and the feldspar porphyry was intruded in the Early Cretaceous (138 ± 2 to 136 ± 2 Ma). The first skarn mineralization occurred at ∼240 Ma and the second vein-type Pb-Zn mineralization took place between 136 and 129 Ma. Thus the Triassic orebodies were overprinted by Early Cretaceous mineralization. The sphalerite and galena from the skarn mineralization have higher δ34S values (-4.7 to +0.3‰) than the sphalerite, galena and aresenopyrite from the vein-type mineralization (-7.5 to -4.2‰), indicating different sulfur sources or ore-forming processes for the two types of mineralization. The Pb isotopic compositions of the two types of ore are very similar, suggesting similar lead sources. Geochemistry and Nd-Pb-Hf isotopic systematics of the igneous rocks in the region show that the Triassic granodiorite was generated from hybridization of mafic and felsic magmas due to strong crust-mantle interaction under the collisional setting that resulted following the closure of the Paleo-Asian Ocean and the collision of North China and Siberian cratons at the end of the Permian; while the

  4. Long-lasting Cadomian magmatic activity along an active northern Gondwana margin: U-Pb zircon and Sr-Nd isotopic evidence from the Brunovistulian Domain, eastern Bohemian Massif

    Science.gov (United States)

    Soejono, Igor; Janoušek, Vojtěch; Žáčková, Eliška; Sláma, Jiří; Konopásek, Jiří; Machek, Matěj; Hanžl, Pavel

    2017-09-01

    Cadomian magmatic complexes of the Brunovistulian Domain crop out at the eastern termination of the Bohemian Massif. However, the age, nature and geotectonic affinity of some of pre-Variscan (meta-)igneous rock complexes from this domain are still unknown. Geochronological and geochemical study of the granitic rocks across the Brunovistulian Domain reveals new information about the timing and nature of this magmatic activity originally situated along the northern margin of Gondwana. Zircon U-Pb data (601 ± 3 Ma, Brno Massif; 634 ± 6 Ma, paraautochtonous core of the Svratka Dome; 568 ± 3 Ma, Bíteš orthogneiss) from the allochtonous Moravicum indicate the prolonged magmatic activity within the Brunovistulian Domain during the Ediacaran. The major- and trace-element and Sr-Nd isotopic signatures show heterogeneous geochemical characteristics of the granitic rocks and suggest a magmatic-arc geotectonic setting. The two-stage Depleted Mantle Nd model ages ( c. 1.3-2.0 Ga) indicate derivation of the granitic rocks from a relatively primitive crustal source, as well as from an ancient and evolved continental crust of the Brunovistulian Domain. These results constrain the magmatic-arc activity to c. 635-570 Ma and provide a further evidence for a long-lived (at least c. 65 Myr) and likely episodic subduction-related magmatism at the northern margin of Gondwana. The presence of granitic intrusions derived from variously mature crustal sources at different times suggests heterogeneous crustal segments to having been involved in the magmatic-arc system during its multistage evolution.

  5. Zircon U-Pb dating of early Palaeozoic monzonitic intrusives from the Goonumbla area, New South Wales

    International Nuclear Information System (INIS)

    Butera, K.M.; Williams, I.S.; Blevin, P.L.; Simpson, C.J.

    2001-01-01

    Zircon U-Pb ages measured on four small intrusions into the succession of Ordovician volcanic rocks that hosts North Parkes Cu-Au mine northwest of Parkes. New South Wales. place limits on the age of the volcanic sequence. The basal Nelungaloo Volcanics are constrained by a cross-cutting monzodiorite to be ≥484.3 ± 2.9Ma (Early Ordovician). Similarly. the overlying basal Goonumbla Volcanics are constrained by another cross-cutting monzodiorite to be ≥450.8 ± 4.2Ma (Middle Ordovician). A later generation of monzonites intruded into the middle and upper Goonumbla Volcanics yield ages of 439.1 ± 4.5 and 438.9 ± 4.7 Ma (Siluro-Ordovician). These various ages are consistent with the ages of fossiliferous sediments within the volcanic sequence. and indicate that both the intrusive and volcanic rocks span an appreciable period of time-neither are the product of a single magmatic episode. Intrusion of the youngest monzonites and mineralisation was virtually contemporaneous. Copyright (2001) Geological Society of Australia

  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

    Science.gov (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

    2016-10-01

    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. Origin and evolution of multi-stage felsic melts in eastern Gangdese belt: Constraints from U-Pb zircon dating and Hf isotopic composition

    Science.gov (United States)

    Guo, Liang; Zhang, Hong-Fei; Harris, Nigel; Pan, Fa-Bin; Xu, Wang-Chun

    2011-11-01

    This integrated study of whole rock geochemistry, zircon U-Pb dating and Hf isotope composition for seven felsic rocks from the Nyingchi Complex in eastern Himalayan syntaxis has revealed a complex magmatic history for the eastern Gangdese belt. This involves multiple melt sources and mechanisms that uniquely identify the tectonic evolution of this part of the Himalayan orogen. Our U-Pb zircon dating reveals five stages of magmatic or anatectic events: 165, 81, 61, 50 and 25 Ma. The Jurassic granitic gneiss (165 Ma) exhibits εHf(t) values of + 1.4 to + 3.5. The late Cretaceous granite (81 Ma) shows variable εHf(t) values from - 0.9 to + 6.2, indicating a binary mixing between juvenile and old crustal materials. The Paleocene granodioritic gneiss (61 Ma) has εHf(t) values of + 5.4 to + 8.0, suggesting that it originated from partial melting of a juvenile crustal material. The Eocene anatexis is recorded in the leucosome, which has Hf isotopic composition similar to that of the Jurassic granite, indicating that the leucosome could be derived from partial melting of the Jurassic granite. The late Oligocene biotite granite (25 Ma) shows adakitic geochemical characteristics, with Sr/Y = 49.3-56.6. The presence of a large number of inherited zircons and negative εHf(t) values suggest that it sourced from anatexis of crustal materials. In contrast to the Gangdese batholiths that are mainly derived from juvenile crustal source in central Tibet, the old crustal materials play an important role for the magma generation of the felsic rocks, suggesting the existence of a crustal basement in the eastern Gangdese belt. These correspond to specific magmatic evolution stages during the convergence between India and Asia. The middle Jurassic granitic gneiss resulted from the northward subduction of the Neo-Tethyan oceanic slab. The late Cretaceous magmatism is probably related to the ocean ridge subduction. The Paleocene-Eocene magmatism, metamorphism and anatexis are

  8. Calymmian magmatism in the basement of the Jauru Terrain (Rondonian - San Ignacio Province), Amazon Craton: U-Pb and Sm-Nd geochemistry and geochronology

    Energy Technology Data Exchange (ETDEWEB)

    Fachetti, Frankie James Serrano; Costa, Ana Claudia Dantas da; Silva, Carlos Humberto da, E-mail: frankiefachetti@hotmail.com, E-mail: acdcosta@ufmt.br, E-mail: chsilva@ufmt.br [Universidade Federal de Mato Grosso (UFMT), Cuiaba, MT (Brazil). Instituto de Ciencias Exatas e da Terra

    2016-11-01

    The Taquarussu Orthogneiss and the Guadalupe Granodiorite, part of the Rondonian-San Ignacio Province basement, southwest of the Amazonian Craton, correspond to oriented bodies with a NW trend. The rocks show granodiorite composition with minor occurrences of coarse grained monzogranites consisting essentially of plagioclase, quartz, microcline, orthoclase and biotite. The accessory minerals are amphibole, titanite, garnet, apatite, epidote, zircon and opaque. The geochemical data indicate that the rocks are classified as granodiorites and monzogranites, with an intermediate to acid magmatism, sub-alkaline character, from the calc-alkaline to the high-K calc-alkaline series, with alumina ratios ranging from metaluminous to lightly peraluminous. The rocks were classified as generated in volcanic islands arc environment and the U-Pb data (SHRIMP zircon) show a concord age 1575 ± 6 Ma. The Sm-Nd model age (T{sub DM}) is 1.63 Ga with εNd (t = 1.57 Ga) ranging from -1.52 to +0.78. These data indicate that these rocks are probably a juvenile crust with a possible contamination of crustal rocks. (author)

  9. Provenance and depositional age of metavolcano-sedimentary sequences of the Santa Terezinha de Goias, based on Sm-Nd and U-Pb zircon single grain

    International Nuclear Information System (INIS)

    Dantas, Ellton Luiz; Jost, Hardy; Fuck, Reinhardt A.; Brod, Jose Afonso; Pimentel, Marcio Martins; Meneses, Paulo Roberto

    2001-01-01

    Supracrustal rocks of the Santa Terezinha de Goias region, Central Brazil, presumably represent a southern extension of the Mara Rosa magmatic arc tectonically juxtaposed to Archean terrains and consist of a meta volcanic unit and a meta sedimentary unit. The aim of the paper is to present and discuss the first U-Pb and Sm-Nd data of rocks belonging to both units of that region. U-Pb data of zircons from a felsic meta volcanic rock inter layered with chlorite-rich schists (metandesites?) yield a concordant age of of 660 My, while the Sm-Nd model age of a variety of chlorite-schists indicate a juvenile age between 1.1 and 1.3 Ga. These data indicate that the meta volcanic rocks are Neo proterozoic in age and may be correlated with rocks of the Mara Rosa magmatic arc. On the other hand, the prevailing provenance of rocks belonging to the meta sedimentary unit indicate a Paleoproterozoic, or older, source. Thus, the deposition of both units are explained by means of the erosion of source-areas of variable age. (author)

  10. Geochemical features and U-Pb dating of trachyandesites of the Ktuts river basin

    International Nuclear Information System (INIS)

    Sahakyan, L.H.

    2015-01-01

    The article deals with the results on geochemical (including isotopes) and U-Pb dating on zircon minerals of trachyandesites from the r. Ktuts basin. Shoshonite series trachyandesites normalised by chondrites have mobile elements enrichment (Rb, Ba, and Th) together with negative HFSE (Nb, Ta) anomalies. The (La/Sm)CN ratio is 6.84 value but the (La/Yb)CN ratio is 38.17, suggesting the presence of residual material at the magmatic source from the deep parts. Neodymium and strontium isotopes have low εNd_(14.5Ma) and high 87"Sr/86"Sr(14.5Ma) ratios, respectively –0.4 and 0.7054. The trachyandesites age determined on the zircon minerals by the U-Pb method is 14,50,2 Ma, which coincides with the magmatism reactivation in the middle-upper Miocene after the collision of Arabic and European plates in the upper Eocene-Oligocene

  11. Detrital zircon ages from southern Norway - implications for the Proterozoic evolution of the southwestern Baltic Shield

    Science.gov (United States)

    Knudsen, T.-L.; Andersen, T.; Whitehouse, M. J.; Vestin, J.

    An ion-microprobe (SIMS) U-Pb zircon dating study on four samples of Precambrian metasediments from the high-grade Bamble Sector, southern Norway, gives the first information on the timing of discrete crust-forming events in the SW part of the Baltic Shield. Recent Nd and Pb studies have indicated that the sources of the clastic metasediments in this area have crustal histories extending back to 1.7 to 2.1Ga, although there is no record of rocks older than 1.6Ga in southern Norway. The analysed metasediments are from a sequence of intercalated, centimetre to 10-metre wide units of quartzites, semi-metapelites, metapelites and mafic granulites. The zircons can be grouped in two morphological populations: (1) long prismatic; (2) rounded, often flattened. The BSE images reveal that both populations consist of oscillatory zoned, rounded and corroded cores (detrital grains of magmatic origin), surrounded by homogeneous rims (metamorphic overgrowths). The detrital zircons have 207Pb/206Pb ages between 1367 and 1939Ma, with frequency maxima in the range 1.85 to 1.70Ga and 1.60 to 1.50Ga. There is no correlation between crystal habit and age of the zircon. One resorbed, inner zircon core in a detrital grain is strongly discordant and gives a composite inner core-magmatic outer core 207Pb/206Pb age of 2383 Ma. Two discrete, unzoned zircons have 207Pb/206Pb ages of 1122 and 1133Ma, representing zircon growth during the Sveconorwegian high-grade metamorphism. Also the μm wide overgrowths, embayments in the detrital cores and apparent ``inner cores'' which represent secondary metamorphic zircon growth in deep embayments in detrital grains, are of Sveconorwegian age. The composite-detrital-metamorphic zircon analyses give generally discordant 206Pb/238U versus 207Pb/235U ratios and maximum 207Pb/206Pb ages of 1438Ma. These data demonstrate the existence of a protocrust of 1.7 to 2.0Ga in the southwestern part of the Baltic Shield, implying a break in the overall westward

  12. U–Pb geochronology and geochemistry of late Palaeozoic volcanism in Sardinia (southern Variscides

    Directory of Open Access Journals (Sweden)

    L. Gaggero

    2017-11-01

    Full Text Available The latest Carboniferous to lower Permian volcanism of the southern Variscides in Sardinia developed in a regional continental transpressive and subsequent transtensile tectonic regime. Volcanism produced a wide range of intermediate–silicic magmas including medium- to high-K calc-alkaline andesites, dacites, and rhyolites. A thick late Palaeozoic succession is well exposed in the four most representative Sardinian continental basins (Nurra, Perdasdefogu, Escalaplano, and Seui–Seulo, and contains substantial stratigraphic, geochemical, and geochronological evidence of the area's complex geological evolution from the latest Carboniferous to the beginning of the Triassic. Based on major and trace element data and LA-ICP-MS U–Pb zircon dating, it is possible to reconstruct the timing of post-Variscan volcanism. This volcanism records active tectonism between the latest Carboniferous and Permian, and post-dates the unroofing and erosion of nappes in this segment of the southern Variscides. In particular, igneous zircon grains from calc-alkaline silicic volcanic rocks yielded ages between 299 ± 1 and 288 ± 3 Ma, thereby constraining the development of continental strike-slip faulting from south (Escalaplano Basin to north (Nurra Basin. Notably, andesites emplaced in medium-grade metamorphic basement (Mt. Cobingius, Ogliastra show a cluster of older ages at 332 ± 12 Ma. Despite the large uncertainty, this age constrains the onset of igneous activity in the mid-crust. These new radiometric ages constitute: (1 a consistent dataset for different volcanic events; (2 a precise chronostratigraphic constraint which fits well with the biostratigraphic data and (3 insights into the plate reorganization between Laurussia and Gondwana during the late Palaeozoic evolution of the Variscan chain.

  13. Role of zircon in tracing crustal growth and recycling

    Science.gov (United States)

    Compston, W.; Williams, I. S.; Armstrong, R. A.; Claoue-Long, J. C.; Kinny, P. D.; Foster, J. J.; Kroener, A.; Pidgeon, R. T.; Myers, J. S.

    Single crystal ion probe ages of zircons is discussed, which allow much better time resolution compared to other geochronological methods, although the technique is not without problems. Rocks from two areas that contain composite zircon populations, including true magmatic zircons as well as a variety of xenocrystic types are described. It is often difficult to distinguish these; xenocrystic zircons, for example, cannot always be identified on the basis of morphology alone. Additional evidence is needed before making age interpretations. Evidence is also presented of zircon growth long after the original time of crystallization, in some cases apparently at temperatures less than 300 C. The spectacular discovery of 4.1 to 4.2 Ga detrital zircons in metaquartzites from the Mount Narryer area of Western Australia is described. Similar zircons with ages as old as 4276 Ma have been found in the nearby Jack Hills area. The source areas or parent lithologies of these zircons have not yet been determined, but the author expects that they may be unrecognized or buried antecedents of the K rich Narryer gneisses. U or Th concentrations of zircon cannot be used to discriminate between felsic and mafic source rocks.

  14. Geology, mineralization, Rb-Sr & Sm-Nd geochemistry, and U–Pb zircon geochronology of Kalateh Ahani Cretaceous intrusive rocks, southeast Gonabad

    Directory of Open Access Journals (Sweden)

    Mohammad Hassan Karimpour

    2013-10-01

    Full Text Available Kalateh Ahani is located 27 km southeast of Gonabad within the Khorasan Razavi province. The area is part of Lut Block. Sub-volcanic monzonitic rocks intruded regional metamorphosed Shemshak Formation (Jurassic age. Magnetic susceptibility of less altered monzonitic rocks is 0.6%., As, Pb and Zn > 1%, Au up to 150 ppb and Sn = 133 ppm. The Sn content of vein in the northern part of Kalateh Ahani (Rud Gaz is > 1%. Based on mineralization, alteration and geochemistry, it seems that Sn mineralization is associated with the Cretaceous monzonitic rocks. Zircon U–Pb dating indicates that the age of the monzonitic rocks associated with mineralization is 109 Ma (Lower Cretaceous. Based on (87Sr/86Sri = 0.71089-0.710647 and (143Nd/144Ndi = 0.512113-0.51227 of the monzonitic rocks, the magma for these rocks were originated from the continental crust. This research has opened new window with respect to Sn-Cu mineralization and exploration within the Lut Block which is associated with Cretaceous granitoid rocks (reduced type, ilmenite series originated from the continental crust.

  15. What can zircon ages from the Jack Hills detrital zircon suite really tell us about Hadean geodynamics?

    Science.gov (United States)

    Whitehouse, Martin; Nemchin, Alexander

    2015-04-01

    As the only direct sample of the Hadean Earth, detrital zircon grains from the Jack Hills, Western Australia, have been the subject of intense investigation over the almost three decades since their discovery. A wide variety of geochemical and isotopic analyses of these grains, as well as their mineral inclusions, have been used variously to support two fundamentally different models for Hadean geodynamics: (i) Some form of (not necessarily modern-style) plate recycling generating felsic (continental-type?) crust at the boundaries [1, 2], or conversely (ii) the persistence of a long-lived, stagnant basaltic lid within which magmatism occurred as a result of internal temperature perturbations and/or impacts [3, 4], a model also generally consistent with a wide range of observations from post-Hadean geochemical reservoirs. Despite the considerable time and resources expended, the majority of these studies uncritically accept the individual U-Pb zircon ages, even though their veracity is key to many of the interpretations [5, 6]. We report here the results of an in-depth evaluation of all published (and new) U-Pb ages from the Jack Hills zircon suite in order to define age populations that can be used with a high degree of confidence in geodynamic interpretations. A notable problem in the interpretation of U-Pb data from ancient zircon grains (including those as young as the Neoarchean) is that disturbance of the systematics even several 100 Ma after crystallization causes data to spread along the concordia curve without becoming discernably discordant within the relatively large error bounds associated with U/Pb ages from in situ dating methods (e.g. SIMS). While 207Pb/206Pb ages are typically more precise, individually they provide no means to detect Pb-loss-induced younging. However, if two or preferably more analyses have been made in the same zircon growth zone, a reasonable evaluation of the possibility of Pb-loss can be made. In the available Jack Hills zircon

  16. Recognition of the Kaweka Terrane in northern South Island, New Zealand : preliminary evidence from Rb-Sr metamorphic and U-Pb detrital zircon ages

    International Nuclear Information System (INIS)

    Adams, C.J.; Mortimer, N.; Campbell, H.J.; Griffin, W.L.

    2011-01-01

    Detrital zircon U-Pb ages and Rb-Sr metamorphic ages from low-grade Torlesse Supergroup metasedimentary rocks from North Canterbury and Marlborough provide preliminary evidence for a continuation of the Kaweka Terrane of the central North Island into the Torlesse Composite Terrane of the South Island. This would extend from the south side of the Wairau Fault in the upper Wairau River valley southwards to the Lake Tennyson and Lake Sumner areas, and as far as Hawarden. Rb-Sr ages indicate Jurassic metamorphism, 170±24 Ma, with initial 87 Sr/ 86 Sr ratios at that time 0.7073 ± 0.0007 i.e. similar to their North Island counterparts. These Kaweka Terrane rocks have detrital zircon ages that follow the distinctive pattern of the Torlesse rocks in general, i.e. substantial (>30%) Permian-Triassic and Precambrian-Early Palaeozoic groupings, but they also have minor youngest age components c. 175-165 Ma which constrain a maximum Early-Middle Jurassic depositional age. In detail, significant, older zircon components are Late Permian-Early Triassic (260-240 Ma) and Late Triassic (220-210 Ma), but the Precambrian-Early Palaeozoic detrital zircon ages are more scattered. The Kaweka Terrane thus forms a linear belt (c. 500 km) along the length of the Torlesse Composite Terrane in both North and South Islands. In the South Island it coincides with the Esk Head Belt, i.e. east of the Rakaia Terrane (Triassic-Late Permian) and west of the Pahau Terrane (Early Cretaceous-Late Jurassic). (author). 61 refs., 4 figs., 2 tabs.

  17. Timing of metamorphism and exhumation in the Nordøyane ultra-high-pressure domain, Western Gneiss Region, Norway: New constraints from complementary CA-ID-TIMS and LA-MC-ICP-MS geochronology

    Science.gov (United States)

    Butler, J. P.; Jamieson, R. A.; Dunning, G. R.; Pecha, M. E.; Robinson, P.; Steenkamp, H. M.

    2018-06-01

    We present the results of a combined CA-ID-TIMS and LA-MC-ICP-MS U-Pb geochronology study of zircon and associated rutile and titanite from the Nordøyane ultra-high-pressure (UHP) domain in the Western Gneiss Region (WGR) of Norway. The dated samples include 4 eclogite bodies, 2 host-rock migmatites, and 2 cross-cutting pegmatites and leucosomes, all from the island of Harøya. Zircon from a coesite eclogite yielded an age of ca. 413 Ma, interpreted as the time of UHP metamorphism in this sample. Zircon data from the other eclogite bodies yielded metamorphic ages of ca. 413 Ma, 407 Ma, and 406 Ma; zircon trace-element data associated with 413 Ma and 407 Ma ages are consistent with eclogite-facies crystallization. In all of the eclogites, U-Pb dates from zircon cores, interpreted as the times of protolith crystallization, range from ca. 1680-1586 Ma, consistent with Gothian ages from orthogneisses in Nordøyane and elsewhere in the WGR. A zircon core age of ca. 943 Ma from one sample agrees with Sveconorwegian ages of felsic gneisses and pegmatites in the western part of the area. Migmatites hosting the eclogite bodies yielded zircon core ages of ca. 1657-1591 Ma and rim ages of ca. 395-392 Ma, interpreted as the times of Gothian protolith formation and Scandian partial melt crystallization, respectively. Pegmatite in an eclogite boudin neck yielded a crystallization age of ca. 388 Ma, interpreted as the time of melt crystallization. Rutile and titanite from 3 samples (an eclogite and two migmatites) yielded concordant ID-TIMS ages of 378-376 Ma. The results are similar to existing U-Pb data from other Nordøyane eclogites (415-405 Ma). In combination with previous pressure-temperature data from the coesite eclogite, these ages indicate that peak metamorphic conditions of 3 GPa/760 °C were reached ca. 413 Ma, followed by decompression to 1 GPa/810 °C by ca. 397 Ma and cooling below ca. 600 °C by ca. 375 Ma. The results are compatible with protracted UHP

  18. Zircon U-Pb dating of Maherabad porphyry copper-gold prospect area: evidence for a late Eocene porphyry-related metallogenic epoch in east of Iran

    Directory of Open Access Journals (Sweden)

    Azadeh Malekzadeh Shafaroudi

    2011-04-01

    Full Text Available Eastern Iran has great potential for porphyry copper deposits, as a result of its past subduction zone tectonic setting that lead to extensive alkaline to calc-alkaline magmatic activity in Tertiary time. Maherabad is the first porphyry Cu-Au prospecting area which is discovered in eastern Iran. This is related to a succession o f monzonitic to dioritic porphyries stocks that were emplaced within volcanic rocks. Monzonitic porphyries have basic role in mineralization. Hydrothermal alteration zones are well developed including potassic, sericitic-potassic, quartz-sericite-carbonate-pyrite, quartz-carbonate-pyrite, silicified-propylitic, propylitic, carbonate and silicified zones. Mineralization occurs as Disseminated, stockwork and hydrothermal breccia. Based on early stage of exploration, Cu is between 179- 6830 ppm (ave. 3200 ppm and Au is up to 1000 ppb (ave. 570 ppb. This prospect is gold- rich porphyry copper deposit. Laser-ablation U-Pb dating of two samples from ore-related intrusive rocks indicate that these two monzonitic porphyries crystallized at 39.0 ± 0.8 Ma to 38.2 ± 0.8 Ma, within a short time span of less than ca. 1 Ma during the middle Eocene. This provides the first precise ages for metallogenic episode of porphyry-type mineralization. Also, the initial 87Sr/86Sr and (143Nd/144Ndi was recalculated to an age of 39 Ma. Initial 87Sr/86Sr ratios for monzonitic rocks are 0.7047-0.7048. The (143Nd/144Ndi isotope composition are 0.512694-0.512713. Initial ε Nd isotope values 1.45-1.81. Based on isotopic data the magma had originated beyond the continental crust. The study will be used for tectonic-magmatic setting and evolution of eastern Iran. Keywords: Lut block, Middle Eocene, Zircon, Geochronology, Laser ablation ICP-MS,

  19. Carboniferous-Permian volcanic evolution in Central Europe-U/Pb ages of volcanic rocks in Saxony (Germany) and northern Bohemia (Czech Republic)

    Czech Academy of Sciences Publication Activity Database

    Hoffmann, U.; Breitkreuz, Ch.; Breiter, Karel; Sergeev, S.; Stanek, K.; Tichomirowa, M.

    2013-01-01

    Roč. 102, č. 1 (2013), s. 73-99 ISSN 1437-3254 Institutional support: RVO:67985831 Keywords : pyroclastic rocks * dykes * stratigraphy * SHRIMP U/Pb ages * Pb/Pb single zircon age * Variscides Subject RIV: DB - Geology ; Mineralogy Impact factor: 2.084, year: 2013

  20. New Constraints for Tectono-Thermal Alpine Evolution of the Pyrenees: Combining Zircon Fission-Track and (U-Th)/He Analyses with Raman Spectrometry and In-Situ K-Ar Geochronology

    Science.gov (United States)

    Waldner, M.; Bellahsen, N.; Mouthereau, F.; Pik, R.; Bernet, M.; Scaillet, S.; Rosenberg, C.

    2017-12-01

    The pyrenean range was formed by the convergence of European and Iberian plates following the inversion of the Mesozoic rifting in the north of Pyrenees. In the Axial Zone, the collision caused an antiformal nappe-stacking of tectonic units. Recent studies pointed out the importance of pre-collision structural and thermal inheritance that may play a major role for orogeny such as: 1) Paleozoic Variscan inheritance; 2) Mesozoic rift-related high geothermal gradients, which are maintained during the onset of convergence in the North Pyrenean Zone. From a mineralogical point of view, pre-collision feldspars have been destabilized and influenced the development of alpine phyllonite in brittle-ductile conditions which suggests a weak crustal behavior during the formation of the orogenic wedge. Our aim is to get a better understanding of alpine deformation and exhumation by coupling different thermochronological, geochronological and thermometric methods. We document the thermal evolution of each tectonic unit by using low-temperature thermochronometers (Zircon Fission Tracks, U-Th/He on zircons including laser ablation profiles). Our data on vertical profiles combined to existing dataset on apatite allows to model alpine exhumation across the Axial zone. Structural observations through alpine thrusts coupled to geochronology (in situ K/Ar on phengites), Raman and chlorite-phengite thermo(baro)metry provide new key data to unravel the alpine evolution of the Pyrenees. According to preliminary ZFT results on granite massifs in the central part of Pyrenean Axial zone (near ECORS profile), exhumation ages potentially indicates a migration of exhumation towards the south. Exhumation ages of the northern massifs seems to have preserved the North Pyrenean Cretaceous rift evolution. Further south, the onset of exhumation is as old as Paleocene, which precedes the Eocene ages of the literature. The low burial estimated in the northern massifs may indicate a high thermal gradient

  1. In-Situ Geochronology: Extending Larims to Pb-Pb Isocrhons

    Science.gov (United States)

    Whitaker, Tom; Anderson, Scott; Levine, Jonathan

    2016-04-01

    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. U-Pb zircon and 40Ar/39Ar geochronology of sericite from hydrothermal alteration zones: new constraints for the timing of Ediacaran gold mineralization in the Sukhaybarat area, western Afif terrane, Saudi Arabia

    Science.gov (United States)

    Harbi, Hesham M.; Ali, Kamal A.; McNaughton, Neal J.; Andresen, Arild

    2018-04-01

    The Sukhaybarat East and Red Hill deposits, in the northeastern part of the Arabian Shield, are mesothermal vein-type gold deposits hosted by late Cryogenian-Ediacaran intrusive rocks of the Idah suites (diorite, tonalite, granodiorite) and, at Sukhaybarat East, also by Ediacaran metasedimentary rocks. Gold mineralization comprises quartz-arsenopyrite veins (Sukhaybarat East), quartz-carbonate-pyrite veins (Red Hill), and subordinate gold-base metal sulfide veins. In the Red Hill deposit, alteration is complicated due to multiple overprinting hydrothermal events and is characteristically affected by pervasive, pink quartz-K-feldspar-hematite alteration which is overprinted by potassic alteration characterized by a quartz-biotite-carbonate-muscovite/sericite-rutile-apatite assemblage. This assemblage is associated with molybdenite veins which appear to form late in the paragenetic sequence and may represent either evolution of the ore fluid composition, or a later, unrelated mineralized fluids. Hydrothermal alteration at the Sukhaybarat East deposit is dominated by quartz-carbonate-sericite-arsenopyrite assemblages. Zircon from ore-hosting tonalite at Sukhaybarat East yields a U-Pb age of 629 ± 6 Ma, and biotite from the same rock gives an 40Ar/39Ar age of 622 ± 23 Ma. The 40Ar/39Ar age is within the uncertainty range for the U-Pb age of the host intrusion and is interpreted as a minimally disturbed cooling age for the tonalite. In the Red Hill area, granodiorite was emplaced at 615 ± 5 Ma, whereas muscovite/sericite separated from a mineralized sample of a quartz-carbonate-pyrite vein, that was overprinted by molybdenite-bearing veinlets, yields an 40Ar/39Ar age of 597 ± 8 Ma. We interpreted this age to represent the maximum age of the molybdenite mineralization and the probable minimum age of gold mineralization in the Red Hill deposit.

  3. LA-ICP-MS zircon U-Pb geochronology to constrain the age of post-Neocomian continental deposits of the Cerrillos Formation, Atacama Region , northern Chile: tectonic and metallogenic implications Geocronología U-Pb en circón por LA-ICP-MS para circunscribir la edad de depósitos continentales post-neocomianos de la Formacion Cerrillos, Región de Atacama, norte de Chile: implicancias tectónicas y metalogénicas

    Directory of Open Access Journals (Sweden)

    Víctor Maksaev

    2009-07-01

    Full Text Available New U-Pb zircon dating of volcamc intercalations in the lower conglomeratic part of the Cerrillos Formation shows that its deposition extended in time at least from 110.7±1.7 to 99.7±1.6 Ma. The significantly younger U-Pb zircon age of 69.5±1.0 Ma obtained for the upper volcamc part of the Cerrillos Formation suggests recurrence of volcanism in the Late Cretaceous instead of continual volcanic activity. A minimum late Maastrichtian age for the Cerrillos Formation and its initial deformation was determined by the U-Pb zircon age range from 66.9±1.0 to 65.2±1.0 Ma for the lower part of the unconformably overlying Hornitos Formation. The new U-Pb data for the Cerrillos and Hornitos formations poses questions about the Campanian-Maastrichtian age range currently ascribed to the latter. The lower part of the Cerrillos Formation represents a major change in the sedimentary regime from previous marine carbonate sedimentation in a back-arc basin until the late Aptian to subsequent coarse alluvial sedimentation and volcanism since the early Albian. The lower part of the Cerrillos Formation is interpreted as the development of coalescent alluvial fans thinning inland, accompanied by volcanism. These developed as the result of transpressive deformation and uplift of the area of the current Coastal Cordillera by late Aptian, leading to subsequent active erosion and sedimentation inland, along with the eastward shift of the magmatic foci in the Región . Amineralizing period of Cu-Au porphyries overlaps in time with the deposition of the Cerrillos Formation in northern Chile; marking also a significant change in the metallogeny of the Andes of northern Chile.Nuevas dataciones U-Pb obtenidas para intercalaciones volcánicas en la parte inferior conglomerádica de la Formacion Cerrillos muestran que su depositacion se extendió en el tiempo al menos desde 110,7±1,7 hasta 99,7±1,6 Ma. La edad U-Pb significativamente más joven de 69,5±1,0 Ma obtenida

  4. Late magmatic stage of the zoned Caleu pluton (Central Chile): insights from zircon crystallization conditions

    Science.gov (United States)

    Molina, P. G.; Parada, M.; Gutierrez, F. J.; Chang-Qiang, M.; Jianwei, L.; Yuanyuan, L.

    2012-12-01

    The Caleu pluton consists of three N-S elongated lithological zones: Gabbro-Diorite Zone (GDZ), Tonalite Zone (TZ) and Granodiorite Zone (GZ); western, middle and eastern portions of the pluton, respectively. The zones are thought to be previously differentiated in a common, isotopically depleted (Sr-Nd), subjacent magma reservoir at a 4 kbar equivalent depth. The emplacement should have occurred at the climax of the Cretaceous rifting. We present preliminary results of U238/Pb206 zircon geochronology; zircon saturation, Tsat(Zrn), and crystallization temperatures (Ti-in-Zrn); as well as relative oxidation states at time of crystallization, based on: (i) the sluggish REE and HFSE subsolidus diffusivities in zircon; (ii) the behavior of Ti4+↔Si4+ and Ce4+↔Zr4+ isovalent replacement, in addition to a constrained TiO2 activity in almost all typical crustal rocks; and (iii) relative oxidation states at time of crystallization, respectively. The latter are obtained by interpolation of the partition coefficients of trivalent (REE) and tetravalent (HFSE) curves in Onuma diagrams for each zircon, and then estimating relative Ce(IV)/Ce(III) ratios. Results obtained from 4 samples (a total of 77 zircon grains) collected from the three mentioned lithological zones indicate U/Pb ages of approximately 99.5 ±1.5 Ma, 96.8 ±0.6 Ma, and 94.4 +2.2 -0.8 Ma; and Ti-in Zrn ranges of ca. 720-870°C, ca. 680-820°C and ca. 750-840°C, for the GDZ, TZ and GZ samples, respectively. On the other hand Tsat(Zrn) of ca. 750-780°C in the TZ, and ca. 830-890°C in the GZ, were obtained. As expected saturation temperatures are similar or higher than Ti-in-Zrn obtained in zircon grains of TZ and GZ, respectively. Cathodoluminiscence images in zircon suggest a magmatic origin, due to absence of complex zoning patterns and fairly well conserved morphologies. Exceptionally the GDZ sample zircons show evidence of inheritance, indicating a xenocrystic and/or antecrystic origin. A relative Ce

  5. Elemental and Isotopic Tomography at Single-Atom-Scale in 4.0 and 2.4 Ga Zircons

    Science.gov (United States)

    Valley, J. W.; Reinhard, D. A.; Snoeyenbos, D.; Lawrence, D.; Martin, I.; Kelly, T. F.; Ushikubo, T.; Strickland, A.; Cavosie, A. J.

    2012-12-01

    Atom probe tomography can determine identity (mass/charge ratio) and 3-D position of individual atoms in minerals such as zircon. These data provide unique information for understanding the thermal history and mechanisms of mineral reaction and exchange, including radiation damage. Nine needle-shaped specimens ~100 nm in diameter (at the apex) were sampled from 2 zircons by FIB and analyzed with a local-electrode atom probe (LEAP), CAMECA LEAP 4000X HR. The LEAP uses pulsed-laser heating to field evaporate the tip of a zircon needle and accelerates the ions into a position-sensitive TOF-MS. With due care for complex isobaric interferences (molecules, multiple ionizations) and background correction, it is possible to individually identify up to 10E8 atoms/needle (36% detection efficiency) by mass/charge (MRP ~ 1000@ m/n=16Da) and position (X-Y-Z coordinates on 0.2 nm scale) (Kelly & Larson 2012). The 3-D distribution of Pb and Y differ at atom-scale in the 2 zircons. Zircon #1 (4007 Ma, Jack Hills, W. Australia, Cavosie 2005, Ushikubo et al. 2008, Bouvier et al. 2011) is homogeneous in Pb and Y. In contrast, incompatible elements, including Pb and Y, are concentrated in equant 5-10 nm dia. domains, spaced ~50 nm apart in zircon #2 (2438 Ma, Albion-Raft R-Grouse Ck core complex, Utah, Strickland et al. 2011). U is homogeneously distributed in both zircons. The analyzed domains suffered 4-8 x 10E15 α-decay events/mg due to U and Th decay and yet both zircons yield >97% concordant U-Pb ages by SIMS, suggesting annealing of radiation damage during the life of the zircons. The 207-Pb/206-Pb ratios for these nm-scale domains, as measured by LEAP, average 0.17 for the 2.4 Ga Zrc2 (3 needles) and 0.43 for the 4.0 Ga Zrc1 (5 needles). These ratios are less precise (±40% 2σ) due to ultra-small sample size, but are in excellent agreement with values measured by SIMS, 0.1684 and 0.4269, respectively. Thus Pb in both zircons is radiogenic. The Pb-Y-rich domains and lack of

  6. U-Pb and Rb-Sr geochronology of the Wedgeport granitoid pluton, southwestern Nova Scotia

    International Nuclear Information System (INIS)

    Cormier, R.F.; Keppie, J.D.; Odom, A.L.

    1988-01-01

    Zircons from biotite monzogranite of the Wedgeport Pluton, intrusive into deformed metasediments of the Cambrian(?) Goldenville formation in the southwestern Meguma Terrane of Nova Scotia, yield concordant U-Pb ages of 316 ± 5 Ma. This is interpreted as the time of intrusion and crystallization. Within the error limits, the 323 ± 0.0056 Rb-Sr analyses of mineral separates of biotite, potassium feldspar, and quartz-plagioclase from several samples yield subparallel, internal isochrons with an average age of 257 ± 8 Ma. Initial ratios of the internal isochrons range from 0.716 to 0.759. A slow-cooling model for the latter data is discarded because the mineral data fall on straight lines. Instead, a reheating event related to plutonism ca 257 Ma ago, which was sufficient to cause local grain-to-grain migration and reequilibration of strontium and rubidium but not large-scale redistribution, is invoked. This reheating is also inferred to be responsible for the hydrothermal alteration and Sn-U mineralization concentrated along the northwestern margin of the pluton. A dextral northeast-southeast shear zone cutting the pluton is also inferred to be ca. 257 Ma old. It may be related to the last stages of westward abduction of the Meguma Terrane. These results provide a clear example of Permo-Carboniferous plutonism in the southwestern Meguma Terrane and suggest a similar interpretation may apply to other anomalously young ages recorded in this area. In light of these results, the Permo-Carboniferous age of the large East Kemptville tin deposit and its location in a dextral shear zone suggest that the association of younger plutonism and shear zones may be a significant factor for economic mineralization. (22 refs., 4 tabs., 3 figs.)

  7. Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

    Science.gov (United States)

    Kunz, Barbara E.; Regis, Daniele; Engi, Martin

    2018-03-01

    Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U-Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P-T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U-Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure.

  8. Geology and U-Pb geochronology of the Kipawa Syenite Complex - a thrust related alkaline pluton - and adjacent rocks in the Grenville Province of western Quebec

    Energy Technology Data Exchange (ETDEWEB)

    Van Breemen, O.; Currie, K.L. [Geological Survey of Canada, Ottawa, Ontario (Canada)]. E-mail: ovanbree@nrcan.gc.ca

    2004-07-01

    The Kipawa Syenite Complex, a thin, folded sheet of amphibole syenite, quartz syenite and minor nepheline syenite, lies along a west-vergent thrust separating a lower slice comprising the Kikwissi granodiorite and biotite tonalite dated at 2717 {sub -11}{sup +15} Ma, and unconformably overlying metasedimentary rocks from an overlying slice containing the Red Pine Chute orthogneiss, an alkali granite gneiss, and the Mattawa Quartzite. The syenite complex, dated at 1033 {+-} 3 Ma, lies within the lower slice but has metasomatically altered the overlying slice. Texturally guided U-Pb spot analyses on partially metasomatised zircons from the alkali granite gneiss yield a cluster of {sup 207}G'Pb/{sup 206}Pb ages at 1389 {+-} 8 Ma, interpreted as the time of igneous crystallization and four ages overlapping the time of syenite emplacement, interpreted as in situ, metasomatic growth. The highest structural slice comprises garnet amphibolite separated from lower slices by the Allochthon Boundary Thrust. Metamorphic grade increases upward from greenschist grade in the biotite tonalite to amphibolite grade (690 {sup o}C, 9 kbar (1 kbar = 100 MPa)) at the lower boundary of the alkali granite. Emplacement of the Kipawa Syenite Complex took place after assembly of the thrust stack had begun and after emplacement of the allochthon or hot slab responsible for the inverted metamorphic gradient. Origin of the syenite is tentatively ascribed to anatexis of material metasomatized by flow of alkaline solutions along a major shear surface. Crystallization of new zircon in the margins of the syenite shows that metasomatism continued from ca. 1035 to 990 Ma, redistributing alkalies, fluorine, rare-earth elements and zirconium. (author)

  9. Evidence for Late Triassic provenance areas and Early Jurassic sediment supply turnover in the Barents Sea Basin of northern Pangea

    Czech Academy of Sciences Publication Activity Database

    Klausen, T. G.; Müller, R.; Sláma, Jiří; Helland-Hansen, W.

    2017-01-01

    Roč. 9, č. 1 (2017), s. 14-28 ISSN 1941-8264 Institutional support: RVO:67985831 Keywords : U-Pb geochronology * detrial zircon * Cenozoic evolution * rocks * metamorphism Subject RIV: DB - Geology ; Mineralogy OBOR OECD: Geology Impact factor: 2.662, year: 2016

  10. Age determination of Precambrian rocks from Greenland: past and present

    International Nuclear Information System (INIS)

    Karlsbeek, F.

    1997-01-01

    For Precambrian rocks as well as for younger igneous intrusions, precise ages can only be obtained by isotopic analysis of minerals or rocks. Isotopic (radiometric) age determination of Greenland rocks began in the early 1960s and has continued since with gradually improving methods. In this contribution, the development of geochronological knowledge of the Precambrian of Greenland is described in historical perspective, and an outline of new results is given. The history of geochronology can be roughly divided into three periods: 1) a period of single-sample K-Ar and Rb-Sr mineral or whole-rock age determinations; 2) a time when most ages were determined with the help of Rb-Sr and Pb-Pb whole-rock isochrones and multi-grain zircon U-Pb isotope data; 3) the present, where 'single' zircon U-Pb data are the preferred method to obtain rock ages. These stages in the development of radiometric dating methods partly overlap in time, and each has yielded very significant contributions to the knowledge of Precambiran evolution in Greenland. (EG)

  11. Geochemistry, mineralogy, and zircon U-Pb-Hf isotopes in peraluminous A-type granite xenoliths in Pliocene-Pleistocene basalts of northern Pannonian Basin (Slovakia)

    Science.gov (United States)

    Huraiová, Monika; Paquette, Jean-Louis; Konečný, Patrik; Gannoun, Abdel-Mouhcine; Hurai, Vratislav

    2017-08-01

    Anorogenic granite xenoliths occur in alkali basalts coeval with the Pliocene-Pleistocene continental rifting of the Pannonian Basin. Observed granite varieties include peraluminous, calcic to peralkalic, magnesian to ferroan types. Quartz and feldspars are dominant rock-forming minerals, accompanied by minor early ilmenite and late magnetite-ulvöspinel. Zircon and Nb-U-REE minerals (oxycalciopyrochlore, fergusonite, columbite) are locally abundant accessory phases in calc-alkalic types. Absence of OH-bearing Fe, Mg-silicates and presence of single homogeneous feldspars (plagioclase in calcic types, anorthoclase in calc-alkalic types, ferrian Na-sanidine to anorthoclase in alkalic types) indicate water-deficient, hypersolvus crystallization conditions. Variable volumes of interstitial glass, absence of exsolutions, and lacking deuteric hydrothermal alteration and/or metamorphic/metasomatic overprint are diagnostic of rapid quenching from hypersolidus temperatures. U-Pb zircon ages determined in calcic and calc-alkalic granite xenoliths correspond to a time interval between 5.7 and 5.2 Ma. Positive ɛHf values (14.2 ± 3.9) in zircons from a 5.2-Ma-old calc-alkalic granite xenolith indicate mantle-derived magmas largely unaffected by the assimilation of crustal material. This is in accordance with abundances of diagnostic trace elements (Rb, Y, Nb, Ta), indicating A1-type, OIB-like source magmas. Increased accumulations of Nb-U-REE minerals in these granites indicate higher degree of the magmatic differentiation reflected in Rb-enrichment, contrasting with Ba-enrichment in barren xenoliths. Incipient charnockitization, i.e. orthopyroxene and ilmenite crystallization from interstitial silicate melt, was observed in many granite xenoliths. Thermodynamic modeling using pseudosections showed that the orthopyroxene growth may have been triggered by water exsolution from the melt during ascent of xenoliths in basaltic magma. Euhedral-to-skeletal orthopyroxene growth

  12. Precambrian U-Pb zircon ages in eclogites and garnet pyroxenites from South Brittany (France): an old oceanic crust in the West European Hercynian belt?

    Science.gov (United States)

    Peucat, J. J.; Vidal, Ph.; Godard, G.; Postaire, B.

    1982-08-01

    U-Pb zircon ages have been determined for two eclogites from the Vendée and for two garnet pyroxenites from the Baie d'Audierne. In an episodic Pb loss model, the two discordia would give upper intercept ages around 1300-1250 Ma and lower intercepts ages of 436-384 Ma. Two interpretations are proposed: (1) The 1250-1300 Ma ages may reflect an unspecified upper mantle event or process; the Paleozoic ages correspond to the tectonic emplacement of an eclogitic mantle fragment into the continental crust. (2) The protolith may have been extracted from the upper mantle 1250-1300 Ma ago and stored in a crustal environment until it was metamorphosed under high-pressure conditions around 400 Ma ago. This latter model is favoured by available geologic and isotopic data. Consequently, we propose that a 1300 Ma old oceanic crust was tectonicly incorporated into a sialic basement during the Proterozoic. This mixture was subsequently subducted during the Paleozoic.

  13. U-Pb ages and geochemistry of zircon from Proterozoic plutons of the Sawatch and Mosquito ranges, Colorado, U.S.A.: Implications for crustal growth of the central Colorado province

    Science.gov (United States)

    Moscati, Richard J.; Premo, Wayne R.; Dewitt, Ed; Wooden, Joseph L.

    2017-01-01

    A broad study of zircons from plutonic rocks of the Sawatch and Mosquito ranges of west-central Colorado (U.S.A.) was undertaken to significantly refine the magmatic chronology and chemistry of this under-studied region of the Colorado province. This region was chosen because it lies just to the north of the suspected arc-related Gunnison-Salida volcano-plutonic terrane, which has been the subject of many recent investigations—and whose origin is still debated. Our new results provide important insights into the processes active during Proterozoic crustal evolution in this region, and they have important ramifications for broader-scope crustal evolution models for southwestern North America.Twenty-four new U-Pb ages and sequentially acquired rare-earth element (REE), U, Th, and Hf contents of zircon have been determined using the sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG). These zircon geochemistry data, in conjunction with whole-rock major- and trace-element data, provide important insights into zircon crystallization and melt fractionation, and they help to further constrain the tectonic environment of magma generation.Our detailed zircon and whole-rock data support the following three interpretations:(1) The Roosevelt Granite in the southern Sawatch Range was the oldest rock dated at 1,766 ± 7 Ma, and it intruded various metavolcanic and metasedimentary rocks. Geochemistry of both whole-rock and zircon supports the contention that this granite was produced in a magmatic arc environment and, therefore, is likely an extension of the older Dubois Greenstone Belt of the Gunnison Igneous Complex (GIC) and the Needle Mountains (1,770–1,755 Ma). Rocks of the younger Cochetopa succession of the GIC, the Salida Greenstone Belt, and the Sangre de Cristo Mountains (1,740–1,725 Ma) were not found in the Sawatch and Mosquito ranges. This observation strongly suggests that the northern edge of the Gunnison-Salida arc terrane underlies the

  14. The history of a continent from U-Pb ages of zircons from Orinoco River sand and Sm-Nd isotopes in Orinoco basin river sediments

    Science.gov (United States)

    Goldstein, S.L.; Arndt, N.T.; Stallard, R.F.

    1997-01-01

    We report SHRIMP U-Pb ages of 49 zircons from a sand sample from the lower Orinoco River, Venezuela, and Nd model ages of the fine sediment load from the main river and tributaries. The U-Pb ages reflect individual magmatic or metamorphic events, the Sm-Nd model ages reflect average crustal-residence ages of the sediment sources. Together they allow delineation of the crust-formation history of the basement precursors of the sediments. The U-Pb ages range from 2.83 to 0.15 Ga, and most are concordant or nearly so. Discrete age groupings occur at ??? 2.8, ??? 2.1, and ??? 1.1 Ga. The oldest group contains only three samples but is isolated from its closest neighbors by a ??? 600 Ma age gap. Larger age groupings at ??? 2.1 and ??? 1.1 Ga make up about a third and a quarter of the total number of analyses, respectively. The remaining analyses scatter along concordia, and most are younger than 1.6 Ga. The ??? 2.8 and ??? 2.1 Ga ages correspond to periods of crust formation of the Imataca and Trans-Amazonian provinces of the Guyana Shield, respectively, and record intervals of short but intensive continental growth. These ages coincide with ??? 2.9 and ??? 2.1 Ga Nd model ages of sediments from tributaries draining the Archean and Proterozoic provinces of the Guyana Shield, respectively, indicating that the U-Pb ages record the geological history of the crystalline basement of the Orinoco basin. Zircons with ages corresponding to the major orogenies of the North Atlantic continents (the Superior at ??? 2.7 Ga and Hudsonian at 1.7-1.9 Ga) were not found in the Orinoco sample. The age distribution may indicate that South and North America were separated throughout their history. Nd model ages of sediments from the lower Orinoco River and Andean tributaries are ??? 1.9 Ga, broadly within the range displayed by major rivers and dusts. This age does not coincide with known thermal events in the region and reflects mixing of sources with different crust-formation ages. The

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

    Energy Technology Data Exchange (ETDEWEB)

    Marques, Suelen Nonata de Souza; Nascimento, Rielva Solimairy Campelo do, E-mail: suelen-marques@hotmail.com, E-mail: rielva@ufam.edu.br [Universidade Federal do Amazonas (UFAM), Manaus, AM (Brazil). Inst. de Geociencias; Souza, Valmir da Silva; Dantas, Elton Luiz, E-mail: vsouza@unb.br, E-mail: elton@unb.br [Universidade de Brasilia (UnB), DF (Brazil). Inst. de Geociencias; Valerio, Cristovao da Silva, E-mail: cristovao@igeo.ufrr.br [Universidade Federal de Roraima (UFRR), Boa Vista, RR (Brazil). Inst. de Geociencias

    2014-07-01

    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)

  16. Isotope geochronology study of the Baiyigou group in west Qinling

    International Nuclear Information System (INIS)

    Yang Junlong; Dong Yibao; Yuan Haihua; Mao Yunian; Min Yongming

    1989-01-01

    Rb-Sr isochron and U-Pb zircon dating shows that the Baiyigou Group in western Qinling belongs to Lower Sinian with an age 734+/-63 Ma. The underlying granophyre gravels carry an U-Pb zircon age 1008.9 Ma (Jinning Qrogeny). The overlying silicious rock, i.e. the lower part of the Taiyangding Formation has Rb-Sr isochron age 535+/- 11 Ma. Lithologic characteristics of the Paiyigou Group voleano-sedimentary rock suite are well comparable with the Wusidaqiao Group in West Sichan, both belonging to products of incipient continental rifting environment. An apparent Early Devonian (400 Ma+/-) tectono-magmatic and metamorphic event was the firstly discovered in this area

  17. U-Pb dating on zircons for the la Borie eclogite (Haut-Allier, France) and consequences for the pre-Variscan evolution of the Western Europe

    Energy Technology Data Exchange (ETDEWEB)

    Ducrot, J.; Lancelot, J.R. (Montpellier-2 Univ., 44 (France). Lab. de Geochimie-Isotopique); Marchand, J. (Nantes Univ., 34 (France). Lab. de Petrologie et Minerologie)

    1983-03-01

    A crystallization age of 432sub(-10)/sup +20/Ma has been determined for the 'La Borie' eclogite (Haut-Allier, France), by U-Pb dating on zircons. This age is yielded by the upper intercept with the Concordia curve. In agreement with recent dates obtained in the southern part of the Massif Central, in Brittany and in Vendee, these data indicate that the high-pressure/high-temperature-metamorphic phase representative of the pre-Variscan evolution of the western Europe, took place at the Silurian/Ordovician boundary. This high-pressure/high-temperature metamorphism could be significant of subduction processes more or less synchronous, preceeding the main continental collision responsible of the Variscan belt.

  18. U-Pb dating on zircons for the la Borie eclogite (Haut-Allier, France) and consequences for the pre-Variscan evolution of the Western Europe

    International Nuclear Information System (INIS)

    Ducrot, J.; Lancelot, J.R.; Marchand, J.

    1983-01-01

    A crystallization age of 432sub(-10) +20 Ma has been determined for the 'La Borie' eclogite (Haut-Allier, France), by U-Pb dating on zircons. This age is yielded by the upper intercept with the Concordia curve. In agreement with recent dates obtained in the southern part of the Massif Central, in Brittany and in Vendee, these data indicate that the high-pressure/high-temperature-metamorphic phase representative of the pre-Variscan evolution of the western Europe, took place at the Silurian/Ordovician boundary. This high-pressure/high-temperature metamorphism could be significant of subduction processes more or less synchronous, preceeding the main continental collision responsible of the Variscan belt. (s.s.). (orig.)

  19. Lead zirconate (PbZrO3 embedded in natural rubber as electroactive elastomer composites

    Directory of Open Access Journals (Sweden)

    Anuvat Sirivat

    2014-11-01

    Full Text Available Perovskite lead zirconate (PbZrO3 was synthesized in an orthorhombic form at a temperature below the Curie temperature, TC. The orthorhombic form is a noncentrosymmetric structure which is capable of spontaneous polarization. Fourier transform infrared (FTIR spectra and X-ray diffraction (XRD patterns confirm the successful synthesis of the lead zirconate; and scanning electron microscopy (SEM micrographs indicate that PbZrO3 particles are moderately dispersed in the natural rubber (NR matrix. Without an electrical field, the particles merely act as a ferroelectric filler, which can absorb and store additional stress. Under an electrical field, particle-induced dipole moments are generated, leading to interparticle interaction and a substantial increase in the storage modulus. At a small amount of lead zirconate particulates present in the natural rubber matrix, at a volume fraction of 0.007306, the electrical conductivity increases dramatically by nearly two orders of magnitude at the electrical frequency of 500 kHz.

  20. ∼2.5 Ga late cratonisation events in Dharwar craton: insights from the gold mineralisation ages

    International Nuclear Information System (INIS)

    Srinivasa Sarma, D.; Ram Mohan, M.; McNaughton, Neal

    2013-01-01

    The history of volcanism, granitic magmatism, and gold mineralization is defined by U-Pb geochronology of magmatic zircons and hydrothermal monazite and xenotime respectively. The felsic volcanic host rocks from Hutti greenstone belt have a U-Pb zircon age of 258 ±7 Ma, about 40 m.y. older than the age of gold mineralization at 2547±10 Ma determined from hydrothermal monazite in the Hutti gold deposit. The syntectonic Kavital granitoid in the Hutti greenstone belt has a U-Pb zircon age of 2545±7 Ma, which overlaps with the timing of gold deposition and is consistent with structural interpretations. Zircon U-Pb ages for a felsic volcanic rock (2,588±10 Ma) and an intrusive granite (e''2,555±6 Ma) in the Gadag greenstone belt in the Western Dharwar Craton. In situ U-Pb dating of monazite and xenotime in gold reefs of the Gadag (2,522±6 Ma) and Ajjanahalli (2,520±9 Ma) gold deposits reveal a previously undated episode of gold mineralization at 2.52 Ga, substantially younger than the 2.55 Ga Hutti deposit in the eastern Dharwar Craton. The Hutti, Gadag and Ajjanahalli gold geochronology suggests that gold mineralization occurred throughout the Dharwar craton some 80 to 120 m.y. later than the major peak of Late Archean world-class orogenic gold mineralization in most other Archean cratons. Although gold mineralization across the craton postdates most of the magmatic activity and metamorphism at upper crustal levels, widespread thermal reworking of the lower middle crust, involving partial melting, metamorphism, and lower crustal granitoid intrusion, occurred concurrently with gold mineralization. It is likely that the large-scale hydrothermal fluid flow that produced widespread gold deposition was also part of this tectono-thermal event during the final stages of cratonization of the Dharwar Craton in southern India. (author)

  1. Fission track dating of kimberlitic zircons

    International Nuclear Information System (INIS)

    Haggerty, S.E.

    1983-01-01

    The only reliable method for dating kimberlites at present is the lengthy and specialized hydrothermal procedure that extracts 206 Pb and 238 U from low-uranium zircons. This paper describes a second successful method by fission track dating of large single-crystal zircons, 1.0-1.5 cm in dimension. The use of large crystals overcomes the limitations imposed in conventional fission track analysis which utilizes crushed fragments. Low track densities, optical track dispersion, and the random orientation of polished surfaces in the etch and irradiation cycle are effectively overcome. Fission track ages of zircons from five African kimberlites are reported, from the Kimberley Pool (90.0 +- 6.5 m.y.), Orapa (87.4 +- 5.7 and 92.4 +- 6.1 m.y.), Nzega (51.1 +- 3.8 m.y.), Koffiefontein (90.9 +- 8.2 m.y.), and Val do Queve (133.4 +- 11.5 m.y.). In addition we report the first radiometric ages (707.9 +- 59.6 and 705.5 +- 61.0 m.y.) of crustal zircons from kimberlites in northwest Liberia. The fission track ages agree well with earlier age estimates. Most of the zircons examined in this study are zoned with respect to uranium but linear correlations are established (by regression analysis) between zones of variable uranium content, and within zones of constant uranium content (by analysis of variance). Concordance between the fission track method and the U/Pb technique is established and we concluded that track fading from thermal annealing has not taken place. Kimberlitic zircons dated in this study, therefore, record the time of eruption. (orig.)

  2. U-Pb Geochronology of non-marine Upper Triassic strata of the Colorado Plateau (western North America): implications for stratigraphic correlation and paleoenvironmental reconstruction

    Science.gov (United States)

    Rasmussen, C.; Mundil, R.; Irmis, R. B.; Keller, C. B.; Giesler, D.; Gehrels, G. E.

    2017-12-01

    The Triassic is a critical period in Earth history that witnessed the origin of modern ecosystems and frequent climate fluctuations, as well as major environmental events such as flood basalt volcanism and bolide impacts. The Chinle Formation contains a primary non-marine archive for past ecosystems in North America due to its fossil richness and well-studied sedimentology. Moreover, within these highly fossiliferous strata, a biotic turnover has been reported that has been hypothesized to coincide with one or more of the aforementioned environmental events. Unfortunately, few radioisotopic ages have been published for the Late Triassic, limiting our ability for lithological and paleoenvironmental correlations. In addition, the superposition of the Chinle Formation remains illusive due to frequent lateral facies changes and discontinuous outcrops across the Colorado Plateau. The 520 m long core 1A of the Colorado Plateau Coring Project from Petrified Forest National Park (PFNP) (Arizona) provides, for the first time, a continuous section of these early Mesozoic sedimentary strata. Many of the sand- and siltstones from this continuous succession throughout most of the Upper Triassic Chinle Formation contain euhedral zircons suitable for U-Pb analyses. We analyzed >300 crystals each from 10 samples using LA-ICPMS; these results indicated abundant Late Triassic crystals that appear to be closely associated with the depositional age of the host rock. We then selected the youngest grains from these samples to obtain precise CA-TIMS U-Pb single zircon ages in order to constrain the maximum depositional ages (using quantitative methods) of these formations. We are able to revise the proposed time scale (based on outcrop samples) for Upper Triassic strata at PFNP and evaluate whether the biotic turnover observed within the Sonsela Member of these strata coincides with the Manicouagan bolide impact event. This revised chronostratigraphic framework allows intercalibration

  3. Archean evolution of Enderby Land (Antarctica) and isotope-geochronological evidences for its ancient history

    International Nuclear Information System (INIS)

    Krylov, D.P.; Belyatskij, B.V.

    1987-01-01

    Revew of published isotope-geochronological data on Ender by Land (Antarctica), which is the region of highly metamorphic formations predominant development which includes ancient rock relicts, is presented. Three tectonic-thermal events present the Archeau evolution in the region. Correlation of isotope-geochronological (U-Pb, Rb-Sr, Sm-Nd) data with micro textural processing allows to estimate tectonic-thermal events age: 3000-3100 about 2900 and about 2500 million years. Metamorphism of 3000-3100 million years age has essentially modified all the isotope systems, while model calculations for evolution of U-Pb, Rb-Sr, Sm-Nd systems have shown that rocks primary formation accurred 3500-3900 million years ago

  4. Petrogenesis of the Yaochong granite and Mo deposit, Western Dabie orogen, eastern-central China: Constraints from zircon U-Pb and molybdenite Re-Os ages, whole-rock geochemistry and Sr-Nd-Pb-Hf isotopes

    Science.gov (United States)

    Chen, Wei; Xu, Zhaowen; Qiu, Wenhong; Li, Chao; Yu, Yang; Wang, Hao; Su, Yang

    2015-05-01

    The Dabie orogen is among the most famous continent-continent collisional orogenic belts in the world, and is characterized by intensive post-collisional extension, magmatism and Mo mineralization. However, the genetic links between the mineralization and the geodynamic evolution of the orogen remain unresolved. In this paper, the Yaochong Mo deposit and its associated granitic stocks were investigated to elucidate this issue. Our new zircon U-Pb ages yielded an Early Cretaceous age (133.3 ± 1.3 Ma) for the Yaochong granite, and our molybdenite Re-Os dating gave a similar age (135 ± 1 Ma) for the Mo deposit. The Yaochong stock is characterized by high silica and alkali but low Mg, Fe and Ca. It is enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs: Rb, K, Th and U), but strongly depleted in heavy REEs, and high field strength elements (HFSEs: Nb, Ta, Ti and Y). The Yaochong granite has initial 87Sr/86Sr ratios of 0.7087-0.7096, and Pb isotopic ratios of (206Pb/204Pb)i = 16.599-16.704, (207Pb/204Pb)i = 15.170-15.618 and (208Pb/204Pb)i = 36.376-38.248. The granite has εNd(t) of -18.0 to -16.3 and εHf(t) values of -26.5 to -20.0. All these data indicate that the Yaochong granite is a high-K calc-alkaline fractionated I-type granite, and may have originated from partial melting of the thickened Yangtze continental crust. The Mo ores also show low radiogenic Pb isotopes similar to the Yaochong stock. Medium Re content in molybdenite (21.8-74.8 ppm) also suggests that the ore-forming materials were derived from the thickened lower crust with possibly minor mixing with the mantle. Similar to the Eastern Dabie orogen, the thickened crust beneath the Western Dabie orogen may also have experienced tectonic collapse, which may have exerted fundamental geodynamic controls on the two-stage Mo mineralization in the region.

  5. Timing of mid-crustal ductile extension in the northern Snake Range metamorphic core complex, Nevada: Evidence from U/Pb zircon ages

    Science.gov (United States)

    Lee, J.; Blackburn, T.; Johnston, S. M.

    2016-12-01

    Metamorphic core complexes (Mccs) within the western U.S. record a history of Cenozoic ductile and brittle extensional deformation, metamorphism, and magmatism, and exhumation within the footwall of high-angle Basin and Range normal faults. Documenting these histories within Mccs have been topics of research for over 40 years, yet there remains disagreement about: 1) whether the detachment fault formed and moved at low angles or initiated at high angles and rotated to a low angle; 2) whether brittle and ductile extensional deformation were linked in space and time; and 3) the temporal relationship of both modes of extension to the development of the detachment fault. The northern Snake Range metamorphic core complex (NSR), Nevada has been central to this debate. To address these issues, we report new U/Pb dates from zircon in deformed and undeformed rhyolite dikes emplaced into ductilely thinned and horizontally stretched lower plate rocks that provide tight bounds on the timing of ductile extension at between 38.2 ± 0.3 Ma and 22.50 ± 0.36 Ma. The maximum age constraint is from the Northern dike swarm (NDS), which was emplaced in the northwest part of the range pre- to syn-tectonic with ductile extension. The minimum age constraint is from the Silver Creek dike swarm (SDS) that was emplaced in the southern part of the range post ductile extensional deformation. Our field observations, petrography, and U/Pb zircon ages on the dikes combined with published data on the geology and kinematics of extension, moderate and low temperature thermochronology on lower plate rocks, and age and faulting histories of Cenozoic sedimentary basins adjacent to the NSR are interpreted as recording an episode of localized upper crustal brittle extension during the Eocene that drove upward ductile extensional flow of hot middle crustal rocks from beneath the NSR detachment soon after, or simultaneous with, emplacement of the NDS. Exhumation of the lower plate continued in a rolling

  6. U-Pb zircon geochronology of the Paleogene - Neogene volcanism in the NW Anatolia: Its implications for the Late Mesozoic-Cenozoic geodynamic evolution of the Aegean

    Science.gov (United States)

    Ersoy, E. Yalçın; Akal, Cüneyt; Genç, Ş. Can; Candan, Osman; Palmer, Martin R.; Prelević, Dejan; Uysal, İbrahim; Mertz-Kraus, Regina

    2017-10-01

    The northern Aegean region was shaped by subduction, obduction, collision, and post-collisional extension processes. Two areas in this region, the Rhodope-Thrace-Biga Peninsula to the west and Armutlu-Almacık-Nallıhan (the Central Sakarya) to the east, are characterized by extensive Eocene to Miocene post-collisional magmatic associations. We suggest that comparison of the Cenozoic magmatic events of these two regions may provide insights into the Late Mesozoic to Cenozoic tectonic evolution of the Aegean. With this aim, we present an improved Cenozoic stratigraphy of the Biga Peninsula derived from a new comprehensive set of U-Pb zircon age data obtained from the Eocene to Miocene volcanic units in the region. The compiled radiometric age data show that calc-alkaline volcanic activity occurred at 43-15 Ma in the Biga Peninsula, 43-17 Ma in the Rhodope and Thrace regions, and 53-38 Ma in the Armutlu-Almacık-Nallıhan region, which are slightly overlapping. We discuss the possible cause for the distinct Cenozoic geodynamic evolution of the eastern and western parts of the region, and propose that the Rhodope, Thrace and Biga regions in the north Aegean share the same Late Mesozoic to Cenozoic geodynamic evolution, which is consistent with continuous subduction, crustal accretion, southwestward trench migration and accompanying extension; all preceded by the Late Cretaceous - Paleocene collision along the Vardar suture zone. In contrast, the Armutlu-Almacık-Nallıhan region was shaped by slab break-off and related processes following the Late Cretaceous - Paleocene collision along the İzmir-Ankara suture zone. The eastern and western parts of the region are presently separated by a northeast-southwest trending transfer zone that was likely originally present as a transform fault in the subducted Tethys oceanic crust, and demonstrates that the regional geodynamic evolution can be strongly influenced by the geographical distribution of geologic features on the

  7. Zircon U-Pb ages and Hf isotopic compositions of neoarchaean granitoids from Karimnagar, Eastern Dharwar craton, Southern India: LA-ICPMS results

    International Nuclear Information System (INIS)

    Babu, E.V.S.S.K.; Vijaya Kumar, T.; Sreenivas, B.; Khadke, Namrata; Vijaya Gopal, B.; Bhaskar Rao, Y.J.

    2015-01-01

    The Archaean Dharwar and Bastar cratons of Peninsular India are separated by the NE-SW trending Godavari Graben. This zone is involved in recurrent rifting. It is generally believed that earlier phase of rifting dates back to Mesoproterozoic and the younger phase is related to the deposition of Permo-carboniferous coal-bearing sediments. A belt of high-grade rocks (∼ 150 x 45 km) - the Karimnagar granulite terrane (KGT) along the southern flank of the graben. The KGT comprises high-grade lithologies such as orthopyroxene-bearing quarto-feldspathic gneiss, amphibolite-granulite facies supracrustal belts that include metamorphosed quartz-arenites, pelites, carbonates, iron formation and mafic rocks as well as several zones of migmatite and younger granitoid intrusives. The study presented new LA-ICPMS zircon U-Pb age data for five granitoid (charnockite and granite) samples (KRN-90-24, KRN-90-30, KRN-13-02, KRN-13-03 and KRN-13-05; a closer constraint of the age crystallization of protoliths and high-grade metamorphism of felsic granulites and orthogneisses from the KGT. Hf isotopic compositions were also obtained for selected zircons from two samples (KRN-90-24 and KRN-90-30) using New Wave UP-213 Laser Ablation system coupled to Nu Plasma HR MC-ICPMS to constraint the nature of the felsic protoliths of the charnockite and granite gneisses in the KGT

  8. The link between volcanism and plutonism in epizonal magma systems; high-precision U–Pb zircon geochronology from the Organ Mountains caldera and batholith, New Mexico

    Science.gov (United States)

    Rioux, Matthew; Farmer, Lang; Bowring, Samuel; Wooton, Kathleen M.; Amato, Jeffrey M.; Coleman, Drew S.; Verplanck, Philip L.

    2016-01-01

    The Organ Mountains caldera and batholith expose the volcanic and epizonal plutonic record of an Eocene caldera complex. The caldera and batholith are well exposed, and extensive previous mapping and geochemical analyses have suggested a clear link between the volcanic and plutonic sections, making this an ideal location to study magmatic processes associated with caldera volcanism. Here we present high-precision thermal ionization mass spectrometry U–Pb zircon dates from throughout the caldera and batholith, and use these dates to test and improve existing petrogenetic models. The new dates indicate that Eocene volcanic and plutonic rocks in the Organ Mountains formed from ~44 to 34 Ma. The three largest caldera-related tuff units yielded weighted mean 206Pb/238U dates of 36.441 ± 0.020 Ma (Cueva Tuff), 36.259 ± 0.016 Ma (Achenback Park tuff), and 36.215 ± 0.016 Ma (Squaw Mountain tuff). An alkali feldspar granite, which is chemically similar to the erupted tuffs, yielded a synchronous weighted mean 206Pb/238U date of 36.259 ± 0.021 Ma. Weighted mean 206Pb/238U dates from the larger volume syenitic phase of the underlying Organ Needle pluton range from 36.130 ± 0.031 to 36.071 ± 0.012 Ma, and the youngest sample is 144 ± 20 to 188 ± 20 ka younger than the Squaw Mountain and Achenback Park tuffs, respectively. Younger plutonism in the batholith continued through at least 34.051 ± 0.029 Ma. We propose that the Achenback Park tuff, Squaw Mountain tuff, alkali feldspar granite and Organ Needle pluton formed from a single, long-lived magma chamber/mush zone. Early silicic magmas generated by partial melting of the lower crust rose to form an epizonal magma chamber. Underplating of the resulting mush zone led to partial melting and generation of a high-silica alkali feldspar granite cap, which erupted to form the tuffs. The deeper parts of the chamber underwent continued recharge and crystallization for 144 ± 20 ka after the

  9. New monazite U-Pb age constraints on the evolution of the Paleoproterozoic Vaasa granitoid batholith, western Finland

    Directory of Open Access Journals (Sweden)

    A.K. Kotilainen

    2016-09-01

    Full Text Available The Vaasa batholith, western Finland, is a large, peraluminous granitoid pluton that crystallized at 1.88–1.87 Ga during the culmination of the Svecofennian orogeny. The batholith has gradual contacts, through metatexites and diatexites, with the enveloping metasedimentary rocks of the Bothnian Belt. We present ID-TIMS U-Pb age data on monazite from granitoids and xenoliths of the Vaasa batholith and combine these with published U–Pb zircon ages in order to shed further light on the evolution of the Vaasa batholith. The apparent monazite ages for seven of the examined samples are 1870–1863 Ma, and 1855±3 Ma for one further sample from the southern part of the batholith. Combined with pre-existing data, the monazite ages of the granitoids are 9 to 18 Ma (face values or 3 to 9 Ma (external errors considered younger than the U–Pb zircon crystallization ages from respective samples. Our new data suggest slow cooling for the Vaasa batholith – the closure/saturation temperature of the monazite U–Pb system was probably reached in ~10 m.y. after the crystallization of magmatic zircon in the examined rocks.

  10. Uranium-lead dating of hydrothermal zircon and monazite from the Sin Quyen Fe-Cu-REE-Au-(U) deposit, northwestern Vietnam

    Science.gov (United States)

    Li, Xiao-Chun; Zhou, Mei-Fu; Chen, Wei Terry; Zhao, Xin-Fu; Tran, MyDung

    2018-03-01

    The Sin Quyen deposit in northwestern Vietnam contains economic concentrations of Cu, Au and LREE, and sub-economic concentration of U. In this deposit, massive and banded replacement ores are hosted in Neoproterozoic metapelite. The paragenetic sequence includes sodic alteration (stage I), calcic-potassic alteration and associated Fe-REE-(U) mineralization (stage II), Cu-Au mineralization (stage III), and sulfide-(quartz-carbonate) veins (stage IV). The Sin Quyen deposit experienced an extensive post-ore metamorphic overprint, which makes it difficult to precisely determine the mineralization age. In this study, zircon and monazite U-Pb geochronometers and the Rb-Sr isochron method are used to constrain the timing of mineralization. Zircon grains in the ore are closely intergrown or texturally associated with hydrothermal minerals of stage II (e.g., garnet, allanite, and hedenbergite). They may contain primary fluid inclusions and display irregular zoning in cathodoluminescence (CL) images. Zircon grains are rich in U (688 to 2902 ppm) and poor in Th (0.2 to 2.9 ppm). Their δ18OV-SMOW values range from 11.9 to 14.0‰, higher than those of typical magmatic zircon. These textural and compositional features imply that zircon precipitated from 18O- and U-rich hydrothermal fluids, coeval with the minerals of stage II. Monazite occurs in close association with stage II magnetite and allanite and has low contents of Th (<2700 ppm), indicative of a hydrothermal origin. Hydrothermal zircon and monazite have indistinguishable U-Pb ages of 841 ± 12 and 836 ± 18 Ma, respectively, representing the timing of Fe-REE mineralization. There is no direct isotopic constraint on the timing of the Cu-Au mineralization, but geological observations suggest that the Cu-Au and Fe-REE ores most likely formed within a single evolved hydrothermal process. In the plot of 87Rb/86Sr vs. 87Sr/86Sr, the composition of bulk-ore and biotite separates from ore lie along a reference line for 30 Ma

  11. Zircon U-Pb ages and Sr-Nd-Hf isotopes of the highly fractionated granite with tetrad REE patterns in the Shamai tungsten deposit in eastern Inner Mongolia, China: Implications for the timing of mineralization and ore genesis

    Science.gov (United States)

    Jiang, Si-Hong; Bagas, Leon; Hu, Peng; Han, Ning; Chen, Chun-Liang; Liu, Yuan; Kang, Huan

    2016-09-01

    The Shamai tungsten deposit is located in the eastern part of the Central Asian Orogenic Belt (CAOB). Tungsten mineralization is closely related to the emplacement of fine- to medium-grained biotite monzogranite (G1) and porphyritic biotite monzogranite (G2) in the Shamai Granite. NW-trending joints and faults host orebodies in the Shamai Granite and Devonian hornfels. The mineralization is characterized by a basal veinlet zone progressing upwards to a thick vein zone followed by a mixed zone, a veinlet zone, and a thread vein zone at the top. The ore-related alteration typically consists of muscovite, greisen, and hornfels. In order to constrain the timing of the Shamai mineralization and discuss the ore genesis, muscovite Ar-Ar, molybdenite Re-Os, and zircon U-Pb geochronological, geochemical, and Sr-Nd-Hf isotopic studies were completed on the deposit. The U-Pb zircon dating yielded weighted mean ages of 153 ± 1 Ma for G1 and 146 ± 1 Ma for G2. Muscovite from a wolframite-bearing quartz vein yielded an Ar-Ar plateau age of 140 ± 1 Ma, whereas two molybdenite samples yielded identical Re-Os model ages of 137 ± 2 Ma. These two ages are younger than the two monzogranites, suggesting a prolonged magmatic-hydrothermal interaction during tungsten mineralization. Major and trace element geochemistry shows that both G1 and G2 are characterized by high SiO2 and K2O contents, high A/CNK values (1.08-1.40), a spectacular tetrad effect in their REE distribution patterns, and non-CHARAC (charge-and-radius-controlled) trace element behavior. This suggests that both G1 and G2 are highly differentiated peraluminous rocks with strong hydrothermal interaction. The Nd-Hf isotope data for the Shamai Granite (εNd(t) between - 1.9 and + 7.4, ɛHf(t) from 5.2 to 12.8) are largely compatible with the general scenario for much of the Phanerozoic granite emplaced in the CAOB. It is here suggested that the Shamai Granite originated from partial melting of a juvenile lower crust with

  12. Data Reduction of Laser Ablation Split-Stream (LASS) Analyses Using Newly Developed Features Within Iolite: With Applications to Lu-Hf + U-Pb in Detrital Zircon and Sm-Nd +U-Pb in Igneous Monazite

    Science.gov (United States)

    Fisher, Christopher M.; Paton, Chad; Pearson, D. Graham; Sarkar, Chiranjeeb; Luo, Yan; Tersmette, Daniel B.; Chacko, Thomas

    2017-12-01

    A robust platform to view and integrate multiple data sets collected simultaneously is required to realize the utility and potential of the Laser Ablation Split-Stream (LASS) method. This capability, until now, has been unavailable and practitioners have had to laboriously process each data set separately, making it challenging to take full advantage of the benefits of LASS. We describe a new program for handling multiple mass spectrometric data sets collected simultaneously, designed specifically for the LASS technique, by which a laser aerosol is been split into two or more separate "streams" to be measured on separate mass spectrometers. New features within Iolite (https://iolite-software.com) enable the capability of loading, synchronizing, viewing, and reducing two or more data sets acquired simultaneously, as multiple DRSs (data reduction schemes) can be run concurrently. While this version of Iolite accommodates any combination of simultaneously collected mass spectrometer data, we demonstrate the utility using case studies where U-Pb and Lu-Hf isotope composition of zircon, and U-Pb and Sm-Nd isotope composition of monazite were analyzed simultaneously, in crystals showing complex isotopic zonation. These studies demonstrate the importance of being able to view and integrate simultaneously acquired data sets, especially for samples with complicated zoning and decoupled isotope systematics, in order to extract accurate and geologically meaningful isotopic and compositional data. This contribution provides instructions and examples for handling simultaneously collected laser ablation data. An instructional video is also provided. The updated Iolite software will help to fully develop the applications of both LASS and multi-instrument mass spectrometric measurement capabilities.

  13. Introduction to isotopic geochronology of pre-quaternary age

    International Nuclear Information System (INIS)

    Poupeau, G.

    1985-01-01

    The historical development of Geochronology is summarized, showing its strong relation to Physics. Considerations about radioactivity and its application to radiochronology are made, including the use of U, Th-Pb dating techniques. A vast list of bibliographical references is given for further consultation. (C.L.B.) [pt

  14. The geotectonic evolution of the Northeastern Minas Gerais, based in geochronological interpretation

    International Nuclear Information System (INIS)

    Siga Junior, O.

    1986-01-01

    This work tries to show the potential of the Rb-Sr, K-Ar, Pb-Pb and U-Pb methods applied to basic geological maping. The different interpretative values of these methodologies contribute to the understanding of the tectonic processes developed in the southeastern border of the Sao Francisco Craton (northeastern Minas Gerais and southern Bahia). The geochronological data for this region represents sampling of the several lithological units characterized and corresponds to about 230 determinations. The radiometric age distribution pattern together with other geological information suggest the subdivision into two main geochronological domains: the external Brazilian domain (at the western portion) and the internal Brazilian domain (at the center-eastern part). (author) [pt

  15. Palynology and detrital zircon geochronology of the Carboniferous Fenestella Shale Formation of the Tethyan realm in Kashmir Himalaya: Implications for global correlation and floristic evolution

    Science.gov (United States)

    Agnihotri, Deepa; Pandita, Sundeep K.; Tewari, Rajni; Ram-Awatar; Linnemann, Ulf; Pillai, S. Suresh K.; Joshi, Arun; Gautam, Saurabh; Kumar, Kamlesh

    2018-05-01

    First palynological data, supplemented by detrital zircon U-Pb ages, from the Fenestella Shale Formation near the Gund Village in the Banihal area of Jammu and Kashmir State, India, provide new insights into the floristic evolution of Gondwana during the Late Palaeozoic, especially in India, from where the Carboniferous-Permian macro- and microfloral records are impoverished. We also present a first approach to the palynological correlation of the Carboniferous-Permian palynoassemblages described from the various Gondwana countries. The palynomorphs from the Fenestella Shale Formation are fairly well preserved and diversified and include 11 genera and 18 species. While the trilete spores and striate bisaccate pollen grains are scarce, monosaccate pollen taxa mainly - Parasaccites, Plicatipollenites and Potonieisporites are dominant. The assemblage is most similar to the Parasaccites korbaensis palynozone of the Lower Gondwana basins of the Indian peninsula and the Stage 2 palynozone of the late Carboniferous of east Australia. Besides, it is comparable with the known Carboniferous assemblages of Pakistan, Yemen and South America; Carboniferous-early Permian assemblages of South Africa and Permian assemblages of Antarctica. The sediment source of the siliciclastic shelf and delta deposits intercalated in the Fenestella Shale Formation is a hinterland in which Precambrian rocks dominantly were exposed and the Th-U ratios of detrital zircons suggest, that most rocks exposed on the erosion level in the hinterland had a felsic composition. The youngest U-Pb zircon age of the investigated fossiliferous strata is 329 ± 16 Ma (late Visean to early Serpukhovian), providing a maximum age of deposition of the studied succession. Based on the affinities of the palynofloral assemblage and earlier palaeontological records, a warm, temperate and arid climate has been inferred for the Fenestella Shale Formation.

  16. The Mesoarchean Tiejiashan-Gongchangling potassic granite in the Anshan-Benxi area, North China Craton: Origin by recycling of Paleo- to Eoarchean crust from U-Pb-Nd-Hf-O isotopic studies

    Science.gov (United States)

    Dong, Chunyan; Wan, Yusheng; Xie, Hangqiang; Nutman, Allen P.; Xie, Shiwen; Liu, Shoujie; Ma, Mingzhu; Liu, Dunyi

    2017-10-01

    Mesoarchean and older potassic granites are important indicators of recycling of ancient continental crust early in Earth's history. This study of integrated whole rock and zircon geochemistry and geochronology reports the age and identification of the source materials of the > 200 km2 Mesoarchean Tiejiashan-Gongchangling granite in the Anshan-Benxi area, North China Craton, the largest pre-Neoarchean granite domain in the craton. SHRIMP U-Pb zircon dating on 15 samples indicates the magmatic crystallization of the granites between 2.95 and 3.0 Ga and reveals a superimposed tectonothermal event at 2.91 Ga. The granites are characterized by high SiO2 and K2O, low CaO, FeOt, MgO and TiO2 with peraluminuous features. They show large variations in (La/Yb)n and strong negative Eu and Ba anomalies and Nb, P and Ti depletions. Whole rock Nd and magmatic zircon Hf isotopic compositions show large variations, but with most having εNd(t) and εHf(t) values recycling of Paleo- to Eoarchean continental material in an intracontinental environment, with little if any contribution from Mesoarchean mantle sources. The sources could be predominantly unaltered ancient gneisses, together with yet to be identified Paleo- to Eoarchean materials affected by early low temperature alteration (weathered rocks or clastic sediment).

  17. 3.3 Ga SHRIMP U-Pb zircon age of a felsic metavolcanic rock from the Mundo Novo greenstone belt in the São Francisco craton, Bahia (NE Brazil)

    Science.gov (United States)

    Peucat, J. J.; Mascarenhas, J. F.; Barbosa, J. S. F.; de Souza, S. L.; Marinho, M. M.; Fanning, C. M.; Leite, C. M. M.

    2002-07-01

    Felsic metavolcanics associated with supracrustal rocks provide U-Pb zircon and Sm-Nd TDM ages of approximately 3.3 Ga, which establish an Archean age of the Mundo Novo greenstone belt. A granodioritic gneiss from the Mairi complex, located on the eastern boundary of the Mundo Novo greenstone belt, exhibits a zircon evaporation minimum age of 3.04 Ga and a Nd model age of 3.2 Ga. These results constrain the occurrence of at least three major geological units in this area: the Archean Mundo Novo greenstone belt, the Archean Mairi gneisses, and the adjoining Paleoproterozoic (<2.1 Ga) Jacobina sedimentary basin. The Jacobina basin follows the same trend as the Archean structure, extending southward to the Contendas-Mirante belt, in which a similar Archean-Paleoproterozoic association appears. We postulate that during the Paleoproterozoic in the eastern margin of the Gavião block, these Archean greenstone belts constituted a zone of weakness along which a late-stage orogenic sedimentary basin developed.

  18. Pervasive remagnetization of detrital zircon host rocks in the Jack Hills, Western Australia and implications for records of the early geodynamo

    Science.gov (United States)

    Weiss, Benjamin P.; Maloof, Adam C.; Tailby, Nicholas; Ramezani, Jahandar; Fu, Roger R.; Hanus, Veronica; Trail, Dustin; Bruce Watson, E.; Harrison, T. Mark; Bowring, Samuel A.; Kirschvink, Joseph L.; Swanson-Hysell, Nicholas L.; Coe, Robert S.

    2015-11-01

    It currently is unknown when Earth's dynamo magnetic field originated. Paleomagnetic studies indicate that a field with an intensity similar to that of the present day existed 3.5 billion years ago (Ga). Detrital zircon crystals found in the Jack Hills of Western Australia are some of the very few samples known to substantially predate this time. With crystallization ages ranging from 3.0-4.38 Ga, these zircons might preserve a record of the missing first billion years of Earth's magnetic field history. However, a key unknown is the age and origin of magnetization in the Jack Hills zircons. The identification of >3.9 Ga (i.e., Hadean) field records requires first establishing that the zircons have avoided remagnetization since being deposited in quartz-rich conglomerates at 2.65-3.05 Ga. To address this issue, we have conducted paleomagnetic conglomerate, baked contact, and fold tests in combination with U-Pb geochronology to establish the timing of the metamorphic and alteration events and the peak temperatures experienced by the zircon host rocks. These tests include the first conglomerate test directly on the Hadean-zircon bearing conglomerate at Erawandoo Hill. Although we observed little evidence for remagnetization by recent lightning strikes, we found that the Hadean zircon-bearing rocks and surrounding region have been pervasively remagnetized, with the final major overprinting likely due to thermal and/or aqueous effects from the emplacement of the Warakurna large igneous province at ∼1070 million years ago (Ma). Although localized regions of the Jack Hills might have escaped complete remagnetization, there currently is no robust evidence for pre-depositional (>3.0 Ga) magnetization in the Jack Hills detrital zircons.

  19. Columbite-Group Minerals from New York Pegmatites: Insights from Isotopic and Geochemical Analyses

    Directory of Open Access Journals (Sweden)

    Marian V. Lupulescu

    2018-05-01

    Full Text Available Columbite crystals from niobium-yttrium-fluorine (NYF pegmatites lacking zircon or containing metamict cyrtolite were analyzed for major and minor elements (Electron Microprobe (EMP, trace elements (Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS, and U-Pb geochronology (Laser AblationMulti-Collector-Inductively Coupled Plasma-Mass Spectrometry (LA-MC-ICP-MS. All four pegmatite localities sampled are hosted by the Proterozoic Fordham gneiss and/or Paleozoic Bedford gneiss (Columbite-(Fe; Kinkel and Baylis localities and the Manhattan Schist of Lower Paleozoic age (Columbite-(Mn; Fort George and Harlem River Drive localities and yield Neoacadian ages. The weighted average U-Pb ages are 372.2 ± 8.2 Ma (Baylis Quarry, 371.3 ± 7.3 and 383.4 ± 8.9 Ma (Kinkel Quarry; 383 ± 15 Ma (185th St. and Harlem River Drive; and 372 ± 10 Ma (Fort George. A partial metamict zircon (“cyrtolite” from the Kinkel Quarry yielded a weighted average U-Pb age of 376.9 ± 4.3 Ma. The Neoacadian ages obtained agree with those determined by thermal ionization mass spectrometry (TIMS for zircon from Lithium-Cesium-Tantalum (LCT pegmatites from Connecticut and Maine. No pegmatites temporally associated with the Taconic orogeny were found. The size, lack of common Pb, uniform U concentrations across crystal cross-sections, sufficient but moderate uranium concentrations, lack of metamictization, and consistency in U-Pb isotopic ratios for columbite samples BCB-COL, NYSM #25232, and NYSM #525.8 suggests they show promise as potential standards for oxide mineral LA-MC-ICP-MS geochronological analyses, however, additional characterization using ID-TIMS would be necessary to develop as such.

  20. U-Pb age of granitoid rock from the quarry Dubna Skala - Mala Fatra Mts

    International Nuclear Information System (INIS)

    Cambel, B.; Shcherbak, N.P.; Bartnitsky, E.N.; Stepanyuk, L.M.

    1990-01-01

    The age of tonalite rock from the quarry Dubna Skala in the Mala Fatra mountains was determined by the U-Pb method. The measurements were carried out at the Institute of Geochemistry and Physics of Minerals in Kiev, USSR. The value measured, 353 m.y., is in a rather good agreement with the data obtained by the Rb-Sr method in core mountain ranges of the Western Carpathians. Nevertheless, in comparison with the Rb-Sr isochronal data from similar rocks and from the same regions, U-Pb geochronologic analyses largely give lower ages. So far, no unique explanation of this fact can be offered; apparently, the geochemical properties of the elements concerned play a major role in this. (author). 1 fig., 1 tab., 30 refs

  1. Ediacaran ( 620 Ma) high grade regional metamorphism in the northern Arabian Nubian Shield: U/Th-Pb monazite ages of the Elat schist

    Science.gov (United States)

    Elisha, Bar; Katzir, Yaron; Kylander-Clark, Andrew

    2017-04-01

    Ediacaran times witnessed a hemisphere-scale orogenesis forming the extensive Pan-African mountain ranges and resulting in the final assembly of Gondwana supercontinent. The Elat metamorphic basement (S Israel) located at the northernmost tip of a major Pan-African orogenic suture, the Arabian Nubian Shield (ANS), comprises amphibolite facies schists and gneisses and was most likely shaped by this major continental collision. However the timing, number and duration of metamorphic events in Elat and elsewhere in the ANS are non-conclusive and a major emphasis was given to pre-Ediacaran island-arc related tectonics. This is mostly because U-Pb dating of zircon, widely used in Elat and elsewhere, is very successful in constraining the ages of the igneous and sedimentary protoliths, but is 'blind' to metamorphism at grades lower than granulite. Here U/Th-Pb dating of monazite, a precise chronometer of metamorphic mineral growth, is systematically applied to the Elat schist and unveils the tectono-metamorphic evolution of the Elat basement. Previous U-Pb dating of detrital zircon has shown that the sedimentary protoliths of the Elat schist are the oldest basement components (≥800 Ma), and detailed structural observations of the schists portrayed a complex deformation history including four successive phases (Shimron, 1972). The earliest three phases were defined as ductile and penetrative, but some of the available geochronological data apparently contradict field relations. In-situ analysis of metamorphic monazites by LASS (Laser Ablation Split Stream) involves simultaneous measurement of U/Th-Pb isotope ratios and REE contents in a single 10 μm sized grain or domain, thus allowing determining the age of specific texture and metamorphic assemblage. Monazite dating of the Elat schist yielded two concordant age clusters at 712±6 and 613±5 Ma. The corresponding REE patterns of the dated monazite grains indicate that porphyroblast growth, either garnet or staurolite

  2. Rapid extensional unroofing of a granite-migmatite dome with relics of high-pressure rocks, the Podolsko complex, Bohemian Massif

    Czech Academy of Sciences Publication Activity Database

    Žák, J.; Sláma, Jiří; Burjak, M.

    2017-01-01

    Roč. 154, č. 2 (2017), s. 354-380 ISSN 0016-7568 Institutional support: RVO:67985831 Keywords : anisotropy of magnetic susceptibility (AMS) * granite-migmatite dome * exhumation * metamorphic core complex * U-Pb zircon geochronology Subject RIV: DB - Geology ; Mineralogy OBOR OECD: Geology Impact factor: 1.965, year: 2016

  3. U-Pb zircon geochronology of intrusive and basement rocks in the Jacurici Valley region, Sao Francisco Craton, BA, Brazil; Geocronologia U-Pb em zircao de rochas intrusivas e de embasamento na regiao do Vale do Jacurici, Craton do Sao Francisco, BA

    Energy Technology Data Exchange (ETDEWEB)

    Silveira, Carlos Jose Sobrinho da; Frantz, Jose Carlos; Marques, Juliana Charao; Roos, Siegbert; Peixoto, Vinicius Medina, E-mail: silveira.carlosjose@hotmail.com, E-mail: jose.frantz@propesq.ufrgs.br, E-mail: juliana.marques@ufrgs.br, E-mail: sig_duda@yahoo.com.br, E-mail: vinimpeixoto@hotmail.com [Universidade Federal do Rio Grande do Sul (UFRGS), Porto Alegre, RS (Brazil). Laboratorio de Geologia Isotopica; Queiroz, Waldemir Jose Alves de, E-mail: waldemirqueiroz@hotmail.com [Companhia de Ferro Ligas da Bahia (FERBASA), Pojuca, BA (Brazil)

    2015-09-15

    The Jacurici Complex, located in the NE of the Sao Francisco Craton, is constituted by several Cr-mineralized mafic-ultramafic N-S bodies, possible fragments of a single sill disrupted during deformation. Some works suggest it is intruded on the Serrinha Block while others consider it in the Salvador-Curaca Belt. The basement on this region is informally divided into paragneisses and orthogneisses; the latter is supposed to be younger considering it is less deformed. Petrography revealed that some of the paragneisses are alkali-feldspar granite strongly milonitized. The orthogneisses occur at the north and consist, at least in part, of monzogranites with heterogeneous deformation, locally of low temperature. U-Pb zircon dating were performed for five representative samples. Just three provided good concordia ages. A mafic rock produced a 2102 ± 5 Ma age and it is petrographically similar to the metanorites described in the Jacurici Complex, being interpreted as the record of the first pulses of the mafic magmatism. A monzogranite yielded a 2995 ± 15 Ma age, older than expected, related to the Serrinha Block. The alkali-feldspar granite yielded a 2081 ± 3 Ma age. The Itiuba Syenite and the pegmatites that crosscut the Jacurici Complex have similar ages. Considering the lack of information about the supracrustal sequence that hosts the intrusive alkaline and mafic-ultramafic rocks at the Ipueira and the Medrado areas, it is possible that part of the terrain belongs to the Salvador-Curaca Belt. We suggest that the Jacurici Complex could be intruded after the tectonic amalgamation between the Serrinha Block and the older part of the Salvador-Curaca Belt and, therefore, could be hosted by both terrains. (author)

  4. Ti-in-zircon thermometry: applications and limitations

    Science.gov (United States)

    Fu, Bin; Page, F. Zeb; Cavosie, Aaron J.; Fournelle, John; Kita, Noriko T.; Lackey, Jade Star; Wilde, Simon A.; Valley, John W.

    2008-08-01

    The titanium concentrations of 484 zircons with U-Pb ages of ˜1 Ma to 4.4 Ga were measured by ion microprobe. Samples come from 45 different igneous rocks (365 zircons), as well as zircon megacrysts (84) from kimberlite, Early Archean detrital zircons (32), and zircon reference materials (3). Samples were chosen to represent a large range of igneous rock compositions. Most of the zircons contain less than 20 ppm Ti. Apparent temperatures for zircon crystallization were calculated using the Ti-in-zircon thermometer (Watson et al. 2006, Contrib Mineral Petrol 151:413-433) without making corrections for reduced oxide activities (e.g., TiO2 or SiO2), or variable pressure. Average apparent Ti-in-zircon temperatures range from 500° to 850°C, and are lower than either zircon saturation temperatures (for granitic rocks) or predicted crystallization temperatures of evolved melts (˜15% melt residue for mafic rocks). Temperatures average: 653 ± 124°C (2 standard deviations, 60 zircons) for felsic to intermediate igneous rocks, 758 ± 111°C (261 zircons) for mafic rocks, and 758 ± 98°C (84 zircons) for mantle megacrysts from kimberlite. Individually, the effects of reduced a_{TiO2} or a_{SiO2}, variable pressure, deviations from Henry’s Law, and subsolidus Ti exchange are insufficient to explain the seemingly low temperatures for zircon crystallization in igneous rocks. MELTs calculations show that mafic magmas can evolve to hydrous melts with significantly lower crystallization temperature for the last 10-15% melt residue than that of the main rock. While some magmatic zircons surely form in such late hydrous melts, low apparent temperatures are found in zircons that are included within phenocrysts or glass showing that those zircons are not from evolved residue melts. Intracrystalline variability in Ti concentration, in excess of analytical precision, is observed for nearly all zircons that were analyzed more than once. However, there is no systematic change in Ti

  5. U-Pb age constraints for the La Tuna Granite and Montevideo Formation (Paleoproterozoic, Uruguay): Unravelling the structure of the Río de la Plata Craton

    Science.gov (United States)

    Pamoukaghlián, Karina; Gaucher, Claudio; Frei, Robert; Poiré, Daniel G.; Chemale, Farid; Frei, Dirk; Will, Thomas M.

    2017-11-01

    The Río de la Plata Craton is a continental block that crops out in Uruguay, eastern Argentina, southernmost Brazil and Paraguay. It comprises in Uruguay the Piedra Alta, Tandilia and Nico Pérez terranes, separated by the Colonia and the Sarandí del Yí megashears. The La Tuna Granite, which intrudes the Araminda metasandstones in the Tandilia Terrane, was considered Cambrian in age and the intruded sandstones were assigned to the Neoproterozoic Piedras de Afilar Formation. We show that the granite is Paleoproterozoic in age and that the host metasandstones do not belong to the Piedras de Afilar Formation, but to the Paleoproterozoic Montevideo Formation. U-Pb LA ICP-MS of zircon ages for the La Tuna Granite yielded a concordant crystallization age of 2156 ± 26 Ma. Furthermore a metamorphic event at 2010 ± 9 Ma is revealed by Pb stepwise leaching dating of monazites. U-Pb detrital zircon ages of the host Araminda metasandstone yield an upper intercept discordia age of 2152 ± 29 Ma, which marks the intrusion of the La Tuna pluton, and which is in accordance with the zircon U-Pb LA ICP MS constraints. A concordant U-Pb detrital zircon age of 2465 ± 40 Ma provides a maximum depositional age constraint for the metapsammites. Comparing quartz arenites of the Ediacaran Piedras de Afilar Formation with the Araminda metaquartzites, we conclude that they are very similar regarding petrology but they differ in age and metamorphic overprint. Detrital zircons in quartz arenites of the Piedras de Afilar Formation show youngest ages of 1.0 Ga. On the other hand, detrital zircons recovered from the Araminda metasandstones and the age of the intruding granite allow interpreting a depositional age between 2465 and 2150 Ma. Nd model ages show crustal residence times in average more than 200 myr older for the Tandilia Terrane both in Uruguay and Argentina, with a significant Neoarchean component, which is lacking in the Piedra Alta Terrane. Whereas the Piedra Alta Terrane was

  6. 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: alan.costa@cprm.gov.br, E-mail: alexandre.dantas@cprm.gov.br [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: marcos@geologia.ufrn.br, E-mail: galindo@geologia.ufrn.br [Universidade Federal do Rio Grande do Norte (UFRN), Natal, RN (Brazil). Departamento de Geologia

    2015-12-15

    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)

  7. At what conditions does zircon grow/dissolve during high-T metamorphism? Relating zircon textures to PT-conditions

    Science.gov (United States)

    Kunz, Barbara E.; Regis, Daniele; Manzotti, Paola; Engi, Martin

    2015-04-01

    A key question in ziconology is when and how zircon grows during metamorphism. To shed light on zircon forming processes and the corresponding PT-conditions during high-T metamorphism a case study was undertaken. The Ivrea Zone (N-Italy) exposes a lower continental crustal section in which a continuous metamorphic field gradient from amphibolite to granulite facies is documented. This field gradient is thought to reflect protracted heating during late Paleozoic times, with a probable high-T peak in the Permian. We present first results from a primarily textural study supported by U-Pb ages, Th/U ratios and Ti-in-Zrn thermometry. Four types of zircon were identified based on their overgrowth proportions and the preservation of detrital cores. Zircon grains were thus classified as Type1 - detrital grains with no overgrowth or very narrow rims (300 Ma) and appears to reflect an early dehydration phase. Rim2b has Permian ages (median 275 Ma), is by far the most common overgrowth type, found in a wide PT-range. Its development appears related to biotite breakdown. Rim3 is texturally indicative of magmatic zircon, occurs only in diatexites. Rim4 is the latest overgrowth and is locally found at all metamorphic grades. Textural features suggest late fluid-related recrystallization of existing zircon. At lowest grade (675±35°C, 6±2 kbar) zircons show type1 only, overgrowths are too thin to clearly identify the rim type. Further upgrade (~700°C, 7 kbar) type1 and type2 dominate. Type2 zircons show rim1, rim2a and occasionally rim4. At the Mu-out isograd (750±50°C, 8.2±1.4 kbar) most zircons are of type2, now with rim2b instead of 2a, in addition to rim1 and rim4. Near and in granulite facies (to 800°C, 8±2 kbar) mostly zircon type2 and type4 are present. While rim1 gets more narrow with increasing metamorphic grade, rim2b grows significantly thicker. Occasionally rim2a and rim4 occur. Close to the Bt-out isograd (~860°C, 9.2±1.7 kbar), mostly type3 and type4 are

  8. Zircon and allanite U-Pb ID-TIMS ages of vaugnerites from the Calzadilla pluton, Salamanca (Spain): dating mantle-derived magmatism and post-magmatic subsolidus overprint

    International Nuclear Information System (INIS)

    López-Moro, F.J.; Romer, R. L.; López-Plaza, M.; González Sánchez, M.

    2017-01-01

    Basic to intermediate high-K, high-Mg mantle-derived rocks occur throughout the Iberian Massif and are particularly important in the Tormes Dome, where vaugnerites form several stocks and small plutons. One of the largest and geochemically most variable among these plutons is the Calzadilla pluton in the Tormes Dome that crystallized at 318 ± 1.4Ma (Bashkirian; U-Pb TIMS zircon). This age reveals that the vaugnerite pluton was emplaced during the transition from late D2 extensional deformation to early D3 contractional deformation (319 to 317Ma). Large-scale extension in the area resulted, on one hand, in extensive anatexis in the crust due to quasiisothermal decompression and mica-dehydration melting and, on the other hand, in the upwelling of the mantle, which induced partial melting of the enriched domains in the lithospheric mantle. The driving reason why crustal and mantle melts were coeval is extension. The U-Pb ID-TIMS age of allanite is not related to the emplacement nor cooling of the Calzadilla vaugnerite, but it seems to be related to a younger subsolidus overprint ca. 275Ma that, in the scale of the Central Iberian Zone, corresponds to a period of hydrothermal alteration, including episyenite formation and tungsten mineralization.

  9. Zircon and allanite U-Pb ID-TIMS ages of vaugnerites from the Calzadilla pluton, Salamanca (Spain): dating mantle-derived magmatism and post-magmatic subsolidus overprint

    Energy Technology Data Exchange (ETDEWEB)

    López-Moro, F.J.; Romer, R. L.; López-Plaza, M.; González Sánchez, M.

    2017-07-01

    Basic to intermediate high-K, high-Mg mantle-derived rocks occur throughout the Iberian Massif and are particularly important in the Tormes Dome, where vaugnerites form several stocks and small plutons. One of the largest and geochemically most variable among these plutons is the Calzadilla pluton in the Tormes Dome that crystallized at 318 ± 1.4Ma (Bashkirian; U-Pb TIMS zircon). This age reveals that the vaugnerite pluton was emplaced during the transition from late D2 extensional deformation to early D3 contractional deformation (319 to 317Ma). Large-scale extension in the area resulted, on one hand, in extensive anatexis in the crust due to quasiisothermal decompression and mica-dehydration melting and, on the other hand, in the upwelling of the mantle, which induced partial melting of the enriched domains in the lithospheric mantle. The driving reason why crustal and mantle melts were coeval is extension. The U-Pb ID-TIMS age of allanite is not related to the emplacement nor cooling of the Calzadilla vaugnerite, but it seems to be related to a younger subsolidus overprint ca. 275Ma that, in the scale of the Central Iberian Zone, corresponds to a period of hydrothermal alteration, including episyenite formation and tungsten mineralization.

  10. U-Pb zircon constraints on the age of the Cretaceous Mata Amarilla Formation, Southern Patagonia, Argentina: Its relationship with the evolution of the Austral Basin

    International Nuclear Information System (INIS)

    Varela, Augusto N; Poire, Daniel G; Martin, Thomas; Gerdes, Axel; Goin, Francisco J; Gelfo, Javier N; Hoffmann, Simone

    2012-01-01

    Despite the abundant fossil content of the Mata Amarilla Formation (Southern Patagonia, Santa Cruz Province, Argentina), its age has always generated a considerable number of questions and debates. The chronological data provided by invertebrates, dinosaurs, fish, turtles, plesiosaurs and fossil flora are contradictory. In this work, twenty U-Pb spot analyses by laser ablation were carried out on the outer parts of the zoned zircon crystals from a tuff layer of the middle section of the Mata Amarilla Formation, yielding a U-Pb concordia age of 96.23±0.71 Ma, which corresponds to the middle Cenomanian. The deposition of the lower section of the Mata Amarilla Formation marks the onset of the foreland stage of the Austral Basin (also known as Magallanes Basin); this transition is characterized by the west-east shift of the depositional systems, which is consistent with the progradation of the Cretaceous fold-and-thrust belt. Thus, the onset of the foreland stage could have occurred between the upper Albian and lower Cenomanian, as the underlying Piedra Clavada Formation is lower Albian in age. On comparing the data obtained with information from the Ultima Esperanza Province in Chile, it can be suggested that the initiation of the closure of the Rocas Verdes Marginal Basin occurred simultaneously

  11. 40Ar/39Ar geochronology of terrestrial pyroxene

    Science.gov (United States)

    Ware, Bryant; Jourdan, Fred

    2018-06-01

    Geochronological techniques such as U/Pb in zircon and baddeleyite and 40Ar/39Ar on a vast range of minerals, including sanidine, plagioclase, and biotite, provide means to date an array of different geologic processes. Many of these minerals, however, are not always present in a given rock, or can be altered by secondary processes (e.g. plagioclase in mafic rocks) limiting our ability to derive an isotopic age. Pyroxene is a primary rock forming mineral for both mafic and ultramafic rocks and is resistant to alteration process but attempts to date this phase with 40Ar/39Ar has been met with little success so far. In this study, we analyzed pyroxene crystals from two different Large Igneous Provinces using a multi-collector noble gas mass spectrometer (ARGUS VI) since those machines have been shown to significantly improve analytical precision compared to the previous single-collector instruments. We obtain geologically meaningful and relatively precise 40Ar/39Ar plateau ages ranging from 184.6 ± 3.9 to 182.4 ± 0.8 Ma (2σ uncertainties of ±1.8-0.4%) and 506.3 ± 3.4 Ma for Tasmanian and Kalkarindji dolerites, respectively. Those data are indistinguishable from new and/or published U-Pb and 40Ar/39Ar plagioclase ages showing that 40Ar/39Ar dating of pyroxene is a suitable geochronological tool. Scrutinizing the analytical results of the pyroxene analyses as well as comparing them to the analytical result from plagioclase of the same samples indicate pure pyroxene was dated. Numerical models of argon diffusion in plagioclase and pyroxene support these observations. However, we found that the viability of 40Ar/39Ar dating approach of pyroxene can be affected by irradiation-induced recoil redistribution between thin pyroxene exsolution lamellae and the main pyroxene crystal, hence requiring careful petrographic observations before analysis. Finally, diffusion modeling show that 40Ar/39Ar of pyroxene can be used as a powerful tool to date the formation age of mafic

  12. Did the Kiruna iron ores form as a result of a metasomatic or igneous process? New U-Pb and Nd data for the iron oxide apatite ores and their host rocks in the Norrbotten region of northern Sweden

    Science.gov (United States)

    Westhues, A.; Hanchar, J. M.; Whitehouse, M. J.; Fisher, C. M.

    2012-12-01

    A number of iron deposits near Kiruna in the Norrbotten region of northern Sweden are of the iron oxide apatite (IOA) type of deposits; also referred to as Kiruna-type deposits. They are commonly considered a subgroup or end-member of iron oxide copper gold (IOCG) deposits, containing no economic grades of copper or gold. Both IOCG and IOA deposits are characterized by abundant low-Ti Fe oxides, an enrichment in REE, and intense sodium and potassium wall-rock alteration adjacent to the ores. Deposits of these types are of a great economic importance, not only for iron, but also for other elements such as rare earth elements (REE) or uranium. Kiruna, the type locality of the IOA type of mineral deposits, is the focus of this study. Despite a century-long mining history and 2500 Mt of iron ore produced in the region to date (with grades of 30 to 70 wt.% Fe), the genesis of these deposits is poorly understood: theories of a magmatic vs. a hydrothermal or metasomatic origin have been debated, and the timing of mineralization of the ores in the Norbotten region has never been directly dated. The results anticipated from this study will provide a better understanding of the nature of the IOA type of mineral deposits and their relation to IOCG deposits such as Olympic Dam in Australia. An array of geochemical methods is used in order to gain insights on the emplacement history of the host rocks, their subsequent alteration, and the ore genesis of these deposits. This includes in situ U/Pb geochronology of zircon, monazite, and titanite to constrain the timing between host rock emplacement, alteration and mineralization. Isotopic data from whole rocks and in situ at mineral scale will provide constraints on the involvement of hydrothermal fluids and their possible sources, as well as on the sources of Fe, U, and the REE. Newly obtained Sm-Nd isotopic data points to distinct source differences between host rocks, ore and alteration related samples. Preliminary in situ U-Pb

  13. Evolution of the European region

    International Nuclear Information System (INIS)

    Jeger, Eh.

    1984-01-01

    The problem on geochronological study of the European region is covered. The most ancient age values are determined by U-Pb methods by zircones from paragneisses. The model of evolution, being in agreement with the data obtained by U-Pb and Rb-Sr methods, is considered. The history of the Schwarzwald development is typical for the continent as a whole. The diagram of evolution of primary 87 Sr/ 86 Sr for orthogneisses and granites in France is given

  14. Early to Middle Jurassic tectonic evolution of the Bogda Mountains, Northwest China: Evidence from sedimentology and detrital zircon geochronology

    Science.gov (United States)

    Ji, Hongjie; Tao, Huifei; Wang, Qi; Qiu, Zhen; Ma, Dongxu; Qiu, Junli; Liao, Peng

    2018-03-01

    The Bogda Mountains, as an important intracontinental orogenic belt, are situated in the southern part of the Central Asian Orogenic Belt (CAOB), and are a key area for understanding the Mesozoic evolution of the CAOB. However, the tectonic evolution of the Bogda Mountains remains controversial during the Mesozoic Era, especially the Early to Middle Jurassic Periods. The successive Lower to Middle Jurassic strata are well preserved and exposed along the northern flank of the Western Bogda Mountains and record the uplift processes of the Bogda Mountains. In this study, we analysed sedimentary facies combined with detrital zircon U-Pb geochronology at five sections of Lower to Middle Jurassic strata to detect the tectonic evolution and changes of provenance in the Bogda area. During Early to Middle Jurassic times, the fluvial, deltaic and lacustrine environments dominated in the western section of the Bogda area. The existence of Early Triassic peak age indicates that the Bogda Mountains did not experience uplift during the period of early Badaowan Formation deposition. The Early Triassic to Late Permian granitoid plutons and Carboniferous volcanic rocks from the Barkol and Santanghu areas were the main provenances. The significant change in the U-Pb age spectrum implies that the Eastern Bogda Mountains initiated uplift in the period of late Badaowan Formation deposition, and the Eastern Junggar Basin and the Turpan-Hami Basin were partially partitioned. The Eastern Bogda Mountains gradually became the major provenance. From the period of early Sangonghe to early Toutunhe Formations deposition, the provenance of the sediments and basin-range frame were similar to that of late Badaowan. However, the Eastern Bogda Mountains suffered intermittent uplift three times, and successive denudation. The uplifts respectively happened in early Sangonghe, late Sangonghe to early Xishanyao, and late Xishanyao to early Toutunhe. During the deposition stage of Toutunhe Formation, a

  15. Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico: COMMENT

    Science.gov (United States)

    Koenig, Alan E.; Lucas, Spencer G.; Neymark, Leonid A.; Heckert, Andrew B.; Sullivan, Robert M.; Jasinski, Steven E.; Fowler, Denver W.

    2012-01-01

    Based on U-Pb dating of two dinosaur bones from the San Juan Basin of New Mexico (United States), Fassett et al. (2011) claim to provide the first successful direct dating of fossil bones and to establish the presence of Paleocene dinosaurs. Fassett et al. ignore previously published work that directly questions their stratigraphic interpretations (Lucas et al., 2009), and fail to provide sufficient descriptions of instrumental, geochronological, and statistical treatments of the data to allow evaluation of the potentially complex diagenetic and recrystallization history of bone. These shortcomings lead us to question the validity of the U-Pb dates published by Fassett et al. and their conclusions regarding the existence of Paleocene dinosaurs.

  16. Age of origin of the polymetamorphosed Copperton Formation, Namaqua-Natal Province, determined by single grain zircon Pb-Pb dating

    International Nuclear Information System (INIS)

    Cornell, D.H.; Griffin, G.; Kroener, A.; Humphreys, H.

    1990-01-01

    The Copperton Formation of the Areachap Group in the Northern Cape Province is a strongly metamorphosed, predominantly metavolcanic unit which contains several massive sulphide deposits. Previous work on this unit has established a complex metamorphic history, the four main phases of which are fairly well dated by a combination of methods; however, the age of origin is still poorly constrained. Zircons extracted from the Smouspan Gneiss Member of the Copperton formation are euhedral and show a crystal habit which indicates an igneous and probably volcanic origin, with no discernable metamorphic component. Whole-grain thermal evaporation on four such zircon grains yielded a precise set of 207 Pb/ 206 Pb data which establishes a 1 285 ± 14 Ma age for the origin of the Smouspan Gneiss and Copperton Formation. Indications of a ∼1 600 Ma age from other parts of the Areachap Group should be treated with caution until more reliable data are obtained. 9 figs., 2 tabs., 29 refs

  17. Geochemical Astro- and Geochronological Constraints on the Early Jurassic

    Science.gov (United States)

    Storm, M.; Condon, D. J.; Ruhl, M.; Jenkyns, H. C.; Hesselbo, S. P.; Al-Suwaidi, A. H.; Percival, L.

    2017-12-01

    The Early Jurassic Hettangian and Sinemurian time scales are poorly defined due to the lack of continuous geochemical records, and the temporal constrain of the Toarcian Oceanic Anoxic Event and associated global carbon cycle perturbation is afflicted by geochemical and biostratigraphical uncertainties of the existing radiometric dates from various volcanic ash bearing sections. Here we present a continuous, orbitally paced Hettangian to Pliensbachian carbon-isotope record of the Mochras drill-core (Cardigan bay Basin, UK). The record generates new insights into the evolution and driving mechanisms of the Early Jurassic carbon cycle, and is contributing to improve the Hettangian and Sinemurian time scale. Furthermore, we introduce a new high-resolution carbon-isotope chemostratigraphy, integrated with ammonite biostratigraphy and new U/Pb single zircon geochronology of the Las Overas section (Neuquén Basin, Argentina). The studied section comprises sediments from the tenuicostatum to Dumortiera Andean Ammonite zone (tenuicostatum to levesqui European standard zones). A stratigraphically expanded negative shift in d13Corg values, from -24‰ down to -32­‰, appears in the tenuicostatum and hoelderi ammonite zone, coeval to the negative excursion in European realm which is associated with the Toarcian Oceanic Anoxic Event. The negative isotope excursion appears concomitant with an increase in sedimentary mercury levels, indicating enhanced volcanic activity. TOC values and elemental data suggest a high sedimentation dilution in the tenuicostatum to pacificum zone. The new geochronological data from several volcanic ash beds throughout the section are further improving the temporal correlation between the Early Toarcian isotope event and causal mechanisms

  18. Zircon U-Pb age, Lu-Hf isotope, mineral chemistry and geochemistry of Sundamalai peralkaline pluton from the Salem Block, southern India: Implications for Cryogenian adakite-like magmatism in an aborted-rift

    Science.gov (United States)

    Renjith, M. L.; Santosh, M.; Li, Tang; Satyanarayanan, M.; Korakoppa, M. M.; Tsunogae, T.; Subba Rao, D. V.; Kesav Krishna, A.; Nirmal Charan, S.

    2016-01-01

    The Sundamalai peralkaline pluton is one among the Cryogenian alkaline plutons occurring in the Dharmapuri Rift Zone (DRZ) of the Salem Block in the Southern Granulite Terrane (SGT) of India. Here we present zircon U-Pb age and Lu-Hf isotopic composition, mineral chemistry and geochemistry of the pluton to explore the petrogenesis and geodynamic implications. Systematic modal variation of orthoclase, Na-plagioclase, Ca-amphibole (ferro-edenite and hastingsite) and quartz developed quartz-monzonite and granite litho units in the Sundamalai pluton. Thermometry based on amphibole-plagioclase pair suggests that the pluton was emplaced and solidified at around 4.6 kbar pressure with crystallization of the major phases between 748 and 661 °C. Estimated saturation temperature of zircon (712-698 °C) is also well within this range. However, apatite saturation occurred at higher temperatures between 835 and 870 °C, in contrast with monazite saturation (718-613 °C) that continued up to the late stage of crystallization. Estimated oxygen fugacity values (log fO2: -14 to -17) indicate high oxidation state for the magma that stabilized titanite and magnetite. The magmatic zircons from Sundamalai pluton yielded a weighted mean 206Pb/238U age of 832.6 ± 3.2 Ma. Geochemically, the Sundamalai rocks are high-K to shoshonitic, persodic (Na2O/K2O ratio > 1), silica-saturated (SiO2:65-72 wt.%), and peralkaline in composition (aluminum saturation index, ASI Sc(2-3 ppm), positive Sr anomaly and predominantly negative zircon εHf(t) values (-10.8 to -9.3 with an average of -10.2) and initial 176Hf/177Hf ratios (0.281947-0.282022) confirm a Paleoproterozoic crustal source. Based on the field and geochemical evidences, we propose that a previously metasomatized mafic lower-crustal source enriched in alkalis has undergone CO2-present partial melting as a result of asthenospheric upwelling beneath an aborted rifting along the DRZ generating the magma that crystallized the Sundamalai rocks

  19. Quantifying time in sedimentary successions by radio-isotopic dating of ash beds

    Science.gov (United States)

    Schaltegger, Urs

    2014-05-01

    global environmental and biotic disturbance (from ash bed analysis in biostratigraphically or cyclostratigraphically well constrained marine sections) with volcanic activity; examples are the Triassic-Jurassic boundary and the Central Atlantic Magmatic Province (Schoene et al. 2010), or the lower Toarcian oceanic anoxic event and the Karoo Province volcanism (Sell et al. in prep.). High-precision temporal correlations may also be obtained by combining high-precision U-Pb dating with biochronology in the Middle Triassic (Ovtcharova et al., in prep.), or by comparing U-Pb dates with astronomical timescales in the Upper Miocene (Wotzlaw et al., in prep.). References Guex, J., Schoene, B., Bartolini, A., Spangenberg, J., Schaltegger, U., O'Dogherty, L., et al. (2012). Geochronological constraints on post-extinction recovery of the ammonoids and carbon cycle perturbations during the Early Jurassic. Palaeogeography, Palaeoclimatology, Palaeoecology, 346-347(C), 1-11. Ovtcharova, M., Bucher, H., Schaltegger, U., Galfetti, T., Brayard, A., & Guex, J. (2006). New Early to Middle Triassic U-Pb ages from South China: Calibration with ammonoid biochronozones and implications for the timing of the Triassic biotic recovery. Earth and Planetary Science Letters, 243(3-4), 463-475. Ovtcharova, M., Goudemand, N., Galfetti, Th., Guodun, K., Hammer, O., Schaltegger, U., Bucher, H. Improving accuracy and precision of radio-isotopic and biochronological approaches in dating geological boundaries: The Early-Middle Triassic boundary case. In preparation. Schoene, B., Schaltegger, U., Brack, P., Latkoczy, C., Stracke, A., & Günther, D. (2012). Rates of magma differentiation and emplacement in a ballooning pluton recorded by U-Pb TIMS-TEA, Adamello batholith, Italy. Earth and Planetary Science Letters, 355-356, 162-173. Schoene, B., Latkoczy, C., Schaltegger, U., & Günther, D. (2010). A new method integrating high-precision U-Pb geochronology with zircon trace element analysis (U-Pb TIMS

  20. Precise age for the Permian-Triassic boundary in South China from high-precision U-Pb geochronology and Bayesian age-depth modeling

    Science.gov (United States)

    Baresel, Björn; Bucher, Hugo; Brosse, Morgane; Cordey, Fabrice; Guodun, Kuang; Schaltegger, Urs

    2017-03-01

    This study is based on zircon U-Pb ages of 12 volcanic ash layers and volcanogenic sandstones from two deep water sections with conformable and continuous formational Permian-Triassic boundaries (PTBs) in the Nanpanjiang Basin (South China). Our dates of single, thermally annealed and chemically abraded zircons bracket the PTB in Dongpan and Penglaitan and provide the basis for a first proof-of-concept study utilizing a Bayesian chronology model comparing the three sections of Dongpan, Penglaitan and the Global Stratotype Section and Point (GSSP) at Meishan. Our Bayesian modeling demonstrates that the formational boundaries in Dongpan (251.939 ± 0.030 Ma), Penglaitan (251.984 ± 0.031 Ma) and Meishan (251.956 ± 0.035 Ma) are synchronous within analytical uncertainty of ˜ 40 ka. It also provides quantitative evidence that the ages of the paleontologically defined boundaries, based on conodont unitary association zones in Meishan and on macrofaunas in Dongpan, are identical and coincide with the age of the formational boundaries. The age model also confirms the extreme condensation around the PTB in Meishan, which distorts the projection of any stratigraphic points or intervals onto other more expanded sections by means of Bayesian age-depth models. Dongpan and Penglaitan possess significantly higher sediment accumulation rates and thus offer a greater potential for high-resolution studies of environmental proxies and correlations around the PTB than Meishan. This study highlights the power of high-resolution radio-isotopic ages that allow a robust intercalibration of patterns of biotic changes and fluctuating environmental proxies and will help recognizing their global, regional or local significance.

  1. Zircon trace element and isotopic (Sr, Nd, Hf, Pb) effects of assimilation-fractional crystallization of pegmatite magma: A case study of the Guangshigou biotite pegmatites from the North Qinling Orogen, central China

    Science.gov (United States)

    Yuan, Feng; Liu, Jia-Jun; Carranza, Emmanuel John M.; Zhang, Shuai; Zhai, De-Gao; Liu, Gang; Wang, Gong-Wen; Zhang, Hong-Yuan; Sha, Ya-Zhou; Yang, Shang-Song

    2018-03-01

    Evidence for open-system magmatic processes related to wallrock assimilation accompanied by fractional crystallization (AFC) is present in the Guangshigou biotite pegmatites, North Qinling Orogen. The biotite pegmatite-gneiss contacts generally coincide with the greatest enrichment of U and Th. Zircon Usbnd Pb dating constrains the crystallization ages of the biotite pegmatite (rim zone-415 ± 2.6 Ma; internal zone-413.5 ± 2.5 Ma), in line with a pyrite Pbsbnd Pb isochron age (413 ± 22 Ma). Metamict areas in zircon show generally elevated concentrations of trace elements and expulsion of radioactive Pb. Internal zone samples, representing uncontaminated magma, have negative to positive zircon ( 413 Ma) εHf(t) (- 1.53 - + 3.24), low εNd(t) values (- 2.4), and old Hf and Nd model ages (tDM2 = 1.5-1.19 Ga, T2DM = 1.35 Ga, respectively), indicating a dominantly recycled Mesoproterozoic lower crustal material with involvement of some juvenile materials in the source region. The magmatic oxygen fugacity (fO2) and crystallization temperatures ranges from - 24.81 to - 13.34 of log fO2 and 570 °C to 793 °C, respectively. Compared to the internal zone, pegmatite rim samples display a variable and lower εNd(t) values (- 3.9 to - 2.8) and T2DM (1.47-1.37 Ga), but similar Hf isotopic compositions, favouring a three-component isotopic mixing model (recycled Mesoproterozoic lower crust materials, juvenile materials, and host gneiss). Pronounced variations of Ti, Y, U, Th, Hf, and REE concentrations in zircon from grain to grain in individual samples and from area to area within individual grains suggest a fluctuating crystallization environment in hybridized magma from which the rim-hosted zircons crystallized. Variable and high radiogenic Pb ratios of pyrites forming in the hybridized magma were inherited from the matrix. Zircons from both zones exhibit similar Hf isotope patterns, indicating the rim-hosted zircons crystallized during the early stage of hybridization of

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

    Science.gov (United States)

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

    2016-04-01

    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. Metamorphic P-T path and zircon U-Pb dating of HP mafic granulites in the Yushugou granulite-peridotite complex, Chinese South Tianshan, NW China

    Science.gov (United States)

    Zhang, Lu; Zhang, Lifei; Xia, Bin; Lü, Zeng

    2018-03-01

    Co-existing granulite and peridotite may represent relics of the paleo-suture zone and provides an optimal opportunity for better understanding of orogeny between two blocks. In this study, we carried out petrological and U-Pb zircon dating investigation on the HP mafic granulites associated with peridotite complex at Yushugou in Chinese South Tianshan. The studied samples include garnet-bearing high-pressure mafic granulites which can be subdivided into two types: Type I orthopyroxene-free and Type II orthopyroxene-bearing granulites and amphibolite. Type I granulite (Y21-2) has a mineral assemblage of garnet (33 vol.%), clinopyroxene (32 vol.%) and plagioclase (30 vol.%); and Type II granulite (Y18-8) has a mineral assemblage of garnet (22 vol.%), clinopyroxene (10 vol.%), orthopyroxene (14 vol.%), plagioclase (45 vol.%) and quartz. Garnet in both granulites exhibits core-rim structure characterized by increasing grossular and decreasing pyrope from core to rim. Petrographic observations and phase equilibrium modeling using THERMOCALC in the NCFMASHTO system for the mafic granulites (Y21-2 and Y18-8) show three stages of metamorphism: Stage I (granulite facies) was recognized by the large porphyroblastic garnet core, with P-T conditions of 9.8-10.4 Kbar and 860-900 °C (Y21-2) and 9.9-10.6 Kbar and 875-890 °C (Y18-8), respectively; Stage II (HP granulite facies) has peak P-T conditions of 12.1 Kbar at 755 °C (Y21-2) and 13.8 Kbar at 815 °C (Y18-8) using mineral assemblages combining with garnet rim compositions with maximum grossular and minimum pyrope contents; Stage III (amphibolite facies) was characterized by the development of calcic amphibole in granulites with temperature of 446-563 °C. Therefore, an anticlockwise P-T path characterized by simultaneous temperature-decreasing and pressure-increasing was inferred for the Yushugou HP mafic granulite. Studies of zircon morphology and inclusions, combined with zircon U-Pb dating and REE geochemistry

  4. U-Pb and Hf isotope analysis of detrital zircons from Mesozoic strata of the Gravina belt, southeast Alaska

    Science.gov (United States)

    Yokelson, Intan; Gehrels, George E.; Pecha, Mark; Giesler, Dominique; White, Chelsi; McClelland, William C.

    2015-10-01

    The Gravina belt consists of Upper Jurassic through Lower Cretaceous marine clastic strata and mafic-intermediate volcanic rocks that occur along the western flank of the Coast Mountains in southeast Alaska and coastal British Columbia. This report presents U-Pb ages and Hf isotope determinations of detrital zircons that have been recovered from samples collected from various stratigraphic levels and from along the length of the belt. The results support previous interpretations that strata in the western portion of the Gravina belt accumulated along the inboard margin of the Alexander-Wrangellia terrane and in a back-arc position with respect to the western Coast Mountains batholith. Our results are also consistent with previous suggestions that eastern strata accumulated along the western margin of the inboard Stikine, Yukon-Tanana, and Taku terranes and in a fore-arc position with respect to the eastern Coast Mountains batholith. The history of juxtaposition of western and eastern assemblages is obscured by subsequent plutonism, deformation, and metamorphism within the Coast Mountains orogen, but may have occurred along an Early Cretaceous sinistral transform system. Our results are inconsistent with models in which an east-facing subduction zone existed along the inboard margin of the Alexander-Wrangellia terrane during Late Jurassic-Early Cretaceous time.

  5. U-Pb geochronology and paleomagnetism of the Neoproterozoic St Simeon dolerite dykes, Quebec: an eastern Laurentian perspective of Ediacaran Rodinia breakup

    Science.gov (United States)

    Pisarevsky, Sergei; Murphy, Brendan; Hamilton, Mike; Söderlund, Ulf; Hodych, Joseph

    2013-04-01

    The St Simeon (SS) mafic dykes (150 km NE of Quebec City) are now dated at 548 ± 1 Ma (U-Pb; baddeleyite). This age is similar to a published LA-ICPMS zircon age of 550 ± 7 Ma for the Mt. St-Anselme (MS) basalts, which supports previous inferences of (i) a genetic relationship between them, (ii) the pene-contemporaneity of OIB-type mafic magmatism in East Laurentia and (iii) the existence of two late Ediacaran plumes that attended the final breakup of Rodinia and opening of the Iapetus Ocean and Tornquist Sea. Both the SS dykes and the MS basalts were sampled for paleomagnetic study. The paleomagnetic pole for SS is similar to the previously published pole for coeval basalts (Skinner Cove, SC) from Newfoundland. Unlike SC, the St Simeon pole represents rocks which are unambiguously coherent tectonically with the Laurentian Craton. This new pole is also coeval with high quality poles from the Winter Coast (Baltica) and provides paleomagnetic constraints on the history of the final breakup of Rodinia and opening of Eastern Iapetus and Tornquist Sea.

  6. On the valency state of radiogenic lead in zircon and its consequences

    DEFF Research Database (Denmark)

    Kramers, J.; Frei, Robert; Newville, M.

    2009-01-01

    nucleus comes to rest. Further, a zircon grain, being small, should remain highly oxidizing in its interior by the constant loss of ß-particles, maintaining the 4+ state of radiogenic Pb. From its effective ion radius, similar to that of Zr4+, and its charge, Pb4+ has to be compatible in the zircon...... not resemble that of PbO2. The arguments why radiogenic Pb should be tetravalent are based on analogies with studies relating to the tetravalent state of 234Th and the hexavalent state of 234U, which show that a-recoil in silicates generates a strongly oxidizing environment at the site where the recoiling......-recoil damaged sites could be leached out by any electrolyte solution that reduces it to the divalent state, making it both incompatible and soluble. Thus, discordia can be generated in weathering. The curious observation that discordant Archaean zircon suites generally define trends to lower intercepts at up...

  7. The Proterozoic of NW Mexico revisited: U-Pb geochronology and Hf isotopes of Sonoran rocks and their tectonic implications

    Science.gov (United States)

    Solari, L. A.; González-León, C. M.; Ortega-Obregón, C.; Valencia-Moreno, M.; Rascón-Heimpel, M. A.

    2018-04-01

    Several Proterozoic basement units crop out in the Sonora State of NW Mexico, and the same can be correlated with crustal provinces of southern Laurentia in the neighboring southwestern USA. Zircon U-Pb and Hf isotopic determinations in more than 300 grains separated from igneous and metaigneous rocks from these units indicate that the crystalline basement in Sonora is made up of different components, which are from west to east: (1) The Caborca-Mojave province to the west, characterized by the so-called Bámori Complex, have U-Pb ages between 1696 and 1772 Ma, with moderately juvenile to slightly evolved ɛHf values, yielding T DM ages of ca. 2.1-2.4 Ga; (2) in the intermediate area, east of Hermosillo, the Palofierral and La Ramada orthogneiss units yield an age of 1640 and 1703 Ma, respectively, both having juvenile ɛHf with the Palofierral overlapping the depleted mantle curve at ca. 1.65 Ga; and (3) in the northeastern Sonora, samples from the southern extension of the Mazatzal province, represented by the Pinal Schist, yielded ages between 1674 and 1694 Ma, with moderately juvenile to juvenile ɛHf values and a T DM age of ca. 1.9 Ga. In addition, a suite of post-tectonic granites was also studied in Caborca (San Luis granite) as well as in northeastern Sonora (Cananea granite), both yielding ages of ca. 1.44 Ga with moderately juvenile ɛHf values ranging from -1 to +8 and T DM dates of ca. 1.8-1.9 Ga and 1.6-1.7 Ga, respectively. These two isotopically contrasting provinces may imply the existence of a Proterozoic paleo-suture. However, if the Palofierral gneiss, of which the Hf signature straddles the depleted mantle array, is taken as the source for the 1.44 Ga Cananea granite, then the location of such a suture zone should lay farther south than the proposed trace of the Mojave-Sonora megashear.

  8. Tectonomagmatic evolution of the proto Andean Margin: Geochemical characterization and zircon U-Pb geochronologic constraints from the Ecuadorian Eastern Cordilleran granitoids

    Science.gov (United States)

    Buchwaldt, R.; Toulkeridis, T.

    2013-05-01

    The timing of pan-Pacific Gondwanide Orogeny in the proto-Andes, and its driving mechanisms are still highly debated and relies predominantly upon whole-rock Rb-Sr and K-Ar chronology and rudimentary mineralogy and geochemistry. In order to decipher these uncertainties we have studied the composition, age and provenance of granitoids along the strike of the Eastern Cordillera of Ecuador and related these deep-seated and surface tectonic processes attending the Late Cretaceous-Palaeogene history of the northern Andes. The plutonic rocks constitute a metaluminous to peraluminous (A/CNK ~ 0.8-1.2), calc-alkaline suite. A unimodal and wide compositional range of the intrusives (49-78 wt. % SiO2) is characteristic of this I-type orogenic suites. Mantle-normalized trace element patterns reveal typical subduction-related signature. Chondrite-normalized REE patterns do not show significant HREE fractionation suggesting the absence of high-pressure residual mineralogy in the source and formation in a "normal thickness", garnet-free crust. Slight Eu anomalies, lowering Sr contents, and concave-up REE patterns of samples dioritic in composition indicate a model involving fractionation of plagioclase, amphibole and pyroxene from a basaltic parent. The analyzed zircon crystals are colorless - transparent ranging in size from 50 to 250 μm. In CL images, 95% of the zircons exhibit oscillatory zonation, characteristic of a magmatic origin. This observation is consistent with the REE zircon composition showing a are characteristic steep positive slope from La to Lu with a significantly positive Ce-anomaly and slight negative Eu-anomaly. There is very little variation in Hf isotopic composition with most of the crust maintains near chondritic Zr/Hf ratios of around 35-40. Our results indicate the development of two tectonic episodes; with the first varying between 210-250 Ma and the second approximate 170-180 Ma. These results are consistent with similar events observed throughout

  9. Long-lasting Cadomian magmatic activity along an active northern Gondwana margin: U-Pb zircon and Sr-Nd isotopic evidence from the Brunovistulian Domain, eastern Bohemian Massif

    Czech Academy of Sciences Publication Activity Database

    Soejono, I.; Janoušek, V.; Žáčková, E.; Sláma, Jiří; Konopásek, J.; Machek, Matěj; Hanžl, P.

    2017-01-01

    Roč. 106, č. 6 (2017), s. 2109-2129 ISSN 1437-3254 Institutional support: RVO:67985530 ; RVO:67985831 Keywords : Cadomian magmatic arc * Brunovistulian Domain * Bohemian Massif * Gondwana margin * U–Pb geochronology * geochemistry Subject RIV: DB - Geology ; Mineralogy; DC - Siesmology, Volcanology, Earth Structure (GFU-E) OBOR OECD: Geology; Volcanology (GFU-E) Impact factor: 2.283, year: 2016

  10. Detrital Zircon Geochronology of Sedimentary Rocks of the 3.6 - 3.2 Ga Barberton Greenstone Belt: No Evidence for Older Continental Crust

    Science.gov (United States)

    Drabon, N.; Lowe, D. R.; Byerly, G. R.; Harrington, J.

    2017-12-01

    The crustal setting of early Archean greenstone belts and whether they formed on or associated with blocks of older continental crust or in more oceanic settings remains a major issue in Archean geology. We report detrital zircon U-Pb age data from sandstones of the 3.26-3.20 Ga Fig Tree and Moodies Groups and from 3.47 to 3.23 Ga meteorite impact-related deposits in the 3.55-3.20 Ga Barberton greenstone belt (BGB), South Africa. The provenance signatures of these sediments are characterized by zircon age peaks at 3.54, 3.46, 3.40, 3.30, and 3.25 Ga. These clusters are coincident either with the ages of major episodes of felsic to intermediate igneous activity within and around the belt or with the ages of thin felsic tuffs reflecting distant volcanic activity. Only 15 of the reported 3410 grains (old zircons could represent felsic rocks in older, unexposed parts of the BGB sequence, but are too few to provide evidence for a continental source. This finding offers further evidence that the large, thick, high-standing, highly evolved blocks of continental crust with an andesitic bulk composition that characterize the Earth during younger geologic times were scarce in the early Archean.

  11. Empirical constraints on the effects of radiation damage on helium diffusion in zircon

    Science.gov (United States)

    Anderson, Alyssa J.; Hodges, Kip V.; van Soest, Matthijs C.

    2017-12-01

    In this study, we empirically evaluate the impact of radiation damage on zircon (U-Th)/He closure temperatures for a suite of zircon crystals from the slowly cooled McClure Mountain syenite of south-central Colorado, USA. We present new zircon, titanite, and apatite conventional (U-Th)/He dates, zircon laser ablation (U-Th)/He and U-Pb dates, and zircon Raman spectra for crystals from the syenite. Titanite and apatite (U-Th)/He dates range from 447 to 523 Ma and 88.0 to 138.9 Ma, respectively, and display no clear correlation between (U-Th)/He date and effective uranium concentration. Conventional zircon (U-Th)/He dates range from 230.3 to 474 Ma, while laser ablation zircon (U-Th)/He dates show even greater dispersion, ranging from 5.31 to 520 Ma. Dates from both zircon (U-Th)/He datasets decrease with increasing alpha dose, indicating that most of the dispersion can be attributed to radiation damage. Alpha dose values for the dated zircon crystals range from effectively zero to 2.15 × 1019 α /g, spanning the complete damage spectrum. We use an independently constrained thermal model to empirically assign a closure temperature to each dated zircon grain. If we assume that this thermal model is robust, the zircon radiation damage accumulation and annealing model of Guenthner et al. (2013) does not accurately predict closure temperatures for many of the analyzed zircon crystals. Raman maps of the zircons dated by laser ablation document complex radiation damage zoning, sometimes revealing crystalline zones in grains with alpha dose values suggestive of amorphous material. Such zoning likely resulted in heterogeneous intra-crystalline helium diffusion and may help explain some of the discrepancies between our empirical findings and the Guenthner et al. (2013) model predictions. Because U-Th zoning is a common feature in zircon, radiation damage zoning is likely to be a concern for most ancient, slowly cooled zircon (U-Th)/He datasets. Whenever possible, multiple

  12. Late Paleozoic to Mesozoic extension in southwestern Fujian Province, South China: Geochemical, geochronological and Hf isotopic constraints from basic-intermediate dykes

    Directory of Open Access Journals (Sweden)

    Sen Wang

    2017-05-01

    Full Text Available The tectonic evolution of SE China block since late Paleozoic remains debated. Here we present a new set of zircon U-Pb geochronological, Lu-Hf isotopic data and whole-rock geochemistry for two stages of basic-intermediate dykes from the southwestern Fujian. The samples were collected from the NE-trending (mainly diabases and NW-trending (mainly diabasic diorites dykes and yielded zircon U-Pb ages of 315 and 141 Ma, with εHf (t values of −8.90 to 7.49 and −23.39 to −7.15 (corresponding to TDM2 values of 850 to 1890 Ma and 737 to 2670 Ma, respectively. Geochemically these rocks are characterized by low TiO2 (0.91–1.73 wt.% and MgO (3.04–7.96 wt.%, and high Al2O3 (12.5–16.60 wt.% and K2O (0.60–3.63 wt.%. Further they are enriched in LREEs and LILEs (Rb, Ba, Th and K, but depleted in HFSEs (Nb, Ta and Zr. The tectonic discrimination analysis revealed that the dykes were formed in an intraplate extensional environment. However, the NW trending dykes show crust-mantle mixed composition, which indicate an extensional tectonic setting with evidence for crustal contamination. The SE China block experienced two main stages of extensional tectonics from late Carboniferous to early Cretaceous. The tectonic evolution of the SE China block from late Devonian to Cretaceous is also evaluated.

  13. Provenance and geochronological insights into Late Cretaceous-Paleogene foreland basin development in the Subandean Zone and Oriente Basin of Ecuador

    Science.gov (United States)

    Gutierrez, E. G.; Horton, B. K.; Vallejo, C.

    2017-12-01

    The tectonic history of the Oriente foreland basin and adjacent Subandean Zone of Ecuador during contractional mountain building in the northern Andes can be revealed through integrated stratigraphic, geochronological, structural, and provenance analyses of clastic sediments deposited during orogenesis. We present new maximum depositional ages and a comprehensive provenance analysis for key stratigraphic units deposited in the western (proximal) Oriente Basin. Detrital zircon U-Pb ages were obtained from Upper Cretaceous and Cenozoic clastic formations from exposures in the Subandean Zone. The sampled stratigraphic intervals span critical timeframes during orogenesis in the Ecuadorian Andes. Cenozoic formations have poorly defined chronostratigraphic relationships and are therefore a primary target of this study. In addition, the newly acquired U-Pb age spectra allow clear identification of the various sediment source regions that fed the system during distinct depositional phases. Maximum depositional ages (MDA) were obtained for five samples from three formations: the Tena (MDA=69.6 Ma), Chalcana (MDA=29.3 Ma), and Arajuno (MDA= 17.1, 14.2, 12.8 Ma) Formations, placing them in the Maastrichtian, early Oligocene, and early-middle Miocene, respectively. Detrital zircon U-Pb ages identify clear signatures of at least four different sources: craton (1600-1300 Ma, 1250-900 Ma), Eastern Cordillera fold-thrust belt (600-450 Ma, 250-145 Ma), Western Cordillera magmatic arc (age spectra of the Upper Cretaceous-Paleogene type sections allow us to recognize variations in the contribution of each recognized source over time. We identify recycled material with two dominant peak ages (1250-900 Ma and 600-450 Ma), material derived from the adjacent uplifted orogen or recycled from foredeep sediments incorporated into the deforming wedge. Finally, an apparent unroofing event is inferred from a 250-145 Ma age peak in the Plio-Pleistocene Mesa-Mera Formation revealing the

  14. U-Pb ages related to uranium mineralization of Lagoa Real, Bahia - Brazil: tectonic implications

    Energy Technology Data Exchange (ETDEWEB)

    Chaves, Alexandre O.; Rios, Francisco J.; Oliveira, Lucilia A.R.; Alves, James V.; Fuzikawa, Kazuo; Neves, Jose M.C.; Chaves, Adriana M.D.V.; Prates, Sonia P. [Centro de Desenvolvimento da Tecnologia Nuclear (CDTN/CNEN-MG), Belo Horizonte, MG (Brazil)]. E-mail: aoc@cdtn.br; Tubrett, Michael [Memorial University of Newfoundland, NL (Canada). Inco Innovation Centre. MicroAnalysis Facility; Matos, Evando C. [Industrias Nucleares do Brasil S.A. (INB), Rio de Janeiro, RJ (Brazil)

    2007-07-01

    The uranium from Lagoa Real (Bahia/Brazil) is associated with syenitic magmatism (1904 {+-}44 Ma minimum age from upper intercept of zircon U-Pb discordia by LA-ICP-MS) belonging to a mafic/ felsic lithologic association linked to the final stages of 2.05-1.80 Ga Orosirian Orogeny, which acted in the Paramirim Block. These syenitic rocks are rich not only in albite, but also in U-rich titanite (source of uranium). In association with the development of the Orosirian ductile shear zones, the syenites were crystallized and deformed. The metamorphic reactions, which include intense recrystallization of magmatic minerals, led uraninite to precipitate (1868 {+-} 69 Ma; U-Pb by LA-ICP MS) under Redox control. A second population of uraninites was also generated by shear zones reactivation during 0.6-0.5 Ga Brasiliano Orogeny (605 {+-} 170 Ma; U-Pb by LA-ICP-MS). The geotectonic implications are: a) the importance of the Orosirian event in the Paramirim Block during paleoproterozoic Sao Francisco/Congo Craton edification and b) the influence of the Brasiliano event in the Paramirim Block during the West-Gondwana assembly processes, indicated by the 605 {+-} 170 Ma uraninite (second population) age and by the 483 {+-} 100 Ma lower intercept of the metamorphosed syenite U-Pb discordia. (author)

  15. Generation of Silicic Melts in the Early Izu-Bonin Arc Recorded by Detrital Zircons in Proximal Arc Volcaniclastic Rocks From the Philippine Sea

    Science.gov (United States)

    Barth, A. P.; Tani, K.; Meffre, S.; Wooden, J. L.; Coble, M. A.; Arculus, R. J.; Ishizuka, O.; Shukle, J. T.

    2017-10-01

    A 1.2 km thick Paleogene volcaniclastic section at International Ocean Discovery Program Site 351-U1438 preserves the deep-marine, proximal record of Izu-Bonin oceanic arc initiation, and volcano evolution along the Kyushu-Palau Ridge (KPR). Pb/U ages and trace element compositions of zircons recovered from volcaniclastic sandstones preserve a remarkable temporal record of juvenile island arc evolution. Pb/U ages ranging from 43 to 27 Ma are compatible with provenance in one or more active arc edifices of the northern KPR. The abundances of selected trace elements with high concentrations provide insight into the genesis of U1438 detrital zircon host melts, and represent useful indicators of both short and long-term variations in melt compositions in arc settings. The Site U1438 zircons span the compositional range between zircons from mid-ocean ridge gabbros and zircons from relatively enriched continental arcs, as predicted for melts in a primitive oceanic arc setting derived from a highly depleted mantle source. Melt zircon saturation temperatures and Ti-in-zircon thermometry suggest a provenance in relatively cool and silicic melts that evolved toward more Th and U-rich compositions with time. Th, U, and light rare earth element enrichments beginning about 35 Ma are consistent with detrital zircons recording development of regional arc asymmetry and selective trace element-enriched rear arc silicic melts as the juvenile Izu-Bonin arc evolved.

  16. Paleogeographic changes across the Pennsylvanian-Permian boundary within the Mid-Continent (USA) inferred from detrital zircon geochronology of continental deposits

    Science.gov (United States)

    Soreghan, M. J.; Soreghan, G. S.

    2017-12-01

    The Permo-Pennsylvanian was characterized by intense orogenesis associated with Pangaean assembly, and profound climate shifts as earth transitioned from full icehouse conditions in the Pennsylvanian-early Permian to collapse into greenhouse conditions by latest Permian time. The modern U.S. Midcontinent was part of equatorial western Pangaea (North America) sandwiched between a continental-scale orogenic zone to the east and south (Appalachian-Ouachita-Marathon orogenic belt) and a series of basement-cored, intra-plate uplifts along western Pangaea (Ancestral Rocky Mountains). Here, we present a compilation of detrital zircon geochronology data from the Permo-Pennsylvanian of the Midcontinent as well as coeval strata of the east and west to explore sediment dispersal and potential tectonic and climatic influences on these provenance signatures. Zircon provenance data come from mostly eolian and fluvial silt- and sandstones of Early Pennsylvanian through Mid Permian age, although some data include marine sandstones. Our new data were acquired by LA-ICPMS at the University of Arizona Laserchron, and predominant age groups include >2500 Ma (Archean), 1600-1800 Ma (Yavapai-Matzatzal), 1300-900 Ma (Grenville), 790-570 Ma (Neoproterozoic), and 480-360 (E-M Paleozoic). However, the relative distributions of these populations exhibit distinctive temporal differences, especially across the Pennsylvanian-Permian boundary, but also spatially in comparison to published data from the Appalachian-Ouachita-Marathon basin, Ancestral Rocky Mountain basins, and the western Pangaean margin. Although the Central Pangaean Mountains, and in particular the Grenville-age basement rocks, were a dominant source of sediment to the Midcontinent, the data suggest an abrupt introduction of Neoproterozoic zircons in the early Permian. This signature also appears within the Ancestral Rocky Mountains region, but is rare along the western margin and the Appalachian basin in the early Permian. This

  17. U-Pb and K-Ar isotopic dating of Sinec (Rimavica) granites (Kohut zone of veporides)

    International Nuclear Information System (INIS)

    Bibikova, E.V.; Grachema, T.V.; Makarov, V.A.

    1988-01-01

    New results of radiometric dating of Sinec (Rimavica) granites by U-Pb and K-Ar method are presented. The K-Ar method (muscovite and biotite) gives the age of 94 m.y., which can be interpreted by tectonics. The U-Pb dating of zircons gave a concordant age of 350±5 m.y., which is a more reliable age than the one obtained from the same samples by Rb-Sr isochrone. The increased age of the Sinec (Rimavica) granites determined by the Sb-Sr method could be caused by the supply of radiogenic Sr from the surrounding older metamorphic rocks to the granites during retrograde metamorphism synchronous with granite autometasomatism. (author). 3 figs., 2 tabs., 15 refs

  18. Zircon Lu-Hf isotope systematics and U-Pb geochronology, whole-rock Sr-Nd isotopes and geochemistry of the early Jurassic Gokcedere pluton, Sakarya Zone-NE Turkey: a magmatic response to roll-back of the Paleo-Tethyan oceanic lithosphere

    Science.gov (United States)

    Karsli, Orhan; Dokuz, Abdurrahman; Kandemir, Raif

    2017-05-01

    The early Mesozoic was a critical era for the geodynamic evolution of the Sakarya Zone as transition from accretion to collision events in the region. However, its complex evolutionary history is still debated. To address this issue, we present new in situ zircon U-Pb ages and Lu-Hf isotope data, whole-rock Sr-Nd isotopes, and mineral chemistry and geochemistry data of plutonic rocks to better understand the magmatic processes. The Gokcedere pluton is mainly composed of gabbro and gabbroic diorite. LA-ICP-MS zircon U-Pb dating reveals that the pluton was emplaced in the early Jurassic (177 Ma). These gabbros and gabbroic diorites are characterized by relatively low SiO2 content of 47.09 to 57.15 wt% and high Mg# values varying from 46 to 75. The samples belong to the calc-alkaline series and exhibit a metaluminous I-type character. Moreover, they are slightly enriched in large ion lithophile elements (Rb, Ba, Th and K) and light rare earth elements and depleted in high field strength elements (Nb and Ti). Gabbroic rocks of the pluton have a depleted Sr-Nd isotopic composition, including low initial 87Sr/86Sr ranging from 0.705124 to 0.705599, relatively high ɛ Nd ( t) values varying from 0.1 to 3.5 and single-stage Nd model ages ( T DM1 = 0.65-0.95 Ga). In situ zircon analyses show that the rocks have variable and positive ɛ Hf ( t) values (4.6 to 13.5) and single-stage Hf model ages ( T DM1 = 0.30 to 0.65 Ga). Both the geochemical signature and Sr-Nd-Hf isotopic composition of the gabbroic rocks reveal that the magma of the studied rocks was formed by the partial melting of a depleted mantle wedge metasomatized by slab-derived fluids. The influence of slab fluids is mirrored by their trace-element characteristics. Trace-element modeling suggests that the primary magma was generated by a low and variable degree of partial melting ( 5-15%) of a depleted and young lithospheric mantle wedge consisting of phlogopite- and spinel-bearing lherzolite. Heat to melt the

  19. Proterozoic to Mesozoic evolution of North-West Africa and Peri-Gondwana microplates: Detrital zircon ages from Morocco and Canada

    Science.gov (United States)

    Marzoli, Andrea; Davies, Joshua H. F. L.; Youbi, Nasrrddine; Merle, Renaud; Dal Corso, Jacopo; Dunkley, Daniel J.; Fioretti, Anna Maria; Bellieni, Giuliano; Medina, Fida; Wotzlaw, Jörn-Frederik; McHone, Greg; Font, Eric; Bensalah, Mohamed Khalil

    2017-05-01

    The complex history of assemblage and disruption of continental plates surrounding the Atlantic Ocean is in part recorded by the distribution of detrital zircon ages entrained in continental sedimentary strata from Morocco (Central High Atlas and Argana basins) and Canada (Grand Manan Island, New Brunswick). Here we investigate detrital zircon from the latest Triassic (ca. 202 Ma) sedimentary strata directly underlying lava flows of the Central Atlantic magmatic province or interlayered within them. SHRIMP (Sensitive High-Resolution Ion MicroProbe) and LA-ICP-MS (Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry) U-Pb ages for zircon range from Paleozoic to Archean with a dominant Neoproterozoic peak, and significant amounts of ca. 2 Ga zircon. These ages suggest a prevailing West African (Gondwanan) provenance at all sampling sites. Notably, the Paleoproterozoic zircon population is particularly abundant in central Morocco, north of the High Atlas chain, suggesting the presence of Eburnean-aged rocks in this part of the country, which is consistent with recent geochronologic data from outcropping rocks. Minor amounts of late Mesoproterozoic and early Neoproterozoic zircon ages (ca. 1.1-0.9 Ga) in Moroccan samples are more difficult to interpret. A provenance from Avalonia or Amazonia, as proposed by previous studies is not supported by the age distributions observed here. An involvement of more distal source regions, possibly located in north-eastern Africa (Arabian Nubian Shield) would instead be possible. Paleozoic zircon ages are abundant in the Canadian sample, pointing to a significant contribution from Hercynian aged source rocks. Such a signal is nearly absent in the Moroccan samples, suggesting that zircon-bearing Hercynian granitic rocks of the Moroccan Meseta block were not yet outcropping at ca. 200 Ma. The only Moroccan samples that yield Paleozoic zircon ages are those interlayered within the CAMP lavas, suggesting an increased dismantling

  20. Origin of an unusual monazite-xenotime gneiss, Hudson Highlands, New York: SHRIMP U-Pb geochronology and trace element geochemistry

    Science.gov (United States)

    Aleinikoff, John N.; Grauch, Richard I.; Mazdab, Frank K.; Kwak, Loretta; Fanning, C. Mark; Kamo, Sandra L.

    2012-01-01

    A pod of monazite-xenotime gneiss (MXG) occurs within Mesoproterozoic paragneiss, Hudson Highlands, New York. This outcrop also contains granite of the Crystal Lake pluton, which migmatized the paragneiss. Previously, monazite, xenotime, and zircon from MXG, plus detrital zircon from the paragneiss, and igneous zircon from the granite, were dated using multi-grain thermal ionization mass spectrometry (TIMS). New SEM imagery of dated samples reveals that all minerals contain cores and rims. Thus TIMS analyses comprise mixtures of age components and are geologically meaningless. New spot analyses by sensitive high resolution ion microprobe (SHRIMP) of small homogeneous areas on individual grains allows deconvolution of ages within complexly zoned grains.Xenotime cores from MXG formed during two episodes (1034 ± 10 and 1014 ± 3 Ma), whereas three episodes of rim formation are recorded (999 ± 7, 961 ± 11, and 874 ± 11 Ma). Monazite cores from MXG mostly formed at 1004 ± 4 Ma; rims formed at 994 ± 4, 913 ± 7, and 890 ± 7 Ma. Zircon from MXG is composed of oscillatory-zoned detrital cores (2000-1170 Ma), plus metamorphic rims (1008 ± 7, 985 ± 5, and ∼950 Ma). In addition, MXG contains an unusual zircon population composed of irregularly-zoned elongate cores dated at 1036 ± 5 Ma, considered to be the time of formation of MXG. The time of granite emplacement is dated by oscillatory-zoned igneous cores at 1058 ± 4 Ma, which provides a minimum age constraint for the time of deposition of the paragneiss.Selected trace elements, including all REE plus U and Th, provide geochemical evidence for the origin of MXG. MREE-enriched xenotime from MXG are dissimilar from typical HREE-enriched patterns of igneous xenotime. The presence of large negative Eu anomalies and high U and Th in monazite and xenotime are uncharacteristic of typical ore-forming hydrothermal processes. We conclude that MXG is the result of unusual metasomatic processes during high grade

  1. 207Pb-206Pb zircon ages of eastern and western Dharwar craton, southern India : Evidence for contemporaneous Archaean crust

    Science.gov (United States)

    Maibam, B.; Goswami, J. N.; Srinivasan, R.

    2009-04-01

    Dharwar craton is one of the major Archaean crustal blocks in the Indian subcontinent. The craton is comprised of two blocks, western and eastern. The western domain is underlain by orthogneisses and granodiorites (ca. 2.9-3.3 Ga) collectively termed as Peninsular Gneiss [e.g., 1] interspersed with older tracts of metasedimentary and metamorphosed igneous suites (Sargur Group and Dharwar Group; [2]). The eastern part of the craton is dominated by Late Archaean (2.50-2.75 Ga) granitoids and their gneissic equivalents. They are interspersed with schist belts (also of Sargur Group and Dharwar Group), which are lithologically similar to the Dharwar Supergroup in the western block, but are in different metamorphic dress. Here we report 207Pb-206Pb age of zircons separated from the metasedimentary and gneissic samples from the two blocks to constrain the evolution of the Dharwar craton during the early Archaean. Detrital zircons of the metasedimentary rocks from both the blocks show a wide range of overlapping ages between ~2.9 to >3.5 Ga. Zircon ages of the orthogneisses from the two blocks showed that most of the analysed grains of the eastern Dharwar block are found to be of the age as old as the western Dharwar gneisses. Imprints of younger events could be discerned from the presence of overgrowths in zircons from the studied samples throughout the craton. Our data suggest that crust forming cycles in the two blocks of the Dharwar craton occurred contemporaneously during the Archaean. References [1] Beckinsale, R.D., Drury, S.A., Holt, R.W. (1980) Nature 283, 469-470. [2] Swami Nath J., Ramakrishnan M., Viswanatha M.N. (1976) Rec. Geol. Surv. Ind., 107, 149-175.

  2. Geochemie a zirkonové U-Pb stáří derflického granodioritu z dyjského masivu

    Czech Academy of Sciences Publication Activity Database

    Svojtka, Martin; Breiter, Karel; Ďurišová, Jana; Ackerman, Lukáš; Veselovský, F.; Šmerda, J.

    2017-01-01

    Roč. 50, č. 1 (2017), s. 17-24 ISSN 0514-8057 R&D Projects: GA ČR GA13-15390S Institutional support: RVO:67985831 Keywords : Bohemian Massif * Cadomian granitoids * Thaya (Dyje) Massif * zircon U-Pb dating Subject RIV: DB - Geology ; Mineralogy OBOR OECD: Geology

  3. Elucidating tectonic events and processes from variably tectonized conglomerate clast detrital geochronology: examples from the Hongliuhe Formation in the southern Central Asian Orogenic Belt, NW China

    Science.gov (United States)

    Cleven, Nathan; Lin, Shoufa; Davis, Donald; Xiao, Wenjiao; Guilmette, Carl

    2017-04-01

    This work expands upon detrital zircon geochronology with a sampling and analysis strategy dating granitoid conglomerate clasts that exhibit differing degrees of internal ductile deformation. As deformation textures within clastic material reflect the variation and history of tectonization in the source region of a deposit, we outline a dating methodology that can provide details of the provenance's tectonomagmatic history from deformation-relative age distributions. The method involves bulk samples of solely granitoid clasts, as they are representative of the magmatic framework within the provenance. The clasts are classified and sorted into three subsets: undeformed, slightly deformed, and deformed. LA-ICPMS U-Pb geochronology is performed on zircon separates of each subset. Our case study, involving the Permian Hongliuhe Formation in the southern Central Asian Orogenic Belt, analyzes each of the three clast subsets, as well as sandstone detrital samples, at three stratigraphic levels to yield a profile of the unroofed provenance. The age spectra of the clast samples exhibit different, wider distributions than sandstone samples, considered an effect of proximity to the respective provenance. Comparisons of clast data to sandstone data, as well as comparisons between stratigraphic levels, yield indications of key tectonic processes, in addition to the typical characteristics provided by detrital geochronology. The clast data indicates a minimal lag time, implying rapid exhumation rates, whereas sandstone data alone would indicate a 90 m.y. lag time. Early Paleozoic arc building episodes appear as Ordovician peaks in sandstone data, and Silurian-Devonian peaks in clast data, indicating a younging of magmatism towards the proximal provenance. A magmatic hiatus starts in the Devonian, correlating with the latest age of deformed clasts, interpreted as timing of collisional tectonics. Provenance interpretation using the correlations seen between the clast and sandstone

  4. The U-Pb age of the Posselandia Diorite, Hidrolina, Goias State, Brazil

    International Nuclear Information System (INIS)

    Jost, Hardy; Pimentel, Marcio M.; Fuck, Reinhard A.; Danni, Jose C.M.

    1993-01-01

    The Posselandia Diorite intrudes Archean granite-greenstone terrains of the region of Hidrolina, Central Goias, Brazil. U-Pb radiometric determinations in two fractions of zircon crystals from the diorite yield an age of 2,146 ± 1,6 Ma, interpreted as the crystallization age of the intrusion. The lack of deformation in the intrusion demonstrates that consolidation of the granite-greenstone terrains in the Hidrolina-Pilar de Goias-Crixas area took place before 2,146 Ma. (author). 3 figs., 2 tabs

  5. Petrological and zircon evidence for the Early Cretaceous granulite-facies metamorphism in the Dabie orogen, China

    Science.gov (United States)

    Gao, Xiao-Ying; Zhang, Qiang-Qiang; Zheng, Yong-Fei; Chen, Yi-Xiang

    2017-07-01

    An integrated study of petrology, mineralogy, geochemistry, and geochronology was carried out for contemporaneous mafic granulite and diorite from the Dabie orogen. The results provide evidence for granulite-facies reworking of the ultrahigh-pressure (UHP) metamorphic rock in the collisional orogen. Most zircons from the granulite are new growth, and their U-Pb ages are clearly categorized into two groups at 122-127 Ma and 188 ± 2 Ma. Although these two groups of zircons show similarly steep HREE patterns and variably negative Eu anomalies, the younger group has much higher U, Th and REE contents and Th/U ratios, much lower εHf(t) values than the older group. This suggests their growth is associated with different types of dehydration reactions. The older zircon domains contain mineral inclusions of garnet + clinopyroxene ± quartz, indicating their growth through metamorphic reactions at high pressures. In contrast, the young zircon domains only contain a few quartz inclusions and the garnet-clinopyroxene-plagioclase-quartz barometry yields pressures of 4.9 to 12.5 kb. In addition, the clinopyroxene-garnet Fe-Mg exchange thermometry gives temperatures of 738-951 °C. Therefore, the young zircon domains would have grown through peritectic reaction at low to medium pressures. The younger granulite-facies metamorphic age is in agreement not only with the adjacent diorite at 125 ± 1 Ma in this study but also the voluminous emplacement of coeval mafic and felsic magmas in the Dabie orogen. Mineral separates from both mafic granulite and its adjacent diorite show uniformly lower δ18O values than normal mantle, similar to those for UHP eclogite-facies metaigneous rocks in the Dabie orogen. In combination with major-trace elements and zircon Lu-Hf isotope compositions, it is inferred that the protolith of mafic granulites shares with the source rock of diorites, both being a kind of mafic metasomatites at the slab-mantle interface in the continental subduction channel

  6. Geochemistry and Geochronology U-Pb SHRIMP of granites from Peixoto de Azevedo: Alta Floresta Gold Province

    International Nuclear Information System (INIS)

    Silva, Fernanda Rodrigues da; Barros, Marcia Aparecida Sant'Ana; Pierosan, Ronaldo; Pinho, Francisco Edigio Cavalcante; Tavares, Carla; Rocha, Jhonattan; Rocha, Mara Luiza Barros Pita; Vasconcelos, Bruno Rodrigo; Dezula, Samantha Evelyn Max

    2014-01-01

    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 N /Yb N » 7.6 to 17.31 and ratios Eu/EU ⁎ between 0.46 – 0.72 for biotite monzogranite and La N /Yb N » 7.13 to 29.09 with ratios Eu/Eu ⁎ 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)

  7. U/Pb (SHRIMP), 207Pb/206Pb, Rb/Sr, Sm/Nd e K/Ar geochronology of granite-greenstone terrains of Gaviao Block: implications for the Proterozoic and Archean evolution of Sao Francisco Craton, Brazil

    International Nuclear Information System (INIS)

    Leal, Luiz Rogerio Bastos

    1998-01-01

    The Gaviao Block (GB) in the northern portion of the Sao Francisco Craton-Northeast of Brazil, constitutes one of the oldest Archean fragments of the South American Platform Archean crust. GB underwent several events of juvenile accretion and reworking of continental crust along its evolutionary history, notably between the Archean and the Paleoproterozoic. 207 Pb/ 206 Pb isotopic analyses were carried out in two zircons populations from strongly migmatized TTG terranes found in the proximity of Brumado: the first population (7 crystals) is taken as representative of the crystallization period of the TTG terranes at 3300 ± 45 Ma; the second (2 crystals) represents the age of the first even of metamorphism/migmatization at 2910 ± 10 Ma. 207 Pb/ 206 Pb analyses in zircons from an outcrop of non-migmatized TTG in the area yielded a 3202 ± 15 Ma age (4 crystals), interpreted to be the crystallization period of the gneiss protolith. Sm/Nd analyses on the TTG rocks of the Brumado region yielded T DM model ages varying between 3.26 and 3.36 Ga and ε Nd (t) between -3.5 and +0.7. These data suggest the occurrence of juvenile accretions to the continental crust during the Archean, with differential involvement of crustal materials. The geochemical data of rare earth elements corresponding to the TTG terranes revealed moderate LRRE contents (La N =83,5), low HREE contents (La N =2,5) and a fairly fractionated pattern (La/Yb) N =34, besides lack of negative Eu anomaly, showing that these rocks have similar compositions to those TTG terranes of cratonic continents, as well as some Archean rocks from CSF (e.g. Sete Voltas, Boa Vista). Finally, the youngest ages present in GB rocks (ca. 1.2-0.45 Ga) represent the role played by tectono thermal events, which produced partial or total rejuvenation of the Rb/Sr and K/Ar isotopic systems during the Espinhaco and Brasiliano cycles. In particular, K/Ar ages illustrate the effect of younger regional cooling episodes related to the

  8. Initiation of the Bukadaban Feng Normal Fault and Implications for the Topographic Evolution of Northern Tibet

    Science.gov (United States)

    Niemi, N. A.; Chang, H.; Li, L.; Molnar, P. H.

    2017-12-01

    The Bukadaban Feng massif in northern Tibet forms the footwall of an east-west trending graben that is kinematically linked to the Kunlun fault. Extension across this graben accommodates left-lateral slip on the Kunlun fault, as evidenced by the 2001 Kunlun earthquake rupture. New geochronologic and thermochronologic data from Bukadaban Feng provide insight into the evolution of this normal fault system. The Bukadaban Feng massif is composed of two plutonic units, an eastern unit of dacitic composition and a western unit of rhyolitic composition. Sixty-five LA-ICP-MS zircon U-Pb age determinations on the rhyolitic unit reveal a range of ages from 873 - 6.3 Ma. CA-TIMS U-Pb zircon geochronology on the nine youngest of these zircons yields an emplacement age of 6.8 Ma. Twenty-seven LA-ICP-MS zircon U-Pb ages on the dacite range from 208 to 7.9 Ma. No coherent population of young zircons was observed, and CA-TIMS analysis was not performed. Zircon (U-Th)/He analysis on the dacite and rhyolite yield ages of 3.9 and 5.0 Ma, respectively, while apatite (U-Th-Sm)/He thermochronology on 5 samples collected from both units along the trace of the normal fault yield ages ranging from 1.4 - 2.6 Ma. The emplacement ages and compositions of plutonic rocks at Bukadaban Feng are consistent with the eruptive timing and geochemistry of silicic volcanic rocks in the graben (Zhang et al., 2012). Silicic magmatism is often associated with the onset of crustal extension, and the combination of plutonism and correlative silicic volcanism provides an indirect constraint on the initiation of this graben at 7 Ma. The distinct zircon (U-Pb) and (U-Th)/He ages indicates that the rocks presently exposed at Bukadaban Feng were emplaced at ambient temperatures in excess of 180°C. The zircon and apatite thermochronologic data require exhumation at rates of 1-2 mm/yr since the late Miocene. A 7 Ma initiation age for the Bukadaban Feng normal fault is consistent with both published estimates of

  9. 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: neto.carla@hotmail.com [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

    2014-03-15

    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)

  10. Paleomagnetism and geochronological studies on a 450 km long 2216 Ma dyke from the Dharwar craton, southern India

    Science.gov (United States)

    Nagaraju, E.; Parashuramulu, V.; Kumar, Anil; Srinivas Sarma, D.

    2018-01-01

    Paleomagnetic and geochronological studies were carried out on a ∼ 450 km long (from 17 sites) N-S striking Paleoproterozoic dyke swarm exposed along a natural crustal cross section of about 10 km (increasing from North to South) in the Dharwar Craton, to study the characteristics of paleomagnetism and geochronology in vertical dimension. U-Pb/Pb-Pb dating on baddeleyite gives a crystallisation age of 2216.0 ± 0.9 Ma for long dyke AKLD. Paleomagnetic data from this well dated ∼ 2216 Ma dyke swarm in Dharwar Craton are of excellent quality. High coercivity and high blocking temperature components are carried by single domain magnetite and show dual polarity remanence directions. Combined normal and reverse polarity remanences on AKLD and other N-S dykes define the most reliable paleomagnetic pole for ∼ 2216 Ma at latitude 36°S and longitude 312°E (A95 = 7°). Though paleomagnetic data is unavailable on other N-S dykes below the Cuddapah basin, high precision geochronology suggest that they are of similar age within errors. Though there is a variation in the crustal depth of Dharwar craton from north to south, consistent Pb-Pb/U-Pb baddeleyite geochronology and paleomagnetic studies along the AKLD established its continuity and preservation along its entire strike length. The virtual geomagnetic poles of these sites confirm a stable remanence and are almost identical to the previously reported paleomagnetic pole and also supported by positive reversal test. Positive paleomagnetic reversal test on these dykes signify that the remanent magnetization is primary and formed during initial cooling of the intrusions. Updated apparent polar wander path of Dharwar craton indicates relatively low drift rate during 2.21-2.08 Ga interval. Magnetogranulometry and SEM studies show that remanent magnetization in this dyke was carried by single domain magnetite residing within silicate minerals.

  11. Geochronology and petrogenesis of the Early Cretaceous A-type granite from the Feie'shan W-Sn deposit in the eastern Guangdong Province, SE China: Implications for W-Sn mineralization and geodynamic setting

    Science.gov (United States)

    Liu, Peng; Mao, Jingwen; Santosh, M.; Bao, Zhian; Zeng, Xiaojian; Jia, Lihui

    2018-02-01

    The Feie'shan greisen-type W-Sn deposit in the eastern Guangdong Province forms part of the Southeastern Coastal Metallogenic Belt (SCMB) in South China. Here we present zircon LA-ICP-MS U-Pb geochronology of the biotite granite which shows a weighted mean 206Pb/238U age of 134.7 ± 2.0 Ma, consistent with the zircon U-Pb, biotite 40Ar-39Ar and molybdenite Re-Os ages in the previous study. The biotite granite is peraluminous and belongs to high-K calc-alkaline type. It is characterized by high SiO2, K2O, F, K2O + Na2O and FeOt/(FeOt + MgO), and low CaO, MgO, TiO2 and P2O5 contents, enrichment in Rb, Cs, Th and U, and depletion in Ba, Sr, Zr, Ti and P, with flat REE patterns and distinctly negative Eu anomalies, showing an A2-type affinity. The rocks also display extremely low Ba, Sr and Ti concentrations and high Rb/Sr, Rb/Ba and low CaO/(Na2O + K2O) ratios, indicating high degree of fractionation. Zircon grains from the granite have low Eu/Eu* and Ce4 +/Ce3 + ratios, suggesting low oxygen fugacity. The highly fractionated and reduced features imply that the Feie'shan mineralization is genetically related to the biotite granite. The εNd(t) values and zircon εHf(t) values of the biotite granite range from - 2.96 to - 1.95 and - 5.69 to 0.62, with two-stage Nd and Hf model ages (TDM2) of 1083 to 1164 Ma and 1150 to 1552 Ma, indicating that they were derived from magma hybridization between anatectic granitic and mantle-derived mafic magmas. In combination with previous studies, we propose a geodynamic model for the 145―135 Ma W-Sn mineral system and related magmatism in the southwestern domain of the SCMB. After ca. 145 Ma, the subduction orientation of the Izanagi plate changed from oblique to parallel with respect to the continental margin resulting in large-scale lithosphere extension and thinning, which led to the upwelling of asthenosphere. The ascending mantle-derived mafic magmas provided not only supplied the heat for crustal remelting but also added

  12. Provenance and depositional age of metavolcano-sedimentary sequences of the Santa Terezinha de Goias, based on Sm-Nd and U-Pb zircon single grain; Proveniencia e idade deposicional de sequencias metavulcano-sedimentares da regiao de Santa Terezinha de Goias, baseada em dados isotopicos Sm-Nd e U-Pb em monocristal de zircao

    Energy Technology Data Exchange (ETDEWEB)

    Dantas, Ellton Luiz; Jost, Hardy; Fuck, Reinhardt A.; Brod, Jose Afonso; Pimentel, Marcio Martins; Meneses, Paulo Roberto [Brasilia Univ., DF (Brazil). Inst. de Geociencias]. E-mail: elton@unb.br

    2001-09-01

    Supracrustal rocks of the Santa Terezinha de Goias region, Central Brazil, presumably represent a southern extension of the Mara Rosa magmatic arc tectonically juxtaposed to Archean terrains and consist of a meta volcanic unit and a meta sedimentary unit. The aim of the paper is to present and discuss the first U-Pb and Sm-Nd data of rocks belonging to both units of that region. U-Pb data of zircons from a felsic meta volcanic rock inter layered with chlorite-rich schists (metandesites?) yield a concordant age of of 660 My, while the Sm-Nd model age of a variety of chlorite-schists indicate a juvenile age between 1.1 and 1.3 Ga. These data indicate that the meta volcanic rocks are Neo proterozoic in age and may be correlated with rocks of the Mara Rosa magmatic arc. On the other hand, the prevailing provenance of rocks belonging to the meta sedimentary unit indicate a Paleoproterozoic, or older, source. Thus, the deposition of both units are explained by means of the erosion of source-areas of variable age. (author)

  13. First results of U-Pb dating of metamorphic rocks of the Greater Antilles arc: age of the Mabujina complex (Cuba)

    International Nuclear Information System (INIS)

    Bibikova, E.V.; Somin, M.L.; Gracheva, T.V.; Makarov, V.A.; Mil'yan, G.; Shukolyukov, Yu.A.; AN SSSR, Moscow

    1988-01-01

    U-Pb-dating of zircons, entering the composition of metamorphic rocks of the Mabujina complex, was conducted in order to solve the problem concerming the place of metamorphic complexes in the structure and tectonic evolution of the Greater Antilles arc. The accuracy of uranium and lead determination was equal to ± 1%, the accuracy of lead isotopic ratio determination with the use of TSN-206A mass-spectrometer- ±0.15%. Isotope data showed, that all examined zircons crystallized about 100 mil. years ago

  14. U-Pb age of the Diana Complex and Adirondack granulite ...

    Indian Academy of Sciences (India)

    For the most concordant analyses, the 207Pb/206Pb ages range between 1115 and 1150 Ma. Detailed petrographic studies revealed that most grains contained at least two phases of zircon growth, either primary magmatic cores enclosed by variable thickness of metamorphic overgrowths or magmatic portions enclosing ...

  15. Mechanisms and timescales of generating eruptible rhyolitic magmas at Yellowstone caldera from zircon and sanidine geochronology and geochemistry

    Science.gov (United States)

    Stelten, Mark; Cooper, Kari M.; Vazquez, Jorge A.; Calvert, Andrew T.; Glessner, Justin G

    2015-01-01

    We constrain the physical nature of the magma reservoir and the mechanisms of rhyolite generation at Yellowstone caldera via detailed characterization of zircon and sanidine crystals hosted in three rhyolites erupted during the (ca. 170 – 70 ka) Central Plateau Member eruptive episode – the most recent post-caldera magmatism at Yellowstone. We present 238U-230Th crystallization ages and trace-element compositions of the interiors and surfaces (i.e., unpolished rims) of individual zircon crystals from each rhyolite. We compare these zircon data to 238U- 230Th crystallization ages of bulk sanidine separates coupled with chemical and isotopic data from single sanidine crystals. Zircon age and trace-element data demonstrate that the magma reservoir that sourced the Central Plateau Member rhyolites was long-lived (150 – 250 kyr) and genetically related to the preceding episode of magmatism, which occurred ca. 256 ka. The interiors of most zircons in each rhyolite were inherited from unerupted material related to older stages of Central Plateau Member magmatism or the preceding late Upper Basin Member magmatism (i.e., are antecrysts). Conversely, most zircon surfaces crystallized near the time of eruption from their host liquids (i.e., are autocrystic). The repeated recycling of zircon interiors from older stages of magmatism demonstrates that sequentially erupted Central Plateau Member rhyolites are genetically related. Sanidine separates from each rhyolite yield 238U-230Th crystallization ages at or near the eruption age of their host magmas, coeval with the coexisting zircon surfaces, but are younger than the coexisting zircon interiors. Chemical and isotopic data from single sanidine crystals demonstrate that the sanidines in each rhyolite are in equilibrium with their host melts, which considered along with their near-eruption crystallization ages suggests that nearly all CPM sanidines are autocrystic. The paucity of antecrystic sanidine crystals relative to

  16. U–Pb, Rb–Sr, and U-series isotope geochemistry of rocks and fracture minerals from the Chalk River Laboratories site, Grenville Province, Ontario, Canada

    International Nuclear Information System (INIS)

    Neymark, L.A.; Peterman, Z.E.; Moscati, R.J.; Thivierge, R.H.

    2013-01-01

    Highlights: • AECL evaluates Chalk River Laboratories site as potential nuclear waste repository. • Isotope-geochemical data for rocks and fracture minerals at CRL site are reported. • Zircons from gneiss and granite yielded U–Pb ages of 1472 ± 14 and 1045 ± 6 Ma. • WR Rb–Sr and Pb–Pb systems do not show substantial large-scale isotopic mobility. • U-series and REE data do not support oxidizing conditions at depth in the past 1 Ma. - Abstract: As part of the Geologic Waste Management Facility feasibility study, Atomic Energy of Canada Ltd. (AECL) is evaluating the suitability of the Chalk River Laboratories (CRL) site in Ontario, situated in crystalline rock of the southwestern Grenville Province, for the possible development of an underground repository for low- and intermediate-level nuclear waste. This paper presents petrographic and trace element analyses, U–Pb zircon dating results, and Rb–Sr, U–Pb and U-series isotopic analyses of gneissic drill core samples from the deep CRG-series characterization boreholes at the CRL site. The main rock types intersected in the boreholes include hornblende–biotite (±pyroxene) gneisses of granitic to granodioritic composition, leucocratic granitic gneisses with sparse mafic minerals, and garnet-bearing gneisses with variable amounts of biotite and/or hornblende. The trace element data for whole-rock samples plot in the fields of within-plate, syn-collision, and volcanic arc-type granites in discrimination diagrams used for the tectonic interpretation of granitic rocks. Zircons separated from biotite gneiss and metagranite samples yielded SHRIMP-RG U–Pb ages of 1472 ± 14 (2σ) and 1045 ± 6 Ma, respectively, in very good agreement with widespread Early Mesoproterozoic plutonic ages and Ottawan orogeny ages in the Central Gneiss Belt. The Rb–Sr, U–Pb, and Pb–Pb whole-rock errorchron apparent ages of most of the CRL gneiss samples are consistent with zircon U–Pb age and do not indicate

  17. U-Pb and Sm-Nd preliminary geochronologic data on the Yaounde series, Cameroon: re-interpretation of the granulitic rocks as the suture of a collision in the ''Centrafrican'' belt

    International Nuclear Information System (INIS)

    Schmus, W.R. Van; Penaye, J.; Toteu, S.F.; Nzenti, J.P.

    1993-01-01

    U-Pb on zircons and Sm-Nd on whole-rock analyses have been applied on the schists, micaschists and granulitic gneisses of the Yaounde series. The results fix the granulitic metamorphism in the Yaounde series at 620±10 Ma and the deposition of the series during the Upper Proterozoic. In addition, another remnant of Lower Proterozoic basement, previously recognized in northern Cameroon, has been identified at Kekem northwest of the Yaounde series. The Yaounde granulitic unit is re-interpreted as the suture of a Pan-African collision between a passive margin (Archean Congo craton and its Birrimian Nyong cover) to the south and an intensive granitized active margin (dismembered eburnian basement and Upper Proterozoic series) to the north. (author). 14 refs., 4 figs

  18. Primary Data on U/Pb-Isotope Ages and Lu/Hf-Isotope Geochemical Systematization of Detrital Zircons from the Lopatinskii Formation (Vendian-Cambrian Transition Levels) and the Tectonic Nature of Teya-Chapa Depression (Northeastern Yenisei Ridge)

    Science.gov (United States)

    Kuznetsov, N. B.; Priyatkina, N. S.; Rud'ko, S. V.; Shatsillo, A. V.; Collins, W. J.; Romanyuk, T. V.

    2018-03-01

    The main results are presented on U/Pb-isotope dating of 100 detrital zircons and, selectively, on the Lu/Hf-isotope system of 43 grains from sandstones of the Lopatinskii formation (the lower stratigraphic level of the Chingasan group). Ages from 896 ± 51 to 2925 ± 38 Ma were obtained with a pronounced maximum of 1890 Ma in the curve of probability density, along with ɛHf estimates from +8.4 to-15.1, which allow one to throw doubt upon the molasse nature of the Lopatinskii formation.

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

    Science.gov (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

    2015-03-01

    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.

  20. Late Paleozoic to Triassic magmatism in the north-central High Andes, Chile: New insights from SHRIMP U-Pb geochronology and O-Hf isotopic signatures in zircon

    Science.gov (United States)

    Hernández González, Álvaro; Deckart, Katja; Fanning, Mark; Arriagada, César

    2014-05-01

    The Chilean High Andes (28o- 31oS) comprises a vast number of late Paleozoic - Triassic granitoids which give information about the last stages of Gondwana assemblage. Particularly, previous studies determined two tectonic configurations during this time: subduction related compressional setting (late Carboniferous - Late Permian) and non-subduction post-collisional extensional setting (Late Permian - Triassic), as the last stage of Gondwana assemblage. However, new O-Hf isotopic data along new U-Pb SHRIMP ages in zircon have shown that this model should be modified and updated to the new analytical data available. δ18O values indicate a strong change in the tectonic configuration approximately 270 Ma (earliest middle Permian) and thus, units can be divided into 2 mayor groups: late Carboniferous to earliest middle Permian and middle Permian to Triassic. The oldest group shows slightly low values of ɛHfi (ca. +1 to -4) with high δ18O (ca. >6.5 o/oo), indicating an elevated supracrustal component and the addition of less radiogenic continental-like material, which along significant residence time (TDM2: Mesoproterozoic) can be interpreted as magmas formed at depth in a subduction-related continental arc, and contaminated with supracrustal material and/or oceanic sediments transported through the subducted slab to the mantle-wedge. Subsequently, middle Permian - Triassic rocks show a wider range of ɛHfi values (ca. +3 to -3) with relatively low, mantle-like δ18O (ca. 4.5-6.5 o/oo), indicating a source of magmas without the addition of supracrustal material for some plutons, whilst for others, a slight input. The higher positive values of ɛHfi can be related to the influence of new juvenile material in the source of some magmas. This isotopic data can be interpreted as rocks formed as the result of melting of an old thinned mafic crust (with mantle-like δ18O values characteristic of this type of rocks) with limited addition of supracrustal material; in

  1. Unraveling the tectonic evolution of a Neoproterozoic-Cambrian active margin in the Ribeira Orogen (Se Brazil): U-Pb and Lu-Hf provenance data

    Energy Technology Data Exchange (ETDEWEB)

    Fernandes, Gabriel Lamounier de F. [Servico Geologico do Estado do Rio de Janeiro (DRM-RJ), Niteroi, RJ (Brazil); Schmitt, Renata; Bongiolo, Everton M.; Mendes, Julio [Universidade Federal do Rio de Janeiro (UFRJ), RJ (Brazil); Basei, Miguel S. [Universidade de Sao Paulo (USP), SP (Brazil)

    2015-07-01

    Full text: The Neoproterozoic-Ordovician Central Ribeira Orogen, in SE Brazil, presents two contrasting tectonic domains in its southern portion: (a) The Arc Domain constituted of Neoproterozoic to Paleozoic magmatic rocks and low P-high T metamorphic para (Sao Fidelis Group) - and ortho- derived units (in Oriental Terrane); and (b) The Basement Domain, constituted of a Paleoproterozoic and Neoproterozoic medium P-high T metamorphic para (Palmital-Buzios Succession)- and ortho-derived units (in Cabo Frio Tectonic Domain). Our work focuses on paraderived rocks sequences from both domains. The provenance analysis using U-Pb and Lu-Hf in zircon grains is presented here as an effective tool to unravel the paleogeography and nature of the pre-collisional sedimentary basins. We performed 505 analyses (U-Pb) on detrital zircon grains and some metamorphic overgrowths from six paragneiss samples. Besides, 141 analyses (Lu-Hf) in six samples only on the detrital zircon grains domains. All samples present a main peak from Neoproterozoic sources (750-570 Ma) and the other minor peak in the Stenian/Tonian periods (1200-850Ma), this indicate an orogenic contribution for this basin. Scarce register from the Mesoproterozoic and two peaks in the Archean/Paleoproterozoic (2.6 and 1.9 Ga) are recognized as a contribution from an ancient continent. The Lu-Hf data reveals a juvenile source for the detrital zircon grains from Buzios Succession while Palmital and Sao Fidelis Group units show a main crustal signature for their detrital zircon population. Based on the U-Pb and Lu-Hf data presented here, plus petrological data, geological correlations, and compilation of data from literature, we propose a tectonic model for the origin of para-derived rocks from the eastern part of the Ribeira Orogen. Starting with an extensional environment of ca. 600 Ma in a back-arc basin (Buzios succession deposition) and continuing as an active margin between 570 and 550 Ma in the fore-arc and prism

  2. Detrital Zircons From the Jack Hills and Mount Narryer, Western Australia: Geochronological, Morphological, and Geochemical Evidence for Diverse >4000 Ma Source Rocks

    Science.gov (United States)

    Crowley, J. L.; Myers, J. S.; Sylvester, P. J.; Cox, R. A.

    2004-05-01

    Detrital zircons from all major clastic units in the Jack Hills and Mount Narryer metasedimentary belts, Western Australia, were analyzed for morphology, internal zoning, inclusion mineralogy, age, and trace element concentrations (latter two obtained by laser-ablation microprobe ICPMS). The results show that zircons were derived from a wide diversity of rocks, including previously described, >4000 Ma grains that are older than any known terrestrial rocks. In three metaconglomerate samples from the western Jack Hills, 4200-3800 Ma zircons ("old grains") comprise 14% of the population, 3800-3600 Ma grains form only 2%, and 3550-3250 Ma zircons ("young grains") are dominant with a significant peak at 3380 Ma. Old and young grains are interpreted as being from similar rock types because they are indistinguishable in trace element concentrations, size (several hundred microns), morphology (subequant, typically fragmented), internal zoning (typically both oscillatory and sector), and U concentration (50-200 ppm). Many of these properties suggest an intermediate plutonic source, whereas an evolved granitic source was previously interpreted from rare-earth element and oxygen isotope data. Detrital zircons in quartzites and metaconglomerates at Mount Narryer differ significantly from zircons from the western Jack Hills. Old grains comprise only 3% (most of which are 4200-4100 Ma), 3800-3600 Ma zircons form 31%, and there are peaks at 3650, 3600, and 3500 Ma. Old and young grains have similar properties that suggest granitic sources, such as elongate prismatic morphology, oscillatory zoning, high U concentrations (100-600 ppm), and xenotime and monazite inclusions. Trace element concentrations are broadly similar to those in Jack Hills zircons, with notable exceptions being generally higher U, smaller Ce and Eu anomalies, and lower Nb/Ta. It is considered unlikely that Jack Hills zircons were derived from granitic gneisses that surround the metasedimentary belts because

  3. Late Archaean-early Proterozoic source ages of zircons in rocks from the Paleozoic orogen of western Galicia, NW Spain

    Energy Technology Data Exchange (ETDEWEB)

    Kuijper, R P; Priem, H N.A. [Laboratorium voor Isotopen-Geologie, Amsterdam (Netherlands); Den Tex, E [Rijksuniversiteit Utrecht (Netherlands). Inst. voor Aardwetenschappen

    1982-08-01

    U-Pb data are reported for nine suites of zircons and three monazites from the Paleozoic orogen in western Galicia: one paragneiss and six orthogneisses from the early Paleozoic basement, and two Carboniferous (ca. 310 Ma old) intrusions of two-mica granite. New whole-rock Rb-Sr analyses, along with earlier data, indicate an age of ca. 470-440 Ma (Ordovician) for the emplacement of the granitic precursors of the orthogneisses. Monazite from the paragneiss also yields an U-Pb age of ca. 470 Ma. From the high initial /sup 87/Sr//sup 86/Sr ratios an involvement of Precambrian continental crust material is evident in the generation of the early Paleozoic suite of granites, while the zircon U-Pb data give evidence of the presence of about 3.0-2.0 Ga old (late Archaean-early Proterozoic) components in the source material. Zircons from the oldest sedimentary rocks in the area, now present as catazonal paragneisses and a likely source for the granites, likewise reveal a provenance age of 3.0-2.0 Ga.

  4. U-Pb SHRIMP ages of volcanic zircons from the Merrions and Turondale Formations, New South Wales, and the early devonian time-scale: a biostratigraphic and sedimentological assessment

    International Nuclear Information System (INIS)

    Packham, G.H.

    2003-01-01

    U-Pb ages for volcanic zircons from the Lower Devonian Turondale and Merrions Formations initially dated by Jagodzinski and Black (1999) have been recalculated by Compston (2000, 2001). The Turondale, Waterbeach and Merrions Formations are, in ascending stratigraphic order, three of the Lower Devonian deep-water formations of the Hill End Trough. The recent discovery of a Lochkovian conodont fauna of the delta zone in derived limestone blocks from the uppermost beds of the Turondale Formation has established a maximum age for that horizon. The principal lithological assemblages of the formations are volcanic (megaturbidite and lavas), epiclastic turbidites and essentially hemipelagic lithologies. The mean zircon ages, determined by Sensitive High-Resolution Ion MicroProbe (SHRIMP), suggest a long duration for the Merrions Formation, 615 m thick and 91% volcanic, and a very much shorter interval for the upper two-thirds of the Turondale Formation, 360 m thick and 34% volcanic, plus the intervening non-volcanic Waterbeach Formation, 512 m thick. A sedimentological model based on scaled depositional rates for the facies has been used to estimate the relative accumulation rates and durations of the succession from the base of the Turondale Formation to the top of the Merrions Formation The model indicates that the Merrions Formation accumulated in a much shorter time than the interval from the top of the lower Turondale Formation to the base of the Merrions Formation. The analytical uncertainties of the Jagodzinski and Black (1999) zircon ages are large enough to be compatible with the sedimentological model, but the smaller analytical uncertainties calculated by Compston (2000, 2001) fall far from the scaled sedimentological model. Grouping together of the lower Merrions Formation and lower Turondale Formation zircon dates in Compston (2000) for a single mid-Lochkovian age is in conflict with the biostratigraphic and sedimentological evidence available and the derived

  5. Tectonic isolation from regional sediment sourcing of the Paradox Basin

    Science.gov (United States)

    Smith, T. M.; Saylor, J.; Sundell, K. E.; Lapen, T. J.

    2017-12-01

    The Appalachian and Ouachita-Marathon mountain ranges were created by a series of tectonic collisions that occurred through the middle and late Paleozoic along North America's eastern and southern margins, respectively. Previous work employing detrital zircon U-Pb geochronology has demonstrated that fluvial and eolian systems transported Appalachian-derived sediment across the continent to North America's Paleozoic western margin. However, contemporaneous intraplate deformation of the Ancestral Rocky Mountains (ARM) compartmentalized much of the North American western interior and mid-continent. We employ lithofacies characterization, stratigraphic thickness, paleocurrent data, sandstone petrography, and detrital zircon U-Pb geochronology to evaluate source-sink relationships of the Paradox Basin, which is one of the most prominent ARM basins. Evaluation of provenance is conducted through quantitative comparison of detrital zircon U-Pb distributions from basin samples and potential sources via detrital zircon mixture modeling, and is augmented with sandstone petrography. Mixing model results provide a measure of individual source contributions to basin stratigraphy, and are combined with outcrop and subsurface data (e.g., stratigraphic thickness and facies distributions) to create tectonic isolation maps. These maps elucidate drainage networks and the degree to which local versus regional sources influence sediment character within a single basin, or multiple depocenters. Results show that despite the cross-continental ubiquity of Appalachian-derived sediment, fluvial and deltaic systems throughout much of the Paradox Basin do not record their influence. Instead, sediment sourcing from the Uncompahgre Uplift, which has been interpreted to drive tectonic subsidence and formation of the Paradox Basin, completely dominated its sedimentary record. Further, the strong degree of tectonic isolation experienced by the Paradox Basin appears to be an emerging, yet common

  6. Accretionary history of the Altai-Mongolian terrane: perspectives from granitic zircon U-Pb and Hf-isotope data

    Science.gov (United States)

    Cai, Keda; Sun, Min; Xiao, Wenjiao

    2014-05-01

    The Central Asian Orogenic Belt (CAOB) consists of many tectonic terranes with distinct origin and complicated evolutionary history. Understanding of individual block is crucial to reconstruct the geodynamic history of the gigantic accetionary collage. This study presents zircon U-Pb ages and Hf isotopes for the granitoid rocks in the Russian Altai mountain range (including Gorny Altai, Altai-Mongolian terrane and CTUS suture zone between them), in order to clarify the timing of granitic magmatism, source nature, continental crustal growth and tectonic evolution. Our dating results suggest that granitic magmatism of the Russian Altai mountain range occurred in three major episodes including 445~429 Ma, 410~360 Ma and ~241 Ma. Most of the zircons within the Paleozoic granitoids present comparable positive ɛHf(t) values and Neoproterozoic crustal model ages, which favor the interpretation that the juvenile crustal materials produced in the early stage of CAOB were probably dominant sources for the Paleozoic magmatism in the region. The inference is also supported by widespread occurrence of short-lived juvenile materials including ophiolites, seamount relics and arc assemblages in the north CAOB. Consequently, the Paleozoic massive granitic rocks maybe not represent continental crustal growth at the time when they were emplaced, but rather record reworking of relatively juvenile Proterozoic crustal rocks although mantle-derived mafic magma was possibly involved to sever as heat engine during granitic magma generation. The Early Triassic granitic intrusion may be product in an intra-plate environment, as the case of same type rocks in the adjacent areas. The positive ɛHf(t) values (1.81~7.47) and corresponding Hf model ages (0.80~1.16 Ga) together with evidence of petrology are consistent with the interpretation that the parental magma of the Triassic granitic intrusion was produced from enriched mantle-derived sources under an usually high temperature condition

  7. U-Pb dating of two granite types from Soultz (Rhine Graben, France); Datation U-Pb des deux facies du granite de Soultz (fosse rhenan, France)

    Energy Technology Data Exchange (ETDEWEB)

    Cocherie, A.; Guerrot, C.; Genter, A. [Bureau de Recherches Geologiques et Minieres (BRGM), 45 - Orleans (France); Fanning, C.M. [Research School of Earth Sciences, ANU, Canberra, (Australia)

    2004-07-01

    Deep boreholes drilled in the basement of the Rhine Graben at Soultz-sous-Forets have shown the presence of an ubiquitous monzogranite. Borehole GPK-2, with a total depth of 5090 m, also intersected a more leucocratic fine-grained two-mica granite, locally present below 4860 m depth and continuously found between 5047 and 5090 m. Thanks to trace elements and in particular to REE (rare-earth elements), it could be shown that the leucocratic rock is a differentiated expression of the potassic magma that was at the origin of the monzogranite pluton. This model agrees with Sr-Nd isotope data, even though a slight contribution from the pre-existing basement should be considered as well. Use of the U-Pb method on monzogranite from EPS-1 - after zircon dissolution - has yielded an emplacement age of 334.0 + 3.8/-3.5 Ma (2 {sigma}). Point dates obtained by SHRIMP II on the rare zircons from the fine-grained granite showed that it was emplaced in a basement with very heterogeneous ages, ranging from Early Proterozoic to Silurian. (author)

  8. Research on geochronology and uranium source of sandstone-hosted uranium ore-formation in major uranium-productive basins, Northern-China

    International Nuclear Information System (INIS)

    Xia Yuliang; Liu Hanbin; Lin Jinrong; Fan Guang; Hou Yanxian

    2004-12-01

    A method is developed for correcting uranium content in uranium ore samples by considering the U-Ra equilibrium coefficient, then a U-Pb isochron is drawn up. By performing the above correction ore-formation ages of sandstone-hosted uranium mineralization which may be more realistic have been obtained. The comparative research on U-Pb isotopic ages of detritic zircon in ore-hosting sandstone and zircon in intermediate-acid igneous rocks in corresponding provenance area indicates that the ore-hosting sandstone is originated from the erosion of intermediate-acid igneous rocks and the latters are the material basis for the formation of the uranium-rich sandstone beds. On the basis of the study on U-Pb isotopic system evolution of the provenance rocks and sandstones from ore-hosting series, it is verified that the uranium sources of the sandstone-hosted uranium deposit are: the intermediate-acid igneous rocks with high content of mobile uranium, and the sandstone bodies pre-concentrated uranium. (authors)

  9. Padrt'stock (Teplá–Barrandian unit, Bohemian Massif): Petrology, geochemistry, U-Pb zircon dating of granodiorite, and Re-Os age and origin of related molybdenite mineralization

    Czech Academy of Sciences Publication Activity Database

    Žák, Karel; Svojtka, Martin; Breiter, Karel; Ackerman, Lukáš; Zachariáš, J.; Pašava, J.; Veselovský, F.; Litochleb, J.; Ďurišová, Jana; Haluzová, Eva

    2014-01-01

    Roč. 59, č. 4 (2014), s. 351-366 ISSN 1802-6222 R&D Projects: GA ČR GA13-15390S Institutional support: RVO:67985831 Keywords : geochemistry * petrology * Re–Os molybdenite dating * Teplá–barrandian unit * U–Pb zircon dating * Variscan granitoids Subject RIV: DD - Geochemistry Impact factor: 1.405, year: 2014

  10. Detrital Zircon Geo- and Thermochronologic Constraints on Late Mississippian-Early Pennsylvanian Sediment Transport and Tectonics, Southwestern Kansas and Northwestern Arkansas

    Science.gov (United States)

    Bidgoli, T. S.; Wang, W.; Moeller, A.; Stockli, D. F.; Watney, L.

    2017-12-01

    The Late Mississippian to Early Pennsylvanian is a critical time interval across the globe, with major changes in tectonics, climate, and eustacy. Transcontinental sediment transport in North America, from the Appalachians to Great Canyon, has been proposed to initiate at this time (Gehrels et al., 2011). In the midcontinent, clastic influx to the Hugoton Embayment and Arkoma Shelf, during widespread carbonate platform deposition, may record evidence for this model, but the limited number of provenance studies has hindered interpretations. To test this model and further constrain sediment dispersal and source-to-sink systems in the midcontinent, we evaluate the provenance of upper Mississippian to middle Pennsylvanian siliciclastic intervals, in two areas, using sandstone component analysis and detrital zircon U-Pb and (U-Th)/He double dating. (1) For the Hugoton Embayment in southwestern Kansas, we focus on sandstones deposited in two valley-filling intervals, the Chester and Morrow. A total of 1100 zircon U-Pb ages have been acquired from samples from 10 boreholes, 6 from Chester and 4 from Morrow. Preliminary analysis suggests that the Chester is characterized by two major zircon U-Pb age clusters of 900-1295 Ma (Grenville) and 390-475 Ma, consistent with sediment delivery from the Appalachian orogen. Morrow strata record a change in provenance, with the presence of two additional age groups, 1300-1500 Ma (24.5%) and 1600-1800 Ma (17.9%), that correspond well with the age of basement rocks in the Granite-Rhyolite and Yavapai-Maztzal provinces, respectively. We ascribe changes in the zircon age spectra and introduction of these grains to the development of local uplifts, like the Nemaha Ridge, in the early Pennsylvanian. Double dating of zircons from these peaks may reveal additional information about these basement sources and the timing of denudation of these uplifts. (2) For the Arkoma shelf, we are analyzing 11 samples, collected from outcrops in northwestern

  11. U-Pb Data On Apatites With Common Lead Correction : Exemples From The Scottish Caledonides

    Science.gov (United States)

    Jewison, E.; Deloule, E.; Villeneuve, J.; Bellahsen, N.; Labrousse, L.; Rosenberg, C.; Pik, R.; Chew, D.

    2017-12-01

    Apatite is a widely used mineral in low-temperature thermochronology (U-Th/He and AFT). The use of apatite in U-Pb geochronology has a great potential, given its closure temperature around 450°C, for orogen thermostructural evolution studies. However, since apatite can accumulate significant amount of initial Pb in its structure, its use can be hindered by the lack of 204 Pb estimations. To work around this, two options are commonly used : either use a ploting sytem that does not require corrected ratios, or use a proxy to estimate 204Pb and use it to correct the ratios. In this study we use a SIMS to mesure 204Pb in order to compare Tera-Wasserburg diagram and corrected ages to examine the cooling pattern in the northern Highlands of Scotland. The Highlands is an extensively studied caledonian collision wedge which results from the closure of the Iapétus Ocean during the Orodivician-Silurian. Two orogenic events are related to this closing, the grampian event (480-460Ma) and the scandian event (435-415 Ma) that culminated in the stacking of major ductile thrusts. The thermal history of thoses nappes are hence complex and the cooling pattern poorly constrained. Corrected apatite U-Pb ages provide new constrains on ductile wedge building and improve our understanding of mid to lower-crustal deformation and orogenic exhumation. Thoses corrected ages yield equivalent errors and mean ages from the classic method. Those data suggest a global cooling younger than previously thought and a sequence departing from a simple forward sequence. We thus present a refined thermal evolution and conceptualize a model of ductile wedge evolution.

  12. Evolution of the Mazatzal province and the timing of the Mazatzal orogeny: Insights from U-Pb geochronology and geochemistry of igneous and metasedimentary rocks in southern New Mexico

    Science.gov (United States)

    Amato, J.M.; Boullion, A.O.; Serna, A.M.; Sanders, A.E.; Farmer, G.L.; Gehrels, G.E.; Wooden, J.L.

    2008-01-01

    New U-Pb zircon ages, geochemistry, and Nd isotopic data are presented from three localities in the Paleoproterozoic Mazatzal province of southern New Mexico, United States. These data help in understanding the source regions and tectonic setting of magmatism from 1680 to 1620 Ma, the timing of the Mazatzal orogeny, the nature of postorogenic maginatism, Proterozoic plate tectonics, and provide a link between Mazatzal subblocks in Arizona and northern New Mexico. The data indicate a period from 1680 to 1650 Ma in which juvenile felsic granitoids were formed, and a later event between 1646 and 1633 Ma, when these rocks were deformed together with sedimentary rocks. No evidence of pre-1680 Ma rocks or inherited zircons was observed. The igneous rocks have ENd(t) from -1.2 to +4.3 with most between +2 and +4, suggesting a mantle source or derivation from similar-aged crust. Nd isotope and trace element concentrations are consistent with models for typical are magmatism. Detrital zircon ages from metasedimentary rocks indicate that sedimentation occurred until at least 1646 Ma. Both local and Yavapai province sources contributed to the detritus. All of the samples older than ca. 1650 Ma are deformed, whereas undeformed porphyroblasts were found in the contact aureole of a previously dated 1633 Ma gabbro. Regionally, the Mlazatzal orogeny occurred mainly between 1654 and 1643 Ma, during final accretion of a series of island arcs and intervening basins that may have amalgamated offshore. Rhyolite magmatism in the southern Mazatzal province was coeval with gabbro intrusions at 1633 Ma and this bimodal magmatism may have been related to extensional processes following arc accretion. ?? 2007 Geological Society of America.

  13. Dating Ore Deposit Using Garnet U–Pb Geochronology: Example from the Xinqiao Cu–S–Fe–Au Deposit, Eastern China

    Directory of Open Access Journals (Sweden)

    Yu Zhang

    2018-01-01

    Full Text Available The large Xinqiao Cu–S–Fe–Au deposit in the Tongling ore district, Eastern China, is characterized by a large-scale stratiform orebody, in which garnet is widely distributed as the main gangue mineral associated with mineralization. Xinqiao garnet can be divided into early (Grt1 and late (Grt2 generations based on extensive back-scattered electron (BSE imaging observations. Laser ablation (LA-ICP-MS trace element and U–Pb isotope composition analyses indicate that uranium occurs homogeneously within the Xinqiao garnet, and Grt1 and Grt2 have weighted average 207Pb-corrected 206Pb/238U ages of 137.0 ± 7.8 Ma (Mean standard weighted deviation (MSWD = 4.9 and 129.6 ± 7.1 Ma (MSWD = 1.6, respectively, similar to the zircon U–Pb age (139.6 ± 1.5 Ma of the Jitou intrusion. These garnet U–Pb ages, combined with the low MnO content and various Y/Ho ratios, suggest that the Xinqiao garnet is likely to have a magmatic hydrothermal replacement origin associated with the Jitou stock. Based on previous studies of the Xinqiao deposit, we infer that the Xinqiao stratiform orebody may have formed from the Early Cretaceous magmatic hydrothermal fluids associated with the Jitou stock, and may have been generated by the Early Cretaceous tectono-thermal event in Eastern China.

  14. Archean crustal growth of the Imataca complex, Amazonian craton: Evidence from U-Pb-Sm-Nd and Rb-Sr geochronology

    International Nuclear Information System (INIS)

    Tassinari, C.C.G.; Teixeira, W; Nutman, A.P; Szabo, G.; Mondin, M.; Sato, K; Santos, A.P; Siso, C.S

    2001-01-01

    The Archean Imataca Complex (IC), NW Amazonian Craton, forms a ENE-trending, fault-bounded block adjacent to the Paleoproterozoic Maroni-Itacai as magmatic arc (2.2 2.0 Ga) (Tassinari and Macambira, 1999). The IC rocks are complexely deformed, exhibiting elongated and symmetrical domes and thrusts combined with isoclinal folds. Transcurrent faults are also important, like the Guri Fault System - a zone of multiple faulting, shearing and mylonitization along the southeastern edge of the IC. In a pre-Pangean reconstruction using paleomagnetic data from rocks of the African counterpart, the Guri System is contiguous to the Sassandra (Ivory coast) and Zednes (Mauritaine) faults, in agreement also with the comparable geologic evolution between the NW Amazonian and the West Africa cratons, during the Archean and Late-Paleoproterozoic. The IC mainly composed of medium- to high grade quartz-feldspathic paragneiss, exhibits extensive mortar, augen, flaser and mylonitic textures. Calc-alkaline gneiss and granitoid rocks of igneous protolith are also present in the IC, as well as dolomitic marbles, orthopyroxene and magnetite quartzites, and BIFs that include huge ore deposits of Algoma type. Moreover, migmatite injections and anatexis (devoid of metasedimentary components) are widespread in the western part of Complex, the largest migmatite mass centered in Cerro La Ceiba. This paper reports zircon U-Pb SHRIMP, Sm-Nd and Rb-Sr isotopic data of different IC rocks in order to investigate their age and geological evolution within the tectonic framework of the Amazonian Craton (au)

  15. Oster pluton (Central Karelia) - the ancient massif of two-feldspar granites of Baltic Shield

    International Nuclear Information System (INIS)

    Kovalenko, A.V.; Rizvanova, N.G.

    2000-01-01

    Geochronological study of two-feldspar granites forming the Oster pluton in the Central Karelia was performed using the methods of U-Pb- and Sm-Nd-dating. U-Pb isotope data for zircons from granite of the Oster pluton correspond to the age of 2876±21 mln. years, whereas Sm-Nd isotope data obtained for three samples of the garnet correspond to the age varying from 3.0 to 3.5 bln. years. The ancient age of granites of the Oster pluton is explained by the presence of a sufficiently powerful crust in the region by that moment [ru

  16. Geochronological Constraints on the Exhumation and Emplacement of Subcontinental Lithospheric Mantle Peridotites in the Westernmost Mediterranean

    Science.gov (United States)

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

    2017-04-01

    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

  17. New evidence for an old idea: Geochronological constraints for a paired metamorphic belt in the central European Variscides

    Science.gov (United States)

    Will, T. M.; Schmädicke, E.; Ling, X.-X.; Li, X.-H.; Li, Q.-L.

    2018-03-01

    New geochronological data reveal a prolonged tectonothermal evolution of the Variscan Odenwald-Spessart basement, being part of the Mid-German Crystalline Zone in central Europe. We report the results from (i) secondary ion mass spectrometry (SIMS) U-Pb dating of zircon, rutile and monazite, (ii) SIMS zircon oxygen isotope analyses, (iii) laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) zircon Lu-Hf isotope analyses and, (iv) LA-ICPMS zircon and rutile trace element data for a suite of metamorphic rocks (five amphibolite- and eclogite-facies mafic meta-igneous rocks and one granulite-facies paragneiss). The protoliths of the mafic rocks formed from juvenile as well as depleted mantle sources in distinct tectonic environments at different times. Magmatism took place at a divergent oceanic margin (possibly in a back-arc setting) at 460 Ma, in an intraoceanic basin at ca. 445 Ma and at a continental margin at 329 Ma. Regardless of lithology, zircon in eclogite, amphibolite and high-temperature paragneiss provide almost identical Carboniferous ages of 333.7 ± 4.1 Ma (eclogite), 329.1 ± 1.8 to 328.4 ± 8.9 Ma (amphibolite), and 334.0 ± 2.0 Ma (paragneiss), respectively. Rutile yielded ages of 328.6 ± 4.7 and 321.4 ± 7.0 Ma in eclogite and amphibolite, and monazite in high-temperature paragneiss grew at 330.1 ± 2.4 Ma (all ages are quoted at the 2σ level). The data constrain coeval high-pressure eclogite- and high-temperature granulite-facies metamorphism of the Odenwald-Spessart basement at ca. 330 Ma. Amphibolite-facies conditions were attained shortly afterwards. The lower plate eclogite formed in a fossil subduction zone and the upper plate high-temperature, low-pressure rocks are the remains of an eroded Carboniferous magmatic arc. The close proximity of tectonically juxtaposed units of such radically different metamorphic conditions and thermal gradients is characteristic for a paired metamorphic belt sensu Miyashiro

  18. Geochronologic inventory of Rio de Janeiro State

    International Nuclear Information System (INIS)

    Fonseca, A.C.

    1995-01-01

    This paper presents a geochronological study on the Rio de Janeiro State rocks, Southeast Brazil. It is based in a great volume of analytical results (Rb-Sr, U-Pb, Sm-Nd, K-Ar, Ar-Ar and fission tracks dating). It also evocates a little doubt on the Juiz de Fora Complex age: a transamazonic age and some doubts on the sedimentation age of Paraiba do Sul and Andrelandia groups, Costeiro Complex and the existence of a correlation between these litho-stratigraphic units. 19 refs., 2 tabs

  19. Geology and geochronology of the sub-Antarctic Snares Islands/Tini Heke, New Zealand

    International Nuclear Information System (INIS)

    Scott, J.M.; Turnbull, I.M.; Sagar, M.W.; Tulloch, A.J.; Waight, T.E.; Palin, J.M.

    2015-01-01

    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 zircon), which intrudes Broughton Granodiorite (c. 114 Ma from U-Pb zircon) on Broughton Island. Rafts of schist within Snares Granite are common on the outlying Western Chain islets, and rare on North East and Broughton islands. Zircon grains extracted from one schistose raft on Broughton Island are prismatic and yield an essentially unimodal age population of c. 116 Ma that is within error of the granodiorite. These properties suggest that the dated raft represents a meta-igneous rock despite its mica-rich nature. Some schistose rocks on the Western Chain contain coarse relict plagioclase phenocrysts and appear to have an igneous protolith. No conclusive metasedimentary rocks have been identified, although sillimanite-bearing mica-rich schist occurs on Rua. Deformation of the crystalline rocks occurred after Snares Granite intrusion and before cooling below muscovite K-Ar closure at 400 ± 50 degrees C at 95 Ma. This period overlaps the age of extensional ductile shear zones on Stewart Island. The discovery of several basaltic dykes, which cut across fabrics and are unmetamorphosed, indicates that volcanic rocks are associated with all Sub-Antarctic island groups. The larger of the islands are overlain by peat, which on North East Island also contains gravel deposits. (author).

  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)

    Science.gov (United States)

    Iaccarino, Salvatore; Montomoli, Chiara; Carosi, Rodolfo; Langone, Antonio

    2013-04-01

    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