Sample records for zircon u-pb dating

  1. Comparison of SHRIMP U-Pb dating of monazite and zircon

    WAN Yusheng; LIU Dunyi; JIAN Ping


    Monazite dating is an important technique in geochronological studies. However, monazite U-Pb dating by SHRIMP is much less popular than zircon in geochronological applications. This paper compares the results of SHRIMP U-Pb dating of monazites and zircons separated from two granite samples, indicating that monazite SHRIMP U-Pb dating at the Beijing SHRIMP Centre is feasible and provides identical results within error.

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

    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.; Lin, Y. T.; Liu, Y.; Tang, G. Q.


    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.

  3. U/Pb Dating of Zircon from the Suvasvesi Impact Structures, Finland

    Schwarz, W. H.; Breutmann, G.; Schmitt, A. K.; Trieloff, M.; Ludwig, T.; Hanel, M.; Buchner, E.; Schmieder, M.; Pesonen, L. J.; Moilanen, J.


    The two Suvasvesi impact structures (Finland), both covered by lakes, forming an apparent crater doublet, were analysed by in-situ U/Pb dating of zircon grains, concluding that the two craters were formed in separate events, ~600 Ma apart.

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

    Takehara, Lucy, E-mail: [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


    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)

  5. High-Precision U-Pb Zircon Dates as Benchmarks in Absolute Time

    Schmitz, M. D.; Bowring, S. A.; Schoene, B.


    High-precision IDTIMS U-Pb zircon dates provide the most precise and accurate isotopic benchmarks in absolute time, due to the concordancy check of the paired U-Pb decay schemes, the precisely measured 235U and 238U decay constants, very high initial parent/daughter ratios, and the robust nature of zircon to loss or gain of U and Pb over geologic time. However, caveats to the use of such zircon dates include the accurate assessment and minimization of random and systematic errors in the analytical methods, and decay constant uncertainties. Unfortunately, there exists little consensus within the U-Pb geochronological community regarding an international zircon standard for the external assessment of interlaboratory reproducibility, while residual questions remain regarding the potential for systematic error in the single available high-precision counting experiment of the U decay constants1. Stringent criteria are imposed on candidates for zircon geochronology standards including the absence of inheritance and Pb-loss at both the single grain scale and the resolution of microbeam techniques. We present an example of the potential and limitations of a possible zircon standard, AS3, from the Duluth Complex, North American Midcontinent Rift2. New data for 27 single zircons are indistinguishable from prior results, with 207Pb/206Pb and upper intercept dates identical within error to a U-Pb concordia date of 1099.1+/-0.2 Ma (+/-1.2 Ma with systematic errors) based on 12 concordant and equivalent analyses. However, we must reiterate that a zircon population exhibiting consistent concordancy remains elusive, as AS3 and all Paleozoic and older standard candidates so far examined contain grains exhibiting Pb-loss, although rigorous selection and preparation of zircons through diamagnetic separation and aggressive abrasion can mitigate this phenomenon. The continued screening of candidate standards by both IDTIMS and SHRIMP techniques should be an organized, international

  6. Hf isotopic compositions of the standard zircons for U-Pb dating

    XU Ping; WU Fuyuan; XIE Liewen; YANG Yueheng


    Using the newly published Yb isotopic abundances and the mass bias relationship between Yb and Hf, we carried out an analysis of Hf isotopes in the standard zircon 91500 by means of 193 nm laser attached to Neptune multi-collector ICP-MS (LA-MC-ICPMS). The obtained Hf isotopic data, in either in situ or line scan modes, are not only identical for different spot sizes, but also are consistent with previously published results obtained on TIMS or other MC-ICPMS machines within errors. This indicates that it is possible to obtain reliable 176Hf/177Hf isotopic ratios for zircon in either in situ or line scan conditions on LA-MC-ICPMS machine, and the applied procedures in our study for elemental interfering correction are appropriate for the purpose of acquiring satisfactory accuracy for Hf isotope analyses. The Hf isotopic compositions of four zircon standards in high spatial resolution U-Pb dating, 91500, CZ3,CN92-1 and TEMORA, are measured, respectively. The obtained 176Hf/177Hf ratios are 0.282316+4 (n = 34, 2σ) for 91500, 0.281704±6 (n = 16, 2σ) for CZ3, 0.282200±6 (n = 20,2σ-) for CN92-1 and 0.282684±14 (n = 24, 2σ) for TEMORA,respectively, with 176Lu/177Hf ratios of ~0.00031, 0.000036,0.00083 and 0.00127. Zircons 91500 and CZ3 show narrower variations in 176Hf/177Hf and 176Lu/177Hf ratios than those of zircons CN92-1 and TEMORA, and thus are appropriate standards for the Hf isotope analysis.

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

    Lucy Takehara


    Full Text Available 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-MultiCollector 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 235U-205Pb 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 Este trabalho apresenta a técnica de abrasão química em zircões do padrão Temora II aplicada em datação de U-Pb por Dissolução Isotópica e Espectrometria de Massa por Ionização Térmica, método DI-ESIT. O emprego deste método tem como princípio diminuir os efeitos da perda secundária de Pb radiogênico antes da dissolução. Em primeira instância, os zircões foram analisados pelo método in situ com uso de Microssonda Laser acoplada a um Espectrômetro de Massa Multicoletor com Plasma Acoplado Indutivamente (MC-ICP-MS, cujo objetivo foi identificar os grãos de zircão de fase de cristalização simples sem sobrecrescimento. Nove cristais de zircão analisados com microssonda a

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

    Pană Dinu


    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.

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

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


    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

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

    Andréa R. Jelinek


    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.

  11. Investigating sources of ignimbrites in the Altiplano-Puna Volcanic Complex using U-Pb dating of zircons

    Kern, J. M.; de Silva, S. L.; Schmitt, A. K.


    Large silicic volcanic fields (LSVFs) are thought to represent the surface expression of upper crustal batholith emplacement, with the spatiotemporal distribution of the vents and eruptions representing the development of the system. The Altiplano-Puna Volcanic Complex (APVC) in the Central Andes is a LSVF active from 11-1 Ma that erupted over 13,000 km3 of magma from large, multicyclic caldera centers and smaller ignimbrite shields during 3 distinct pulses of volcanism at 8.4, 5.5, and 4.0 Ma. Links to the magmatic system beneath are being pursued through U-Pb zircon dating of APVC ignimbrites. Initial results comprise 61 238U/206Pb zircon ages of mostly marginal crystal domains from five APVC ignimbrites-the 0.98 ± 0.03 Ma Purico, 3.96 ± 0.08 Ma Atana, 4.0 ± 0.9 Ma Toconao, 4.09 ± 0.02 Ma Puripicar, and 8.33 ± 0.06 Ma Sifon ignimbrites-dated by high-resolution secondary ionization mass spectrometry (SIMS). Each zircon analyzed was less than 350 μm in length and cathodoluminescence images reveal zonations within individual zircons, though significant core-rim age differences are rare. The ~1 Ma Purico ignimbrite displays multiple zircon age populations significantly predating the 40Ar/39Ar eruption age, but younger than ages from the nearby large-volume Atana ignimbrite erupted from La Pacana caldera. Some peaks do, however, coincide with later resurgent activity within La Pacana as expressed by the 2.7 Ma Cerro Bola dome. Zircon ages in the Atana ignimbrite are indistinguishable from its eruption, while those from the 4.0 Ma Toconao ignimbrite-the volatile-rich cap of the Atana magma chamber-contains three populations of xenocrystic zircons from the Proterozoic-Ordivician, ~13 Ma, and ~9 Ma. The ~9 Ma zircons correlate with K-Ar ages from an underlying ignimbrite, whereas the 13 Ma xenocrysts likely have a plutonic source. The Purico ignimbrite thus provides direct evidence of zircon inheritance from previous eruption cycles, while the Toconao records a

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

    Chemale Junior, Farid, E-mail: [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


    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)

  13. SHRIMP zircon U-Pb dating in Jingshan "migmatitic granite", Bengbu and its geological significance

    XU Wenliang; WANG Qinghai; YANG Debin; LIU Xiaochun; GUO Jinghui


    The petrographic characteristics of Jingshan "migmatitic granite" and the occurrence of the magmatic zircons indicate that the granite was formed by normal crystallization of felsic melts. All zircons in the granite have inherited cores and fine-scale oscillatory zoning rims of magmatic origin. It is realized that the granite was formed at 160.2±1.3 Ma through dating magmatic zircons. The generation of the granitic magma could be related to the lithospheric mantle and/or lower crust delamination after the ultrahigh pressure metamorphism (UHPM) in Triassic. Most inherited zircons yield the ages of 217.1±6.6 Ma, which is consistent with the peak UHPM in the Dabie-Sulu orogenic belt. Some of the inherited zircons (433-722 Ma) constitute a discordia line with the upper intercept age of 850+85/-68 Ma and a lower intercept age of 261+100/-140 Ma. These ages imply that the granite could be derived from the partial melting of the crustal materials of the South China Block that was intensively superimposed by the UHPM. The UHPM could be the reason for the major Pb loss at ±220 Ma.

  14. U-Pb zircon dating constraints on formation time of Qilian high-grade metamorphic rock and its tectonic implications

    XU WangChun; ZHANG HongFei; LIU XiaoMing


    In order to constrain the formation time of high-grade metamorphic rocks in the Qilian Mountains, U-Pb zircon dating was carried out by using LA-ICPMS technique for a paragneiss of the Hualong Group in the Qilian Mountains basement series and a weakly foliated granite that intruds into the Hualong Group. Zircons from the paragneiss consist dominantly of detrital magma zircons with round or sub-round shape. They have 207Pb/206Pb ages mostly ranging from 880 to 900 Ma, with a weighted mean age of 891±9 Ma, which is interpreted as the magma crystallization age of its igneous provenance and can be taken as a lower age limit for the Hualong Group. Magma crystallization age for the weak-foliated granite is 875±8 Ma, which can be taken as an upper age limit for the Hualong Group. Accordingly, the formation time of the Hualong Group is constrained at sometime between 875 and 891 Ma. A few zircons from both paragneiss and weak-foliated granite display old inherited ages of 1000 to 1700 Ma and young metamorphic ages of Early Paleozoic. The zircon age distribution pattern confirms that the Qilian Mountains and the northern margin of Qaidam Basin had a united basement, with geotectonic affinity to the Yangtze Block. The results also reveal that sediments of the Hualong Group formed by rapid accumulation due to rapid crustal uplift-erosion. This process may result from intensive Neoproterozoic orogenesis due to assembly of the suppercontinent Rodinia.

  15. Sediment provenance, reworking and transport processes in the Indus River by U-Pb dating of detrital zircon grains

    Alizai, Anwar; Carter, Andrew; Clift, Peter D.; VanLaningham, Sam; Williams, Jeremy C.; Kumar, Ravindra


    We present new major and trace element data, together with U-Pb ages for zircon sand grains from the major tributaries of the Indus River, as well as the adjacent Ghaggar and Yamuna Rivers and from bedrocks within the Sutlej Valley, in order to constrain the origin of the sediment reaching the Arabian Sea. Zircon grains from the upper Indus are generally younger than 200 Ma and contrast with those from the eastern tributaries eroded from Himalayan sources. Grains younger than 15 Ma, which typify the Nanga Parbat Massif, comprise no more than 1-2% of the total, even in the upper Indus, showing that this terrain is not a major sediment producer, in contrast with the Namche Barwe Massif in the eastern Himalayan syntaxis. The Sutlej and Yamuna Rivers in particular are very rich in Lesser Himalayan-derived 1500-2300 Ma zircons, while the Chenab is dominated by 750-1250 Ma zircons, mostly eroded from the Greater Himalaya. The upper Indus, Chenab and Ravi yield zircon populations broadly consistent with the outcrop areas, but the Jhelum and the Sutlej contain many more 1500-2300 Ma zircons than would be predicted from the area of Lesser Himalayan rock within their drainages. A significant population of grains younger than 200 Ma in the sands of the Thar Desert indicates preferential eolian, monsoon-related transport from the Indus lower reaches, rather than reworking from the local rivers. Modelling of observed zircon ages close to the delta contrasts with modern water discharge. The delta is rich in zircons dating 1500-2300 Ma, while discharge from modern rivers carrying such grains is low. The modest size of the Sutlej, the richest source of these materials in the modern system, raises the possibility that the compositionally similar Yamuna used to flow westwards in the recent past. Our data indicate a non-steady state river with zircon transport times of 5-10 k.y. inferred from earlier zircon dating of delta sands. The modern delta zircons image an earlier, likely

  16. SHRIMP zircon U-Pb dating for subduction-related granitic rocks in the northern part of east Junggar, Xinjiang


    SHRIMP U-Pb zircon dating on the Xileketehalasu granodiorite porphyry and Kalasayi monodiorite porphyry that intrude middle Devonian Beitashan Formation at the north part of east Junggar region shows that they were formed at 381±6 Ma and 376±10 Ma respectively. They are interpreted as subduction-related granitic rocks, which is the first report that the isotopic ages for the granitic rocks range from 350 to 390 Ma. Another determined age for the Kalasayi monodiorite porphyry is 408±9 Ma, representing the age of underlain Lower Devonian volcanic rocks. Thus, the U-Pb dates suggest that the northeastward subduction of Junggar ocean from southwest occurred at 408 to 376 Ma (the real interval may be larger). Because the ore-bearing porphyry intruded following the formation of the volcanic rocks of middle Devonian Beitashan Formation, their tectonic setting is similar to the Andes Mountains that hosts world-class porphyry copper deposits, and the researched area could be regarded as a potential area for prospecting large porphyry copper deposits.

  17. Mesoproterozoic Continental Arc Type Granite in the Central Tianshan Mountains: Zircon SHRIMP U-Pb Dating and Geochemical Analyses

    YANG Tiannan; LI Jinyi; SUN Guihua; WANG Yanbin


    The Central Tianshan belt in northwestern China is a small Precambrian block located in the southern part of the Central Asia Orogenic Belt (CAOB), which is considered as "the most voluminous block of young continental crust in the world" that comprises numerous small continental blocks separated by Paleozoic magmatic arcs. The Precambrian basement of the central Tianshan Mountains is composed of volcanic rocks and associated volcano-sedimentary rocks that were intruded by granitic plutons. Geochemical analyses demonstrate that the granitic plutons and volcanic rocks were generated in the Andean-type active continental arc environment like today's Chile, and the zircon U-Pb SHRIMP dating indicates that they were developed at about 956 Ma, possibly corresponding to the subduction of the inferred Mozambique Ocean under the Baltic-African super-continent.

  18. Age and timing of the Permian mass extinctions: U/Pb dating of closed-system zircons.

    Mundil, Roland; Ludwig, Kenneth R; Metcalfe, Ian; Renne, Paul R


    The age and timing of the Permian-Triassic mass extinction have been difficult to determine because zircon populations from the type sections are typically affected by pervasive lead loss and contamination by indistinguishable older xenocrysts. Zircons from nine ash beds within the Shangsi and Meishan sections (China), pretreated by annealing followed by partial attack with hydrofluoric acid, result in suites of consistent and concordant uranium/lead (U/Pb) ages, eliminating the effects of lead loss. The U/Pb age of the main pulse of the extinction is 252.6 +/- 0.2 million years, synchronous with the Siberian flood volcanism, and it occurred within the quoted uncertainty.

  19. Zircon SHRIMP U-Pb dating for olivine gabbro at Wangmuguan in the Beihuaiyang zone and its geological significance

    LIU Yican; LI Shuguang; GU Xiaofeng; HOU Zhenhui


    Zircon SHRIMP U-Pb dating was performed for olivine gabbro at Wangmuguan in the Beihuaiyang zone of the Dabie orogen and its country rock (garnet-bearing epidote-mica-quartz schist).The results show that the gabbro was crystallized at 635±5 Ma, in the late Neoproterozoic rather than in the late Paleozoic as previously suggested; its country rocks formed at 464±7 Ma, younger than the enclosed gabbro. The U-Pb age for the gabbro is in good agreement with ages for tuff interbedded with sediments from the Doushantuo Formation in the South China Block and late-Neoproterozoic basic dyke swarms distributed on a large scale over areas of Suizhou to Zaoyang of Hubei Province in the northern margin of the South China Block. This suggests a large-scale magmatic activity occurred at the late Neoproterozoic in the South China Block, so that the gabbro at Wangmuguan in the western segment of the Beihuaiyang zone is geotectonic affinity to the northern margin of the South China Block. Since the olivine gabbro occurs within the schist of Ordovician protolith with tectonic contact between them but forming in different tectonic settings, it is concluded that the late-Neoproterozoic gabbro was detached from the Precambrian basement of the South China Block during the Triassic subduction of the South China Block, and tectonically thrusted over the metamorphosed rocks in the southern margin of the North China Block.

  20. The existing time of Sihetun vertebrate in western Liaon-ing, China--Evidence from U-Pb dating of zircon


    In different opinions, the stratigraphic time of the feathered dinosaurs and early birds in Sihetun and its neighboring areas in Beipiao, western Liaoning is appointed to different epoches of eras, such as Late Jurassic, Early Cretaceous and Late Jurassic-Early Cretaceous. Even the recently dating data are still very different. This note first reported the U-Pb age of (125.2±0.9) Ma of zircon separated from tuff of Sihetun vertebrates horizon. The age reveals that Sihetun vertebrate belongs to Early Cretaceous. The method is more objective, because of the strong ability of disturbance resistance and high U-Pb blocking temperature of zircons.

  1. Zircon SHRIMP U-Pb Dating, Geochemical Characteristics and Tectonic Significance of Granitic Gneisses in Amdo, Tibet

    Lu Lu; Zhenhan Wu; Zhen Zhao; Daogong Hu; Peisheng Ye


    The Amdo microcontinent is located within the middle of Bangong-Nujiang suture (BNS) zone in the shape of lens. The basic geological research restricts geologists from understand-ing the histories of tectonic evolution of BNS and regional geology more deeply. This paper system-atically studies the geochronology and geochemistry of granitic gneisses from Amdo basement. These data provide constraints on formation age, source characteristics and tectonic setting of their protolith. The SHRIMP zircon U-Pb dating is operated for granitic gneisses. Samples AGS-2 and AGS-3 (granitic gneiss) yield average zircon U-Pb ages of 485±14 and 487±6 Ma, respectively. These ages should represent the formation age of protolith and indicate that they are formed in the Early Ordovician. Granitic gneisses are characterized by high SiO2, Na2O, K2O and Al2O3, low Fe and Mg, enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), deple-tion in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), with negative Eu anomaly. The Rittmann index (σ) is 1.77 to 2.60, less than 3.3. The aluminum saturation index (A/CNK) values range from 0.88 to 1.26. These features suggest that protolith of granitic gneisses from Amdo basement show characteristics of calc-alkaline and S-type granite, and they could be de-rived from partial melting of metamorphic greywackes in the upper crust of low maturity. The tec-tonic setting is syn-collision. These all suggest that the formation of protolith of granitic gneisses from Amdo are caused by the Early Paleozoic orogeny, which could be related to proto-Tethyan oceanic subduction along Gondwana continental margins, and does not result from the production of Pan-African orogenesis.

  2. SHRIMP Zircon U-Pb Dating of the Tongshi Magmatic Complex in Western Shandong and Its Geological Implications

    HU Huabin; MAO Jingwen; LIU Dunyi; NIU Shuyin; WANG Yanbin; LI Yongfeng; SHI Ruruo


    The SHRIMP U-Pb zircon dating result of the Tongshi magmatic complex in western Shandong is presented in this paper.The Tongshi magmatic complex comprises fine-grained porphyritic diorite and syenitic porphyry.Eighteen analyses for fine-grained porphyritic diorite gave two concordia ages,in which ten analyses constitute the young age group,giving 206Pb/238U ages ranging from 167.9 Ma to 183 Ma with a weighted mean age of 175.7±3.8 Ma,and the other eight yielded 207Pb/206Pb ages of 2502 Ma to 2554 Ma with a weighted mean 2518±11 Ma.Two analyses for syenitic porphyry gave ages of 2485 Ma and 2512 Ma,respectively.The age of 175.7±3.8 Ma indicates that the crystallization of the Tongshi magmatic complex occurred in the Middle Jurassic,whereas that of 2518±11 Ma is interpreted as the age of inherited magmatic zircons in the Neoarchean Wutai period.

  3. The Dabie Orogen as the early Jurassic sedimentary provenance: Constraints from the detrital zircon SHRIMP U-Pb dating

    LI Renwei; WAN Yusheng; CHENG Zhenyu; ZHOU Jianxiong; XU Yunhua; LI Zhong; JIANG Maosheng


    The SHRIMP U-Pb ages of detrital zircon from the oldest Mesozoic strata, the Fanghushan Fomation, in the Hefei Basin range from 200 Ma to ca. 2500 Ma, which indicates that the Dabie Orogen as the early Jurassic sedimentary provenance was complex. The composition of the Dabie Orogen includes: the Triassic high pressure-ultrahigh pressure metamorphic rocks, of which the detrital zircon ages are from 234 Ma to 200 Ma; the rocks possibly related to the Qinling and Erlangping Groups representing the southern margin of the Sino-Korean craton in the Qinling and Dabie area, of which the detrital zircon has an age of 481-378 Ma; the Neoproterozoic rocks originated from the Yangtze croton, of which the detrital zircon ages are 799-721 Ma old; and the rocks with the detrital zircon ages of ca. 2000 Ma and ca. 2500 Ma, which could be the old basement of the Yangtze craton.

  4. Chronology and Sources of Mesozoic Intrusive Complexes in the Xuzhou-Huainan Region, Central China:Constraints from SHRIMP Zircon U-Pb Dating

    XU Wenliang; WANG Qinghai; LIU Xiaochun; WANG Dongyan; GUO Jinghui


    SHRIMP zircon U-Pb dating in the Liguo and Jiagou intmsives indicates that they were formed at ~130 Ma in the Early Cretaceous. Most inherited zircons in the Liguo intrusive were formed at 2509±43 Ma. Most inherited and detrital zircons in the Jiagou intrusive were formed at ~2500 Ma, ~2000 Ma and ~1800 Ma. The SHRIMP zircon U-Pb dating in two gneiss xenoliths from the Jiagou intrusive yields the ages of 2461+22 Ma and 2508±15 Ma, respectively. The dating results from inherited and detrital zircons in the intmsives and the gneiss xenoliths imply that the magmas could be derived from the partial melting of the basement of the North China Block (NCB). The magmatism is strong and extensive in the periods from 115 to 132 Ma, which is of typical bimodal characteristics. It is suggested that the lithospheric thinning in the eastern North China Block reached its peak in 115-132 Ma.

  5. Sm-Nd and zircon SHRIMP U-Pb dating of Huilanshan mafic granulite in the Dabie Mountains and its zircon trace element geochemistry

    HOU; Zhenhui; LI; Shuguang; CHEN; Nengsong; LI; Qiuli; LIU


    The mafic granulites from Huilanshan are outcropped on the center of the Luotian dome in the northern Dabie Mountains. The Sm-Nd isochron defined by granulite-facies metamorphic minerals (garnet + clinopyroxene + hypersthene) yields an age of 136(±)18 Ma indicating the early Cretaceous granulite-facies metamorphism. The cathodoluminescence (CL) images of zircons from the granulite show clearly core-mantle-rim structures. The zircon cores are characterized by typical oscillatory zoning and highly HREE enriched patterns, which suggests their magma origin. Some zircon cores among them with little Pb loss give SHRIMP U-Pb ages ranging from 753 to 780 Ma, which suggests that the protolith of Huilanshan granulite is Neoproterozoic mafic rocks. The zircon mantles usually cut across the oscillatory zone of the zircon cores have 3―10 times lower REE, Th, U, Y, Nb and Ta contents than the igneous zircon cores but have high common Pb contents. These characteristics suggest that they were formed by hydrothermal alteration of the igneous zircons. The part of zircon mantles with little Pb loss give a similar SHRIMP U-Pb age (716―780 Ma) to the igneous zircon cores, which implies that the hydrothermal events occurred closely to the magmatic emplacement. In view of the strong early Cretaceous magmatism in the Luotian dome, consequently, the Huilanshan mafic granulite was formed by heating of the Neoproterozoic mafic rocks in mid-low crust, which caused the granulite-facies metamorphism underneath the Dabie Mountains. The similarity between the granulite metamorphic age (136±18 Ma) defined by Sm-Nd isochron and K-Ar age of 123―127 Ma given by amphible from the gneiss in Luotian dome suggests a rapid uplifting of the Luotian dome, which may result in further exhumation of the ultrahigh pressure metamorphic rocks in the Dabie Mountains.

  6. Fingerprinting the K/T impact site and determining the time of impact by U-Pb dating of single shocked zircons from distal ejecta

    Krogh, T. E.; Kamo, S. L.; Bohor, B. F.


    U-Pb isotopic dating of single 1 - 3 micrograms zircons from K/T distal ejecta from a site in the Raton Basin, Colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site. Data for the least shocked zircons are slightly displaced from the 544 +/- 5 Ma primary age for a component of the target site, while those for highly shocked and granular grains are strongly displaced towards the time of impact at 65.5 +/- 3.0 Ma. Such shocked and granular zircons have never been reported from any source, including explosive volcanic rocks. Zircon is refractory and has one of the highest thermal blocking temperatures; hence, it can record both shock features and primary and secondary ages without modification by post-crystallization processes. Unlike shocked quartz, which can come from almost anywhere on the Earth's crust, shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers. With U-Pb zircon dating, it is now possible to correlate ejecta layers derived from the same target site, test the single versus multiple impact hypothesis, and identify the target source of impact ejecta. The ages obtained in this study indicate that the Manson impact site, Iowa, which has basement rocks that are mid-Proterozoic in age, cannot be the source of K/T distal ejecta. The K/T distal ejecta probably originated from a single impact site because most grains have the same primary age.

  7. Uplift of Northwest Margin of Tibetan Plateau: Indicated by Zircon LA ICP-MS U-Pb Dating of Conglomerate from Mazartagh, Tarim Basin

    Si Jialiang; Li Haibing; Pei Junling; Pan Jiawei


    Radiometric age dating of detrital zircons is highly advantageous for analysis of the depositional environment and to identify source areas. Aiming at the uplift and denudation of the surrounding ranges, LA ICP-MS U-Pb analysis has been performed on zircon grains from a conglomerate collected at the Lower Pilocene of Mazartagh, which is in the center of the Tarim basin, Xinjinng (新疆), China. A wide range of ages mainly failing into three groups was yielded: 200-500, 800-1 100, and 1 800-2 000 Ma. Zircon features principally indicate magmatic origin. According to the comparison between the analyzed zircons with those from surrounding orogenic belts, the younger grains are mainly related to the west while the older ones are to the regions more eastward. The variations might imply the W-E propagation tectonic activation and uplift of the surrounding orogenic belts. The west segment uplifted and was denuded firstly, driven by the approximate W-E height difference, upon the denudation, transportation and deposition, acting as the source of young zircons analyzed. With the eastward spreading of tectonic movement, the segment more to the east rose, the meridional relief in creased rapidly and began to control the flow direction, then more zircons joined in the Pliocene in Mazartagh. It is difficult to definitely explain the source of grains with similar values to that from the Altyn Mountain region, more detailed data and chronological ages with higher precision will be helpful for making more credible conclusion.

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

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


    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

  9. Timing of the terrestrial Permian-Triassic boundary biotic crisis: Implications from U-Pb dating of authigenic zircons

    YU JianXin; LI HuiMin; ZHANG SuXin; YANG FengQing; FENG QingLai


    The Late Permian to Early Triassic transition represents one of the most important Phanerozoic mass extinction episodes.The cause of this event is still in debate between catastrophic and gradual mechanisms.This study uses the U-Pb method on zircons from the uppermost Permian/lowermost Triassic clay deposits at Chahe (Guizhou Province,SW China) to examine time constraints for this event.The results of both this and previous studies show that the ages of Bed 68a and 68c (the upper clay bed of the terrestrial Permian-Triassic boundary (PTB)) respectively are 252.6~2.8 and 247.5±2.8 Ma.This age (within the margin of error) almost accords with the upper clay bed (Bed 28) age of Meishan and the eruption age of Tunguss Basalt,and is so far the most accurate age obtained from terrestrial PTB.The claystone of Bed 68 was formed in the earliest Triassic.The biotic crisis occurred at nearly the same time in terrestrial and marine environments during Permian-Triassic interval; however the extinction patterns and processes are different.The extinction pattern of the terrestrial plants shows a major decline at the PTB after long-term evolution,followed by a retarded extinction of the relicts in the earliest Triassic.

  10. Timing of the terrestrial Permian-Triassic boundary biotic crisis:Implications from U-Pb dating of authigenic zircons


    The Late Permian to Early Triassic transition represents one of the most important Phanerozoic mass extinction episodes. The cause of this event is still in debate between catastrophic and gradual mechanisms. This study uses the U-Pb method on zircons from the uppermost Permian/lowermost Triassic clay deposits at Chahe (Guizhou Province, SW China) to examine time constraints for this event. The results of both this and previous studies show that the ages of Bed 68a and 68c (the upper clay bed of the terrestrial Permian-Triassic boundary (PTB)) respectively are 252.6±2.8 and 247.5±2.8 Ma. This age (within the margin of error) almost accords with the upper clay bed (Bed 28) age of Meishan and the eruption age of Tunguss Basalt, and is so far the most accurate age obtained from terrestrial PTB. The claystone of Bed 68 was formed in the earliest Triassic. The biotic crisis occurred at nearly the same time in terrestrial and marine environments during Permian-Triassic interval; however the extinction patterns and processes are different. The extinction pattern of the terrestrial plants shows a major decline at the PTB after long-term evolution, followed by a retarded extinction of the relicts in the earliest Triassic.

  11. Paleoproterozoic lower crust beneath Nushan in Anhui Province: Evidence from zircon SHRIMP U-Pb dating on granulite xenoliths in Cenozoic alkali basalt

    HUANG Xiaolong; XU Yigang; LIU Dunyi; JIAN Ping


    Zircon SHRIMP U-Pb dating was carried out for an intermediate granulite xenolith in Cenozoic alkali basalt from Nushan. The results suggest that the lower crust beneath Nushan may have formed at about 2400-2200 Ma, and have been subjected to granulite-facies metamorphism at 1915±27 Ma. The old age of the Nushan lower crust is consistent with the geochemical similarities between Nushan granulite xenoliths and Archean-Paleoproterozoic granulite terrains in the NorthChina craton, but it is not distinguishable from high-grade metamorphic rocks in the Yangtze craton where such old ages were also reported. Significant Pb-lossoccurs in the Nushan zircons, implying important influence of widespread Mesozoic to Cenozoic underplating in East China on the lower crust beneath Nushan.

  12. Age of the Lava Creek supereruption and magma chamber assembly at Yellowstone based on 40Ar/39Ar and U-Pb dating of sanidine and zircon crystals

    Matthews, Naomi E.; Vazquez, Jorge A.; Calvert, Andrew T.


    The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff. Tephra from the Lava Creek eruption is a key Quaternary chronostratigraphic marker, in particular for dating the deposition of mid Pleistocene glacial and pluvial deposits in western North America. To resolve the timing of eruption and crystallization history for the Lava Creek magma, we performed (1) 40Ar/39Ar dating of single sanidine crystals to delimit eruption age and (2) ion microprobe U-Pb and trace-element analyses of the crystal faces and interiors of single zircons to date the interval of zircon crystallization and characterize magmatic evolution. Sanidines from the two informal members composing Lava Creek Tuff yield a preferred 40Ar/39Ar isochron date of 631.3 ± 4.3 ka. Crystal faces on zircons from both members yield a weighted mean 206Pb/238U date of 626.5 ± 5.8 ka, and have trace element concentrations that vary with the eruptive stratigraphy. Zircon interiors yield a mean 206Pb/238U date of 659.8 ± 5.5 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high U concentration cores that likely grew from highly evolved melt. The occurrence of distal Lava Creek tephra in stratigraphic sequences marking the Marine Isotope Stage 16–15 transition supports the apparent eruption age of ∼631 ka. The combined results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up-temperature evolution for Yellowstone's subvolcanic reservoir in the 103−104 year interval before eruption.

  13. U-Pb dating of zircon from the Central Zone of the East Kunlun Orogen and its implications for tectonic evolution


    LA-ICP-MS and SHRIMP U-Pb dating of zircons from orthogneisses and amphibolite from the Central Zone of the Kunlun Orogen is reported in this paper. One orthogneiss sample has metamorphic zircons yielding weighted average 206Pb/238U age of 517.0 +5.0/-6.0 Ma, and the other orthogneiss sample con- tains zircons with inherited magmatic cores giving three population 207Pb/206Pb ages of 955 Ma, 895 Ma and 657 Ma for the magmatic protolith, and metamorphic recrystallized rims with peak 206Pb/238U ages of 559 +12/?17 Ma and 516 ± 13 Ma. The amphibolite yielded three populations of weighted average 206Pb/238U age of 482.0 +10/?8.0 Ma, 516.2 ± 5.8 Ma and 549 ± 10 Ma for the metamorphic zircons. These dating results recorded the tectonothermal events that occurred in the early Paleozoic and the Pre- cambrian time. The records of the Cambrian magmatic-metamorphic event in the Qinling Orogen, the Altyn Tagh belt, north margin of the Qaidam Block and the Kunlun Orogen suggest that continental assembly probably occurred in the early evolutionary history of the Proto-Tethys.

  14. U-Pb dating of zircon from the Central Zone of the East Kunlun Orogen and its implications for tectonic evolution

    CHEN NengSong; SUN Min; WANG QinYan; ZHANG KeXin; WAN YuSheng; CHEN HaiHong


    LA-ICP-MS and SHRIMP U-Pb dating of zircons from orthogneisses and amphibolite from the Central Zone of the Kunlun Orogen is reported in this paper. One orthogneiss sample has metamorphic zircons yielding weighted average 206Pb/238U age of 517.0 +5.0/-6.0 Ma,and the other orthogneiss sample contains zircons with inherited magmatic cores giving three population 207Pb/206Pb ages of 955 Ma,895 Ma and 657 Ma for the magmatic protolith,and metamorphic recrystallized rims with peak 206pb/238U ages of 559 +12/-17 Ma and 516 + 13 Ma. The amphibolite yielded three populations of weighted average 206Pb/238U age of 482.0 +10/-8.0 Ma,516.2 ± 5.8 Ma and 549 ± 10 Ma for the metamorphic zircons. These dating results recorded the tectonothermal events that occurred in the early Paleozoic and the Precambrian time. The records of the Cambrian magmatic-metamorphic event in the Qinling Orogen,the Altyn Tagh belt,north margin of the Qaidam Block and the Kunlun Orogen suggest that continental assembly probably occurred in the early evolutionary history of the Proto-Tethys.

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

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


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

  16. SHRIMP single zircon U-Pb dating of the Kongling high-grade metamorphic terrain: Evidence for >3.2 Ga old continental crust in the Yangtze craton

    GAO; Shan; (


    [1]Liu, G. L., New progress in the geochronology of the Kongling terrain, Regional Geology of China, 1987, 1: 95.[2]Zheng, W. Z., Liu, G. L., Wang, X. W., Geochronology of the Archean Kongling terrain, Bull. Yichang Inst. Geol. Miner. Resour. (in Chinese), 1991, 16: 97-105.[3]Yuan, H. H., Zhang, Z. L., Liu, W. et al., Dating of zircons by evaporation method and its application, Mineral. Petrol. (in Chinese), 1991, 11: 72.-79[4]Ling, W. L., Gao, S., Zheng, H. F. et al., Sm-Nd isotopic dating of Kongling terrain, Chinese. Sci. Bull., 1998, 43(1): 86-89.[5]Gao, S., Ling, W. L., Qiu, Y. et al., Contrasting geochemical and Sm-Nd isotopic compositions of Archean metasediments from the Kongling high-grade terrain of the Yangtze craton: evidence for cratonic evolution and redistribution of REE during crustal anatexis, Geochim. Cosmochim. Acta, 1999, 63: 2071-2088.[6]Gao, S., Zhang, B. R., The discovery of Archean TTG gneisses in northern Yangtze craton and their implications, Earth Sci. (in Chinese, with English abstract), 1990, 15: 675-679.[7]Dong, S. B., Metamorphism and Its Relation to the Crustal Evolution in China (in Chinese), Beijing: Geological Publishing House, 1986. [8]Composton, W., Williams, I. S., Meyer, C., U-Pb geochronology of zircons from lunar breccia 73217 using sensitive high mass-resolution ion microprobe, J. Geophys. Res., 1984, 89(B): 252-534.[9]Williams, I. S., Composton, W., Black, L. P et al., Unsupported radiogenic Pb in zircon: a case of anomalously high Pb-Pb, U-Pb and Th-Pb ages, Contrib. Mineral. Petrol., 1984, 88: 322-327.[10] Nelson, D. R., Evolution of the Archean granite-greenstone terrains of the Eastern Goldfileds, Western Australia: SHRIMP U-Pb zircon constraints, Precambrian Res., 1997, 83: 57-81.[11] Ling, W. L., Geochronology and crustal growth of the Paleoproterozoic basements along the northern margin of the Yangzte craton, Earth Sci., 1996, 21(5): 491—493.

  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

    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)


    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 SHRIMP zircon dating of high-grade rocks from the Upper Allochthonous Terrane of Bragança and Morais Massifs (NE Portugal); geodynamic consequences

    Mateus, A.; Munhá, J.; Ribeiro, A.; Tassinari, C. C. G.; Sato, K.; Pereira, E.; Santos, J. F.


    Bragança and Morais Massifs are part of the mega-klippen ensemble of NW Iberia, comprising a tectonic pile of four allochthonous units stacked above the Central-Iberian Zone autochthon. On top of this pile, the Upper Allochthonous Terrane (UAT) includes different high-grade metamorphic series whose age and geodynamic meaning are controversial. Mafic granulites provided U-Pb zircon ages at 399 ± 7 Ma, dating the Variscan emplacement of UAT. In contrast, U-Pb zircon ages of ky- and hb-eclogites, felsic/intermediate HP/HT-granulites and orthogneisses (ca. 500-480 Ma) are identical to those of gabbros (488 ± 10 Ma) and Grt-pyroxenites (495 ± 8 Ma) belonging to a mafic/ultramafic igneous suite that records upper mantle melting and mafic magma crustal underplating at these times. Gabbros intrude the high-grade units of UAT and did not underwent the HP metamorphic event experienced by eclogites and granulites. These features and the zircon dates resemblance among different lithologies, suggest that extensive age resetting of older events may have been correlative with the igneous suite emplacement/crystallisation. Accordingly, reconciliation of structural, petrological and geochronological evidence implies that the development and early deformation of UAT high-grade rocks should be ascribed to an orogenic cycle prior to ≈ 500 Ma. Undisputable dating of this cycle is impossible, but the sporadic vestiges of Cadomian ages cannot be disregarded. The ca. 500-480 Ma time-window harmonises well with the Lower Palaeozoic continental rifting that trace the Variscan Wilson Cycle onset and the Rheic Ocean opening. Subsequent preservation of the high heat-flow regime, possibly related to the Palaeotethys back-arc basin development (ca. 450-420 Ma), would explain the 461 ± 10 Ma age yielded by some zircon domains in felsic granulites, conceivably reflecting zircon dissolution/recrystallisation till Ordovician times, long before the Variscan paroxysm (ca. 400-390 Ma). This

  19. Zircon SHRIMP U-Pb dating of meta-diorite from the basement of the Songliao Basin and its geological significance

    WANG Ying; ZHANG Fuqin; ZHANG Dawei; MIAO Laicheng; LI Tiesheng; XIE Hangqiang; MENG Qingren; LIU Dunyi


    The basement of the Songliao Basin mainly contains low-grade metamorphic rocks and granites. It has been long disputed whether the basin has Precambrian metamorphic basement. This is a report of zircon SHRIMP U-Pb dating results of a meta-diorite sample, which was taken from the Si-5 drilling hole in the southern portion of the Songliao Basin. The SHRIMP analyses indicate that the meta-diorite with a weighted mean 207Pb/206Pb age of 1839±7 Ma (2σ, n = 8) was emplaced during Paleo-Proterozoic time. Additionally, the meta-diorite has old Nd model ages (TDM1:2999Ma; TDM2:2849Ma). These data suggest that the southern part of the Songliao Basin do possess Precambrian basement.

  20. SHRIMP U-Pb Zircon Dating of the Tula Granite Pluton on the South Side of the Altun Fault and Its Geological Implications

    WU Suoping; WU Cailai; WANG Meiying; CHEN Qilong; Joseph L. WOODEN


    The Tula A2-subtype granite pluton is located between the Altun fault and its branching fault.According to the geological, geochemical, REE and trace elements characteristics, it belongs to the A2 (PA) subtype granite. The SHRIMP U-Pb zircon dating gives a result of 385.2+8.1 Ma, which is located between the Middle and Late Devonian in the international stratigraphic chart, and can be regarded as the crystallization age of the Tula granite. The study indicates that the Tula area was in a local extensional environment in the end of the Middle Devonian, and that environment was probably related to the synchronized strike-slip activity of the Altun fault.

  1. Geochemistry and LA-ICP-MS zircon U-Pb dating of the Dongjiahe ophiolite complex from the western Bikou terrane

    LAI; ShaoCong; LI; YongFei; QIN; JiangFeng


    The Dongjiahe ophiolite complex occurring in the western Bikou terrane that is composed chiefly of serpentinite, listwanitizational peridotite, gabbro, cumulus gabbro, and sub-alkaline meta-basalt, possesses a rock association of typical ophiolite sequence. The metaperidotite is depleted in light rare earth element (LREE), whereas the gabbro and meta-basalt from the studied ophiolite sequence, generated by the same parental magmas those have close affinity to the MORB (Mid-ocean ridge basalt), their REE and immobile elements patterns imply an ocean in the northern margin of the Yangtze plate during the Neoproterozoic period. The zircon LA-ICP-MS U-Pb dating for the gabbro yields a weighted mean age of 839.2±8.2Ma, suggesting that the basin occurred during the Neoproterozoic period.

  2. U-Pb dating of zircon from the bed parallel anatectic granitic intrusion in the Baoban group in Hainan Island and the tectonic implication

    DING; Shijiang; HU; Jianmin; SONG; Biao; CHEN; Mulun; XIE


    The petrological and geochemical features of the bed parallel granitic intrusion in the Pre-Cambrian Baoban group in Hainan Island attest to the anatexis origin of the granites. U-Pb dating analyses of zircons from the anatectic granite and the biotite two-feldspar gneiss in the Baoban group, using SHRIMP II in the Beijing Ion-probe Center, acquire 206Pb/238U ages of 368±3.5 Ma (of granite, 95% confidence level, MSDW=1.23) and 362.9±6.1 Ma (of gneiss, 95% confidence level, MSDW = 2.04) respectively. The two late Devonian ages indicate consistently a tectonic- thermal event experienced in Hainan Island, and are the first discovered record of the Devonian geological process in the district. This event possibly resulted from the deep thermal-dynamic process when the Gondwana continent began to break up in the Devonian period.

  3. SHRIMP zircon U-Pb dating for volcanic rocks of the Dasi Formation in southeast Hubei Province, middle-lower reaches of the Yangtze River and its implications

    XIE Guiqing; MAO Jingwen; LI Ruiling; ZHOU Shaodong; YE Huishou; YAN Quanren; ZHANG Zusong


    The Jinniu Basin in southeast Hubei,located at the westernmost part of middle-lower valley of the Yangtze River, is one of the important volcanic basins in East China. Volcanic rocks in the Jinniu Basin are distributed mainly in the Majiashan Formation, the Lingxiang Formation and the Dasi Formation, consisting of rhyolite, basalt and basaltic andesite, (trachy)-basalt and basaltic trachy-andesite and (trachy)-andesite and (trachy)-dacite and rhyolite respectively, in which the Dasi volcanism is volumetrically dominant and widespread. The Dasi volcanic rocks were selected for SHRIMP zircon U-Pb dating to confirm the timing of volcanism. The results indicate that there exist a large amount of magmatic zircons characterized by high U and Th contents in the volcanic rocks. The concordia ages for 13 points are 128±1Ma (MSWD = 3.0). On account of the shape of zircons and Th/U ratios, this age is considered to represent the crystallization time of the Dasi volcanism. The volcanic rocks in the Dasi, Majiashan and Lingxiang Formations share similar trace element and REE partition patterns as well as Sr-Nd isotopic compositions. In combination with the regional geology, it is proposed that the southeast Hubei volcanic rocks were formed mainly during the Early Cretaceous, just like other volcanic basins in middle-lower Yangtze valley. A lithospheric extension is also suggested for tectonic regime in this region in the Cretaceous Period.

  4. U-Pb SHRIMP Dating of Zircon from Quartz Veins of the Yangshan Gold Deposit in Gansu Province and Its Geological Significance

    QI Jinzhong; YUAN Shisong; LIU Zhijie; LIU Dunyi; WANG Yanbin; LI Zhihong; GUO Junhua; SUN Bin3


    The Yangshan gold deposit is a super-large fine-grained disseminated gold deposit located in southern Gansu Province. Its metallogenic age has been determined by using the cathodoluminescence image and ion probe U-Pb dating techniques. It is found that zircons from quartz veinlet of the fine-grained disseminated gold ore show characters of magmatic origin with prism idiomorphism, oscillatory zoning and dominant Th/U ratios of 0.5-1.5. Three main populations of zircons are obtained, giving average 206pb/238U ages of 197.6±1.7 Ma, 126.9±3.2 Ma and 51.2±l.3 Ma respectively. The first age corresponds to the K-Ar age of the plagiogranite dike, while the latter two ages indicate that buried Cretaceous and Tertiary intrusives exist in the orefield, suggesting that the Yangshan gold deposit was genetically related to the three magmatic hydrothermal activities. By contrast, zircons from coarse gold-bearing quartz vein in the mining area are much older than the host rock, indicating that the vein was formed earlier and was not contaminated by later magmatic fluids. It is concluded that the coupling of multiperiodic hydrothermal activities in the mining area has contributed a lot to mineralization of the Yangshan gold deposit.

  5. Geochemistry, zircon U-Pb dating and Hf isotopies composition of Paleozoic granitoids in Jinchuan, NW China: Constraints on their petrogenesis, source characteristics and tectonic implication

    Zeng, Renyu; Lai, Jianqing; Mao, Xiancheng; Li, Bin; Ju, Peijiao; Tao, Shilong


    Granitoids are widely distributed in Jinchuan at the southwestern margin of the North China plate, which is also an important area of mineral deposits. The research subject of this article are two Paleozoic granitoids, a cataclastic syenogranite and a granodiorite porphyry. This study presents whole rock geochemistry and zircon U-Pb-Hf isotope data for the two granitoids to determine their petrogenesis, source characteristics and tectonic significance. The cataclastic syenogranite is characterized by metaluminous composition with high potassium, and LaN/YbN from 39 to 48. The composition with strong negative Eu anomalies and Zircon saturation temperatures (TZr) from 947 to 1072 °C classify this intrusion as an A-type granite. The granodiorite porphyry is metaluminous with high sodium, sub-alkaline, LaN/YbN ratios from 27 to 32. These I-type intrusions have no Eu anomalies and TZr ranges from 818 to 845 °C. Both the cataclastic syenogranite and granodiorite porphyry show enrichment of LREE and LILE and depletion of HREE and HFSE, except Hf and Zr. Using single zircon LA-ICP-MS U-Pb dating, the emplacement age of the cataclastic syenogranite and granodiorite porphyry are determined at 433.4 ± 3.7 Ma and 361.7 ± 4.6 Ma, respectively. Zircons from the cataclastic syenogranits have uniform negative εHf(t) values (-11 ± 0.5 to -9 ± 0.5), implying the involvement of an old Palaeoproterozoic crustal source in magma genesis. The zircons from the granodiorite porphyry have εHf(t) values that range from -8 ± 1.0 to +10 ± 0.6, suggesting heterogeneous source materials involving both juvenile and ancient crust reworked crustal components. Based on the geological significance of granites at the southwestern margin of the North China plate, the closure of the North Qilian Ocean occurred at ∼444 Ma. Geochemical features suggest that the cataclastic syenogranite and granodiorite porphyry formed in an intraplate extensional and compressional setting, respectively. Hence

  6. Single Zircon LA-ICP-MS U-Pb Dating of the Guandimiao and Wawutang Granitic Plutons in Hunan, South China and Its Petrogenetic Significance

    CHEN Weifeng; CHEN Peirong; ZHOU Xinmin; HUANG Hongye; DING Xing; SUN Tao


    The Guandimao and Wawutang plutons are located at the center of Hunan, South China.The former is mainly composed of biotite monzonitic granites/granodiorites and two-mica monzonitic granites, but the latter only consists of biotite monzonitic granites. The zircon ages of 203.0±1.6 Ma (biotite monzonitic granites) and 208.0±3.2 Ma (two-mica monzonitic granites) for the Guandimao pluton and 204±3 Ma for the Wawutang pluton obtained with the LA-ICP-MS U-Pb dating indicate that they were formed during the late Indosinian. In consideration of other geochronological data from Indosinlan rocks of South China and adjacent regions, it is inferred that the two plutons were derived from crustal materials by decompressional melting in a post-collisional tectonic setting during spontaneous thinning of the thickened curst. Moreover, the inherited zircon age of 1273±k57 Ma from the Wawutang pluton indicates that the source of the two plutons is related to the early Proterozoic crustal basement.

  7. U-Pb (SHRIMP-II) isotope dating of zircons from alkali rocks of Vitim province, West Transbaikalia

    Doroshkevich, A. G.; Ripp, G. S.; Sergeev, S. A.


    A U-Pb (SHRIMP-II) geochronological study of rocks from the Sayzhenski, Snezhny and Nizhne-Burulzayski massifs, incorporated into the Vitim alkali province (Sayzhenski Complex) has been made. The acquired data indicate that the rocks formed in the interval of 520-486 Ma. The proximity of their age to accretionary-collision processes in the Central Asian fold belt, accompanied by intràplate magmatism, has been noted. Two independent stages in the evolution of Vitim Province magmatism are suggested: Early Paleozoic (520-485 Ma) and Late Paleozoic (350-290 Ma).

  8. SHRIMP U-Pb dating of detrital zircons in paragneiss from Oki-Dogo Island, western Japan

    TSUTSUMI, Yukiyasu; YOKOYAMA, Kazumi; HORIE, Kenji; TERADA, Kentaro; HIDAKA, Hiroshi


    We obtained the radiometric ages of detrital zircons from two samples of paragneiss from Oki-Dogo Island, Japan, from the 238U/206Pb ratio and isotopic composition of Pb determined using a Sensitive...

  9. SHRIMP U-Pb dating of detrital zircons in paragneiss from Oki-Dogo Island, western Japan

    TSUTSUMI, Yukiyasu; YOKOYAMA, Kazumi; HORIE, Kenji; TERADA, Kentaro; HIDAKA, Hiroshi


      We obtained the radiometric ages of detrital zircons from two samples of paragneiss from Oki-Dogo Island, Japan, from the 238U/206Pb ratio and isotopic composition of Pb determined using a Sensitive...

  10. The timing of final closure along the Changchun-Yanji suture zone: Constraints from detrital zircon U-Pb dating of the Triassic Dajianggang Formation, NE China

    Wang, Bin; Zhou, Jian-Bo; Wilde, Simon A.; Zhang, Xing-Zhou; Ren, Shou-Mai


    The Dajianggang Formation is located in the Changchun-Yanji suture zone of central Jilin Province and unconformably overlies the Changchun-Yanji Accretionary Complex (CYAC), which is a mélange resulting from subduction of the Jiamusi-Khanka Block (JKB) beneath the North China Craton (NCC). LA-MC-ICP-MS U-Pb dating of detrital zircon from four samples of the formation yields ages of 2516 to 216 Ma. Zircons with U-Pb ages at 2516-2501 Ma and 1897-1832 Ma indicate a provenance from Precambrian basement rocks of the NCC. The 525-482 Ma ages indicate a provenance from metamorphic rocks of Late Pan-African age in the JKB that have a tectonic affinity to the Central Asian Orogenic Belt (CAOB). Zircon grains with ages of 383-314 Ma and 275-250 Ma were likely derived from the underlying CYAC. The youngest population has a peak age of ca. 225 Ma, which together with Late Triassic fossils, suggests that deposition of the Dajianggang Formation was Late Triassic or younger. This result supports the view that the final collision of the JKB and NCC along the Changchun-Yanji suture took place before the Late Triassic. Furthermore, this closure time is at least 10-20 Ma later than closure along the Solonker-Xar Moron-Changchun suture in the Late Permian. We thus establish that the Changchun-Yanji suture is not related to the collision between the Siberia Craton (SC) and the NCC but was instead related to the Paleo-Pacific plate subduction. Consequently, the Changchun-Yanji suture is not the eastward extension of the Solonker-Xar Moron-Changchun suture as previously considered, but the southern margin of the Jilin-Heilongjiang high-pressure metamorphic belt (Ji-Hei HP belt), and resulted from westward subduction of the Paleo-Pacific Ocean. Thus, the Late Triassic marked the switch in subduction from the Paleo-Asian Ocean to the Paleo-Pacific Ocean in NE China.

  11. Zircon SHRIMP U-Pb dating for the Cangshuipu volcanic rocks and its implications for the lower boundary age of the Nanhua strata in South China

    WANG Jian; LI Xianhua; DUAN Taizhong; LIU Dunyi; SONG Biao; LI Zhongxiong; GAO Yonghua


    The continental volcanic rocks and volcaniclastic sedimentary conglomerates of the Cangshuipu Formation occur well in Yiyang of Hunan Province, consisting of a wedge-shaped succession of Neoproterozoic strata that overlie with high-angle unconformity the flysch turbidites of the Lengjiaxi Group in the Upper Mesoproterozoic Eonothem. SHRIMP zircon U-Pb dating gives a weighted mean age of 814 ( 12 Ma for the dacitic volcanic agglomerates from the lowest part of the volcanic rocks in the Cangshuipu Formation. This age is younger than previously reported values of 921-933 Ma for the volcanic rocks from the Cangshuipu Formation. Our new dating represents the lower boundary age of the Neoproterozoic System in the studied area. The younger age for the Cangshuipu volcanic rocks is supported by the following observations: (1) Lower Neoproterozoic strata (814-1000 Ma) are missing in the studied area; (2) the Nanhua rift system was initiated at about 820 Ma; and (3) an age of ~820 Ma may be taken as the lower boundary timing of the Nanhua System (even Neoproterozoic) in South China.

  12. LA-ICPMS Zircon U-Pb Dating in the Jurassic Daohugou Beds and Correlative Strata in Ningcheng of Inner Mongolia

    LIU Yanxue; LIU Yongqing; ZHANG Hong


    LA-ICPMS Zircon U-Pb dating is applied to volcanic rocks overlying and underlying the Salamander-bearing bed in the Daohugou beds of Ningcheng in Inner Mongola and Reshuichang of Lingyuan and Mazhangzi of Jianping in western Liaoning. The results indicate that the youngest age of the rocks in Daohugou of Ningcheng is 158 Ma, and the oldest one is 164 Ma. Synthesized researches indicate that the salamander-bearing beds in Daohugou of Ningcheng, Reshuichang of Lingyuan and Mazhangzi of Jianping were developed in the same period. The Daohugou beds were formed in the geological age of 164-158 Ma of the middle-late Jurassic. Whilst, the Daohugou beds and its correlative strata should correspond to the Tiaojishan Formation (or Lanqi Formation) of the middle Jurassic in northern Hebei Province and western Liaoning Province, based on the disconformity between the Daohugou beds and its overlaying beds of the Tuchengzi Formation of Late Jurassic and the Jehol Beds of early Cretaceous, and the disconformity between the Daohugou Beds and its underlying Jiulongshan Formation, which is composed of conglomerate, sandstone, shale with coal and thin coal beds.

  13. SHRIMP zircon U-Pb dating from K-bentonite in the top of Ordovician of Wangjiawan Section, Yichang, Hubei,China

    HU YanHua; ZHOU JiBin; SONG Biao; LI Wei; SUN WeiDong


    The Global boundary Stratotype Section and Point (GSSP) for the base of the Hirnantian Stage (the latest stage of the Ordovician System) is defined at a point 0.39m below the base of the Kuanyinchiao Bed in the Wangjiawan North Section, which is the upper most "golden spike" of the Ordovician.However, this "golden spike" is lack of reliable geochronology data. This article gives a sensitive high resolution ion microprobe (SHRIMPII) zircon U-Pb dating for a K-bentonite sample from the Kuanyinchiao Bed in the Wangjiawan North Section. The age of the K-bentonite sample is 443.2+1.6 Ma, that is to say, the isotopic age of the uppermost of Hirnantian Stage, the point of Ordovician-Silurian boundary,should be near to, but slightly younger than 443.2±1.6 Ma. This age is identical to the Ordovician-Silurian boundary age 443.7±1.5 Ma as declared by International Commission on Stratigraphy (ICS). So, this research provides some good geochronlogical data for the Hirnantian Stage and the Ordovician-Silurian boundary as well as the global correlation.

  14. Emplacement age for the mafic-ultramafic plutons in the northern Dabie Mts. (Hubei): Zircon U-Pb, Sm-Nd and 40Ar/39Ar dating


    The protoliths of mafic-ultramafic plutons in the northern Dabie Mts. (NDM) (Hubei) include pyroxenite and gabbro. The zircon U-Pb dating for a gabbro suggests that emplacement of mafic magma took place in the post-collisional setting at the age of 122.9(0.6 Ma. It is difficult to obtain a reliable Sm-Nd isochron age, due to disequilibrium of the Sm-Nd isotopic system. Two hornblende 40Ar/39Ar ages of 116.1(1.1 Ma and 106.6(0.8 Ma may record cooling of metamorphism in the mafic-ultramafic plutons in Hubei below 500(C. The hornblende 40Ar/39Ar ages for the mafic-ultramafic rocks in Hubei are evidently 15-25 Ma younger than those for the same rocks in Anhui, indicating that there is a diversity of the cooling rates for the mafic-ultramafic rocks in Hubei and Anhui. The difference in their cooling rates may be controlled by the north-dipping normal faults in the NDM. The intense metamorphism occurring in the mafic-ultramafic rocks in Hubei may result from the Yanshanian magmatic reheating and thermal fluid action induced by the Cretaceous migmatization. The geochemical similarity of these mafic-ultramafic rocks wherever in Hubei and Anhui may be attributed to the same tectonic setting via an identical genetic mechanism.

  15. SHRIMP U-Pb zircon dating for lamprophyre from Liaodong Peninsula: Constraints on the initial time of Mesozoic lithosphere thinning beneath eastern China

    JIANG Yaohui; JIANG Shaoyong; ZHAO Kuidong; NI Pei; LING Hongfei; LIU Dunyi


    It is undebated fact that the lithospheric mantle beneath eastern China was considerably thinned during the Mesozoic time. However, it has no adequate evidence for the exact timing when the lithosphere thinning started. The Liaodong Peninsula is located in the eastern segment of the North China Craton and is one of the important domains to explore the event of lithosphere thinning. SHRIMP U-Pb zircon dating and geochemical study were carried out for the lamprophyre dike swarm that intruded into the magnesite ore-beds in the Dashiqiao Formation of Paleoproterozoic Liaohe Group at the Huaziyu magnesite ore district, Liaodong Peninsula. The results indicate that these lamprophyre dikes were intruded in late Jurassic (155±4 Ma) and show some geochemical characteristics of potassic magmas. It is now accepted that the lithosphere thinning took place in the late Mesozoic, and the peak thinning stage occurred in early Cretaceous (130―120 Ma). Considering the potassic mafic magmatism marking the onset of the lithospheric thinning, we therefore suggest that the studied late Jurassic potassic lamprophyre dike swarm could imply that the late Jurassic is the time that lithosphere thinning started.

  16. Zircon LA-ICP-MS U-Pb dating and Sr-Nd isotope study of the Guposhan granite complex, Guangxi, China

    GU Shengyan; HUA Renmin; QI Huawen


    Zircon U-Pb dating by the LA-ICP-MS method was applied to determining the ages of different units of the Guposhan granite complex, among which the East Guposhan unit is 160.8±1.6 Ma, the West Guposhan unit is 165.0±1.9 Ma, and the Lisong unit is 163.0±1.3 Ma in age. Much similarity in ages of the three units has thus proved that the whole Guposhan granite complex was formed in the same period of time. They were the products of large-scale granitic magmatism through crust-remelting in the first stage of the Middle Yanshanian in South China. However, the three units have differences both in petrology and in geochemistry. Besides the differences in major, trace and rare-earth elements, they are distinct in their Rb-Sr and Sm-Nd isotopic compositions. The East Guposhan unit and Lisong unit and its enclaves have a similar (87Sr/86Sr)i value of 0.7064 with an average of εNd(t)=-3.03, indicating that more mantle material was evolved in the magma derivation; whereas the West Guposhan unit has a higher (87Sr/86Sr)i value of 0.7173 but a lower εNd(t) value of -5.00, and is characterized by strong negative Eu anomalies and higher Rb/Sr ratios, suggesting that its source materials were composed of relatively old crust components and new mantle-derived components. In addition, an inherited zircon grain in the East Guposhan unit (GP-1) yielded a 206Pb/238U age of 806.4 Ma, which is similar to the ages of the Jiulin cordierite granite in northern Jiangxi and of the Yinqiao migmatic granite in Guangxi in the HZH granite zone. All this may provide new evidence for Late Proterozoic magmatism in the HZH granite zone.

  17. Zircon U/Pb Dating of Cretaceous Adakitic Volcanic Rocks in the Eastern Part of North Dabie Mountains

    薛怀民; 董树文; 刘晓春


    Mesozoic volcanic rocks in the eastern part of the North Dabie Mountains are rich inNa ( Na2O=4.03%, Na2O/K2O = 1.31 ), Sr (865μg/g) and Ba ( 1361μg/g) , and high inSr/Y ratio (66.1) but low in Nb, Y and HREE. They have experienced strong fractionation ofREE [ (La/Yb)N = 26.6 ], and are similar to adakite in geochemical characteristics. The U-Pbdating of zircon from the volcanic rocks is ( 129.2 + 2.6) Ma, belonging to Early Cretaceous.These rocks are similar to the volcanic rocks of North Huaiyang not only in age and rare-earth el-ement and trace element geochemistry, but also in the formation temperature and pressure of theminerals. The results indicated that the delamination of mountain root and underplating of mafic-ultramafic magma had happened in the Dabie orogen before Early Cretaceous. Mesozoic mag-matism was intense in the North Dabie Mountains, including the intrusion of mafic-ultramaficmagma, uplifting of gneiss dome, explosion of volcanic rocks and intrusion of granitic magma.The Mesozoic volcanic rocks in the eastern part of the North Dabie Mountains may be one part ofthe Mesozoic volcano-intrusive complex belt of North Huaiyang. The existence of Mesozoic vol-canic remnant cap means the denudation of the Dabie orogenic belt was not very strong sinceEarly Cretaceous.

  18. SHRIMP U-Pb zircon dates from igneous rocks from the Fontana Lake region, Patagonia: Implications for the age of magmatism, Mesozoic geological evolution and age of basement

    A.P. Rolando


    Full Text Available In the eastern margin of the Patagonian Andes and between 44° 30´S and 45° 30´S (Fontana Lake region, Middle Jurassic to Early Cretaceous volcanic and sedimentary rocks were intruded by granitic bodies during the Cretaceous. The reconstruction of the Jurassic-Cretaceous magmatic evolution in the Fontana Lake region and in the adjacent Patagonian Batholith was made possible by the consideration of the following characteristics: distribution in time and space of several intrusive bodies, retro-arc basin formation and volcanic intensity. U-Pb SHRIMP dating of zircon crystals from an ignimbrite, a dacitic porphyry and two granitoid rocks yielded dates of 148.7 ± 2.3, 144.5 ± 1.6, 117 ± 1.7 and 99.6 ± 2.8 Ma, respectively. The Cerro Bayo Ignimbrite (148.7 ± 2.3 Ma, Late Jurassic was included in the Lago La Plata Formation; this unit hosts an epithermal ore deposit. The Laguna Escondida dacitic porphyry (144.5 ± 1.6 Ma, Jurassic-Cretaceous boundary intruded metasedimentary rocks of the Lago La Plata Formation; this sub-volcanic body can chronologically be linked to the Patagonian Batholith. After the Jurassic volcanic events, a retro-arc basin formed in the eastern sector of the Patagonian Range at about 140-115 Ma (Late Berriasian-Barremian and magmatism ceased during this event. The dating of granitoids (117 ± 1.7 and 99.6 ± 2.8 Ma in the Fontana Lake region confirms a temporal magmatic continuity with the Patagonian Batholith. These dates also are in agreement with the volcanic rocks of the Divisadero Group and epithermal deposits in the region (La Ferrocarrilera deposit. One of the analyzed granitoids (Dedo Chico, 99.6 ± 2.8 Ma has inherited zircon crystals of about 2,100 and 3,410 Ma, in agreement with other previous isotopic evidence for the occurrence of an underlying Precambrian basement in the region.

  19. Timing of Magma Mixing in the Gangdisê Magmatic Belt during the India-Asia Collision:Zircon SHRIMP U-Pb Dating

    MO Xuanxue; DONG Guochen; ZHAO Zhidan; GUO Tieying; WANG Liangliang; CHEN Tao


    Abundant mafic microgranular enclaves (MMEs) extensively distribute in granitoids in the Gangdisê giant magmatic belt, within which the Quxu batholith is the most typical MME-bearing pluton. Systematic sampling for granodioritic host rock, mafic microgranular enclaves and gabbro nearby at two locations in the Quxu batholith, and subsequent zircon SHRIMP Ⅱ U-Pb dating have been conducted. Two sets of isotopic ages for granodioritic host rock,mafic microgranular enclaves and gabbro are 50.4±1.3 Ma, 51.2±1.1 Ma, 47.0±1 Ma and 49.3±1.7 Ma, 48.9±1.1 Ma,49.9±1.7 Ma, respectively. It thus rules out the possibilities of mafic microgranular enclaves being refractory residues after partial melting of magma source region, or being xenoliths of country rocks or later intrusions.Therefore, it is believed that the three types of rocks mentioned above likely formed in the same magmatic event, i.e., they formed by magma mixing in the Eocene (c. 50 Ma). Compositionally, granitoid host rocks incline towards acidic end member involved in magma mixing, gabbros are akin to basic end member and mafic microgranular enclaves are the incompletely mixed basic magma clots trapped in acidic magma. The isotopic dating also suggested that huge-scale magma mixing in the Gangdise belt took place 15-20 million years after the initiation of the India-Asia continental collision, genetically related to the underplating of subduction-collision-induced basic magma at the base of the continental crust. Underplating and magma mixing were likely the main process of mass-energy exchange between the mantle and the crust during the continental collision, and greatly contributed to the accretion of the continental crust, the evolution of the lithosphere and related mineralization beneath the portion of the Tibetan Plateau to the north of the collision zone.

  20. Paleo-Pacific subduction-accretion: Evidence from Geochemical and U-Pb zircon dating of the Nadanhada accretionary complex, NE China

    Zhou, Jian-Bo; Cao, Jia-Lin; Wilde, Simon A.; Zhao, Guo-Chun; Zhang, Jin-Jiang; Wang, Bin


    The Nadanhada Terrane, located along the eastern margin of Eurasia, contains a typical accretionary complex related to paleo-Pacific plate subduction-accretion. The Yuejinshan Complex is the first stage accretion complex that consists of meta-clastic rocks and metamafic-ultramafic rocks, whereas the Raohe Complex forms the main parts of the terrane and consists of limestone, bedded chert, and mafic-ultramafic rocks embedded as olistolith blocks in a weakly sheared matrix of clastic meta-sedimentary rocks. Geochemical data indicate that the Yuejinshan metabasalts have normal mid-ocean ridge basalt (N-MORB) affinity, whereas the Raohe basaltic pillow lavas have an affinity to ocean island basalts (OIB). Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses of gabbro in the Raohe Complex yield a weighted mean 206Pb/238U zircon age of 216 ± 5 Ma, whereas two samples of granite intruded into the complex yield weighted mean 206Pb/238U zircon ages of 128 ± 2 and 129 ± 2 Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb zircon analyses of basaltic pillow lava in the Raohe Complex define a weighted mean age of 167 ± 1 Ma. Two sandstone samples in the Raohe Complex record younger concordant zircon weighted mean ages of 167 ± 17 and 137 ± 3 Ma. These new data support the view that accretion of the Raohe Complex was between 170 and 137 Ma, and that final emplacement of the Raohe Complex took place at 137-130 Ma. The accretion of the Yuejinshan Complex probably occurred between the 210 and 180 Ma, suggesting that paleo-Pacific plate subduction was initiated in the Late Triassic to Early Jurassic.

  1. SHRIMP Zircon U-Pb Dating of Gabbro and Granite from the Huashan Ophiolite, Qinling Orogenic Belt, China: Neoproterozoic Suture on the Northern Margin of the Yangtze Craton

    SHI Yuruo; LIU Dunyi; ZHANG Zongqing; MIAO Laicheng; ZHANG Fuqin; XUE Hongmei


    The recently identified Huashan ophiolitic mélange was considered as the eastern part of the Mianliie suture in the Qinling orogenic belt. SHRIMP zircon U-Pb geochronology on gabbro from the Huashan ophiolite and granite intruding basic volcanic rocks indicates crystallization ages of 947±14Ma and 876±17 Ma respectively. These ages do not support a recently proposed Hercynian Huashan Ocean, but rather favor that a Neoproterozoic suture assemblage (ophiolite) is incorporated into the younger (Phanerozoic) Qinling orogenic belt.

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

    Azadeh Malekzadeh Shafaroudi


    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,

  3. Late Eocene to Early Miocene Andean uplift inferred from detrital zircon fission track and U-Pb dating of Cenozoic forearc sediments (15-18°S)

    Decou, A.; von Eynatten, H.; Dunkl, I.; Frei, D.; Wörner, G.


    Timing, amount, and mechanisms of uplift in the Central Andes have been a matter of debate in the last decade. Our study is based on the Cenozoic Moquegua Group deposited in the forearc basin between the Western Cordillera and the Coastal Cordillera in southern Peru from ˜50 to ˜4 Ma. The Moquegua Group consists mainly of mud-flat to fluvial siliciclastic sediments with upsection increasing grain size and volcanic intercalations. Detrital zircon U-Pb dating and fission track thermochronology allow us to refine previous sediment provenance models and to constrain the timing of Late Eocene to Early Miocene Andean uplift. Uplift-related provenance and facies changes started around 35 Ma and thus predate major voluminous ignimbrite eruptions that started at ˜25 by up to 10 Ma. Therefore magmatic addition to the crust cannot be an important driving factor for crustal thickening and uplift at Late Eocene to Early Oligocene time. Changes in subduction regime and the subducting plate geometry are suggested to control the formation of significant relief in the area of the future Western Cordillera which acts as an efficient large-scale drainage divide between Altiplano and forearc from at least 15.5 to 19°S already at ˜35 Ma. The model integrates the coincidence of (i) onset of provenance change no later than 35 Ma, (ii) drastic decrease in convergence rates at ˜40, (iii) a flat-subduction period at around ˜40 to ˜30 Ma leading to strong interplate coupling, and (iv) strong decrease in volcanic activity between 45 and 30 Ma.

  4. U-Pb zircon and CHIME monazite dating of granitoids and high-grade metamorphic rocks from the Eastern and Peninsular Thailand - A new report of Early Paleozoic granite

    Kawakami, T.; Nakano, N.; Higashino, F.; Hokada, T.; Osanai, Y.; Yuhara, M.; Charusiri, P.; Kamikubo, H.; Yonemura, K.; Hirata, T.


    In order to understand the age and tectonic framework of Eastern to Peninsular Thailand from the viewpoint of basement (metamorphic and plutonic) geology, the LA-ICP-MS U-Pb zircon dating and the chemical Th-U-total Pb isochron method (CHIME) monazite dating were performed in the Khao Chao, Hub-Kapong to Pran Buri, and Khanom areas in Eastern to Peninsular Thailand. The LA-ICP-MS U-Pb zircon dating of the garnet-hornblende gneiss from the Khao Chao area gave 229 ± 3 Ma representing the crystallization age of the gabbro, and that of the garnet-biotite gneisses gave 193 ± 4 Ma representing the timing of an upper amphibolite facies metamorphism. The CHIME monazite dating of pelitic gneiss from the Khao Chao gneiss gave scattered result of 68 ± 22 Ma, due to low PbO content and rejuvenation of older monazite grains during another metamorphism in the Late Cretaceous to Tertiary time. The U-Pb ages of zircon from the Hua Hin gneissic granite in the Hub-Kapong to Pran Buri area scatter from 250 Ma to 170 Ma on the concordia. Granite crystallization was at 219 ± 2 Ma, followed by the sillimanite-grade regional metamorphism at 185 ± 2 Ma. Monazite in the pelitic gneiss from this area also preserves Early to Middle Jurassic metamorphism and rejuvenation by later contact metamorphism by non-foliated granite or by another fluid infiltration event in the Late Cretaceous to Tertiary time. The Khao Dat Fa granite from the Khanom area of Peninsular Thailand gave a U-Pb zircon age of 477 ± 7 Ma. This is the second oldest granite pluton ever reported from Thailand, and is a clear evidence for the Sibumasu block having a crystalline basement that was formed during the Pan-African Orogeny. The Khao Pret granite gives U-Pb zircon concordia age of 67.5 ± 1.3 Ma, which represents the timing of zircon crystallization from the granitic melt and accompanied sillimanite-grade contact metamorphism against surrounding metapelites and gneisses. Metamorphic rocks in the Doi Inthanon area

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

    Malihe Nakhaei


    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

  6. Genesis of zircon and its constraints on interpretation of U-Pb age

    WU Yuanbao; ZHENG Yongfei


    Zircon U-Pb dating is the most commonly used method for isotopic geochronology. However, it has been a difficult issue when relating zircon U-Pb ages to metamorphic conditions in complex metamorphic rocks. Much progress has been made in the past decade with respect to the genesis of zircon and its constraints on interpretation of U-Pb age. Three methods have been proposed to link zircon U-Ph age to metamorphic conditions: ( I ) internal structure; (ii)trace element feature; (iii) mineral inclusion composition.Magmatic zircon shows typical oscillatory zoning and/or sector zoning, whereas metamorphic zircon has internal structures such as no zoned, weakly zoned, cloudy zoned,sector zoned, planar zoned, and patched zoned ones. Zircons formed in different geological environments generally have characteristic internal structures. Magmatic zircons from different rock types have variable trace element abundances,with a general trend of increasing trace element abundances in zircons from ultramafic through mafic to granitic rocks.Zircons formed under different metamorphic conditions have different trace element characteristics that can be used to relate their formation to metamorphic conditions. It is an effective way to relate zircon growth to certain P-T conditions by studying the trace element partitioning between coexisting metamorphic zircon and garnet in high-grade metamorphic rocks containing both zircon and garnet. Primary mineral inclusions in zircon can also provide unambiguous constraints on its formation conditions. Therefore,interpretation of zircon U-Pb ages can be constrained by its internal structure, trace element composition, mineral inclusion and so on.

  7. LA-ICP-MS zircon U-Pb dating and phenocryst EPMA of dikes, Guocheng, Jiaodong Peninsula: Implications for North China Craton lithosphere evolution


    Widespread dike swarm, including diorite-, monzonite-porphyry and lamprophyre, intruded in the al- tered breccia gold deposits along basin marginal faults, Guocheng, Jiaodong Peninsula. Petrography exhibits biotite enclaves in amphibole phenocrysts and the presence of acicular apatites in these dikes. Electron probe microanalyses (EPMA) show that the amphibole and clinopyroxene phenocryst’s mantle in diorite porphyry and lamprophyre respectively has sharply higher MgO (Mg#) and Cr2O3 contents in contrast to their cores. The plagioclase phenocryst in monzonite porphyry has reverse zoning. These results indicate that the magma mixing between mantle-derived mafic and crust-derived felsic magmas occurred in the original process of the dikes. Zircon cathodoluminescence (CL) images show well-developed magmatic oscillatory zones and the acquired LA-ICP-MS zircon U-Pb weighted mean 206Pb/238U ages are 114±2 Ma (MSDW=1.5) for monzonite porphyry (GS1) and 116±1 Ma (MSDW=0.8) for diorite porphyry (GS2), respectively. Earlier magmatic events in the northwest Jiaodong Peninsula represented by some inherited or captured zircons also occur in these dikes. Magmatic zircons from GS1 and GS2 display consistent chondrite-normalized REE patterns and Nb/Ta values, implying that they may share a similar or same source. HREE enrichment and obvious negative Eu anomalies of these zircons preclude garnet presented in their source. Our results, combined with preciously pub- lished data, indicate that dike intrusion and gold mineralization among quartz vein, altered tectonite and altered breccia gold deposits are broadly contemporaneous throughout the Jiaodong Peninsula. These also imply that the intensive crust-mantle interaction and asthenospheric underplating had oc- curred in the Early Cretaceous in the Peninsula, together with foundering of lower crust in the early Mesozoic, representing the different stages of lithosphere thinning in the North China Craton (NNC).

  8. LA-ICP-MS zircon U-Pb dating and phenocryst of dikes, Guocheng, Jiaodong Peninsula: Implications for North China Craton lithosphere evolution

    TAN Jun; WEI JunHao; GUO LingLi; ZHANG KeQing; YAO ChunLiang; LU JianPei; LI HongMei


    Widespread dike swarm, including diorite-, monzonite-porphyry and lamprophyre, intruded in the altered breccia gold deposits along basin marginal faults, Guocheng, Jiaodong Peninsula. Petrography exhibits biotite enclaves in amphibole phenocrysts and the presence of acicular apatites in these dikes.Electron probe microanalyses (EPMA) show that the amphibole and clinopyroxene phenocryst's mantle in diorite porphyry and lamprophyre respectively has sharply higher MgO (Mg#) and Cr203 contents in contrast to their cores. The plagioclase phenocryst in monzonite porphyry has reverse zoning. These results indicate that the magma mixing between mantle-derived mafic and crust-derived felsic magmas occurred in the original process of the dikes. Zircon cathodoluminescence (CL) images show well-developed magmatic oscillatory zones and the acquired LA-ICP-MS zircon U-Pb weighted mean 206pb/238U ages are 114±2 Ma (MSDW=1.5) for monzonite porphyry (GS1) and 116±1 Ma (MSDW=0.8) for diorite porphyry (GS2), respectively. Earlier magmatic events in the northwest Jiaodong Peninsula represented by some inherited or captured zircons also occur in these dikes. Magmatic zircons from GSl and GS2 display consistent chondrite-normalized REE patterns and Nb/Ta values, implying that they may share a similar or same source. HREE enrichment and obvious negative Eu anomalies of these zircons preclude garnet presented in their source. Our results, combined with preciously published data, indicate that dike intrusion and gold mineralization among quartz vein, altered tectonite and altered breccia gold deposits are broadly contemporaneous throughout the Jiaodong Peninsula.These also imply that the intensive crust-mantle interaction and asthenospheric underplating had occurred in the Early Cretaceous in the Peninsula, together with foundering of lower crust in the early Mesozoic, representing the different stages of lithosphere thinning in the North China Craton (NNC).

  9. 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

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


    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.

  10. Zircon U-Pb dating and geochemical study of the Xianggou granite in the Ma’anqiao gold deposit and its relationship with gold mineralization

    LEE; Ben


    Single zircon LA-ICP-MS U-Pb dating and lithogeochemical studies have been performed on the Xianggou monzonitic granitic porphyry outcropped in the Ma’anqiao gold deposit.A weighted average U-Pb age of 242.0±0.8 Ma for Xianggou monzonitic porphyry has been obtained.This corresponds with the conclusions of previous studies indicating a syn-orogenic age (242±21 Ma) of the Qinling Orogenic Belt,suggesting that the formation of the Xianggou granite should be associated with the collisional event of the North China Plate and the Yangtze Plate in the Indosinian period.The Xianggou granite is characterized by the high silicon and alkali of high K calc-alkaline series granites.It is rich in Al (Al2O3=14.49%-15.61%) and Sr (457.10-630.82 ppm),poor in Y (<16 ppm) and HREE (Yb<0.45 ppm),and exhibits high ratios of Sr/Y (76.24-97.34) and (La/Yb)N (29.65-46.10),as well as strongly fractionated REE patterns.These geochemical characteristics suggest the Xianggou granite can be classified as C-type adakitic rock.The initial Sr isotope ratios for the Xianggou granite vary from 0.70642 to 0.70668,εNd(t) values from -4.54 to -3.98,and TDM values from 1152 Ma to 1220 Ma.The low εNd(t) and ISr and high TDM values,as well as Na2O/K2O ratios of the Xianggou granite are close to 1 (Na2O/K2O=0.95-1.10),indicating that it is not an I-type adakite formed by partial melting of the subducting oceanic crust,nor adakitic rock formed by melting of the underplated basaltic lower crust,but the product of partial melting of the nonunderplated basaltic thickened lower crust.Zircons from the Xianggou pluton have a homogeneous Hf isotopic composition with negative εHf(t) values (between -9.7 and -5.9,with an average of -6.9),indicating that the rock-forming materials were mostly extracted from the ancient crust,not from the depleted mantle.The Xianggou monzonitic granitic porphyry is rich in LILE and LREE and depleted in HSFE,HREE and Y;the composition of trace element and REE are similar to

  11. Emplacement age for the mafic-ultramafic plutons in the northern Dabie Mts. (Hubei): Zircon U-Pb, Sm-Nd and 40Ar/39Ar dating

    WANG; Jianghai


    transfer for the dissolution of zircons in a multi-sample capsule at high-pressure, in Short Papers of the Fourth International Conference on Geochronology, Cosmochronology, and Isotope Geology (ed. Zartman, R. E.), USGS Open-File Rep., 1978, 78-701: 233-234.[23]Mao, C. X., Zhu, N. J., Zhu, B. Q. et al., Measurement techniques and application of the Sm-Nd isotopic system, Geochimica, 1989, (1): 36-42.[24]Ludwig, R. K., Isoplot--a plotting and regression program for radiogenic-isotope data, version 2.57, USGS Open-File Rep, 1992, 91-445: 40.[25]Steiger, R. H., J?ger, E., Subcommission on geochronology: Conventions on the use of decay constants in geo- and cosmo-chronology, Earth Planet. Sci. Lett., 1977, 36: 359-362.[26]Dai, T. M., Hong, A. S., 40Ar/39Ar dating and some isotopic determinations on Himalayan biotite from granitoids in southern Tibet, Geochimica, 1982, (1): 48-55.[27]McDougall, I., Harrison, T. M., Geochronology and Thermochronology by the 40Ar/39Ar Method, New York: Oxford University Press, 1999, 1-269.[28]Chen, T. Y., Niu, B. G., Liu, Z. G. et al., Isotopic geochronology of metamorphism and Yanshanian magmatism within the Dabie Shan, Acta Geol. Sinica, 1991, 65(4): 329-335.[29]Chen, J. F., Xie, Z., Liu, S. S., Cooling age of Dabie orogen, China determined by 40Ar-39Ar and fission track techniques, Sci. in China, Ser. B, 1995, 38(6): 749-757.[30]Li, S., Wang, T., Geochemistry of Granitoids in the Tongbai-Dabie Mountains, Wuhan: China University of Geosciences Press, 1991, 1-208.[31]Hacker, B. R., Ratschbacher, L., Webb, L. et al., U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling-Dabie orogen, China, Earth Planet. Sci. Lett., 1998, 161: 215-230.[32]Deng, S. X., Wang, J. H., Sun, M. et al., Types of leucosomes in the migmatites and their U-Pb ages at Fenghuangguan, Hubei, Geochimica, 1997, 26(2): 75-86.[33]Ratschbacher, L., Hacker, B. R., Webb, L. E. et al., Exhumation of the ultrahigh

  12. Mineral inclusions and SHRIMP U-Pb dating of zircons from the Alamas nephrite and granodiorite: Implications for the genesis of a magnesian skarn deposit

    Liu, Yan; Zhang, Rongqing; Zhang, Zhiyu; Shi, Guanghai; Zhang, Qichao; Abuduwayiti, Maituohuti; Liu, Jianhui


    Extending approximately 1300 km and located in the Western Kunlun Mountains, the Hetian nephrite belt is the largest nephrite belt in the world and contains approximately 11 major deposits and more than 20 orebodies including the Alamas deposit. Hetian nephrite deposits can be classified as Mg-skarn deposits with Precambrian dolomitic marble host rock and green, green-white and white nephrite zones are distributed gradually in the zone of a granodiorite pluton. The green nephrite is mainly predominately composed of tremolite with generally minor to trace constituents of diopside, grossularitic garnet, actinolite and other minerals. Also green nephrite has higher content of TFe2O3, than green-white and white nephrites have. We subdivided the zircons from the green nephrites into four types, depending on their internal textures, mineral inclusions, and SHRIMP U-Pb ages. Type I zircons are round instead of idiomorphic in shape and lack obvious zoning. Type II and IV zircons have broad, clear oscillatory zoning and are hypidiomorphic or idiomorphic in shape; they contain inclusions of diopside, tremolite, chlorite and calcite. Most Type III zircons are narrow rims (nephrite deposit. The partially recrystallization of zircons during skarn formation possibly lead to some younger individual ages (406.5 to 308 Ma). In the Western Kunlun Mountain, both Buya granite and Alamas grandiorite are high Ba-Sr granites and crystallized in Western Kunlun Orogen. The Buya granite formed at about 430 Ma in a post-orogenic tectonic environment. Considering Alamas granodiorite formed at about 12 Ma younger than that of Buya granite and it is convincible that Alamas granodiorite also formed at a post-orogenic tectonic environment. Together with the evolution of Western Kunlun Mountain, it is also possible that high Ba-Sr Alamas granodiorite and the nephrite deposit formed in the post-orogenic stage. Most zircons in the Alamas granodiorite and green nephrite have high Th/U ratios (> 0

  13. LA-ICPMS Zircon U-Pb Dating for Three Indosinian Granitic Plutons from Central Hunan and Western Guangdong Provinces and Its Petrogenetic Implications

    PENG Bingxia; WANG Yuejun; FAN Weiming; PENG Touping; LIANG Xinquan


    The LA-ICPMS zircon U-Pb geochronology of three typically Indosinian granitic plutons yielded weighted mean 206Pb/238U ages of 214.1±5.9 Ma and 210.3±4.7 Ma for the biotite monzonitic granites from the Xiema and Xiangzikou plutons in Hunan Province, and 205.3±1.6 Ma for biotite granite from the Napeng pluton, western Guandong Province, respectively, showing a similar late Indosinian age of crystallization. In combination with other geochronological data from Indosinian granites within the South China Block (SCB), it is proposed that those late Indosinian granites with an age of ~210 Ma and the early Indosinian granites (230-245 Ma) have the similar petrogenesis in identical tectonic setting. The Indosinian granites within the SCB might be the products of anatexis of the thickening crust in a compressive regime. These data provide a further understanding for the temporal and spatial distribution of the Indosinian granites and the dynamic evolution of the SCB.

  14. A combined study of SHRIMP U-Pb dating, trace element and mineral inclusions on high-pressure metamorphic overgrowth zircon in eclogite from Qinglongshan in the Sulu terrane

    LI Qiuli; LI Shuguang; HOU Zhenhui1; HONG Jian; YANG Wei1


    Methods recently advanced for discrimination on the genesis of metamorphic zircon, such as analysis of mineral inclusions and trace elements, provide us powerful means to distinguish zircon overgrowth during high-pressure metamorphism. Zircons in ultrahigh-pressure eclogite from Qinglongshan in the Sulu terrane were studied by the SHRIMP U-Pb method in combining with trace element and mineral inclusion analyses. No inherited core was identified in the analyzed zircons by means of cathodoluminescence images. The occurrence of high-pressure metamorphic mineral inclusions in zircon, such as garnet, omphacite, rutile, and the flat HREE pattern in zircon indicate that the zircon formed at high-pressure metamorphic conditions. Therefore, a weighted average U-Pb age of 227.4 ± 3.5 Ma obtained from such a kind of zircon is interpreted to represent the timing of peak metamorphism for the Qinglongshan eclogite.

  15. Integrated single crystal laser ablation U/Pb and (U-Th)/He dating of detrital accessory minerals - Proof-of-concept studies of titanites and zircons from the Fish Canyon tuff

    Horne, Alexandra M.; van Soest, Matthijs C.; Hodges, Kip V.; Tripathy-Lang, Alka; Hourigan, Jeremy K.


    Excimer laser technologies enable a rapid and effective approach to simultaneous U/Pb geochronology and (U-Th)/He thermochronology of a wide range of detrital accessory minerals. Here we describe the 'laser ablation double dating' (LADD) method and demonstrate its viability by applying it to zircon and titanite crystals from the well-characterized Fish Canyon tuff. We found that LADD dates for Fish Canyon zircon (206Pb/238U - 28.63 ± 0.11 Ma; (U-Th)/He - 28.38 ± 0.73 Ma) are statistically indistinguishable from those obtained through established, traditional methods of single-crystal dating. The same is true for Fish Canyon titanite LADD dates: 206Pb/238U - 28.08 ± 0.90 Ma; (U-Th)/He - 27.98 ± 0.86 Ma. As anticipated, given that LADD involves the analysis of smaller amounts of material than traditional methods, it yields dates with higher analytical uncertainty. However, this does not substantially reduce the utility of the results for most applications to detrital datasets. An important characteristic of LADD is that it encourages the chemical characterization of crystals by backscattered electron, cathodoluminescence, and/or Raman mapping prior to dating. In addition, by permitting the rapid and robust dating of crystals regardless of the degree of their abrasion during sedimentary transport, the method theoretically should yield dates that are more broadly representative of those of the entire population of detrital crystals in a natural sample.

  16. Petrology and U-Pb zircon dating of intrusive rocks from A, C-south, and Dardvay districts, Sangan iron stone mine, Khaf

    Abbas Golmohammadi


    Full Text Available Sangan magnetite skarn mine is located 300 km southeast of Mashhad, along the eastern part of Khaf-Drouneh volcanic-plutonic belt. Granitoids from three areas within the Sangan mine A, C-North and Dardvay were studied. Within the study area, three intrusive rocks including biotite-hornblende monzonite porphyry, biotite syenite and syenogranite were recognized. Based on field cross cutting, absence of garnet-magnetite skarn around the contact, and alteration by younger hydrothermal fluids, these granitoids are older than the magnetite skarn. U-Pb zircon age of the granitoid is 42 Ma (Middle Eocene. Magnetic susceptibility of the granitoids are 310-900 × 10-5 (SI units and therefore, they are classified as belonging to the magnetite-series (oxidant type I-type. Chemically, these granitoids are metaluminous, alkali-calcic to alkali and shoshonite to ultrapotassic. Enrichment of LREE, relative to HREE and enrichment of LILE (Sr, Cs, Rb, K and Ba relative to HFSE (Nb, Ta, Ti, Hf and Zr indicate that the magma formed in island arc setting. This magma originated from low partial melting (<1% of garnet-spinel lherzolite (low garnet and was contaminated in the continental crust. Based on the ratio of (La/YbN 6.23-34.73, indicate that small amount of garnet was left in the source rock. High ration of Rb/Sr, LILE/HFSE and high content of K2O, Th and Nb indicative of continental crust contamination. Petrognesis and geochronology of Sangan granitoid will help to know more about tectonomagmatic of Khaf-Kashmar-Drouneh volcanic-plutonic belt and their mineralization.

  17. Geochemical Characteristics and LA-ICP-MS Zircon U-Pb Dating of Amphibolites in the Songshugou Ophiolite in the Eastern Qinling

    LIU Liang; CHEN Danling; ZHANG Anda; ZHANG Chengli; YUAN Honglin; LUO Jinhai


    in basalts from other regions of the world. The LA-ICP-MS trace element and U-Pb isotopic analyses show that the zircon grains from the amphibolites are similar to the typical magmatic zircon in terms of their very low U and Th contents (62.36-0.10 μg/g and 78.47-0.003 μtg/g, respectively). Seven pits from the core and core-mantle parts of the zircon grains yielded an average weighted 206Pb/238U age of 973+35 (2σ) Ma with the Th/U ratios range from 0.01 to 8.38 and mostly greater than 0.23. This age is consistent within the error range with the whole-rock Sm-Nd isochron age of 1030±46 Ma for the same kind of rocks reported by Dong et al. (1997a). In a combined analysis with the zircon positions on the CL images and the corresponding Th/U ratios, the age of 973±35 Ma is probably the formation age of tholeiite, the protolith of the Songshugou amphibolite. The geochronological determination gives further evidence that the Songshugou ophiolite was formed during the Neoproterozoic. In addition, there is one pit from the rim of a zircon grain giving a 206pb/238U age of 572+ 199 (1 σ) Ma with a Th/U ratio of 0.08. It may represent the age of the accretionary zircon in the amphibolite-facies metamorphism.

  18. LA-ICP-MS U-Pb Zircon Dating for Felsic Granulite,Huangtuling Area, North Dabieshan: Constraints on Timing of Its Protolith and Granulite-Facies Metamorphism, and Thermal Events in Its Provenance

    Chen Nengsong; Liu Rong; Sun Min; Li Huimin; He Lei; Wang Qinyan; Zhang Hongfei


    Information about the protolith of the Huangtuling granulite in North Dabieshan has been unavailable. The complex evolution history of the rock and its host basement must be further discussed. LA-ICP-MS U-Pb dating was conducted on three textural domains in zircon from a high-temperature, high-pressure felsic granulite in the Huangtuling area, North Dabieshan, Central China. The metamorphic growth-derived detrital zircon domain yields a 207Pb/206Pb age in the range of (2 493±54) -(2 500±180) Ma. The magmatic genesis-derived detrital zircon domain gives a 207Pb/206Pb age ranging from 2 628 Ma to 2 690 Ma, with an oldest 206Pb/238U age of (2 790±150) Ma. The metamorphic overgrowth or metamorphic recrystallization zircon domain yields a dicsordia with an upper intercept age of (2 044.7±29.3) Ma. Compositions of the mineral assemblage, major element geochemistry, and especially the complex interior texture of the zircon suggest that the prololith of the felsic granulite is of sedimentary origin. Results show that the protolith material of the granulite came from a provenance with a complex thermal history, i.e. ~2.8 Ga magmatism and ~2.5 Ga metamorphism, and was deposited in a basin not earlier than 2.5 Ga. The high-temperature and high-pressure granulite-facies metamorphic age was precisely constrained at (2.04±0.03) Ga, which indicates the granulite in Huangtuling area should be a relict of a Paleoproterozoic UHT (ultrahigh temperature) metamorphosed slab.

  19. Timing and sources of granite magmatism in the Ribeira Belt, SE Brazil: Insights from zircon in situ U-Pb dating and Hf isotope geochemistry in granites from the São Roque Domain

    Janasi, Valdecir de Assis; Andrade, Sandra; Vasconcellos, Antonio Carlos B. C.; Henrique-Pinto, Renato; Ulbrich, Horstpeter H. G. J.


    Eight new in situ U-Pb zircon age determinations by SHRIMP and LA-MC-ICPMS reveal that the main granitic magmatism in the São Roque Domain, which is largely dominated by metaluminous high-K calc-alkaline monzogranites with subordinate peraluminous leucogranites, occurred between 604 ± 3 and 590 ± 4 Ma. This small temporal range is ca. 20-30 Ma younger than previously admitted based on U-Pb TIMS dates from literature, some of which obtained in the same occurrences now dated. The observed discrepancy seems related to the presence of small Paleoproterozoic inherited cores in part of the zircon populations used for TIMS multigrain dating, which could also respond for the unusually high (up to 10 Ma) uncertainty associated with most of these dates. The younger age range now identified for the São Roque granite magmatism has important implications for the evolution of the Ribeira Fold Belt. Whilst previously admitted ages ca. 620-630 Ma substantiated correlations with the widespread and intensely foliated high-K calc-alkaline granitoid rocks of the neighbor Socorro-Guaxupé Nappe (potentially associated with an accretionary continental margin), the ˜600-590 Ma interval seems more consistent with a late deformation tectonic setting. Strongly negative ɛHf(t) characterize the magmatic zircons from the São Roque Domain granites. An eastward increase from -22 in the São Roque Granite to -11 in the Cantareira Granite and neighboring stocks suggests an across-domain shift in granite sources. Such eastward younging of sources, also indicated by Sm-Nd isotope data from granites and supracrustal sequences in neighboring domains, is suggestive that some of the first-order limits and discontinuities in this belt are not defined by the strike-slip fault systems traditionally taken to separate distinct domains. Although the negative ɛHf(t) and ɛNd(t) indicate sources with long crustal residence for all studied granite plutons, the observed range is more radiogenic than the

  20. The temporal and spatial distribution of magmatism during lower crustal accretion at an ultraslow-spreading ridge: High-precision U-Pb zircon dating of ODP Holes 735B and 1105A, Atlantis Bank, Southwest Indian Ridge

    Rioux, Matthew; Cheadle, Michael J.; John, Barbara E.; Bowring, Samuel A.


    Ocean Drilling Program Hole 735B at Atlantis Bank on the Southwest Indian Ridge sampled 1508 m of plutonic oceanic crust, hosted in the footwall of an oceanic detachment fault. We present new high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon dates from samples spanning the length of Hole 735B, and from the shallower adjacent Hole 1105A (158 m). The new dates provide the most complete and precise record of both the spatial and temporal distribution of magmatism during accretion of the lower oceanic crust to date. Whole rock and mineral geochemistry from Hole 735B define three main igneous series. Weighted mean 206Pb/238U dates suggest each igneous series intruded beneath the preceding series. Weighted mean 206Pb/238U dates range from 12.175 to 11.986 Ma in Series 1; 11.974 to 11.926 Ma in Series 2; and 11.936 to 11.902 Ma in Series 3 (±0.015 to 0.069 Ma). Weighted mean 206Pb/238U dates from Hole 1105A range from 11.9745 to 11.9573 Ma (±0.0082 to 0.0086 Ma). The Hole 1105A dates are coeval with Series 2 in Hole 735B, consistent with previous correlations of Fe-Ti oxide-rich layers between the two holes, suggesting individual magmatic series formed sheet-like bodies that were ≥250 m thick and extended ≥1.1 km parallel to the ridge axis (E-W) and ≥0.48 km in the spreading direction (N-S). The data suggest a total duration of magmatism in Hole 735B of ≥ 0.214 ± 0.032 Ma, corresponding to accretion over a horizontal distance of ≥ 2.6 ± 0.4 km. The crust at Atlantis Bank was formed during active detachment faulting, and the successive underplating of each magmatic unit may have been favored in this environment. The combined U-Pb dates, and reported Ti-in-zircon temperatures, are consistent with magmatic cooling rates of 103-104 °C/Ma over the temperature interval of 900-700 °C.

  1. 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

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


    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

  2. Refined depositional history and dating of the Tongaporutuan reference section, north Taranaki, New Zealand: new volcanic ash U-Pb zircon ages, biostratigraphy and sedimentation rates

    Maier, K.L.; Crundwell, Martin P.; Coble, Matthew A.; Kingsley-Smith, Peter R.; Graham, Stephan A.


    This study presents new radiometric ages from volcanic ash beds within a c. 1900 m thick, progradational, deep-water clastic slope succession of late Miocene age exposed along the north Taranaki coast of the North Island, New Zealand. The ash beds yield U–Pb zircon ages ranging from 10.63 ± 0.65 Ma to 8.97 ± 0.22 Ma. The new ages are compatible with and provide corroboration of New Zealand Tongaporutuan Stage planktic foraminiferal and bolboformid biostratigraphic events identified in the same section. The close accord between these two age datasets provides a stratigraphically consistent and coherent basis for examining margin evolution. The arrival of a prograding clastic wedge and ensuing upward shoaling is recorded by sedimentation rates c. 2000 m/Ma–1 that are an order of magnitude higher than sedimentation rates on the precursor deep basin floor. This outcrop study provides new constraints for interpreting analogous subsurface deposits in Taranaki Basin and complements the regional late Miocene biostratigraphic dating framework.

  3. Zircon U-Pb dating, geochemical and Sr-Nd-Hf isotopic characteristics of the Jintonghu monzonitic rocks in western Fujian Province, South China: Implication for Cretaceous crust-mantle interactions and lithospheric extension

    Li, Bin; Jiang, Shao-Yong; Lu, An-Huai; Zhao, Hai-Xiang; Yang, Tang-Li; Hou, Ming-Lan


    Comprehensive petrological, in situ zircon U-Pb dating, Ti-in-zircon temperature and Hf isotopic compositions, whole rock geochemical and Sr-Nd isotopic data are reported for the Jintonghu monzonitic intrusions in the western Fujian Province (Interior Cathaysia Block), South China. The Jintonghu monzonitic intrusions were intruded at 95-96 Ma. Their Sr-Nd-Hf isotopic compositions are similar to the coeval and nearby enriched lithospheric mantle-derived mafic and syenitic rocks, indicating that the Jintonghu monzonitic rocks were likely derived from partial melting of the enriched mantle sources. Their high Nb/Ta ratios (average 21.6) suggest that the metasomatically enriched mantle components were involved, which was attributed to the modification of slab-derived fluid and melt by the subduction of the paleo-Pacific Plate. The presence of mafic xenoliths, together with geochemical and isotopic features indicates a mafic-felsic magma mixing. Furthermore, the Jintonghu intrusions may have experienced orthopyroxene-, biotite- and plagioclase-dominated crystallization. Crust-mantle interaction can be identified as two stages, including that the Early Cretaceous mantle metasomatism and lithospheric extension resulted from the paleo-Pacific slab subduction coupled with slab rollback, and the Late Cretaceous crustal activation and enhanced extension induced by dip-angle subduction and the underplating of mantle-derived mafic magma.

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

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


    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.

  5. U-Pb Dating of Calcite by LA-ICPMS

    Hacker, B. R.; Kylander-Clark, A. R.; Holder, R. M.; Nuriel, P.


    An emerging frontier area in geochronology is U-Pb dating of carbonate minerals by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The spate of papers over the last few years applying LA-ICPMS to carbonate dating stems from the capability of LA-ICPMS to deal with the variable, and often low, U/Pb ratios of carbonate. LA-ICPMS is an excellent tool for efficiently screening out samples with low U/Pb ratios and provides the ability to measure many spots with different U/Pb ratios and obtain dates free of assumptions about the composition of common Pb. Because this technique is in its infancy, important questions remain. What percentage of carbonate samples have high enough U/Pbc ratios that they can be dated? What percentage of samples yield isochronous datasets? What are the limits on precision and accuracy of carbonate U/Pb dates? What is the best analytical method in the absence of isotopically homogeneous reference materials? Through the generosity of our colleagues we have acquired 8 reference materials ranging in age from 3 to 250 Ma. We have analyzed 125 unknowns from a variety of locations using a 193 nm ns laser with an 80-100 μm spot and a Nu Plasma HR-ES. We measure 207Pb/206Pb using NIST 614 glass and then calculate a 206Pb/238U correction factor based on the measured vs. known ages of the reference materials. Sixty of these samples ( 50%) have high enough U/Pb ratios that they can be dated. There is great heterogeneity among the sample suites: some have no datable samples, whereas one suite of 68 samples yielded 53 datable rocks. Of the samples with high U/Pbc ratios, a majority yielded isochronous U-Pb data, indicating that the U-Pb system closed at a given time and was not subsequently disturbed.

  6. Zircon U-Pb Dating of Yushugou Terrain of High-Pressure Granulite Facies in Southern Tianshan Mountain and Its Geological SIgnificance


    The Yushugou terrain of high-pressure granulite facies in southern Tianshan Mountain is composed mainly of an ophiolite suite.Most selected zirocons are round or elliptical in shape,and some are of tetragonal prism with round edges.The granulometric analyses show that they are well sorted in sedimentation.ZrO2/HfO2 ratios in zircons range from 45 to 57.these characters,together with the petrologic and geochemical characters of plagioclase-garnet-orthopyroxenite bearing zircons,indicate that the protolith of plagioclase-garnet-orthopyroxenite may be derived mainly from volcanic base surge sedimentary debris in oceanic islands and from clays formed by seafloor weathering.Zircons are simply of pyroclastic debris.The ophiolite formation age of (440±18)Ma and the first-stage metamorphic age(amphibolite or granulite facies) of (364±5) Ma were obtained with a method of multiple grains in different groups and a method of concordia plot.These ages provide important information on the temporal and spatial occurrence of southern Paleoxoic Tianshan Ocean,The subduction rate of the oceanic crust and the formation mechanism of ophiolite of granulite facies.

  7. Zircon-apatite U-Pb geochronology, zircon Hf isotope composition and geochemistry of granite batholith in the northern Mexico: Implications for Tectonomagmatic evolution of southern Cordillera.

    Mahar, M. A.; Goodell, P.


    We present the zircon-apatite U-Pb ages and zircon Hf isotope composition of the granite batholith exposed at the western boundary of Chihuahua. Granidiorite samples were analyzed from both, north and south of the Rio El Fuerte and Sinforosa Lineament. Based on previous studies, the WWN-EES trending Sinforosa Lineament is proposed as the manifestation of a terrane boundary between Seri in the north and Tahue terrane in the south. Zircon U-Pb data indicate that the magmatism spans a time period of 36 Ma from 89 to 53 Ma to the north of the Sinforosa Lineament while granodiorites in the south of the Sinforosa Lineament are dated at 59 Ma. The U-Pb apatite ages are variable in the north of the Sinforosa Lineament and range from 86-51 Ma. These apatite dates are 1-28 Ma younger than the corresponding zircon U-Pb crystallization ages. This indicates variable cooling rates and moderate to shallow emplacement. In contrast, in the south of the Sinforosa Lineament, the U-Pb apatite ages (64-59 Ma) are indistinguishable from the zircon U-Pb age (59 Ma), indicating rapid cooling and shallow emplacement. Zircon morphology and U-Pb dating revealed the absence of inherited component in the zircon ages, as no inheritance of any age has been observed. Most of the northwestern Mexico is underlain by Precambrian-Paleozoic-Jurassic basement. However, in the study area, U-Pb dating does not support the involvement of the older basement in generating the granite magmas. The weighted mean initial ɛHf (t) isotope composition of granodiorites on both sides of the Sinforosa Lineament varies from +2 to +5. However, Hf isotope composition in the south of the Sinforosa Lineament is more heterogeneous and relatively evolved with weighted Mean ɛHf (t) = +1.45. The Hf isotope composition is consistent with the previously reported near bulk silicate Sr-Nd isotope values. We suggest that the magmatic rocks in this region are not derived from melting of a felsic older crust beneath the batholith

  8. U-Pb zircon dating, geochemical and Sr-Nd-Hf isotopic compositions of mafic intrusive rocks in the Motuo, SE Tibet constrain on their petrogenesis and tectonic implication

    Pan, Fa-Bin; Zhang, Hong-Fei; Xu, Wang-Chun; Guo, Liang; Luo, Bi-Ji; Wang, Shuai


    Mafic intrusive rocks are widely exposed in the Motuo tectono-magmatic belt, southeast Lhasa terrane. LA-ICP-MS U-Pb zircon dating shows that they have magma crystallization ages of 69 and ca. 50 Ma. These mafic intrusive rocks are characterized by variable SiO2 (44.60-57.60 wt.%), high Al2O3 (17.19-20.86 wt.%), and low MgO (1.85-5.38 wt.%) with Mg# of 31-55. Their chemical composition is comparable with low-MgO high-Al basalts to basaltic andesites. They are enriched in LILEs (Rb, Ba, K) and LREE and depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), with weakly evolved Sr-Nd-Hf compositions (whole-rock (87Sr/86Sr)0 = 0.7064 to 0.7086, εNd(t) = - 3.41 to + 1.22, and zircon εHf(t) = - 3.8 to + 6.4). The mafic rocks were derived from partial melting of metasomatized lithospheric mantle. Geochemical and Sr-Nd-Hf isotopic data show that they were insignificantly modified during magma emplacement. We provide a different secular evolution of the lithospheric mantle beneath the eastern part relative to the central part of the southern Lhasa terrane. Sr-Nd isotopic composition of the Motuo Late Cretaceous-Eocene mafic rocks argues that they were derived from partial melting of a relatively homogeneous and depleted lithospheric mantle. We propose that the Late Cretaceous delamination resulted in the replacement of ancient lithospheric mantle by the juvenile homogeneous lithospheric mantle in the eastern Lhasa terrane. The foundered ancient materials may subsequently re-fertilize the lithospheric mantle not only in the eastern Lhasa terrane but also in the surrounding areas.

  9. 锡林浩特杂岩中斜长角闪岩锆石U-Pb年代学及Hf同位素研究%U-Pb Dating and Hf Isotopic Composition of Zircons in Amphibolite from the Xilinhot Complex

    王善辉; 陈岳龙; 李大鹏


    锡林浩特杂岩是一套经历强变形变质作用的变质岩,主要由黑云斜长片麻岩、角闪斜长片麻岩、斜长角闪片麻岩及变粒岩夹透镜状或脉状斜长角闪岩组成.查明其形成年龄、变质年龄对深入认识中亚造山带的形成演化具有重要意义.本次研究对锡林浩特杂岩中斜长角闪岩进行了SHRIMP锆石U-Pb测年和LA-MC-ICP-MS锆石Hf同位素组成分析,给出了锆石SHRIMP U-Pb的加权平均年龄为(316 ±4) Ma,该年龄代表斜长角闪岩的原岩形成年龄,表明锡林浩特杂岩不是前寒武纪地质体.通过锆石Hf同位素分析,εHf(t)值主要为正值(+4.1~+17.0),揭示其物源可能来自亏损地幔或壳幔混合.根据亏损地幔和古老地壳的锆石Hf同位素进行两端元的混合计算,得到斜长角闪岩原岩岩浆的源区以幔源增生组分为主体(经过计算壳幔混合比例大约1∶2).部分锆石的模式年龄在1.8 ~2.5 Ga之间,表明斜长角闪岩的原岩在形成过程中有古元古代地壳物质的加入,通过兴蒙造山带和锡林浩特杂岩中斜长角闪岩的Hf模式年龄对比可知,物源可能来自兴蒙造山带内部.基于Hf同位素的两端元混合计算表明该区在晚古生代存在地壳增生.根据野外观察和室内岩石特征分析,该套杂岩可能是一套经历强变形与变质作用的晚古生代火山-沉积建造.%The Xilinhot complex is mainly composed of biotite-plagioclase gneiss, hornblende-plagioclase gneiss, amphibolitic gneiss and granulite with lenticular or vein amphibolite rocks, whose formation and metamorphosed age and petrogenesis have been debating. Accurate determination of its formation and metamorphosed age is of great significance to understanding the formation and evolution of the Hinggan-Mongolia Orogenic Belt. Through zircon SHRIMP U-Pb dating and LA-MC-ICP-MS Hf isotopic analysis on zircons in plagioclase amphibolite from the Xilinhot complex, weighted average value of

  10. Zircon U-Pb Dating of Dayishan and Tashan Plutons in Hunan Province and Its Tectonic Implications%湖南大义山、塔山岩体锆石U-Pb年龄及其构造意义

    李勇; 张岳桥; 苏金宝; 李建华; 董树文


    The distribution and intrusion of Early Mesozoic granites in central South China such as Dayishan pluton and Tashan pluton are closely related to folded structure in this region. Precise chronologic studies can constrain the age of the tectonic deformation. This paper report zircon U-Pb LA-ICP-MS ages of the southern part of Dayishan pluton based on dating of 3 samples:the age of the eastern margin is (171.8±1.9) Ma, that of the western margin is (169.9±1.8) Ma and that of the central part is (164.2±2.6) Ma. The zircon U-Pb LA-ICP-MS age of the eastern part of Tashan pluton is (247±5.9) Ma. Dayishan pluton experienced two phases of intrusion between 164.2 and 171.8 Ma and formed a composite pluton;after that, it experienced rapid uplift prior to 125 Ma, and then it was rapidly raised to the surface at the cooling rate of 1°C/Ma. Dayishan pluton and Tashan pluton recorded Indosinian and Yanshanian magmatic activities. The EW-trending structural zone controlled Indosinian granites, such as Yangmingshan and Tashan; the ages of Dayishan and Qitianling plutons constrain the Yanshanian tectonic deformation, and the formation of the NNE-trending folds in South China should be earlier than (171.8±1.9) Ma.%华南中部地区大义山、塔山等早中生代花岗岩的展布和侵入特征与这一时期褶皱构造之间存在着非常密切的关系,对这些岩体进行精确年代学研究,可以为构造变形时代提供年代学约束。大义山岩体南部地区锆石U-Pb LA-ICP-MS年龄为东缘(171.8±1.9) Ma、西缘(169.9±1.8) Ma、中部(164.2±2.6) Ma,塔山岩体东段为(247±5.9) Ma。大义山岩体在164.2—171.8 Ma之间经历两个期次的侵入,形成了一个复式岩体,岩体形成后至125 Ma经历了快速隆升,之后以接近1°C/Ma的冷却速率抬升到地表。大义山和塔山岩体记录了华南早中生代印支和燕山两期重要的岩浆活动。阳明山、塔山岩体等E—W向展布岩体受

  11. New insights from U Pb zircon dating of Early Ordovician magmatism on the northern Gondwana margin: The Urra Formation (SW Iberian Massif, Portugal)

    Solá, A. R.; Pereira, M. F.; Williams, I. S.; Ribeiro, M. L.; Neiva, A. M. R.; Montero, P.; Bea, F.; Zinger, T.


    The Central Iberian-Ossa-Morena transition zone (SW Iberian Massif) represents a segment of the northern Gondwana margin with a long geodynamic evolution, characterized by the superposition of Cadomian and Variscan events. The Early Ordovician is mainly represented by porphyritic felsic volcaniclastic rocks (the Urra Formation) that pass up into a siliciclastic sediments typical of the Central Iberian Zone (Lower Ordovician Armorican Quartzite Formation). The Urra Formation unconformably overlies the previously deformed and metamorphosed Ediacaran sediments of the Série Negra (with Ossa-Morena Zone paleogeographic affinity). New SHRIMP zircon data obtained from the Urra Formation volcaniclastic rocks indicate an Early Ordovician age ( 206Pb/ 238U ages ranging from 494.6 ± 6.8 Ma to 488.3 ± 5.2 Ma) for this magmatic event. The inherited zircon cores indicate the presence of multicycle protoliths with different Precambrian ages: Neoproterozoic (698-577 Ma), Paleoproterozoic (2.33 Ga) and Paleoarchean (3.2-3.3 Ga). There is a noticeable lack of Meso- to Neoarchean and Mesoproterozoic ages. The data support the hypothesis that the volcaniclastic rocks were derived by partial melting of Cadomian basement (linked to a West African Craton provenance). The Urra Formation volcaniclastic rocks have rhyolitic to dacitic compositions, are peraluminous and similar to calc-alkaline high-K series suites elsewhere. Isotopic signatures present a wide range of values ( 87Sr/ 86Sr) t = 0.7085-0.7190, more restricted ɛNd t (- 2.65 to - 0.35) and δ18O = 9.63-10.34‰, compatible with magmas derived from crustal rocks, including portions of the lower crust. Some samples show disturbance of the Rb-Sr system as shown by unrealistic values for ( 87Sr/ 86Sr) t < 0.703, probably due to Variscan deformation and metamorphism. The volcaniclastic rocks with a significant sedimentary contribution (upper unit) are distinguished from the others by the lowest values of ɛNd t (- 5.53 to - 4

  12. Zircon U-Pb geochronology of basement metamorphic rocks in the Songliao Basin

    PEI FuPing; XU WenLiang; YANG DeBin; ZHAO QuanGuo; LIU XiaoMing; HU ZhaoChu


    Zircon LA-ICP MS U-Pb dating of six metamorphic rocks and a metagranite (breccia) from southern basement of the Songliao Basin are reported in order to constrain the formation ages of basement. The basement metamorphic rocks in the Songliao Basin mainly consist of metagabbro (L45-1), amphibolite (SN117), metarhyolitical tuff (G190), sericite (Ser) schist (N103), chlorite (Chi) schist (T5-1), biotite (Bi)-actinolite (Act)-quartz (Q) schist (Y205), and metagranite (L44-1). The cathodoluminesence (CL)images of the zircons from metagabbro (L45-1) and metagranite (L44-1) indicate that they have cores of magmatic origin and rims of metamorphic overgrowths. Their U-Pb isotopic ages are 1808±21 Ma and 1873±13 Ma, respectively. The zircons with oscillatory zoning from amphibolite (SN117) and Chi schist (T5-1), being similar to those of mafic igneous rocks, yield ages of 274 ± 3.4 Ma and 264 ± 3.2 Ma, respectively. The zircons from metarhyolitical tuff (G190) and Ser schist (N103) display typical magmatic growth zoning and yield ages of 424 ± 4.5 Ma and 287 ± 5.1Ma, respectively. Most of zircons from Bi-Act-Q schist (Y2O5) are round in shape and different in absorption degree in the CL images, implying their sedimentary detritals. U-Pb dating yield concordant ages of 427 ± 3.1Ma, 455 ± 12 Ma, 696 ± 13 Ma,1384±62 Ma, 1649±36 Ma, 1778±18 Ma, 2450±9 Ma, 2579±10 Ma, 2793±4 Ma and 2953±14 Ma. The above-mentioned results indicate that the Precambrian crystalline basement (1808-1873 Ma) exists in the southern Songliao Basin and could be related to tectonic thrust, and that the Early Paleozoic (424-490 Ma) and Late Paleozoic magmatisms (264-292 Ma) also occur in the basin basement, which are consistent with the ages of the detrital zircons from Bi-Act-Q schist in the basement.

  13. SHRIMP U-Pb Dating of Zircons of a Dark Eclogite and a Garnet-bearing Gneissic Granitic Rock from Bixiling, EasternDabie Area, Anhui Province: Isotope Chronological Evidence of Neoproterozoic UHP Metamorphism


    The paper reports SHRIMP U-Pb zircon data of a dark eclogite and a post-eclogite garnet-bearing gneissic granitic rock from the Bixiling area, Yuexi County, Anhui Province, in the eastern Dabie Mountains. The eclogite, which is metamorphosed basic tuff, contains very scarce zircons in omphacite or garnet, but more zircons in quartz. They usually exhibit a double-layered texture, as shown clearly in cathodoluminescence images. Their inner main parts give a206 Pb/238U age of 757±7Ma, representing the approximate age of the high-pressure (HP)-ultrahighpressure (UHP) metamorphic event duing which the eclogite was formed. The outer peripheral parts of the zircons,which have been modified by late-stage fluids, give an age of 223±3 Ma. The granitic rock contains more zircons of anatectic origin found mostly in feldspar and quartz and usually also showing a similar composite texture. The inner main parts of the anatectic zircons with oscillatory zoning give a 206Pb/238U age of 727±15Ma for the approximate age of the emplacement of the granitic rock, and their outer parts, an age of 219±3 Ma for a similar or even the same fluid event. It is thus suggested that the HP-UHP metamorphism of the Bixiling eclogite facies rocks took place during the Neoproterozoic Jinningian,and the Indosinian age values may only represent a late event in the nature of fluid activity.

  14. LiDAR offshore structural mapping and U/Pb zircon/monazite dating of Variscan strain in the Leon metamorphic domain, NW Brittany

    Le Gall, Bernard; Authemayou, Christine; Ehrhold, Axel; Paquette, Jean-Louis; Bussien, Denise; Chazot, Gilles; Aouizerat, Arthur; Pastol, Yves


    An exceptional structural picture of the immerged Variscan basement, offshore the Leon metamorphic domain, is supplied by high-resolution LiDAR and echosounder data recorded in the Molène archipelago, western Brittany (France). Various types of fabrics are identified and, from in situ rock sample analyses further combined with field structural data, are interpreted on a lineament trajectory map as the trace of magmatic and tectonic structures. Our onshore/offshore study leads us to propose a two-phase kinematic model that emphasizes the role of a strike-slip duplex in an EW-trending relay zone linking the North Armorican and Pierres Noires ductile shear zones (NASZ, PNSZ). Dextral shearing occurred within a transtensional setting, synchronously with magmatic intrusions (St-Renan granite and an offshore gabbro-diorite complex) dated at 314-320 Ma by new U-Th/Pb ages. It post-dated an early regional foliation related to top-to-the-NE ductile transpressional shearing. Our study emphasizes the key role of strike-slip tectonics in the NW part of the Armorica Variscan belt.

  15. Palaeoproterozoic continental arc magmatism, and Neoproterozoic metamorphism in the Aravalli-Delhi orogenic belt, NW India: New constraints from in situ zircon U-Pb-Hf isotope systematics, monazite dating and whole-rock geochemistry

    Kaur, Parampreet; Zeh, Armin; Chaudhri, Naveen


    Presently, the extent, origin and petrogenesis of late Palaeoproterozoic (ca. 1.85 Ga) magmatism in the north-central Aravalli-Delhi orogenic belt, NW India and subsequent metamorphic overprints are poorly constrained. Results of new in situ zircon U-Pb-Hf isotope analyses in combination with whole-rock elemental and isotopic data provide the first hard evidence that granitoid magmatism occurred in a continental magmatic arc setting between 1.86 and 1.81 Ga. The Hf-Nd model ages of 3.0-2.6 Ga and inherited zircon grains of 3.3-2.5 Ga indicate abundant reworking of Archaean crust. Flat HREE patterns with negative Eu anomalies furthermore reveal that the granitoids were generated from garnet-free and plagioclase-rich sources at shallow depths. Significant isotope variation among granitoid samples (εHft = -3.7 to -9.0; εNdt = -4.8 to -7.9) indicate that the reworked Archaean crust was not completely homogenised during the Palaeoproterozoic. This is best reflected by zircon Hf-isotope variation of ca. 9.5 epsilon units within the oldest granitoid sample. Zircon grains from this sample define three discrete Hf-isotope groups at εHf1.86Ga = -8.9, -4.8 and -1.6. These are interpreted to result from mixing of zircon-saturated magmas derived from three distinct sources within the crust prior to solidification. A monazite U-Pb isochron age of 868 ± 4 Ma from one of the granitoid samples furthermore indicates that the Aravalli fold belt was affected by an important post-magmatic overprint, perhaps related to the widespread metasomatic, granulite metamorphic and/or magmatic events during the same time span.

  16. Zircon U-Pb age of the Shacun gabbro body,Yuexi,Dabie orogen and its geological implications


    The single grain zircon U-Pb age of the Shacun mafic-ultramafic intrusion,Yuexi,North Dabie is (128.1±2.0) Ma.This date indicates that the intrusion was implaced at Yanshanian when most Mesozoic granitoids were intruded.It was not the syn-collisional intrusion related to the subduction of the Yangtze Block and ultrahigh-pressure metamorphism at Indosinian.

  17. Zircon U-Pb ages for Wulian granites in northwest Sulu and their tectonic implications


    The Wulian complex occurring in the north-western part of the Sulu orogen consists of metaigneous and metasedimentary rocks at greenschist-facies. Zircon U-Pb dating for three granite intrusions within it yields concordant ages of (672±4), (742±9) and (747±14) Ma, respectively. These ages indicate that the granites are the Neoproterozoic intrusions in the northern margin of the Yangtze plate, and correspond to the magmatic complex at Luzhenguan in the eastern part of the Beihuaiyang belt in the Dabie orogen. They were tectonic slices scraped off from the Yangtze plate during Triassic subduction and thus belong to a part of the accretionary wedge of the Yangtze plate subduction. The discovery of extensive Neoproterozoic magmatic rocks in the Wulian area suggests that the suture location between the North China and the Yangtze plates lies north of the Wulian complex.

  18. SHRIMP U-Pb dating of the preeruption growth history of zircons from the 340 ka Whakamaru Ignimbrite, New Zealand: Evidence for >250 k.y. magma residence times

    Brown, Stuart J. A.; Fletcher, Ian R.


    The Whakamaru group ignimbrites are products of one of the largest caldera-forming eruptions from the central Taupo Volcanic Zone, New Zealand, ca. 340 ka. Imaging of zircons separated from low-silica rhyolite pumice from Whakamaru group ignimbrite reveals a complex magmatic crystallization history; some grains contain resorbed cores. Cores and rims of individual zircons were dated using the sensitive high-resolution ion microprobe. Due to the high U contents of the zircons, 206Pb count rates were sufficiently high to obtain 206Pb/238U ages with precisions of 5% 10% (1 σS) using longer than usual 206Pb+ count times, and this has allowed zircon core and rim ages to be distinguished. Zircon rim and outer growth zone ages range from within 1 σS error of the eruption age to 449 ± 20 ka, whereas zircon cores produced ages to 608 ± 20 ka, indicating magmatic residence times exceeding 250 k.y. for some zircons. The data are consistent with a prolonged evolution of the magma system over hundreds of thousands of years, involving magma replenishment and mixing with residual phases of older silicic magma chambers, in addition to crystal fractionation processes, prior to eruption. The study has shown that 206Pb/238U dating using high-resolution ion microprobe can be used to determine zircon preeruptive growth histories in Quaternary lavas and pyroclastic rocks.

  19. 滇东南建水火山岩 LA-ICP-MS 锆石 U-Pb 测年、地球化学特征及地质意义%LA-ICP-MS Zircon U-Pb Dating, Geochemical Characteristics of Volcanic Rocks in Jianshui, Southeast Yunnan and Their Geological Implications

    陈超; 周涛; 胡维云; 陈桂凡; 李华亮; 张林友; 李德威


    滇东南建水地区位于师宗‒弥勒构造带的南段,区内出露一套玄武岩–安山岩–英安岩–流纹岩组合。研究这套火山岩的形成时间和形成环境对认识滇东南构造格局有着重要意义。本文首次对建水火山岩进行 LA-ICP-MS 锆石 U-Pb 测年,获得两个样品的年龄分别为261.9±2.2 Ma (MSWD=0.80)、264.8±1.7 Ma (MSWD=1.12),属中二叠世晚期,代表建水火山岩的形成时间。建水玄武岩与安山岩具有低 TiO2含量(0.50%~0.88%)、高 Mg#(52.0~64.5)、弱富集 LREE((La/Yb)N=1.42~3.44)、富集 LILE(Rb, Th, U, Pb)、轻微亏损 HFSE(Nb, Ta, Ti)的特点,具典型岛弧玄武岩的特征;英安岩和流纹岩高ΣREE 含量(139.5×10–6~313.6×10–6)、富集 LREE((La/Yb)N=4.16~9.78)和 LILE(Rb, Ba, Th, U)、亏损 HFSE(Nb, Ta)、强亏损 Sr、Ti、Y 等元素的特点与典型的岛弧流纹岩相似,但高钾(K2O 含量平均7.73%)、钙碱性(δ=0.93~2.94)、强过铝质(A/CNK=1.13~2.10)的特点使之区别于一般岛弧酸性火山岩,而具有一些上地壳部分熔融形成的 S 型花岗岩的特点。综合建水火山岩岩石组合、地球化学和区域地质背景,认为建水火山岩形成于活动大陆边缘的弧后盆地伸展环境,是地幔部分熔融形成的玄武质岩浆结晶分异与上地壳混染作用的共同结果,与滇桂交界处岛弧火山岩、两广交界处岛弧玄武岩等同为哀牢山洋向北俯冲的产物。%The age and tectonic setting of a set of basalt-andesite-dacite-rhyolite in Jianshui, Southeast Yunnan, south of the Shizong-Mile structural belt is studied in this paper. LA-ICP-MS zircon U-Pb dating of two samples yield 261.9±2.2 Ma (MSWD=0.80) and 264.8±1.7 Ma (MSWD=1.12) respectively. The basalts and andesites are characterized by low TiO2 (0.50%–0.88%), high Mg# (52.0–64.5), slightly enriched LREE ((La/Yb)N=1.42–3.44), enriched LILE (Rb, Th, U, Pb) and slightly depleted HFSE (Nb, Ta, Ti), which are typical of

  20. Chronological record of the Early Mesozoic underplating in the northern margin of North China-U-Pb chronometry of zircons in the Late Mesozoic andesite from Western Liaoning


    Cathodoluminescence spectroscopic analysis of zircons in the uppermost sanukites of the early Cretaceous Yixian Formation in Western Liaoning reveals zoning structures of the zircons in the volcanic rocks. LA-ICP-MS chronometry shows that the central part of the zircons is remnant zircon with a U-Pb dating of 254 Ma, which is consistent with the age of the Early Mesozoic underplating granulites, and that the periphery is zircon crystallizing out of the host magma with a U-Pb dating of 116 Ma, which is highly consistent with the Ar-Ar dating of the host volcanic rocks. Compositions of the clinopyroxenes xenocrystals in the sanukites are similar to those of the clinopyroxenes in the underplating granulites, indicating that the granulites accreted to the lower crust in the Early Mesozoic have been reformed and disrupted by the upwelling diapers of the asthenosphere and taken part in formation of the sanukites in the Late Mesozoic.

  1. Disturbed Sr and Nd Isotope Systematics in Zircons With Concordant SHRIMP U-Pb Ages

    Weaver, K. L.; Bennett, V. C.; Depaolo, D. J.; Mundil, R.


    Little is known about the Sr- and Nd-isotopic systematics of zircon. With slow diffusion rates and a high resistance to weathering, zircon should preserve accurate age information and initial Sr and Nd isotopic ratios. As a common accessory mineral, it could provide petrogenetic information for rocks that have been altered, weathered, or metamorphosed. We have investigated the Sm-Nd and Rb-Sr systematics of zircons from unmetamorphosed granitic rocks that have yielded concordant U-Pb SHRIMP (Sensitive High Resolution Ion Microprobe) ages and have depleted mantle signatures for Nd and Sr isotopes. Zircon populations from mantle-derived igneous rocks with ages of 0.1, 1.7, and 3.8 Ga were chosen for Sr and Nd isotopic analysis. Low concentrations (Sr, 4 to 8 ppm and Nd, 6 to 12 ppm) and small grain size necessitate the use of multigrain aliquots. Meaningful results can be obtained only if all of the zircons in the rock are a coherent population with homogeneous ages throughout and among grains. Zircon U-Pb ages were characterized using the SHRIMP RG, and trace element concentrations were measured by LA-ICPMS. The populations are homogeneous and the material ablated by the ion beam ( ˜~20 μ m spot size) shows little evidence of lead loss. Results on zircons of 100 Ma and 1700 Ma indicate that both the Rb-Sr and Sm-Nd systems have been severely disturbed. For the 1700 Ma granitic rocks from the Yavapai sequence of Arizona, zircon Sm-Nd apparent ages are ca. 1000 Ma! Leaching was used to remove contributions from adhering or included minerals, but leached residues that presumably most closely approximate the composition of the pure zircon (e.g. have high Sm/Nd) are no less disturbed than unleached samples. Despite the U-Pb SHRIMP ages indicating a closed system, the zircons have failed to preserve a reasonable age or initial isotopic composition for Sr and Nd, indicating that parts of the crystal might be severely affected by radiation damage resulting in disturbed

  2. 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

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


    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

  3. Temporal-spatial Distribution and Tectonic Implications of the Batholiths in the Gaoligong-Tengliang-Yingjiang Area, Eastern Tibet: Constraints from Zircon U-Pb Ages and Hf Isotopes

    XU Yi-gang; YANG Qi-jun; LAN Jiang-bo; HUANG Xiao-long; LUO Zhen-yu; SHI Yu-ruo; XIE Lie-wen


    @@ Zircon U-Pb dating reveals three episodes of plutonism, and more importantly a southwestward magmatic migration in the Gaoligong-Tengliang-Yingjiang area (west Yunnan), east of the eastern Himalayan Syntaxis.

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


    Zircon U-Pb ages are reported for three samples of intrusive rocks in Khondalite series in the Sanggan area, North China craton. The age of meta-granite is dated as 2005±9 Ma, implying that the sedimentary sequences in Khondalites series formed before 2.0Ga. The age of 1921 ±1Ma for the meta-diorite constrain the age of granulite facies metamorphism younger than this date. The age of 1892±10 Ma for garnet granite is obtained, but the granite crystallization age seems a little younger than the date considering the morphology of zircons. On the basis of these dates and of a concise review of previous age data, it is inferred that the Khondalite series was subjected to granulite facies metamorphism at about 1.87Ga together with tonalitic granulites and HP basic granulites in the Sanggan area.

  5. Zircon U-Pb ages of the basement rocks beneath the Songliao Basin, NE China


    The basement of the Songliao Basin is mainly composed of slightly-metamorphosed or unmetamorphosed Paleozoic strata, granites and gneiss. Petrographical studies indicate that the gneiss was originally the granitic intrusions which were deformed in the later stage. One undeformed granitic rock sample gives a U-Pb age of (305±2) Ma, and the mylonitic granite yields a U-Pb age of (165±3) Ma. Both of the two samples contain no inherited zircon, which suggests that there is no large-scale Precambrian crystalline basement beneath the Songliao Basin.

  6. Provenance and sediment dynamics within river basins in Western Peru through detrital zircons U-Pb ages

    Camille, Litty; Pierre, Lanari; Marco, Burn; Fritz, Schlunegger


    U-Pb dating of detrital zircons from clastic sediments by LA-ICPMS has become a popular method in sedimentary correlation and provenance studies. Because of remarkable durability, detrital zircons may be reworked through multiple sedimentary cycles and provide an ideal material to study the sedimentary provenance in rivers and the erosional characteristics. The Western side of the Peruvian Andes has experienced multiple pluvial periods induced phases of erosion and the formation of subsequent cut-and-fill terrace sequences since the Pleistocene. The aim of the study is to estimate the source areas of the terrace and modern deposits to infer changes in sediment dynamics through time and correlate them with the climatic change and especially precipitation patterns. To this extent, we determined the provenance of 4 dated terrace deposits along with modern sediments from the same streams by matching detrital-zircon ages with crystallization ages of source rocks. Age populations of detrital zircons are derived using U-Pb LA-ICP-MS analysis of about 50 zircons. Results show changes in the sediment provenance through time. Nowadays, sediment source areas are mainly located on the uppermost reach of the rivers whereas during the Pleistocene, sediment source areas were both located in the headwaters and along the middle reach of the rivers. These differences in terms of provenance could correlate with a change in precipitation locations and rates. Indeed a scenario where the locus of precipitation occurrence shifted from the middle reaches including the Altiplano during the past, to the Altiplano only as observed today, along with higher precipitation rates during the periods of terraces formation, offers an explanation to explain the erosional patterns recorded by detrital zircons.

  7. The Cadomian Orogeny and the opening of the Rheic Ocean: The diacrony of geotectonic processes constrained by LA-ICP-MS U Pb zircon dating (Ossa-Morena and Saxo-Thuringian Zones, Iberian and Bohemian Massifs)

    Linnemann, Ulf; Pereira, Francisco; Jeffries, Teresa E.; Drost, Kerstin; Gerdes, Axel


    Cadomian orogenic processes and their continuum to the opening of the Rheic Ocean were modeled by making use of new LA-ICP-MS U-Pb ages from detrital zircons of sedimentary rocks of Late Neoproterozoic (Ediacaran) and Cambro-Ordovician sediments of the Ossa-Morena Zone (Iberian Massif) compared with those from the Saxo-Thuringian Zones (Bohemian Massif). Presented data constrain a diachrony of Cadomian and related geotectonic processes along the northern realm of the Gondwana Supercontinent. Early stage of Cadomian evolution is characterized by a continental magmatic arc at the periphery of the West African Craton and a related back-arc basin opened at c. 590 to 570 Ma. Diacronic arc-continent collision was caused by oblique vector of subduction and started first in the East of Peri-Gondwana at c. 560-570 Ma and resulted at c. 543 Ma in the formation of a short-lived Cadomian retro-arc basin in the Saxo-Thuringian Zone. In contrast, more to the West in the Ossa-Morena Zone, the Cadomian back-arc basin was longer active, at least until c. 545 Ma. In that region, final magmatic pulse of the Cadomian magmatic arc at c. 550 Ma is documented by new zircon data. Closure of the Cadomian back-arc basin and arc-continent collision in the Ossa-Morena Zone occurred between c. 545 Ma and the overall onset of Cambrian plutonism at c. 540 Ma. A mid-oceanic ridge was subducted underneath the Cadomian orogen accompanied by slab break-off of the subducted oceanic plate. Oblique incision of the oceanic ridge into the continent caused the formation of rift basins during the Lower to Middle Cambrian (c. 530-500 Ma). This process continued and finally caused the opening of the Rheic Ocean documented by thick Lower Ordovician siliciclastic sediments and a final magmatic event at c. 490-485 Ma. Opening of the Cambrian rift basin and of the Rheic Ocean again was diachronic and started from the West of Peri-Gondwana and expanded eastward.

  8. Zircon SHRIMP U-Pb age of garnet olivine pyroxenite at Hujialin in the Sulu terrane and its geological significance

    GAO Tianshan; CHEN Jiangfeng; XIE Zhi; YANG Shenghong; YU Gang


    Garnet olivine pyroxenite at Hujialin is situated in the Sulu ultrahigh pressure (UHP) metamorphic belt, Shandong Province. Most of the zircon separated from the rock is well crystallized, prismatic and granular with a length to width ratio of 1︰1.3-1︰2.5. CL and BSE images show the magmatic oscillatory zoning in the zircon. Th/U ratio ranges from 0.99 to 2.81. These suggest a magmatic origin for the zircon studied. SHRIMP dating yields 206Pb/238U ages of 207-223 Ma, with a weighted average of 216±3 Ma. This age corresponds to zircon growth during exhumation of UHP slab and thus the timing of amphibolite-facies retrogression. The garnet olivine pyroxenite was wrapped and brought to the crust by the UHP slab during exhumation, and then suffered from metasomatism by fluid from the UHP slab itself. The zircon U-Pb age records the timing of the crystallization of metasomatic melt. Therefore, fluid that was released during exhumation of deeply subducted continental slab may be the important source for zircon growth.

  9. A comparison of zircon U-Pb age results of the Red Clay sequence on the central Chinese Loess Plateau

    Gong, Hujun; Nie, Junsheng; Wang, Zhao; Peng, Wenbin; Zhang, Rui; Zhang, Yunxiang


    Single grain zircon U-Pb geochronology has demonstrated great potentials in extracting tectonic and atmospheric circulation signal carried by aeolian, fluvial, and fluviolacustrine sediments. A routine in this sort of studies is analyzing 100-150 grains and then compares zircon U-Pb age spectra between the measured sample and the potential sources. Here we compared the zircon U-Pb age results of the late Miocene-Pliocene Red Clay sequence of two neighboring sites from the Chinese Loess Plateau where similar provenance signal is expected. Although the results from the 5.5 Ma sediment support this prediction, the results from the 3 Ma sediment at these two sites differ from each other significantly. These results emphasize the importance of increasing analysis number per sample and combining the zircon U-Pb geochronology with other provenance tools in order to get reliable provenance information.

  10. U-Pb zircon geochronology of Caledonian age orthogneisses dredged from the Chukchi Borderland, Arctic Ocean.

    Brumley, K.; Miller, E. L.; Mayer, L. A.; Wooden, J.; Grove, M.


    Over 500 kilos of metamorphic rock was dredged from outcrops along a steep normal fault scarp in the central Chukchi Borderland in 2009 (HLY0905) from water depths of between about 2500-1400m. The rocks in the dredge included broken angular cobbles and boulder-sized samples of amphibolites, orthogneisses, and granitoids of the same amphibolite facies metamorphic grade, as well as gravel to small cobble-sized ice rafted debris of various rock types. Zircons were separated from two of the orthogneiss samples, and single grain zircon U-Pb ages were determined by SHRIMP analysis to be 428 Ma ± 3.4 for both samples (N=60). Several zircon grains had distinct igneous cores that ranged in age from about 928-1200 Ma (n=7) with two older grains (1700, 1760 Ma). The Caledonian orogenic belt developed in the Ordovician to Devonian affecting northern Europe, Greenland and Arctic Canada. Caledonian deformational trends continue into the Arctic and disappear at the rifted margin of the Arctic Ocean. Syn-orogenic magmatism in the Barents region date deformation in this region during the Caledonian event to have occurred between about 450-410 Ma (Johansson et al., 2005; Gee et al., 2006; Gee and Tebenkov, 2004). Grenville age plutons (900-1250Ma) that were later involved in Caledonian deformation and intruded by 410-450 Ma aged plutons are found on western and eastern Svalbard (Johansson et al., 2005), eastern Greenland, and the Pearya Terrane of northern Ellesmere Island (Trettin,1986, 1992). The Franklinian basement of Arctic Alaska and Canada do not share these Grenvillian ages (Trentin et al, 1987). This suggests that the inherited zircon cores in the Chukchi Borderland orthogneisses were derived, at least in part, from an older Grenvillian basement like that of Pearya, Svalbard and parts of Greenland, or through sediments eroded from these sources, and later intruded by Caledonian aged granites. This constrains the pre-rift location of the Chukchi Borderland to be within the

  11. Zircon U-Pb dating of metabasic rocks in the Zanhuang metamorphic complex and its geological significance%赞皇变基性岩中锆石的U-Pb定年及其地质意义

    肖玲玲; 王国栋


    赞皇变质杂岩区位于阜平杂岩南部,地处华北克拉通中部造山带的中段,和中部带北段杂岩一样,是洞悉华北克拉通前寒武纪基底构造演化历史的一个重要窗口.研究区变基性岩可分为斜长角闪岩和角闪斜长片麻岩两种,二者均以似层状方式产于黑云斜长片麻岩或长英质片麻岩中,斜长角闪岩亦可呈透镜状,二者后期与围岩一起共同经历了高角闪岩相变质作用.斜长角闪岩和角闪斜长片麻岩中普遍存在变质锆石,锆石SIMS U-Pb原位定年获得的207Pb/206Pb谐和年龄表明,赞皇变基性岩记录了约18.5亿年(1842±21 Ma、1868±29 Ma)的一次较为广泛的变质作用事件,结合赞皇变质杂岩的构造背景和变质演化特征,推测该期变质作用事件与古元古代末期华北克拉通东部陆块和西部陆块间的俯冲碰撞作用有关.%The Zanhuang metamorphic complex is significant for understanding the Precambrian tectonics and evolution of the North China Craton. Metabasic rocks composed of amphibolite and amphibole-bearing plagio-clase gneiss as thin layers or lenses are widely distributed within felsic gneisses and biotite-plagioclase gneisses. These metabasic rocks together with country rocks underwent high-amphibolite facies metamorphism, and abundant metamorphic zircons were formed during this thermal event. SIMS U-Pb chronological analysis indicates that metamorphic zircons record peak high-amphibolite facies metamorphism at 1 868~1 842 Ma. These Paleo-proterozoic metamorphic ages are largely in accordance with metamorphic ages of ~ 1 850 Ma produced from high-pressure granulites in the northern segment of the Trans-North China Orogen, indicating that a significant subduction-collision event did occur in the Trans-North China Orogen between the eastern block and the western block during Paleoproterozoic ( ~ 1 850 Ma).

  12. 广东连平大顶铁矿区石背岩体 LA-ICP-MS锆石 U-Pb 年龄及地质意义%LA-ICP-MS zircon U-Pb dating of Shibei pluton at Dading iron mine area in Lianping,Guangdong and its geological significance

    林小明; 李宏卫; 黄建桦; 娄峰


    Dading iron mine in Lianping,Guangdong province is well known as the largest iron ore de-posit,and is also the most typical skarn type deposit in South China.The LA-ICP-MS zircon U-Pb age of coarse-grain porphyritic biotite granite and medium-grain porphyritic biotite granite from Dading pluton, closely related to mineralization in Dading mining area,are (174.3 ±3.6)Ma (MSWD =1.6)and (176.9 ±4.3)Ma (MSWD =2.6),respectively,and it shows the emplacement age of Shibei pluton belonged to the early Jurassic-middle Jurassic,and it was the product of the first magma activity of yans-hanian period.The Shibei pluton was formed in extrusion tectonic setting,in the process of subduction of the Pacific plate under Eurasian plate in the early Mesozoic.Its source mainly from crust,formed by crustal material melting and magma consolidation,similar to the Fogang pluton,is crustal transformation type (S-type)granite.%广东连平大顶铁矿被誉为华南第一大铁矿床,为华南地区最为典型的矽卡岩型矿床。对大顶铁矿区与成矿作用有关的石背岩体粗粒斑状黑云母花岗岩和中粒斑状黑云母花岗岩进行 LA-ICP-MS 锆石 U-Pb 定年,获得的同位素年龄分别为(174.3±3.6)Ma (MSWD =1.6)和(176.9±4.3)Ma (MSWD =2.6),表明石背岩体侵位时代为早侏罗世晚期-中侏罗世早期,是燕山第一幕岩浆活动之产物。石背岩体形成于早中生代古太平洋板块向欧亚板块俯冲过程中的挤压构造背景,其物源主要来自于地壳,为壳内物质熔融-岩浆固结而成,与佛冈岩体相似,均为陆壳改造型(S -型)花岗岩。

  13. Dating prograde fluid pulses during subduction by in situ U-Pb and oxygen isotope analysis

    Gauthiez-Putallaz, Laure; Rubatto, Daniela; Hermann, Jörg


    The Dora-Maira whiteschists derive from metasomatically altered granites that experienced ultrahigh-pressure metamorphism at ~750 °C and 40 kbar during the Alpine orogeny. In order to investigate the P-T-time-fluid evolution of the whiteschists, we obtained U-Pb ages from zircon and monazite and combined those with trace element composition and oxygen isotopes of the accessory minerals and coexisting garnet. Zircon cores are the only remnants of the granitic protolith and still preserve a Permian age, magmatic trace element compositions and δ18O of ~10 ‰. Thermodynamic modelling of Si-rich and Si-poor whiteschist compositions shows that there are two main fluid pulses during prograde subduction between 20 and 40 kbar. In Si-poor samples, the breakdown of chlorite to garnet + fluid occurs at ~22 kbar. A first zircon rim directly overgrowing the cores has inclusions of prograde phlogopite and HREE-enriched patterns indicating zircon growth at the onset of garnet formation. A second main fluid pulse is documented close to peak metamorphic conditions in both Si-rich and Si-poor whiteschist when talc + kyanite react to garnet + coesite + fluid. A second metamorphic overgrowth on zircon with HREE depletion was observed in the Si-poor whiteschists, whereas a single metamorphic overgrowth capturing phengite and talc inclusions was observed in the Si-rich whiteschists. Garnet rims, zircon rims and monazite are in chemical and isotopic equilibrium for oxygen, demonstrating that they all formed at peak metamorphism at 35 Ma as constrained by the age of monazite (34.7 ± 0.4 Ma) and zircon rims (35.1 ± 0.8 Ma). The prograde zircon rim in Si-poor whiteschists has an age that is within error indistinguishable from the age of peak metamorphic conditions, consistent with a minimum rate of subduction of 2 cm/year for the Dora-Maira unit. Oxygen isotope values for zircon rims, monazite and garnet are equal within error at 6.4 ± 0.4 ‰, which is in line with closed

  14. Determination of U-Pb age and rare earth element concentrations of zircons from Cenozoic intrusions in northeastern China by laser ablation ICP-MS

    YUAN Honglin; WU Fuyuan; GAO Shan; LIU Xiaoming; XU Ping; SUN Deyou


    Using the in situ zircon U-Pb dating method of LA-ICPMS, we analyzed the 31 Ma old SHRIMP U-Pb age of the Yongsheng nepheline syenite from southern Jilin Province under different spot sizes. The obtained ages are comparable with that of SHRIMP in both accuracy and precision. The age is also identical to that of the Yinmawanshan gabbro from the Liaodong Peninsula within error. Both the Yongsheng nepheline syenite and the Yinmawanshan gabbro represent the youngest known exposed intrusions in northeastern and even eastern China. The results indicate the Eocene mantle-derived magmatic underplating, and the rapid crustal uplifting of this region since 30 Ma. The analyses also document extremely high LREE concentrations and relatively flat REE patterns for the zircons from the Yongsheng nepheline syenite, which represent a new type of zircon REE pattern.

  15. Evidence for multi-cycle sedimentation and provenance constraints from detrital zircon U-Pb ages: Triassic strata of the Lusitanian basin (western Iberia)

    Pereira, M. F.; Gama, C.; Chichorro, M.; Silva, J. B.; Gutiérrez-Alonso, G.; Hofmann, M.; Linnemann, U.; Gärtner, A.


    Laser ablation ICP-MS U-Pb analyses were conducted on detrital zircons of Triassic sandstone and conglomerate from the Lusitanian basin in order to: i) document the age spectra of detrital zircon; ii) compare U-Pb detrital zircon ages with previous published data obtained from Upper Carboniferous, Ordovician, Cambrian and Ediacaran sedimentary rocks of the pre-Mesozoic basement of western Iberia; iii) discuss potential sources; and iv) test the hypothesis of sedimentary recycling. U-Pb dating of zircons established a maximum depositional age for this deposit as Permian (ca. 296 Ma), which is about sixty million years older compared to the fossil content recognized in previous studies (Upper Triassic). The distribution of detrital zircon ages obtained points to common source areas: the Ossa-Morena and Central Iberian zones that outcrop in and close to the Porto-Tomar fault zone. The high degree of immaturity and evidence of little transport of the Triassic sediment suggests that granite may constitute primary crystalline sources. The Carboniferous age of ca. 330 Ma for the best estimate of crystallization for a granite pebble in a Triassic conglomerate and the Permian-Carboniferous ages (< ca. 315 Ma) found in detrital zircons provide evidence of the denudation of Variscan and Cimmerian granites during the infilling of continental rift basins in western Iberia. The zircon age spectra found in Triassic strata are also the result of recycling from the Upper Carboniferous Buçaco basin, which probably acted as an intermediate sediment repository. U-Pb data in this study suggest that the detritus from the Triassic sandstone and conglomerate of the Lusitanian basin is derived from local source areas with features typical of Gondwana, with no sediment from external sources from Laurussia or southwestern Iberia.

  16. First U-Pb isotopic data on zircon from andesite of the Saf'yanovka Cu-bearing massive sulfide deposit (Middle Urals)

    Ronkin, Yu. L.; Pritchin, M. E.; Soroka, E. I.; Gerdes, A.; Puchkov, V. N.; Busharina, S. V.


    New results of U-Pb LA ICP-MS dating of zircon from andesite samples cropping out on the western wall of the Saf'yanovka quarry (57°22'58.88″ N, 61°31'50.85″ E) in the synonymous Cu-Zn-bearing massive sulfide deposit of the Urals type are considered. The position of data points of the U-Pb systematics in the 207Pb/235U-206Pb/238U plot determines a cluster practically corresponding to the concordant U-Pb age: 422.8 ± 2.0 Ma. This date indicates for the first time the presence of Pridolian volcanogenic rocks in the East Urals megazone of the Middle Urals.

  17. First U-Pb detrital zircon ages from Numidian sandstones in Southern Apennines (Italy): Evidences of African provenance

    Fornelli, Annamaria; Micheletti, Francesca; Langone, Antonio; Perrone, Vincenzo


    Two samples of quartz-rich sandstones collected in the Numidian Flysch of Southern Apennines (Italy) have been studied to highlight the provenance of detritus using radiometric dating by LA-ICP-MS of detrital zircons and to compare the obtained ages with those of the Betic and Maghrebian Chains. The provenance of quartzose detritus from European or African Plates is still debated in these Chains, accordingly the ages of the detrital zircons can contribute significantly to discriminate the origin of the quartzose supply. The U-Pb zircon ages (n = 47) vary from 3047 ± 13 Ma (Mesoarchean) to 516 ± 19 Ma (Cambrian). The predominance of Paleo-Proteozoic ages (2500-1600 Ma) and the lack of Hercynian and Alpine ones suggest a provenance of the Numidian supply from North-African cratonic areas during the early-middle Langhian, when the Numidian successions of Southern Apennines were deposited. In addition, a cluster of ages at 773 ± 24 Ma and 668 ± 12 Ma in one sample and at 664 ± 17 Ma in the other sample, calculated on zircon domains with magmatic zoning, testify to an important contribution from Neo-proterozoic "granitic" rocks widely outcropping in the North-African Craton. The age data on detrital zircons from Numidian sandstones in Southern Apennines overlap those found in the Numidian sandstones widespread in the Betic Cordillera and in the Maghrebian Chain from south Spain to Sicily. This suggests that the entire depositional zone in which Numidian Flysch deposited, was fed from a southerly source represented by the African Craton where Archean, Proterozoic and Cambrian rocks widely crop out from the Atlantic coast to the Hoggar and Tibesti Massifs. Finally, it must be outlined that a Meso-Archean zircon age (3047 Ma) has been found in the Numidian Flysch of the Southern Apennines whereas in the Numidian Flysch of the Maghrebian Chain, zircons older than Paleo-proterozoic (1840 Ma) have not yet been found.

  18. Provenance records of the North Jiangsu Basin,East China:Zircon U-Pb geochronology and geochemistry from the Paleogene Dainan Formation in the Gaoyou Sag

    Chun-Ming; Lin; Xia; Zhang; Ni; Zhang; Shun-Yong; Chen; Jian; Zhou; Yu-Rui; Liu


    Detailed zircon U-Pb dating and whole-rock geochemical analyses were carried out on the sedimentary rocks of the Paleogene Dainan Formation from Gaoyou Sag in the North Jiangsu Basin,East China.Whole-rock rare earth element characteristics suggest that the provenance was mainly from the Late Proterozoic low-grade metamorphic felsic rocks in the Dabie-Sulu orogenic belt,with the parent rocks probably being the I-type high-potassium granite gneiss.Cathodoluminescence images indicate that most of the detrital zircons are originally magmatic.A few zircons show overgrowths,indicating multiple-episode tectonic events.The U-Pb age distribution patterns of the detrital zircons suggest four main magmatic episodes in the provenance:Late Archean-Early Proterozoic(2450-2600 Ma),Early Proterozoic(1700-1900 Ma),Late Proterozoic(700-850 Ma),and Late Paleozoic-Mesozoic(100-300 Ma).These zircon U-Pb age and whole-rock geochemical results suggest that the sediments of the Dainan Formation were mainly sourced from the recycled orogenic belts within and/or around the North Jiangsu Basin,including the basement of the Yangtze Block,the Neoproterozoic rocks in the Dabie-Sulu orogenic belt,and the Mesozoic igneous rocks in the south part of Zhangbaling Uplift.

  19. Molybdenite Re/Os dating, zircon U-Pb age and geochemistry of granitoids in the Yangchuling porphyry W-Mo deposit (Jiangnan tungsten ore belt), China: Implications for petrogenesis, mineralization and geodynamic setting

    Mao, Jingwen; Xiong, Bikang; Liu, Jun; Pirajno, Franco; Cheng, Yanbo; Ye, Huishou; Song, Shiwei; Dai, Pan


    The Yangchuling W-Mo deposit, located in the Jiangnan porphyry-skarn (JNB) tungsten ore belt, is the first recognized typical porphyry W-Mo deposit in China in the 1980's. Stockworks and disseminated W-Mo mineralization occur in the roof pendant of a 0.3 km2 monzogranitic porphyry stock that intruded into a granodiorite stock, hosted by Neoproterozoic phyllite and slate. LA-ICPMS zircon U-Pb analyses suggest that of the monzogranitic porphyry and granodiorite were formed at 143.8 ± 0.5 Ma and 149.8 ± 0.6 Ma, respectively. Six molybdenite samples yielded a Re-Os weighted mean age of 146.4 ± 1.0 Ma. Geochemical data show that both granodiorite and monzogranitic porphyry are characterized by enrichment of large ion lithophile elements (LILE) relative to high field strength elements (HFSE), indicating a peraluminous nature (A/CNK = 1.01-1.08). Two granitoids are characterized by a negative slope with significant light REE/heavy REE fractionation [(La/Yb)N = 8.38-23.20] and negative Eu anomalies (Eu/Eu* = 0.69-0.76). The P2O5 contents of the Yangchuling granitoids range from 0.12% to 0.17% and exhibit a negative correlation with SiO2, reflecting that they are highly fractionated I-type. They have high initial 87Sr/86Sr ratios (0.7104-0.7116), low negative εNd(t) (- 5.05 to - 5.67), and homogeneous εHf(t) between - 1.39 and - 2.17, indicating similar sources. Additionally, two-stage Nd model ages (TDM2) of 1.3-1.4 Ga and two-stage Hf model ages (TDM2) of 1.2-1.3 Ga are consistent, indicating that Neoproterozoic crustal rocks of the Shuangqiaoshan Group could have contributed to form the Yangchuling magmas. Considering the two groups of parallel Late Mesozoic ore belts, namely the Jiangnan porphyry-skarn tungsten belt (JNB) in the south and the Middle-Lower Yangtze River porphyry-skarn Cu-Au-Mo-Fe ore belt (YRB) in the north, the Nanling granite-related W-Sn ore belt (NLB) in the south, the neighboring Qin-Hang porphyry-skarn Cu-Mo-hydrothermal Pb-Zn-Ag ore belt (QHB

  20. SHRIMP U-Pb geochronology of the detrital zircons from the Longshoushan Group and its tectonic significance

    KuoAn TUNG; HoungYi YANG; LIU DunYi; ZHANG JianXin; ChienYuan TSENG; WAN YuSheng


    Sixty-two geologically meaningful U-Pb dates were obtained by using SHRIMP technique for the detrital zircons in three mettasedimentary rocks from stratigraphically uppermost parts of the Longshoushan Group in the present study.Eighty percents of these dates range from 1.7 Ga to 2.2 Ga with a peak at 1.8-2.0 Ga and twenty percents from 2.3 Ga to 2.7 Ga.The youngest detrital zircon is dated at 1724±19Ma which is interpreted as the maximum depositional age of the metasedimentary rocks.Therefore,the age for the diagenesis and lithification of the original sedimentary rocks of the Longshoushan Group before the metamorphism must be younger than 1724±19 Ma.Comparison of the age histograms of these detrital zircons with the ages of the igneous rocks on the surrounding older massifs suggests that the sediments of the Longshoushan Group were most likely derived from the Alaxa Block and Tarim Craton.This implies that the affinity between Alaxa Block and Tarim Craton was strong and that they might have been a unified craton during middle-early Proterozoic time

  1. LA-MC-ICP-MS zircon U-Pb dating of Longxianggai pluton in Dachang of Guangxi and its geological significance.%广西大厂笼箱盖复式岩体的LA-MC-ICP-MS锆石U-Pb年龄及其地质意义

    梁婷; 王登红; 侯可军; 李华芹; 黄惠明; 蔡明海; 王东明


    The Longxianggai pluton is a multiple-intruded in the middle of the Dachang ore field of Guangxi. Although the former researchers had carried out some dating on the rock intruded in some stages, so far systematic chronology study about its multi-stage magmatic intrusions has not done. Based on analyzing the interior structures of zircons by the CL technology, in-situ zircon U-Pb dating was performed with LA-MC-ICP-MS and reliable 2O6 Pb/238 U age data in fine-grained zircon were obtained. The results show that Longxianggai pluton was formed in three staged, that is, 103. 8 ~ 102Ma,96. 6-93. 86Ma and 91 -85. Ima, and 96. 6 ~93. 86Ma is the main magmatic period. In the first stage, medium-fine-grained biotite granite with porphyritic (96. 6 ±2. 5Ma) and fine-grained biotite granite with porphyritic (94.3 ±2. 2Ma) was formed. In the second stage, medium-fine-grained equigranular biotite granite (96. 1 ±2. Oma). In the third stage, the porphyraceous biotite granite (93. 86 ±0. 84Ma) was formed. The boundary between the rocks is clear and has no chilled margin, which indicates that magmatism forming Longxiagngai pluton is a continuous pulse process. The formation of zinc-copper ore body and tin-polymetallic ore body is closely related with the second magmatic period of the Longxianggai pluton and the mineralization process was completed in a short time. This information period of the Longxianggai pluton is the same as the large-scale mineralization of 110 ~ 80Ma in eastern China, and its digenesis and mineralization may be related to lithosphere extension.

  2. SHRIMP zircon U-Pb dating of the Huangfengzhai pluton in eastern Nanling and its geological implications%南岭东段黄峰寨岩体SHRIMP锆石U-Pb年龄及地质意义

    范飞鹏; 肖惠良; 陈乐柱; 蔡逸涛; 李海立; 鲍晓明; 周延


    黄峰寨岩体位于南岭钨锡多金属成矿带的东段,对该岩体中的石英二长岩进行 SHRIM P 锆石 U‐Pb 年龄测定、岩石学和地球化学研究。石英二长岩的 SHRIMP 锆石 U‐Pb 年龄为231±3 Ma ,属中三叠世晚期。岩石学和地球化学特征表明,该岩体具有高硅、富铝、高钾、中碱、高钙的特征,属高钾钙碱性系列弱过铝质 S 型花岗岩类;微量元素 Sr 、Ba 、Nb 、P 、Ti 强烈亏损,Rb 、Th 、Ce 、Zr 、Sm 相对富集;稀土元素总量高,轻稀土富集,轻、重稀土分异明显,Eu 中等负异常;Hf(t)为-15.25~-2.88,Hf 两阶段模式年龄值为1.87~2.22Ga ,表明黄峰寨岩体为上地壳底部基性岩部分熔融的产物,推测该岩体可能形成于同碰撞构造环境。%The Huangfengzhai pluton is located in the eastern section of tungsten‐tin polymetallic met‐allogenic belt in the Nanling region .This study carried out SHRIM P zircon U‐Pb dating and lithological and geochemical anlyzses for quartz monazites from the Huangfengzhai pluton .The results show that a zircon U‐Pb age of quartz monzonite from the Huangfengzhai pluton is 231 ± 3 Ma ,belonging to late Mid‐dle Triassic .Lithological ,mineralogical and geochemical analyses suggest that the pluton is rich in Si ,Al and Ca ,with medium alkali ,indicating that the pluton belongs to high‐K ,calc‐alkaline series ,weakly‐per‐luminous S‐type granitoids .The pluton is also depleted in Sr ,Ba ,Nb ,P and Ti ,but relatively enriched in Rb ,Th ,Ce ,Zr and Sm ,with high ΣREE content ,distinct LREE and HREE differentiation ,and weak negative Eu anomalies .Zircon εHf(t) values range from - 15 .25 to - 2 .88 and Hf two‐stage model age values are 1 .87 ~ 2 .22 Ga ,indicating that the Huangfengzhai pluton was derived from partial melting of mafic rocks at the bottom of upper crust .It can be inferred that the Huangfengzhai pluton might form at a

  3. U-Pb zircon age of the foliated garnet-bearing granites in western Dabie Mountains,Central China


    U-Pb zircon dating on two foliated gar net-bearing granite samples in the western Dabie ultra high-pressure (UHP) metamorphic unit yields concordant ages of (234±4) Ma and (227±5) Ma, respectively. These ages, following the UHP peak metamorphism, represent the magma emplacement ages for the foliated garnet-bearing granites. This, for the first time, shows that there are the Triassic granites in the Dabie Mountains. The foliated gar net-bearing granites resemble A-type granite in geochemical characteristics, indicating that they were formed in exten sional geodynamic setting. The magma formation reflects a reheating event in the Dabie orogenic belt and it enhances the transfer of tectonic regime from collision into extension and promotes the rapid exhumation into lower crust for the UHP metamorphic rocks.

  4. Resolving high precision U-Pb ages from Tertiary plutons with complex zircon systematics

    Mcclelland, William C.; Mattinson, James M.


    Success in establishing high precision crystallization ages with the U/Pb zircon method is highly dependent on the ability to isolate or remove the effects of inherited xenocrystic components or secondary lead-loss, or the combined effects of both. Current "conventional" single crystal and ion probe (SHRIMP) methods are successfully applied to Paleozoic and older samples but typically suffer from imprecision when applied to younger (Mesozoic-Cenozoic) samples due to generally reduced radiogenic lead concentrations. Here we apply a series of intense partial dissolution steps to multigrain zircon fractions from a series of Paleocene tonalitic to granodioritic intrusive rocks from southeastern Alaska. The zircon systematics are complicated by both minor inheritance and postcrystallization lead-loss. Physical and isotopic evidence demonstrate that the partial dissolution steps preferentially remove outer layers susceptible to lead loss, as well as core regions containing inherited components. The final residues are often hollow shells of low-U primary igneous zircon that yield highly precise, reproducible, and concordant ages. The resulting age determinations commonly statistically differ at the 95% confidence level from ages based on apparently concordant, but less precise conventional isotope dilution analyses, the uncertainties of which masked minor, subtle isotopic complexities. This observation strongly cautions against basing age assignments of samples yielding slightly discordant data on (1) single "concordant" analyses accompanied by an array of discordant data or (2) the mean of several 206Pb/ 238U or 207Pb/ 206Pb ages. The step-wise dissolution technique allows age determinations on young, relatively low U and Pb zircons at a resolution not currently possible with techniques such as single-grain conventional or spot ion probe analyses. Widespread application of the technique will likely prove instrumental in resolving detailed magmatic histories of igneous

  5. Zircon LA-ICP-MS U-Pb dating of amphibolite in the southern margin of Altyn Tagh,China and its geological implication%阿尔金山南缘清水泉地区斜长角闪岩锆石LA-ICP-MS U-Pb测年及其地质意义

    唐卓; 马中平; 李向民; 孙吉明


    Altyn Tagh Group is the oldest metamorphosed strata in the Altyn Tagh area. Because of lack of precise isotopic dating,some researchers porposed that this Group formed in Neoproterozoic and there were not existence of Neoarchean to Paleoproterozoic metamorphosed basement in Altyn Tagh area. This paper provides the zircon LA-ICP-MS U-Pb isotopic dating result of the amphibolite from Altyn Tagh Group in the southern margin of Altyn Tagh and discusses its geological significance. The CL images of zircons from the amphibolite show that most of the zircons present the typical characteristics of metamorphic zircons with multi-faceted shape structure and without girdle structure and core-rim structure of magmatic zircons and with lower Th/U ratios. Zircon LAICP-MS U-Pb isotopic dating give a weighted mean 207Pb/206Pb age of 1827±13Ma(MSWD=4.4), with 207Pb/206Pb ages ranging from (1786±10) Ma to (1877±12) Ma of 19 analysing plots and suggest that the epoch of metamorphism of the amphibolite is Later Paleoproterozoic. The new data of isotopic chronology indicated that there existed a tectothermal event which corresponds to Ltiliang orogenic movement in China and maybe related to the converging or breakup event of global Columbia suppercontinent in Paleoproterozoic. The result also indicate that there are Neoarchean-Paleoproterozoic metamorphosed basement in the Altyn Tagh area and provide a new data for research Precambrian teotothermal events and the evolution of Altyn Tagh belt.%阿尔金群为阿尔金构造带的古老变质基底,由于缺乏精确的同位素测年数据,对其形成时代尚存争议.运用LA-ICP-MS锆石U-Pb定年分析方法,对阿尔金山南缘清水泉地区阿尔金群中的斜长角闪岩进行了年代学测定.并初步探讨其地质意义.锆石CL图像具有多晶面面状结构,无岩浆锆石具有的震荡环带和核边结构.Th/U比值大多都较低(<0.1),显示变质锆石的特点.18个锆石颗粒的19个测点给出的207Pb

  6. Multi-stage evolution of gneiss from North Dabie:evidence from zircon U-Pb chronology

    XIE Zhi; GAO Tianshan; CHEN Jiangfeng


    TTG gneiss is a common rock to outcrop in the northern part of the Dabie orogen, a few of which are closely associated with eclogites that experienced the Triassic ultrahigh pressure metamorphism. Although they were thermally metamorphosed by a large-scale magma activity in this region at the Early Cretaceous, it is unclear whether or not they are also affected by the Triassic metamorphism during continental subduction and exhumation. In order to resolve this issue, SHRIMP zircon U-Pb dating was carried out for the host gneiss of eclogites in North Dabie. The results show that cores from the gneiss have an age of 746±31 Ma, consistent with the protolith ages of granitic gneisses in the Dabie orogen. Zircon overgrowing with different U and Th concentrations give concordant ages of 212±21 and 120±11 Ma, respectively. Th/U ratios of overgrown zircons are both lower than 0.1, suggesting a metamorphic genesis. The present results suggest that the gneiss in North Dabie has the similar protolith ages of Neoproterozoic to those granitic gneisses elsewhere in the Dabie orogen, and experienced not only the Triassic metamorphism but also the thermal metamorphism due to the Early Cretaceous magmatism. This provides an important insight into the geodynamic evolution of gneissic rocks in the Dabie orogen.

  7. LA-ICP-MS zircon U-Pb dating of the gold-bearing quartz veins in the Bainaimiao Au deposit of Inner Mongolia and its geological significance%内蒙古白乃庙金矿含金石英脉LA-ICP-MS锆石U-Pb测年及其地质意义

    冯晓曦; 姚书振; 段明; 王佳营; 曲凯; 魏佳林; 李光耀; 朱强


    Cathodoluminescence image (CL) and zircon LA-ICP-MS U-Pb dating techniques were used to study the metallogenic age of the Bainaimiao gold deposit.According to the results obtained, all zircons from the gold-bearing quartz veins show clear core-mantle-rim texture, the core-mantles are of magmatic and metamorphic characteristics,the rims of zircons show the features of hydro-thermal zircon with clear core-mantle-rim texture, non-luminescent rim, and euhedral and subhedral crystals, Th/U ratios vary in the range of 0.2~0.5,and the results of the rim dating are 421.4~447.6Ma, suggesting that the Bainaimiao gold deposit formed in Si-lurian. The ages are accordant to ages of granodiorite porphyry, which has similar zircon core-mantle CL features. It is thus held that the gold deposit was closely related to tectonic-magmatism, post-magmatic hydrothermal activities and durative orogenesis resulting from the intense subduction of Early Paleozoic paleo-Asian oceanic plate towards the northern margin of North China craton.%为了厘定内蒙古白乃庙金矿床的成矿时代,对矿区主要含金石英脉中热液蚀变锆石进行了LA-ICP-MS U-Pb测年。结果表明,锆石热液蚀变特征明显,具有清晰的核-幔-边结构,锆石核、幔均为岩浆锆石或变质锆石,热液蚀变增生边呈自形-半自形结构,Th/U值集中于0.2~0.5之间,测年数据为421.4~447.6Ma,认为金矿形成时代为志留纪。从区域成矿规律分析,初步认为白乃庙金矿床的形成与早古生代古亚洲板块向华北地块强烈俯冲引起的构造-岩浆活动密切相关。

  8. Geochemistry and U-Pb zircon geochronology of Late-Mesozoic lavas from Xishan, Beijing

    YUAN; Honglin; LIU; Xiaoming; LIU; Yongsheng; GAO; Shan; LING; Wenli


    Zircon U-Pb dating by both SHRIMP and LA-ICP-MS and geochemical study of the Tiaojishan Formation and the Donglintai Formation from Xishan, Beijing, reveal that ages of upper lavas of Tiaojishan Formation and Middle of Donglintai Formation are 137.1±4.5 Ma(2σ) and 130-134 Ma, respectively. The fomer is slightly older than the latter and the age difference between these two formations is less than 5 Ma. These lines of evidence prove that the two volcanoes erupted within a short time. The age of the Tiaojishan Formation from Xishan, Beijing is distinctively different from that of the Chende Basin. This indicated that the ages of Tiaojishan lavas varied in different regions. The Tiaojishan Formation consists of typical adakite (SiO2=56%, Na2O = 3.99-6.17, Na2O/K2O = 2.2-3.1, Sr = 680-1074×10-6, Y = 13.2-16.3×10-6, Yb = 1.13-1.52×10-6, Sr/Y = 43-66), high-Mg adakite and high-Mg andesite (Mg# = 54-55). Features of continental crust of adakite from the Tiaojishan Formation and its syngeneric middle silicic vocanic rocks, such as typical Nd-Ta negative abnormality and Pb possive abnormality, indicate that these lavas are originated from partial melts of continental crust. These results suggest that the adakite from the Tiaojishan Formation of Xishan, Beijing derived from thickened eclogitic lower crust and lithosphere beneath the North China craton at mesozoic that was foundered into the aesthenosphere, and subsequenctly partially melted and interacted with mantle olivine during melts upward migration. The age of lavas from the Tiaojishan Formation restrained the foundation which should last at least until 137 Ma. Lavas of the Donglintai Formation are rhyolith and andesite with normal Mg# and thus they did not interact with the mantle. These lavas represent remobilized melts of lower crust material caused by mantle aesthenosphere upwelling migration induced by foundation.

  9. Zircon U-Pb Dating of the Lamprophyre in the Xikuangshan Mining District, Central Hunan and its Geological Implications%湘中锡矿山矿区煌斑岩中捕获锆石U-Pb定年及其地质意义

    彭建堂; 胡阿香; 张龙升; 雷文艳; 阳杰华; 林芳梅


    The giant Xikuangshan deposit, located in the Xiangzhong Basin, Central Hunan, is the largest antimony deposit in the world. In spite of numerous detailed geological investigations and geochemical researches have been carried out, the sources and tectonic background of the giant ore deposit is unclear. The lamprophyre dyke occurring in the eastern part of the mining district probably can shed light on these issues. LA-ICP-MS U-Pb isotopic dating has been conducted on zircon grains collected from the lamprophyre dyke in the Xikuangshan mining district. The zircon grains are usually subhedral or sub-rounded, and exhibit typical oscillatory or sector zoning under cathodoluminescence. Both its the morphology and geochemical signatures (Th/U ratio and REE chondrite-normalized distribution pattern) of these zircon grains reveals that they are predominately of magmatic origin, and were probably captured from the wall rocks during magma ascending. The zircon U-Pb ages mainly vary in the range of 830 Ma to 800 Ma, which coincide well with the ages of the Neoproterozoic Lengjiaxi Group and Banxi Group occurred in the adjacent Xufeng Range, implying the possible existence of underlying Precambrian clastic basement in the Xiangzhong Basin. The inferred Precambrian basement lays foundation for the further understanding for tracing the ore-forming metal of the giant Xikuangshan deposit and determining the tectonic nature of the Xiangzhong Basin. The Neoproterozoic basement underlying in the Xiangzhong Basin might have provided Sb for the giant ore accumulation in the Xikuangshan mining district. Tectonically, the Xiangzhong Basin should belong to the Yangtze Block rather than the Cathaysian Block as was previously considered. The lamprophyre in the Xikuangshan mining district formed under the geodynamic setting of crustal extension, and emplaced at a depth of at least 5 kilometers.%利用 LA-ICP-MS 技术,对湘中锡矿山煌斑岩中的锆石进行 U-Pb 同位素定年。研

  10. Geochemistry And U-Pb Dating Of The Magmatism In Eastern Srednogorie Zone, SE Europe

    Georgiev, S.; von Quadt, A.; Heinrich, C. A.; Peytcheva, I.


    In the Carpathian-Balkan segment of southeastern Europe a prominent magmatic belt of Upper Cretaceous calc-alkaline rocks extends from southern Romania through eastern Serbia and all across Bulgaria. This more than 1000 km belt is known as the ABTS (Apuseni-Banat-Timok-Srednogorie) magmatic and metallogenic belt and it hosts most of the major Cu-(Au) porphyry deposits in Europe. The formation of this belt is broadly related to the northwards subduction of former Tethyan Ocean beneath the European continental margin, but the complex regional geodynamic evolution remains not well understood. We present new geochemical and geochronological data for the Eastern Srednogorie zone - the easternmost parts of the ABTS belt that displays specific features in terms of tectonics, magmatism and metallogeny. Magmatism in this zone is the most voluminous, basic and K-rich, compared to the other parts of the ABTS belt. Mafic igneous rocks prevail in this zone with mostly intrusive varieties to the south and mostly volcanic rocks in the northern part. Across the belt from south to north a compositional change can be traced from tholeiitic, Ca-alkaline, high-K Ca-alkaline, shoshonitic to high-K transitional rocks. This is interpreted to represent different parts of the former island-arc system: from south to north the fore-arc, axial part and back- arc part. The absolute abundances and ratios of trace and REE elements are consistent with this petrochemical zonality. Trace element chondrite normalized patterns display a marked Nb-Ta trough. The REE elements patterns show light-REE enrichment and a relatively weak Eu anomaly. We conducted LA-ICPMS and high precision TIMS U-Pb dating of single zircon grains from different plutons in the southern intrusive part. The results show that the intrusive bodies crystallized during the Campanian in a relatively narrow time span: from 81 to 78 Ma, with the exception of one diorite (Monastery Heights) pluton dated at 86 Ma. Inherited concordant

  11. U-Pb (Zircon Ages of Metavolcanic Rocks From the Itaiacoca Group: Tectonic Implications

    Oswaldo Siga Jr.


    Full Text Available The main aim of this work is to present and discuss the U-Pb ages obtained for zircon grains from metavolcanic rocks of theItaiacoca Group. The Itaiacoca Group is a metavolcano-sedimentary sequence, which occurs as a narrow belt between theCunhaporanga granitic batholith to the northwest and the Itapirapuã shear zone to the south and southwest, which separates thesequence from the Três Córregos granite batholith and metasedimentary rocks of the Açungui Group. Geological studies of thesouthern part of the Itaiacoca belt led to the recognition of three units, represented (from base to top by metawackes with animportant volcanic component, metacarbonate, and metapelitic and metapsammitic rocks. The U-Pb geochronological analyses ofzircon grains from two outcrops of metavolcanic rocks yield ages of 628 ± 18 Ma (SHRIMP and 636 ± 30 Ma (conventional multigrainanalyses. These ages are quite close to the metamorphic event recorded in the Itaiacoca Group (628 – 610 Ma, suggesting ashort interval between the formation of these rocks and closure of the basin. Furthermore, this volcanism is very close to the age offormation of the Três Córregos (630 Ma and Cunhaporanga (590 Ma granitic batholiths, admitted as associated with a probablemagmatic arc. Such an isotopic pattern characterizes a Neoproterozoic tectonic scenario involving volcanism, metamorphism andgranitic plutonism, interpreted here as the final stages in the evolution of the Itaiacoca Basin.

  12. SHRIMP U-Pb zircon age of the youngest exposed pluton in eastern China


    Analysis using the Sensitive High Resolution Ion Microprobe (SHRIMP) shows that the Yongsheng pluton, located to the south of Huadian County, Jilin Province, in the North China Craton, has a zircon U-Pb isotopic emplacement age of (31.6±13) Ma. It is therefore the youngest exposed pluton so far recognized in eastern China. Although geochemical data indicate intensive crystal fractionation, the Sr-Nd isotopic features suggest that the magmatic source region of the lithospheric mantle was weakly depleted. This implies that the previous enriched lithospheric mantle had been replaced by juvenile asthenospheric mantle before or during the Palaeogene. This recognition has great significance for future work on the Mesozoic-Cenozoic geological evolution and deep geological processes in eastern China.

  13. A polyphase metamorphic evolution for the Xitieshan paragneiss of the north Qaidam UHP metamorphic belt, western China: In-situ EMP monazite- and U-Pb zircon SHRIMP dating

    Zhang, C.; Roermund, H.L.M. van; Zhang, L.; Spiers, C.


    In-situ electron microprobe (EMP) U–Th–Pbmonazite-, sensitive high-resolution ion microprobe (SHRIMP) zircon analyses, metamorphic phase equilibrium (Domino/Theriak)- and geothermobarometric calculations are performed on kyanite/sillimanite-bearing garnet biotite gneisses forming part of the dominan

  14. SHRIMP zircon U-Pb ages and tectonic implications of igneous events in the Ereendavaa metamorphic terrane in NE Mongolia

    Miao, Laicheng; Zhang, Fochin; Baatar, Munkhtsengel; Zhu, Mingshuai; Anaad, Chimedtseren


    The Ereendavaa metamorphic terrane in NE Mongolia has long been considered as a Pre-Altaid block or a Precambrian cratonic terrane with a Paleoproterozoic basement overlain by Neoproterozoic-Cambrian rocks, but the idea has not been supported by any isotopic dating. Sensitive high resolution ion microprobe (SHRIMP) zircon U-Pb dating on gneisses, amphibolite and schists (mylonites) of the Ereendavaa terrane suggests that the terrane mainly formed during Early Paleozoic (495-464 Ma) and Late Paleozoic-Early Mesozoic (295-172 Ma). A minor amount of Precambrian rocks might have been involved in the formation of the protoliths of these rocks, as shown by Precambrian inherited zircons (1796-794 Ma). The new age data also suggest that the Ereendavaa terrane experienced at least two periods of magmatism: (1) Early Paleozoic (495-464 Ma) and (2) Late Paleozoic-Early Mesozoic (295-172 Ma), which are probably produced by the subduction of the Paleo Asian Ocean in the south and the subduction of the Mongol-Okhotsk Ocean in the north, respectively. The mylonitized granite (172 Ma) and undeformed pegmatite (163 Ma) are interpreted to be syn- and post-kinematic products. The new age data constrain the closure of the Mongol-Okhotsk Ocean at mid-Jurassic.

  15. Detrital zircon U-Pb and Hf isotopic data from the Liuling Group in the South Qinling belt: Provenance and tectonic implications

    Liao, Xiao-ying; Wang, Ya-wei; Liu, Liang; Wang, Chao; Santosh, M.


    The Liuling Group is exposed in the Northern part of the South Qinling orogenic belt. LA-ICP-MS U-Pb analysis of detrital zircons from the meta-sandstones in this Group yields ages ranging between 400 Ma and 3200 Ma, with three prominent age clusters at 500-400 Ma, 850-700 Ma and 1000-900 Ma. A few older zircon populations with U-Pb ages of 1750-1450 Ma, 2000 Ma and 2600-2400 Ma are also present. Age data integrated with cathodoluminescence, trace element data and εHf(t) values of zircon grains show that the Liuling sediments have a complex source. Source rocks mainly include Early Neoproterozoic and Early Paleozoic granitoids, together with minor ultra-high pressure/high pressure (HP-UHP) metamorphic rocks, and paragneiss in the North Qinling belt, and Middle-Late Neoproterozoic magmatic rocks in the South Qinling belt. The dominant population of detrital zircon grains with ages between 500 Ma and 400 Ma show the characteristics of both magmatic and metamorphic zircons. They show three age clusters at 497 Ma, 451 Ma, and ca. 420 Ma and show marked correlation with the three stages of Palaeozoic magmatism, as well as with the peak and retrograde HP-UHP metamorphic stages in the North Qinling belt. This correlation demonstrates that these Early Palaeozoic granitoids and HP-UHP metamorphic rocks in the North Qinling belt were already exhumed to the surface, underwent erosion prior to Middle Devonian time and were then deposited in an extensional basin. Based on the results from detrital zircon U-Pb dating, combined with geochemical data and the regional geology, the deposition of Liuling sediments is inferred to have occurred in a post-orogenic extensional basin, rather than a subduction-related fore-arc basin or a foreland basin formed during or after continental collision.

  16. Paleoproterozoic basement beneath the southern Jiangxi Province: Evidence from U-Pb ages and Lu-Hf isotopes in zircons from the Doushui Iamprophyre

    YU JinHai; WANG LiJuan; O'REILLY S Y; SHU LiangShu; UN Tao


    This paper presents geochemical analyses of a lamprophyre intruding the Caledonian Doushui granite body in Shangyou County, southern Jiangxi Province. U-Pb dating and Hf-isotope analyses are especially carried out for zircons from it. Petrological and geochemical features show that the lamprophyre belongs to a high-K, weakly alkaline pyroxene-biotite iamprophyre. It is characterized by high Mg# (0.74),Ni (253 μg/g) and Cr (893 μg/g) contents, and also enriched in incompatible elements, such as REE, Rb, Sr, Ba and K. It is suggested that its primitive melt probably derived from enriched mantle metasomatized by non-crust-derived agent. Morphology and LAM-ICPMS dating results of zircons in the lam prophyre indicate that they are xenocrysts captured from deep crustal rocks. Most xenocrystic zircons formed at -1.86 Ga, and a few in different Phanerozoic periods. The Paleoproterozoic zircons are of magmatic origin and have similar 176Hf/177Hf and 176Lu/177Hf ratios, indicating that they probably are from the same igneous basement rock. The ages and Hf-isotope compositions of zircons from this basement rock are similar to those of Paleoproterozoic Danzhu granite in the southern Zhejiang Province, but extremely distinguished from the Nanling basement, suggesting that the study area probably is a westward extension part of Paleoproterozoic Wuyishan terrane in the eastern Cathaysia Block. These Paleoproterozoic zircons have low Hf-isotope compositions, characteristic of crustal source. Hf model ages of these zircons and the presence of older inherited cores within them, coupled with available other data reported by previous studies, suggest that the source of Paleoproterozoic magma is Neoarchaean crust, implying the existence of older basement in the Wuyishan terrane. U-Pb ages and Hf isotopes of five Phanerozoic zircons indicate that Paleoproterozoic basement underwent multi-reworking occurring in Caledonian, Indosinian and early Yanshanian, respectively. The Caledonian

  17. Zircon U?Pb ages in Myanmar: Magmatic?metamorphic events and the closure of a neo-Tethys ocean?

    Mitchell, Andrew; Chung, Sun-Lin; Oo, Thura; Lin, Te-Hsien; Hung, Chien-Hui


    Previous isotopic age studies have concluded that the main metamorphism in the Mogok Metamorphic belt (MMB) in Myanmar was Tertiary and caused by collision with or underthrusting by India. We present here results of zircon U-Pb age determinations on 18 samples from Myanmar, largely on intrusive rocks from the north-trending 550 km long Shan Scarps segment of the sickle-shaped MMB. The oldest determination is 491 Ma on orthogneiss north of Mandalay, interpreted as a protolith age and similar to the stratigraphic age of the Bawdwin Volcanics. Augen gneiss at Kyaukse has a 114 Ma protolith age. Our zircon U-Pb ages on undeformed intrusive rocks in the MMB range from Cretaceous to Miocene; the oldest (128 Ma), on a diorite near Yebokson, implies an earliest Cretaceous minimum age for regional metamorphism here. Younger ages on undeformed intrusions include 91 Ma on diorite at Mokpalin, 72 Ma on the weakly foliated garnet-bearing Nattaung granite, 44 and 48 Ma respectively on the MEC and Sedo granites, and 17-20 Ma on granite dykes. For the 128 Ma diorite, Sr and Nd isotopes indicative of a continental crust component, and mapped continuity of metamorphic host rocks, suggest correlation with mid early Cretaceous plutons in the eastern Transhimalayas. Zircon U-Pb ages for the MEC, Sedo and other granites within the MMB and Slate belt help to define a late Cretaceous to early Eocene age range for mostly ilmenite series peraluminous granites within the Western Granite belt and more restricted Western tin belt of Myanmar and southern Thailand. In the Wuntho-Popa arc west of the dextral Sagaing Fault, our new U-Pb zircon age of 105 Ma on diorite at Salingyi supports previous K-Ar ages (106-93 Ma) for pre-Tertiary intrusions. In the Cretaceous this arc may have occupied the Andaman Sea and continued northwards through the Mokpalin diorites and west of the MMB to the 103-80 Ma I-type plutons in the Gandise batholith of southern Tibet. We interpret the MMB and Slate belt as

  18. SHRIMP U-Pb zircon geochronology and its implications on the Xilin Gol Complex, Inner Mongolia, China

    SHI Guanghai; LIU Dunyi; ZHANG Fuqin; JIAN Ping; MIAO Laicheng; SHI Yuruo; TAO Hua


    The Xilin Gol Complex, consisting of deformed and metamorphosed rocks, was exposed as a large geological unit within the Central Asian Orogenic Belt, but its forming and subsequent deformed and metamorphic time has been an issue of little consensus. Petrographic analyses and SHRIMP dating on biotite-plagioclase gneiss, one of the major rocks within the Xilin Gol Complex, in southeast Xilinhot City, Inner Mongolia, China, where the Xilin Gol Complex was identified and named, yield its lower limit age of 437 ( 3 Ma (2--) by its magmatic zircon SHRIMP U-Pb dating, and an upper limit age of 316 ( 3Ma (2--), which was constrained by SHRIMP dating of magmatic zircons from adjacent undeformed garnet-bearing granite which intruded the Complex. The Complex was thus determined to be formed and subsequently deformed/metamorphosed from the late Ordovician-early Silurian to the mid-Carboniferous. Consequently, it is not the Precambrian terrane as previously considered by most geologists. More or less, the major rock--biotite- plagioclase gneiss within the Complex is more likely to be Paleozoic fore-arc turbidite formation before metamorphism and intensive deformation, in which the detrital zircons gave sporadic Precambrian ages as old as up to 3.1 Ga. The source of the turbidite formation is multiple, which may be derived either from the North China Craton, or from the South- Mongolia Micro-continent, or probably came from a potential and undiscovered in situ terranes aged 600-800 Ma or even up to ca 3.1 Ga near the Complex.

  19. Petrography and U-Pb zircon geochronology of Caledonian age orthogneisses dredged from the Chukchi Borderland, Arctic Ocean

    Brumley, K.; Miller, E. L.; Mayer, L. A.; Wooden, J. L.; Dumitru, T. A.


    The Caledonian orogenic belt developed in the Ordovician to Devonian affecting northern Europe, Greenland and Arctic Canada. Caledonian deformational trends continue into the Arctic region disappearing at the rifted margin of the Arctic Ocean. Syn-orogenic magmatism in the Barents region date deformation of Grenvillian basement during the Caledonian event to be between about 450-410 Ma (Johansson et al., 2005; Gee et al., 2006; Gee and Tebenkov, 2004). Over 500 kilos of metamorphic rock was dredged from outcrops in the central Chukchi Borderland in 2009 (HLY0905) that included amphibolites and orthogneisses metamorphosed to amphibolite facies continuing to deform under greenschist conditions. Zircons were separated from two orthogneiss samples from this dredge, and single grain zircon U-Pb ages were determined by SHRIMP analysis be 428 Ma ± 3.4 for both samples (N=60). One sample had several zircon grains with distinct igneous cores that ranged in age from about 850-1000 Ma. Grenville age plutons (1000-910 Ma) that were later involved in Caledonian deformation and intruded by 450-410 Ma aged plutons are found on western and eastern Svalbard (Johansson et al., 2005), eastern Greenland, and the Pearya Terrane of northern Ellesmere Island (Trettin,1986, 1992). The Franklinian basement of Arctic Alaska and Canada do not share these Grenvillian ages (Trentin et al, 1987). This suggests that the inherited zircon cores in the Chukchi Borderland orthogneisses were derived, at least in part, from an older Grenvillian basement like that of Pearya, Svalbard and parts of Greenland and later intruded by Caledonian aged granites.

  20. Constraining a SHRIMP U-Pb age: micro-scale characterization of zircons from Saxonian Rotliegend rhyolites

    Nasdala, Lutz; Götze, Jens; Pidgeon, Robert T.; Kempe, Ulf; Seifert, Thomas

    We present results of a detailed investigation of zircons from two rhyolites from St. Egidien and Chemnitz, Saxony, using a combination of microprobe techniques (SHRIMP ion probe, Raman microprobe, SEM: SE, BSE, and CL imaging). These rhyolites belong to the so-called ``lower volcanics'', which is the older of two series of Late Variscan volcanic rocks occurring in the Saxonian Sub-Erzgebirge basin (Germany). The purpose of the present contribution is to demonstrate that detailed characterization of zircons, as provided by the different micro-techniques, facilitates soundest interpretation of geochronological data. The zircons (at most 40 to 80 m in size) show oscillatory growth zoning, with reversely correlated CL and BSE signal intensities. These zircons are interpreted to have grown during crystallization of the rhyolite because, apart from some cracking, they do not appear to have experienced any alteration since the time of their growth: The shapes of the zircons and their internal structures revealed by CL and BSE imaging appear to be magmatic, and neither annealing of the accumulated alpha-decay damage nor disturbance of the U-Pb system is observed. The SHRIMP ion probe measurements on the zircons gave a Permian 206Pb/238U age of 278 +/- 5 Ma (95% confidence). The concordance of this age is supported by the correlation between the low degrees of metamictization (estimated from Raman parameters) and the accumulated alpha fluxes (calculated from SHRIMP data). The 278 Ma zircon age is interpreted to represent the age of the ``lower rhyolites'' series and, with that, the age of postkinematic Late Variscan volcanism in the Sub-Erzgebirge basin, which has been related to anorogenic extension and uplift as a result of intracontinental rifting. Because of genetic association of rhyolites in the Sub-Erzgebirge basin and Li-F granites and lamprophyres in the neighbouring Erzgebirge, the rhyolite age also indirectly contributes to the understanding of the geological

  1. 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

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


    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

  2. SHRIMP U-Pb zircon dates from igneous rocks from the Fontana Lake region, Patagonia: Implications for the age of magmatism, Mesozoic geological evolution and age of basement Datación de circón por U-Pb SHRIMP en rocas ígneas de la región del lago Fontana, Patagonia: Implicancia para la edad del magmatismo, la evolución geológica mesozoica y edad del basamento

    A.P. Rolando


    Full Text Available In the eastern margin of the Patagonian Andes and between 44° 30´S and 45° 30´S (Fontana Lake region, Middle Jurassic to Early Cretaceous volcanic and sedimentary rocks were intruded by granitic bodies during the Cretaceous. The reconstruction of the Jurassic-Cretaceous magmatic evolution in the Fontana Lake region and in the adjacent Patagonian Batholith was made possible by the consideration of the following characteristics: distribution in time and space of several intrusive bodies, retro-arc basin formation and volcanic intensity. U-Pb SHRIMP dating of zircon crystals from an ignimbrite, a dacitic porphyry and two granitoid rocks yielded dates of 148.7 ± 2.3, 144.5 ± 1.6, 117 ± 1.7 and 99.6 ± 2.8 Ma, respectively. The Cerro Bayo Ignimbrite (148.7 ± 2.3 Ma, Late Jurassic was included in the Lago La Plata Formation; this unit hosts an epithermal ore deposit. The Laguna Escondida dacitic porphyry (144.5 ± 1.6 Ma, Jurassic-Cretaceous boundary intruded metasedimentary rocks of the Lago La Plata Formation; this sub-volcanic body can chronologically be linked to the Patagonian Batholith. After the Jurassic volcanic events, a retro-arc basin formed in the eastern sector of the Patagonian Range at about 140-115 Ma (Late Berriasian-Barremian and magmatism ceased during this event. The dating of granitoids (117 ± 1.7 and 99.6 ± 2.8 Ma in the Fontana Lake region confirms a temporal magmatic continuity with the Patagonian Batholith. These dates also are in agreement with the volcanic rocks of the Divisadero Group and epithermal deposits in the region (La Ferrocarrilera deposit. One of the analyzed granitoids (Dedo Chico, 99.6 ± 2.8 Ma has inherited zircon crystals of about 2,100 and 3,410 Ma, in agreement with other previous isotopic evidence for the occurrence of an underlying Precambrian basement in the region.En la margen oriental de la cordillera Patagónica, entre los 44° 30´ L.S. y 45° 30´ L.S. (región del lago Fontana, rocas volc

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

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


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

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

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


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

  5. In-Situ U-Pb Dating of Apatite by Hiroshima-SHRIMP: Contributions to Earth and Planetary Science.

    Terada, Kentaro; Sano, Yuji


    The Sensitive High Resolution Ion MicroProbe (SHRIMP) is the first ion microprobe dedicated to geological isotopic analyses, especially in-situ analyses related to the geochronology of zircon. Such a sophisticated ion probe, which can attain a high sensitivity at a high mass resolution, based on a double focusing high mass-resolution spectrometer, designed by Matsuda (1974), was constructed at the Australian National University. In 1996, such an instrument was installed at Hiroshima University and was the first SHRIMP to be installed in Japan. Since its installation, our focus has been on the in-situ U-Pb dating of the mineral apatite, as well as zircon, which is a more common U-bearing mineral. This provides the possibility for extending the use of in-situ U-Pb dating from determining the age of formation of volcanic, granitic, sedimentary and metamorphic minerals to the direct determination of the diagenetic age of fossils and/or the crystallization age of various meteorites, which can provide new insights into the thermal history on the Earth and/or the Solar System. In this paper, we review the methodology associated with in-situ apatite dating and our contribution to Earth and Planetary Science over the past 16 years.

  6. Geochemistry and U-Pb geochronology of detrital zircons in the Brujas beach sands, Campeche, Southwestern Gulf of Mexico, Mexico

    Tapia-Fernandez, Hector J.; Armstrong-Altrin, John S.; Selvaraj, Kandasamy


    This study investigated the bulk sediment geochemistry, U-Pb ages and rare earth element (REE) geochemistry of one hundred detrital zircons recovered from the Brujas beach sands in southwestern Gulf of Mexico to understand the provenance and age spectra. The bulk sediments are high in Zr and Hf contents (∼1400-3773 ppm and ∼33-90 ppm, respectively) suggested the abundance of resistant mineral zircon. The chondrite normalized REE patterns of the bulk sediments are less fractionated with enriched low REE (LREE; LaCN/SmCN = ∼491-693), depleted heavy REE (HREE; GdCN/YbCN = ∼44-69) and a negative Eu anomaly (Eu/Eu∗ = ∼0.44-0.67) suggested that the source rock is felsic type. The results of this study revealed highly varied contents of Th (∼4.2-321 ppm), U (∼20.7-1680 ppm), and Hf (∼6970-14,200 ppm) in detrital zircons compared to bulk sands. The total REE content (∼75 and 1600 ppm) and its chondrite-normalized pattern with positive Ce and negative Eu anomalies as well as low Th/U ratio of zircon grains indicated that they were dominantly of magmatic origin. U-Pb data of zircons indicated two age populations, with predominance of Permian-Triassic (∼216-286 Ma) and Neoproterozoic (∼551-996 Ma). The Permian-Triassic zircons were contributed by the granitoids and recycled metasedimentary rocks of the Chiapas Massif Complex. The major contribution of Neoproterozoic zircons was from the Chaucus, Oaxacan, and Chiapas Massif Complexes in Grenville Province, southern Mexico. U-Pb ages of zircons from the Brujas beach are consistent to the reported zircon ages from the drainage basins of Usumacinta, Coatzacoalcos, and Grijalva Rivers in southern Mexico, suggesting that the sediments delivered by the rivers to the beach area are vital in defining the provenance of placers.

  7. U-Pb provenance ages of shocked zircons from the K-T boundary, Raton Basin, Colorado

    Premo, W. R.; Izett, G. A.


    U-Pb isotopic systematics from analyses of single zircons identify at least two provenance ages, approximately 575 Ma and approximately 330 Ma, for zircons from the impact layer of the K-T boundary, Raton Basin, Colorado. These data are a preliminary confirmation of results reported from the same layer. The zircon provenance ages provide a unique signature for identification of the source crater since igneous rocks of these ages (or sedimentary rocks derived from them) must characterize part of the impact stratigraphy.

  8. Provenance from zircon U-Pb age distributions in crustally contaminated granitoids

    Bahlburg, Heinrich; Berndt, Jasper


    The basement of sedimentary basins is often entirely covered by a potentially multi-stage basin fill and therefore removed from direct observation and sampling. Melts intruding through the basin stratigraphy at a subsequent stage in the geological evolution of a region may assimilate significant volumes of country rocks. This component may be preserved in the intrusive body either as xenoliths or it may be reflected only by the age spectrum of incorporated zircons. Here we present the case of an Ordovician calc-alkaline intrusive belt in NW Argentina named the "Faja Eruptiva de la Puna Oriental" (Faja Eruptiva), which in the course of intrusion sampled the unexposed and unknown basement of the Ordovician basin in this region, and parts of the basin stratigraphy. We present new LA-ICP-MS U-Pb ages on zircons from 9 granodiorites and granites of the Faja Eruptiva. The main part of the Faja Eruptiva intruded c. 445 Ma in the Late Ordovician. The zircon ages obtained from the intrusive rocks have a large spread between 2683.5 ± 21.6 and 440.0 ± 4.9 Ma and reflect the underlying crust and may be interpreted in several ways. The inherited zircons may have been derived from the oldest known unit in the region, the thick siliciclastic turbidite successions of the upper Neoproterozoic-lower Cambrian Puncoviscana Formation, which is inferred to represent the basement of the NW Argentina. The basement to the Puncoviscana Formation is not known. Alternatively, the inherited zircons may reflect the geochronological structure of the entire unexposed Early Paleozoic crust underlying this region of which the Puncoviscana Formation was only one component. This crust likely contained rocks pertaining to and detritus derived from earlier orogenic cycles of the southwestern Amazonia craton, including sources of Early Meso- and Paleoproterozoic age. Detritus derived, in turn, from the Faja Eruptiva intrusive belt reflects the origin of the granitoids as well as the inherited

  9. In situ simultaneous determination of trace elements,U-Pb and Lu-Hf isotopes in zircon and baddeleyite

    XIE LieWen; ZHANG YanBin; ZHANG HuiHuang; SUN JingFeng; WU FuYuan


    This paper describes a combined method of simultaneously measuring U-Pb and Lu-Hf isotopes as well as trace elements in Phalaborwa baddeleyite and 91500, GJ-1, TEMORA-1 and SK10-2 zircons by means of Neptune MC-ICPMS and Agilent Q-ICPMS connected to a 193 nm excimer laser ablation system. Material ablated by laser was carried in different proportions into Q-ICPMS for U-Pb isotopic and trace elemental and MC-ICPMS for Lu-Hf isotopic compositions. Experiments indicate that different propor-tions of ablated material for the Q-ICPMS and MC-ICPMS (6∶4, 5∶5 and 4∶6 respectively) do not show any bias for the zircon/baddeleyite U-Pb age, Lu-Hf isotope and trace elemental compositions within ana-lytical errors. Using 40-60 μm spot size, the obtained U-Pb ages of Phalaborwa baddeleyite, 91500, GJ-1, TEMORA and SK10-2 zircons are 2065~15 (2σ, n=20), 1063±6 (2σ, n=19), 613±6 (2σ,=20), 416±5 (2σ, n=20) and 32.6±0.5 (2σ, n=20) Ma, respectively. The 176Hf/177Hf ratios are 0.281231±24 (2SD, n=20), 0.282310±35 (2SD, n=19), 0.282028±34 (2SD, n=20), 0.282687±34 (2SD, n=20) and 0.282752±53 (2SD, n=20), respectively. The obtained trace elemental compositions are identical to the reference values. Therefore, this kind of technique makes it possible to simultaneously obtain the U-Pb age, Lu-Hf iso-topes and trace elemental compositions of zircon and baddeleyite, which could be an important tool in solving problems in earth sciences.

  10. Paleozoic accretionary orogenesis in the eastern Beishan orogen: constraints from zircon U-Pb and 40Ar/39Ar geochronology

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


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

  11. Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Paleoproterozoic Granulite-Facies Metamorphism and Archean Crustal Remnant in the Dabie Orogen

    Lei Nengzhong; Wu Yuanbao


    Zircon U-Pb age, trace elements, and Hf isotopes were determined for granulite and gneiss at Huaugtuling (黄土岭), which is hosted by ultrahigh-pressure metamorphic rocks in the Dabie(大别) orogen, east-central China. Cathodolumineseence (CL) images reveal core-rim structure for most zircons in the granulite. The cores show oscillatory zoning, relatively high Th/U and 176 Lu/177 Hf ratios, and high rare earth element (HREE)-enriched pattern, consistent with magmatic origin. They gave a weighted mean 207 Pb/206 Pb age of (2 766±9) Ma, dating magma emplacement of protolith. The rims are characterized by sector ur planar zoning, low Th/U and 176 Lu/177 Hf ratios, negative Euanomalies and flat HREE patterns, consistent with their formation under granulite-facies metamorphicconditions. Zircon U-Pb dating yields an age of (2 029±13) Ma, which is interpreted as a record ofmetamorphic event during the assembly of the supercontinent Columbia. The gneiss has a protolith ageof (1982±14) Ma, which is similar to the zircon U-Pb age for the granulite-facies metamorphism,suggesting complementary processes to granulite-facies metamorphism and partial melting. A fewinherited cores with igneous characteristics have 207 pb/206 Pb ages of approximately 3.53, 3.24, and 2.90Ga, respectively, suggesting the presence of Mesoarchean to Paleoarchean crustal remnants. A fewTriassic and Cretaceous metamorphic ages were obtained, suggesting the influences by the Triassiccontinental collision and postcollisional collapse in response to the Cretaceous extension. Comparingwith abundant occurrence of Triassic metamorphic zircons in ultrahigh-pressure eclogite and granitehydrous melt is evident for zircon growth in theHuangtuling granulite and gneiss during thecontinental collision. The magmatic protolithzircons from the granulite show a large variationin 176 Hf/177 Hf ratios from 0.280 809 to 0.281 289,corresponding to era(t) values of-7.3 to 6.3 andHf model ages of 2.74 to 3.34 Ga. The 2

  12. U-Pb zircon geochronology and Nd-Hf-O isotopic systematics of the Neoproterozoic Hadb adh Dayheen ring complex, Central Arabian Shield, Saudi Arabia

    Ali, Kamal A.; Jeon, Heejin; Andresen, Arild; Li, Shuang-Qing; Harbi, Hesham M.; Hegner, Ernst


    A combined study of single zircon U-Pb dating, Hf-O zircon isotopic analyses and whole-rock Nd isotopic compositions was carried out to infer the magma sources of Neoproterozoic post-collisional A-type granitoids in Saudi Arabia. U-Pb zircon dating of magmatic zircons of two samples from the Hadb adh Dayheen ring complex yielded ages of 625 ± 11 Ma for a hornblende-biotite granite sample, and 613 ± 4 Ma for a monzogranite sample. The granitic rocks show initial εNd values of + 4.1 to + 5.3 and εHf of + 4.5 to + 8.4 that are lower than those of a model depleted mantle (εHf ~+ 14 and εNd ~+ 6.5) and consistent with melting of subduction-related crustal protoliths that were formed during the Neoproterozoic assembly of the Arabian-Nubian Shield (ANS). Crustal-model ages (Hf-tNC) of 0.81 to 1.1 Ga are inconsistent with depleted-mantle Nd model ages of 0.71 to 0.81 Ga and indicate that the post-collisional Hadb adh Dayheen granites were derived mostly from juvenile crust formed in Neoproterozoic time. Single zircons data show a wide range in δ18O values from + 3.2‰ to + 6.4‰, possibly indicating crystallization of zircon from magma derived from magmatic rocks altered by meteoric water in a magma chamber-caldera system.

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

    GUO; Jinghui


    on the Early Precambrian grey gneiss and Khondalitein Nei Mongol-Shanxi-Hebei junction region, north China, Acta Petrologica Sinica (in Chinese with English abstract), 1996, 12(2): 329-342.[19]Guo, J. H., Shi, X., Bian, A. G. et al., Pb isotopic composition of feldspar and U-Pb age of zircon from early Proterozoic granite in Sanggan area, North China craton: Metamorphism, crustal melting and tectono-thermal event, Acta Petrologica Sinica (in Chinese with English abstract), 1999, 15(2): 199-207.[20]Krogh, T. E., A low contamination method for the hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations, Geochim. Cosmochim. Acta, 1973, 37: 485-494,[21]Xu, R. H., The single zircon U-Pb dating method, Acta Petrologica Sinica (in Chinese with English abstract), 1989, 5(2):88-93.[22]Stacey, J. S., Kramers, J. D., Approximation of terrestrial lead isotopes evolution by a two-stage model, Earth Planet. Sci.Lett., 1975, 26: 207-221.[23]Wiedenbeck, M., An example of reverse discordance during ion microprobe zircon dating: an artifact of enhanced ion yields from a radiogenic labile Pb, Chem. Geol., 1995, 125: 197-218.[24]Williams, I. S., Compston, W., Black, L. P. et al., Unsupported radiogenic Pb in zircon: a cause of anomalously high Pb-Pb, U-Pb and Th-Pb ages, Contrib. Mineral. Petrol., 1984, 88: 322-327.[25]Chen, F., Siebel, W., Satir, M., Zircon and zircons; unveiled by the stepwise leach experiment (abstr.), EUG-I 1 (European Union of Geosciences), Strasbourg, France, 2001, 37.[26]Pidgeon, R. T., Aftalion, M., Cogenetic and inherited zircon U-Pb systems in granites: Paleozoic granites of Scotland and England, in Crustal Evolution of Northwestern Britain and Adjacent Regions (eds. Bowes, R. D., Leake, B. E.), Geological Journal (special issue), 1978, 10: 183-220.[27]Schenk, V., U-Pb and Rb-Sr radiometric dates and their correlation with metamorphic events in the granulite facies basement of the

  14. 浙江石浦下白垩统灰岩微相分析与锆石U-Pb年代学研究%Microfacies and Zircon U-Pb Dating of the Lower Cretaceous Shipu Limestone in Xiangshan, Eastern Zhejiang

    王学寅; 胡文瑄; 胡广; 姚素平; 曹剑; 王小林; 谢小敏


    Through the synthetic lithofacies studies for the Shipu limestone section located in Xiangshan, Zhejiang Province, four types microfacies can be recognized, i. e. bioclasts micrite, oolite micritic limestone, stromatolite marlite, and thin lamina-banded limestone, and they formed in tidal flat, oolitic beach and shallow environments, accordingly. Based on the researches of the cyclostratigrapy, the Shipu limestone can be divided into four sedimentary cycles which record three major transgressions. Furthermore, zircon LA-ICP-MS U-Pb dating has been conduced for the tuff beds from the base and top of the section for the first time. The new radiometric age data indicated that the Shipu limestones deposited in the Early Cretaceous age from 114. 2 Ma to 96. 3Ma. We believe that these results should stir up a new thought on the depositional environment and tectonic background of Early Cretaceous in southeast of China%通过对浙江象山石浦灰岩剖面系统的岩相学研究,总结出4种基本灰岩微相类型,即生物碎屑泥晶灰岩、泥晶鲕粒灰岩或鲕粒泥晶灰岩、叠层石泥灰岩、薄层一条带状泥灰岩,其形成于潮坪、鲕粒滩和浅海等三种沉积环境.结合整个剖面岩性变化规律,划分出4个沉积旋回,反映了3次主要海侵过程.并利用LA-ICP-MS锆石U-Pb 同位素定年技术,首次获得了该剖面下部凝灰岩夹层中锆石U-Pb年龄为114±2Ma,确定了该剖面形成时代为早白垩世.这一成果对于深化我国东南沿海早白垩世构造—沉积环境的研究具有重要意义.

  15. Cretaceous Volcanic Events in Southeastern Jilin Province, China: Evidence from Single Zircon U-Pb Ages

    CHEN Yuejun; SUN Chunlin; SUN Yuewu; SUN Wei


    Mesozoic volcanic rocks in southeastern Jilin Province are an important component of the huge Mesozoic volcanic belt in the northeastern area. Study of the age of their formation is of great significance to recognize Mesozoic volcanic rule in northeastern China. Along with the research of rare Mesozoic biota and extensive Mesozoic mineralization in western Liaoning, a number of researchers have focused on Mesozoic volcanic events. The authors studied the ages of the Cretaceous volcanic rocks in southeastern Jilin Province using single Zircon U-Pb. The result shows that the Sankeyushu Formation volcanic rocks in the Tonghua area are 119.2 Ma in age, the Yingcheng Formation in the Jiutai area 113.4±3.1 Ma, the Jinjiatun Formation in Pinggang Town of Liaoyuan City and the Wufeng volcanic rocks in the Yanji area 103.2±4.7 Ma and 103.6±1 Ma, respectively. Combined with the data of recent publication on volcanic rocks ages; the Cretaceous volcanic events in southeastern Jilin Province can be tentatively subdivided into three eruption periods: 119 Ma, 113 Ma and 103 Ma. The result not only provides important chronology data for subdividing Mesozoic strata in southeastern Jilin Province, establishing Mesozoic volcanic event sequence, discussing geological tectonic background, and surveying the relation between noble metals to the Cretaceous volcanic rocks, but also otters important information of Mesozoic volcanism in northeastern China.

  16. New U-Pb zircon and 40Ar/39Ar muscovite age constraints on the emplacement of the Lizio syn-tectonic granite (Armorican Massif, France)

    Tartese, Romain; Poujol, Marc; Ruffet, Gilles; Boulvais, Philippe; Yamato, Philippe; Kosler, Jan


    International audience; LA-ICP-MS U-Pb analyses performed on zircon grains from the Lizio granite yielded an emplacement age of 316 6 Ma. Typical S-C structures show that the Lizio granite was emplaced contemporaneously with dextral shearing along the northern branch of the South Armorican Shear Zone and that it was therefore active at that time. 40Ar/39Ar analyses performed on muscovite grains yielded plateau dates ranging between 311.5 and 308.2 Ma. Muscovite chemistry is typical of primary...


    高林志; 丁孝忠; 庞维华; 张传恒


    On the basis of the new remarked the Precambrian stratigraphical column of China, which is discussed by the National Commission on Stratigraphy of China on Aug. , 2010. The Changcheng System is limited in 1.8~1.6 Ga. The Jixian System is limited in 1. 6~1.4 Ga. Unnamed system (1. 4~1.0 Ga) only developed the Xiamaling Formation in the Jixian section. The Qingbaikou System is limited in 1.0~0. 78 Ga. The Nanhua System is in 780 ~635 Ma and The Sinian System is in between 635 ~ 542 Ma.There three important markers in the new geologic time scale. 1) The Changcheng System and the Jixian System are reformed in it; 2) According to the SHRIMP zircon U-Pb dating, the unnamed system is left in the time scale; and 3) the lower boundary age of the Nanhua System is limited at 780 Ma. Because the big problems is the strata division and correlation in restudy of new regional geology in each provinces by Chinese Geological Survey and according to the new geologic time scale of Meso and Neoproterozoic of China, it is big changes in Precambrian strata correlation and tectonic explanation. According to new zircon U-Pb dating found in North China Plate, Yangtze Plate and Talimu Plate, it primary shows the volcanic province and distribution. Dealing with the basic global geodynimics, it is useful to set up a basic late Precambrian section of unify, practical and high precision chronological dating.%报道了全国地层委员会的中-新元古代地层年表新标定的方案:即长城系限定在1.8~1.6 Ga;蓟县系限定在1.6~1.4 Ga;待建系1.4~1.0 Ga;青白口系限定在1.0~0.78 Ga;南华系限定在780~635 Ma及震旦系限定在635~542 Ma.新年表突出3个方面的标定:1)对长城系和蓟县系的年代学限定;2)依据蓟县剖面的新年龄信息,在中国年表中表示了"待建系"的位置; 3)对南华系下限的限定.依据新地层年表,有关的地层对比和构造背景解疑将发生重大变化.中国华北古陆、扬子古陆

  18. Amplifying Earth history: Zircon U-Pb geochronology by ID-TIMS at the 0.1 ‰ level using new 1013 ohm resistors

    Wotzlaw, Jörn-Frederik; von Quadt, Albrecht


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

  19. Analysis of U-Pb, O, Hf, and trace elements of horizontally oriented outer and inner zones of zircons from the Boulder batholith, Montana

    Aleinikoff, J. N.; Lund, K.; Du Bray, E. A.; Wooden, J. L.; Kozdon, R.; Kita, N.; Valley, J. W.; Kamenov, G. D.; Mueller, P. A.


    The Late Cretaceous Boulder batholith, southwestern MT, is composed of the Butte Granite and at least a dozen smaller granodiorite to syenogranite plutons. These plutons (81-73 Ma) were dated by zircon U-Pb geochronology using the SHRIMP. Typically for SIMS analysis of igneous zircon, the analytical spot is located midway between core and rim on an area that displays fine oscillatory zoning, thus sampling a ~25-30 µm area of vertically oriented zones to a depth of 1-2 µm. For this study, preliminary LA-ICP-MS analysis of Hf isotopes in zircons from several plutons suggested that some grains show significant variation (>5 ɛHf units) between inner and outer zones. This finding instigated a detailed investigation in which data for multiple isotopic systems (U-Pb, trace elements, O, and Hf ) were collected from horizontally oriented zones. Zircons were mounted in epoxy but not ground or polished. Reflected light, profilometer, and CL images were used to select homogeneous crystal faces. The outermost parts of 12-15 grains each from 12 plutons were analyzed consecutively for O and U-Pb isotope ratios, and trace element concentrations using an ion microprobe. The grains were then ground to half-thickness and O, U-Pb, and trace elements were measured at the centers of previously analyzed grains. The final step was Hf isotopic analysis by LA-ICP-MS which ablated a hole completely through the remaining half-grains. Although measurements of these isotopes from the outsides and insides of selected zircons is a limited form of depth-profiling, it enables acquisition of a very large, more precise data set than typical depth profiling. For all samples, U-Pb ages of zircon from interior and exterior zones are not resolvably different at ± 1% (2-sigma). However, in several samples a few outliers were identified, suggesting that interior parts in some grains formed later, during or after growth of the exterior parts of other grains. Thus, zircon growth was not an episodic

  20. 西藏冈底斯厅宫铜矿侵入岩成因:LA-ICP-MS 锆石U-Pb 年龄、地球化学及 Sr-Nd-Pb 同位素证据%Petrogenesis of Intrusions in Tinggong Copper Deposit,Gangdese, Xizang(Tibet):Evidence from LA-ICP-MS Zircon U-Pb Dating, Petrogeochemical,and Sr-Nd-Pb Isotopic Composition

    黄勇; 丁俊; 李光明; 董随亮; 黄瀚霄; 崔晓亮; 戴婕; 闫国强


    studies of above major granites are benefited for future exploration.LA-ICP-MS zircon U-Pb dating,major and trace elements,Rb-Sr,Sm-Nd,Pb isotope composition were test by this study.The results show that the age of above granites are 50.32 Ma,48.34 Ma,and 49.41 Ma,respectively,which are apparently older than that of mineralization.Above stocks belong to high-K calc-alkaline,weakly peraluminous,I-type granites.The REE patterns indicate a LREE rich,with weak /no HREE fractionation V-type model and strong depleted δEu,which are reverse with SiO2 content.All granites are rich in Th,U,K,Cs,and Rb,depleted in Ba,Nb,Sr,Ti.Whole rock Pb isotopic compositions show that calculated n(206 Pb)/n(204 Pb)n(207 Pb)/n(204 Pb),n(208 Pb)/n(204 Pb) are 18.414 ~18.5137,15.616 ~15.7039,38.6394 ~39.0029,respectively,indicating a high radio-Pb isotope composition.Calculated n(87 Sr)/n(86 Sr)and n(143 Nd)/n(144 Nd)restrict in narrow range:0.70320 ~0.70477, 0.51260 ~0.51265,respectively,with calculated positive εNd (t)(0.51 ~1.44).The above results indicate that spread magna are not related to the mineralization and derived from new lower crust/depleted mantle during the roll-back and break-off of northward subducted ocean crust,undergoing AFC process during emplace.

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

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


    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

  2. 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)

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


    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).

  3. Tectono-magmatic evolution of the Chihuahua-Sinaloa border region in northern Mexico: Insights from zircon-apatite U-Pb geochronology, zircon Hf isotope composition and geochemistry of granodiorite intrusions

    Mahar, Munazzam Ali; Goodell, Philip C.; Feinstein, Michael Nicholas


    We present the whole-rock geochemistry, LA-ICP-MS zircon-apatite U-Pb ages and zircon Hf isotope composition of the granodioritic plutons at the southwestern boundary of Chihuahua with the states of Sinaloa and Sonora. These granodiorites are exposed in the north and south of the Rio El Fuerte in southwest Chihuahua and northern Sinaloa. The magmatism spans over a time period of 37 Ma from 90 to 53 Ma. Zircons are exclusively magmatic with strong oscillatory zoning. No inheritance of any age has been observed. Our new U-Pb dating ( 250 analyses) does not support the involvement of older basement lithologies in the generation of the granitic magmas. The U-Pb apatite ages from granodiorites in southwest Chihuahua vary from 52 to 70 Ma. These apatite ages are 1 to 20 Ma younger than the corresponding zircon U-Pb crystallization ages, suggesting variable cooling rates from very fast to 15 °C/Ma ( 800 °C to 500 °C) and shallow to moderate emplacement depths. In contrast, U-Pb apatite ages from the Sinaloa batholith are restricted from 64 to 61 Ma and are indistinguishable from the zircon U-Pb ages range from 67 to 60 Ma within the error, indicating rapid cooling and very shallow emplacement. However, one sample from El Realito showed a larger difference of 20 Ma in zircon-apatite age pair: zircon 80 ± 0.8 Ma and apatite 60.6 ± 4 Ma, suggesting a slower cooling rate of 15 °C/Ma. The weighted mean initial εHf (t) isotope composition (2σ) of granodiorites varies from + 1.8 to + 5.2. The radiogenic Hf isotope composition coupled with previous Sr-Nd isotope data demonstrates a significant shift from multiple crustal sources in the Sonoran batholithic belt to the predominant contribution of the mantle-derived magmas in the southwest Chihuahua and northern Sinaloa. Based on U-Pb ages, the absence of inheritance, typical high Th/U ratio and radiogenic Hf isotope composition, we suggest that the Late Cretaceous-Paleogene magmatic rocks in this region are not derived from

  4. U-Pb zircon age for the Daohugou Biota at Ningcheng of Inner Mongolia and comments on related issues

    LIU Yongqing; LIU Yanxue; JI Shu'an; YANG Zhiqing


    SHRIMP U-Pb zircon dating was carried out for the Daohugou Biota near Ningcheng of Inner Mongolia and for lavas overlying or underlying salamander-bearing strata at Reshuitang in Lingyuan of West Liaoning. The results suggest that the Daohugou Biota occurred at an interval from 168 Ma to 164-152 Ma. Both the Daohugou Biota and the salamander-bearing fossil assemblage are the same biota and thus developed from 168 to 152 Ma, i.e.from late Middle Jurassic to the early Late Jurassic.The Daohugou Biota-bearing rocks, resting on the Jiulongshan Formation in disconformity and being overlain in unconformity by Late Jurassic Tuchengzi Formation and Early Cretaceous rocks containing the Jehol Biota, are mainly composed of volcanic-sedimentary rocks in a normal sequence. It is recommended that the Daohugou Biota and the related stratigraphy should be correlated with the Tiaojishan Formation (Lanqi Formation in West Liaoning) or its synchronous rocks. It is suggested that the Daohugou Biota and the Jehol Biota would be neither taken into one biota nor considered as the earliest elements of the Jehol Biota. The Daohugou Biota and the related rocks and the Yixian Formation were respectively formed in different periods of volcanic-sedimentary tectonics.

  5. Petrogenesis of keratophyes in the Pingshui Group,Zhejiang: Constraints from zircon U-Pb ages and Hf isotopes

    CHEN ZhiHong; XING GuangFu; GUO KungaYi; DONG YongGuan; CHEN Rong; ZENG Yong; LI LongMing; HE ZhengYu; ZHAO Ling


    Zircon LA-ICP-MS U-Pb ages and Hf isotopic as well as whole-rock geochemical data are reported for keratophyes in the Pingshui Group, Zhejiang. The results are used to discuss their petrogenesis and geological significance. The keratophyes were dated at 904±8 to 906±10 Ma. These intermediate-felsic rocks are characterized by high LREE contents end depletion of HREE and HFSE (e.g., Nb, Ta, Ti, P), resembling arc-derived rocks. The keratophyes exhibit positive εHf(t) values of 8.6 to 15.4, consistent with their εNd(t) values of 6.4 to 7.9 but far away from those of crust-derived rocks. Such features indicate that they were likely originated from prompt reworking of juvenile crust by arc-continent collision during the early-Neoproterozoic assembly between the Cathaysia and Yangtze Blocks. Combining with their Hf model ages, we suggest that there may exist not only remarkable growth of juvenile crust at ca.1.3-1.1 Ga but also production of juvenile arc-derived crust along the southeastern margin of the Yangtze Block (e.g., the Pingshui area) at ca.1.0-0.9 Ga.

  6. SHRIMP U-Pb in zircon geochronology of granitoids from Myanmar: temporal constraints on the tectonic evolution of Southeast Asia

    Barley, M. E.; Zaw, Khin


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

  7. LA-ICPMS U-Pb Ages of Zircon from Metaleucosomes, Olongbuluke Microcontinent,North Qaidam, and Implications on the Response to the Global Rodinia Supercontinent Assembly Event in NW China

    Wang Shuqing; Li Xiaoyan; Chen Nengsong; Wang Xinyu; Wang Qinyan; Liu Xiaoming


    U-Pb dating was conducted on different domains of zircons from metamorphosed leucosomes in Delingha (德令哈) complex, the lower basement rocks of the Olongbuluke (欧龙布鲁克) microcontinent, North Qaidam, in order to review its complex tectonothermal history. The zircon core is comprised of highly-modified magmatic zircon relicts, the zircon mantle was produced in response to anatexis of a Late Protoproterozoic thermal event; age and isotopic composition of both the zircon core and the zircon mantle have been seriously disturbed due to the thermal event related with growth of the zircon overgrowth rim. The 207 Pb/206 Pb apparent age of the overgrowth rim was estimated to be ~ 1 030 Ma. This Late Mesoproterozoic thermal event has been interpreted as a response to the global Rodinia supercontinent assembly event in the Olongbuluke microcontinent,Northwest China.

  8. SHRIMP Zircon U-Pb Chronology and Geochemistry of the Henglingguan and Beiyu Granitoids in the Zhongtiao Mountains, Shanxi Province

    YU Shengqiang; LIU Shuwen; TIAN Wei; LI Qiugen; FENG Yonggang


    Henglingguan and Beiyu metamorphic granitoids, distributed in the northwest of the Zhongtiaoshan Precambrian complex, comprise trondhjemites and calc-alkaline monzogranites,displaying intrusive contacts with the Archean Zhaizi TTG gneisses. And the Beiyu metamorphic granitoids consist mainly of trondhjemites, distributed at the core of the Hujiayu anticline fold. New SHRIMP zircon U-Pb dating data show that the weighted mean 207Pb/206Pb ages are 2435.9 Ma and 2477 Ma for the Henglingguan metamorphic calc-alkaline monzogranites and Beiyu metamorphic trondhjemites, respectively, and reveal ~2600 Ma inherited core in magmatic zircons. Whole-rock geochemical data indicate that all the Henglingguan and Beiyu metamorphic trondhjemites and calcalkaline monzogranites belong to the metaluminous medium- and high-potassium calc-alkaline series.These rocks are characterized by relatively high total alkali contents (Na2O+K2O, up to 9.08%),depleted Nb, Ta, P and Ti, and right-declined REE patterns with moderate to high LREEs/HREEs fractionation (the mean ratio of (La/Yb)n = 25). The Henglingguan and Beiyu metamorphic trondhjemites display negative Rb, Th and K anomalies in the multi-element spider diagrams normalized by primitive mantle. Sm-Nd isotopic data reveal that these granitoids have initial εNd(t)=-1.2 to +2.4 and Nd depleted mantle model ages of TMD = 2622 Ma-2939 Ma. All these geochemical features indicate that these granitoids were formed in an continent-marginal arc, and the trondhjemites mainly originated from partial melting of juvenile basaltic materials and, howbeit, the Henglingguan metamorphic calc-alkaline monzogranites derived from recycling of materials in the ancient crust under a continent-marginal arc. The granitic magma underwent contamination and fractional crystallization during their formation.

  9. SHRIMP U-Pb zircon age of tuff from the Kunyang Group in central Yunnan: Evidence for Grenvillian orogeny in South China

    ZHANG ChuanHeng; GAO LinZhi; WU ZhenJie; SHI XiaoYing; YAN QuanRen; LI DaJian


    Whether or not Grenvillian orogeny occurred in South China still remains highly controversial because high-quality, discriminating data are lacking, and therefore, the key to resolve this matter is to find datable volcanic and/or sedimentary rocks related to Grenvillian orogeny. Such rocks are apparently present in the Fuliangpeng Member from the lower-middle part of Kunyang Group in central Yunnan;here the unit is more than 100 m thick and consists of andesitic ignimbrite, tuffite, terrigeous clastic rocks and carbonates. These volcanic rocks, developed south of the Sibao fold-thrust belts, represent the earliest calc-alkaline volcanic activity in late Precambrian time from central Yunnan and are coeval with both a change in sedimentary facies from detritus to carbonates and the beginning of seismite development elsewhere. Two samples for SHRIMP analysis were collected from this volcanic unit.Sample G3-29-2, from the bottom of Fuliangpeng Member, is an ignimbrite, and about 100 zircon crystals recovered from it have euhedral shapes and display relatively simple sector zonation under cathodoluminescent (CL) imaging, suggesting a magmatogenic origin. Twenty-five of the zircons were analyzed and a weighed-mean U-Pb age of 1032±9 Ma was obtained. Sample G3-29-3 from uppermost part of Fuliangpeng Member is a tuffite, and many rounded, evidently detrital zircons were recovered.Nine of these zircons were analyzed, and the oldest single-grain U-Pb zircon age is 1938±26 Ma, implying that Paleoproterozoic basement developed in Cathaysia. The dating result, combined with the geotectonic research on the Fuliangpeng Member, leads us to conclude that late Mesoproterozoic orogenic volcanic activity occurred in the western part of South China, and that the related collision of Yangtze and Cathaysian cratons was an integral part of the assembly of Rodinia.

  10. SHRIMP zircon U-Pb age and significance of Early Paleozoic volcanic rocks in East Kunlun orogenic belt, Qinghai Province, China


    Early Paleozoic volcanic rocks in Nuomuhong area occurred as basalt slice and meta-volcanic slice. SHRIMP zircon U-Pb dating of the basalt slice and the meta-volcanic slice show that the age of the basalt slice is 419±5 Ma, and that of the meta-volcanic slice is 401± 6 Ma. These ages directly testify that there existed Early Paleozoic ocean-continent transform in East Kunlun, the basalt slice was formed in an extensional mid-ocean ridge setting and the meta-volcanic rock slice was formed in an extrusion subduction and collision setting. The inherited zircon age of 1734 Ma in volcanic rocks reflects that the base of East Kunlun may be Middle Proterozoic.

  11. Age and genesis of the Myanmar jadeite: Constraints from U-Pb ages and Hf isotopes of zircon inclusions

    QIU ZhiLi; WU FuYuan; YANG ShuFeng; ZHU Min; SUN JinFeng; YANG Ping


    Myanmar jadeite (jadeitite) is well known for its economical value and distinctive tectonic locality within the collisional belt between India and Eurasian plates. However, it is less studied for its genesis and geodynamic implications due to precipitous topography, adverse weather and local military conflicts in the area. By means of combined ICP-MS and LA-MC-ICPMS techniques, we have carried out in-situ trace elements, U-Pb and Lu-Hf isotopes for zircon inclusions in a piece of jadeite gem sample. CL imaging suggests that the zircons are metasomatic in origin, and contain mineral inclusions of jadeite and omphacite. Seventy-five analyses on 16 grains of the zircons yield a U-Pb age of 158±2 Ma. The Myanmar zircons differ from other types in that they have no significant Eu anomalies despite high HREE concentrations. Measured 176Hf/177Hf ratios range from 0.282976 to 0.283122, with an average value of 0.283066±7;εHf(t) value of 13.8±0.3 (n=75). These results indicate that the Myanmar jadeite was formed in the Late Jurassic, probably by interaction of fluid released from subducted oceanic slab with mantle wedge. Therefore, its formation has no genetic relationship to the continental collision between Indian and Euroasian plates.

  12. Zircon U-Pb and Lu-Hf isotope study of the Neoproterozoic Haizhou Group in the Sulu orogen: Provenance and tectonic implications

    Zhou, Jian-Bo; Wilde, Simon A.; Liu, Fu-Lai; Han, Jie


    The Neoproterozoic Haizhou Group crops out sporadically in the Sulu orogen in east-central China. It is divided into the Jinping and Yuntai formations and consists of quartzite, quartz schist, marble and graphite- and apatite-bearing sequences. Major and trace element data for quartz schist from the two formations indicate that these rocks have a greywacke protolith and have been deposited during strong tectonic activity. LA-ICPMS U-Pb dating of detrital zircon yields ages of 635 to 1074 Ma for three samples from the Jinping Formation and 611 to 943 Ma for two samples from the Yuntai Formation. More than 78% of the detrital zircons from the two formations have U-Pb ages grouped between 700 and 890 Ma, with two clusters peaking at 758 Ma and 828 Ma, respectively. This indicates that their provenance is magmatic rocks of Neoproterozoic age that have a tectonic affinity to the South China Block (SCB). A few older zircon populations with peak U-Pb ages at 943 and 1074 Ma are also present. A younger population shows peaks at 661 and 611 Ma. This suggests that deposition of the Haizhou Group was later than ~ 611 Ma rather than during the Mesoproterozoic as previously thought. Zircon Lu-Hf isotope data collected from the same U-Pb sites show negative ɛHf(t) values of - 22.8 to - 7.4 and Hf model ages of 2341 to 3100 Ma. This indicates that the Neoproterozoic magmatic rocks were derived from reworking of ancient Paleoproterozoic to Archean crust. The results support the contention that the Haizhou Group is similar to the Wulian Group at the northwestern edge of the Sulu orogen, both having a SCB affinity, but that the Penglai Group does not belong to the SCB because of the absence of Neoproterozoic ages. This lends support to the conclusion that the Triassic suture between the North China and South China blocks is located along the Baichihe-Yantai Fault, which lies north of the Wulian Complex and south of the Jiaobei Terrane; thus the Wulian-Yantai Fault is not the suture

  13. The onset of flood basalt volcanism, Northern Paraná Basin, Brazil: A precise U-Pb baddeleyite/zircon age for a Chapecó-type dacite

    Janasi, Valdecir de Assis; de Freitas, Vivian Azor; Heaman, Larry H.


    We report the first U-Pb baddeleyite/zircon date for a felsic volcanic rock from the Paraná Large Igneous Province in south Brazil. The new date of 134.3 ± 0.8 Ma for a hypocrystalline Chapecó-type dacite from Ourinhos (northern Paraná basin) is an important regional time marker for the onset of flood basalt volcanism in the northern and western portion of the province. The dated dacite was erupted onto basement rocks and is overlain by a high-Ti basalt sequence, interpreted to be correlative with Pitanga basalts elsewhere. This new U-Pb date for the Ourinhos dacite is consistent with the local stratigraphy being slightly older than the few reliable step-heating 40Ar/39Ar dates currently available for overlying high-Ti basalts (133.6-131.5 Ma). This indicates an ~ 3 Ma time span for the building of the voluminous high-Ti lava sequence of the Paraná basin. On the other hand, it overlaps the 40Ar/39Ar dates (134.8-134.1 Ma) available for the stratigraphically older low-Ti basalt (Gramado + Esmeralda types) and dacite-rhyolite (Palmas type) sequences from South Brazil, which is consistent with the short-lived character of this volcanism and its rapid succession by the high-Ti sequence.

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

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


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

  15. SHRIMP zircon U-Pb ages for the Paleoproterozoic metamorphic-magmatic events in the southeast margin of the North China Craton

    GUO SuShu; LI ShuGuang


    A garnet-pyroxene bearing amphibolite as a xenolith hosted by the Mesozoic igneous rocks from Xuzhou-Suzhou area was dated by zircon SHRIMP U-Pb method, which yields a metamorphic age of Archean metamorphic complex named Wuhe group in the Bengbu uplift give a metamorphic U-Pb age Bengbu uplift give a magma crystallization U-Pb age of 2054 ± 22 Ma. Both the Xuzhou-Suzhou area and Bengbu uplift are located in the southeastern margin of the North China Craton. Therefore, these ages indicate that there is a Paleoproterozoic tectonic zone in the southeastern margin of the North China Craton, and its metamorphic and magmatic ages are consistent with those of the other three Paleoproterozoic tectonic zones in the North China Craton. In view of the large scale sinistral strike-slip movement occurred at the Mesozoic along the Tan-Lu fault zone, the position of the eastern Shandong area, which is a south section of the Paleoproterozoic Jiao-Liao-Ji Belt, was correlated to Xuzhou-Suzhou-Bengbu area prior to movement of the Tan-Lu fault zone. This suggests that the Xuzhou-Suzhou-Bengbu Paleoproterozoic tectonic zone might be a southwest extension of the Paleoproterozoic Jiao-Liao-Ji Belt.

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

    Oliveira, Alisson Lopes; Hauser, Natalia; Pimentel, Marcio Martins; Matteini, Massimo, E-mail: [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)


    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)

  17. SHRIMP zircon U-Pb and Re-Os dating of No. 10 intrusive body and associated ores in Pobei mafic-ultramafic belt of Xinjiang and its significance%新疆坡北基性-超基性岩带10号岩体SHRIMPU-Pb和矿石Re-Os同位素定年及其意义

    李华芹; 梅玉萍; 屈文俊; 蔡红; 杜国民


    Located in Beishan rift in the northeastern part of Tarim plate, the Pobei No. 10 mafic-ultramafic body in-truded into Early Carboniferous Hongliuyuan Formation. It is large in size and well differentiated, thus serving as one of the most important intrusive bodies in search for copper-nickel sulfides. According to its emplacement strata, it was previously thought that this intrusive body was formed in Late Carboniferous-Early Permian. The uthors performed dating by using such techniques as zircon U-Pb, disseminated ores Re-Os and Sr-Nd isotopic tracer with the purpose of systematic determination of the magmatie and mineralization ages, on such a basis, discussed the metallogenic significance of systematic differences between the ages dated by different isotopic sys-tems. Radiometric dating of the gabbro from the Pobei No. 10 mafic-ultramafie body yielded rather scattered zir-con age information, but obtained a main body zircon SHRIMP U-Pb age of (289± 13)Ma(95 % confidence, MSWD=4.9, n = 8), which is interpreted as the formation age of the host intrusive. The disseminated ores give an apparent Re-Os isochron age of (413 ± 20) Ma with an initial 187Os/188Os ratio of (0.226 ± 0.032). However, it is found that the apparent Re-Os isochron ages of disseminated ores are older than both the forma-tion age of the host intrusions and the ages of Pobei No. 10 mafic-ultramafic emplacement strata, which indicates that the apparent Re-Os isochron ages of disseminated ores are not reliable. It is suggested that the Pobei No. 10 mafic-ultramafic body and the deposit were formed at about 280 Ma ago. Because of heterogeneity in initial ~(187)Os/~(188)Os ratios of disseminated ores caused by crust contamination, the apparent dates older than the true age were obtained. Cu-Ni mineraliztion shows a close time-space relationship with the mafic-ultramafic intrusive body, implying a product of magmatic liquation. It is therefore concluded that the intrusion of Pobei No. 10 mafic

  18. Petrography and U-Pb detrital zircon geochronology of metasedimentary strata dredged from the Chukchi Borderland, Amerasia Basin, Arctic Ocean

    Brumley, K.; Miller, E. L.; Mayer, L. A.; Andronikov, A.; Wooden, J. L.; Dumitru, T. A.; Elliott, B.; Gehrels, G. E.; Mukasa, S. B.


    In 2008-2009, twelve dredges were taken aboard the USCGC Healy from outcrops along the Alpha Ridge, Northern Chukchi Borderland, Northwind Ridge and the Chukchi Plateau in the Arctic Ocean as part of the U.S. Extended Continental Shelf Project. To ensure sampling of outcrop, steep bathymetric slopes (>40°) with little mud cover were identified with multibeam sonar and targeted for dredging. The first dredge from Alpha Ridge yielded volcaniclastic sedimentary rocks deposited from a phreatomagmatic eruption in shallow water (<200m). This is inconsistent with tectonic reconstructions suggesting that the Alpha Ridge was created as an oceanic plateau on deep oceanic crust of the Canada Basin. Another dredge, taken from the northern tip of Northwind Ridge, yielded metasedimentary rocks deformed under greenschist facies conditions (chlorite+white mica). These rocks are intruded and/or overlain by mid-Cretaceous alkalic basalts, also taken in this dredge, and dated by 40Ar/39Ar (plagioclase separate) to be 112±1 Ma. The metasedimentary rocks, from this single dredge, range in grain size from mud to coarse sandstone and grit which all contain grains and sub-angular clasts of volcanic, plutonic, metamorphic and fine grained sedimentary rocks as well as monocrystalline quartz, potassium feldspar, and plagioclase. All of these samples display the same bedding to foliation angle and lithology, which further indicates that they were dredged from in situ outcrop and are not random samples of ice rafted debris. Based on grain size variations and graded beds, they are interpreted as Silurian gravity flow deposits fed by proximal syn-orogenic and/or magmatic arc sources. Detrital zircons were separated from four sandstone samples of the Northwind Ridge dredge, and their U-Pb single grain ages determined by LA-MC-ICPMS and SHRIMP, (N= 393). Their detrital zircon populations are dominated by euhedral first-cycle zircon ca. 430 and 980 Ma with lesser older recycled zircons between

  19. Better U-Pb Zircon Standards for SIMS and LA-ICPMS? Preliminary Results of Detailed Characterization and Pre-treatment using CA-TIMS

    Mattinson, J. M.; Hourigan, J.; Wooden, J. L.


    U-Pb zircon dating using SIMS and LA-ICPMS requires calibrations against natural zircons of known age. Ideally, such zircons should be perfectly concordant -- free from Pb-loss, inheritance, or any other complications, and also accurately dated. In practice this can be problematic. Complexities can be difficult to detect by SIMS and LA-ICPMS analysis if they are at or below the level of precision of an individual analysis. In addition, conventional TIMS dating, used to date the standards, can be limited by residual minor Pb loss that has not been removed by conventional "pretreatments" such as air abrasion. CA-TIMS (Mattinson, 2005, Chem Geol 220, 47-66) utilizes high-T annealing of natural radiation damage, followed by partial dissolution in HF, and has proven highly successful at complete removal of zircon domains that have experienced Pb loss, both from the outer rims and also from the deep interiors of zircon grains. Thus, it is useful for detailed characterization of existing and potential zircon standards. Here, we also investigate the potential of CA-TIMS for pre-treating zircon standards prior to SIMS and LA-ICPMS analysis. Multi-step CA-TIMS analyses of Temora-2, R-33, and a possible new standard from the Klamath Mountains reveal minor Pb loss in the first few partial dissolution steps, then yield excellent 206Pb*/238 plateau ages, and concordant 207Pb*/206Pb* ages for the remaining steps. Additional aliquots of these zircons were then "pretreated" by CA-TIMS, with sufficient partial dissolution to remove all vestiges of Pb loss, based on the earlier experiments. A sample of AS-57 was pre-treated "blind" because it was received just prior to a scheduled SHRIMP session. All samples were then prepared in a single mount for SHRIMP analysis. Our preliminary results from a round-robin Stanford SHRIMP-RG session using Temora-2, R-33, AS-57, and the Klamath sample are very promising. The Temora-2 and Klamath zircons in particular yielded excellent reproducibility

  20. LA-ICPMS U-Pb Dating of Zircons from Tuffs of the Upper Permian-Middle Triassic in Youjiang Basin,Guangxi Province and Its Geological Significance%右江盆地上二叠统—中三叠统凝灰岩年龄及其地质意义

    黄虎; 杨江海; 杜远生; 黄宏伟; 黄志强; 谢春霞; 胡丽沙


    Upper Permian-Middle Triassic sections in Youjiang basin contain many volcanic tuff horizons. Zircon LA-ICP-MS U-Pb dating of 4 volcanic tuff samples from the bottom of the Luolou and Baifeng formations in the Jiuxu and Lizhou areas, Guangxi Province, yield the depositional ages of 252. 8 + 1. 8, 253. 0 + 2. 4, 247. 2+1. 5 and 239. 3±2. 7 Ma, respectively, which, together with the inherited or xenocrystal zircon ages (with peak values of 264 and 267 Ma), indicate that episodical volcanism occurred during Middle Permian Gudalupian and Middle Triassic Bithynian in southwestern China The appearance of the inherited zircon ages (260-270 Ma) from the volcanic tuffs around the Permian-Triassic boundary (PTB) indicate the volcanic events in South China may have caused the End-Permian extinction. The depositional age of 247. 2 + 1. 5 Ma from the volcanic tuff at the bottom of the Middle Triassic Baifeng Formation indicates it is more appropriate to place the Early to Middle Trissic boundary at 247. 2 Ma. The petrographical study of the volcanic tuffs indicates the Youjiang basin expierenced the tectonic transition from extension to compression after Middle Triassic, and especially after Bithynian, which resulted in the great tectonic subsidence of the basin.%右江盆地上二叠统—中三叠统地层中常含有多层凝灰岩,对采自广西河池九圩及田林利周罗楼组和百逢组底部的4件凝灰岩样品进行锆石原位LA-ICPMS方法U-Pb同位素定年,结果显示凝灰岩的形成年龄分别为252.8±1.8Ma、253.0±2.4 Ma、247.2±1.5Ma和239.3±2.7Ma,指示晚二叠世末期到中三叠世Bithynian期华南地块西南缘存在多期火山活动,其可能与华南西南缘陆块的裂解与汇聚有关.凝灰岩中继承或捕虏锆石年龄(260~270Ma)记录了中晚二叠世峨眉山大火成岩省岩浆活动或华南西南缘岛弧火山活动事件.凝灰岩的岩相学研究表明,中三叠世特别是Bithynian期之后右江盆地由伸展

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



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

  2. Determinations of rare earth element abundance and U-Pb age of zircons using multispot laser ablation-inductively coupled plasma mass spectrometry.

    Yokoyama, Takaomi D; Suzuki, Toshihiro; Kon, Yoshiaki; Hirata, Takafumi


    We have developed a new calibration technique for multielement determination and U-Pb dating of zircon samples using laser ablation-inductively coupled plasma mass spectrometry (ICPMS) coupled with galvanometric optics. With the galvanometric optics, laser ablation of two or more sample materials could be achieved in very short time intervals (~10 ms). The resulting sample aerosols released from different ablation pits or different solid samples were mixed and homogenized within the sample cell and then transported into the ICP ion source. Multiple spot laser ablation enables spiking of analytes or internal standard elements directly into the solid samples, and therefore the standard addition calibration method can be applied for the determination of trace elements in solid samples. In this study, we have measured the rare earth element (REE) abundances of two zircon samples (Nancy 91500 and Prešovice) based on the standard addition technique, using a direct spiking of analytes through a multispot laser ablation of the glass standard material (NIST SRM612). The resulting REE abundance data show good agreement with previously reported values within analytical uncertainties achieved in this study (10% for most elements). Our experiments demonstrated that nonspectroscopic interferences on 14 REEs could be significantly reduced by the standard addition technique employed here. Another advantage of galvanometric devices is the accumulation of sample aerosol released from multiple spots. In this study we have measured the U-Pb age of a zircon sample (LMR) using an accumulation of sample aerosols released from 10 separate ablation pits of low diameters (~8 μm). The resulting (238)U-(206)Pb age data for the LMR zircons was 369 ± 64 Ma, which is in good agreement with previously reported age data (367.6 ± 1.5 Ma). (1) The data obtained here clearly demonstrate that the multiple spot laser ablation-ICPMS technique can become a powerful approach for elemental and isotopic

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

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


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

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

    Ehsan Salati


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

  5. 黑龙江完达山地区358高地岩金矿床花岗斑岩LA-ICP-MS锆石U-Pb定年及其地质意义%LA-ICP-MS zircon U-Pb dating of granite-porphyry from 358 Upland rock gold deposit in Wanda Mountain area, Heilongjiang Province and its geological significance

    王庆磊; 杨言辰; 李骞; 马晓阳; 谈艳; 张国宾


    The 358 Upland rock gold deposit is located at the east edge of the Wanda orogenic belt, being part of the Circum-Pacif-ic metallogenic belt. The authors selected the granite porphyry closely associated with mineralization in the study area to conduct a systematic study of geology, petrography, element geochemistry and LA-ICP-MS zircon U-Pb dating results. According to the re-sults obtained, the granite porphyry is rich in silicon, sodium, potassium, aluminum and poor in magnesium and calcium, the Rittman index (σ) is 1.12 to 2.00(σ1.1). The granite-porphyry is enriched in LILE, relatively depleted in Ba, Nb and Ta, severely depleted in Sr, P and Ti; enriched in LREE, and depleted in HREE, with Eu anomalies. These characteristics reveal that the granite-porphyry is peraluminous, and the S type granite and magma might have been of shell source. LA-ICP-MS zircon dating methods were used to determine the age of the granite-porphyry. It is thus concluded that the gold metallogenic epoch should be late Early Cretaceous. Combined with the tectonic discrimination diagrams, the porphy-ry-granite shows the characteristics of post-collisional granite, and the formation of the porphyry might have been closely related to the post-collisional extension of the Pacific plate.%选取黑龙江省完达山地区358高地与成矿关系密切的花岗斑岩,进行系统的地质、岩相学、元素地球化学及LA-ICP-MS锆石U-Pb定年研究.结果显示:花岗斑岩具有富硅、钠、钾、铝,贫镁、钙的特征,里特曼指数(σ)为1.12~2.00(σ1.1);富集大离子亲石元素,亏损Nb、Ta,严重亏损Sr、P、Ti;轻稀土元素富集,重稀土元素亏损,具有负Eu异常.这些特征揭示,花岗斑岩是过铝质的S型花岗岩,岩浆来源可能为壳源.LA-ICP-MS锆石定年获得花岗斑岩的成岩年龄为128±1Ma,推断金矿成矿时代应为早白垩世晚期.构造判别图解显示,花岗斑岩具有碰撞后花岗岩的特征,斑岩体的形成很可能

  6. Zircon SHRIMP U-Pb ages of the "Xinghuadukou Group" in Hanjiayuanzi and Xinlin areas and the "Zhalantun Group" in Inner Mongolia, Da Hinggan Mountains

    MIAO LaiCheng; LIU DunYi; ZHANG FuQin; FAN WeiMing; SHI YuRuo; XIE HangQiang


    A report is presented of SHRIMP zircon U-Pb dating data of meta-igneous and meta-sedimentary rocks of the Xinghuadukou Group (Xinlin-Hanjiayuanzi area, Heilongjiang Province) and meta-volcanic rocks of the Zhalantun Group (Zhalantun district, Inner Mongolia). The SHRIMP analyses show that the meta-igneous rocks from the Xinghuadukou Group formed at 506±10-547±46 Ma, belonging to Early-Middle Precambrian, whereas the meta-sedimentary rocks yielded detrital zircons, with ages of 1.0-1.2, 1.6-1.8 and 2.5-2.6 Ga, indicative of deposition age at least <1.0 Ga.Meta-basic volcanic rocks from the Zhalantun Group have a formation age of 506±3 Ma. These data suggest that both the Xinghuadukou and Zhalantun Groups formed during Cambrian and/or Neoproterozoic time, rather than Paleoproterozoic time as previously thought. Early Precambrian inherited zircons in the meta-igneous rocks and numerous Precambrian detrital zircons in the meta-sedimentary rocks imply that these rocks were formed proximal to older crust. It is inferred that the Xinghuadukou and Zhalantun Groups represent Cambrian and/or Neoproterozoic volcano-sedimentary sequences formed in an active continental margin setting.

  7. Zircon U-Pb Ages from an Ultra-High Temperature Metapelite, Rauer Group, East Antarctica: Implications for Overprints by Grenvillian and Pan-African Events

    Wang, Yanbin; Tong, Laixi; Liu, Dunyi


    SHRIMP U-Pb dating of zircon from an ultra-high temperature (UHT, ~1000 °C) granulite-facies metapelite from the Rauer Group, Mather Peninsula, east Antarctica, has yielded evidence for two episodes of metamorphic zircon growth, at ~1.00 Ga and ~530 Ma, and two episodes of magmatism in the source region for the protolith sediment, at ~2.53 and ~2.65 Ga, were identified from the zircon cores. Successive zircon growth at ~1.00 Ga and ~530 Ma records a sequence of distinct, widely spaced high-temperature metamorphic and/or anatectic events related to Grenvillian and Pan-African orogenesis. This study presents the first robust geochronological evidence for the timing of UHT metamorphism of the Rauer Group, supporting arguments that the peak UHT metamorphic event occurred at ~1.00 Ga and was overprinted by a separate high-grade event at ~530 Ma. The new age data indicate that the UHT granulites of the Rauer Group experienced a complex, multi-stage tectonothermal history, which cannot simply be explained via a single Pan-African (~500 Ma) high-grade tectonic event. This is critical in understanding the role of the eastern Prydz Bay region during the assembly of the east Gondwana supercontinent, and the newly recognized inherited Archaean ages (~2.53 and ~2.65 Ga) suggest a close tectonic relationship between the Rauer Group and the adjacent Archaean of the Vestfold Hills

  8. Trace element signature and U-Pb geochronology of eclogite-facies zircon, Bergen Arcs, Caledonides of W Norway

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

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

  9. U-Pb ases and Hf isotopes for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southwestern margin of the North China Craton

    DIWU ChunRong; SUN Yong; YUAN HongLin; WANG HongLiang; ZHONG XingPing; LIU XiaoMing


    In situ U-Pb dating and Lu-Hf isotopic analysis were carried out for detrital zircons from quartzite in the Paleoproterozoic Songshan Group on the southern margin of the North China Craton (NCC). The re-sults provide further constraints on the crustal formation and evolution history of NCC. Four 207Pb/206Pb age populations were obtained from 99 analyses, with clusters at ~3.40 Ga, 2.77-2.80 Ga, ~2.50 Ga and 2.34 Ga, respectively. The 3.40 Ga old zircons have similar Hf isotopic compositions to those from Ar-chean rocks in the Jidong and Anshan areas of NCC. However, crustal remnants older than 3.6 Ga have been identified in the southern margin of NCC, the South China Craton, the northwestern part of the Qinling Orogen and its adjacent area. Thus, it is not easy to trace the source rock from which the 3.40 Ga detrital zircons were derived. It can be inferred that the crustal remnants older than 3.40 Ga might have been widely distributed in the North China Craton. The 2.77-2.80 Ga zircons make up a relatively small proportion and have the highest ε (t) values (up to 6.1±1.6), consistent with the Hf isotopic composition of the depleted mantle at 2.83 Ga. Their single-stage Hf model age of 2.83 Ga is close to their crystallized age, suggesting that their source rocks were extracted from the contemporaneous depleted mantle. The ~2.50 Ga zircon grains constitute about 85% of the total grain population and their Hf isotopic compositions indicate major growth of juvenile crust at ~2.50 Ga but minor reworking of ancient crust. The youngest zircon dated in this study gave an U-Pb age of 2337±23 Ma, which can be considered the maximum depositional age of the formation of the Songshan Group.

  10. Palaeoproterozoic U Pb SHRIMP zircon age from basement rocks in Bangladesh: A possible remnant of the Columbia supercontinent

    Hossain, Ismail; Tsunogae, Toshiaki; Rajesh, Hariharan M.; Chen, Bin; Arakawa, Yoji


    We present new U-Pb SHRIMP zircon geochronological data for basement rocks in Bangladesh, and discuss the relationship with the formation of the Columbia supercontinent. Euhedral zircons from a diorite sample yield a concordia age of 1730 ± 11 Ma, which is interpreted as the crystallization age. The Palaeoproterozoic age of the examined basement rock and the common occurrences of similar ˜1.7-Ga geologic units in the Central Indian Tectonic Zone and Meghalaya-Shillong Plateau in Indian Shield suggest their apparent continuation. This, together with the occurrence of similar ˜1.7-Ga geologic units in the Albany-Fraser belt in Australia and East Antarctica, are used to suggest that the basement rocks in Bangladesh formed towards the final stages of the assembly of the Columbia supercontinent.

  11. Constraints on U-Pb Dating of Detrital Zircon of the Maximum Depositional Age for Upper Permian to Lower Triassic Strata in Xishan,Taiyuan%太原西山上二叠统-下三叠统地层最大沉积年龄的碎屑锆石U-Pb定年约束

    刘超; 孙蓓蕾; 曾凡桂


    Permian to Triassicis an important turning point of geological history during the transition period of Paleozoic to Mesozoic,in which the largest mass extinction events in geological history happened.As the transition stage of Permian to Triassic,the era of Shiqianfeng Formation is crucial to define the boundary of Permian to Triassic.In order to determine the era of Shiqianfeng Formation in Xishan,Taiyuan,North China,the laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) was applied to date the U-Pb age of detrital zircon.Four sandstone samples (SNF,XM,K8 ,LJG)from the marker layers of strata division and correlation from the Xiashihezi Formation,Shiqianfeng Formation and Liujiagou Formation were analyzed.Four sandstone samples SNF,XM,K8,LJG obtained youngest graphical detrital zircon age were 270 Ma,296 Ma,250 Ma,262 Ma,respectively.The results show that the deposition time of Shinaofeng sandstone(SNF)was not earlier than 270Ma,the deposition time of K8 sandstone(K8)was not earlier than 250Ma.Therefore,Shiqianfeng Formation of Xishan,Taiyuan should belong to the Lower Triassic,and the boundary of Permian to Triassic lied under the K8 sandstone.%二叠纪-三叠纪之交是古生界向中生界过渡的重要地质历史转折时期,发生了地史时期规模最大的生物灭绝事件。石千峰组作为华北地台二叠系-三叠系过渡阶段沉积,其时代归属对二叠系-三叠系界线的准确划分至关重要。本文应用LA-MC-ICPMS对华北中部太原西山晚二叠世-早三叠世上石盒子组、石千峰组、刘家沟组中4件砂岩样品进行碎屑锆石U-Pb定年,4件砂岩样品(SNF、XM、K8、LJG)分别给出了最年轻锆石峰值年龄270Ma、296Ma、250Ma、262Ma,表明太原西山上石盒子组师脑峰砂岩(SNF)的沉积时间不早于270Ma,石千峰组底部K8砂岩(K8)沉积时间不早于250Ma。因此,太原西山地区石千峰组应归属于早三叠世

  12. 新疆库鲁克塔格阿訇开里得南石炭纪花岗岩LA-ICP-MS锆石U-Pb定年及其地质意义%Zircon LA-ICP-MS U-Pb dating of South Ahongkailide Carbonifereous granite in Kuruktag of Xinjiang and its geological implications

    尼加提·阿布都逊; 郭瑞清; 帕拉提·阿布都卡迪尔; 贾晓亮; 秦切; 朱志新; 王克卓


    对出露于新疆库鲁克塔格地块西段的阿訇开里得南花岗岩进行了详细的岩石学、地球化学及锆石U-Pb定年研究,并探讨了其构造背景及地质意义.岩体LA-ICP-MS锆石U-Pb原位定年获得年龄为340.6Ma±5.7Ma,属于石炭纪花岗岩.地球化学分析显示岩体具高硅、高铝、高钾,贫钠、低钙、低磷、低钛和贫镁的特征,铝饱和指数(A/CNK)为1.14~2.31,CIPW标准刚玉分子大于1%,属高钾钙碱性过铝质花岗岩.在稀土元素球粒陨石标准化配分模式图上,轻稀土元素(LREE)富集、重稀土元素(HREE)亏损,轻重稀土元素分馏明显,Eu出现中等负异常(δEu=0.52~0.62);在微量元素原始地幔标准化蛛网图上,明显富集大离子亲石元素Rb、Ba、Th和K,亏损高场强元素Nb、Ta、Ti.综合分析研究认为,岩体的形成与加厚地壳引起的地壳重熔有关,反映了塔里木板块与伊犁-中天山板块陆-陆碰撞的岩浆活动.%The South Ahongkailide Carboniferous granite is exposed in the west segment of Kuruktag block in Xinjiang. This paper has studied petrographical, geochemical and zircon U-Pb chronolgical characteristics of this granite as well as its petrogenesis and geological significance. Zircon LA-ICP-MS dating yielded the age of 340.6Ma±5.7Ma, implying that the magmatic crystallization epoch of the granite is Carboniferous. Geochemical analyses show that the granite has high SiO2, A12O3, K2O but relatively low CaO, P2O5, TiO2 and MgO. The ratios of A/CNK vary from 1.14 to 2.31, suggesting high-K calc- alkaline peraluminous syn-collision granite. The chondrite-normalized REE patterns show that the granite is enriched in LREE but depleted in HREE, with high fractionations between LREE and HREE and moderate negative Eu anomalies ((δEu=0.52~0.62). In addition, the granite is relatively rich in such LILEs as Rb,Ba.Th and K, but poor in such HFSEs as Nb, Ta and Ti, with relative enrichment of Ce in the primitive mantle

  13. Zircon SHRIMP U-Pb dating and geological significance of granite in the Baogeda Ula Mo (W) mining area, Inner Mongolia, China%内蒙古宝格达乌拉钼(钨)矿区花岗岩锆石SHRIMPU-Pb年龄及地质意义

    刘勇; 聂凤军; 刘翼飞; 侯万荣


    本文首次对新发现的宝格达乌拉钼(钨)矿区中含矿花岗岩的锆石进行了SHRIMP U-Pb定年,获得9个锆石颗粒的206Pb/238U年龄的加权平均值为240.9±2.5Ma,MSW值为1.7,上述数值表明岩体形成时代应为印支期.根据野外实地勘查以及进一步室内研究工作,可以初步推测出,该地区在中生代时期曾经发生过古陆块内部拉张作用,由此所诱发的大规模构造-岩浆活动及相关流体活动为该区钼(钨)矿床的形成提供了物质、动力和热力来源.区内含矿花岗岩即是此次构造-岩浆作用产物.除此之外,由于宝格达乌拉地区位于早期西伯利亚板块和华北板块相互俯冲和碰撞的交界处,区域内花岗岩体在形成过程中受到了多期次岩浆热液作用的影响,因此在对区域内锆石年龄及相关微量元素进行测定时发现,该区域内含矿岩体岩性复杂,表现出了一定的继承性.这也为研究人员对二连浩特-东乌珠穆沁旗一带矿床成因作用的进一步研究提供了重要参考.%It's the first time that the zircon SHRIMP U-Pb dating of granite in the Baogeda Ula Mo (W) mining area which is found recently, and obtain the weighted average age of the nine zircons is 240. 9 ± 2. 5 Ma, the MSW is 1. 7, Indosinian. According to the fieldwork and indoor studies, we could speculate that the extension of ancient land mass in Mesozoic occurred in this area. Large scale tectonic-magmatic activities and related to fluid which induced by the extension provide the material, power and heating sources for the deposit in Baogeda Ula Mo (W) mining area. The granite in this area is the result of tectonic-magmatic activities. Beyond that, we found the ore-bearing rock with complex lithological characters and some succession characters. It' s because of that this area is at the junction after the subduction and collision between Early Siberian plate and the North China plate. Its also provide important reference

  14. 大兴安岭中段玛尼吐组火山岩LA-ICP-MS锆石U-Pb年龄及地球化学特征%W.Zircon U-Pb dating and geochemical study of volcanic rocks in Manitu Formation of central Da Hinggan Mountains

    李世超; 徐仲元; 刘正宏; 李永飞; 王兴安; 张超; 范志伟


    In this study, LA-ICP-MS zircon U-Pb dating results and geochemical data were obtained from the volcanic rocks in Mani-tu Formation of Chaihe area lying in southwest Zhalantun City of central Da Hinggan Mountains. Zircon U-Pb dating results show that they were formed in early Cretaceous(135Ma). The volcanic rocks belonging chemically to the shoshonite series have SiO2 59.5%~70.8% and are enriched in alkali and potassium, with Rittman index from 3.2 to 4.7(σ=3.2~4.7). In addition, they are characterized by low total rare earth elements (EREE=112×10-6~172×10-6), significant fractionations of REE(La/Yb)N=6.5~9.2), moderate negative Eu anomalies (δEu=0.54~0.77), enrichment of large ion lithophile elements (LILEs, such as Ba and K) and light rare earth elements (LREEs) and depletion of high field strength elements (HFSEs, such as Nb, Ti and P). The values of such iron family elements as Cr, Co and Ni are all low (Cr=6.55×10-3, Cr=6.97×10-6, Ni=4.35× 10-6 on average). Recent researches show that the volcanic rocks might have been formed in an extensional setting after the closure of the Mongol-Okhotsk Ocean. Considering the regional resources, the authors have reached the conclusion that the volcanic rocks in Manitu Formation of central Da Hinggan Mountains region were formed in two structural environments. Total REE amounts are relatively law(∑REE=112×10-6~M72×10-6), fractionation between LREE and HREE (La/Yb)N=6.5~9.2) is obvious, and there exist moderate negative Eu anomalies (δEu=0.54~0.77). They are enriched in high LILE (e.g., Ba, K) and LREE, and depleted in HFSE (e.g., Nb, Ti, P) and iron family elements such as Cr, Co and Ni(Cr=6.55×10-6, Cr=6.97×10-6, Ni=4.35×10-6 on average). The research above shows that the tectonic background of Manitu Formation volcanics in the study area are formed on the lithospheric extensional structure after the closure orogeny of Mongolia -Ehucike, which is similar to the north section of the Great Xing

  15. U-Pb zircon constraints on the age and provenance of the Rocas Verdes basin fill, Tierra del Fuego, Argentina

    Barbeau, David L.; Gombosi, David J.; Zahid, Khandaker M.; Bizimis, Michael; Swanson-Hysell, Nicholas; Valencia, Victor; Gehrels, George E.


    The Late Jurassic to Early Cretaceous Rocas Verdes basin constitutes one of the most poorly understood components of the southernmost Andes. As a result, accurate reconstructions and interpretations of deformation associated with the Andean orogeny and the kinematics of Scotia arc development also remain poorly constrained. In this data brief, we report U-Pb zircon ages from sandstones of the Rocas Verdes basin fill and from a crosscutting pluton in the southernmost Andes of Argentine Tierra del Fuego. Detrital samples contain predominant Early to early Middle Cretaceous (circa 130-105 Ma) U-Pb zircon age populations, with very small or single-grain middle Mesozoic and Proterozoic subpopulations. A very small subpopulation of Late Cretaceous ages in one sample raises the unlikely possibility that parts of the Rocas Verdes basin are younger than perceived. A sample from a crosscutting syenitic pegmatite yields a crystallization age of 74.7 +2.2/-2.0 Ma. The data presented herein encourage further geochronologic evaluation of the Rocas Verdes basin in order to better constrain the depositional ages and provenance of its contents.

  16. U-Pb ages of detrital zircon and provenances of Red Clay in the Chinese Loess Plateau

    Gong, Hujun; Xie, Wenbin; Zhang, Rui; Zhang, Yunxiang


    The Chinese Loess Plateau (CLP), which is located in the central part of China, mainly contains two sequences: the Quaternary loess-paleosol sequence and the late Miocene-Pliocene Red Clay sequence. U-Pb ages of detrital zircon from the Quaternary loess-paleosol sequences indicate that the current spatial pattern of central Asian aridification was largely established during the early Quaternary. U-Pb ages of detrital zircon from the underlying Red Clay sequences demonstrate that the northern source regions were important contributors for the dust since the late Miocene, except for the periods of ∼8 Ma and ∼5.5 to ∼4 Ma. These two periods correspond to increased and decreased dust deposition in the North Pacific, respectively, ascribed to the late Miocene aridification of the Qaidam Basin (in the west of the CLP) and the early Pliocene aridification of the Tarim Basin (in the west of the CLP). We attribute the increase of the Qaidam Basin-derived North Pacific dust fluxes to the uplift of the NE Tibetan Plateau. Such uplift also controlled the climatic and environmental changes in central Asian during the late Miocene. This study highlights the dynamical process of the central Asian aridification during the late Miocene-Pliocene.

  17. Zircon U-Pb Age Determination of Volcanic Eruptions in Lutao and Lanyu in the Northern Luzon Magmatic Arc

    Wen-Yu Shao


    Full Text Available This paper reports for the first time zircon U-Pb ages of volcanic rocks and sands from Lutao and Lanyu, two islets off SE Taiwan in the north Luzon arc. The samples include (1 seven andesites from four volcanic units and three river/beach sands from Lutao and (2 five basaltic andesites from four volcanic units and two river/beach sands from Lanyu. The Lutao andesites contain abundant magmatic zircons, aging from ~1.54 to ~1.24 Ma for individual sample, which yielded an overall mean 206Pb/238U age of 1.31 ±± 0.03 Ma (n = 190, MSWD = 2.6. This is slightly older than, or broadly coincident with, a mean 206Pb/238U age of 1.23 ±± 0.03 Ma (n = 103, MSWD = 1.9 given by detrital zircons from the three sands. The Lanyu volcanics appear to have less abundant magmatic zircons, aging from ~2.72 to ~2.35 Ma for individual sample, which yielded an overall mean 206Pb/238U age of 2.61 ±± 0.13 Ma (n = 11, MSWD = 1.8. This accords with a mean 206Pb/238U age of 2.69 ±± 0.11 Ma (n = 34, MSWD = 4.7 obtained by detrital zircons from the two sands. The age data suggest that in Lutao and Lanyu the major volcanic eruptions occurred at ~1.3 and ~2.6 Ma, respectively. Moreover, volcanic samples from both islets contain various amounts of older inherited zircons, ~11% in Lutao and up to ~82% in Lanyu, which together with detrital zircons from the sands show main age peaks at ~150 Ma and ~1.9 and ~2.5 Ga, consistent with the notion for a _ continental crust involved in the genesis of the northern Luzon magmatic arc.

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

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


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

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

    Reza Monazzami Bagherzadeh


    Full Text Available Introduction The Bornaward area is located in the Northeastern Iran (in the Khorasan Razavi province 28 km northwest of the city of Bardaskan at 57˚ 46΄ to 57˚ 52΄ N latitude and 35˚ 21΄ to 35˚ 24΄E longitude. The Taknar structural zone, situated in the North central Iranian micro continent, is part of the Lut block (Forster, 1978. The Taknar zone is an allochthonous block bounded by the Darouneh and Taknar major faults. Much of this zone consists of metarhyolite-rhyodacite volcanic rocks, and rhyolitic tuff with interlayers of sandstone and dolomite (Taknar Formation. Analytical Results ICP-MS analysis of REE and minor elements of samples of the Bornaward metarhyolites was carried out at the ACME Laboratory in Vancouver, Canada. U-Pb dating of the metarhyolites was performed on isolated zircons in Crohn's Laser Lab, in Arizona (Gehrels et al., 2008. Measurement of Rb, Sr, Sm and Nd isotopes and (143Nd/144Ndi and (87Sr/86Sri ratios took place in the radioisotope laboratory of the University of Aveiro in Portugal. Petrography The volcanic rocks are porphyritic, commonly containing phenocrysts of orthoclase and rarely sanidine, quartz and intermediate plagioclase in a groundmass of fine-grained quartz and feldspar. An alteration has produced oriented needles of sericite and clay minerals, clusters of fine-grained green biotite and clots of epidote and chlorite. Geochemistry The compositions of the volcanic rocks are calc alkaline and high K- calc alkaline. The obtained Shand index (Al2O3/( CaO+Na2O+K2O is above 1.1, in the peraluminous S-type granite field (Chappell and White, 2001. Plotted on the TAS diagram (Middlemost, 1994, all the metarhyolite-rhyodacite samples are located in the sub-alkaline field and the majority fall into the rhyolite group. The metarhyolite-rhyodacites show enrichment of LREE with a moderately ascending pattern ((La/YbN=2.51-10.11 and La=46.45-145.48. Europium shows a negative anomaly (Eu/Eu*=0.23-0.71. U-Pb

  20. Detrital zircon U-Pb geochronology and provenance of the Carboniferous-Permian glaciomarine pebbly slates in the Tibetan Plateau

    Wang, Q.; Zhu, D.; Zhao, Z.; Chung, S.; Li, C.; Sui, Q.; Fu, X.; Mo, X.


    Glaciomarine diamictites (including pebbly slate, pebbly siltstone, and pebbly sandstone) in the Tibetan Plateau are widely interpreted to have been associated with the deglaciation of the Indian continent. Guiding by zircon cathodoluminescence images, we determined U-Pb ages for detrital zircons from five typical Carboniferous-Permian pebbly slate samples from the Qiangtang, Lhasa, and Tethyan Himalaya of the Tibetan Plateau. The age distributions of detrital zircons from two samples (180 analyses) from Qiwu and Gangma Tso of the Qiangtang Terrane are similar, with two main age peaks ca. 579 and ca. 816 Ma and one minor age peak ca. 2490 Ma. Two samples (177 analyses) from Jiangrang and Damxung of the Lhasa Terrane define similar age distributions with two main age peaks ca. 539 and ca. 1175 Ma. Ages of detrital zircons from one sample (110 analyses) from Kangmar of the Tethyan Himalaya display main age peaks ca. 535, ca. 949, and ca. 2490 Ma. The ca. 816-Ma detrital zircons from the Qiangtang Terrane were most likely derived from the Lesser Himalaya, and the ca. 950-Ma detrital zircons from the Tethyan Himalaya might have been sourced from the High Himalaya, Eastern Ghats Province of the Indian plate and the Rayner Province of East Antarctica. The distinctive ca. 1175-Ma age population characteristic of zircons in the pebbly slates from the Lhasa Terrane is identical to the detrital zircons from the late Paleozoic sandstones (Zhu et al., 2011a) and the inherited zircons from the Mesozoic peraluminous granites (Zhu et al., 2011b) in this terrane, but significantly absent in the pebbly slates from both the Qiangtang and the Tethyan Himalayan terranes. The ca. 1175-Ma detrital zircons in the Lhasa Terrane were most likely sourced from the Albany-Fraser-Wilkes in southwestern Australia and East Antarctica. These new data obtained in this study reveal a distinct difference of detrital zircon provenance for the coeval Carboniferous-Permian glaciomarine pebbly slates

  1. U-Pb zircon geochronology, petrochemical and Sr-Nd isotopic characteristic of Late Neoproterozoic granitoid of the Bornaward Complex (Bardaskan-NE Iran)

    Bagherzadeh, R. Monazzami; Karimpour, M. H.; Farmer, G. Lang; Stern, C. R.; Santos, J. F.; Rahimi, B.; Heidarian Shahri, M. R.


    The Bornaward Granitoid Complex (BGC) in the Taknar Zone is located in the northeast of Central Iranian Block. The BGC consists of granite, alkaligranite, syenogranite, leucogranite, granophyre, monzogranite, granodiorite, tonalite and diorite that have intruded into the center of Taknar Zone. These intrusive rocks affected by low grade metamorphism. Because of there are no reliable isotope dating data, for the Bornaward Granitoid Complex rocks have been proposed discordant ages (Jurassic, Cretaceous or even younger ages) by many studies. In the present study, new isotopic information based on zircon U-Pb dating has revealed the origin and time of the formation of the BGC. These new results do not confirm previously proposed ages. The results obtained from zircon U-Pb dating of the BGC rocks suggest late-Neoproterozoic (Precambrian) age (540-550 Ma). The Bornaward Granitoid Complex is middle-high metaluminous to lower-middle peraluminous and belongs to tholeiite, calc-alkaline to high-K calc-alkaline rock series with enrichment in LIL (Cs, Rb and Ba, U, K, Zr, Y, Th) and depletion in HIL (Sr and Nb, Ta, Ti) elements. Chondrite-normalized Rare Earth Elements (REE) plots indicate minor enrichment of LREE compared to HREE, and strong negative anomaly of Eu compared to other Rare Earth Elements. Furthermore, initial 87Sr/86Sr and 143Nd/144Nd range from 0.70351 to 0.71689 and 0.511585 to 0.512061, respectively, and initial εNd isotope values for granite, granodiorite and diorite range from -6.73 to 2.52. These all indicate that the BGC has derived from partial melting of distinct basement source regions with very high initial 87Sr/86Sr and undergoing extensive crustal contamination (S-type granite).

  2. 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

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


    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

  3. Geochemistry and SHRIMP Zircon U-Pb Age of Post-Collisional Granites in the Southwest Tianshan Orogenic Belt of China: Examples from the Heiyingshan and Laohutai Plutons

    LONG Lingli; GAO Jun; WANG Jingbin; QIAN Qing; XIONG Xianming; WANG Yuwang; WANG Lijuan; GAO Liming


    The Heiyingshan granite and the Laohutai granite plutons exposed in the Southwest Tianshan resemble A-type granites geochemically. Analysis shows that the both are ferron calc-alkalic peraluminous or ferron aikali-calcic peraluminous with a relatively high concentration of SiO2 (>70%), high alkali contents (Na2O + K2O = 7.14%-8.56%; K2O>N2O; A/CNK = 0.99-1.20), and pronounced negative anomales in Eu, Ba, Sr, P and Ti. A SHRIMP zircon U-Pb age of 285±4 Ma was obtained for the Heiyingshan hornblende biotite granite intrusion. The geochemical and age dating data reported in this paper indicate that these granites were formed during the post-collisional crustal extension of the Southwest Tianshan orogenic belt, in agreement with the published data for the granites in the South Tianshan.

  4. Zircon U-Pb Dating and Ore-bearing Properties Evaluation of Yanshanian Shuitou Pluton in Southern Jiangxi Province%赣南燕山期水头岩体的锆石铀-铅年代学研究及其含矿性评价

    于扬; 陈振宇; 陈郑辉; 侯可军; 赵正; 许建祥; 张家菁; 曾载淋


    水头岩体是目前赣南地区缺少精确年龄数据的复式岩体之一.对赣南水头岩体中的锆石样品进行了U-Pb法年代学研究,测年结果显示该岩体主体年龄为(142.7±0.6)Ma,属于燕山期的产物,并不是前人认为的加里东期.但也存在前期(海西期和加里东期)的同位素年龄信息记录,说明该岩体是复式岩体,存在至少三期岩浆活动.对水头岩体进行了岩石学、地球化学研究,简要论述了岩石成因及含矿性.水头岩体主体为黑云母花岗岩,为过铝质钾玄岩系,轻稀土和重稀土中等分馏,负Eu异常明显.初步认为水头岩体是原地重熔交代作用形成的复式岩体.岩石化学资料数据显示,该岩体具备稀土成矿母岩的基本条件.%The Shuitou pluton is one of the composite plutons without accurate dating age in Southern Jiangxi province. Zircon samples taken from the Shuitou pluton were selected for U-Pb dating. The obtained age of (142.7 ± 0.6) Ma confirmed that this granitic pluton belongs to the Yanshanian era and not the Caledonian era as a previous study has reported. Other old ages were also obtained for this pluton (such as Hercynian and Caledonian), which indicates that there were at least three periods of magmatic activity. This study indicates that Shuitou pluton is a composite pluton. Combining the pelrological and geochemical study on this pluton, petrogenesis and ore-bearing properties are illustrated briefly in this paper. The Shuitou pluton mainly consists of biotite granites, which can be attributed to the peraluminous shoshonitic series with a middle fraclionation of light rare earth element (LREE) and heavy rare earth element (HREE), and obvious Eu anomalies. These data indicate that the Shuitou pluton is a composite pluton formed by the result of in-situ-melting of granite. The geochemical data from this study show that this pluton has major mineralization conditions of mother rock for REE deposits.

  5. 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

    Henrichs, Isadora A.; Frantz, Jose Carlos; Marques, Juliana C.; Castoldi, Marco S., E-mail:, E-mail:, E-mail: [Universidade Federal do Rio Grande do Sul (UFRS), Porto Alegre, RS (Brazil). Instituto de Geociencias; Ordonez-Carmona, Oswaldo, E-mail: [Universidad Nacional de Colombia, Medellin (Colombia). Facultad de Minas; Sato, Kei, E-mail: [Universidade de Sao Paulo (USP), Sao Paulo, SP (Brazil). Centro de Pesquisas Geocronologicas


    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)

  6. High-pressure granulite from Western Kunlun,northwestern China:Its metamorphic evolution,zircon SHRIMP U-Pb ages and tectonic implication


    High-pressure mafic granulites occurring as lenticular bodies within garnet-amphibolites in Kangxiwar Fault have been first reported in this paper. The P-T conditions of two metamorphic stages were ob-tained using calibrated geothermal barometers and ThermoCalc Program. The peak metamorphic con-dition of these high-pressure granulites is about 760―820℃,1.0―1.2 GPa and the retrograde meta-morphic condition is about 620―720℃,0.7―0.8 GPa. The petrological studies show that they have a near-isobaric cooling P-T path which suggests that the Western Kunlun underwent initial crustal thickening,subsequent exhumation and cooling. The SHRIMP zircon U-Pb dating gives two groups of ages for high-pressure granulites. One is 177±6 Ma which is obtained from the rim of the zircon. We consider this age should be the metamorphic age. And the other is 456±30 Ma which is obtained from the core of the zircon and should be the protolith age. The formation of these high-pressure granulites in western Kunlun is closely correlated with the evolution of the Paleo-Tethys and has important im-plications for the research on Tethys and Paleo-Asian tectonic zone.

  7. High-pressure granulite from Western Kunlun, northwestern China: Its metamorphic evolution, zircon SHRIMP U-Pb ages and tectonic implication

    QU JunFeng; ZHANG LiFei; AI YongLiang; L(U) Zeng; WANG JianPing; ZHOU Hui; WANG ShiYan


    High-pressure mafic granulites occurring as lenticular bodies within garnet-amphibolites in Kangxiwar Fault have been first reported in this paper. The P-T conditions of two metamorphic stages were obtained using calibrated geothermal barometers and ThermoCalc Program. The peak metamorphic condition of these high-pressure granulites is about 760-820℃, 1.0-1.2 GPa and the retrograde metamorphic condition is about 620-720℃, 0.7-0.8 GPa. The petrological studies show that they have a near-isobaric cooling P-T path which suggests that the Western Kunlun underwent initial crustal thickening, subsequent exhumation and cooling. The SHRIMP zircon U-Pb dating gives two groups of ages for high-pressure granulites. One is 177±6 Ma which is obtained from the rim of the zircon. We consider this age should be the metamorphic age. And the other is 456±30 Ma which is obtained from the core of the zircon and should be the protolith age. The formation of these high-pressure granulites in western Kunlun is closely correlated with the evolution of the Paleo-Tethys and has important implications for the research on Tethys and Paleo-Asian tectonic zone.

  8. U-Pb geochronology of zircon and monazite from Mesoproterozoic granitic gneisses of the northern Blue Ridge, Virginia and Maryland, USA

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


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

  9. Mesozoic Bimodal Volcanic Suite in Zhalantun of the Da Hinggan Range and Its Geological Significance:Zircon U-Pb Age and Hf Isotopic Constraints


    Mesozoic bimodal volcanic rocks of basaltic andesite and rhyolite are widely distributed in the Da Hinggan Range, but their petrogenetic relationships and geodynamic implications are rarely constrained. Detailed studies on doleritic and porphyry dikes in the Zhalantun area indicate that they display features of magma mixing, suggesting their coeval formation. In situ zircon U-Pb dating shows that the porphyry was emplaced in the Early Cretaceous with a 206Pb/238U age of 130±1 Ma. Zircons from the dolerite also yield an Early Cretaceous emplacement age of 124±2 Ma although some inherited zircons have been identified. These age results indicate that the Early Cretaceous was an important period of magmatism in the Da Hinggan Range. Zircons from porphyry are characterized by positive value of εHf(t) as high as 10.3±0.5 with Hf depleted mantle model age of 349-568 Ma, whereas magmatic zircons from the dolerite have εHf(t) value of 11.0±1.4 with Hf depleted mantel model age of 342-657 Ma,consistent with those from the porphyry. Considering other data on the geological evolution of this area,it is concluded that the mafic magma originated from the partial melting of Paleozoic enriched lithospheric mantle, whereas the felsic magma came from recycling of juvenile crust formed during the Paleozoic. Both of the protoliths are closely related to the subduction of the Paleo-Asian Ocean during the Paleozoic, indicating that the Paleozoic is an important period of large-scale crustal growth in the area.

  10. Parent zonation in thermochronometers - resolving complexity revealed by ID-TIMS U-Pb dates and implications for the application of decay-based thermochronometers

    Navin Paul, Andre; Spikings, Richard; Chew, David; Daly, J. Stephen; Ulyanov, Alexey


    High temperature (>350℃) U-Pb thermochronometers primarily use accessory minerals such as apatite, titanite and rutile, and assume that daughter isotopes are lost by thermally activated volume diffusion while the parent remains immobile. Studies exploiting such behaviour have been successfully used to reconstruct thermal histories spanning several hundred million years (e.g. Cochrane et al., 2014). However, outliers in date (ID-TIMS) vs diffusion length space are frequently observed, and grains are frequently found to be either too young or too old for expected thermal history solutions using the diffusion data of Cherniak et al. (2010). These deviations of single grain apatite U-Pb dates from expected behaviour could be caused by a combination of i) metamorphic (over-)growth, ii) fluid-aided Pb mobilisation during alteration/recrystallization, iii) parent isotope zonation, iv) metamictisation, and v) changes in diffusion length with time (e.g. fracturing). We present a large data set from the northern Andes of South America, where we compare apatite U-Pb ID-TIMS-(TEA) data with LA-ICP-MS element maps and in-situ apatite U-Pb LA-(MC)-ICP-MS dates. These are combined with U-Pb zircon and 40Ar/39Ar (muscovite) data to attempt to distinguish between thermally activated volume diffusion and secondary overgrowth/recrystallization. We demonstrate that in young (e.g. Phanerozoic) apatites that have not recrystallized or experienced metasomatic overgrowths, U-Pb dates are dominantly controlled by volume diffusion and intra-crystal uranium zonation. This implies that ID-TIMS analyses of apatites with zoned parent isotope distributions will not usually recover accurate thermal history solutions, and an in-situ dating method is required. Recovering the uranium distribution during in-situ analysis provides a means to account for parent zonation, substantially increasing the accuracy of the modelled t-T-paths. We present in-situ data from apatites where scatter in date v

  11. LA-ICP-MS Zircon U-Pb Dating of the Dacite Porphyry from Zijinshan Cu-Au Deposit and its Metallogenetic Implications%福建紫金山矿田晚中生代英安玢岩形成时代及其成矿意义

    胡春杰; 黄文婷; 包志伟; 梁华英; 王春龙


    The dacite porphyry occurs in the volcanic caldera of Zijinshan ore field and closely associates with the Cu-Au mineralization.The LA-ICP-MS zircon U-Pb dating of two dacite porphyry samples yields weighted mean ages of ( 105.0 ±0.7) Ma and ( 105.0 ±2.2) Ma,respectively.These results suggest that the dacite porphyry was formed proximately contemporarily with the other two Mo and Cu mineralized granite porphyries,i.e.,the Sifang and Luoboling granite porphyries.The dacite and granite intrusives are characterized by high Ce4+/Ce3+ ratios,which indicates that the magmatic system was of oxidative and therefore consistent with the huge Cu-Mo-Au ore-forming potential of the dacite and granite porphyries.Moreover,the similarities of age,geochemical characteristics and mineralization demonstrate that the Cu-Mo-Au mineralization in the Zijinshan ore field is most likely a unified porphyry ore-forming system.%福建紫金山Cu-Mo-Au-Ag多金属矿田是一个完整的斑岩-浅成低温热液矿床成矿体系.英安玢岩主要分布在紫金山Au-Cu矿床内火山机构及其附近,与高品位Au-Cu矿体空间上联系密切,是矿田内火山-侵入岩浆过程的一个重要环节.采自矿区的两个英安玢岩样品LA-ICP-MS锫石U-Pb定年结果分别为(105.0±0.7) Ma(MSWD =0.83)和(105.0±2.2) Ma (MSWD=1.70),其形成时代与区内Mo-Cu矿化的萝卜岭和四方岩体形成时代相近.三个岩体形成时代及地球化学特征的相似性表明它们应为同源岩浆演化的产物,它们均具有较高的锆石Ce4+/Ce3+值,即岩浆体系具有较高的氧逸度,与区内斑岩成矿体系巨量金属堆积的地质事实相一致.英安玢岩的精确定年表明,紫金山Cu-Au矿与萝卜岭Mo-Cu矿及四方岩体有关的Mo矿床为同一斑岩成矿体系的产物.

  12. Petrography and U-Pb Zircon Geochronology of Geological Units of the Mesa de Cocodite, Península de Paraguaná, Venezuela

    Mendi, D.; Baquero, M. L.; Oliveira, E. P.; Urbani, F.; Pinto, J.; Grande, S.; Valencia, V.


    Several continental crust units crop out in The Mesa de Cocodite, central Paraguaná Peninsula, Northwestern Venezuela, including a newly mapped quartz-feldspar gneiss that intruded the El Amparo Pluton, a major low-graded metamorphic unit of Permian age. It is unconformably overlying by Late Jurassic phyllites of the Pueblo Nuevo Formation. All these units are cross-cut by narrow dykes. This contribution focuses on the petrography and LA-ICP-MS U-Pb zircon dating of the igneous units, with the aim of constraining magmatism and its tectonic significance in the area. The quartz-feldspar gneiss consists of albite, bluish-smoked quartz, muscovite-chlorite, epidote and zircon. The enclosing El Amparo Pluton is typically a coarse-grained and thick-banded metagranodiorite, containing andesine, quartz, hornblende, epidote, K-feldspar, biotite, chlorite, titanite and zircon. The younger dykes consist of fine-grained, porphyritic hornblende diorites that contain numerous hornblende phenocrysts. The main minerals are andesine, hornblende, quartz, and chlorite. Accessory minerals include zircon, epidote and opaques. A felsic gneiss xenolith collected from the dykes contain quartz, K-feldspar, chlorite, epidote, albite and zircon. Zircon population in the quartz-feldspar gneiss displays a discordia trend, ranging in age from 1050-750 Ma but mostly around 950-900 Ma, which may represent a peak high-grade metamorphism in the area. The El Amparo Pluton provides a concordant Permian age (271.3±6.5 Ma), which is comparable with previous reported U-Pb ages in titanite. Because the porphyry dykes cross-cut all the units in the area, they should be post-Late Jurassic in Age; however, all dated zircons from the dykes are in the range of 1200-750 Ma. The absence of younger ages can be attributed to either formation during a relatively low temperature magmatic event, which generated very narrow younger zircon rims only and thus undetectable with the technique used, or a completely lack

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

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


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

  14. Zircon U-Pb age and geochemical constraints on the origin of the Birjand ophiolite, Sistan suture zone, eastern Iran

    Zarrinkoub, Mohammad Hossein; Pang, Kwan-Nang; Chung, Sun-Lin; Khatib, Mohammad Mahdi; Mohammadi, Seyyed Saeid; Chiu, Han-Yi; Lee, Hao-Yang


    The Birjand ophiolite is one of the ophiolitic complexes in the northern Sistan suture zone, eastern Iran, which marks the closure of an enigmatic branch of the Neotethys Ocean (called the Sistan Ocean) associated with the collision between the Lut and Afghan continental blocks. Ophiolitic rocks in the suture zone occur as collisional block-to-block geological terranes, or as blocks within tectonic mélange. We present zircon U-Pb age, and whole-rock geochemical and Sr-Nd isotopic data of the Birjand ophiolite to understand its formation age and magma genesis in this poorly constrained part of the Alpine-Himalayan orogenic belt. Varieties of the ophiolitic rocks include clinopyroxene-bearing harzburgites, harzburgites, dunites, massive and pillow basalts, dolerites, gabbros and leucogabbros. Using laser ablation inductively coupled plasma-mass spectrometry, zircon separates from two leucogabbros yield U-Pb ages of 113 ± 1 and 107 ± 1 Ma, which are interpreted to represent the formation ages of the ophiolite. The clinopyroxene-bearing harzburgites can be explained as melting residues after low to moderate extraction of MORB-type melts, whereas the harzburgites require more than one melting event to explain their trace element depletion. Most mafic rocks of the ophiolite display MORB-like trace element patterns and high ɛNd(t) (+ 3.4 to + 8.4), features consistent with derivation from MORB-source mantle. Magma genesis can be explained by low to moderate degrees of melting of moderately depleted spinel peridotite and variable interaction with enriched mantle melts. The Birjand ophiolite represents a MORB-type ophiolite and part of the Middle Cretaceous oceanic lithosphere of the Sistan Ocean that closed during the destruction of a narrow arm of the Neotethys Ocean.

  15. 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

    Costa, Felipe Grandjean da; Araujo, Carlos Eduardo Ganade de; Vasconcelos, Antonio Maurilio, E-mail:, E-mail:, E-mail: [Servico Geologico do Brasil (CPRM), Fortaleza, CE (Brazil); Amaral, Wagner da Silva, E-mail: [Universidade Federal do Ceara (UFC), Fortaleza, CE (Brazil). Departamento de Geologia; Rodrigues, Joseneusa Brilhante, E-mail: [Servico Geologico do Brasil (CPRM), Brasilia, DF (Brazil)


    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)

  16. Multiple provenance of rift sediments in the composite basin-mountain system: Constraints from detrital zircon U-Pb geochronology and heavy minerals of the early Eocene Jianghan Basin, central China

    Wu, Lulu; Mei, Lianfu; Liu, Yunsheng; Luo, Jin; Min, Caizheng; Lu, Shengli; Li, Minghua; Guo, Libin


    Zircon U-Pb geochronology and heavy minerals are used in combination to provide valuable insights into the provenance of the early Eocene Jianghan Basin, central China. Five samples for zircon U-Pb dating and eighty-five samples for heavy mineral analysis were collected from drill cores or cuttings of the Xingouzui Formation. Most analyzed zircons are of magmatic origin, with oscillatory zoning. Detrital zircons from sample M96 located on eastern basin have two dominant age groups of 113-158 Ma and 400-500 Ma, and the other samples located on southern basin have three prominent age populations at 113-158 Ma, 400-500 Ma and 700-1000 Ma. Samples on different parts of the basin show distinct differences in heavy mineral compositions and they apparently divide into two groups according to the content of rutile (higher or lower than 4%). The spatial variations of zircon-tourmaline-rutile (ZTR) indices are marked by some noticeable increasing trends from basin margins to the inner part of the basin. Compared with the potential source areas, this study clarifies the multiple source characteristics of the Jianghan basin in the composite basin-mountain system. The majority of clastic material was supplied from the north source area through rift-trough sediment-transport pathways, and the eastern, southern and northwestern source areas also contributed detritus to the basin. This clastic material is broadly dispersed in the basin. The early Eocene paleogeography implies that rift architecture and rifting process had an important influence on sediment dispersal. This study shows that integrated zircon U-Pb geochronology and heavy mineral analysis is a useful and powerful method to identify sediment provenance.

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

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


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

  18. Timing and origin of migmatitic gneisses in south Karakoram: Insights from U-Pb, Hf and O isotopic record of zircons

    Mahar, Munazzam Ali; Mahéo, Gweltaz; Goodell, Philip C.; Pavlis, Terry L.


    The timing and origin of partial melting in collision belts is crucial to understand the thermotectonic evolution and the relationship between HT metamorphism and magmatism in over-thickened crust. In the present study, we used the in-situ isotopic (Hf, O and U-Pb) record of zircons to investigate the timing and origin of migmatitic gneisses exposed in the core of the Dassu dome in south Karakoram. The new U-Pb zircon dating identified the Proterozoic inherited cores (1.8-1.9 Ga and 2.3-2.5 Ga) surrounded by a Neogene overgrowth with ages ranging from ∼6 to ∼20 Ma. These ages imply that the partial melting in the Karakoram Metamorphic Complex lasted from >20 Ma to ∼6 Ma and can be correlated with the Miocene magmatism in the adjacent Baltoro region. Oxygen isotopic data from Proterozoic inherited cores (1.8-1.9 Ga) and Neogene overgrowths are indistinguishable and generally vary from 8‰ to 9.5‰. These values are slightly higher than the most igneous zircons (6.5-8‰, Valley et al., 2005) indicating an igneous precursor with heavy initial O composition that later might have equilibrated with low temperature environment or some involvement of supracrustal material is likely. However, a few low U/Th, relatively old inherited cores (2.3-2.5 Ga) showed mantle-like (δ18O = 5.3 ± 0.6‰, Valley et al., 2005) values of δ18O = 5.5 ± 2.7‰. The present-day weighted mean εHf (0) of the Proterozoic inherited cores ranges from -50 ± 1.0 to -44.3 ± 1.2. In contrast, the Neogene rims are 15-20 ε-units higher than the inherited core with present-day εHf (0) = -30.6 ± 0.9. This implies that the Hf composition of the Neogene overgrowth is not controlled exclusively by the dissolution of the inherited cores and that contamination by external melts is likely. We suggest a contribution from the Neogene, less-evolved magmatism in the Baltoro region (εHf (0) = ∼-4 to -10). The elevated oxygen composition is not consistent with the contribution from pristine

  19. The stages and duration of the Kieveiskoe and Fedorovskoe Pt-Pd deposits formation: U-Pb zircon data (Kola Peninsula)

    Nitkina, Elena


    The Kola Peninsula is one of the unique geological provinces both in Russia and in the world, where Pt-Pd Kieveiskoe and Fedorovskoe deposits have been discovered (Mitrofanov, 2005). Several deposits within the Northern and Southern belts contain first hundreds of tons of estimated platinum metal resources, allowing us to ascribe the intrusions of the belts to the class of large igneous province (Schissel et al., 2002; Mitrofanov, 2005). The Kieveiskoe and Fedorovskoe deposits belong to the Pt-bearing Fedorovo-Pansky layered intrusion which is situated in the central part of the Kola Peninsula and is one of 14 major Early Proterozoic layered massifs of the Northern belt occurring at the border between Early Proterozoic volcano-sedimentary rift sequences and Archaean basement gneisses (Zagorodny, Radchenko, 1983; Bayanova, 2004). The aim of this report is to summarize all U-Pb data for Kieveiskoe and Fedorovskoe deposits including single grain dating with Pb205 tracer. At present, the following ages have been defined for the different stages of the massif evolution: 2526 - 2516 Ma - pyroxenite and gabbro of the Fedorovskoe deposit magma chamber (Nitkina, 2006), 2515 - 2518 Ma - Pt-bearing gabbro of Federovskoe stratiforme deposit; 2505 - 2496- 2485 Ma (Bayanova, 2004; Nitkina, 2006) - gabbro-norite and gabbro of the main phase of the Kieveiskoe deposit magma chamber and disseminated platinum-metal mineralization and relatively rich Cu-Ni sulphide mineralization in the basal part of the Kieveiskoe and Fedorovskoe non-stratiforme deposits; ca. 2470 Ma (Bayanova, 2004) - pegmatoid gabbro-anothosite and, probably, fluid-associated rich platinum-metal ores of the Lower Layered Horizon (Kieveiskoe deposit); ca. 2447+/-12 Ma (U-Pb zircon and baddeleyte (Bayanova, 2004)) - anorthositic injections and, probably, local lens-like rich Pt-Pd accumulations of the Upper layered Horizon (Kieveiskoe deposit). The U-Pb zircon ages of the massif evolution stages corroborate the

  20. 东准噶尔晚古生代次火山侵入岩锆石SHRIMP U-Pb年龄及Sr-Nd同位素地球化学%Zircon SHRIMP U-Pb Dating and Sr-Nd Isotopic Geochemistry of Late Paleozoic Subvolcanic-Intrusive Rocks in Eastern Junggar

    张峰; 徐涛; 郭晓东; 范俊杰; 张栋; 潘爱军; 朝银银


    Late Paleozoicvolcanic-subvolcanic-intrusive rocks in Eastern Junggar are developed.Songkaersu granodiorite porphyry,as a part of them,formed in the southeast Kalamali tectonic belt.Petrography,SHRIMP zircon U-Pb and Sr-Nd isotope indicate that all zircons selected are magmatic, and the zircons U-Pb concordant ages of two samples may be sub-divided into many groups,of which the youngest concordant age is 305.5~316.3Ma which can be interpreted as the for-mation age of Songkaersu granodiorite porphyry.On the other hand,others ages such as early Cambrian to early Carbonif-erou are consistent with the ages of oceanic-crust and island-arc magmatic rocks around the Junggar Basin.Moreover,the positiveεNd(t)(3.0 to 15.1), medium 87Sr/86Sr initial ratios (0.703 861 to 0.713 151), and smaller T2DM (412 to 839) may imply that the eastern basement of the Junggar Basin was tectonically amalgamated by underplating of mantle-derived substances and different Paleozoic remnant oceanic crust and arc complexes upon the minor Precambrian continental crust . Songkaersu granodiorite porphyry may result from the interaction of a amount of new-born lower crust and minor asthenospheric mantle or depleted lithospheric mantle,and the formation mechanism of them perhaps might be related to the subduction of the Paleo-Asian oceanic plate under the Kazakhstan-Junggar plate and result in back-arc spreading.%东准噶尔地区石炭纪巴塔玛依内山组火山岩-次火山侵入岩发育。本文以卡拉麦里构造带东南部松喀尔苏地区次火山侵入岩--花岗闪长斑岩为研究对象,通过岩石学、SHRIMP锆石U-Pb和Sr-Nd同位素方面的研究发现,所有锆石都具岩浆成因,锆石U-Pb年龄可分为多组,其中最小的一组年龄为305.5~316.3 Ma,代表岩体的形成时间,其他年龄多与准噶尔地区产出的具洋壳和岛弧性质的岩浆岩时代一致。岩石具正的εNd(t)(3.0~15.1),中等的87Sr/86Sr初始值(0

  1. Isotope U-Pb age on single zircon and REE distribution in rocks and zircon from paleoproterozoic Kandalaksha-Kolvitsa complex Baltic shield

    Steshenko, Ekaterina; Bayanova, Tamara; Drogobuzhskaya, Svetlana; Lyalina, Ludmila; Serov, Pavel; Chashchin, Viktor; Elizarov, Dmitriy


    Kandalaksha-Kolvitsa paleoproterozoic complex located in the N-E part of Baltic shield and consists of three zones. Marginal zone (mesocratic metanorite) lies at the base of the massif. Main zone is composed of leucocratic metagabbro. The upper zone is alteration of mataanorthosite and leucocratic metagabbro. All rocks were subjected to granulate and anorthositic metamorphism. Age of magmatic crystallization of the massif was determined for the first time, using the U-Pb isotope method for single zircon grains. Three fractions of single zircons from anorthosite of the Kandalaksha massif gave precise U-Pb age of 2435.5 ± 4.8 Ma. For the first time REE concentration (WR) was determined using a quadrupole mass spectrometer (Agilent 7500 ce ICP-MS) in the main varieties of rocks of the Kandalaksha-Kolvitsa paleoproterozoic complex. Anorthosite and leucocratic metagabbros (main zone) are characterized by a flat spectrum distribution of HREE, which were normalized by [1]. The REE pattern is characterized by significant positive anomalies of Eu ((Eu / Eu *)n = 3.72-3.91) in anorthosite and leucogabbros and 7.26 - in ortoamfibolitah. General content of individual elements that are common for this type of rocks: Cen = 5.82-8.54, Ybn = 1.54-1.58, which indicates that the process of crystallization of the rock occurred with predominant accumulation of plagioclase. According to geochemical and Nd-Sr isotopic data (ISr=0.702 - 0.706, ɛNd(T) = +1 - (-3)) Kandalaksha Kolvitsa complex, appear to have a general plume source with Paleoproterozoic layered intrusions of the Baltic Shield [2] Distribution of REE (ELAN-9000 ICP-MS) in zircon have a typical magmatic species: a positive Ce, negative Eu anomaly and HREE flat spectrum. Titanium content in zircons were measured for the calculation of their crystallization temperature with 8350C. These data are evidence of magmatic origin of zircon [3]. The scientific researches are supported by RFBR (projects № 15-35-20501, № 16

  2. SHRIMP U-Pb dating and Hf isotopic analyses of Middle Ordovician meta-cumulate gabbro in central Qiangtang, northern Tibetan Plateau


    Metabasites consisting of metamorphic ultra-mafic rocks, cumulate gabbro, gabbro (diabase), basalt, and plagiogranite are exposed at the Taoxinghu area in central Qiangtang, northern Tibetan Plateau. Zircon SHRIMP U-Pb dating for the cumulate gabbro yields a weighted mean age of 467±4 Ma, which is the oldest and most reliable magmatic age in this area. Zircon 176Hf/177Hf ratios range from 0.282615 to 0.282657, with εHf(t) values of 5.02±0.28, indicating that the cumulate gabbro was mainly derived from the depleted mantle. In addition, geochemical data of metabasites suggest that they have similar characteristics to those in the mid-ocean ridge basalts (MORB). The Taoxinghu metabasites may represent the fragment of Early Paleozoic ophiolite in the "Central Uplift" of the Qiangtang, northern Tibetan Plateau.

  3. U-Pb ages and morphology of zircons from different granites within the Saxonian Granulite Massif

    Sagawe, Anja [Senckenberg Naturhistorische Sammlungen Dresden (Germany). Oeffentlichkeitsarbeit; Gaertner, Andreas; Hofmann, Mandy; Linnemann, Ulf [Senckenberg Naturhistorische Sammlungen Dresden (Germany). Sektion Geochronologie


    The Saxonian Granulite Massif comprises various granitoid intrusions with different stages of deformation but of similar ages. However, there is only little knowledge about the magmatic source of these rocks. Combining the external and internal morphology of zircons and taking into consideration their Th-U values allows the differentiation of the granitoids into at least two groups of distinct evolution.

  4. U/Pb ages on detrital zircons in the southern central Andes Neogene foreland (36°-37°S): Constraints on Andean exhumation

    Sagripanti, Lucía; Bottesi, Germán; Naipauer, Maximiliano; Folguera, Andrés; Ramos, Victor A.


    U/Pb dating on detrital zircons was performed in the Pampa de Carrizalito depocenter of the Late Miocene foreland basin associated with the Southern Central Andes orogenic front. This reveals Andean and pre-Andean components in magmatic derived zircons inhomogeneously distributed through the sequence. Andean, Grenville, Pampean, Famatinian and Gondwanic components reveal a complex source distribution from either the Main Andes, Coastal Cordillera and basement foreland areas. These are discussed showing different patterns in the context of the Andean orogenic cycle. Cretaceous and Jurassic components that are partly related to Mesozoic batholiths, developed at the western slope of the Andes at these latitudes, have a very contrasting behavior through the sequence: While Jurassic grains are represented from base to top, Cretaceous ones dilute upwardly. This is explained through the progressive uplift of the Southern Central Andes that could have created a barrier to Cretaceous and Jurassic detritus, while the older ones could have had either an alternative source area represented by the inverted rift system of the Huincul Ridge in the foreland area and the Cordillera del Viento in the hinterland area or the reworking of Jurassic sedimentary sequences of the Neuquén basin. Finally, a progressive enrichment in pre-Andean components to the top of the sequence is interpreted as related to the development of a broken foreland and the consequent rapid expansion of the orogenic front at the time of development of a slab shallowing setting in the region as shown by previous works.

  5. U Pb zircon (TIMS and SIMS) and Sm Nd whole-rock geochronology of the Gour Oumelalen granulitic basement, Hoggar massif, Tuareg shield, Algeria

    Peucat, Jean Jacques; Drareni, Amar; Latouche, Louis; Deloule, Etienne; Vidal, Philippe


    Two major granulitic units are recognized in the Gour Oumelalen area. One of the units is composed partially of Archean gneisses (Red Gneiss complex) with U-Pb zircon SIMS and TIMS ages of approximately 2.7 Ga. Although they were formed from 3.0- to 3.2-Ga-old precursors, as indicated by Nd model ages, we find no evidence of any older history (≈3.5 Ga) as suggested by previous Pb-Pb ages. The other formation (Gour Oumelalen supergroup) is a metasedimentary sequence at least partly of Paleoproterozoic age, as indicated by zircon dates of a metavolcanic rock at approximately 2.2 Ga. A later magmatic event is recorded at approximately 1.9 Ga in both units and related to coeval granulite-facies metamorphism that affected both units. Nd model ages at approximately 2.0 Ga suggest an accretion of juvenile crust formation at that time. The existence of T DM Nd model ages intermediate between 2.5 and 2.9 Ga could result from the mixing of 3.2 and 2.0-Ga-old material or may reflect separate events.

  6. Zircon U-Pb ages of olivine pyroxenite xenolith from Hannuoba:Links between the 97-158 Ma basaltic under-plating and granulite-facies metamorphism

    LIU Yongsheng; YUAN Honglin; GAO Shan; HU Zhaochu; WANG Xuance; LIU Xiaoming; LIN Wenli


    U-Pb zircon dating by LA-ICP-MS and SHRIMP for one olivine pyroxenite yields complex age populations including Mesozoic ages of 97-158 Ma and 228 ±8.7 Ma, Early Paleozoic ages of 418-427 Ma, Paleoproterozoic age of 1844±13 Ma, Neoarchean age of 2541 ± 54 Ma and middle Archean age of 3123 ± 4.4 Ma. The 97-158 Ma and 228 ± 8.7 Ma zircons show typical igneous oscillatory zonation in CL images, suggesting two episodes of magmatic events. Overlapping of the 97-158 Ma ages with that of granulite xenoliths indicates that the Mesozoic granulite-facies metamorphism was induced by heating from the basaltic underplating at the base of the lower crust. Both processes lasted at least from about 158 to 97 Ma. Ages of 418-427 Ma could be records of the subduction of Mongolia oceanic crust under the North China craton. Ages of 1.84 Ga,2.54 Ga and 3.12 Ga correspond to the three important crust-mantle evolutionary events in the North China craton,and imply preservation of Precambrian lower crust in the present-day lower crust.

  7. SHRIMP U-Pb zircon ages of granite gneiss and paragneiss from Oki-Dogo island, southwest Japan, and their tectonic implications

    Cho, D.-L.; Takahashi, Y.; Yi, K.; Lee, S. R.


    The Hida belt, located in the northern part of southwest Japan, is considered to be a rarely preserved cratonic remnant originated from the easternmost continental margin of the Eurasian continent before the major build-up of the Japanese Islands. It is thus one of the key tectonic elements to unravel the continental accretion processes of the East Asia that have been almost accomplished at the end of the Permo-Triassic time. The gneiss in the Oki-Dogo Island is considered as a western extension of the Hida gneiss on the basis of similarities in both lithofacies and metamorphic ages. However, recent geochronologic investigations on these gneisses raised a question against the simple tectonic correlation between the Hida belt and Oki-Dogo Island (e.g., Tsutsumi et al., 2006). In this work we report new SHRIMP U-Pb zircon ages obtained from two granite gneiss and a paragneiss (Oki gneiss) from Oki-Dogo island. Samples include garnet-bearing granite gneiss (OKI8), granite gneiss (OKI14) and garnet-biotite gneiss (OKI13). Most of zircon grains in OKI8 and OKI14 show oscillatory zoning patterns. They give almost equivalent Late Paleoprotrozoic ages of 1867±16 Ma (n=20, MSWD=2.0) and 1881±17 Ma (n= 20, MSWD=4.4) which are interpreted to date the emplacement ages of the granite protoliths. A few of zircon grains from sample OKI8 show recrystallized rims yielding Triassic metamorphic age of 235.0±6.1 Ma (n=9, MSWD=1.8). Zircon grains from OKI13 have overgrowth rims with low Th/U ratio, and gave a Late Paleoprotrozoic metamorphic age of 1867±16 Ma (n=18, MSWD=3.3) marking the timing of the paragneiss. Most of inherited cores of OKI13 zircons showing magmatic zoning patterns with high Th/U ratio gave a spectrum ranging in ages from Late Archean (up to 2693 Ma) to Early Paleotroterozoic. The Permo-Triassic metamorphic age (~235 Ma) obtained here is well corroborated with those reported from the Hida belt. However, formation ages (~1.9 Ga) of the Oki Gneisses are clearly

  8. U-Pb geochronology of zircon and polygenetic titanite from the Glastonbury Complex, Connecticut, USA: An integrated SEM, EMPA, TIMS, and SHRIMP study

    Aleinikoff, J.N.; Wintsch, R.P.; Fanning, C.M.; Dorais, M.J.


    U-Pb ages for zircon and titanite from a granodioritic gneiss in the Glastonbury Complex, Connecticut, have been determined using both isotope dilution thermal ionization mass spectrometry (TIMS) and the sensitive high resolution ion microprobe (SHRIMP). Zircons occur in three morphologic populations: (1) equant to stubby, multifaceted, colorless, (2) prismatic, dark brown, with numerous cracks, and (3) elongate, prismatic, light tan to colorless. Cathodoluminescence (CL) imaging of the three populations shows simple concentric oscillatory zoning. The zircon TIMS age [weighted average of 207Pb/206Pb ages from Group 3 grains-450.5 ?? 1.6 Ma (MSWD=1.11)] and SHRIMP age [composite of 206Pb/238 U age data from all three groups-448.2 ?? 2.7 Ma (MSWD = 1.3)], are interpreted to suggest a relatively simple crystallization history. Titanite from the granodioritic gneiss occurs as both brown and colorless varieties. Scanning electron microscope backscatter (BSE) images of brown grains show multiple cross-cutting oscillatory zones of variable brightness and dark overgrowths. Colorless grains are unzoned or contain subtle wispy or very faint oscillatory zoning. Electron microprobe analysis (EMPA) clearly distinguishes the two populations. Brown grains contain relatively high concentrations of Fe2O3, Ce2O3 (up to ~ 1.5 wt.%), Nb2O5, and Zr. Cerium concentration is positively correlated with total REE + Y concentration, which together can exceed 3.5 wt.%. Oscillatory zoning in brown titanite is correlated with variations in REE concentrations. In contrast, colorless titanite (both as discrete grains and overgrowths on brown titanite) contains lower concentrations of Y, REE, Fe2O3, and Zr, but somewhat higher Al2O3 and Nb2O5. Uranium concentrations and Th/U discriminate between brown grains (typically 200-400 ppm U; all analyses but one have Th/U between about 0.8 and 2) and colorless grains (10-60 ppm U; Th/U of 0-0.17). In contrast to the zircon U-Pb age results, SHRIMP U-Pb

  9. U-Pb zircon age from the base of the Ediacaran Doushantuo Formation in the Yangtze Gorges,South China: constraint on the age of Marinoan glaciation

    ChongyuYin; FengTang; YongqingLiu; LinzhiGao; PengjuLiu; YushengXing; ZhiqingYang; YushengWan; ZiqiangWang


    The reported new U-Pb age by sensitive high-resolution ion microprobe (SHRIMP Ⅱ) on zircon was obtained from a tuff sample at the basal Doushantuo Formation in the Jiuqunao section, which situated at the western limb of the Huangling anticline in the Yangtze Gorges in Zigui, Hubei, South China. Eighteen spots of zircons were analyzed and they form two clusters: one includes three spots, with an inherited age of 784+ 15 Ma (MSWD=0.05); the other consists of 15 spots and gives a weighted mean 206pb/238U age of 628.3±5.8 Ma (MSWD=0.86). It is the first SHRIMP U-Pb zircon age obtained nearly the base of the Doushantuo Formation of Ediacaran and represents a maximum age of the Doushantuo Formation It also forms an age constraint on the upper limit age of the Nantuo (Marinoantype) glaciation.

  10. HFSE (High Field Strength Elements)-transport and U-Pb-Hf isotope homogenization mediated by Ca-bearing aqueous fluids at 2.04 Ga: Constraints from zircon, monazite, and garnet of the Venetia Klippe, Limpopo Belt, South Africa

    Zeh, A.; Gerdes, A.


    Results from laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) and isotope dilution (ID) analyses of minerals and rocks from a single outcrop of the Venetia Klippe of the Limpopo Belt indicate that the U-Pb and Hf isotope system homogenized on the decimetre scale under amphibolite-facies conditions of ⩽645 ± 25 °C and ⩽7.0 ± 1.1 kbar, i.e. in the presence of an aqueous fluid phase. For a metabasite sample, homogenization is supported by isotope analyses of metamorphic zircon, garnet, and whole rock, which yield a six-point Lu-Hf isochron age of 2039.7 ± 3.4 Ma, with initial 176Hf/177Hf of 0.28126 ± 0.00001, and a U-Pb zircon age of 2042 ± 10 Ma. The occurrence of a few inherited magmatic zircon cores with ages up to 2705 Ma, and with significantly lower initial 176Hf/177Hf of 0.28112, however, indicate that homogenization was incomplete. For a chlorite-biotite-garnet schist isotope homogenization is reflected by within error identical zircon and monazite U-Pb ages of 2045 ± 10 Ma and 2041 ± 8 Ma, respectively, and by a zircon-garnet-whole rock Lu-Hf isochron age of 2083 ± 63 Ma, with an initial 176Hf/177Hf of 0.28140 ± 0.00003. Contemporaneous formation of metamorphic zircon, monazite and garnet in the chlorite schist is not only supported by the isotope data, but also by chlorite inclusions in all three minerals, and by inclusions of metamorphic zircon in garnet. The inclusion textures and the identical initial 176Hf/177Hf support the conclusion that metamorphic zircon grains precipitated from an aqueous fluid phase, after dissolution of zirconium-bearing phases elsewhere, followed by a major HFSE transport, and Hf isotope homogenization. This fluid perhaps was Ca-bearing, as is suggested by the fact that garnet in the schist sample is the only Ca-bearing phase, and that metamorphic monazite, dating the metamorphic peak, is partially replacement by apatite. The fact that the metamorphic zircon rims in the metabasite sample have

  11. Continental Arc Magmatism and its Abrupt Termination by Ridge Subduction or Ridge Jump Along the Proto-Pacific Margin of Gondwana, Marie Byrd Land, Antarctica: A Zircon U-Pb Study

    Mukasa, S. B.


    The Paleozoic and Mesozoic development and subsequent fragmentation of Gondwanaland's Pacific margin is recorded in igneous and metamorphic rocks cropping out in the Marie Byrd Land (MBL) continental block of West Antarctica, recognized on geologic and paleomagnetic grounds to comprise a distinct microplate. Widespread occurrence of metaluminous granitoids dated by the zircon U-Pb method as mid- to late Paleozoic shows that convergence-related magmatism dominated the early evolution of this margin. Dates for granodiorites, monzogranites and granites from the Ruppert and Hobbs Coasts of western MBL reveal a prolonged period of subduction-related calc-alkaline magmatism between at least 320 ñ 3 Ma (age of the oldest granodiorite dated) and 110 ñ1 Ma (the age of the Mt. Prince granite). The latter is intruded by swarms of mafic and intermediate dikes believed to record the onset of rifting that led to separation of the New Zealand microcontinent from MBL. The dikes have been dated by zircon U-Pb at 101 ñ 1 Ma. Thus, the regime along the Ruppert and Hobbs Coasts had shifted from subduction-related to rift-related magmatism within a mere ~9-m.y. period. In the Kohler Range and the Pine Island Bay areas of eastern MBL, the calc-alkaline magmatism did not terminate until 96 ñ 1 Ma, based on U-Pb dating of zircons from one granitoid sample, or 94 ñ 3 Ma based on zircons from another. No continental separation occurred to the east of MBL. The margins of the Thurston Island and Antarctic Peninsula blocks went directly from convergent to inactive. With their zircon U-Pb ages clustering around 100 ñ 2 Ma, dike-free "anorogenic" syenites and quartz syenites along the Ruppert and Hobbs Coasts show that the transition to extensional magmatism was rapid in the west. This is also reflected by the fact that from the onset of rifting at 101 ñ 1 Ma to formation of oceanic crust between MBL and Greater New Zealand (Campbell Plateau, Chatham Rise, North Island and South Island

  12. Ion microprobe U-Pb dating and REE abundance of biogenic apatite

    Sano, Y.; Terada, K.; Ueki, S.


    If the direct U-Pb dating of a fossil itself is possible, the method could have great impact on stratigraphic studies in establishing the absolute chronology of sedimentary sequences. Micro fossil ?conodont? are candidates for this purpose since they consist of apatite (Ca2(PO5)3 (F,Cl,OH)), which would uptake U, Th and Pb after sedimentation no longer than a few million years and is supposed to remain closed to U and Pb under relatively low effective closure temperature. We report here results of direct ion microprobe U-Th-Pb dating of two conodonts; Trichognathus from Kinderhookian stage of Mississippian sedimentary sequence from Illinois Basin region in North America and Panderodus from a Llandoverian sedimentary sequence on Langkawi Island, northern Malaysia. Secondary purpose of the study is to indicate in situ analysis of all REE on the same spots of U-Pb measurements. Samples were cast into epoxy resin discs with a few grains of standard apatite, PRAP, derived from an alkaline rock of Prairie Lake circular complex in the Canadian Shield and polished until they were exposed through their mid-sections. U, Th and REE abundances, and Pb isotopic compositions were measured by using SHRIMP installed at Hiroshima University. Thirteen spots on Trichognathus yield a 238U/206Pb isochron age of 323+/-36 Ma, which is consistent with the depositional and early diagenetic ages. Fifteen spots on Panderodus give 232Th/208Pb isochron age of 429+/-50 Ma, which is again comparable to an early Silurian. Shale-normalized REE of Trichognathus shows a broadly flat pattern from light to middle REE and a decrease from middle to heavy REE with negative anomalies of Ce and Eu. In contrast Panderodus indicates a concave-shape pattern with middle REE enrichment. These characteristics are probably due to a different formation environment as suggested by other workers.

  13. Small-Volume U-Pb Zircon Geochronology by Laser Ablation-Multicollector-ICP-MS


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

  14. Impact of hydrothermal alteration on the U-Pb isotopic system of zircons from the Fangcheng syenites in the Qinling orogen, Henan Province, China

    BAO Zhiwei; WANG Qiang; BAI Guodian; ZHAO Zhenhua


    Disturbance of the zircon U-Pb isotopic system has been investigated extensively, but mostly in lab, in the last decades. Here, we reported a field-based study on intensive sericitization, K-feldsparthization and the impacts of mylonitization on zircons from the Fangcheng syenites.The Fangcheng syenites occur in the eastern part of the Qinling orogen and consist mainly of aegirine-augite syenite, aegirine nepheline syenite, biotite syenite and hornblende nepheline syenite. Zircons from the slightly sericitized aegirine augite syenite are colorless, transparent crystals and exhibit well-developed oscillatory and sector zoning on the cathodoluminescence (CL) images which are typical of magmatic zircons from alkaline rocks. Zircon U-Pb determinations by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) showed that the syenite was formed in Neoproterozoic time, the weighted average of 206Pb/238U ages is 844.3±1.6 Ma (MSWD=0.86). In contrast, the hydrothermally altered zircons (hydrothermal zircon) from the intensively sericitized, K-feldsparthized, and weakly mylonitized aegirine augite syenite are conglomerates, yellowish to brown in color, generally translucent and internally textureless. The CL and backscatter electron (BSE) images of hydrothermal zircons exhibit fractured, textureless or mosaic textures, and occasionally show "sponge texture" with the veinlets and inclusions of K-feldspar; however, relicts of magmatic oscillatory zoning can still be discerned locally in individual grains. LA-ICPMS analyses of the hydrothermal zircons demonstrated that the zircons are chemically inhomogeneous, with enhanced and widely varied Pb, U, and Th contents. The U and Th contents of the hydrothermal zircons are estimated to be 32×10-6-1550×10-6 and 188×10-6-4059×10-6, respectively, with Th/U ratios within the range of 0.7-44.9. 206Pb/238U apparent ages of the hydrothermal zircons are negatively correlated with the contents of U, and radiogenic and

  15. The India-Asia collision in north Pakistan: Insight from the U-Pb detrital zircon provenance of Cenozoic foreland basin

    Ding, Lin; Qasim, Muhammad; Jadoon, Ishtiaq A. K.; Khan, Muhammad Asif; Xu, Qiang; Cai, Fulong; Wang, Houqi; Baral, Upendra; Yue, Yahui


    The northernmost exposures of sub-Himalayan Cenozoic strata in the Hazara-Kashmir syntaxial region of north Pakistan comprises the Paleocene-Eocene marine strata in the lower part and Oligocene-Miocene nonmarine strata in the upper part. This study provides the detrital zircon U-Pb geochronology of the Cenozoic strata in this area. The strong resemblance of U-Pb age spectra of Paleocene Hangu, Lockhart and Patala formations with those of Himalayan strata indicate an Indian plate provenance. The first appearance of Pakistan at ca. 56-55 Ma.

  16. Zircon U-Pb Dating and Mineralization Prospective of the Triassic Qingxi Pluton in Southern Jiangxi Province%赣南印支期清溪岩体的锆石U-Pb年代学研究及其含矿性评价

    于扬; 陈振宇; 陈郑辉; 侯可军; 赵正; 许建祥; 张家菁; 曾载淋


    清溪岩体是赣南地区缺少精确年龄数据的含稀土岩体之一.对赣南清溪岩体中的锆石样品进行了U-Pb法年代学研究,测年结果显示该岩体主体年龄为229.3±0.8 Ma和227.0±1.0 Ma,属于印支期产物,并不是前人认为的属于加里东期.对清溪岩体进行了岩石学、地球化学的研究,简要论述了其含矿性,认为清溪岩体主体为黑云母花岗岩,为过铝质钾玄岩系,轻稀土和重稀土中等分馏,负Eu异常不明显.岩石化学资料数据显示,该岩体稀土总量较高,具备成矿母岩的基本条件.%The Qingxi pluton is a rare earth element mineralized granite located in southern Jiangxi province.Zircons from the Qingxi granite were analyzed using MC-ICP-MS method,which yield U-Pb ages of 229.3 ±0.8 Ma and 227.0 ±1.0 Ma,respectively.The new ages confirm that the granite was intruded in Indosinian.The Qingxi pluton consists mainly of biotite granite,which is of peraluminous shoshonitic and characterized by mildly fractionated REE without obvious Eu anomalies.∑REE of this pluton is rather high (269 × 10 ~6 -298×10 ~6),which is one of the prerequisites for REE mineralization in the weathering crust.

  17. Zircon U-Pb Dating and Tectonic Significance of the Shipingchuan Granite in Southeastern Zhejiang Province, SE China%浙东南石平川花岗岩体LA-ICP-MS锆石U-Pb年代学及构造意义

    李艳军; 魏俊浩; 姚春亮; 鄢云飞; 谭俊; 付乐兵; 潘锦勃; 李伟


    The Shipingchuan K-feldspar granite is situated in south-central volcanic belt of southeastern Zhejiang Province. LA-ICP-MS zircon U-Pb dating yields a weighted mean ~(206)Pb/~(238)U age of 102.5±1.2 Ma (MSDW=2.1)for the Shipingchuan granite intrusion,implying its end of early Cretaceous crystallization time. Petrography and chemical compositions show that it belongs to high-K calc-alkaline series characterized by high silicon,enriched in kalium and aluminum but depleted in iron,magnesium,phosphorus and titanium.SiO_2 and K_2O contents range from 74.79% to 77.79% and 4.26% to 7.97% respectively. A/CNK values vary from 0.98 to 1.10,indicating a metaluminous to weakly peraluminous character. The rocks are enriched in Rb,Th,U and K,but depleted in Sr,Ba,Nb,P and Ti. REEs are characterized by intermediate negative Eu anomalies with Eu/Eu* =0.33~0.50 and exhibit right-dipping "V" patterns with LREE enrichment. [n(~(87)Sr)/n(~(86)Sr)]I and ε_(Nd)(t) values (t=102.5Ma)of the Shipingchuan intrusion range from 0.70801 to 0.71012 and -8.76 to -8.63 respectively,showing the intrusion is the result of mixing by crustal and mantle materials.Mineralogy and geochemistry suggest that the Shipingchuan K-feldspar granite intrusion is a highly fractionated I-type granite. Trace elements features display that the pluton was a post-collisional granite and formed in an extensional tectonic setting in the end of early Cretaceous. These results indicate the Shipingchuan granite is related to back-arc extension due to high angle subduction of the paleo-Pacific plate,caused by northward movement of Indian plate.%石平川钾长花岗岩体位于浙江沿海火山岩带的中南部,LA-ICP-MS锆石U-Pb定年结果为102.5±1.2 Ma(MSDW=2.1),形成于早白垩世末期.岩相学及化学成分显示其属高钾钙碱性岩系,具有高硅富钾和铝低铁、镁、磷和钛特征,SiO_2含量为74.79%~77.79%,K_2O为4.26%~7.97%,铝指数A/CNK=0.98~1.10,属准铝质-弱过铝质岩石.富

  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)

    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


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

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


    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

  20. U-Pb dating of speleogenetic dolomite: A new sulfuric acid speleogenesis chronometer

    Victor Polyak


    Full Text Available The 1100-meter Big Room elevation level of Carlsbad Cavern, New Mexico USA, formed 4 Ma by hypogenic sulfuric acid speleogenesis (SAS. The age of the Big Room level of 4.0 ± 0.2 Ma was previously determined by dating alunite, a byproduct of speleogenesis, using the 40Ar/39Ar method. Duplication of these results is possible by radiometric dating of other byproducts interpreted to be speleogenetic (a byproduct of speleogenesis such as calcite and dolomite in certain settings. XRD and TEM analyses of sample 94044, a piece of crust collected within the Big Room level of SAS just below Left Hand Tunnel indicate that this dolomite sample we interpret to be speleogenetic is as well-ordered crystallographically as the Permian bedrock dolomite, possibly reflecting its SAS origin. Three U-Pb analyses were performed on subsamples A1, A2, and A3 of sample 94044, and two, A1 & A2, produced out-of-secular equilibrium results due to the presence of authigenic quartz and/or later re-distribution of uranium in the dolomite crust, which prevented the calculation of an isochron age. Because subsample 94044-A3 exhibited δ234U and 230Th/238U values consistent with secular equilibrium, we were able to generate a 238U/204Pb-206Pb/204Pb model age of 4.1 ± 1.3 Ma on the dolomite crust (94044 that we interpret to be reliable. The 4.1 Ma age of the speleogenetic dolomite crust agrees with the 4 Ma 40Ar/39Ar age for the timing of speleogenesis of the Big Room level. While 40Ar/39Ar-dating of speleogenetic alunite- and jarosite-group minerals remains the primary way to determine absolute timing of hypogenic SAS, here we demonstrate that U-Pb dating of speleogenetic dolomite can be used to compliment or independently measure the timing of SAS. This method of dating SAS could be applicable in caves where the more soluble SAS-indicator minerals such as gypsum, alunite, and jarosite have been removed.

  1. Timing and tectonic implications of the Pan-African Bangangte syenomonzonite, West Cameroon: Constraints from in-situ zircon U-Pb age and Hf-O isotopes

    Tchouankoue, Jean Pierre; Li, Xian-Hua; Belnoun, Rose Noel Ngo; Mouafo, Lucas; Ferreira, Valderez Pinto


    The Bangangte pluton is a SW-NE elongated (5 × 20 km) massif located in the southeastern part of the Pan-African North Equatorial Fold Belt in Cameroon, consisting of two units with dominant monzonites in the south and syenites in the north. SIMS U-Pb zircon dating yields consistent emplacement ages of 585 ± 4 Ma and 583 ± 4 Ma for the southern unit and the northern unit, respectively. The Bangangte rocks display typical shoshonitic compositions characterized by Na2O + K2O > 5 wt%, K2O/Na2O ∼2, enrichment in LILE and LREE, but depletion in HFSE. Rocks from both units have similar O-Hf isotopes, with the monzonite zircons from the southern unit showing slightly higher δ18O (7.0 ± 0.4‰) but lower εHf(t) (-15.3 ± 1.4) value than the syenite zircons from southern unit (δ18O = 6.0 ± 0.4‰; εHf(t) = -14.0 ± 2.0). They were generated by partial melting of an enriched mantle source metasomatized by previous subduction processes, accompanied by crystal fractionation of pyroxene, Ti-Fe oxides and apatite, as well as crustal contamination to varying degrees. These rocks display a transitional geochemical feature of the subduction-related and within-plate shoshonites, suggesting that they were most likely emplaced in a post-collisional setting at the waning stage of the Pan-African orogeny.

  2. Zircon U-Pb ages and O-Nd isotopic composition of basement rocks in the North Qinling Terrain, central China: evidence for provenance and evolution

    Liu, Bing-Xiang; Qi, Yue; Wang, Wei; Siebel, Wolfgang; Zhu, Xi-Yan; Nie, Hu; He, Jian-Feng; Chen, Fukun


    The Qinling Group was previously interpreted as the oldest Precambrian basement unit of the North Qinling Terrain, recording its formation and early crustal evolution. The Qinling Group consists predominantly of gneisses, amphibolites, and marbles, which underwent multi-phase deformation and metamorphism. In order to better constrain the provenance and tectonic setting of this group and the evolution of the North Qinling orogenic belt, in situ U-Pb dating and oxygen isotopic analysis of zircons in combination with whole-rock geochemistry and Sr-Nd isotope analysis was performed on the two dominant rock types, amphibolite and felsic gneiss. Felsic gneisses exhibit enrichment of LREEs and LILEs (Rb, Ba, Th, K, Pb), negative Eu anomalies and depletion of HFSEs (Nb, Ta, P, Ti). The rocks have slightly elevated δ18O values (6.5-9.3 ‰) and initial ɛ Nd values of -4.6 corresponding to two-stage Nd model age of 1.99 Ga. Amphibolites are also enriched in LILEs and LREEs and depleted in Nb and Ta and have homogeneous δ18O values (5.0-6.0 ‰), but higher initial ɛ Nd values (2.8-3.3) and younger two-stage Nd model ages (1.29-1.24 Ga) compared to the gneisses. The zircon age record indicates that the gneisses and amphibolites were formed in a ~960 Ma volcanic arc environment rather than in a rift setting as previously suggested. A major metamorphic event took place during the Early Paleozoic. Based on the age spectrum of detrital zircons, the Qinling Group is interpreted as an autonomous geological unit, which was mainly derived mostly from 1,000 to 900 Ma old granitoid rocks. The North Qinling Terrain can be regarded as a remnant of the Grenville orogenic belt with an early Neoproterozoic evolution different from that of the North and South China blocks.

  3. Timing of the Wudangshan, Yaolinghe volcanic sequences and mafic sills in South Qinling: U-Pb zircon geochronology and tectonic implication

    LING WenLi; REN BangFang; DUAN RuiChun; LIU XiaoMing; MAO XinWu; PENG LianHong; LIU ZaoXue; CHENG JianPing; YANG HongMei


    The Wudangshan, Yaolinghe volcanic-sedimentary sequences and doleritic-gabbroic sills comprise the largest exposed Precambrian basement in South Qinling. Zircons separated from 5 volcanic-pyroclas-tic samples of the Wudangshan Group, 2 volcanic samples of the Yaolinghe Group and one sample for the mafic sills were used for U-Pb dating by laser ablation-inductively coupled plasma mass spec-trometry (LA-ICPMS). The results reveal that the Wudangshan volcanic sequence was formed at (755±3) Ma (a weighted mean from the 5 samples, MSWD=0.47), whereas the Yaolinghe volcanic suite and the mafic sill were crystallized at (685:L±5) (2 samples, MSWD=0.36) and (679±J:3) Ma (MSWD=1.6), respectively, which are equal to each other within analysis errors. These ages are markedly younger than those previously documented for the rocks. The newly obtained ages for the Wudangshan and Yaolinghe Groups are identical to those of the bottom Liantuo and slightly older than those of the Nantuo Forms-tions, respectively, lower strata of the Nanhua (middle to late Neoproterozoic) stratotype section in eastern Three Gorges, Yangtze craton. A range of inherited magmatic zircons was recognized with ages of 830 to 780 Ma, which are typical of Neoprotzrozoic magmatisms recorded along the margins and interior of the Yangtze craton. Thus, there is Neoproterozoic basement comprising 830-780 Ma igne-ous suites in South Qinling; the inherited zircons were detrital sediments derived from the northern margin of the Yangtze craton. Accordingly, it is suggested that the South Qinling is a segment of the Yangtze craton before the Qinling Orogeny.

  4. In situ zircon Hf isotopic, U-Pb age and trace element study of monzonite xenoliths from Pingquan and Fuxin basalts: Tracking the thermal events of 169 Ma and 107 Ma in Yanliao area

    ZHENG; Jianping; ZHANG; Ruisheng; YU; Chunmei; TANG; Huay


    K-Ar ages of the Mesozoic (92-100 Ma) Fuxin alkalic basalts (western Liaoning Province) and the Tertiary (23-45 Ma) Pingquan alkalic basalts (eastern Hebei Province), and the results of in situ zircon U-Pb dating, Hf isotope and the trace elements from three monzonite xenoliths carried in the alkalic basalts are reported. The crust-mantle interaction occurring in the Yanshan intracontinental orogenic belt is also discussed. Fuxin zircons show highly uniform U-Pb age ((169±3) Ma). More than 95% Pingquan zircons display the age of (107±10) Ma except two are 2491 Ma and 513 Ma respectively. Zircons with the ages of (169±3) Ma have εHf close to zero. εHf of the zircons with the ages of (107±10) Ma are mainly at -11.5--16.3, showing the crustal derivation. Fuxin zircons contain low Nb, Ta, Sr, Th, U contents, low and narrow Hf model ages (0.87-1.00 Ga), which reflect that the source materials of the monzonite xenoliths are young to Pingquan (focus at (1.28±0.08) Ga). High contents of the incompatible elements, and wide range of Hf model ages (0.89-2.56 Ga) in Pingquan zircons suggest a more complex source and the highly crustal maturity in their petrogenesis. Comprehensive information including the published data indicates that J3-K1 is the key period of the deep processes and shallow tectonic reverse in the Yanliao area. However, the processes were highly heterogeneous in spatial and in temporal.

  5. In situ zircon Hf isotopic, U-Pb age and trace element study of monzonite xenoliths from Pingquan and Fuxin basalts:Tracking the thermal events of 169 Ma and 107 Ma in Yanliao area

    ZHENG Jianping; ZHANG Ruisheng; YU Chunmei; TANG Huayun; ZHANG Pei


    K-Ar ages of the Mesozoic (92-100 Ma) Fuxin alkalic basalts (western Liaoning Province) and the Tertiary (23-45 Ma) Pingquan alkalic basalts (eastern Hebei Province), and the results of in situ zircon U-Pb dating, Hf isotope and the trace elements from three monzonite xenoliths carried in the alkalic basalts are reported. The crust-mantle interaction occurring in the Yanshan intracontinental orogenic belt is also discussed. Fuxin zircons show highly uniform U-Pb age ((169±3) Ma). More than 95% Pingquan zircons display the age of (107±10) Ma except two are 2491 Ma and 513 Ma respectively. Zircons with the ages of (169±3) Ma have εHf close to zero. εHf of the zircons with the ages of (107±10) Ma are mainly at -11.5--16.3, showing the crustal derivation. Fuxin zircons contain low Nb, Ta, Sr, Th, U contents, low and narrow Hf model ages (0.87-1.00 Ga), which reflect that the source materials of the monzonite xenoliths are young to Pingquan (focus at (1.28±0.08) Ga). High contents of the incompatible elements, and wide range of Hf model ages (0.89-2.56 Ga) in Pingquan zircons suggest a more complex source and the highly crustal maturity in their petrogenesis. Comprehensive information including the published data indicates that J3-K1 is the key period of the deep processes and shallow tectonic reverse in the Yanliao area. However, the processes were highly heterogeneous in spatial and in temporal.

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

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


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

  7. Tracing source terranes using U-Pb-Hf isotopic analysis of detrital zircons: provenance of the Orhanlar Unit of the Palaeotethyan Karakaya subduction-accretion complex, NW Turkey

    Ustaömer, Timur; Ayda Ustaömer, Petek; Robertson, Alastair; Gerdes, Axel


    Sandstones of the Late Palaeozoic-Early Mesozoic Karakaya Complex are interpreted to have accumulated along an active continental margin related to northward subduction of Palaeotethys. The age of deposition and provenance of the sandstones are currently being determined using radiometric dating of detrital zircons, coupled with dating of potential source terranes. Our previous work shows that the U-Pb-Hf isotopic characteristics of the sandstones of all but one of the main tectonostratigraphic units of the Karakaya Complex are compatible with a provenance that was dominated by Triassic and Permo-Carboniferous magmatic arc-type rocks, together with a minor contribution from Lower to Mid-Devonian igneous rocks (Ustaömer et al. 2015). However, one of the tectono-stratigraphic units, the Orhanlar Unit, which occurs in a structurally high position, differs in sedimentary facies and composition from the other units of the Karakaya Complex. Here, we report new isotopic age data for the sandstones of the Orhanlar Unit and also from an extensive, associated tectonic slice of continental metamorphic rocks (part of the regional Sakarya Terrane). Our main aim is to assess the provenance of the Orhanlar Unit sandstones in relation to the tectonic development of the Karakaya Complex as a whole. The Orhanlar Unit is composed of shales, sandstone turbidites and debris-flow deposits, which include blocks of Devonian radiolarian chert and Carboniferous and Permian neritic limestones. The sandstones are dominated by rock fragments, principally volcanic and plutonic rocks of basic-to-intermediate composition, metamorphic rocks and chert, together with common quartz, feldspar and mica. This modal composition contrasts significantly with the dominantly arkosic composition of the other Karakaya Complex sandstones. The detrital zircons were dated by the U-Pb method, coupled with determination of Lu-Hf isotopic compositions using a laser ablation microprobe attached to a multicollector

  8. Scanning ion imaging - a potent tool in SIMS U -Pb zircon geochronology

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


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

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

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


    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

  10. U/Pb zircon geochronology and tempo of the end-permian mass extinction

    Bowring; Erwin; Jin M W Martin YG; Davidek; Wang


    The mass extinction at the end of the Permian was the most profound in the history of life. Fundamental to understanding its cause is determining the tempo and duration of the extinction. Uranium/lead zircon data from Late Permian and Early Triassic rocks from south China place the Permian-Triassic boundary at 251.4 +/- 0.3 million years ago. Biostratigraphic controls from strata intercalated with ash beds below the boundary indicate that the Changhsingian pulse of the end-Permian extinction, corresponding to the disappearance of about 85 percent of marine species, lasted less than 1 million years. At Meishan, a negative excursion in delta13C at the boundary had a duration of 165,000 years or less, suggesting a catastrophic addition of light carbon.

  11. Zircon U-Pb SHRIMP Ages from the Late Paleozoic Turpan-Hami Basin, NW China

    Xiang Mao; Jianghai Li; Huatian Zhang


    Permian volcanic rocks are widely distributed in the Turpan-Hami Basin, which is part of the Central Asian orogenic belt. Here we present SHRIMP zircon data for the rhyolite in Well Baocan 1, one of the deepest wells in the basin. The 283.9±2.7 Ma reported in our study provides the best precise age determination for the Yierxitu Formation, the oldest Permian layer of Hami Depression, one of the three substructural units of the Turpan-Hami Basin, and a potential hydrocarbon reservoir in this ba-sin. Our data refines earlier imprecise39Ar-40Ar ages and shows that the volcanic rocks both inside the Turpan-Hami Basin and along its margin are almost coeval. We delineate a collisional orogenesis, and the new age of 283.9±2.7 Ma may limit the latest time of the collision orogenesis.

  12. Tectonic implications of U-Pb zircon ages of the himalayan orogenic belt in nepal

    DeCelles; Gehrels; Quade; LaReau; Spurlin


    Metasedimentary rocks of the Greater Himalaya are traditionally viewed as Indian shield basement that has been thrust southward onto Lesser Himalayan sedimentary rocks during the Cenozoic collision of India and Eurasia. Ages determined from radioactive decay of uranium to lead in zircon grains from Nepal suggest that Greater Himalayan protoliths were shed from the northern end of the East African orogen during the late Proterozoic pan-African orogenic event. These rocks were accreted onto northern Gondwana and intruded by crustal melts during Cambrian-Ordovician time. Our data suggest that the Main Central thrust may have a large amount of pre-Tertiary displacement, that structural restorations placing Greater Himalayan rocks below Lesser Himalayan rocks at the onset of Cenozoic orogenesis are flawed, and that some metamorphism of Greater Himalayan rocks may have occurred during early Paleozoic time.

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

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


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

  14. High-precision U-Pb zircon age from the Anfiteatro de Ticó Formation: Implications for the timing of the early angiosperm diversification in Patagonia

    Perez Loinaze, Valeria S.; Vera, Ezequiel I.; Passalia, Mauro G.; Llorens, Magdalena; Friedman, Richard; Limarino, Carlos O.; Césari, Silvia N.


    The Baqueró Group is one of the most relevant units regarding the study of the early diversification of angiosperms in South America. Whereas the age of the upper part of the Group, namely the Punta del Barco Formation, has been recently dated at 114.67 ± 0.18 Ma, the rest of the unit still lacks precise dating. In this contribution a CA-TIMS U-Pb zircon age of 118.23 ± 0.09 Ma for a tuff interlayered with fossiliferous rocks of the Anfiteatro de Ticó Formation (lower part of the Baqueró Group) is reported. This age constrains the duration of deposition of the Baqueró Group to approximately 4 Ma and provides new evidence for the age interpretation of the previously described angiosperm flora and associated pollen assemblages from this unit, until now interpreted as early Aptian or possibly Barremian in age. The Aptian age of the Baqueró Group allows a better comparison between the paleofloras from this southernmost region.

  15. Late Cretaceous volcanic arc system in Southwest Korea: Occurrence, lithological characteristics, SHRIMP zircon U-Pb age, and tectonic implications

    Koh, Hee Jae; Kwon, Chang Woo


    In the southwest region of the Korean Peninsula, four large volcanoes, the Buan, Seonunsan, Wido, and Beopseongpo, with a maximum diameter of ca 20 km, form a distinct topographic undulation along the NE-SW-trending Hamyeol Fault. These volcanics comprise various types of pyroclastic, sedimentary, and lava/intrusive rocks, and are interpreted as remnants of calderas resulting from various volcanic eruptions, indicating that Hamyeol Fault, together with crustal extension, played an important role in volcano formation in this region. SHRIMP U-Pb ages of zircon isolated from each volcanics are as follows. For Buan Volcanics, Cheonmasan Tuff 87.23 ±0.92 Ma, Udongje Tuff 86.79 ±0.71 Ma, Seokpo Tuff 87.30 ±0.99 Ma and Yujeongje Tuff 86.66 ±0.93 Ma. For Seonunsan Volcanics, Gyeongsusan Tuff 84.9 ±1.1 Ma and Yeongije Tuff 86.61 ±0.67 Ma. These ages indicate that the four volcanics were formed in the Late Cretaceous. The ages are comparable to those of the volcanic rocks of the Aioi and Arima groups in Southwestern Japan, suggesting that the Late Cretaceous volcanic arc systems developed in a NE-SW direction from the Japanese Islands to the southwestern part of the Korean Peninsula caused by regional magmatism together with crustal deformation as reflected by occurrence of the volcanic rocks along the Hamyeol Fault.

  16. U Pb zircon and monazite geochronology of Variscan magmatism related to syn-convergence extension in Central Northern Portugal

    Valle Aguado, B.; Azevedo, M. R.; Schaltegger, U.; Martínez Catalán, J. R.; Nolan, J.


    The Viseu area is located in the Central Iberian Zone of the Iberian Variscan Belt and hosts numerous post-thickening, collision-related granitoids intruded into upper and middle crustal levels. The present paper reports high precision U-Pb zircon and monazite ages for four plutons of the Viseu area: the syn-kinematic granitoids of Maceira (314±5 Ma), Casal Vasco (311±1 Ma) and Junqueira (307.8±0.7 Ma) and the late-kinematic biotite monzogranites of Cota (306±9 Ma). This points to a synchronous emplacement of the different syn-kinematic plutons shortly followed by the intrusion of the late-kinematic granites and shows that the Upper Carboniferous plutonism occurred within a short time span of ca. 10 million years. The ascent of granite magmas took place after an extensional tectonic event (D 2) and is coeval with dextral and sinistral crustal-scale transcurrent shearing (D 3). Field and petrographical evidence suggest a narrow time-span between peak T metamorphic conditions and the intrusion of granitic melts which implies very fast uplift rates accommodated through active tectonic exhumation. Magma compositions evolve through time, reflecting an increasing involvement of mid-crustal sources and the underplating effect of an upwelling asthenospheric mantle at the base of a thinning and stretching continental crust.

  17. The Paleogene California River: Evidence of Mojave-Uinta paleodrainage from U-Pb ages of detrital zircons

    Davis, S.J.; Dickinson, W.R.; Gehrels, G.E.; Spencer, J.E.; Lawton, T.F.; Carroll, A.R.


    U-Pb age spectra of detrital zircons in samples from the Paleogene Colton Formation in the Uinta Basin of northeastern Utah and the Late Cretaceous McCoy Mountains Formation of southwestern Arizona (United States) are statistically indistinguishable. This finding refutes previous inferences that arkosic detritus of the Colton was derived from cratonic basement exposed by Laramide tectonism, and instead establishes the Cordilleran magmatic arc (which also provided sediment to the McCoy Mountains Formation) as the primary source. Given the existence of a north-south-trending drainage divide in eastern Nevada and the north-northeast direction of Laramide paleoflow throughout Arizona and southern Utah, we infer that a large river system headed in the arc of the Mojave region flowed northeast ~700 km to the Uinta Basin. Named after its source area, this Paleogene California River would have been equal in scale but opposite in direction to the modern Green River-Colorado River system, and the timing and causes of the subsequent drainage reversal are important constraints on the tectonic evolution of the Cordillera and the Colorado Plateau. ?? 2010 Geological Society of America.

  18. Permian volcanisms in eastern and southeastern margins of the Jiamusi Massif, northeastern China: zircon U-Pb chronology, geochemistry and its tectonic implications

    MENG En; XU WenLiang; YANG DeBin; PEI FuPing; Yu Yang; Zhang XingZhou


    LA-ICP-MS zircon U-Pb dating and geochemical data for the Late Paleozoic volcanic rocks from eastern and southeastern margins of the Jiamusi Massif are presented to understand the regional tectonic evolution. Zircons from eight representative volcanic rocks are euhedral-subhedral in shape and dis-play striped absorption and fine-scale oscillatory growth zoning as well as high Th/U ratios (0.33-2.37), implying a magmatic origin. The dating results show that the Late Paleozoic volcanic rocks in the study area can be divided into two stages, I.e., the Early Permian (a weighted mean 206pb/238U age of 288 Ma) and the Middle Permian volcanisms (a weighted mean 206Pb/238U age of 268 Ma). The former is com-posed mainly of basalt, basaltic-andesite, andesite and minor dacite. They are characterized by low SiO2 contents, high Mg# (0.40-0.59), enrichment in Na (Na2O/K2O = 1.26-4.25) and light rare earth elements (LREEs), relative depletion in heavy rare earth elements (HREEs) and high field strength ele-ments (HFSEs), indicating that an active continental margin setting could exist in the eastern margin of the Jiamusi Massif in the Early Permian. The latter consists mainly of rhyolite and minor dacite with high SiO2 (77.23%-77.52%), low MgO (0.11%-0.14%), enrichment in K2O (Na2O/K2O ratios <0.80) and Rb, Th, U and depletion in Eu, Sr, P and Ti, implying a crust-derived origin. Therefore, it is proposed that the Middle Permian volcanic rocks could have formed under the collision of the Jiamusi and the Khanka Massifs.

  19. Géochronologie U-Pb sur zircons et géochimie (Pb, Sr et Nd) du socle de la chaîne de Songpan-Garze (Chine)

    Roger, Françoise; Calassou, Sylvain


    The basement of the Songpan-Garze fold belt has been dated at 829 ± 9 Ma (2σ) using the U-Pb method on zircon fractions. This age is interpreted as the emplacement age, and together with Pb, Sr and Nd isotope data, suggests that this belt was part of the Yangtsé craton (South China block). The emplacement of these calc-alkaline Upper Proterozoic granitoids may be linked to a south-dipping subduction under the South China block during the Jinnigian orogeny (1000-850 Ma).

  20. SHRIMP Zircon U-Pb and Molybdenite Re-Os Dating of the Skarn-type Tungsten Deposits in Southern Jiangxi Province, China, and Geological Implications: Examplified by the Jiaoli and Baoshan Tungsten Polymetallic Deposits%赣南矽卡岩型钨矿成岩成矿年代学及地质意义——以焦里和宝山矿床为例

    丰成友; 曾载淋; 王松; 梁景时; 丁明


    Skarn-type tungsten deposit is a very important type of tungsten mineral resources in southern Jiangxi province.Based on detailed field investigation and analysis on major elements,trace elements,rare earth elements and radiometric dating,the ages of tungsten mineralization and petrogenesis of ore-bearing granitoids,and tectonic environment of the skarn-type tungsten deposits in southern Jiangxi province were studied.SHRIMP zircon U-Pb age for the medium to fine-gained porphyritic granodiorite from the Jiaoli ore district is 164.4 ±1.1 Ma.Molybdenite Re-Os isochron age for the related Jiaoli skarn tungsten deposit is 170.6 ±4.6 Ma.The SHRIMP zircon U-Pb age of the Baoshan medium to fine-grained granite is 156.6 ±3.9 Ma,whereas the molybdenite Re-Os mean model age for the related Baoshan skarn-type tungsten deposit is 161.0 ±1.9 Ma.Thus,W-Sn mineralization and granite crystallization taking place in Middle to Late Jurassic were contemporaneous.These new ages,combined with those available in the literature,indicate that the ages of the granites and related W-Sn ores are Middle to Late Jurassic with a peak at 150 to 160 Ma.It can be concluded that tungsten mineralization occurred under Jurassic intraplate extensional environment which was a phase of lithospheric thinning in southern China.%矽卡岩型钨矿是赣南地区的一种重要钨矿床类型.本文以焦里和宝山两个典型的矽卡岩型钨多金属矿床为例,开展了系统的成岩成矿年代学和岩石地球化学研究,结合区域钨锡矿床最新年代学研究成果,探讨了赣南地区钨矿的成岩成矿时代及形成构造环境.结果表明,焦里矽卡岩型钨多金属矿区斑状花岗闪长岩SHRIMP锆石U-Pb年龄为164.4±1.1 Ma,辉钼矿Re-Os等时线年龄为170.6 ±4.6 Ma;宝山矽卡岩型钨矿区花岗岩SHRIMP锆石U-Pb年龄为156.6±3.9 Ma,辉钼矿Re-Os模式年龄加权平均值为161.0±1.9 Ma,厘定了它们的成岩成矿年龄为中-晚侏罗世,同一矿

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

    Léo A. Hartmann


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

  2. Large southward motion and clockwise rotation of Indochina throughout the Mesozoic: Paleomagnetic and detrital zircon U-Pb geochronological constraints

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


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

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

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


    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

  4. In situ ion microprobe U-Pb dating and REE abundances of a carboniferous conodont

    Sano, Yuji; Terada, Kentaro

    We report here in situ ion microprobe U-Pb dating of a conodont micro-fossil using an ion microprobe method. Thirteen spots on the single fragment of the Carboniferous conodont (size: approximately 800 µm × 100 µm) yield a 238U/206Pb isochron age of 323±36 Ma and a Tera-Wasserburg concordia intercept age of 332±44 Ma in a three-dimensional 238U/206Pb-207Pb/206 Pb-204Pb/206Pb diagram. These ages are consistent with the depositional and early diagenetic ages of the fossil in its host Mississippian sedimentary sequence within experimental error. The success of the method depends on the chemical fractionation of U from Pb within a hundred-µm length scale and the consequent variations in Pb isotopic compositions due to radioactive decay. Shale-normalized rare earth element (REE) abundances of two spots on the same sample show flat patterns from light REE to middle REE and decreases from middle REE to heavy REE with negative anomalies of both Ce and Eu. The REE characteristics are significantly different from those of Devonian conodonts reported by other workers, suggesting discrepant redox states and/or formation environments.

  5. Exotic island arc Paleozoic terranes on the eastern margin of Gondwana: Geochemical whole rock and zircon U-Pb-Hf isotope evidence from Barry Station, New South Wales, Australia

    Manton, Ryan J.; Buckman, Solomon; Nutman, Allen P.; Bennett, Vickie C.


    Early Paleozoic intra-oceanic terranes crop out along the Peel-Manning Fault System, in the southern New England Orogen, NSW Australia. These are the Cambrian ophiolitic Weraerai terrane and the Siluro-Devonian island arc Gamilaroi terrane. There has been debate whether these terranes formed at the Gondwana margin or if they are intra-oceanic, and were accreted to Gondwana later in the Paleozoic. Major-trace-REE elemental data indicate Weraerai terrane formed in a supra-subduction environment. Rare zircons extracted from Weraerai terrane gabbro-plagiogranite suites at Barry Station yield a U-Pb zircon date of 504.9 ± 3.5 Ma with initial εHf values of + 11.1 indicating a juvenile source. Amphibole-bearing felsic dykes and net-vein complexes are also found within the gabbro with a U-Pb zircon date of 503.2 ± 5.7 Ma and initial εHf values of + 11.6. These are coeval in age with their host rocks and we propose they represent partial melts of the mafic crust during the circulation of seawater. The Gamilaroi trondhjemites of prehnite-pumpellyite-greenschist metamorphic grade terrane yielded very few zircons with an age of 413 ± 8.7 Ma. Zircon initial εHf values range from + 5.0 to + 2.9, indicating an input from an evolved crustal source, unlike the purely oceanic Weraerai terrane. Gamilaroi terrane trondhjemites are enriched in LREE have low K2O and K2O/Na2O ratios and strong negative Nb anomalies consistent with supra-subduction zone environments. Multiple subduction zones may well have existed within the Panthalassa Ocean during the early-mid Paleozoic with the Weraerai-Gamilaroi being accreted onto the Gondwanan margin during the latest Devonian.

  6. Petrogenesis and significance of the Hongshan syenitic pluton,South Taihang: zircon SHRIMP U-Pb age, chemical compositions and Sr-Nd isotopes


    The Hongshan syenitic pluton (South Taihang) is mainly made up of hornblende syenite and minor granite. SHRIMP zircon dating yields a U-Pb age of 135 ± 2.7 Ma for the emplacement of the pluton, while biotite Rb-Sr isotopic data give an isochron of 120.3 ± 2.4 Ma. Hongshan syenites show low silica, high alkalis and LILE such as Sr, and exhibit negligible Eu anomalies or slightly positive anomalies in the REE patterns. The syenites show quite enriched isotopic compositions with Isr from 0. 7052 to 0. 7102 and εNd( t ) from - 7.5 to - 11. 1. Petrogenesis of the pluton can be that partial melting of an enriched lithospheric mantle gave birth to an alkali basaltic magma, which subsequently underplated in the lower crust and experienced a coupled fractionation of ferromagnesian phases like pyroxene and hornblende and minor ( < 10 % ) contamination of lower continental crust, producing the Hongshan syenites that partially evolved into granite through combined fractionation of hornblende and feldspar. Our data are not in agreement with a previous model that the syenites originated from melting of lower crust in a thickened crust circumstance, and thus do not support the speculation of "East China Plateau" in the Mesozoic.

  7. Triassic U-Pb age for zircon from granites in the Tonghua area and its response to the Dabie-Sulu ultrahigh-pressure collisional orogenesis

    LU Xiaoping; WU Fuyuan; ZHAO Chengbi; ZHANG Yanbin


    Single-grain zircon U-Pb dating was carried out to constrain the emplacement timing of granitic plutons at Chaxinzi, Xiaoweishahe and Longtou in the Tonghua area, south of Jilin Province. The results show that these plutons formed in the Triassic with ages of 203-217 Ma. Geological and geochemical characteristics indicate that the plutons are composed of quartz diorite and granite. The former was derived from partial melting of mafic lower crust, whereas the latter originated from thickened crust with garnet as the residue in the source. It appears that protoliths of these two types of granitits are different although they have the same emplacement age. Considering that these plutons are petrologically different from the coeval granites in the Xingmeng (Xing'an-Mongolian) to Jihei (Jilin-Heilongjiang) orogenic belt in the north, it is suggested that their formation was related to the Dabie-Sulu ultrahigh-pressure collisional orogenesis since their ages are only 10-20 Ma younger than timing of the ultrahigh-pressure metamorphism, but comparable to that of the first rapid exhumation of the ultrahigh-pressure metamorphic rocks and the emplacement of the post-collisional granites.

  8. A Modern Analog to the Depositional Age Problem: Zircon and Apatite Fission Track and U-Pb Age Distributions by LA-ICP-MS

    Donelick, H. M.; Donelick, M. B.; Donelick, R. A.


    Sand from three river systems in North Idaho (Snake River near Lewiston, Clearwater River near Lewiston and the Salmon River near White Bird) and two regional ash fall events (Mt. Mazama and Mt. St. Helens) were collected for zircon U-Pb detrital age analysis. Up to 120 grains of zircon per sample were ablated using a Resonetics M-50 193 nm ArF Excimer laser ablation (LA) system and the Pb, Th, and U isotopic signals were quantified using an Agilent 7700x quadrupole inductively coupled plasma-mass spectrometer (ICP-MS). Isotopic signals for major, minor, and trace elements, including all REEs, were also monitored. The youngest zircon U-Pb ages from the river samples were approximately 44 Ma; Cenozoic Idaho Batholith and Precambrian Belt Supergroup ages were well represented. Significant common Pb contamination of the Clearwater River sample (e.g., placer native Cu was observed in the sample) precluded detailed analysis of the zircon U-Pb ages but no interpretable ages <44 Ma were observed. Interestingly, not one of the river samples yielded zircon U-Pb ages near 0 Ma, despite all three catchment areas having received significant ash from Mt. St. Helens in 1980, and Mount Mazama 7,700 years ago, and no doubt other events during the Quaternary. Work currently in progress seeks to address bias against near 0 Ma ages in the catchment areas due to: a) small, local ash fall grain sizes and b) overwhelming number of older grains relative to the ash fall grains. Data from Mt. St. Helens ash from several localities near the mountain (Toutle River and Maple Flats, WA) and several far from the mountain (Spokane, WA; Princeton, ID; Kalispell, MT) and Mt. Mazama ash fall deposits near Lewiston, ID and Spokane, WA will be presented to address these possibilities. Additionally, fission track and U-Pb ages from apatites collected from these river and ash fall samples will also be shown to help constrain the problem.

  9. Cretaceous-Cenozoic tectonic history of the Jiaojia Fault and gold mineralization in the Jiaodong Peninsula, China: constraints from zircon U-Pb, illite K-Ar, and apatite fission track thermochronometry

    Deng, Jun; Wang, Changming; Bagas, Leon; Carranza, Emmanuel John M.; Lu, Yongjun


    The Jiaojia Fault (JJF) in the Jiaodong area of eastern China is an important NNE-trending structure that is subsidiary to the regional Tancheng-Lujiang (Tan-Lu) Fault Zone, and hosts >1200 t of gold reserves contained in disseminated and stockwork ore, dominantly in the footwall of the fault. We present new zircon U-Pb, apatite fission track, and illite K-Ar data along the JJF and have delineated its tectonic history focusing on its formation and reactivation. Zircon U-Pb dating shows that the Shangzhuang granite is a composite body with ages between 132 ± 1 and 127 ± 1 Ma. Illite K-Ar ages for the fault's gouge range from 83 ± 2 to 68 ± 2 Ma, and the measured apatite fission track ages for ores are between 55 and 21 Ma. Previous zircon U-Pb geochronology and structural studies suggest that the JJF was originally activated in the Jurassic during 160-150 Ma as a sinistral fault. The JJF was a normal fault in the Early Cretaceous due to NW-SE orientated tension and NE-SW compression, which lasted from 135 to 120 Ma. This was followed by sinistral strike-slip faulting due to NW-SE compression and NE-SW tension during 120-110 Ma, and it changed to normal displacement at ca. 110 Ma. Our apatite fission track data analysis and thermal modeling of representative samples suggest that there was a subsequent dextral reactivation of the fault at ca. 55 Ma. Previous age data of ca. 130-110 Ma for gold mineralization along the JJF coincides with the Early Cretaceous magmatism and is coeval with the transition from normal faulting to sinistral strike-slip faulting of the JJF in Early Cretaceous, which is interpreted to be due to changing direction of the subducting Pacific Plate.

  10. U-Pb detrital zircon age patterns of Cenozoic clastic sedimentary rocks in Trinidad and its implications

    Xie, Xiangyang; Mann, Paul


    The Cenozoic evolution of northern South America can be simplified as a diachronous, west-to-east change from north-facing passive margin, to active convergence and transcurrent plate margin. As the current eastern end of the Caribbean-South America plate boundary, the Trinidad area records the most recent tectonic regime transition. Documenting the provenance of Cenozoic clastic rocks in Trinidad provides insights into the spatial and temporal relationships between mountain range uplifting, sediment dispersal, and drainage system development along the eastern end of the Caribbean-South American plate boundary. Four Eocene to Pliocene age outcropping sandstone samples were collected from Trinidad and 545 detrital zircon grains were analyzed using the U-Pb LA-ICP-MS method. A total of 404 analyses with less than 10% discordance were used for the final interpretation. Results show that the age distribution of the Eocene to the Early Oligocene samples is very restricted and dominated by Precambrian age grains with age peaks at ~ 1400 Ma, ~ 1800 Ma, and ~ 2000 Ma, typically derived from the Guyana Shield. In contrast, the Late Oligocene and younger samples show much broader and mixed age distribution that includes Mesozoic, Paleozoic, and Precambrian peaks. This age shift was interpreted as arriving of the Great Caribbean Arc and oblique collision between the South America and Caribbean plates. Continuing collision uplifted the Andes belts to the west. The Eastern Cordillera of Colombia, the Merida Andes, and the northern Venezuela coastal ranges then became the second primary sources. Offshore sedimentation switched from the passive margin with multiple small drainage systems to the active convergent and transcurrent plate margin with a single large river-dominated delivery system.

  11. Middle Permian Seamount from Xiahe Area, Gansu Province, Northwest China: Zircon U-Pb Age, Biostratigraphy and Tectonic Implications

    Kou Xiaohu; Zhang Kexin; Zhu Yunhai; Chen Zhongqiang; Lin Qixiang; Chen Fenning; Huang Jinyuan


    The well-preserved seamount buildups are documented from the northwestern Qinling (秦岭) orogenic belts, Northwest China. The study sections are located in the Ganjia (甘加) area of the Xiahe (夏河) County, Gansu (甘肃) Province. The dark basalt and overlying massive reef carbonate characterize the Xiahe seamount buildup. Basalts are dominated by the olivine type of rocks and bear distinct porphyritic textures, and fumarole and amygdaloidal structures. The basaits are dominated by SiO2(up to 48.49 wt.%-52.29 wt.%) followed by (Na2O+ K2O) (3.80 wt.%-4.96 wt.%) and TiO2 (2.04 wt.%-2.52 wt.%). They are featured by considerably high content of Ti. The tholeilteseries rocks dominate the basalts, while calc-alkali-series rocks are also present. The REE of the basalts shows the LREE-enrichment type with distinct positive Eu abnormal. The trace elements of the basalts are characterized by the lack of P and high content of Ti. These geochemical signals suggest that the Xiahe basalts were formed in an ocean-island setting. The LA ICP-MS zircon U-Pb age of the basalts is 267.6±5 Ma, which is reinforced by the presnce of the fusuHnid Neoschwagerina Zone of the Wordian (Middle Permian) in the limestone interbeds of the basalts. Integration of petrological and geochemical studies of seamount basaits and lateral correlation of seamount buildups reveals that the Qinling-qilian-Kunlun orogenic belts were probably the archipelagtc oceans during the Permian.

  12. Petrogenesis of Jurassic fractionated I-type granites in Southeast China: Constraints from whole-rock geochemical and zircon U-Pb and Hf-O isotopes

    Zhang, Yang; Yang, Jin-Hui; Sun, Jin-Feng; Zhang, Ji-Heng; Chen, Jing-Yuan; Li, Xian-Hua


    Whole-rock geochemical and Sr-Nd-Hf isotopic data and zircon in situ U-Pb ages and Hf-O isotopes are reported for twelve granitic intrusions from inland to coastal areas of the Cathaysia Block, Southeast (SE) China, in order to constrain their magma sources and petrogenesis. They are mainly composed of biotite granites with minor granodiorites. Zircon U-Pb dating by LA-ICPMS and SIMS methods gave emplacement ages of 165-154 Ma for these rocks, which are metaluminous to weakly peraluminous (A/CNK = 0.95-1.10), with variable SiO2 contents and high K2O contents. They are all depleted in high field strength elements (HFSEs) and enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and show negative Sr, Ba and Eu anomalies in the spidergrams. Petrographic and geochemical features suggest the granodiorites that contain hornblende to be typical of I-type, while the biotite granites without hornblende are fractionated I-type granites. Their variable major and trace element concentrations are due to fractional crystallization of feldspar, biotite, hornblende, and accessory minerals, including apatite, allanite and monazite. Moreover, our data indicate significant variations in isotopic compositions of these granitoids from inland to coastal areas. The inland granites, from the Lapu, Fogang, Xinfengjiang and Baishigang plutons, have relatively high initial 87Sr/86Sr ratios (0.7105-0.7155) and low εNd(t) (-11.2 to -7.3) and εHf(t) (-10.9 to -6.3) values for whole rocks, and variable δ18O values of 7.69-9.25‰ and εHf(t) values of -13.5 to -6.7 for zircons. In contrast, the coastal granites, from the Longwo, Wushikeng, Lianhuashan, Shigushan, Chiliao, Hulutian, Mantoushan and Fenghuang plutons, show relatively low initial 87Sr/86Sr ratios (0.7056-0.7110) and high εNd(t) (-7.6 to -6.2) and εHf(t) (-6.5 to -2.8) values for whole-rocks, together with εHf(t) values of -7.2 to 3.9 and δ18O values of 6.69-8.38‰ for zircons. The spatial

  13. Direct U-Pb dating of Cretaceous and Paleocene dinosaur bones, San Juan Basin, New Mexico: COMMENT

    Koenig, Alan E.; Lucas, Spencer G.; Neymark, Leonid A.; Heckert, Andrew B.; Sullivan, Robert M.; Jasinski, Steven E.; Fowler, Denver W.


    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.

  14. U-Pb Zircon geochronology of the Cambro-Ordovician metagranites and metavolcanic rocks of central and NW Iberia

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


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

  15. Age of nephrite-bearing dikes of the Uzunkyr Belt (South Urals): Local U-Pb isotope analysis of zircon and Sr-Nd isotope data of rock-forming minerals

    Arkhireev, I. E.; Makagonov, E. P.; Belyatskii, B. V.; Maslennikov, V. V.


    Through local U-Pb isotope analysis of zircon and Sir-Need data on rock-forming minerals, the age of nephrite-bearing monzonite-diorite dikes of the Uzunkyr Belt has been determined. The derived datings coincide with known geological events that took place in the Phanerozoic on the territory of the South Urals. Xenogenic zircons prove the participation of the Upper Ordovician units in the tectonic structure of the studied area. Devonian zircons are associated with assimilation of subvolcanic rocks which are middle and basic in composition and whose formation time correlates with the appearance of the subduction zone with the Magnitogorsk island arc above it. Early Carboniferous datings indicate the relationship between dike formation and formation of the continental arc-shaped structure to which the Syrostan massif (monzodiorite-granite formation) belongs. The age range of the Uzunkyr nephrite-bearing dikes coincides with that of intrusives (350-336 Ma) of the Magnitogorsk Belt, where formation of gabbro series was also changed by formation of subalkali and alkali igneous rocks. According to the analogous data on zircon datings from metamorphic rocks of the Il'menogorskii Complex, the given territory later evolved as a whole.

  16. SHRIMP U-Pb zircon geochronology of granulites at Rimana (Southern Tibet) in the central segment of Himalayan Orogen

    LI Dewei; LIAO Qunan; YUAN Yanming; WAN Yusheng; LIU Demin; ZHANG Xionghua; YI Shunhua; CAO Shuzhao; XIE Defan


    High-pressure mafic granulites, occurring as lenses within gneisses and quartzite in the central segment of the Himalayan orogen, were dated using SHRIMP U-Pb technique. 13 analyses out of a total of 15 are plotted along a concordia line and yield a mean 206Pb/238U age of 17.6 ± 0.3 Ma. This age indicates adiabatic decompression and a metamorphic event associated with rapid uplift of granulites in a tectonic environment resulted from the collision between India and Eurasia, synchronous with large-scale thrusting, extension, detachment as well as emplacement of leucogranite. One analysis gives a 206Pb/238U age of 29.5 ± 0.4 Ma that is interpreted to represent the timing of the final closure of the Neo-Tethys. Another age is 1991 ± 26 Ma that represents the age of the protolith of the granulites. In summary, dating results show that granulites in this area underwent multiphase metamorphism and complex geological evolution.

  17. Origin and evolution of the Tengchong block, southeastern margin of the Tibetan Plateau: Zircon U-Pb and Lu-Hf isotopic evidence from the (meta-) sedimentary rocks and intrusions

    Li, Dapeng; Chen, Yuelong; Hou, Kejun; Luo, Zhaohua


    U-Pb ages and Hf isotopic data from detrital zircons of Gaoligongshan metamorphic complex and the Carboniferous Menghong Group and igneous zircons from intrusions constrain the origin, tectonic affinity (East Gondwana), crustal evolution processes, and the properties of regional high-grade metamorphic rocks in the Tengchong block of southeastern China. Three granites intruded into the Tengchong block at 72 Ma, 114 Ma and 122 Ma. Detrital zircons range in age from Archean to Late Ordovician for both the Carboniferous Menghong Group and the Gaoligongshan metamorphic complex. Analyses for these two units yield similar age clusters at 2.5 Ga, 1.6 Ga, 1.17 Ga, 0.95 Ga, and 0.65-0.5 Ga as well as parallel Hf isotopic distributions. The protolith of the studied Gaoligongshan complex in the Tengchong block should deposit in the Late Paleozoic. Detrital zircon age distribution patterns of the Carboniferous Menghong Group and the Gaoligongshan complex show dominant younger Grenvillian age peaks at 0.95 Ga, indicating the strong paleogeographic connection of the Tengchong block with the Indian margin. The Hf isotopic comparison of both detrital and igneous dated-zircon shows that the Tengchong block can be represented by the post-Archean Indian continental margin. After 250 Ma, the intensive magma events affected the region and considerable juvenile material accreted to the crust of the Tengchong block.

  18. U-Pb Ages of Lunar Apatites

    Vaughan, J.; Nemchin, A. A.; Pidgeon, R. T.; Meyer, Charles


    Apatite is one of the minerals that is rarely utilized in U-Pb geochronology, compared to some other U-rich accessory phases. Relatively low U concentration, commonly high proportion of common Pb and low closure temperature of U-Pb system of apatite inhibit its application as geochronological tool when other minerals such as zircon are widely available. However, zircon appear to be restricted to certain type of lunar rocks, carrying so called KREEP signature, whereas apatite (and whitlockite) is a common accessory mineral in the lunar samples. Therefore, utilizing apatite for lunar chronology may increase the pool of rocks that are available for U-Pb dating. The low stability of U-Pb systematics of apatite may also result in the resetting of the system during meteoritic bombardment, in which case apatite may provide an additional tool for the study of the impact history of the Moon. In order to investigate these possibilities, we have analysed apatites and zircons from two breccia samples collected during the Apollo 14 mission. Both samples were collected within the Fra Mauro formation, which is interpreted as a material ejected during the impact that formed the Imbrium Basin.

  19. 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

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


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

  20. U-Pb zircon and geochemical evidence for bimodal mid-Paleozoic magmatism and syngenetic base-metal mineralization in the Yukon-Tanana terrane, Alaska

    Dusel-Bacon, C.; Wooden, J.L.; Hopkins, M.J.


    New SHRIMP (sensitive, high-resolution ion microprobe) U-Pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic Yukon-Tanana terrane in east-central Alaska help define the tectonic setting of mid-Paleozoic magmatism and syngenetic hydrothermal Zn-Pb-Ag mineralization along the ancient Pacific margin of North America. We compare data from similar greenschist-facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks, in the Wood River area of the Alaska Range and the Salcha River area of the Yukon-Tanana Upland, and from amphibolite-facies augen gneiss and mafic gneiss (amphibolite) in the Goodpaster River area of the upland. Allowing for analytical uncertainties, igneous crystallization age ranges of 376-353 Ma, 378-346 Ma, and 374-358 Ma are indicated by 13 new SHRIMP U-Pb dates for the Wood River, Salcha River, and Goodpaster River areas, respectively. Bimodal magmatism is indicated by Late Devonian crystallization ages for both augen gneiss (371 ?? 3 and 362 ?? 4 Ma) and associated orthoamphibolite (369 ?? 3 Ma) in the upland and by stratigraphic interleaving of mafic and felsic rocks in the Alaska Range. Metabasites in all three study areas have elevated HFSE (high field strength element) and REE (rare earth element) contents indicative of generation in a within-plate (extensional) tectonic setting. Within-plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the Bonnifield district in the Wood River area and for metarhyolite tuff interlayered with the carbonaceous Nasina assemblage, which hosts sedimentary exhalative sulfide occurrences in the Salcha River area. Most of the other felsic metaigneous samples from the Alaska Range and the Yukon-Tanana Upland have geochemical signatures that are similar to those of both average upper continental crust

  1. SHRIMP Dating and Recrystallization of Metamorphic Zircons from a Granitic Gneiss in the Sulu UHP Terrane

    LI Hongyan


    An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite-gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very low Th/U ratios (0.01-0.08) of the zircons indicate that they were formed by metamorphic recrystallization. Strongly in contrast with previously published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks where protolith ages of 600-800 Ma are commonly recorded, only metamorphic age of 218+5 Ma, defined by 18 analytical spots either in rim or in core of zircons, are recorded in this granitic gneiss. This age represents the time of the complete metamorphic recrystallization overprint on primary magmatic zircons. The recrystallization was derived by the UHP metamorphism,and was strengthened by the early stage of retrograde metamorphic fluid activity.

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

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


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

  3. Petrogenesis of subvolcanic rocks from the Khunik prospecting area, south of Birjand, Iran: Geochemical, Sr-Nd isotopic and U-Pb zircon constraints

    Samiee, Somayeh; Karimpour, Mohammad Hassan; Ghaderi, Majid; Haidarian Shahri, Mohammad Reza; Klöetzli, Urs; Santos, José Francisco


    The Khunik prospecting area is located 106 km south of Birjand in eastern Iran, and is considered as an epithermal gold prospecting area. The mineralization is related to subvolcanic rocks. There are several outcrops of subvolcanic intrusions in the area which intruded into Paleocene-Eocene volcanic rocks (andesite, trachy-andesite and pyroclastic rocks). Petrographic studies indicate that subvolcanic rocks consist mainly of diorite, monzonite, quartz-monzonite, monzodiorite and quartz-monzodiorite. Mineralogically, these rocks contain plagioclase, K-feldspar, amphibole, pyroxene, biotite and quartz. Geochemically, they have features typical of high-K calk-alkaline to shoshonitic and are metaluminous, and also belong to magnetite granitoid series (I-type). Primitive mantle normalized trace element spider diagrams display enrichment in LILE, such as Rb, Ba, and Cs, compared to HFSE. Chondrite-normalized REE plots show moderately LREE enriched patterns (7.45 < LaN/YbN < 10.54), and no significant Eu anomalies. Tectonic discrimination diagrams also show affinities with modern convergent margin magmas, suggesting that magmas of Khunik area formed in volcanic arc setting related to subduction of the oceanic crust under the Lut Block plate. The initial 87Sr/86Sr ratios (0.704196-0.704772) and εNdi values (+1.3 to +3.3) are compatible with an origin of the parental melts in a supra-subduction mantle wedge. Zircon U-Pb dating by LA-ICP-MS indicates the age of 38 ± 1 Ma (late Eocene) for subvolcanic units that are related to mineralization. A biotite granodiorite porphyry is the testimony of the youngest magmatic activity in the area, with an age of 31 ± 1 Ma (early Oligocene). The represented dates are interpreted as magmatic crystallization ages of subvolcanic intrusions.

  4. Early Cretaceous U-Pb zircon ages for the Copiapó plutonic complex and implications for the IOCG mineralization at Candelaria, Atacama Region, Chile

    Marschik, Robert; Söllner, Frank


    Four of the major plutons in the vicinity of the Candelaria mine (470 Mt at 0.95% Cu, 0.22 g/t Au, 3.1 g/t Ag) and a dike-sill system exposed in the Candelaria open pit have been dated with the U-Pb zircon method. The new geochronological data indicate that dacite magmatism around 123 Ma preceded the crystallization of hornblende diorite (Khd) at 118 ± 1 Ma, quartz-monzonite porphyry (Kqm) at 116.3 ± 0.4 Ma, monzodiorite (Kmd) at 115.5 ± 0.4 Ma, and tonalite (Kt) at 110.7 ± 0.4 Ma. The new ages of the plutons are consistent with field relationships regarding the relative timing of emplacement. Plutonism temporally overlaps with the iron oxide Cu-Au mineralization (Re-Os molybdenite ages at ˜115 Ma) and silicate alteration (ages mainly from 114 to 116 and 110 to 112 Ma) in the Candelaria-Punta del Cobre district. The dated dacite porphyry and hornblende diorite intrusions preceded the ore formation. A genetic link of the metallic mineralization with the quartz-monzonite porphyry and/or the monzodiorite is likely. Both of these metaluminous, shoshonitic (high-K) intrusions could have provided energy and contributed fluids, metals, and sulfur to the hydrothermal system that caused the iron oxide Cu-Au mineralization. The age of the tonalite at 110.7 Ma falls in the same range as the late alteration at 110 to 112 Ma. Tonalite emplacement may have sustained existing or driven newly developed hydrothermal cells that caused this late alteration or modified 40Ar/39Ar and K/Ar systematic in some areas.

  5. Formation Age of the Marble in the Khondalite Series in Jining, Inner Mongolia: Evidence of the LA-ICP-MS Zircon U-Pb Dating of Felsic Gneiss%内蒙集宁地区孔兹岩系中大理岩的形成时代——长英质片麻岩中LA-ICP-MS锆石U-Pb测年的证据

    徐仲元; 范志伟; 刘正宏; 李世超; 王挽琼; 张超; 马越; 王英男


    Our research area is located in Jining,the eastern section of the Khondalite belt in the western block of the North China craton,where the khondalite series is composed of garnet-biotite gneiss in the lower part and marble in the upper part.Several layers of felsic gneiss interbedded with thin marble layers are found,and the parental rock of the felsic gneiss is defined as feldsparthic sandstone by petrographical study,zircon characteristics and restoring the protolithic types.After LA-ICP-MS zircon U-Pb dating of the felsic gneiss,72 zircon data are obtained from 70 zircon grains,among which the youngest detrital zircon data is (2 091±20) Ma that can represent the lower limit of felsic gneiss formation age; And the weighted average 207Pb/206 Pb apparent age of the 19 metamorphic zircon concentrated between 1.8-1.9 Ga is (1 852±15) Ma(MSWD=0.7),can represent the upper limit of deposition age of the original rock of the felsic gneiss.Based on the previous research results,we could conclude that in Jining area,marbles in the khondalite series were formed in the Paleoproterozoic Era between (1 852±15) Ma and (2 091±20) Ma,and underwent the regional metamorphism in (1 852±15) Ma.This suggests that marbles are the import.ant component of the khondalite series; In the Khondalite belt across the western block of the North China craton,a large scale of granitic intrusion had existed in the source region of protolith of the khondalite series.However,there are different material Source regions in every part of the belt; the protoliths of the khondalite series is not the in-situ deposition of fragmentary materials provided by the nearby Archean rocks,but alien tectonic rock sheet mingled with the nearby Archean rocks by Late Paleoproterozoic tectonic processes.%研究区位于华北克拉通西部孔兹岩带东段的集宁一带,区内的孔兹岩系由下部的榴云片麻岩组合和上部的大理岩组合组成.在大理岩组合的底部发现了数层与薄层大

  6. Integrated in situ U-Pb Age and Hf-O Analyses of Zircon from the Northern Yangtze Block: New Insights into the Neoproterozoic Low-δ18O Magmas in the South China Block

    Yang, Y. N.; Wang, X. C.; Li, Q. L.; Li, X. H.


    The oxygen isotopic composition of Neoproterozoic magmas from the northern Yangtze Block holds a key for the origin of large-scale 18O depletion in the HP and UHP metamorphic rocks in the Dabie-Sulu orogenic belt, northern margin of the South China Block. We report here the integrated in situ U-Pb dating and O-Hf isotope analyses of zircon grains from sedimentary and volcanic rocks of the late Neoproterozoic Suixian Group (SG) from the northern Yangtze Block. Detrital zircon grains display age peaks of 0.73-0.74 Ga, 0.79 Ga, and 2.0 Ga. Zircon U-Pb ages together with Hf-O isotopic composition indicate provenance of SG dominantly from proximal Neoproterozoic igneous rock and likely hidden Paleoproterozoic basement along the northern margin of the Yangtze Block. The zircon δ18O values from SG range from 10.5‰ to 1.3‰. Zircon grains with negative δ18O value, typical result of magma-ice interaction, were not identified in this study. The major phase of low-δ18O (< 4‰) magmas initiated at ca. 780 Ma, long before the first glaciation event (< 715 Ma) in the South China Block. Thus caution should be taken when using low-δ18O zircon grains to infer cold climate. Low-δ18O zircon grains have large ranges of ɛHf(t) values, varying from -15.5 to 10.7, concentrating on negative ɛHf(t). This strongly argues against the possibility that the low-δ18O magma was produced by partial melting of high-temperature hydrothermally altered oceanic crust because this model predicted MORB-like Hf isotopes for the resultant low-δ18O magmas. This study emphasizes that high-T water-rock interaction and continental rifting tectonic setting are essential to generate abundant low-δ18O magmas. The important application of our study is to confirm that most of negative-δ18O zircons identified in HP and UHP metamorphic rocks may not have been inherited from their Neoproterozoic protoliths.

  7. Zircon U-Pb geochronological framework of Qitianling granite batholith, middle part of Nanling Range, South China


    The Qitianling granite batholith (QGB) is located in the southern Hunan Province, middle part of the Nanling Range, South China. Its total exposure area is about 520 km2. Based on our 25 single grain zircon U-Pb age data and 7 published data as well as the geological, petrological, and space distribution characteristics, we conclude that QGB is an Early Yanshanian (Jurassic) multi-staged composite pluton. Its formation process can be subdivided into three major stages. The first stage, emplaced at 163―160 Ma with a peak at about 161 Ma, is mainly composed of hornblende-biotite monzonitic granites and locally biotite granites, and distributed in the eastern, northern, and western peripheral parts of the pluton. The second stage, emplaced at 157―153 Ma with a peak at 157―156 Ma, is mainly composed of biotite granites and locally containing hornblende, and distributed in the middle and southeastern parts of the pluton. The third stage, emplaced at 150―146 Ma with a peak at about 149 Ma, is mainly composed of fine-grained (locally porphyritic) biotite granites, and distributed in the middle-southern part of the pluton. Each stage can be further disintegrated into several granite bodies. The first two intrusive stages comprise the major phase of QGB, and the third intrusive stage comprises the additional phase. Many second stage fine-grained granite bosses and dykes intruded into the first stage host granites with clear chilling margin-baking phenomena at their intrusive contacts. They were emplaced in the open fracture space of the earlier stage consolidated rocks. Their isotopic ages are mostly 2―6 Ma younger than their hosts. Conceivably, the time interval from magma emplacement, through cooling, crystallization, solidification, up to fracturing of the earlier stage granites cannot exceed 2―6 Ma. During the Middle-Late Jurassic in the Qitianling area and neighboring Nanling Range, the coeval granitic and basic-intermediate magmatic activities were widely

  8. Cretaceous and Jurassic intrusions in Crimean Mountains: The first data of U-Pb (SIMS SHRIMP) dating

    Morozova, E. B.; Sergeev, S. A.; Savelev, A. D.


    U-Pb geochronological studies of igneous rocks of the Crimean Mountains were carried out for the first time. The ages of magma crystallization determined for gabbro-dolerites of the Dzhidair and Pervomaiskii intrusions point to the injection of these rocks during the Middle Jurassic Aalenian-Bajocian stage of magmatism. The Berriasian-Valanginian and Aptian age of sill-like bodies within the mass of volcanogenic-sedimentary rocks presumes the necessity to reconsider the common notion of an exclusively Albian magmatic event during the Cretaceous. High-precision U-Pb dating of magma intrusions allowed us to verify the age of Middle Jurassic magmatism and to distinguish the new Early Cretaceous Berriasian-Valanginian magmatism stage of basic composition.

  9. Volcano-climate interactions during the PETM and U-Pb dating from the Fur Formation, Denmark

    Jones, Morgan; Augland, Lars; Svensen, Henrik; Tegner, Christian; Planke, Sverre; Willumsen, Pi


    The Palaeocene-Eocene Thermal Maximum (PETM) at ~56 Ma is one of the most extreme global warming events in Earth's history. Vast quantities of CO2 and CH4 were released to the atmosphere, causing a rapid (within 20,000 years) 5-6 °C warming that persisted for ~170,000 years. The PETM occurred during an abnormally warm period in Earth history, and was followed by other hyperthermal events later in the Eocene. The PETM also coincided with the second and major pulse of magmatism from the North Atlantic Igneous Province (NAIP) during the break-up of Laurentia and Eurasia at 54-56 Ma. Evidence of explosive volcanism is prevalent at Fur, an island in northern Denmark, where over 180 distinct >1 cm thick ash horizons are preserved in a shallow marine succession (numbered from #-39 to #+140). These ash layers are believed to have originated from volcanic centres in east Greenland and western UK, which indicates that that they were formed during very large volcanic eruptions. Here we present the results of two key sections of the Fur Formation: a beach section at Stolleklint that includes sediment deposited syn-PETM, and a quarry section at Jenshøj that encompasses post-PETM sediments. A detailed chemostratigraphic log of δ13C (TOC) and δ15N values through the section suggests that there may be part of sequence missing, either through glaciotectonism or a depositional hiatus. The δ13C values around ash #-33 are typical of the PETM negative carbon isotope excursion (-31 to -32 ‰), yet are back to background values of ~-26 ‰ just four metres up section. The quarry section around ash #+19 displays further variations in δ13C values, suggesting a coincidence with one of the later Eocene hyperthermal events. High precision U-Pb dating of magmatic zircons from ash layer #+19 conforms to a post-PETM depositional age, giving a mean deposition rate of approximately ~5 cm kyr-1. The correlation between the largest ash layers (#-33 and #+19) and the largest perturbations to

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

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


    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

  11. (U-Th)/He and U-Pb double dating constraints on the interplay between thrust deformation and basin development, Sevier foreland basin, Utah

    Pujols, E.; Stockli, D. F.; Horton, B. K.; Steel, R. J.; Constenius, K. N.


    The degree of connectivity between thrust-belt deformation and foreland basin evolution has been a matter of debate for decades. This is in part due to the lack of temporal constraints on the relationship between thrust-belt deformation and associated deposition. New high-resolution zircon (U-Th)-(Pb-He) double dating of pre- and syn-tectonic sedimentary strata along the Sevier thrust front and basin provide an unprecedented geochronological framework to temporally and spatially link the Sevier foreland basin stratigraphy to deforming hinterland sources. Results improve constraints on timing and magnitude of deformation, depositional ages, sediment dispersal and sources. In Late Cretaceous proximal deposits of the Indianola Group (IG) and Canyon Range Conglomerates (CRC), detrital zircon U-Pb (zUPb) and (U-Th)/He ages (ZHe) chronicle the sequential unroofing of the Charlestone-Nebo Salient (CNS) and Canyon Range (CR) duplexes. Furthermore, short ZHe depositional lag-times indicate rapid hinterland exhumation (>1km/my) associated with active thrusting during Cenomanian and Coniacian-Santonian times as supported by bedrock ZHe ages in the CNS and CR thrust sheets. Detrital zircon analyses on the Late Cretaceous marine Book Cliffs strata suggest a more complex source-to-sink evolution compared to the time-equivalent IG and CRC proximal strata due to mixing of multi-source detrital zircons, sediment recycling and more prominent volcanic input. Nonetheless, the overall cooling history recorded in the Book Cliffs clearly reflects three hinterland exhumational phases, an early phase derived from the frontal thrusts and two additional phases with more integrated hinterland ZHe signatures. These three short lag-time phases correlate with fast clastic progradational wedges in the Sevier foreland. These results strengthen the role played by hinterland deformation on clastic progradation and elucidate the temporal relationship between thrusting and foreland basin architecture.

  12. 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

    Rioux, Matthew; Farmer, G. Lang; Bowring, Samuel A.; Wooton, Kathleen M.; Amato, Jeffrey M.; Coleman, Drew S.; Verplanck, Philip L.


    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 final eruption. Calculated magmatic

  13. U-Pb (LA-PIMMS) Ages of Inherited Zircons from Early Palaeozoic Granitoids of the W Sudetes, N Bohemian Massif, Central Europe: Implications for Neoproterozoic Continental Reconstructions

    Crowley, Q. G.; Patocka, F.; Kachlík, V.


    A U-Pb laser ablation plasma ionisation multi-collector mass spectrometry (LA-PIMMS) geochronological study of zircons from early Palaeozoic (meta)granitoids of the Czech W Sudetes (E Saxothuringian Zone), NW Bohemian Massif, was carried out in order to determine the range of inherited age spectra preserved in these lithologies. Backscattered SEM images indicate that many zircons have distinct cores and rims. The majority of inherited zircon components yield concordant U-Pb ages that fall into the following age ranges: (1) 520-770 Ma, (2) 1.9-2.2 Ga and (3) ca. 3.0 Ga. These three age populations are typical of the W African Craton and the Armorican Terrane Assemblage of Europe. The age spectra correspond to Cadomian, Birimian / Icartian / Eburnean / Burkinian and Leonian events respectively. Some previous Pb-Pb zircon and whole rock Nd studies of similar lithologies from the W Sudetes (e.g. Hegner &Kröner, 2000) have attributed the presence of Mesoproterozoic 207Pb/206Pb ages to a peri-Amazonian provenance. Although some zircons from this study have yielded apparent Mesoproterozoic ages, they are discordant and can be resolved into early Palaeozoic to Neoproterozoic lower intercept and Palaeoproterozoic to Archaean upper intercept components. This unequivocally proves that an inherited Grenvillian component does not exist in these lithologies. We therefore favour derivation of the Saxothuringian zone and associated members of the Armorican Terrane Assemblage from a W African Craton Gondwanan setting. References: Hegner, E, &Kröner, A. 2000. Review of Nd data and xenocrystic and detrital ages from the pre-Variscan basement in the Eastern Bohemian Massif: speculations on palinspastic reconstructions. In: Franke, W., Altherr, R., Haak, V. &Oncken, O. (eds.), Orogenic Processes: Quantification and Modelling in the Variscan Belt of Central Europe Geological Society of London Special Publication, 179, 113-129.

  14. Laser Ablation Split Stream (LASS) U-Pb & Lu-Hf Isotope Analysis of Detrital Zircons from the Old Red Sandstone, NW Svalbard: Implications for Northern Caledonian Paleogeography

    Beranek, L. P.; Gee, D. G.; Fisher, C. M.


    The Svalbard archipelago consists of three Caledonian provinces that were assembled by thrusting and transcurrent faulting during the Silurian and Devonian in a location directly northeast of the Greenland Caledonides. Syn- to post-orogenic alluvial strata, referred to as the Old Red Sandstones, filled pull-apart basins adjacent to the transcurrent faults and comprise cover assemblages that help constrain the timing of the Caledonian orogeny. To further investigate the tectonic history and paleogeography of the Raudfjorden-Liefdefjorden-Woodfjorden area of Spitsbergen, NW Svalbard, we analyzed rock samples of the Old Red Sandstones and underlying Precambrian basement complexes for detrital zircon analysis. Laboratory studies of the Old Red Sandstones include the novel Laser Ablation Split Stream (LASS) technique, which allows for simultaneous U-Pb & Lu-Hf isotope analysis of zircon crystals. Lower Devonian Red Bay Group strata contain a range of early Neoproterozoic to Neoarchean detrital zircons with prominent age peaks c. 960, 1050, 1370, 1450, 1650, and 2700 Ma; subordinate Ordovician (c. 460-490 Ma) and Cryogenian (c. 650 Ma) detrital zircons occur in a subset of the samples. Underlying Precambrian metasedimentary rocks are composed of similar earliest Neoproterozoic to Neoarchean age populations, which argues for much of the Red Bay Group to be derived from local basement rocks during thrusting and other faulting. The U-Pb ages and Hf isotope compositions of Paleozoic to Neoarchean detrital zircons are consistent with Arctic crustal evolution, and support the hypothesis that northwestern and northeastern provinces of the Svalbard Caledonides are extruded fragments of the northeast Greenland allochthons. The new Hf isotope results further allow paleogeographic and stratigraphic comparisons with rock assemblages proximal to the North Atlantic Caledonides during the Silurian-Devonian, including the Pearya terrane of Ellesmere Island, Alexander terrane of NW

  15. SHRIMP U-Pb zircon age, geochemistry and Nd isotope of the Guandaoshan pluton in SW Sichuan: Petrogenesis and tectonic significance

    LI; Xianhua(李献华); LI; Zhengxiang(李正祥); ZHOU; Hanwen(周汉文); LIU; Ying(刘颖); LIANG; Xirong(梁细荣); LI; Wuxian(李武显)


    SHRIMP U-Pb zircon age, geochemical and Nd isotopic data are reported for the Neoproterozoic Guandaoshan pluton in the Yanbian region, SW Sichuan. This pluton is of typical I-type granite and emplaced at (857±13) Ma. Geochemical and Nd isotopic characters suggest that the pluton was generated by partial melting of pre-existing, young (late Mesoproterozoic to early Neoproterozoic) low-K tholeiitic protolith within an intraplate anorogenic setting. The Guandaoshan pluton probably records the earliest magmatism induced by the proposed ca. 860-750 Ma mantle superplume beneath the supercontinent Rodinia.

  16. Cryogenian U-Pb (SHRIMP I) zircon ages of anorthosites from the upper sequences of Niquelandia and Barro Alto Complexes, Central Brazil

    Correia, Ciro Teixeira Vicente; Girardi, Antonio Vitorio; Basei, Miguel Angelo Stipp, E-mail: ccorrei@usp.b, E-mail: girardi@usp.b, E-mail: baseimas@usp.b [Universidade de Sao Paulo (IG/USP), SP (Brazil). Inst. de Geociencias. Dept. de Mineralogia e Geotectonica; Nutman, Allen, E-mail: [Australian National University, Camberra (Canada). Research School of Earth Science


    The Niquelandia Complex comprises two main superposed sequences dipping westward: the lower (LS), at the eastern, and the upper (US), at the western part of the body. The Complex is either interpreted as a single body, or as two distinct unrelated layered massifs. New SHRIMP U-Pb determinations on igneous zircon grains of anorthosites from Niquelandia US and from the upper portion of the Barro Alto Complex indicate crystallization ages of 833 {+-} 21 Ma and 733 {+-} 25 Ma, respectively, thus supporting Cryogenian Neoproterozoic ages for the igneous crystallization of the US unit of Niquelandia and for the Barro Alto anorthosites. (author)

  17. Grenvillian orogeny in the Southern Cathaysia Block: Constraints from U-Pb ages and Lu-Hf isotopes in zircon from metamorphic basement

    WANG LiJuan; YU JinHai; S.Y. O'REILLY; W.L. GRIFFIN; SUN Tao; WEI ZhenYang; JIANG ShaoYong; SHU LiangShu


    Metamorphic basement rocks in the Cathaysia Block are composed mainly of meta-sediments with different ages. New zircon U-Pb geochronological results from the meta-sedimentary rocks exposed in the Zengcheng and Hezi areas, southern Cathaysia Block, show that they consist dominantly of early Neoproterozoic (1.0-0.9 Ga) materials with minor Paleo- to Mesoproterozoic and late Neoproterozoic (0.8-0.6 Ga) components, suggesting that the detritus mostly come from a Grenvillian orogen. The youngest detrital zircon ages place a constraint on the deposition time of these sediments in Late Neoproterozoic. Zircon Hf isotopic compositions indicate that the Grenvillian zircons were derived from the reworking of Mesoproterozoic arc magmatic rocks and Paleoproterozoic continental crust, implying an arc-continent collisional setting. Single-peak age spectra and the presence of abundant euhedral Grenvillian zircons suggest that the sedimentary provenance is not far away from the sample location. Thus, the Grenvillian orogen probably preexisted along the southern margin of the Cathaysia Block, or very close to the south. Similarity in the ages of Grenvillian orogeny and the influence of the assembly of Gondwana in South China with India and East Antarctic are discussed, with suggestion that South China was more likely linked with the India-East Antarctica continents in Early Neoproterozoic rather than between western Laurentia and eastern Australia.

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

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


    The Urumieh-Dokhtar magmatic belt of Central Iran runs parallel to the Zagros orogenic belt and has been resulted from Neotethys ocean subduction underneath Eurasia. The Bahr Aseman volcanic-plutonic complex (BAC), covering an area 2000 km2 in the Kerman magmatic belt (KMB) in the southern section of the Urumieh-Dokhtar belt, has long been considered as the earliest manifestation of extensive Cenozoic arc magmatism in KMB. The nature and timing of the magmatism, however, is poorly constrained. An area 1000 km2, in BAC and adjacent Razak volcaniclastic complex and Jebal Barez-type granitoids, was mapped and sampled for geochemistry and geochronology. Andesite and basaltic andesite are the main volcanic components in the study area; plutonic bodies vary from tonalite to quartz diorite, granodiorite and biotite-granite. The rocks in BAC display dominantly normal calc-alkaline character. On spider diagrams, the rocks are characterized by enrichments in LILE relative to HFSE and enrichments in LREE relative to HREE. These features suggest a subduction related setting for the BAC. LaN/YbN ratios for the intrusive and volcanic rocks range from 1.41 to 5.16 and 1.01 to 6.42, respectively. These values are lower than those for other known granitoids in KMB, namely the abyssal, dominantly Oligocene Jebal Barez-type (LaN/YbN = 1.66-9.98), and the shallow, dominantly late Miocene Kuh Panj-type (LaN/YbN = 12.97-36.04) granitoids. This suggests a less evolved magma source for the BAC igneous rocks. In Y vs. Nb and Th/Yb vs. La/Yb discrimination diagrams, an island-arc setting is defined for the BAC rocks. The rocks further plot in primitive island-arc domain in Nb vs. Rb/Zr and Y/Nb vs. TiO2 diagrams. The BAC volcanic and plutonic rocks yielded zircon U-Pb ages of 78.1 to 82.7 Ma and 77.5 to 80.8 Ma, respectively. Zircon U-Pb dating of volcanic rocks and granitoids from the adjacent Razak complex and the Jebal Barez-type granitoids indicated 48.2 Ma and 26.1 Ma ages

  19. U-Pb zircon geochronology and geochemical constraints on the age and origin of late Neoarchean leucosomes in migmatites from the Maevatanana area, Madagascar

    Yang, Xi-An; Liu, Shan-Bao; Hou, Ke-Jun


    Migmatites represent the dominant lithological facies in the Maevatanana area of Madagascar. The migmatites are composite rocks with hybrid metamorphic and magmatic features, comprising 60% melanosome bands that are interlayered and/or tectonically interleaved with 40% of centimeter-to decimeter-scale quartz-feldspar leucosome veins. The leucosome and granite veins are bordered by a series of gently dipping shear zones in the migmatites. U-Pb zircon dating shows that the leucosome veins formed at 2548.3 ± 5.3 Ma and are coeval with intrusion of the granite veins at 2552.2 ± 6.1 Ma. Furthermore, the leucosome and granite veins have uniform chondrite-normalized rare earth element (REE) patterns which are characterized by light REE enrichment relative to heavy REE, and negative or slightly positive Eu anomalies (δEu = 0.61-1.13). Primitive-mantle-normalized trace element patterns of the leucosome and granite veins are enriched in large-ion lithophile elements (Ba and K) and show pronounced depletions in high-field strength elements (Th, U, Ta, Nb, Zr, Hf, and Ti), suggesting that the leucosome and granite veins in the migmatites are syntectonic and synigneous intrusives. Primitive-mantle-normalized trace element patterns of the leucosome veins and melanosome bands in the migmatites are similar and suggest that the granite veins in the migmatites were generated by partial melting of medium-to high-K metabasalt, whereas the melanosome bands originated through metamorphism of calc-alkaline basalts. As such, the leucosome veins were likely generated by partial melting of melanosome bands in the migmatites. Calculated zircon saturation temperatures (TZr) of the leucosome veins are 599-685 °C, which represents the magma crystallization temperatures, and the pressures of magma formation range from 1.2 to 3.2 GPa. These P-T estimates for magma formation and crystallization are consistent with those of 605-658 °C and 1.22-1.43 GPa for metamorphism of the melanosome bands

  20. Zircon U-Pb age, Hf isotope and geochemistry of Carboniferous intrusions from the Langshan area, Inner Mongolia: Petrogenesis and tectonic implications

    Liu, Min; Zhang, Da; Xiong, Guangqiang; Zhao, Hongtao; Di, Yongjun; Wang, Zhong; Zhou, Zhiguang


    Late Paleozoic was a critical period for the tectonic evolution of the northern margin of the Alxa-North China craton, but the evolutionary history is not well constrained. The Carboniferous intrusions in the Langshan area in the western part of the northern margin of the Alxa-North China craton are mainly composed of tonalite, quartz diorite, olivine gabbro and pyroxene peridotite. Zircon LA-ICP-MS U-Pb dating indicates that the Langshan Carboniferous intrusions were emplaced at ca. 338-324 Ma. The quartz diorites are characterized by high amounts of compatible trace elements (Cr, Ni and V) and high Mg# values, which may suggest a significant mantle source. The positive Pb and negative Nb-Ta-Ti anomalies, the variable εHf(t) (-6.9 to 2.0) values and the old Hf model ages (1218-1783 Ma) imply some involvement of ancient continental materials in its petrogenesis. The tonalite has relatively high Sr/Y ratios, low Mg#, Yb and Y contents, features of adakite-like rocks, negative εHf(t) values (-9.8 to -0.1) and older Hf model ages (1344-1953 Ma), which suggest significant involvement of ancient crust materials and mantle-derived basaltic component in its petrogenesis. The high Mg# values, high Cr and Ni contents, and low Zr and Hf contents of the mafic-ultramafic rocks show evidence of a mantle source, and the relatively low zircon εHf(t) values (-5.9 to 3.2) might point to an enriched mantle. The trace element characteristics indicate the influence of subducted sediments and slab-derived fluids. In the tectonic discrimination diagrams, all the rocks plot in subduction-related environment, such as volcanic arc and continental arc. Considering the regional geology, we suggest that the Carboniferous intrusions in the Langshan area were likely emplaced during the late stage of the southward subduction of the Paleo-Asian Ocean plate, which formed a continental arc along the northern margin of the Alxa-North China craton.

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

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


    (2σ external uncertainty), determined relative to the astronomically dated A1 tephra sanidine, is interpreted as the ACR eruption age. This age is supported by CA-TIMS U–Pb zircon dating, guided by LA-ICPMS trace element analyses, titanium-in-zircon (TiZR) thermometry, and cathodoluminescence (CL...

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

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


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

  3. Age and origin of post collision Baltoro granites, south Karakoram, North Pakistan: Insights from in-situ U-Pb, Hf and oxygen isotopic record of zircons

    Mahar, Munazzam Ali; Mahéo, Gweltaz; Goodell, Philip C.; Pavlis, Terry L.


    Origin of post collision plutonism is critical to understand the tectonothermal evolution of the over thickened continental crust in collision zones. This has proven difficult to reconcile with the conventional whole rock geochemical and field based studies alone. We report in-situ study of zircon U-Pb, Hf and O isotopes from five samples of the Baltoro Plutonic Unit (BPU) in south Karakoram. The plutonic unit is the western part of the southern Asian margin of the India-Asia convergent zone. Baltoro granites and a biotite-rich enclave yielded similar and overlapping U-Pb ages ranging from 26 to 15 Ma. Hafnium isotopic composition (εHf (0)) is very heterogeneous ranging from - 17.1 to + 4.4 while the oxygen isotopic composition of the granites is homogeneous with mean δ18O ranging from 7.2 to 9.4‰. Based on U-Pb geochronology and Hf-O isotopic composition, the involvement of two main sources is suggested (1) Cretaceous calc-alkaline Karakoram crust and (2) Karakoram gneisses. Moreover, possible involvement of metasomatized Asian lithospheric mantle is supported by elevated oxygen composition of granites and identical Hf composition of biotite-rich enclave to the mantle derived Baltoro lamprophyre. However, direct contribution from juvenile pristine mantle is unlikely as no juvenile mantle type Hf and oxygen values were obtained. This also precludes the involvement of southward juvenile arc related component of Kohistan-Ladakh batholith. Our new U-Pb and Hf data are comparable to the Mesozoic Karakoram batholith, Miocene two-mica leucogranites in the Pangong Range and magmatism from the Lhasa terrane in south Tibet, suggesting a genetic link between the Karakoram and the rocks to the east. This magmatic event is best explained by lower crust partial melting promoted by both thermal equilibration following crustal thickening and heat advection by ultrapotassic magmas associated with the breakoff of the Indian continental margin.

  4. Geochemistry and zircon U-Pb ages of granulite xenolith from Tuoyun basalts, Xinjiang: Implications for the petrogenesis and the lower crustal nature beneath the southwestern Tianshan

    ZHENG Jianping; LUO Zhaohua; YU Chunmei; YU Xiaolu; ZHANG Ruisheng; LU Fengxiang; LI Huimin


    The granulitic xenoliths discovered in the late Cretaceous basaltic rocks from Tuoyun basin, western Xinjiang Province contain the assemblage of Opx + Cpx + Pl ± Grt ± Qtz ± Kfs. Mineral chemistry, petrochemistry and zircon U-Pb ages from the xenoliths have been reported in the paper. Petrogenesis, the lower crustal nature and the mantle-crust interaction reflected by the granulites are discussed as well. Tuoyun granulites are mainly mafic with few intermediate components, and represent the magmatic products underwent granulite-facies metamorphism in lower crustal condition. Equilibrium temperature and the maximum pressure estimates of the granulites are 910 ± 35℃ and 13.5 × 108 Pa respectively, indicating that the crust beneath southwestern Tianshan had the thickness of less than 44 km and high geotherm (e.g. > 80 mW/m2) when the host magma erupted. During their formation, the fractional crystallization of olivine, pyroxene, plagioclase and ilmenite, plagioclase accumulation and the contamination of felsic components or fluid metasomatism processes had happened. Metamorphic zircon U-Pb age of 253 Ma may record the crust-mantle interaction caused by the orogenic root delamination beneath the southwestern Tianshan.

  5. U-Pb zircon SHRIMP data from the Cana Brava layered complex: new constraints for the mafic-ultramafic intrusions of Northern Goiás, Brazil

    Giovanardi, T.; Girardi, V. A. V.; Correia, C. T.; Sinigoi, S.; Tassinari, C. C. G.; Mazzucchelli, M.


    The Cana Brava Complex is the northernmost and least well known layered intrusion of a discontinuous belt of mafic-ultramafic massifs within the Brasilia Belt, which also comprises theNiquelândia and Barro Alto complexes. Available geochronological data from a range of techniques (K/Ar, Ar/Ar, Rb/Sr, Sm/Nd and U/Pb) provide a range of possible ages (time span from 3.9 Ga to 450 Ma), hence a precise and reliable age for the Cana Brava Complex is still lacking. Also, preliminary isotopic and geochemical data of the Cana Brava Complex suggest a significant crustal contamination, which could have affected bulk-rock Sr and Nd systematics resulting in meaningless age determinations. In this paper, we present new U-Pb SHRIMP zircon analyses from four samples of different units of the Cana Brava Complexwhich suggest that the intrusion occurred during the Neoproterozoic, between 800 and 780 Ma, i.e. at the same age ofNiquelândia. Discordant older 206Pb/238U ages are provided by inherited zircons, and match the age of the metamorphism of the embedding Palmeirópolis Sequence.

  6. SIMS U-Pb zircon age of a tuff layer in the Meishucun section, Yunnan, southwest China: Constraint on the age of the Precambrian-Cambrian boundary


    Determination of the age of the Precambrian-Cambrian boundary is critical in understanding early evolution of life on Earth. SIMS U-Pb zircon analyses of the Bed 5 tuff layer of the Meishucun section were carried out closely following the guidance of cathodoluminescence images, and the majority of analyses were conducted on the oscillatory zircon grains. Thirteen measurements yield a highly reliable Concordia U-Pb age of 536.7 ± 3.9 Ma for the Bed 5 horizon. A grand mean of 206Pb/238U age of 535.2± 1.7 Ma (MSWD = 0.53) is calculated based on 13 concordant SIMS measurements of this study and 4 nano-SIMS measurements of Sawaki et al., which is the best estimate of the deposition age of the tuff layer within Bed 5 in the Meishucun section. This age has provided a robust age constraint on the significant Precambrian-Cambrian boundary strata in southern China, which independently suggested the placement of the Precambrian-Cambrian boundary at the bottom of the Xiaowaitoushan Member (Marker A).

  7. U-Pb zircon geochronology and Sr-Nd isotopic composition of the Inchope orthogneiss in Mozambique: Age constraints and petrogenetic implications

    Manjate, Vicente Albino


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

  8. U-Pb zircon geochronology of rocks in the Salinas Valley region of California: A reevaluation of the crustal structure and origin of the Salinian block

    Barth, A.P.; Wooden, J.L.; Grove, M.; Jacobson, C.E.; Pedrick, J.N.


    The Salinian block in the Salinas Valley region of central California consists of arc granitic and metasedimentary rocks (schist of Sierra de Salinas) sandwiched between coeval high-pressure, low-temperature me??lange belts. U-Pb zircon ages of three granitic plutons from this region range from 88 to 82 Ma, and coexisting biotite yielded 40Ar/39Ar cooling ages of 76-75 Ma. The U-Pb ages from detrital zircons indicate derivation of the protolith of the schist from a 117-81 Ma igneous provenance. Muscovite and biotite 40Ar/ 39Ar cooling ages of 72-68 Ma from the nearby schist are distinctly younger than those from the granitic plutons. These data indicate that deposition and metamorphism of the schist occurred after emplacement of adjacent granitic rocks, contradicting the prevailing view that the schist comprises the local framework for the Salinian arc. We propose that the schist of Sierra de Salinas was thrust beneath the Salinian magmatic arc along a Campanian thrust fault that has not been recognized. This hypothesis implies that the Salinian arc originated as a klippe of basement rocks derived from the vicinity of the western Mojave Desert. Thrusting initiated southeastward-migrating Laramide tectonism of a style similar to that which formed the Vincent thrust and the latest Cretaceous and Paleocene Pelona and Orocopia Schists of southern California and southwestern Arizona.

  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

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


    High-grade rocks of the Wilmington Complex, northern Delaware and adjacent Maryland and Pennsylvania, contain morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia. The arc complex has been divided into several members, the protoliths of which include both intrusive and extrusive rocks. Metasedimentary rocks are interlayered with the complex and are believed to be the infrastructure upon which the arc was built. In the Wilmingto n Complex rocks, both igneous and metamorphic zircons occur as elongate and equant forms. Chemical zoning, shown by cathodoluminescence (CL), includes both concentric, oscillatory patterns, indicative of igneous origin, and patchwork and sector patterns, suggestive of metamorphic growth. Metamorphic monazites are chemically homogeneous, or show oscillatory or spotted chemical zoning in backscattered electron images. U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date complexly zoned zircon and monazite. All but one member of the Wilmington Complex crystallized in the Ordovician between ca. 475 and 485 Ma; these rocks were intruded by a suite of gabbro-to-granite plutonic rocks at 434 ?? Ma. Detrital zircons in metavolcanic and metasedimentary units were derived predominantly from 0.9 to 1.4 Ga (Grenvillian) basement, presumably of Laurentian origin. Amphibolite to granulite facies metamorphism of the Wilmington Complex, recorded by ages of metamorphic zircon (428 ?? 4 and 432 ?? 6 Ma) and monazite (429 ?? 2 and 426 ?? 3 Ma), occurred contemporaneously with emplacement of the younger plutonic rocks. On the basis of varying CL zoning patterns and external morphologies, metamorphic zircons formed by different processes (presumably controlled by rock chemistry) at slightly different times and temperatures during prograde metamorphism. In addition, at least three other thermal episodes are

  10. U-Pb isotopic ages and Hf isotope composition of zircons in Variscan gabbros from central Spain: evidence of variable crustal contamination

    Villaseca, Carlos; Orejana, David; Belousova, Elena; Armstrong, Richard A.; Pérez-Soba, Cecilia; Jeffries, Teresa E.


    Ion microprobe U-Pb analyses of zircons from three gabbroic intrusions from the Spanish Central System (SCS) (Talavera, La Solanilla and Navahermosa) yield Variscan ages (300 to 305 Ma) in agreement with recent studies. Only two zircon crystals from La Solanilla massif gave slightly discordant Paleoproterozoic ages (1,848 and 2,010 Ma). Hf isotope data show a relatively large variation with the juvenile end-members showing ɛHfi values as high as +3.6 to +6.9 and +1.5 to +2.9 in the Navahermosa and Talavera gabbros, respectively. These positive ɛHfi values up to +6.9 might represent the composition of the subcontinental mantle which generates these SCS gabbros. This ɛHfi range is clearly below depleted mantle values suggesting the involvement of enriched mantle components on the origin of these Variscan gabbros, and is consistent with previous whole-rock studies. The presence of zircons with negative ɛHfi values suggest variable, but significant, crustal contamination of the gabbros, mainly by mixing with coeval granite magmas. Inherited Paleoproterozoic zircons of La Solanilla gabbros have similar trace element composition (e.g. Th/U ratios), but more evolved Hf-isotope signatures than associated Variscan zircons. Similar inherited ages have been recorded in zircons from coeval Variscan granitoids from the Central Iberian Zone. Granitic rocks have Nd model ages (TDM) predominantly in the range of 1.4 to 1.6 Ga, suggesting a juvenile addition during the Proterozoic. However, Hf crustal model ages of xenocrystic Proterozoic zircons in La Solanilla gabbro indicate the presence of reworked Archean protoliths (TDM2 model ages of 3.0 to 3.2 Ga) incorporated into the hybridized mafic magma.

  11. Petrogenesis of highly fractionated I-type granites in the Zayu area of eastern Gangdese, Tibet: Constraints from zircon U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopes


    The Cretaceous granitoids in the middle and northern Gangdese, Tibet are generally interpreted as the products of anatexis of thickened deep crust genetically associated with the Lhasa-Qiangtang collision. This paper reports bulk-rock major element, trace element and Sr-Nd isotopic data, zircon U-Pb age data, and zircon Hf isotopic data on the Zayu pluton in eastern Gangdese, Tibet. These data shed new light on the petrogenesis of the pluton. Our SHRIMP zircon U-Pb age dates, along with LA-ICPMS zircon U-Pb age dates recently reported in the literature, indicate that the Zayu pluton was emplaced at about 130 Ma, coeval with Early Cretaceous magmatic rocks in other areas of eastern Gangdese (e.g., Rawu, Baxoi areas) and the Middle Gangdese. The Zayu pluton samples lack amphibole and muscovite, and are compositionally characterized by high SiO2 (69.9%―76.8%), K2O (4.4%―5.7%), and low P2O5 (0.05%―0.12%). These samples also have A/CNK values of 1.00-1.05, and are enriched in Rb, Th, U, and Pb, and depleted in Ba, Nb, Ta, Sr, P, Ti, and Eu. These geochemical features suggest that the Zayu pluton samples are metaluminous to slightly peraluminous and are of highly fractionated I-type granite. The Zayu pluton samples have high εNd(t) values (-10.9--7.6) and low initial 87Sr/86Sr ratios (0.7120- 0.7179) relative to melts derived from mature continental crust in the Gangdese (e.g., Ningzhong Early Jurassic strongly peraluminous granite). The Zayu pluton samples are heterogeneous in zircon εHf(t) values (-12.8--2.9), yielding ancient zircon Hf crustal model ages of 1.4―2.0 Ga. The data obtained in this study together with the data in the recent literature suggest that the Early Cretaceous granitoids in eastern Gangdese represent the eastward extension of the Early Cretaceous magmatism in the middle Gangdese, and that the Lhasa micro-continent block with ancient basement may extend for ~2000 km from east to west. Zircon Hf isotopic data and bulk-rock zircon

  12. Petrogenesis of highly fractionated Ⅰ-type granites in the Zayu area of eastern Gangdese,Tibet:Constraints from zircon U-Pb geochronology,geochemistry and Sr-Nd-Hf isotopes

    ZHU DiCheng; MO XuanXue; WANG LiQuan; ZHAO ZhiDan; NIU Yaoling; ZHOU ChangYong; YANG YueHeng


    The Cretaceous granitoids in the middle and northern Gangdese,Tibet are generally interpreted as the products of anatexis of thickened deep crust genetically associated with the Lhasa-Qiangtang collision.This paper reports bulk-rock major element,trace element and Sr-Nd isotopic data,zircon U-Pb age data,and zircon Hf isotopic data on the Zayu pluton in eastern Gangdese,Tibet.These data shed new light on the petrogenesis of the pluton.Our SHRIMP zircon U-Pb age dates,along with LA-ICPMS zircon U-Pb age dates recently reported in the literature,indicate that the Zayu pluton was emplaced at about 130 Ma,coeval with Early Cretaceous magmatic rocks in other areas of eastern Gangdese (e.g.,Rawu,Baxoi areas) and the Middle Gangdese.The Zayu pluton samples lack amphibole and muscovite,and are compositionally characterized by high SiO2 (69.9%-76.8%),K20 (4.4%-5.7%),and low P2O5 (0.05%-0.12%).These samples also have A/CNK values of 1.00-1.05,and are enriched in Rb,Th,U,and Pb,and depleted in Ba,Nb,Ta,Sr,P,Ti,and Eu.These geochemical features suggest that the Zayu pluton samples ere metaluminous to slightly peraluminous and are of highly fractionated Ⅰ-type granite.The Zayu pluton samples have high εNd(t) values (-10.9-7.6) and low initial 87Sr/86Sr ratios (0.7120-0.7179) relative to melts derived from mature continental crust in the Gangdese (e.g.,Ningzhong Early Jurassic strongly peraluminous granite).The Zayu pluton samples are heterogeneous in zircon εHf(t) values (-12.8-2.9),yielding ancient zircon Hf crustal model ages of 1.4-2.0 Ga.The data obtained in this study together with the data in the recent literature suggest that the Early Cretaceous granitoids in eastern Gangdese represent the eastward extension of the Early Cretaceous magmatism in the middle Gangdese,and that the Lhasa micro-continent block with ancient basement may extend for ~2000 km from east to west.Zircon Hf isotopic data and bulk-rock zircon saturation temperature (789-821℃) indicate that

  13. Zircon U-Pb Geochronology, Hf Isotopic Composition and Geological Implications of the Neoproterozoic Huashan Group in the Jingshan Area, Northern Yangtze Block, China

    Yang, Z.; Yang, K.


    In the northern Yangtze Block, a clear angular unconformity between the Mesoproterozoic sequences (e.g. Dagushi Group) and the overlying Neoproterozoic strata (e.g. Huashan Group) marks the the Jinning orogeny. A combined study of Lu-Hf isotopes and U-Pb ages for detrital zircons from Huashan Group can provide information on the crustal evolution of sedimentary provenances and the timing of the Jinning orogeny. Detrital zircons from Huashan Group have two major U-Pb age populations of about 2.0Ga, 2.65Ga, and three subordinate age groups of about 0.82Ga, 2.5Ga, 2.9Ga with minor >3.0Ga ages. The youngest five analyses yield a weighted average age of 816±9Ma, which is consistent with that of interlayered basalt (824±9Ma, Deng et al., 2013) and roughly defines the minimum depositional age of Huashan Group. Detrital zircons of Huashan Group mostly have two stage Hf isotope model ages (TDM2) between 3.0 to 3.3Ga, indicating that the northern Yangtze Block experienced significant continental crustal growth during the Paleo- to Meso-archean. Similar U-Pb ages of detrital zircons have been obtained from Precambrian sedimentary rocks in the northern Yangtze Block from previous studies (Liu et al., 2008; Guo et al., 2014 and references therein). Recently, ca. 2.65Ga A-type granites had been reported from the Kongling and Huji area, which likely record the thermally stable lithosphere (Chen et al., 2013; Zhou et al., 2015). In combination with this study, it documents the widespread 2.6-2.7Ga magmatic rocks in the northern Yangtze Block. Zhao et al. (2013) demonstrated both the ca. 850Ma tonalite and trondhjemite of the Huangling igneous complex were formed in a continental arc setting. This suggests the Miaowan-Huashan oceanic basin proposed by Bader et al. (2013) has not been closed at ca. 850Ma. This evidence, together with the depositional age of the Huashan Group, indicates the Jinning orogeny took place at 850-820 Ma. [1] Bader et al., 2013 Tectonics [2] Deng et al

  14. Testing alternative tectonic models of Palaeotethys in the E Mediterranean region: new U-Pb and Lu-Hf isotopic analyses of detrital zircons from Late Carboniferous and Late Triassic sandstones associated with the Anatolide and Tauride blocks (S Turkey)

    Ustaömer, Timur; Ayda Ustaömer, Petek; Robertson, Alastair; Gerdes, Axel


    Alternative tectonic models of Palaeotethys during Late Palaeozoic-Early Mesozoic time infer: 1. southward subduction beneath the north margin of Gondwana; 2. northward subduction beneath the south margin of Eurasia, or 3. double subduction (northwards and southwards), at least during Late Carboniferous. U-Pb and Lu-Hf isotopic analysis of detrital zircons, extracted from sandstones, can provide strong indications of age and identity of source terranes. Here, we consider the provenance of both Late Carboniferous and Late Triassic sandstones from both relatively allochthonous and relatively autochthonous units that are all spatially associated with the Anatolide and Tauride continental blocks. The relatively allochthonous units are sandstones (3 samples) from the Late Carboniferous Aladaǧ Nappe (Tauride; in the east), the Konya Complex (Anatolide; central area) and the Karaburun Mélange (Tauride-related; in the west). The relatively autochthonous units are Late Triassic sandstones (4 samples) from the Üzümdere Formation, the Kasımlar Formation (both western Taurides) and the Güvercinlik Formation (Karaburun Peninsula-Tauride related; far west). The Late Carboniferous sandstones from the three relatively allochthonous units are dominated by Precambrian zircon populations, the age distribution of which suggests derivation from two contrasting source regions: First, a NE African-type source (i.e. Saharan craton) for the sandstones of the Konya Mélange and the Aladaǧ Nappe because these sediments have prominent zircon populations dated at 0.5-0.7, 0.8 and 0.9-1.1 Ga. Palaeozoic zircons are minimal in the sandstones of the Aladaǧ Nappe and the Konya Complex (3 and 5% of the whole data, respectively) and are confined to Cambrian to Ordovician. Secondly, a contrasting NW African-type source is inferred for sandstone from the Karaburun Mélange because of the marked absence of Tonian-Stenian zircons and the predominance of ~2 Ga zircons over ~2.5 Ga zircons. In

  15. Zircon U-Pb ages, Hf isotope data, and tectonic implications of Early-Middle Triassic granitoids in the Ailaoshan high-grade metamorphic belt of Southeast Tibet

    Wu, Wenbin; Liu, Junlai; Chen, Xiaoyu; Zhang, Lisheng


    The Ailaoshan tectonic belt, where the effects of the Paleo-Tethyan ocean evolution and Indian-Eurasian plate collision are superimposed, is one of the most significant geological discontinuities in western Yunnan province of southeast Tibet. An Ailaoshan micro-block within the belt is bounded by the Ailaoshan suture zone to the west and the Red River Fault to the east, and consists of low- and high-grade metamorphic belts. Late Permian-Middle Triassic granitoids that are widely distributed to the west of the Ailaoshan suture zone and within the Ailaoshan micro-block may yield significant information on the Tethyan tectonic evolution of the Ailaoshan tectonic belt. This study reports new LA-ICP-MS zircon U-Pb geochronology and Hf isotope data of four granitoids from the Ailaoshan high-grade metamorphic belt. Zircon grains from the Yinjie granitoid do not have inherited cores and yield a weighted mean U-Pb age of 247.1 ± 2.0 Ma. The zircon ɛ Hf( t) values range from 7.8 to 12.1, and Hf model ages from 775 to 546 Ma, indicating that the granitoid was derived from juvenile crust. The rims of zircons from the Majie and Yuanjiang granitoids yield weighted mean U-Pb ages of 239.5 ± 1.8 and 237.9 ± 2.6 Ma, respectively, whereas the cores yield ages of 1608-352 Ma. The ɛ Hf( t) values of zircon rims range from -20.4 to -5.3, yielding Hf model ages from 2557 to 1606 Ma and suggesting that the source magma of the Majie and Yuanjiang granitoids was derived from ancient crust. An additional granitoid located near the Majie Village yields a zircon U-Pb age of 241.2 ± 1.0 Ma. Based on our geochronological and geochemical data, combined with geological observations, we propose that the Ailaoshan micro-block was derived from the western margin of the Yangtze block, and is comparable to the Zhongzan and Nam Co micro-blocks. The presence of late Permian mafic rocks with rift-related geochemical characteristics within the Ailaoshan micro-block, together with granitoids derived

  16. Zircon U-Pb ages, Hf isotope data, and tectonic implications of Early-Middle Triassic granitoids in the Ailaoshan high-grade metamorphic belt of Southeast Tibet

    Wu, Wenbin; Liu, Junlai; Chen, Xiaoyu; Zhang, Lisheng


    The Ailaoshan tectonic belt, where the effects of the Paleo-Tethyan ocean evolution and Indian-Eurasian plate collision are superimposed, is one of the most significant geological discontinuities in western Yunnan province of southeast Tibet. An Ailaoshan micro-block within the belt is bounded by the Ailaoshan suture zone to the west and the Red River Fault to the east, and consists of low- and high-grade metamorphic belts. Late Permian-Middle Triassic granitoids that are widely distributed to the west of the Ailaoshan suture zone and within the Ailaoshan micro-block may yield significant information on the Tethyan tectonic evolution of the Ailaoshan tectonic belt. This study reports new LA-ICP-MS zircon U-Pb geochronology and Hf isotope data of four granitoids from the Ailaoshan high-grade metamorphic belt. Zircon grains from the Yinjie granitoid do not have inherited cores and yield a weighted mean U-Pb age of 247.1 ± 2.0 Ma. The zircon ɛ Hf(t) values range from 7.8 to 12.1, and Hf model ages from 775 to 546 Ma, indicating that the granitoid was derived from juvenile crust. The rims of zircons from the Majie and Yuanjiang granitoids yield weighted mean U-Pb ages of 239.5 ± 1.8 and 237.9 ± 2.6 Ma, respectively, whereas the cores yield ages of 1608-352 Ma. The ɛ Hf(t) values of zircon rims range from -20.4 to -5.3, yielding Hf model ages from 2557 to 1606 Ma and suggesting that the source magma of the Majie and Yuanjiang granitoids was derived from ancient crust. An additional granitoid located near the Majie Village yields a zircon U-Pb age of 241.2 ± 1.0 Ma. Based on our geochronological and geochemical data, combined with geological observations, we propose that the Ailaoshan micro-block was derived from the western margin of the Yangtze block, and is comparable to the Zhongzan and Nam Co micro-blocks. The presence of late Permian mafic rocks with rift-related geochemical characteristics within the Ailaoshan micro-block, together with granitoids derived

  17. Orogenesis and Basin Development: U-Pb Detrital Zircon Age Constraints on Evolution of the Late Paleozoic St. Marys Basin, Central Mainland Nova Scotia.

    Murphy; Hamilton


    The St. Marys Basin, along the southern flank of the composite Late Paleozoic Magdalen Basin in the Canadian Appalachians and along the Avalon-Meguma terrane boundary, contains Late Devonian-Early Carboniferous continental clastic rocks of the Horton Group that were deposited in fluvial and lacustrine environments after the peak of the Acadian orogeny. SHRIMP II (Geological Survey of Canada) data on approximately 100 detrital zircons from three samples of Horton Group rocks from the St. Marys Basin show that most of the zircons have been involved in a multistage history, recycled from clastic rocks in the adjacent Meguma and Avalonian terranes. Although there is a minor contribution from Early Silurian (411 Ma) and Late Devonian suites (ca. 380-370 Ma), Neoproterozoic (ca. 700-550 Ma) and Paleoproterozoic (ca. 2.0-2.2 Ga) zircon populations predominate, with a minor contribution from ca. 1.0-, 1.2-, and 1.8-Ga zircons. Published U-Pb single-zircon analyses on clastic sedimentary rocks indicate that the Meguma and Avalon terranes have different populations of detrital zircons, sourced from discrete portions (Amazonian and West African cratons) of the ancient Gondwanan margin. Both terranes contain Neoproterozoic and Late Archean populations. The SHRIMP data, in conjunction with published sedimentological and geochemical data, indicate that the Horton Group basin-fill sediments are largely the result of rapid uplift and erosion of Meguma terrane metasedimentary and granitoid rocks immediately to the south of the St. Marys Basin during the waning stages of the Acadian orogeny. Regional syntheses indicate that this uplift occurred before and during deposition and was a consequence of dextral ramping of the Meguma terrane over the Avalon terrane along the southern flank of the Magdalen Basin.

  18. Simultaneous in-situ determination of U-Pb age and trace elements in zircon by LA-ICP-MS in 20 μm spot size

    LIU XiaoMing; GAO Shan; DIWU ChunRong; YUAN HongLin; HU ZhaoChu


    Simultaneous in-situ determination of U-Pb agea and 20 trace elements of three international zircon standards (91500, GJ1 and TEMORA 1) and one laboratory zircon standard (SK10-2) separated from Cenozoic fine-grained gabbro was carried out on quadrupole ICP-MS equipped with 193 nm excimer laser in 20 μm spot size. The weighted mean 208Pb/238U ages of g1500, GJ1 and TEMORA 1 are 1064.4±4.8 Ma (2σ), 603.2±2.4 Ma (2σ) and 418.2±2.4 Ma (2σ), respectively. The relative standard deviations (RSDs) of 206Pb/238U ages (2σ) are less than 2.2% for single measurements and 0.6% for weighted means. The obtained weighted mean 206Pb/238U ages of three atandard zircons agree with the recommended values within 2σσerror. The weighted mean 206Pb/238U age of SK10-2 is 31.42±0.25 Ma (2σ)and the RSDs of 206Pb/238U ages (2σ) are between 2.4% and 5.7% for single measurements and less than 0.8% for weighted mean. The obtained weighted mean 206Pb/238U age is in good agreement with the age obtained by Yuan (2004). Trace element concentrations of NIST612 and NIST614 obtained under the same LA-ICP-MS operating conditions agree with the recommended values within analytical error. The results indicate that it is possible to measure U-Pb age and trace elements aimultaneously by LA-ICP-MS in a small spot size of 20 μm.

  19. SHRIMP zircon U-Pb dating and geochemical characteristics of Shihama granite pluton in northeastern Hunan province%湘东北地区石蛤蟆花岗岩体SHRIMP U—Pb年龄及地球化学特征

    姚宇军; 马铁球; 周柯军; 柏道远


    分布于湖南东北部的石蛤蟆岩体侵位于新元古代地层中。由微细粒斑状黑云母花岗闪长岩和细粒斑状黑云母二长花岗岩等两期侵入体组成。通过锆石SHRIM PU--Pb法测得岩体侵位年龄为157土2Ma(2d),MSWD=0.98,成岩时代为晚侏罗世。SiO2=68.26%~68.53%,K2O/Na2O=1.37~1.59,岩石属镁质、准铝质-微过铝质、高钾钙碱性-钾玄岩系列;岩石明显富集大离子亲石元素,亏损高场强元素,Rb/Sr较低(0.40~0.56);乏REE较高(171.48~183.81),Eu为弱负异常(δEu=0.86~0.93),(La/Yb)N=27.11~45.87;具较高的eNd值(-5.11)和高T2DM(1.63Ga)。综合研究表明,石蛤蟆花岗岩为混合源高钾钙碱性花岗岩类(KCG),其花岗岩浆有大量幔源物质加入。讨论认为岩体形成于构造体制转换下的地球动力学背景,是造山晚期张弛作用下的产物。%Distributed in northeastern Hunan province and intruded in Neoproterozoic strata, Shihama granite pluton is mainly composed of micro-fine-grained porphyritic biotite granodiorites of early stage and fine- grained porphyritic biotite monzonitic granites of late stage. SHRIMP zircon dating of the granites gives an age of 157±2 Ma which belongs to Late Jurassic. The granites belong to magnesian, metaluminous-weak- ly peraluminous, high-K calc-alkaline-shoshonite series, with SIO2=68.26% -68.53M and K2O/Na2O= 1.37 - 1.59. The rocks show enriched large-ion lithophile elements, depleted high field-strength ele- ments, low values of Rb/Sr(0.40-0.56), high values of ∑REE (171.48-183.81), with δEu=0.86-- 0. 93, (La/Yb)N=27. 11-45.87, εNd=5.11, T2DM=1. 63 Ga. Studies indicate that the Shihama gran- ites belong to high-K calc-alkaline granites (KCG) originated form the crust-mantled mixed source ivolving large amount of mantle source materials. It is believed that the granite pluton is the result of

  20. U Pb dating of the end of the Pan-African orogeny in the Tuareg shield: the post-collisional syn-shear Tioueine pluton (Western Hoggar, Algeria)

    Paquette, J. L.; Caby, R.; Djouadi, M. T.; Bouchez, J. L.


    The Tioueine pluton intrudes the Neoproterozoic series of the Iskel terrane, located in the Tuareg shield, western Hoggar. The consistency of the internal structures as well as the nature and organization of the associated microstructures demonstrate that the Tioueine pluton was emplaced syn-kinematically while N-S strike-slip shear zones were active. The syn-tectonic emplacement of the Tioueine massif implies that this pluton, although belatedly crystallized, entirely belongs to the concept of post-collisional magmatism. In order to date precisely the late Pan-African tectono-metamorphic event in the studied area, an U-Pb age of 523±1 Ma was obtained from abraded zircons of a late quartz-syenite from the Tioueine pluton. This early Cambrian age is younger than the other plutons of the Tuareg shield, which were mainly emplaced between 630 Ma and 580 Ma. This dating also shows that the Tuareg shield was not a single coherent block at 525 Ma, but rather an amalgam of active terranes moving each other along major shear zones. Finally, the Tioueine massif represents probably the final welding of the Tuareg shield assembly of terranes and consequently the end of the post-collisional orogenic episode in the whole Pan-African belt.

  1. Eburnean and Pan-African granitoids and the Raghane mega-shear zone evolution: Image analysis, U-Pb zircon age and AMS study in the Arokam Ténéré (Tuareg shield, Algeria)

    Nouar, O.; Henry, B.; Liégeois, J. P.; Derder, M. E. M.; Bayou, B.; Bruguier, O.; Ouabadi, A.; Amenna, M.; Hemmi, A.; Ayache, M.


    In the Arokam Ténéré, the three studied massifs of granitoids are located around the N-S oriented Raghane mega-shear zone, which separates two of the main domains of the Tuareg shield, the Saharan metacraton and the Central Hoggar. The field observations, AMS analyses and U-Pb zircon dating were completed by a study of Landsat images because of the scarcity of outcrops in several parts of the studied area. The image analysis allows to distinguish Arokam-East and Abdou granitoids in the eastern plutonic complex. It has also shown that the western plutonic complex corresponds to two different intrusions: the Yvonne granite and granodiorite. This is confirmed by the magnetic fabric that presents different characteristics in the different granitoids. U-Pb zircon dating and field observations show that the Arokam-West basement granite is much older (1915 Ma) than the Hanane granodiorite (582 Ma) in the central plutonic complex. Arokam-East and -West granites then belong to the Eburnean basement. The magnetic fabric of these granites is mainly associated with a post-magmatic deformation, probably of Pan-African age. Yvonne granodiorite is likely contemporaneous of the main displacement along the Raghane shear zone. The Yvonne granite (594 Ma) and Hanane granodiorite have a fabric similar to that previously obtained in most plutons of the Tiririne area. This fabric is related to the regional stress field, associated with the activity of the Raghane shear zone, during the late-magmatic phase in the plutons. On the contrary, the magnetic fabric of the Abdou pluton is still reflecting only the magma flow.

  2. 西藏嘎拉勒夕卡岩型金(铜)矿床地球化学特征与锆石的LA-ICP-MS定年及意义%Geochemical characteristics and zircon LA-ICP-MS U-Pb dating of Galale skarn gold(copper) deposit,Tibet and its significance

    吕立娜; 崔玉斌; 宋亮; 赵元艺; 曲晓明; 王江朋


    嘎拉勒金(铜)矿床位于西藏班公湖—怒江缝合带的西段,为夕卡岩型金(大型)铜(小型)矿床。矿区主要岩石为巨斑状石英闪长岩、中细粒花岗闪长岩与闪长玢岩。该矿床岩石样品的REE分异程度较高,Eu呈弱负异常,w(Al2O3)为14.45%~15.84%,铝过饱和指数A/CNK为1.33~1.46,均大于1,表明为S型花岗岩。经微量元素的相关分析,该矿岩体为板内花岗岩,形成期接近于同碰撞期。锆石的LA-ICP-MS定年结果表明巨斑状石英闪长岩的年龄为(131±17)Ma,中细粒花岗闪长岩为(86.52±0.41)Ma,两个闪长玢岩的年龄分别为(88.59±0.45)和(88.09±0.41)Ma,其中巨斑状石英闪长岩为非成矿岩体,其测年数据代表了成岩时代下限;矿体只产在中细粒花岗闪长岩和闪长玢岩岩体与灰岩和粉砂岩夹硅质岩的接触带,为燕山期成矿。由测年结果可将该矿床的岩浆演化过程大致分为两个阶段,早白垩世非成矿岩体巨斑状石英闪长岩岩体的侵入和晚白垩世成矿岩体的形成,发生在白垩纪班公湖—怒江中特提斯洋构造演化的第4个阶段弧-陆碰撞造山阶段。%The Galale gold(copper) deposit is located in the west section of Bangong lake-Nujiang suture zone in Tibet.It is a gold(of large scale) and copper(of small scale) skarn deposit.There are mainly huge porphyritic quartz diorite,middle-fine grain granodiorite and diorite porphyrite in the mine area.The REE differential degree of rock sample in this ore deposit has a relatively higher value,and Eu negative anomaly is weak.The content of Al2O3 is 14.45%-15.84%.The aluminum supersaturation index(A/CNK) ranges between 1.33 and 1.46,which is greater than 1,indicating its being S-type granite.According to the analysis of trace elements,the magmatic body belongs to MPG.The forming stage is close to the collision stage.The result of zircon LA-ICP-MS dating indicates that the age of

  3. Integrated U-Pb zircon and palynological/palaeofloristic age determinations of a Bashkirian palaeofjord fill, Quebrada Grande (Western Argentina)

    Valdez Buso, Victoria; di Pasquo, Mercedes; Milana, Juan Pablo; Kneller, Benjamin; Fallgatter, Claus; Junior, Farid Chemale; Gomes Paim, Paulo Sérgio


    This work presents a new age framework for the main Bashkirian glacio-eustatic transgression in Argentina, including the first absolute age for the Jejenes Formation, San Juan Province, based on radiometric dating of a crystal-rich tuff, supported by palynological and palaeofloristic studies, and presented within a revised palaeogeographic setting. The Jejenes Formation represents the glacial to postglacial fill of the Quebrada Grande palaeofjord carved in the Eastern Precordillera. The succession has been subdivided into five stages, the youngest of which suggests a previously unrecognised glacial event for this locality. Six productive levels for palynology were found within proglacial strata, and in the base and top of the succeeding interglacial stage. Palynoassemblages are characterized by poorly preserved trilete spores and monosaccate pollen grains along with a large amount of terrestrial phytoclasts. Main species indicating the Raistrickia densa-Convolutispora muriornata SubZone (DMa SZ) are Vallatisporites ciliaris, Cristatisporites rollerii, C. stellatus, C. chacoparanensis, C. inconstans and monosaccates such as Circumplicatipollis plicatus. This DMa SZ is estimated as Serpukhovian/Bashkirian and characterizes the glacial-related Guandacol Formation and equivalents units of the western Paganzo Basin. A tuffaceous level in the proglacial unit, bearing platyspermic seeds, plant remains and palynomorphs, yielded first-cycle volcanic zircons that were analysed by SHRIMP. An absolute age of 321.3 ± 5.3 Ma confirms a Bashkirian age for the main postglacial transgression in the Paganzo Basin, and offers a novel calibration for the palynoassemblages of DMa SZ that occurs elsewhere in Western Argentina.

  4. Combined U Pb and Hf isotope LA-(MC-)ICP-MS analyses of detrital zircons: Comparison with SHRIMP and new constraints for the provenance and age of an Armorican metasediment in Central Germany

    Gerdes, Axel; Zeh, Armin


    Uranium-lead ages obtained by LA-ICP-MS analyses of zircon cores from a high-grade Armorican metasediment from the Mid-German Crystalline Rise, Central Germany, yield results which are identical to, but more precise than those previously obtained by SHRIMP dating. This is mainly due to the fact that SHRIMP analyses are more sensitive than LA-ICP-MS analyses to common Pb contamination on the surface of the grain mount. The new U-Pb ages, in combination with in-situ Hf isotope analyses of zircon, provide the first evidence that detrital zircons within Armorican sediments crystallized in both juvenile and evolved magmatic rocks during the Archaean at 2.7-2.9 Ga, the Palaeoproterozoic at 1.8-2.1 Ga, and the Neoproterozoic/Early Palaeozoic at 500-720 Ma. In addition, zircons were formed at ca. 1.0 Ga by remelting of Palaeoproterozoic crust during the Grenville orogeny. The U-Pb dataset shows an age gap between 1.8 and 1.0 Ga, which is characteristic of Armorican sediments, and indicates that the metasediment protolith is younger than Late Cambrian. In addition, the data support previous conclusions that sediments constituting the Armorican terrane assemblage were derived from three crustal sources. Dominant sources were the Avalonian-Cadomian belt (ca. 45%), situated at the northern margin of Gondwana during the Neoproterozoic, and the West-African and/or eastern Amazonian cratons (ca. 50%). The Grenville belt was a minor source (< 5%). Variation of ɛHf( t) values of the Neoproterozoic/Early Paleozoic zircons indicates two periods of increased juvenile magma formation, one at 595-575 Ma and a second at 515-500 Ma. The older event is coeval with the formation of the Avalonian-Cadomian magmatic arc, whereas the younger event can be related to the break-up of the northern Gondwana margin in Cambrian/Ordovician times. In between, at around 545 Ma, only recycling of older crustal material took place.

  5. U-Pb zircon, geochemical and Sr-Nd-Hf-O isotopic constraints on age and origin of the ore-bearing intrusions from the Nurkazgan porphyry Cu-Au deposit in Kazakhstan

    Shen, Ping; Pan, Hongdi; Seitmuratova, Eleonora; Jakupova, Sholpan


    Nurkazgan, located in northeastern Kazakhstan, is a super-large porphyry Cu-Au deposit with 3.9 Mt metal copper and 229 tonnage gold. We report in situ zircon U-Pb age and Hf-O isotope data, whole rock geochemical and Sr-Nd isotopic data for the ore-bearing intrusions from the Nurkazgan deposit. The ore-bearing intrusions include the granodiorite porphyry, quartz diorite porphyry, quartz diorite, and diorite. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating indicates that the granodiorite porphyry and quartz diorite porphyry emplaced at 440 ± 3 Ma and 437 ± 3 Ma, respectively. All host rocks have low initial 87Sr/86Sr ratios (0.70338-0.70439), high whole-rock εNd(t) values (+5.9 to +6.3) and very high zircon εHf(t) values (+13.4 to +16.5), young whole-rock Nd and zircon Hf model ages, and consistent and slightly high zircon O values (+5.7 to +6.7), indicating that the ore-bearing magmas derived from the mantle without old continental crust involvement and without marked sediment contamination during magma emplacement. The granodiorite porphyry and quartz diorite porphyry are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) and depleted in high-field strength elements (HFSE), Eu, Ba, Nb, Sr, P and Ti. The diorite and quartz diorite have also LILE and LREE enrichment and HFSE, Nb and Ti depletion, but have not negative Eu, Ba, Sr, and P anomalies. These features suggest that the parental magma of the granodiorite porphyry and quartz diorite porphyry originated from melting of a lithospheric mantle and experienced fractional crystallization, whereas the diorite and quartz diorite has a relatively deeper lithospheric mantle source region and has not experienced strong fractional crystallization. Based on these, together with the coeval ophiolites in the area, we propose that a subduction of the Balkhash-Junggar oceanic plate took place during the Early Silurian and the ore-bearing intrusions and associated Nurkazgan

  6. Jurassic cooling ages in Paleozoic to early Mesozoic granitoids of northeastern Patagonia: 40Ar/39Ar, 40K-40Ar mica and U-Pb zircon evidence

    Martínez Dopico, Carmen I.; Tohver, Eric; López de Luchi, Mónica G.; Wemmer, Klaus; Rapalini, Augusto E.; Cawood, Peter A.


    U-Pb SHRIMP zircon crystallization ages and Ar-Ar and K-Ar mica cooling ages for basement rocks of the Yaminué and Nahuel Niyeu areas in northeastern Patagonia are presented. Granitoids that cover the time span from Ordovician to Early Triassic constitute the main outcrops of the western sector of the Yaminué block. The southern Yaminué Metaigneous Complex comprises highly deformed Ordovician and Permian granitoids crosscut by undeformed leucogranite dikes (U-Pb SHRIMP zircon age of 254 ± 2 Ma). Mica separates from highly deformed granitoids from the southern sector yielded an Ar-Ar muscovite age of 182 ± 3 Ma and a K-Ar biotite age of 186 ± 2 Ma. Moderately to highly deformed Permian to Early Triassic granitoids made up the northern Yaminué Complex. The Late Permian to Early Triassic (U-Pb SHRIMP zircon age of 252 ± 6 Ma) Cabeza de Vaca Granite of the Yaminué block yielded Jurassic mica K-Ar cooling ages (198 ± 2, 191 ± 1, and 190 ± 2 Ma). At the boundary between the Yaminué and Nahuel Niyeu blocks, K-Ar muscovite ages of 188 ± 3 and 193 ± 5 Ma were calculated for the Flores Granite, whereas the Early Permian Navarrete granodiorite, located in the Nahuel Niyeu block, yielded a K-Ar biotite age of 274 ± 4 Ma. The Jurassic thermal history is not regionally uniform. In the supracrustal exposures of the Nahuel Niyeu block, the Early Permian granitoids of its western sector as well as other Permian plutons and Ordovician leucogranites located further east show no evidence of cooling age reset since mica ages suggest cooling in the wake of crystallization of these intrusive rocks. In contrast, deeper crustal levels are inferred for Permian-Early Triassic granitoids in the Yaminué block since cooling ages for these rocks are of Jurassic age (198-182 Ma). Jurassic resetting is contemporaneous with the massive Lower Jurassic Flores Granite, and the Marifil and Chon Aike volcanic provinces. This intraplate deformational pulse that affected northeastern

  7. Protracted, coeval crust and mantle melting during Variscan late-orogenic evolution: U-Pb dating in the eastern French Massif Central

    Laurent, Oscar; Couzinié, Simon; Zeh, Armin; Vanderhaeghe, Olivier; Moyen, Jean-François; Villaros, Arnaud; Gardien, Véronique; Chelle-Michou, Cyril


    The late stages of the Variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt, which makes it akin to "hot" orogens. These processes resulted in the development of large granite-migmatite complexes, the largest of which being the 305-300-Ma-old Velay dome in the eastern French Massif Central (FMC). This area also hosts a wide range of late-Variscan plutonic rocks that can be subdivided into four groups: (i) cordierite-bearing peraluminous granites (CPG); (ii) muscovite-bearing peraluminous granites (MPG); (iii) K-feldspar porphyritic, calc-alkaline granitoids (KCG) and (iv) Mg-K-rich (monzo)diorites and lamprophyres ("vaugnerites"). New results of LA-SF-ICP-MS U-Pb zircon and monazite dating on 33 samples from all groups indicate that both granites and mafic rocks emplaced together over a long period of 40 million years throughout the Carboniferous, as shown by intrusion ages between 337.4 ± 1.0 and 298.9 ± 1.8 Ma for the granitoids, and between 335.7 ± 2.1 and 299.1 ± 1.3 Ma for the vaugnerites. Low zircon saturation temperatures and abundant inherited zircons with predominant late Ediacaran to early Cambrian ages indicate that the CPG and MPG formed through muscovite or biotite dehydration melting of ortho- and paragneisses from the Lower Gneiss Unit. The KCG and vaugnerites contain very few inherited zircons, if any, suggesting higher magma temperatures and consistent with a metasomatized lithospheric mantle source for the vaugnerites. The KCG can be explained by interactions between the CPG/MPG and the vaugnerites, or extensive differentiation of the latter. The new dataset provides clear evidence that the eastern FMC was affected by a long-lived magmatic episode characterized by coeval melting of both crustal and mantle sources. This feature is suggested here to result from a lithospheric-scale thermal anomaly, triggered by the removal of the lithospheric mantle root. The spatial

  8. Protracted, coeval crust and mantle melting during Variscan late-orogenic evolution: U-Pb dating in the eastern French Massif Central

    Laurent, Oscar; Couzinié, Simon; Zeh, Armin; Vanderhaeghe, Olivier; Moyen, Jean-François; Villaros, Arnaud; Gardien, Véronique; Chelle-Michou, Cyril


    The late stages of the Variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt, which makes it akin to "hot" orogens. These processes resulted in the development of large granite-migmatite complexes, the largest of which being the 305-300-Ma-old Velay dome in the eastern French Massif Central (FMC). This area also hosts a wide range of late-Variscan plutonic rocks that can be subdivided into four groups: (i) cordierite-bearing peraluminous granites (CPG); (ii) muscovite-bearing peraluminous granites (MPG); (iii) K-feldspar porphyritic, calc-alkaline granitoids (KCG) and (iv) Mg-K-rich (monzo)diorites and lamprophyres ("vaugnerites"). New results of LA-SF-ICP-MS U-Pb zircon and monazite dating on 33 samples from all groups indicate that both granites and mafic rocks emplaced together over a long period of 40 million years throughout the Carboniferous, as shown by intrusion ages between 337.4 ± 1.0 and 298.9 ± 1.8 Ma for the granitoids, and between 335.7 ± 2.1 and 299.1 ± 1.3 Ma for the vaugnerites. Low zircon saturation temperatures and abundant inherited zircons with predominant late Ediacaran to early Cambrian ages indicate that the CPG and MPG formed through muscovite or biotite dehydration melting of ortho- and paragneisses from the Lower Gneiss Unit. The KCG and vaugnerites contain very few inherited zircons, if any, suggesting higher magma temperatures and consistent with a metasomatized lithospheric mantle source for the vaugnerites. The KCG can be explained by interactions between the CPG/MPG and the vaugnerites, or extensive differentiation of the latter. The new dataset provides clear evidence that the eastern FMC was affected by a long-lived magmatic episode characterized by coeval melting of both crustal and mantle sources. This feature is suggested here to result from a lithospheric-scale thermal anomaly, triggered by the removal of the lithospheric mantle root. The spatial

  9. U-Pb LA-SF-ICP-MS zircon geochronology of the Serbo-Macedonian Massif, Greece: palaeotectonic constraints for Gondwana-derived terranes in the Eastern Mediterranean

    Meinhold, Guido; Kostopoulos, Dimitrios; Frei, Dirk; Himmerkus, Felix; Reischmann, Thomas


    The Pirgadikia Terrane in northern Greece forms tectonic inliers within the Vardar suture zone bordering the Serbo-Macedonian Massif to the southwest. It comprises Cadomian basement rocks of volcanic-arc origin and very mature quartz-rich metasedimentary rocks. U-Pb laser ablation sector-field inductively-coupled plasma mass spectrometry analyses of detrital zircons from the latter reveal a marked input from a Cadomian-Pan-African source with minor contribution from Mesoproterozoic, Palaeoproterozoic and Archaean sources. The metasedimentary rocks are correlated with Ordovician overlap sequences at the northern margin of Gondwana on the basis of their maturity and zircon age spectra. The Pirgadikia Terrane can be best interpreted as a peri-Gondwana terrane of Avalonian origin, which was situated close to the Cadomian terranes in the Late Neoproterozoic-Early Palaeozoic, very much like the Istanbul Terrane. The second unit investigated is the Vertiskos Terrane, which constitutes the major part of the Serbo-Macedonian Massif in Greece. It comprises predominantly igneous rocks of Silurian age and minor metasedimentary rocks of unknown age and provenance. U-Pb analyses of detrital zircons from a garnetiferous mica schist of the Vertiskos Terrane indicate derivation from 550 to 1,150 Ma-old source rocks with a major Cadomian peak. This, combined with minor input of >1,950 Ma-old zircons and the absence of ages between ca. 1.2 and 1.7 Ga suggests a NW Africa source. The protolith age of the garnetiferous mica schist is presumably Early Ordovician. One sample of garnet-bearing biotite gneiss, interpreted as meta-igneous rock, comprises predominantly subhedral zircons of igneous origin with late Middle Ordovician to Silurian ages. We suggest that the rock association of the Vertiskos Terrane is part of an ancient active-margin succession of the Hun superterrane, comparable to successions of the Austro- and Intra-Alpine Terranes. The new data of this study provide evidence

  10. Geochemistry and U-Pb zircon geochronology of the pegmatites in Ede area, southwestern Nigeria: A newly discovered oldest Pan African rock in southwestern Nigeria

    Adetunji, A.; Olarewaju, V. O.; Ocan, O. O.; Ganev, V. Y.; Macheva, L.


    Field and petrographic studies, whole rock geochemistry and in-situ LA-ICP-MS geochemical and isotopic U-Pb measurements on zircons have been performed on granitic pegmatites of Ede area, southwestern Nigeria with a view to characterize them, determining their mineralization potentials, petrogenetic attributes and emplacement age. The pegmatites are hosted by migmatite gneiss complex, biotite-muscovite schist and associated quartzite. The textural and mineralogical characteristics of these pegmatites indicate the occurrence of two main varieties, namely, muscovite pegmatite and garnet pegmatite. Of less importance are inclusions and pods of graphic granite, quartz-microcline aplitic and pegmatitic bodies. At the present level of erosion, the parent igneous rocks of the pegmatites are not exposed. The two dominant pegmatite varieties show slightly different chemical peculiarities but similar peraluminous character. The average K/Rb ratios of 165 and 163, respectively, for muscovite and garnet pegmatites combined with other trace element compositions are indicative of affinity to muscovite class of pegmatite which are generally not promising for rare elements mineralization. However, the unusually high concentration of bismuth in the zircons indicates Bi mineralization in the area which can either be in the pegmatites or host rocks. The Nb/Ta ratios for both muscovite and garnet pegmatites range from 0.7 to 15.2 and 1.0 to 14.8, respectively. These Nb/Ta ratios and Eu anomalies are statistically similar for both pegmatites. These probably indicate the pegmatites crystallized from a common source but separated into crystallization paths that produced different pegmatite varieties through liquid-liquid immiscibity mechanism. In-situ measurements of REE, P, Y, Nb, Hf, Ta, Bi, Th and U of individual zircon grains show the existence of two chemically and texturally different domains which are indicative of alteration that may be due to interface-coupled dissolution

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

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


    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.

  12. Repeated granitoid intrusions during the Neoproterozoic along the western boundary of the Saharan metacraton, Eastern Hoggar, Tuareg shield, Algeria: An AMS and U-Pb zircon age study

    Henry, B.; Liégeois, J. P.; Nouar, O.; Derder, M. E. M.; Bayou, B.; Bruguier, O.; Ouabadi, A.; Belhai, D.; Amenna, M.; Hemmi, A.; Ayache, M.


    The N-S oriented Raghane shear zone (8°30') delineates the western boundary of the Saharan metacraton and is, with the 4°50' shear zone, the most important shear zone in the Tuareg shield. It can be followed on 1000 km in the basement from southern Aïr, Niger to NE Hoggar, Algeria. Large subhorizontal movements have occurred during the Pan-African orogeny and several groups of granitoids intruded during the Neoproterozoic. We report U-Pb zircon datings (laser ICP-MS) showing that three magmatic suites of granitoids emplaced close to the Raghane shear zone at c. 790 Ma, c. 590 and c. 550 Ma. A comprehensive and detailed (158 sites, more than 1000 cores) magnetic fabric study was performed on 8 plutons belonging to the three magmatic suites and distributed on 200 km along the Raghane shear zone. The main minerals in all the target plutons do not show visible preferential magmatic orientation except in narrow shear zones. The AMS study shows that all plutons have a magnetic lineation and foliation compatible with the deformed zones that are zones deformed lately in post-solidus conditions. These structures are related to the nearby mega-shear zones, the Raghane shear zone for most of them. The old c. 793 Ma Touffok granite preserved locally its original structures. The magnetic structures of the c. 593 Ma Ohergehem pluton, intruded in the Aouzegueur terrane, are related to thrust structures generated by the Raghane shear zone while it is not the case of the contemporaneous plutons in the Assodé-Issalane terrane whose structures are only related to the subvertical shear zones. Finally, the c. 550 Ma granite group has magnetic structure related to the N-S oriented Raghane shear zone and its associated NNE-SSW structures when close to them, but NW-SE oriented when further. These NW-SE oriented structures appear to be characteristic of the late Neoproterozoic evolution of the Saharan metacraton and are in relation to the convergence with the Murzuq craton. This

  13. Using zircon U-Pb ages to constrain the provenance and transport of heavy minerals within the northwestern shelf of the South China Sea

    Zhong, Lifeng; Li, Gang; Yan, Wen; Xia, Bin; Feng, Yuexing; Miao, Li; Zhao, Jianxin


    Numerous ore-grade heavy mineral placer deposits occur in the northern South China Sea region. Previous studies on these deposits have focused on the heavy-mineral ore resources themselves, but the provenance and transport pathways of these heavy minerals are poorly constrained. This paper presents U-Pb ages of detrital zircons from sediments within the northwestern shelf of the South China Sea, and uses this new dataset to determine the provenance and transport pathway of the sediments. Zircons in sediments from ten areas of the northwestern shelf exhibit distinct age populations, suggesting that they have multiple provenances. Zircons in sediments from the Pearl River Mouth, Shangchuan Island, and Moyang River Mouth areas all have an obvious peak of Mesozoic ages, indicating that they have similar sediment provenances; i.e., mainly from the Pearl River, and to a lesser degree from the Moyang River. Zircons in sediments from the areas around the Jian River Mouth and Leizhou Bay, and off Hainan Island have an early Paleozoic age, suggesting that the sediments predominantly originate from the Yunkai massif. Zircons of the sediments from the remaining four areas, the Leiqiong Strait, Wanquan River Mouth, Qiongdongnan, and the Outer Shelf, have Yanshannian and Indosinian age peaks in addition to an obvious early Paleozoic population, implying mixed provenances, including the Yunkai massif and Hainan Island. The sediment transport may have involved two hydrodynamic conditions in two distinct stages. First, the Guangdong Longshore Current carried the river sediments to where they dispersed in the inner shelf; subsequently, wave-induced strong currents further transported sandy sediments southeastward to the outer shelf. In addition to explaining the provenance and transport pathways of heavy minerals within the northwestern shelf of the South China Sea, these results provide new information relevant to exploration for heavy-mineral placer deposits.

  14. Zircon U-Pb age and Hf isotope of Quanyishang A-type granite in Yichang: Signification for the Yangtze continental cratonization in Paleoproterozoic

    XIONG Qing; ZHENG JianPing; YU ChunMei; SU YuPing; TANG HuaYun; ZHANG ZhiHai


    Zircon U-Pb age and Hf isotope, and major and trace element compositions were reported for granite at Quanyishang, which intruded into the Kongling complex in Yichang, Hubei Province. The results show that the Quanyishang granite is rich in silicon and alkalis but poor in calcium and magnesium, and displays enrichment in Ga, Y, Zr, Nb but depletion in Sr and Ba, exhibiting the post-orogenic A-type affinity. 90% zircons from the granite are concordant, and give a middle Paleoproterozoic magmatic crystallization age (mean 1854 Ma). Initial Hf isotope ratios (176Hf/177Hf)1 of the middle Paleoproterozoic zircons range from 0.280863 to 0.281134 and they have negative εHf(t) values with a minimum of -26.3. These zircons give the depleted mantle model ages (TDM) of 2.9-3.3 Ga (mean 3.0 Ga), and the average crustal model ages (Tcrust) of 3.6-4.2 Ga (mean 3.8 Ga)., A Mesoarchean grain with 207pb/206Pb age of 2859 Ma has a slightly high TDM (3.4 Ga) but similar Tcrust (3.8 Ga) to the Paleoproterozoic zircons. All these data suggest that the source materials of the Quanyishang A-type granite are unusually old, at least ≥2.9 Ga (even Eoarchean). The event of crustal remelting, which resulted in the formation of the Quanyishang granite in the middle Paleoproterozoic, recorded the cratonization of the Yangtze conti-nent. The process may have relation to the extension and collapse of the deep crust with Archean ages, in response to the transition stage of the assembly and breakup of the Columbia supercontinent.

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

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


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

  16. LA-ICPMS zircon U-Pb geochronological constraints on the tectonothermal evolution of the Early Paleoproterozoic Dakendaban Group in the Quanji Block, NW China

    WANG QinYan; CHEN NengSong; LI XiaoYan; HAO Shuang; CHEN HaiHong


    The Ouanji Block, situated between the northern margin of the Oaidam Block and the South Qilian orogenic belt in the NE Qinghai-Tibet Plateau, China, is thought to represent a remnant continental crust. In this study, LA-ICPMS U-Pb analyses of detrital zircon grains from two mesosomes in the migmatitic Dakendaban Group yield ages of 2467+281-26 Ma and 2474+66/-52 Ms, respectively. Zircon grains from a leucosome give two distinct ages of 2471+18/-16 Ma and 1924+14/-15 Ms. Zircon from a granitic pegmatite that intruded into the Dakendaban Group yields an age of 2427+44/-38 Ms. These data suggest that the Early Paleoproterozoic Dakendaban Group deposited between ~2.43 to ~2.47 Ga and has been subject to an intrusive event at 2.43 Ga, and regional metamorphism-anatexis at 1.92 Ga. The common lower intercept age of ~0.9 Ga probably records a significant Early Neoproterozoic event in the Quanji Block.

  17. U-Pb-Hf zircon study of two mylonitic granite complexes in the Talas-Fergana fault zone, Kyrgyzstan, and Ar-Ar age of deformations along the fault

    Konopelko, D.; Seltmann, R.; Apayarov, F.; Belousova, E.; Izokh, A.; Lepekhina, E.


    A 2000 km long dextral Talas-Fergana strike-slip fault separates eastern terranes in the Kyrgyz Tien Shan from western terranes. The aim of this study was to constrain an age of dextral shearing in the central part of the fault utilizing Ar-Ar dating of micas. We also carried out a U-Pb-Hf zircon study of two different deformed granitoid complexes in the fault zone from which the micas for Ar dating were separated. Two samples of the oldest deformed Neoproterozoic granitoids in the area of study yielded U-Pb zircon SHRIMP ages 728 ± 11 Ma and 778 ± 11 Ma, characteristic for the Cryogenian Bolshoi Naryn Formation, and zircon grains analyzed for their Lu-Hf isotopic compositions yielded εHf(t) values from -11.43 to -16.73, and their calculated tHfc ages varied from 2.42 to 2.71 Ga. Thus varying Cryogenian ages and noticeable heterogeneity of Meso- to Paleoproterozoic crustal sources was established for mylonitic granites of the Bolshoi Naryn Formation. Two samples of mylonitized pegmatoidal granites of the Kyrgysh Complex yielded identical 206Pb/238U ages of 279 ± 5 Ma corresponding to the main peak of Late-Paleozoic post-collisional magmatism in the Tien Shan (Seltmann et al., 2011), and zircon grains analyzed for their Lu-Hf isotopic compositions yielded εHf(t) values from -11.43 to -16.73, and calculated tHfc ages from 2.42 to 2.71 Ga indicating derivation from a Paleoproterozoic crustal source. Microstructural studies showed that ductile/brittle deformation of pegmatoidal granites of the Kyrgysh Complex occurred at temperatures of 300-400 °C and caused resetting of the K-Ar isotope system of primary muscovite. Deformation of mylonitized granites of the Bolshoi Naryn Formation occurred under high temperature conditions and resulted in protracted growth and recrystallization of micas. The oldest Ar-Ar muscovite age of 241 Ma with a well defined plateau from a pegmatoidal granite of the Kyrgysh Complex is considered as a “minimum” age of dextral motions

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

    Cordani Umberto G.


    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.

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

    Cordani, Umberto G; Nutman, Allen P; Andrade, Antonio S; Santos, José F; Azevedo, Maria do Rosário; Mendes, Maria Helena; Pinto, Manuel S


    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.

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

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


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

  1. Zircon U-Pb geochronology and petrogenesis of metabasites from the western Beihuaiyang zone in the Hong'an orogen, central China: Implications for detachment within subducting continental crust at shallow depths

    Liu, Yi-Can; Liu, Li-Xiang; Li, Yuan; Gu, Xiao-Feng; Song, Biao


    Deformed low-grade metabasites from the western segment of the Beihuaiyang zone in the Hong'an orogen, central China can be divided into two types, i.e. meta-gabbro and meta-basalt. These lithologies have been studied by using whole-rock elemental and Sr-Nd-Pb isotopic analyses, and zircon SHRIMP U-Pb dating. Concordant zircon U-Pb ages of 631 ± 5 Ma and 623 ± 14 Ma are obtained for the meta-gabbros, consistent with a previously reported U-Pb age of 635 ± 5 Ma. The meta-basalt was dated to have a protolith age of middle Neoproterozoic (∼750 Ma) and a metamorphic age at ca. 240 Ma. The all studied metabasites occur as block or slice within a metamorphosed Ordovician volcanic zone (originally named as the Dingyuan Formation) and are in tectonic contact to each other. The gabbro and basalt emplaced at ∼630 Ma and ∼750 Ma, respectively in a continental rifting setting, whereas their present country rocks were erupted at ∼465 Ma in an arc setting. The Pb-isotope compositions of the low-grade meta-gabbros and meta-basalts are similar to those from the Dabie ultrahigh-pressure (UHP) meta-igneous rocks with an upper continental crust affinity. The protolith ages of the studied relatively low-grade meta-basic rocks are in good agreement not only with ages for two episodes of middle and late Neoproterozoic mafic and felsic magmatism in the Suizhou to Zaoyang areas at the northern margin of the South China Block, but are also in agreement with the protolith ages of UHP meta-igneous rocks in the Dabie-Sulu orogenic belt. Therefore, these Neoproterozoic low-grade metabasites are considered to be exotic and they may have been detached and offscraped from the subducting upper crust of the South China Block at shallow depths during continental collision in the Triassic. They were subsequently exhumed in the initial stage of continental subduction, and thrusted over the Paleozoic metamorphosed rocks in the southern margin of the North China Block or as foreign slices

  2. Baddeleyite and zircon U Pb ages from the Kærven area, Kangerlussuaq: Implications for the timing of Paleogene continental breakup in the North Atlantic

    Holm, Paul Martin; Heaman, Larry M.; Pedersen, Lise E.


    We report U-Pb zircon and baddeleyite age results for two mafic units in the Kangerlussuaq area which are part of the Paleogene East Greenland igneous province; the Kangerlussuaq macrodyke, which represents a feeder to some flood basalts, and the nearby Kærven Gabbro. Baddeleyite from the macrodyke yielded a concordant age of 54.7 ± 0.4 (2 σ) Ma and this is among the most precise age determinations from the province. This age is interpreted to reflect the time of emplacement. The macrodyke belongs to a swarm of dykes emplaced perhaps in a failed arm during continental rifting, and the age predates the completion of continental breakup at the end of emplacement of the Plateau Basalts of the East Greenland eruption sequence. Geochemically the macrodyke is correlated with the Milne Land Formation lavas. The magma experienced slight crustal contamination. Based on the age and geochemistry of the macrodyke, the age of the Skaergaard intrusion and Skrænterne Formation lavas, an age span of 1-2 Ma is confirmed for the extrusion of the entire Plateau Basalts. U-Pb analysis of zircon from the Kærven Gabbro yields an age of 53.0 ± 0.3 (2 σ) Ma. The Kærven Gabbro was intruded after extension had ceased in the area and we suggest that the post plateau basalt magmatism did not take place at a distinct later time, but instead that igneous activity continued for some time at a lower rate. Slightly fractionated HREEs indicate that the Kærven Gabbro magma was extracted at relatively shallow levels in the mantle.

  3. Mesoproterozoic island arc magmatism along the south-eastern margin of the Indian Plate: Evidence from geochemistry and zircon U-Pb ages of mafic plutonic complexes

    Subramanyam, K. S. V.; Santosh, M.; Yang, Qiong-Yan; Zhang, Ze-ming; Balaram, V.; Reddy, U. V. B.


    The Prakasam Igneous Province within the Nellore Schist Belt (NSB) preserves important imprints of mafic magmatism along the south-eastern margin of the Indian plate. Here we report petrology, geochemistry and zircon U-Pb age data from three gabbro plutons namely Purimetla, Kanigiri and P C Palle which intruded into the high grade rocks of the region. LA-ICP-MS U-Pb data on zircons from the three plutons reveal prominent late Mesoproterozoic ages of 1334 ± 15 Ma, 1338 ± 27 Ma and 1251.2 ± 9.4 Ma. The cumulative 207Pb/206Pb mean age of 1315 ± 11 Ma is interpreted to represent the timing of mafic magmatism in the Prakasam Igneous Province. These rocks show adcumulus to mesocumulus and poikilitic textures indicating fractional crystallization of plagioclase and clinopyroxenes in the Purimetla pluton whereas the Kanigiri and P C Palle intrusions possess hornblende and biotite suggesting the role of water during partial melting. The rocks show LREE enrichment (∑LREE/∑HREE = 2.2-15.0), marked Eu-anomalies (Eu/Eu∗ = 0.8-2.2) and fractionated patterns (LaN/YbN = 3-79). Primitive mantle normalised trace element spider diagrams indicate subduction modified arc signatures with LILE enrichment and depletion of Nb, Ti and Zr relative to Th and La. Tectonic discrimination diagrams show arc magmatic affinities for the three gabbro plutons consistent with subduction zone setting. We propose a tectonic model involving intra oceanic island arc accretion during late Mesoproterozoic along the eastern margin of the Indian continent.

  4. Coupled zircon Lu-Hf and U-Pb isotopic analyses of the oldest terrestrial crust, the >4.03 Ga Acasta Gneiss Complex

    Bauer, Ann M.; Fisher, Christopher M.; Vervoort, Jeffrey D.; Bowring, Samuel A.


    The Acasta Gneiss Complex of the Northwest Territories, Canada, contains some of the earliest terrestrial continental crust and thus provides a critical sample set for characterization of crust-forming processes on the early Earth. Here we report the results of a combined Lu-Hf and U-Pb isotopic study of zircons from predominantly felsic orthogneisses from the Acasta Gneiss Complex that crystallized between ∼4.0 and 2.9 Ga, many of which contain complex zoning and therefore require an analytical treatment suited to distinguish amongst compositionally distinct age and Hf isotopic domains. To ensure the reliability of the analyses and of subsequent geologic interpretations, we employed the laser ablation split-stream (LASS) technique to concurrently measure the Lu-Hf and U-Pb isotopic systems in zircon. Our results confirm prior findings of precursor Hadean crust (>4.0 Ga) in the source of these rocks and the continued involvement of this reservoir until ∼3.6 Ga. We present evidence for the input of relatively more juvenile material at ∼3.6 Ga, which we suggest corresponds to a fundamental change in the source of the magmas. This study extends the lower bound of the published Acasta Hf isotopic record from 3.6 Ga to 2.9 Ga and demonstrates that the ∼3.6 Ga-2.9 Ga interval is largely represented by reworking of relatively juvenile ∼3.6 Ga crust and the diminution of the >4.0 Ga crustal signal. Significantly, there is no evidence that rocks within the Acasta Gneiss Complex were derived from a strongly depleted mantle.

  5. U-Pb zircon geochronology of ''brasiliano'' granitoids from the Serido orogenic belt (Borborema Province, NE Brazil). Ages U-Pb sur zircon de granitoides ''brasilianos'' de la ceinture du Serido (Province Borborema, NE Bresil)

    Leterrier, J.; Bertrand, J.M. (Centre National de la Recherche Scientifique (CNRS), 54 - Nancy (France). Centre de Recherches Petrographiques et Geochimiques); Pin, C. (Clermont-Ferrand-2 Univ., 63 - Aubiere (France)); Jardim de Sa, E.


    Diorites previously attributed to an early stage of the 'brasiliano' plutonic evolution of the Serido belt yielded U-Pb zircon age of 579 [+-]7 Ma. The age of associated granites is slightly younger although a synchronous emplacement of granites and diorites is suggested by field relationships. Such an age difference may result from difficulties to distinguish 'crystallization age' from 'emplacement age', which is still to be determined precisely. (authors). 16 refs., 3 figs., 1 tab.

  6. Provenance variability along the Early Ordovician north Gondwana margin: Paleogeographic and tectonic implications of U-Pb detrital zircon ages from the Armorican Quartzite of the Iberian Variscan belt

    Shaw, J.; Gutiérrez-Alonso, G.; Johnston, S.T.; Pastor-Galán, D.


    Detrital zircon laser ablation–inductively coupled plasma–mass spectrometry U-Pb age data from the Lower Ordovician Armorican Quartzite (deformed passive margin strata of Gondwanan affinity) of the Iberian Massif are presented herein. The S-shaped coupled Iberian oroclines defined within these zones

  7. Subduction-related Late Carboniferous to Early Permian Magmatism in the Eastern Pontides, the Camlik and Casurluk plutons: Insights from geochemistry, whole-rock Sr-Nd and in situ zircon Lu-Hf isotopes, and U-Pb geochronology

    Karsli, Orhan; Dokuz, Abdurrahman; Kandemir, Raif


    Late Carboniferous to early Permian granitoid rocks represent a volumetrically minor component of the Eastern Pontide lithosphere, but they preserve useful information about the region's tectonomagmatic history. The Casurluk and Camlik plutons primarily consist of gabbro, gabbroic diorite, diorite, monzogabbro, monzodiorite and monzonite, which intrude early to middle Carboniferous granitic basement rocks in the region. In this study, we use in situ zircon U-Pb ages and Lu-Hf isotopic values, whole-rock Sr-Nd isotopic values, and mineral chemistry and geochemistry of these plutons to determine petrogenesis and crustal evolution; we also discuss geodynamic implications. LA-ICP-MS zircon U-Pb dating of magmatic zircons from the rocks suggests that the plutons were emplaced during the late Carboniferous to early Permian (302 Ma). The metaluminous and I-type intrusive rocks belong to the high-K calc-alkaline series. In addition, they are relatively enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs); they are depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs), such as Nb and Ti. All of the samples have homogeneous initial ISr values (0.70675 to 0.70792) and low εNd (t) values (- 5.1 to - 3.3). Zircons from the rocks of both plutons have uniform negative to slightly positive εHf (t) values (- 3.5 to 1.4) and old Hf two-stage model ages (1323 to 1548 Ma), implying that they have the same source, as well as suggesting the involvement of old enriched lithospheric mantle materials during their magma genesis. These results, combined with the εHf (t) values and two-stage model ages, demonstrate that the primary magmas were derived from partial melting of old lithospheric mantle material metasomatized by subduction-related fluids. Considering other regional geological data from the Sakarya Zone where these plutons formed, we conclude that late Carboniferous to early Permian magmatism in the area

  8. African/Amazonian Proterozoic correlations of Iberia: A detrital zircon U-Pb study of early Cambrian conglomerates from the Sierra de la Demanda (northern Spain)

    ÁBalos, B.; Gil Ibarguchi, J. I.; SáNchez-Lorda, M. E.; Paquette, J. L.


    Unfoliated conglomerates define the base of an Early Cambrian transgressive system tract in the Sierra de la Demanda. Correlations allow us to bracket the corresponding sechron between 532 Ma and 520-521 Ma. These conglomerates contain sandstone and metamorphic quartzite pebbles carrying detrital tourmaline, rutile and zircon grains of plutonic or medium- to high-grade metamorphic derivation. Zircon detrital grains exhibit concordant or sub-concordant U/Pb ages clustered in various groups, including Neoarchean (2.52-2.56 Ga), Paleoproterozoic (1.71-2.02 Ga), and Mesoproterozoic (1.47 and ca. 1.1-1.0 Ga), the latter representative of orogenic magmatism related to Rodinia supercontinental assembly. The Neoproterozoic is represented by concordant ages in the range 750-880 Ma and by Cryogenian discordant ages. Ediacaran zircons cluster in two subsets ranging between 590 and 680 Ma and 560-585 Ma, both including several concordant ages. They reflect formation of juvenile crust in magmatic arc and back-arc basin settings. Zircon ages younger than 520-525 Ma postdate the depositional age of the conglomerate and may represent Hercynian overprinting. Bibliographic data overlooked in other provenance studies indicate that Mesoproterozoic relics as those presented here should no longer be considered of exotic origin with respect to a Gondwanan (West African) affinity of the Neoproterozoic to early Paleozoic of Iberia. The proposed source area, the "Ebro Massif" of central-north Iberia, currently is concealed under a kilometer-thick Paleozoic or younger cover. Its tectonic organization would compare to that of the North African or Amazonian cratons (including Mesoproterozoic components), rather than to the Neoproterozoic arc settings described in northwest and southwest Iberia.

  9. Complexity of In-situ zircon U-Pb-Hf isotope systematics during arc magma genesis at the roots of a Cretaceous arc, Fiordland, New Zealand

    Milan, L. A.; Daczko, N. R.; Clarke, G. L.; Allibone, A. H.


    Zircons from seventeen samples of Western Fiordland Orthogneiss (WFO) diorites and three samples of country rock (two schists and one Darran Suite diorite) from the lowermost exposed sections of the Median Batholith, Fiordland, New Zealand, were analysed for in-situ U-Pb and Hf-isotopes. The WFO represents the deeper levels of Early Cretaceous continental arc magmatism on the Pacific margin of Gondwana, marking the final stage of long-lived arc magmatism on the margin spanning the Palaeozoic. The WFO plutons were emplaced at high-P (mid to deep crust at c. 8-12 kbar) between 124 and 114 Ma. Minor very high-P (c. 18 kbar) WFO eclogite and omphacite granulite facies orthogneiss (Breaksea Orthogneiss) are inferred to have crystallised in the base of thickened crust at c. 124 Ma. Zircons from the Breaksea Orthogneiss are considered to be variably affected by Pb-loss due to emplacement of the adjacent (Malaspina) Pluton at c. 114 Ma. By identifying Pb-loss, magmatic ages were able to be inferred in respect to apparent Pb-loss ages. Hf isotope data for the WFO define an excursion to less radiogenic Hf isotope ratios with time, reflecting increased recycling of an old source component. Peaks at c. 555, 770 and 2480 Ma, determine the age spectra of inherited populations of zircons within the WFO. This contrasts with detrital zircon patterns in country rocks of the Takaka terrane, which include peaks at c. 465 Ma, and 1250-900 Ma that are absent in the WFO inheritance pattern. These results indicate a previously unrecognised Precambrian lower crustal component of New Zealand. Recycling of this lower crust became increasingly important as a source for the final stage or Mesozoic arc magmatism along this segment of the palaeo-Pacific margin of Gondwana.

  10. Heavy mineral compositions and zircon U-Pb ages of Cenozoic sandstones in the SW Qaidam basin, northern Tibetan Plateau: Implications for provenance and tectonic setting

    Zhu, Wen; Wu, Chaodong; Wang, Jialin; Zhou, Tianqi; Li, Jijun; Zhang, Changhao; Li, Linlin


    The Qaidam basin is the largest intermountain basin within the Tibetan Plateau. The spatio-temporal evolution of the basin and the provenances of the deposits in the SW Qaidam basin were greatly influenced by the boundary ranges. Comprehensive research is needed to identify the relationships between the basin and the surrounding ranges. Therefore, 4682 heavy mineral data from 70 wells and 720 detrital zircon ages from 8 samples were used to investigate the provenances of the deposits in the SW Qaidam basin and to further constrain the tectonic evolution of the boundary ranges. The heavy mineral data, zircon U-Pb ages, and paleocurrent data indicate that the Qimen Tagh Range and the Altyn Tagh Range were two provenances of the sediments deposited in the SW Qaidam basin during the Cenozoic. The variations of heavy mineral assemblages and ZTR index (100 ∗ (zircon + tourmaline + rutile)/total transparent heavy minerals) on the northern flank of the Qimen Tagh Range suggest that the Qimen Tagh Range experienced rapid uplift in the Early to Middle Miocene. Furthermore, according to the greatly increased quantities of epidote and hornblende sourced from the Altyn Tagh Range and the obvious decrease in the ZTR index values, the Altyn Tagh Range underwent fast uplift in the Late Oligocene to Early Miocene. In addition, based on the ages of the granitic plutons in the Altyn Tagh Range and the zircon ages in the Ganchaigou and Qigequan sections, the strike-slip movement of the Altyn Tagh Fault (ATF) initiated during or no later than the Middle Eocene, resulting in the ATF offsets of ca. 200 km between the Middle Eocene and the Miocene and of ca. 345 km between the Middle Eocene and the present.

  11. LA-ICP-MS U-Pb Zircon Geochronology of Basic Dikes within Maxianshan Rock Group in the Central Qilian Orogenic Belt and Its Tectonic Implications

    He Shiping; Wang Hongliang; Chen Junlu; Xu Xueyi; Zhang Hongfei; Ren Guangming; Yu Jiyuan


    A large number of basic dikes, which indicate an important tectonic-magmatic event in the eastern part of the Central Qilian (祁连) orogenic belt, were found from Maxianshan (马衔山) rock group, Yongjing (永靖) county, Gansu (甘肃) Province, China. According to the research on the characteristics of geology and petrology, the basic dike swarms, widely intruded in Maxianshan rock group,are divided into two phases by the authors. U-Pb isotope of zircons from the basic dikes above two phases is separately determined by LA-ICP-MS in the Key Laboratory of Continental Dynamics of Northwest University, China and the causes of formation of the zircons are studied using CL images.The formation age of the earlier phase of metagabbro dikes is (441.1±1.4) Ma (corresponding to the early stage of Early Silurian), and the age of the main metamorphic period is (414.3±1.2) Ma (corresponding to the early stage of Early Devonian). The formation age of the later phase of diabase dike swarms is (434±1.0) Ma (corresponding to the late stage of Early Silurian). The cap- tured-zircons from diabase dike swarms saved some information of material interfusion by Maxianshan rock group (207pb/206Pb apparent ages are (2325±3)-(2573±6) Ma), and some zircons from diabase dike swarms also saved impacted information by tectonic thermal event during the late period of Caledonian movement (206pb/238U apparent ages are (400±2)-(429±2) Ma). By combining the results of the related studies, the basic dikes within Maxianshan rock group were considered to be formed in the transfer period, from subductional orogeny towards collisional orogeny, which represents geological records of NW-SE extension during regional NE-SW towards intense compression in the Central Qilian block.

  12. U-Pb Ages and Hf Isotope Compositions of Detrital Zircons from the Sandstone in the Early Cretaceous Wawukuang Formation in the Jiaolai Basin, Shandong Province and its Tectonic Implications%胶莱盆地早白垩世瓦屋夼组砂岩中碎屑锆石U-Pb-Hf同位素组成及其构造意义

    霍腾飞; 杨德彬; 许文良; 王枫; 刘海彬; 师江朋


    LA-ICP-MS U-Pb dating and in situ Hf isotope analysis were carried out for the detrital zircons to constrain the depositional age and provenance of the Wawukuang Formation, which is believed as the earliest unit of the Laiyang Group in the Jiaolai Basin, and its implications. Most of these detrital zircons from the feldspar quartz sandstone in the Wawukuang Formation are magmatic in origin, which are euhedral-subhedral and display oscillatory zoning in CL images; whereas few Late Triassic detrital zircons are metamorphic in origin and structureless in CL images. U-Pb isotopic dating of 82 zircon grains yields age populations at ca. 129 Ma, 158 Ma, 224 Ma, 253 Ma, 461 Ma, 724 Ma, 1851 Ma and 2456 Ma. U-Pb dating and Hf isotopic results indicate that:1) the Wawukuang Formation deposited during the Early Cretaceous (129–106 Ma);2) the detrital zircons with the ages of 1851 Ma and 2456 Ma mainly sourced from the Precambrian basement rocks of the North China Craton;the Neoproterozoic (729–721 Ma) magmatic zircons and the Late Triassic (226–216 Ma) metamorphic zircons sourced from the Su-Lu terrane; The Late Paleozoic detrital zircons could source from the Late Paleozoic igneous rocks in the northern margin of the North China Craton;the Late Triassic (231–223 Ma) magmatic zircons and the 158–129 Ma zircons sourced from the coeval igneous rocks in the Jiaobei and Jiaodong;3) the deposition age and provenance of the Jiaolai Basin are different from those of the Hefei Basin; 4) the recognition of clastic sediments from the Su-Lu terrane in the Wawukuang Formation suggests that the Su-Lu terrane was under denudation in the Early Cretaceous.%本文报道了胶莱盆地莱阳群最底部瓦屋夼组长石石英砂岩中碎屑锆石的LA-ICP-MS U-Pb年代学和原位Hf同位素分析结果,进而约束了瓦屋夼组砂岩的沉积时代和物源及其构造意义。碎屑锆石多数呈自形–半自形晶,发育岩浆生长环带,暗示它们为岩浆成因

  13. Monazite U-Th-Pb EPMA and zircon U-Pb SIMS chronological constraints on the tectonic, metamorphic, and thermal events in the inner part of the Variscan orogen, example from the Sioule series, French Massif Central

    Do Couto, Damien; Faure, Michel; Augier, Romain; Cocherie, Alain; Rossi, Philippe; Li, Xian-Hua; Lin, Wei


    In the northern Variscan French Massif Central, the Sioule metamorphic series exposes from top to bottom the tectonic superposition of the Upper Gneiss Unit (UGU), Lower Gneiss Unit (LGU), and Para-autochthonous Unit (PAU). The nappe stacking developed throughout two prograde syn-metamorphic events: D1 is a top-to-the-SW shearing coeval with a probable Devonian migmatization and D2 is a top-to-the-NW shearing event. Both events were completed before the unconformable deposition of the undeformed and unmetamorphosed "Tufs anthracifères" formation, dated at ca 330 Ma (Late Visean). Furthermore, the UGU experienced a high-pressure metamorphism ascribed to a D0 event during which eclogite or granulite crystallized in several parts of the UGU. Monazite U-Th-Pb and zircon U-Pb SIMS datings were carried out in order to constrain the ages of these D0, D1, and D2 tectono-metamorphic events. These new geochronological results are placed in a P-T-t diagram constructed for the UGU, LGU, and PAU. Monazite sampled in UGU, LGU, and PAU rocks yields similar 365-350 Ma ages consistent with the D2 event dated in other places of the French Massif Central. A zoned monazite grain from a granulitic paragneiss yields 416 ± 15 and 362 ± 14 Ma ages interpreted as those of the D0 and D2 events, respectively. Zircon from the same granulitic paragneiss yields SIMS ages at 343 ± 2 and 328 ± 2 Ma that are interpreted as recrystallization processes associated with post-thickening thermal events, possibly recording the onset of orogenic collapse of the Northern Massif Central. It is worth to note that neither monazite nor zircon recorded the D1 event.

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

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


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

  15. Sandstone provenance and U-Pb ages of detrital zircons from Permian-Triassic forearc sediments within the Sukhothai Arc, northern Thailand: Record of volcanic-arc evolution in response to Paleo-Tethys subduction

    Hara, Hidetoshi; Kunii, Miyuki; Miyake, Yoshihiro; Hisada, Ken-ichiro; Kamata, Yoshihito; Ueno, Katsumi; Kon, Yoshiaki; Kurihara, Toshiyuki; Ueda, Hayato; Assavapatchara, San; Treerotchananon, Anuwat; Charoentitirat, Thasinee; Charusiri, Punya


    Provenance analysis and U-Pb dating of detrital zircons in Permian-Triassic forearc sediments from the Sukhothai Arc in northern Thailand clarify the evolution of a missing arc system associated with Paleo-Tethys subduction. The turbidite-dominant formations within the forearc sediments include the Permian Ngao Group (Kiu Lom, Pha Huat, and Huai Thak formations), the Early to earliest Late Triassic Lampang Group (Phra That and Hong Hoi formations), and the Late Triassic Song Group (Pha Daeng and Wang Chin formations). The sandstones are quartzose in the Pha Huat, Huai Thak, and Wang Chin formations, and lithic wacke in the Kiu Lom, Phra That, Hong Hoi and Pha Daeng formations. The quartzose sandstones contain abundant quartz, felsic volcanic and plutonic fragments, whereas the lithic sandstones contain mainly basaltic to felsic volcanic fragments. The youngest single-grain (YSG) zircon U-Pb age generally approximates the depositional age in the study area, but in the case of the limestone-dominant Pha Huat Formation the YSG age is clearly older. On the other hand, the youngest cluster U-Pb age (YC1σ) represents the peak of igneous activity in the source area. Geological evidence, geochemical signatures, and the YC1σ ages of the sandstones have allowed us to reconstruct the Sukhothai arc evolution. The initial Sukhothai Arc (Late Carboniferous-Early Permian) developed as a continental island arc. Subsequently, there was general magmatic quiescence with minor I-type granitic activity during the Middle to early Late Permian. In the latest Permian to early Late Triassic, the Sukhothai Arc developed in tandem with Early to Middle Triassic I-type granitic activity, Middle to Late Triassic volcanism, evolution of an accretionary complex, and an abundant supply of sediments from the volcanic rocks to the trench through a forearc basin. Subsequently, the Sukhothai Arc became quiescent as the Paleo-Tethys closed after the Late Triassic. In addition, parts of sediments of

  16. Geology, mineralization, U-Pb dating and Sr-Nd isotope geochemistry of intrusive bodies in northeast of Kashmar

    Alireza Almasi


    Full Text Available Alireza Almasi1, Mohammad Hassan Karimpour1*, Khosrow Ebrahimi Nasrabadi1, Behnam Rahimi1, Urs KlÖtzli2 and Jose Francisco Santos3 Introduction The study area is located in central part of the Khaf- Kashmar-Bardeskan belt which is volcano-plutonic belt at the north of the Dorouneh fault in the north of Lut block. The north of the Lut block is affected by tectonic rotation and subduction processes which occur in the east of Iran (Tirrul et al., 1983. The magmatism of Lut block begins in Jurassic and continues in Tertiary (Aghanabati, 1995. Karimpour (Karimpour, 2006 pointed out the Khaf-Kashmar-Bardeskan belt has significant potential for IOCG type mineralization such as Kuh-e-Zar, Tannurjeh, and Sangan (Karimpour, 2006; Mazloumi, 2009. The data gathered on the I-type intrusive rocks include their field geology, petrography, U–Pb zircon dating and Sr–Nd isotope and also alteration and mineralization in the study area. Materials and methods - Preparation of 150 thin sections of rock samples for study of petrography and alteration of the intrusive rocks. - Magnetic susceptibility measuring of intrusive rocks. - U-Pb dating in zircon of I-type intrusive rocks by Laser-Ablation Multi Collector ICP-MS method. - Sr-Nd analysis on 5 samples of I-type intrusive rocks by Multi-Collector Thermal Ionization Mass Spectrometer (TIMS VG Sector 54 instrument. - Mineralography and paragenetic studies of ore-bearing quartz veins and geochemical analysis for 28 samples. - Production of the geology, alteration and mineralization maps by scale: 1:20000 in GIS. Results Oblique subduction in southern America initiated an arc-parallel fault and shear zones in the back of continental magmatic arc (Sillitoe, 2003. Because of this event, pull-apart basins were formed and high-K to shoshonitic calc-alkalineI- and A-type magmatism occur (Sillitoe, 2003. Most important deposits accompany with this magmatism are Au-Cu deposits types and Fe-Skarns (Sillitoe, 2003. We have

  17. Cretaceous high-pressure metamorphism and low pressure overprint in the Sistan Suture Zone, eastern Iran: Additional temperature estimates for eclogites, geological significance of U-Pb zircon ages and Rb-Sr constraints on the timing of exhumation

    Kurzawa, Timon; Bröcker, Michael; Fotoohi Rad, Gholamreza; Berndt, Jasper; Lisker, Frank


    The Sistan Suture Zone, eastern Iran, includes blocks and lenses of eclogite-, blueschist- and/or epidote amphibolite-facies rocks that provide an excellent opportunity to examine the exhumation history of oceanic HP/LT rocks and their retrograde derivatives. Zr-in-rutile thermometry of eclogites corroborates previous interpretations suggesting metamorphic temperatures of ca. 550-600 °C during the HP stage in the Sistan area. Flat HREE distribution patterns and Ti-in-zircon temperatures of ca. 500-600 °C document that zircon in eclogite is of metamorphic origin. REE patterns of zircon from felsic meta-igneous rocks do not allow to distinguish between a magmatic or metamorphic origin, but relatively low temperatures indicated by Ti-in-zircon thermometry (ca. 500-600 °C) and the close similarity of zircon (U-Pb) and white mica (Rb-Sr, Ar-Ar) ages favor a metamorphic zircon origin. Previously published isotopic ages of the felsic rocks cannot unambiguously be linked to the eclogite- and/or blueschist-facies P-T conditions due to the absence of unequivocal mineralogical and petrological evidence. Instead, these rocks may record contemporaneous metamorphic processes that took place at a different depth within the subduction complex, or may indicate active ridge subduction and/or melt formation in the subduction zone at relatively low pressures. Biotite-based internal Rb-Sr isochrons of newly dated epidote amphibolite and biotite-albite gneisses indicate ages of ca. 74-80 Ma, either dating fluid-infiltration-induced formation of biotite during relatively fast uplift, or the time of final passage through the effective biotite closure temperature. Rb-Sr ages of phengite from both an epidote amphibolite and a biotite-albite gneiss yield ages that correspond to the HP/LT stage. This outcome, combined with textural evidence for derivation from eclogitic precursors documents that white mica ages of some strongly overprinted Sistan rocks are compromised by inheritance and do

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

    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.


    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. Formation history and protolith characteristics of granulite facies metamorphic rock in Central Cathaysia deduced from U-Pb and Lu-Hf isotopic studies of single zircon grains

    YU Jinhai; ZHOU Xinmin; Y. S. O'Reilly; ZHAO Lei; W. L. Griffin; WANG Rucheng; WANG Lijuan; CHEN Xiaomin


    The petrochemical as well as zircon U-Pb and Lu-Hf isotopic studies of granulite facies metamorphic rock from the Taoxi Group in eastern Nanling Range, Central Cathaysia indicate that its protolith is the sedimentary rock with low maturation index. The clastic materials are mostly from middle Neoproterozoic (~736 Ma) granitoid rocks with minor Neoarchaean and Paleoproterozoic rocks. The timing of this Neoproterozoic magmatism is in agreement with the second period of magmatism widespread surrounding the Yangtze Block. Hf isotopic data indicate that the Neoproterozoic granitoids resulted from the recycled Paleoproterozoic mantle-derived crustal materials. The sedimentary rock was deposited in Late Neoproterozoic Era, and carried into low crust in Early Paleozoic. The partial melting of the meta-sedimentary rock took place at about 480 Ma and subsequently granulite facies metamorphism occurred at ca. 443 Ma. The zircons forming during this time interval (Early Paleozoic) show large Hf isotope variations, and their -Hf(t) values increase from -13.2 to +2.36 with decreasing age, suggesting the injection of mantle-derived materials during partial melting and metamorphism processes in the Early Paleozoic. Calculation results show that this metamorphic rock, if evolved to Mesozoic, has similar isotopic composition to the nearby Mesozoic high Si peraluminous granites, implying that this kind of granulite facies metamorphic rock is probably the source material of some Mesozoic peraluminous granitoids in eastern Nanling Range.

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

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


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

  1. Electron microprobe Th-U-Pb monazite dating and metamorphic evolution of the Acaiaca Granulite Complex, Minas Gerais, Brazil

    Medeiros Junior, Edgar Batista; Marques, Rodson Abreu, E-mail:, E-mail: [Universidade Federal do Espirito Santo (UFES), Alegre, ES (Brazil). Departamento de Geologia; Jordt-Evangelista, Hanna; Queiroga, Glaucia Nascimento, E-mail:, E-mail: [Universidade Federal de Ouro Preto (UFOP), Ouro Preto, MG (Brazil). Escola de Minas. Departamento de Geologia; Schulz, Bernhard, E-mail: [TU Bergakademie - Institute of Mineralogy, Freiberg - Saxony (Germany)


    The Acaiaca Complex (AC) is located in southeastern Minas Gerais state, and comprises felsic, mafic, ultramafic, and aluminous granulite as well as lower grade gneisses and mylonite. The complex is distributed over an area of ca. 36 km by 6 km, surrounded by amphibolite facies gneisses of the Mantiqueira Complex (MC). The discrepancy in the metamorphic grade between both complexes led to the present study aiming to understand the metamorphic history of the AC by means of geothermobarometry calculations and electron microprobe Th-U-Pb monazite dating. Estimates of the metamorphic conditions of the granulite based on conventional geothermobarometry and THERMOCALC resulted in temperatures around 800 deg C and pressures between of 5.0 and 9.9 kbar and a retro metamorphic path characterized by near-isobaric cooling. Part of the granulite was affected by anatexis. The melting of felsic granulite resulted in the generation of pegmatites and two aluminous lithotypes. These are: 1) garnet-sillimanite granulite with euhedral plagioclase and cordierite that show straight faces against quartz, and is the crystallization product of an anatectic melt, and 2) garnet-kyanite-cordierite granulite, which is probably the restite of anatexis, as indicated by textures and high magnesium contents. Th-U-Pb monazite geochronology of two granulite samples resulted in a metamorphic age around 2060 Ma, which is similar to the age of the MC registered in the literature. The similar Paleoproterozoic metamorphic ages of both complexes lead to the conclusion that the Acaiaca Complex may be the high grade metamorphic unit geochronological related to the lower grade Mantiqueira Complex. (author)

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

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


    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

  3. Structural evolution and U-Pb SHRIMP zircon ages of the Neoproterozoic Maria da Fé shear zone, central Ribeira Belt - SE Brazil

    Zuquim, M. P. S.; Trouw, R. A. J.; Trouw, C. C.; Tohver, E.


    The Maria da Fé Shear Zone (MFSZ) is a sinistral strike-slip kilometric-scale structure developed in the late Neoproterozoic during the assembly of Gondwana. The MFSZ development is related to the NW-SE collision between the São Francisco Paleocontinent and the Rio Negro Magmatic Arc, which formed the Ribeira Belt. This paper describes the shear zone in detail, concluding that the orientation and age are consistent with NW-SE shortening during the afore mentioned collision. A U-Pb SHRIMP Concordia age of 586.9 ± 8.7 Ma is reported from zircon grains of a granitic dyke that crystallised synkinematically to the main tectonic activity of the shear zone. Another group of zircon grains from the same sample generated an upper intercept age of 2083 ± 43 Ma anchored in the younger Concordia age. These zircon grains are interpreted as relict grains of the basement from which the granite dyke was generated by partial melting. The temperature during mylonitization in the MFSZ was estimated in the range from 450 to 600 °C, based on microstructures in quartz and feldspar. An earlier collision in the same region, between 640 and 610 Ma, led to an extensive nappe-stack with tectonic transport to ENE, integrating the southern Brasilia Belt. One of the thrust zones between these nappes in the studied area is the Cristina Shear Zone with mylonites that were generated under upper amphibolite to granulite facies conditions. Brittle-ductile E-W metric-scale shear zones are superimposed on the MFSZ, which were active in similar, but probably slightly cooler, metamorphic conditions (≈500 °C).

  4. Zircon SHRIMP dating of sodium alkaline rocks from Maomaogou area of Huili County in Panxi,SW China and its geological implications

    LIU Hongying; XIA Bin; ZHANG Yuquan


    Alkali-feldspar syenites from the Maomaogou area of Huili County in Panxi of Sichuan Province, SW China are sodium alkaline rocks, the minerals of which are mainly composed of calcic ferroaugite, ferroamphibole and biotite. Most of the zircons (>80%) selected for SHRIMP U-Pb dating are magmatic compound zircons and they have relict inner cores and growing outer shells. The age of zircon SHRIMP U-Pb dating is 224±8 Ma, suggesting that the Maomaogou alkali-feldspar syenite was formed in the late Triassic period. At the same time, two old age periods of relict cores of zircon are determined, they are the Archean age (2692-2818 Ma) and Neoproterozoic age (622-691 Ma),respectively. These periods indicate that the cores derived from the crystal basement. Furthermore, the Archean age of zircon provides the first reliable SHRIMP U-Pb zircon dating evidence of >2.8 Ga basement in the Xikang-Yunnan Axis of southwest China.

  5. Possible early Neoproterozoic magmatism associated with slab window in the Pingshui segment of the Jiangshan-Shaoxing suture zone: Evidence from zircon LA-ICP-MS U-Pb geochronology and geochemistry


    We report here geochemical data, U-Pb zircon ages, and Hf isotopes for the high-Mg diorites (HMDs), Nb-enriched basaltic porphyrys (NEBPs) and plagiogranites (PLAGs) in the Pingshui segment of the Jiangshan-Shaoxing suture zone. The HMDs are characterized by high Mg# (>60), high Na and LREE contents, depletion of HREE and HFSE, and pronounced positive εNd(t) values of 7.0 to 7.7, similar to some adakitic high-Mg andesites. The NEBPs are relatively Na-rich (Na2O/K2O>6) and display high abundances of P2O5 (~1.00%), TiO2 (~3.08%) and HFSE (e.g., Nb=9.53-10.27 ppm). Their Nd isotopic compositions (εNd(t)=6.8-8.0) are comparable to those of the HMDs. The PLAGs are metaluminous (A/CNK=0.84-0.89) and sodic (Na2O/K2O>10). Their depletion in HFSE (e.g., Nb, Ta) is consistent with "SSZ-type" plagiogranite. Zircon LA-ICP-MS U-Pb dating yields an age of 932±7 Ma for the HMD, 916±6 Ma for the NEBP, and 902±5 Ma for the PLAG, respectively, indicating that they were products of early Neoproterozoic magmatism. The PLAGs exhibit relatively high zircon Hf isotopes and positive εHf(t) values of 11.0 to 16.2, consistent with their Nd isotopic data (εNd(t)=7.5-8.4). Such features are similar to those of oceanic plagiogranites in ophiolites and distinct from those of crust-derived granites. The PLAGs were most likely derived from partial melting of subducted oceanic crust in an active continental margin. Considering these results in the context of the regional geology, we suggest that a slab window in the subducting oceanic crust between the Yangtze Block and Cathaysia Block was possibly the principal cause for the unique arc magmatism in the area. The upwelling asthenosphere below the slab window may have provided significant thermodynamic conditions.

  6. Possible early Neoproterozoic magmatism associated with slab window in the Pingshui segment of the Jiangshan-Shaoxing suture zone: Evidence from zircon LA-ICP-MS U-Pb geochronology and geochemistry

    CHEN ZhiHong; GUO KunYi; DONG YongGuan; CHEN Rong; LI LongMing; LIANG YiHong; LI ChunHai; YU XiMing; ZHAO Ling; XING GuangFui


    We report here geochemical data, U-Pb zircon ages, and Hf Isotopes for the high-Mg diorites (HMDs), Nb-enriched basaltic porphyrys (NEBPs) and plagiogranitee (PLAGs) in the Pingshui segment of the Jiangshan-Shaoxing suture zone. The HMDs are characterized by high Mg# (>60), high Na and LREE contents, depletion of HREE and HFSE, and pronounced positive εNd(t) values of 7.0 to 7.7, similar to some adakitic high-Mg andesites. The NEBPs are relatively Na-rich (Na2O/K2O>6) and display high abundances of P2Os5(~1.00%), TiO2(~3.08%) and HFSE (e.g., Nb=9.53-10.27 ppm). Their Nd isotopic compositions (εNd(t)=6.8-8.0) are comparable to those of the HMDs. The PLAGs are metaluminous (AJCNK=0.84-0.89) and sodic (Na2O/K2O>10). Their depletion in HFSE (e.g., Nb, Ta) Is consistent with "SSZ-type" plagiogrenite. Zircon LA-ICP-MS U-Pb dating yields an age of 932±7 Ma for the HMD, 916±6 Ma for the NEBP, and 902±5 Ma for the PLAG, respectively, indicating that they were products of early Neoproterozoic magmatism. The PLAGs exhibit relatively high zircon Hf isotopes and positive εHf(t) values of 11.0 to 16.2, consistent with their Nd isotopic data (trod(f)=7.5-8.4). Such features are similar to those of oceanic plagiogranites in ophiolites and distinct from those of crust-derived granites. The PLAGs were most likely derived from partial melting of subducted oceanic crust in an active conUnen- tal margin. Considering these results in the context of the regional geology, we suggest that a slab window in the subducting oceanic crust between the Yangtze Block and Cathaysia Block was possibly the principal cause for the unique arc magmatism in the area. The upwelling aathenosphere below the slab window may have provided significant thermodynamic conditions.

  7. Petrology and U-Pb zircon geochronology of bimodal volcanic rocks from the Maierze Group, northern Tibet: Constraints on the timing of closure of the Banggong-Nujiang Ocean

    Fan, Jian-Jun; Li, Cai; Xie, Chao-Ming; Wang, Ming; Chen, Jing-Wen


    We present new zircon U-Pb dates, major and trace element chemistry, and Hf isotopic compositions for bimodal volcanic rocks of the Maierze Group (MG) in the Maierze area of the Southern Qiangtang-Baoshan block, northern Tibet. We discuss the implications of these data for the evolution of this region. The MG bimodal volcanic rocks consist of basalts and dacites that yield LA-ICP-MS zircon U-Pb ages of 122 and 120 Ma, respectively. The MG basalts have light rare earth element (LREE)-enriched chondrite-normalized REE patterns (LaN/YbN = 13-14), high Ti/V ratios (45-64), high Zr (190-270 ppm) and Nb (22-41 ppm) concentrations, and Zr/Y ratios (7-9) that are similar to those of within-plate basalts. The MG basalts also have low MgO and total Fe2O3 (TFe2O3) concentrations, significant enrichments in the LREE and the light ion lithophile elements (LILEs; Rb, Ba, Th, U and Pb), and weak depletions in the high field strength elements (HFSE; Nb, Ta, and Ti), all of which are clearly evident in primitive-mantle-normalized multi-element variation diagrams. The MG dacites are more enriched in the LILE (e.g., Rb, Ba, Th, U, K, and Pb) and more depleted in the HFSE (e.g., Nb, Ta, and Ti) than the MG basalts. Moreover, the dacites have variable zircon εHf(t) values (- 6.3 to + 6.3). These features indicate that the parental magma for the MG basalts was likely derived from an enriched lithospheric mantle source that was contaminated by subduction-related fluids or melts. In contrast, the MG dacites were derived from mixing of the MG basaltic magma with a second magma derived from partial melting of the continental crust. The geochemical and Hf isotopic characteristics of the MG bimodal volcanic rocks suggest that they formed during the initial stages of development of a back-arc basin. From south to north, the Bangong-Nujiang suture zone, the Duolong gold-rich porphyry copper deposit, and the Maierze bimodal rocks are interpreted to represent a remnant of a complete volcanic arc

  8. U-Pb zircon and Re-Os molybdenite geochronology from La Caridad porphyry copper deposit: insights for the duration of magmatism and mineralization in the Nacozari District, Sonora, Mexico

    Valencia, V. A.; Ruiz, J.; Barra, F.; Geherls, G.; Ducea, M.; Titley, S. R.; Ochoa-Landin, L.


    Uranium-lead zircon (laser ablation multi-collector ICP-MS spot analysis) ages from La Caridad porphyry copper deposit in the Nacozari District, Northeastern Sonora, Mexico, suggest a short period of magmatism, between 55.5 and 52.6 Ma. Two U-Pb ages from the mineralized quartz monzonite unit, showing different textural characteristics, yielded indistinguishable crystallization ages (~54 Ma), and indicate that the intrusion responsible for the mineralization occurred as a single large complex unit, instead of multiple pulses of magmatism. Some zircons analyzed also show inherited ages in cores recording dates of 112-124 Ma, 141-166 Ma and 1.4 Ga. The Re-Os molybdenite ages from the potassic and phyllic hydrothermal alteration veins yielded identical ages within error, 53.6±0.3 Ma and 53.8±0.3 Ma, respectively (weighted average of 53.7±0.21 Ma), supporting a restricted period for the mineralization. The geochronological data thus indicate a short-lived magmatic and hydrothermal system. The inherited zircons of Precambrian and Late Jurassic-Mid Cretaceous age found in the intrusive rocks of La Caridad deposit, can be explained considering two possible scenarios within the tectonic/magmatic evolution of the area. The first scenario considers the presence of a Precambrian anorogenic granitic basement that is intruded by Mesozoic (Jurassic-Cretaceous) units present beneath the La Caridad deposit. The second scenario suggests that the Mesozoic Glance Conglomerate Formation of Arizona underlies the Paleocene volcanic-igneous pile in the La Caridad area.

  9. The tectonic evolution of the Irtysh tectonic belt: New zircon U-Pb ages of arc-related and collisional granitoids in the Kalaxiangar tectonic belt, NW China

    Hong, Tao; Klemd, Reiner; Gao, Jun; Xiang, Peng; Xu, Xing-Wang; You, Jun; Wang, Xin-Shui; Wu, Chu; Li, Hao; Ke, Qiang


    Precise geochronological constraints of the Irtysh tectonic belt situated between the Saur Island Arc and the Altay Terrane are crucial to a better understanding of the tectonic evolution of the Central Asian Orogenic Belt (CAOB). Recently, we discovered repeatedly deformed arc-related and collisional granitoids in the Kalaxiangar tectonic belt (KTB), which is located in the eastern part of the Irtysh tectonic belt. In this study, we report new whole-rock geochemical, zircon U-Pb and Hf isotopic data of the arc-related and collisional granitoids. Our data reveal that 1) arc-related granodioritic porphyries formed at ca. 382-374 Ma. Recrystallized zircon grains from a (ultra-)mylonitic granodiorite of the Laoshankou zone in the southern KTB display a U-Pb age of ca. 360 Ma; 2) syn-collisional granodioritic porphyries, which distribute along faults and parallel to the cleavage, were emplaced at ca. 367-356 Ma, with εHf(t) values varying from + 7.8 to + 14.2 and Hf model ages from 873 to 459 Ma; 3) a post-collisional A-type granodioritic porphyry, which crosscuts the NW-NNW trending schistosity of the metasedimentary country rocks at a low angle, has an age of ca. 324-320 Ma, while the εHf(t) values range from + 7.6 to + 14.4 with Hf model ages from 850 to 416 Ma; 4) post-collisional strike-slip A-type granite dykes, exposed along strike-slip faults, gave ages between 287 and 279 Ma, whereas the εHf(t) values range from + 4.9 to + 12.7 and the Hf model ages from 995 to 500 Ma; and 5) A-type biotite granite dykes, which intruded along conjugate tension joints, have ages of 274-271 Ma, and εHf(t) values from + 1.5 to + 13.2 with Hf model ages from 1196 to 454 Ma. Consequently, we propose that the collision between the Saur Island Arc and the Altay Terrane occurred in the Early Carboniferous (ca. 367-356 Ma) and the subsequent post-collisional tectonic process continued to the Late Carboniferous (ca. 324-320 Ma). It is further suggested that the Irtysh tectonic belt

  10. Zircon Trace Element Contents and Refined U-Pb Crystallization Ages for the Tatoosh Pluton, Mount Rainier National Park, Washington Cascades

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


    The 7x12 km Tatoosh pluton south of Mount Rainier consists of 4 petrographic/compositional phases, here termed Nisqually, Reflection, Pyramid, and Stevens, that intrude Tertiary volcanic and sedimentary wall and roof rocks; contacts between the 4 intrusive units are rarely exposed. We used the USGS-Stanford SHRIMP- RG to analyze, in a continuous session, zircons from each of 6 quartz monzodiorite (qmd), quartz monzonite (qm), or granodiorite (grd) samples for 206Pb/238U ages and, concurrently, U, Th, Hf, and REE concentrations. A round-robin procedure yielded statistically robust geochronological results. Ages that we reported previously (FM07) were compromised by instrument instability and by calibration differences between analytical sessions. Between 11 and 31 new analyses of zircons from each sample were evaluated using the TuffZirc and Umix Ages routines of Isoplot 3.41 (Ludwig, 2003). TuffZirc solidification ages for the intrusions are: Nisqually grd (Paradise Valley; 65.4% SiO2) 17.29 +0.37/-0.24 Ma, Nisqually grd (Christine Falls; 66.4%) 17.70 +0.30/-0.16 Ma, Reflection qm (Pinnacle Peak trail; 66.6%) 18.38 +0.45/-0.28 Ma, Pyramid qmd (58.5%) 18.58 +0.20/-0.15 Ma, Stevens grd (Stevens Canyon; 67.8%) 19.15 +0.15/-0.12 Ma, and Stevens grd (south of Louise Lake; 69.3%) 19.20 +0.31/-0.26 Ma (U-Th initial-disequilibrium corrected, ±2σ). Precision of the U-Pb data limits rigorous identification of antecrysts to those with ages ~1 Myr > solidification ages. Antecryst ages that produce subsidiary modes in relative probability diagrams for the two Stevens samples give weighted mean values of 20.18 ±0.26 Ma and 20.07 ±0.18 Ma. Wide ranges in trace element concentrations and ratios indicate that many analyzed zircons grew in highly fractionated residual liquids in high-crystallinity environments. Concentrations of Th and U in Tatoosh zircons vary by two orders of magnitude, cores tend to have higher Th, U, and Th/U than rims, and overgrowths that fill reentrants

  11. Evaluating the provenance of Permian-Triassic and Palaeocene-Eocene ash beds by high precision U-Pb and Lu-Hf isotopic analyses of zircons: linking local sedimentary records to global events

    Eivind Augland, Lars; Jones, Morgan; Planke, Sverre; Svensen, Henrik; Tegner, Christian


    Zircons are a powerful tool in geochronology and isotope geochemistry, as their affinity for U and Hf in the crystal structure and the low initial Pb and Lu allow for precise and accurate dating by U-Pb ID-TIMS and precise and accurate determination of initial Hf isotopic composition by solution MC-ICP-MS analysis. The U-Pb analyses provide accurate chronostratigraphic controls on the sedimentary successions and absolute age frames for the biotic evolution across geological boundaries. Moreover, the analyses of Lu-Hf by solution MC-ICP-MS after Hf-purification column chemistry provide a powerful and robust fingerprinting tool to test the provenance of individual ash beds. Here we focus on ash beds from Permian-Triassic and Palaeocene successions in Svalbard and from the Palaeocene-Eocene Thermal Maximum (PETM) in Fur, Denmark. Used in combination with whole rock geochemistry from the ash layers and the available geochemical and isotopic data from potential source volcanoes, these data are used to evaluate the provenance of the Permian-Triassic and Palaeocene ashes preserved in Svalbard and PETM ashes in Denmark. If explosive eruptions from volcanic centres such as the Siberian Traps and the North Atlantic Igneous Province (NAIP) can be traced to distal basins as ash layers, they provide robust tests of hypotheses of global synchronicity of environmental changes and biotic crises. In addition, the potential correlation of ash layers with source volcanoes will aid in constraining the extent of explosive volcanism in the respective volcanic centres. The new integrated data sets will also contribute to establish new reference sections for the study of these boundary events when combined with stable isotope data and biostratigraphy.

  12. Petrogenetic setting of andsites in Rongna ore block, Tiegelong Cu (Au-Ag) deposit, Duolong ore concentration area, Tibet:Evidence from zircon U-Pb LA-ICP-MS dating and petrogeochemistry of andsites%西藏多龙矿集区铁格隆南铜(金银)矿床荣那矿段安山岩成岩背景:来自锆石U-Pb年代学、岩石地球化学的证据

    王勤; 卫鲁杰; 冯军; 李力; 唐菊兴; 方向; 林彬; 宋扬; 王艺云; 杨欢欢; 杨超; 李彦波


    In this paper, the authors studied the andsites in Rongna ore block, Tiegelong Cu (Au-Ag) deposit, Duolong ore concentration area, Tibet, and used the zircon U-Pb data and petrogeochemistry of andsites to identify the petrogenetic age and tectonic setting of the andsites. The weighted average age of zircon U-Pb is 110.1 ± 0.7 Ma (MSWD=2.3) , which represents the petrogenesis age. An analysis of the whole rock sample shows that SiO2 values are 59.5%-61.48%, Al2O3 values are 16.14%-17.11%and Na2O+K2O values are 6.02%-6.63%;Mg# is of intermediate value. LILE such as Ba, Rb, Th, K are rich, HFSE such as Ta, Nb, P, Ti are relatively poor and characterized by Rb positive anomaly and obvious Zr, Hf negative anomalies. Petrological and geochemical studies show that the andesites are high potassium calc-alkaline rocks exhibiting the characteristics of Andean-type active continental margin volcanic rocks. They resulted probably from partial melting of subducted oceanic slab as well as miscibility with crustal material during the upward migration of the magma in the case that subducted oceanic slab didn’t interact with the mantle. They were the magma products of the northward subduction of Bangong Co-Nujiang Ocean and Lhasa and the collision collage process of Qiangtang terrane. In combination with other dating data and drilling core catalog data, the authors hold that the formation of andesites was later than the rock-forming and ore-forming stage of the Tiegelongnan high sulfur epithermal Cu (Au-Ag) deposits, which seems to have been the important condition for the preservation of the ore deposit.%本文主要对出露与西藏多龙矿集区铁格隆南矿区荣那矿段的安山岩开展了锆石U−Pb定年、岩石地球化学测试,旨在确定该套火山岩的成岩年龄及其形成的构造背景。安山岩的LA−ICP−MS锆石U−Pb加权平均年龄为(110.1±0.7) Ma (MSWD=2.3),代表了其成岩年龄。岩石SiO2为59.5%~61.48%;Al2O3为16.14%~17.11%

  13. New structural and U-Pb zircon data from Anafi crystalline basement (Cyclades, Greece): constraints on the evolution of a Late Cretaceous magmatic arc in the Internal Hellenides

    Martha, Silviu O.; Dörr, Wolfgang; Gerdes, Axel; Petschick, Rainer; Schastok, Janina; Xypolias, Paraskevas; Zulauf, Gernold


    The Asterousia Crystalline Complex consists of Late Cretaceous amphibolite facies metamorphic rocks and associated granitoids, which can be found in exposures on Crete and the Cyclades (Greece). It is attributed to the Uppermost Unit and therefore to the Pelagonian domain of the Internal Hellenides. The tectonometamorphic evolution of this unit is still a matter of debate. We present new structural and petrological data of Asterousia-type rocks and greenschist facies metamorphic rocks from the island of Anafi in the southern Aegean Sea as well as U-Pb zircon ages of granitoids from Anafi. The crystalline sequence of Anafi rests on top of Eocene flysch and comprises from bottom to top: (a) Anafi Greenschist; (b) Anafi Amphibolite Group (orthoamphibolite with intercalations of metasedimentary rocks at the base); and (c) Chalepa Group (amphibolite facies metasediments with slices of serpentinite and granitoids). LA-ICP-MS and ID-TIMS 206Pb/238U zircon ages of granodiorite from the Chalepa Group reveal several similar zircon populations suggesting continuous emplacement of granitoids inside a magmatic arc from ca. 72.5 to 79 Ma. The minimum emplacement age of granodioritic magma, deduced from the 206Pb/238U median age of the youngest zircon population, is 72.6 +0.1/-0.2 Ma. Deformation (micro)fabrics of granodiorite result from low strain obtained at T > 600 °C. This along with the U-Pb ages and published K-Ar ages indicates intrusion of the plutonic rocks at deep structural levels followed by very slow cooling. Monzogranitic dykes cutting through granodiorite in north-eastern Anafi are undeformed and yielded a 206Pb/238U median age of 69.9 +0.7/-0.7 Ma. Based on the new and published data, the following implications for the tectonometamorphic evolution on Anafi can be made: (1) obduction and accretion of mantle slices (serpentinite) to the Asterousia-type rocks were prior to amphibolite facies metamorphism; (2) intrusion of granitoids during the middle to late

  14. SHRIMP zircon U-Pb dating for gabbro from the Tiding ophiolite in Tibet

    WANG Ran; XIA Bin; ZHOU Guoqing; ZHANG Yuquan; YANG Zhiqing; LI Wenqian; WEI Dongliang; ZHONG Lifeng; XU Lifeng


    @@ Ophiolite is fragments of oceanic lithosphere formed by obduction and its formation age is very important for reconstructing the formation and evolution of ancient oceanic basin and the configuration of ancient plate tectonics. The Yarlung Zangbo suture zone is the eastern part of the Tethyan suture zone between the India and Eurasian plates.

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

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


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

  16. 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

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


    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

  17. Zircon U-Pb geochronology,Hf isotopic composition and geological implications of the rhyodacite and rhyodacitic porphyry in the Xiangshan uranium ore field,Jiangxi Province,China


    The Xiangshan uranium ore field is the largest volcanic rock hosted uranium deposit in China.The host rock is a volcanic intrusive complex,including rhyodacite,porphyroclastic lava and late stage sub-volcanic rocks.In this study,zircons from an early stage rhyodacite and a late stage rhyodacite porphyry were dated by SHRIMP and LA-ICP-MS U-Pb methods,and their Hf isotopic compositions were measured by LA-MC-ICP-MS.206Pb/238U ages of 135.1±1.7 and 134.8±1.1 Ma were obtained for the rhyodacite and rhyodacitic porphyry,respectively.These accurate ages indicate that the Xiangshan volcanic-intrusive complex formed in the Early Cretaceous rather than in the Late Jurassic,as concluded in some previous studies.By the Early Cretaceous,the tectonic setting of the area has evolved into a back-arc extensional setting,possibly related to subduction of the paleo-Pacific plate.The close ages of the(early) eruptive rhyodacite and the(late) hypabyssal rhyodacitic porphyry shows that the Xiangshan volcanism was intensive and concentrated in a short time.Zircons from the rhyodacite show negative εHf(t) values of-5.7 to-8.5,with Hf depleted mantle model ages between 1550 and 1720 Ma,whereas zircons from the rhyodacitic porphyry yield εHf(t) values of-6.9 to-10.1 and Hf model ages between 1621 and 1823 Ma.These zircon Hf model ages are similar to the whole rock Nd model ages(1486 to 1911 Ma).Combined with other geochemical characteristics,the Xiangshan rhyodacite and rhyodacitic porphyry may have been derived from partial melting of the Paleo-Mesoproterozoic metamorphic rocks from the Xiangshan basement,without any significant addition of mantle-derived magma.Contribution of basement of this age is also supported by finding a Paleoproterozoic xenocrystic zircon core in the rhyodacite sample.

  18. Th-U-Pb{sub T} dating by electron probe microanalysis, Part I. Monazite: analytical procedures and data treatment

    Vlach, Silvio Roberto Farias [Universidade de Sao Paulo (USP), Sao Carlos, SP (Brazil). Inst. de Geociencias. Dept. de Mineralogia e Geotectonica], e-mail:


    Dating methodology by the electron probe microanalyser (EPMA) of (Th, U)-bearing minerals, highlighting monazite, acquired greater than ever importance in literature, particularly due to its superior spatial resolution, as well as versatility, which allow correlating petrological processes at times registered only in micro-scales in minerals and rocks with absolute ages. Although the accuracy is inferior to the one achieved with conventional isotopic methods in up to an order of magnitude, EPMA is the instrument that allows the best spatial resolution, reaching a few {mu}m{sup 3} in some conditions. Quantification of minor and trace elements with suitable precision and accuracy involves the own instrumental and analytical set-ups and data treatment strategies, significantly more rigorous when compared with those applied in conventional analyses. Th-U-Pb{sub T} dating is an example of these cases. Each EPMA is a unique machine as for its instrumental characteristics and respective automation system. In such a way, analytical procedures ought to be adjusted for laboratory specific cities. The analytical strategies and data treatment adopted in the Electronic Microprobe Laboratory from Instituto de Geociencias of Universidade de Sao Paulo, Brazil, with a JEOL JXA8600S EPMA, and a ThermoNoran-Voyager 4.3 automation system, are presented and compared with the ones used in other laboratories. The influence of instrumental factors and spectral overlaps on Th, U, and Pb quantification is discussed. Applied procedures to interference correction, error propagation, data treatment, and final chemical age presentation as well as to sampling and analyses are emphasized. Some typical applications are discussed, drawing attention to the most relevant aspects of electron microprobe dating. (author)

  19. The LA-ICP-MS zircons U-Pb ages and geochemistry of the Baihua basic igneous complexes in Tianshui area of West Qinling

    PEI; XianZhi; DING; SaPing; ZHANG; GuoWei; LIU; HuiBin; LI; ZuoChen; LI; GaoYang; LIU; ZhanQing; MENG; Yong


    Baihua meta-igneous complex consists mainly of pyroxenite-gabbro(diorite)-diorite-quartz diorite. They form a complete comagmatic evolutionary series. The geochemical characteristics of basic-interme- diate basic igneous rocks indicate that they belong to a tholeiite suite. The REE distribution pattern is nearly flat type and LREE is slightly enriched type, and their primitive mantle-normalized and MORB-normalized trace element spider diagrams are generally similar; the LIL elements (LILE) Cs, Ba, Sr, Th and U are enriched, but Rb, K and the HFSEs Nb, P, Zr, Sm, Ti and Y are relatively depleted. All these show comagmatic evolution and origin characteristics. The tectonics environment discrimination of trace element reveals that these igneous complexes formed in an island-arc setting. The LA-ICP-MS single-zircons U-Pb age of Baihua basic igneous complex is 434.6±1.5 Ma (MSWD = 1.3), which proves that the formation time of the island-arc type magmatite in the northern zone of West Qinling is Late Ordovician or Early Silurian, also reveals that the timing of subduction of paleo-ocean basin represented by the Guanzizhen ophiolite and resulting island-arc-type magmatic activities is probably Middle-Late Ordovician to Early Silurian.

  20. New material of microfossils from the Ediacaran Doushantuo Formation in the Zhangcunping area,Yichang, Hubei Province and its zircon SHRIMP U-Pb age

    LIU PengJu; YIN ChongYu; GAO LinZhi; TANG Feng; CHEN ShouMing


    The Zhangcunping area is located at the north limb of the Huangling anticline in Yichang, Hubei Province. Here, the sedimentary succession of the Ediacaran Doushantuo Formation is similar with that in the Weng'an area, Guizhou Province. A large number of new microfossils (mainly acanthoacritarchs) from the Doushantuo Formation in this area are reported in this paper. The fossil assemblage shows similarity as the phosphatized biota of the Doushantuo Formation at Wang'an, Guizhou Province and the silicified biota of the Doushantuo Formation at the Yangtze Gorges, which suggests that the Zhangcunping area is a key for correlation of the Doushantuo Formation between the Weng'an area,Guizhou Province and the Yangtze Gorges. Besides, a new zircon SHRIMP U-Pb age (614.0±7.6 Ma) is first obtained from a horizon underneath the exposed surface in the middle of the Doushantuo Formation in the Zhangcunping area. This age not only provides a new datum for subdivision of the Ediacaran Doushantuo Formation, but also indicates that the age of the exposed surface in the middle of the Doushantuo Formation in the Yangtze Platform should be posterior to 614.0±7.6 Ma. Due to the horizon of the Weng'an biota situated above the exposed surface, the age of the Wang'an biota should be posterior to 614.0±7.6 Ma as well.

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

    Dingling Huang


    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.

  2. Geochemical characteristics and zircon U-Pb isotopic ages of island-arc basic igneous complexes from the Tianshui area in West Qinling

    PEI Xianzhi; LI Zuochen; LIU Huibin; LI Gaoyang; DING Saping; LI Yong; HU Bo; GUO Junfeng


    The Liushuigou intermediate-basic meta-igneous complex at Guanzizhen, Tianshui area, is mainly composed of metagabbro, metagabbro diorite and metadiorite, while the Baihua basic meta-igneous complex consists mainly of pyroxenite, gabbro (gabbro diorite), diorite and quartz diorite.They form a relatively complete comagmatic evolutionary series. The geochemical characteristics of intermediate-basic igneous rocks indicate that they belong to a tholeiite suite.Their chondrite-normalized REE patterns are nearly flat and are LREE-slightly enriched type, and their primitive mantle-normalized and MORB-normalized trace element spider-grams are generally similar; the LILEs Cs, Ba, Sr, Th and U are enriched, while Rb and K and the HFSEs Nb, P, Zr,Sm, Ti and Y are depleted. All these show comagmatic evolu-tionary and genetic characteristics. The tectonic environment discrimination by trace element reveals that these igneous complexes formed in an island-arc setting. The Thermal Ionization Mass Spectrometry (TIMS) single-grain zircon U-Pb age for the Liushuigou intermediate-basic meta-igneous rocks in the Guanzizhen area is (507.5 ± 3.0) Ma, represent-ing the age of these igneous complexes, which indicates that island-arc-type magmatite rocks in the northern zone of West Qinling are Late Cambrian and also reveals that the timing of subduction of the paleo-ocean basin represented by the Guanzizhen ophiolite and resulting island-arc-type magrnaticactivity are probably Late Cambrian to Early Ordovician.

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

    Longlong Zhao


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

  4. In situ SIMS U-Pb dating of hydrothermal rutile: reliable age for the Zhesang Carlin-type gold deposit in the golden triangle region, SW China

    Pi, Qiaohui; Hu, Ruizhong; Xiong, Bin; Li, Qiuli; Zhong, Richen


    The contiguous region between Guangxi, Guizhou, and Yunnan, commonly referred to as the Golden Triangle region in SW China, hosts many Carlin-type gold deposits. Previously, the ages of the gold mineralization in this region have not been well constrained due to the lack of suitable minerals for radiometric dating. This paper reports the first SIMS U-Pb age of hydrothermal rutile crystals for the Zhesang Carlin-type gold deposit in the region. The hydrothermal U-bearing rutile associated with gold-bearing sulfides in the deposit yields an U-Pb age of 213.6 ± 5.4 Ma, which is within the range of the previously reported arsenopyrite Re-Os isochron ages (204 ± 19 to 235 ± 33 Ma) for three other Carlin-type gold deposits in the region. Our new and more precise rutile U-Pb age confirms that the gold mineralization was contemporaneous with the Triassic W-Sn mineralization and associated granitic magmatism in the surrounding regions. Based on the temporal correlation, we postulate that coeval granitic plutons may be present at greater depths in the Golden Triangle region and that the formation of the Carlin-type gold deposits is probably linked to the coeval granitic magmatism in the region. This study clearly demonstrates that in situ rutile U-Pb dating is a robust tool for the geochronogical study of hydrothermal deposits that contain hydrothermal rutile.

  5. Th-U-PbT Dating by Electron Probe Microanalysis, Part I. Monazite: Analytical Procedures and Data Treatment

    Silvio Roberto Farias Vlach


    Full Text Available Dating methodology by the electron probe microanalyser (EPMA of (Th, U-bearing minerals, highlighting monazite,acquired greater than ever importance in literature, particularly due to its superior spatial resolution, as well as versatility,which allow correlating petrological processes at times registered only in micro-scales in minerals and rocks with absoluteages. Although the accuracy is inferior to the one achieved with conventional isotopic methods in up to an order of magnitude,EPMA is the instrument that allows the best spatial resolution, reaching a few μm3 in some conditions. Quantifi cation ofminor and trace elements with suitable precision and accuracy involves the own instrumental and analytical set-ups anddata treatment strategies, signifi cantly more rigorous when compared with those applied in conventional analyses. Th-U-PbTdating is an example of these cases. Each EPMA is a unique machine as for its instrumental characteristics and respectiveautomation system. In such a way, analytical procedures ought to be adjusted for laboratory specifi cities. The analyticalstrategies and data treatment adopted in the Electronic Microprobe Laboratory from Instituto de Geociências of Universidadede São Paulo, Brazil, with a JEOL JXA8600S EPMA, and a ThermoNoran-Voyager 4.3 automation system, are presentedand compared with the ones used in other laboratories. The infl uence of instrumental factors and spectral overlaps on Th,U, and Pb quantifi cation is discussed. Applied procedures to interference correction, error propagation, data treatment, andfi nal chemical age presentation as well as to sampling and analyses are emphasized. Some typical applications are discussed,drawing attention to the most relevant aspects of electron microprobe dating.

  6. Cassiterite LA-MC-ICP-MS U/Pb and muscovite 40Ar/39Ar dating of tin deposits in the Tengchong-Lianghe tin district, NW Yunnan, China

    Chen, Xiao-Cui; Hu, Rui-Zhong; Bi, Xian-Wu; Li, Hui-Min; Lan, Jiang-Bo; Zhao, Cheng-Hai; Zhu, Jing-Jing


    The Tengchong-Lianghe tin district in northwestern Yunnan, China, is an important tin mineralization area in the Sanjiang Tethyan Metallogenic Domain. There are three N-S trending granite belts in the Tengchong-Lianghe area, with emplacement ages ranging from Early Cretaceous to Late Cretaceous and Early Cenozoic. Tin mineralization is spatially associated with these granitic rocks. However, the petrogenetic link between the tin deposits and the host granites is not clear because of the lack of age data for the tin mineralization. We investigate the possibility of direct dating of cassiterite from three typical tin deposits in the Tengchong-Lianghe tin district, using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). In situ LA-MC-ICP-MS dating of seven cassiterite samples from the Lailishan (LLS-1 and LLS-2), Xiaolonghe (XLH, WDS, DSP, and HJS), and Tieyaoshan (TYS) tin deposits yielded well-defined 206Pb/207Pb-238U/207Pb isochron ages. To assess the accuracy of the in situ U/Pb dating of cassiterite, 40Ar/39Ar dating of coexisting muscovite (in samples LLS-1, DSP, and TYS) was also performed. The cassiterite in situ U/Pb ages (47.4 ± 2.0, 71.9 ± 2.3, and 119.3 ± 1.7 Ma, respectively) are in excellent agreement with the coexisting muscovite 40Ar/39Ar ages (48.4 ± 0.3, 71.9 ± 1.4, and 122.4 ± 0.7 Ma, respectively). The U/Pb ages of cassiterite combined with the 40Ar/39Ar ages of muscovite indicate that there are three tin mineralization events in this district: the Lailishan tin deposit at 47.4 ± 2.0 to 52 ± 2.7 Ma, the Xiaolonghe tin deposit at 71.6 ± 2.4 to 3.9 ± 2.0 Ma, and the Tieyaoshan tin deposit at 119.3 ± 1.7 to 122.5 ± 0.7 Ma. These ages are highly consistent with the zircon U/Pb ages of the host granites. It is su.ggested that the Cretaceous tin mineralization might have taken place in a subduction environment, while the Early Tertiary tin metallogenesis was in a postcollisional geodynamic setting.

  7. Variable microstructural response of baddeleyite to shock metamorphism in young basaltic shergottite NWA 5298 and improved U-Pb dating of Solar System events

    Darling, James R.; Moser, Desmond E.; Barker, Ivan R.; Tait, Kim T.; Chamberlain, Kevin R.; Schmitt, Axel K.; Hyde, Brendt C.


    The accurate dating of igneous and impact events is vital for the understanding of Solar System evolution, but has been hampered by limited knowledge of how shock metamorphism affects mineral and whole-rock isotopic systems used for geochronology. Baddeleyite (monoclinic ZrO2) is a refractory mineral chronometer of great potential to date these processes due to its widespread occurrence in achondrites and robust U-Pb isotopic systematics, but there is little understanding of shock-effects on this phase. Here we present new nano-structural measurements of baddeleyite grains in a thin-section of the highly-shocked basaltic shergottite Northwest Africa (NWA) 5298, using high-resolution electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) techniques, to investigate shock-effects and their linkage with U-Pb isotopic disturbance that has previously been documented by in-situ U-Pb isotopic analyses. The shock-altered state of originally igneous baddeleyite grains is highly variable across the thin-section and often within single grains. Analyzed grains range from those that preserve primary (magmatic) twinning and trace-element zonation (baddeleyite shock Group 1), to quasi-amorphous ZrO2 (Group 2) and to recrystallized micro-granular domains of baddeleyite (Group 3). These groups correlate closely with measured U-Pb isotope compositions. Primary igneous features in Group 1 baddeleyites (n = 5) are retained in high shock impedance grain environments, and an average of these grains yields a revised late-Amazonian magmatic crystallization age of 175 ± 30 Ma for this shergottite. The youngest U-Pb dates occur from Group 3 recrystallized nano- to micro-granular baddeleyite grains, indicating that it is post-shock heating and new mineral growth that drives much of the isotopic disturbance, rather than just shock deformation and phase transitions. Our data demonstrate that a systematic multi-stage microstructural evolution in

  8. Opening of the Tethys in southwest China and its significance to the breakup of East Gondwanaland in late Paleozoic: Evidence from SHRIMP U-Pb zircon analyses for the Garzê ophiolite block

    YAN Quanren; WANG Jianguo; ZHANG Dehui; ZHAO Jian; WANG Zongqi; LIU Shuwen; LI Qiugen; ZHANG Hongyuan; WANG Tao; LIU Dunyi; SHI Yuruo; JIAN Ping


    SHRIMP U-Pb zircon analyses for a gabbro sample from the Garzê ophiolite block yielded a mean 206Pb/238U age of 292±4 Ma, which indicated that the spreading time of the Garzê-Litang Tethys was most likely at the earliest Permian. Combined with previous studies, we suggest that the opening of the Tethys in southwest China was derived from breakup of the East Gondwanaland in the late Paleozoic.

  9. 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

    Jiang, Si-Hong; Bagas, Leon; Hu, Peng; Han, Ning; Chen, Chun-Liang; Liu, Yuan; Kang, Huan


    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

  10. Single crystal U-Pb zircon age and Sr-Nd isotopic composition of impactites from the Bosumtwi impact structure, Ghana: Comparison with country rocks and Ivory Coast tektites.

    Ferrière, Ludovic; Koeberl, Christian; Thöni, Martin; Liang, Chen


    The 1.07 Myr old Bosumtwi impact structure (Ghana), excavated in 2.1-2.2 Gyr old supracrustal rocks of the Birimian Supergroup, was drilled in 2004. Here, we present single crystal U-Pb zircon ages from a suevite and two meta-graywacke samples recovered from the central uplift (drill core LB-08A), which yield an upper Concordia intercept age of ca. 2145 ± 82 Ma, in very good agreement with previous geochronological data for the West African Craton rocks in Ghana. Whole rock Rb-Sr and Sm-Nd isotope data of six suevites (five from inside the crater and one from outside the northern crater rim), three meta-graywacke, and two phyllite samples from core LB-08A are also presented, providing further insights into the timing of the metamorphism and a possibly related isotopic redistribution of the Bosumtwi crater rocks. Our Rb-Sr and Sm-Nd data show also that the suevites are mixtures of meta-greywacke and phyllite (and possibly a very low amount of granite). A comparison of our new isotopic data with literature data for the Ivory Coast tektites allows to better constrain the parent material of the Ivory Coast tektites (i.e., distal impactites), which is thought to consist of a mixture of metasedimentary rocks (and possibly granite), but with a higher proportion of phyllite (and shale) than the suevites (i.e., proximal impactites). When plotted in a Rb/Sr isochron diagram, the sample data points (n = 29, including literature data) scatter along a regression line, whose slope corresponds to an age of 1846 ± 160 Ma, with an initial Sr isotope ratio of 0.703 ± 0.002. However, due to the extensive alteration of some of the investigated samples and the lithological diversity of the source material, this age, which is in close agreement with a possible "metamorphic age" of ∼ 1.8-1.9 Ga tentatively derived from our U-Pb dating of zircons, is difficult to consider as a reliable metamorphic age. It may perhaps reflect a common ancient source whose Rb-Sr isotope

  11. Mid-Neoproterozoic intraplate magmatism in the northern margin of the Southern Granulite Terrane, India: Constraints from geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes

    Deeju, T. R.; Santosh, M.; Yang, Qiong-Yan; Pradeepkumar, A. P.; Shaji, E.


    The northern margin of the Southern Granulite Terrane in India hosts a number of mafic, felsic and alkaline magmatic suites proximal to major shear/paleo-suture zones and mostly represents magmatism in rift-settings. Here we investigate a suite of gabbros and granite together with intermediate (dioritic) units generated through mixing and mingling of a bimodal magmatic suite. The massive gabbro exposures represent the cumulate fraction of a basic magma whereas the granitoids represent the product of crystallization in felsic magma chambers generated through crustal melting. Diorites and dioritic gabbros mostly occur as enclaves and lenses within host granitoids resembling mafic magmatic enclaves. Geochemistry of the felsic units shows volcanic arc granite and syn-collisional granite affinity. The gabbro samples show mixed E-MORB signature and the magma might have been generated in a rift setting. The trace and REE features of the rocks show variable features of subduction zone enrichment, crustal contamination and within plate enrichment, typical of intraplate magmatism involving the melting of source components derived from both depleted mantle sources and crustal components derived from older subduction events. The zircons in all the rock types show magmatic crystallization features and high Th/U values. Their U-Pb data are concordant with no major Pb loss. The gabbroic suite yields 206Pb/238U weighted mean ages in the range of 715 ± 4-832.5 ± 5 Ma marking a major phase of mid Neoproterozoic magmatism. The diorites crystallized during 206Pb/238U weighted mean age of 724 ± 6-830 ± 2 Ma. Zircons in the granite yield 206Pb/238U weighted mean age of 823 ± 4 Ma. The age data show broadly similar age ranges for the mafic, intermediate and felsic rocks and indicate a major phase of bi-modal magmatism during mid Neoproterozoic. The zircons studied show both positive and negative εHf(t) values for the gabbros (-6.4 to 12.4), and negative values for the diorites (-7

  12. Zircon U-Pb geochronology and geochemistry of low-grade metamorphosed volcanic rocks from the Dantazi Complex: Implications for the evolution of the North China Craton

    Ge, Songsheng; Zhai, Mingguo; Li, Tiesheng; Peng, Peng; Santosh, M.; Shan, Houxiang; Zuo, Pengfei


    The late Neoarchean witnessed the cratonization of the North China Craton (NCC) through amalgamation of several micro-blocks to form a coherent basement. The Archean orthogneisses and supracrustal rocks in this craton have experienced various grades of metamorphism ranging up to upper amphibolite and granulite facies at ∼2500 Ma. Recently, a suite of low-grade metamorphosed (greenschist to lower amphibolite facies) volcanic rocks was discovered in the late Neoarchean Dantazi Complex in northern Hebei province. These meta-volcanic rocks consist of bimodal basalt-andesite and trachyte-dacite with a SiO2 gap between 54.4 wt.% and 60.7 wt.%. Here we report SHRIMP zircon U-Pb ages of 2490 ± 19 Ma (MSWD = 2.0) and 2502 ± 8 Ma (MSWD = 0.83) from the meta-mafic and meta-felsic volcanics, respectively, representing the timing of igneous activity. All the meta-mafic volcanic rocks display coherent trace element and REE patterns which are characterized by enriched LILE and LREE but depleted HFSE and HREE ((La/Yb)N = 6.29-15.10). Combining these trace element features with the positive zircon εHf(t) values (+1.3 to +6.6), we propose that the mafic rocks were likely derived from partial melting of a previously metasomatized lithospheric mantle. In the primitive mantle-normalized diagram, the felsic rocks display uniform patterns enriched in LILE but depleted in Nb and Ta, similar to those of lower crust. Furthermore, their strongly fractionated REE ((La/Yb)N = 15.24-61.20), lower HREE concentrations (Yb = 0.47-1.65 ppm) and positive zircon εHf(t) values (+1.6 to +5.3) suggest that they were derived from partial melting of the lower crust with garnet in the residue. This coeval occurrence of metasomatized mantle-derived mafic magmas and potassic felsic magmas from different source regions reflects an intracontinental extensional setting during the late Neoarchean to earliest Paleoproterozoic following the cratonization of the NCC. Our new data, combined with previous

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

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


    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

  14. LA-ICP-MS U-Pb dating of rutile inclusions within corundum (ruby and sapphire): new constraints on the formation of corundum deposits along the Mozambique belt

    Sorokina, Elena S.; Rösel, Delia; Häger, Tobias; Mertz-Kraus, Regina; Saul, John M.


    Direct dating of corundum and the establishment of age constraints for corundum crystal growth are restricted due to the absence of a geochronometer. Syngenetic mineral inclusions may help define minimum ages for corundum growth. Here, we present results of in situ LA-ICP-MS U-Pb dating applied to rutile inclusions in corundum from localities along the Mozambique belt. U-Pb dating of rutile inclusions gave ages of 533 ± 11 Ma (2 σ) for the Gitonga pit (John Saul ruby mine) and 526 ± 13 Ma for the Aqua Ruby mine, both in the Mangari area of Kenya, and an age of 499 ± 12 Ma for ruby from the Morogoro area, Tanzania. These ages are interpreted as cooling ages that set a minimum age for corundum growth following metamorphism along the East-African Antarctic Orogen, and might be used for petrogenetic tracing of corundum in placer deposits.

  15. Source and mode of the Permian Panjal Trap magmatism: Evidence from zircon U-Pb and Hf isotopes and trace element data from the Himalayan ultrahigh-pressure rocks

    Rehman, Hafiz Ur; Lee, Hao-Yang; Chung, Sun-Lin; Khan, Tahseenullah; O'Brien, Patrick J.; Yamamoto, Hiroshi


    We present an integrated study of LA-ICP-MS U-Pb age, Hf isotopes, and trace element geochemistry of zircons from the Himalayan eclogites (mafic rocks) and their host gneisses (felsic rocks) from the Kaghan Valley in Pakistan in order to understand the source and mode of their magmatic protoliths and the effect of metamorphism. Zircons from the so-called Group I (high-pressure) eclogites yielded U-Pb mean ages of 259 ± 10 Ma (MSWD = 0.74), whereas those of Group II (ultrahigh-pressure) eclogites yielded 48 ± 3 Ma (MSWD = 0.71). In felsic gneisses the central or core domains of zircons yielded ages similar to those from Group I eclogites but zircon overgrowth domains yielded 47 ± 1 Ma (MSWD = 1.9). Trace element data suggest a magmatic origin for Group I-derived (having Th/U ratios: > 0.5) and metamorphic origin for Group II-derived (Th/U rocks (Panjal Traps) with almost no contribution from the ancient crustal material. The similar ƐHf(t) values, identical protolith ages and trace element compositions of zircons in felsic (granites or rhyolites) and mafic (basalt and dolerite) rocks attest to a bimodal magmatism accounting for the Panjal Traps during the Permian. Later, during India-Asia collision in Eocene times, both the felsic and mafic lithologies were subducted to mantle-depths (> 90 km: coesite-stable) and experienced ultrahigh-pressure metamorphism before their final exhumation.

  16. Fossil pollen analysis of U-Pb-dated speleothems: a new approach to understanding Pliocene terrestrial climate evolution

    Sniderman, K.; Woodhead, J. D.; Porch, N.


    The nature of terrestrial environments in the Southern Hemisphere during the warm Pliocene is poorly known. This is not only because there are few published fossil records, but also because many of the existing records have very limited age control. For example, in Australia, the ages of most putative 'mid-Pliocene' fossil pollen records are based solely on biostratigraphic correlation. These correlations for the most part do not have the resolution to differentiate late Miocene from early Pliocene environments, let alone to pinpoint a Myrtaceae, with no sign of extensive cheonopod shrubland. Climate reconstructions based on presence of taxa now absent from the Nullarbor suggest that mean annual rainfall and/or summer rainfall were 50-100% higher than today. Second, vegetation near the Miocene/Pliocene boundary (at c. 5.5 Ma) was dominated by Gyrostemonaceae and Casuarinaceae. We interpret this transition from earliest Pliocene to mid-late Pliocene vegetation as an increase in biological productivity, from possibly very sparse woodland to forest, presumably in response to increased effective moisture in the mid-late Pliocene. Hence climate evolution within the Pliocene was substantial enough to drive complete biome turnover. Explanations of the mechanisms that drove Pliocene warmth thus need to explain not only why the Pliocene warm interval was terminated by Pleistocene cooling, but also why the mid-late Pliocene differed substantially from the earliest Pliocene. Our novel use of U/Pb dating of speleothem pollen records demonstrates that previous syntheses of 'mid-Pliocene' vegetation may, in regions with poor age control, have conflated biomes growing within different climatic regimes during different stages within the Pliocene.

  17. Evolution Of An Upper Crustal Plutonic-Volcanic Plumbing System:Insights From High Precision U-Pb Zircon Geochronology Of Intracaldera Tuff And Intrusions In Silver Creek Caldera, Arizona, USA

    Zhang, T.; Mundil, R.; Miller, C. F.; Miller, J. S.; Paterson, S. R.


    Study of both plutonic and volcanic regimes in one single magmatic system is a powerful approach towards obtaining a more complete view of the long-term evolution of magma systems. The recently discovered Silver Creek caldera is the source of the voluminous Peach Spring Tuff (PST) (Ferguson, 2008) and presents a unique opportunity to study a field laboratory of a linked plutonic-volcanic system. This relict west-facing half caldera is predominantly filled with trachytic intracaldera tuff with the caldera margin intruded by several petrologically distinct hypabyssal intrusions. These include porphyritic granite with granophyric texture, felsic leucogranite, porphyritic monzonite exposed on NE side of the caldera that is zoned from more felsic to more mafic, and quartz-phyric dikes that intrude the caldera fill. We present preliminary single zircon ages from 4 samples that have been analyzed using the CA-TIMS method after thermal annealing and chemical leaching (Mattinson 2005), including 1 sample from intracaldera tuff and 3 samples from caldera-related intrusions. 3-D total U/Pb isochron ages from all four samples fall within a range of 18.32-18.90 Ma with uncertainties between 0.09 and 0.39 Ma, although some of them lack precision and are compromised by elevated common Pb. For example, zircon from the dated porphyritic monzonite yields an age of 18.32±0.42 Ma (MSWD=2.7) where the excess scatter may result from real age dispersion and/or different compositions of the common Pb contribution. The PST had been dated to ~18.5 Ma by 40Ar/39Ar techniques (Nielson et al., 1990). In order to be compared to U/Pb ages the 40Ar/39Ar age must be adjusted for a revised age for the then used flux monitor (MMbh-1) and corrected for the now quantified systematic bias between 40Ar/39Ar and U/Pb ages (Renne et al., 2010), which results in a corrected age of 18.8 Ma. Thus, the ages for our samples match that of the PST within error. Based on current results, the age difference

  18. LA--ICP--MS zircon U--Pb age of Dongliang pluton in Balinyouqi,Inner Mongolia and its geochemical characteristics%内蒙古巴林右旗东梁岩体 LA--ICP--MS 锆石 U--Pb 定年及地球化学特征

    王璐; 赵庆英; 李鹏川; 李子昊; 邱士龙; 田子龙


    The study of the petrologic,chronological and geochemical characteristics of Dongliang pluton in Balinyouqi,Inner Mongolia shows that the lithology of Dongliang pluton is of quartz diorite,while the LA--ICP--MS zircon U --Pb dating shows the weighted average age as (251.9 ±4.4) Ma,representing the Dongliang pluton formed in the Early Triassic.The geochemical characteristics indicate that the rocks are enriched in Si,Al,Na, and depleted in Mg,K.The A /CNK is 0.98 ~1.11,showing the pluton is of calc-alkalic metaluminous-weakly peraluminous rocks.The quartz diorite is rich in LREE,depleted in HREE and without Eu,Ce anomaly,which shows the characteristics of high Sr,low Y and Yb,belonging to adakite rocks.Combined with previous resear-ches,the authors believe that the magma of Dongliang pluton may be originated from the partial melting of juvenile crust caused by the collision and suturing of Siberian Plate and North China Plate.%本文对内蒙古巴林右旗东梁岩体进行的岩石学、年代学和地球化学研究显示,东梁岩体岩性为石英闪长岩,LA-ICP-MS 锆石 U-Pb 定年加权平均年龄为(251.9±4.4)Ma,指示东梁岩体形成于早三叠世。地球化学特征表明岩石富硅、铝、钠,贫镁、钾,A /CNK =0.98~1.11,属于钙碱性准铝质-弱过铝质岩石,轻稀土富集,重稀土亏损,不具 Eu 和 Ce 异常,具有高 Sr、低 Y、低 Yb 的特征,属于埃达克岩。结合区域已有研究成果,认为东梁岩体的岩浆可能来源于西伯利亚板块和华北板块碰撞拼合所导致的新生地壳部分熔融。

  19. Primera edad U-Pb en circón usando LA-ICP-MS de un dique traquiandesítico emplazado en el granito tipo-A Los Árboles, Sierras Pampeanas Orientales First age U-Pb zircon LA-ICP-MS from trachyandesitic dyke emplaced in the A-type Los Árboles pluton, Eastern Sierras Pampeanas

    Juan A Dahlquist


    Full Text Available La edad de cristalización de diques subvolcánicos emplazados en plutones graníticos carboníferos, Sierras Pampeanas Orientales, permanece incierta. La primera edad U-Pb en circón usando LA-ICP-MS obtenida en un dique que intruye al plutón Los Árboles (sierra de Fiambalá, revela una edad de cristalización de 311 ± 3 Ma.The crystallization age of subvolcanic dykes emplaced in Carboniferous granitic plutons of the Sierras Pampeanas Orientales remains uncertain. The first age precise U-Pb zircon LA-ICP-MS obtained from a dyke emplaced in the Los Árboles pluton (Sierra de Fiambalá yield a crystallization age of 311 ± 3 Ma.

  20. Zircon U-Pb SHRIMP ages of high-pressure granulite in Yushugou ophiolitic terrane in southern Tianshan and their tectonic implications

    ZHOU Dingwu; SU Li; JIAN Ping; WANG Runsan; LIU Xiaoming; LU Guanxiang; WANG Juli


    Zircons from two high-pressure granulite samples from the Yushugou ophiolitic terrane, southern Tianshan have been investigated by cathodoluminescence, LAICPMS and ion microprobe (SHRIMP) for their internal textures, trace elemental compositions and in situ dating. The weighted mean ages of these two samples are 392±7 and 390±11 Ma, respectively, representing the granulite-facies metamorphic age of the ophiolitic terrane, and indicating that the southern Tianshan ocean initiated its northward subduction since the early Devonian.

  1. Synchronous formation of the metamorphic sole and igneous crust of the Semail ophiolite: New constraints on the tectonic evolution during ophiolite formation from high-precision U-Pb zircon geochronology

    Rioux, Matthew; Garber, Joshua; Bauer, Ann; Bowring, Samuel; Searle, Michael; Kelemen, Peter; Hacker, Bradley


    The Semail (Oman-United Arab Emirates) and other Tethyan-type ophiolites are underlain by a sole consisting of greenschist- to granulite-facies metamorphic rocks. As preserved remnants of the underthrust plate, sole exposures can be used to better understand the formation and obduction of ophiolites. Early models envisioned that the metamorphic sole of the Semail ophiolite formed as a result of thrusting of the hot ophiolite lithosphere over adjacent oceanic crust during initial emplacement; however, calculated pressures from granulite-facies mineral assemblages in the sole suggest the metamorphic rocks formed at >35 km depth, and are too high to be explained by the currently preserved thickness of ophiolite crust and mantle (up to 15-20 km). We have used high-precision U-Pb zircon dating to study the formation and evolution of the metamorphic sole at two well-studied localities. Our previous research and new results show that the ophiolite crust formed from 96.12-95.50 Ma. Our new dates from the Sumeini and Wadi Tayin sole localities indicate peak metamorphism at 96.16 and 94.82 Ma (±0.022 to 0.035 Ma), respectively. The dates from the Sumeini sole locality show for the first time that the metamorphic rocks formed either prior to or during formation of the ophiolite crust, and were later juxtaposed with the base of the ophiolite. These data, combined with existing geochemical constraints, are best explained by formation of the ophiolite in a supra-subduction zone setting, with metamorphism of the sole rocks occurring in a subducted slab. The 1.3 Ma difference between the Wadi Tayin and Sumeini dates indicates that, in contrast to current models, the highest-grade rocks at different sole localities underwent metamorphism, and may have returned up the subduction channel, at different times.

  2. Initial subduction of the Paleo-Pacific Oceanic plate in NE China: Constraints from whole-rock geochemistry and zircon U-Pb and Lu-Hf isotopes of the Khanka Lake granitoids

    Liu, Kai; Zhang, Jinjiang; Wilde, Simon A.; Zhou, Jianbo; Wang, Meng; Ge, Maohui; Wang, Jiamin; Ling, Yiyun


    Northeast China is located in the eastern part of the Central Asian Orogenic Belt (CAOB) and was influenced by Paleo-Pacific subduction during the Mesozoic. Abundant granitoids from the late Paleozoic to early Mesozoic in NE China record this process, including the Khanka Lake granitoids, which resulted in extensive growth of continental crust in the area. However, the question of how and when the Paleo-Pacific tectonic system began to affect NE China is still highly controversial. The Khanka Lake granitoids can be subdivided into two main components based on their geochemical characteristics, namely granodiorite and syenogranite. The granodiorite has a U-Pb age of 249 Ma and is adakite-like (enriched in LREE and LILEs with high Mg#, Sr, La/Yb, Sr/Y and Na2O/K2O), with zircon εHf(t) values of - 0.65 to 1.61, produced by the magma mixing between melting of the lower continental crust and juvenile basaltic magma. The syenogranite has zircon U-Pb ages of 209 to 199 Ma and geochemical features of highly fractionated I-type granites, with high SiO2, total alkalis and low Mg (and Mg#), Fe, Cr and Ni, and positive zircon εHf(t) of 1.72 to 5.12, indicating an origin from remelting of juvenile crust. The granitoids were intruded by felsic veins between 195 and 184 Ma with positive zircon εHf(t) from 0.57 to 5.32. The εHf(t) values of the granitoids become more positive as the zircon U-Pb ages become younger, suggesting continuous melting of juvenile crust during subduction. It is concluded that the Khanka Lake granitoids record the early stage of subduction of the Paleo-Pacific Oceanic plate, which commenced at least ca. 250 Ma ago.

  3. Tectonic evolution of the Irtysh collision belt: New zircon U-Pb ages of deformed and collisional granitoids in the Kalaxiangar area, NW China

    Tao, Hong; Jun, Gao; Xingwang, Xu; Klemd, Reiner


    The CAOB is thought to have formed by multiple accretion and collision of various microcontinents, island arcs, oceanic plateaus and accretionary wedges due to the closure of the Paleo-Asia Ocean [1, 2, 3]. The Irtysh collision belt is located at the middle-western part of the CAOB and generally thought to be the result of the collision of the Sawuer Island arc and the Altay Terrane, subsequent to the consumption of the Early Paleozoic Junggar Ocean, a branch of Paleo-Asia Ocean. Therefore, the exact timing of the Irtysh collision belt is crucial for a better understanding of the tectonic evolution of this collision belt and will provide constraints on the evolution of the CAOB. Recently, we discovered various collisional granitoids in the Kalaxiangar tectonic belt (KTB), which is located in the eastern part of the Irtysh collision belt. In this contribution, we report new geochemical whole-rock, zircon U-Pb and Hf isotopic data of the arc-related and collisional granitoids. Our new results reveal that 1) the arc-related granodioritic porphyries formed at ca. 374 Ma. Furthermore, recrystallized zircons from the granodioritic mylonite and ultramylonite of the Laoshankou ductile deformation zone have a similar U-Pb age of ca. 360 Ma; 2) the syn-collisional granodioritic porphyries, which distribute along cleavege, were emplaced at ca. 355 Ma; 3) the post-collisional A-type granodioritic porphyry, which cuts the NW-NNW trending schistosity at a low angle, has an age of ca. 323 Ma, ɛHf(t) values from + 7.5 to + 14.4, and young Hf model ages between 387 and 658 Ma; 4) the post-collisional A-type granite dykes, which are exposed along strike-slip faults, have ages between 282.5 and 279.2Ma, ɛHf(t) values from + 4.8 to + 12.6, and Hf model ages between 436 and 729 Ma; 5) the A-type biotite granite dykes that intruded along conjugate tension joints have ages between 273.9 and 271.4 Ma, ɛHf(t) values from + 1.1 to + 12.8, and Hf model ages between 393 and 979 Ma. In

  4. SHRIMP U-Pb dating of recurrent Cryogenian and Late Cambrian-Early Ordovician alkalic magmatism in central Idaho: Implications for Rodinian rift tectonics

    Lund, K.; Aleinikoff, J.N.; Evans, K.V.; duBray, E.A.; deWitt, E.H.; Unruh, D.M.


    Composite alkalic plutonic suites and tuffaceous diamictite, although discontinuously exposed across central Idaho in roof pendants and inliers within the Idaho batholith and Challis volcanic-plutonic complex, define the >200-km-long northwest-aligned Big Creek-Beaverhead belt. Sensitive highresolution ion microprobe (SHRIMP) U-Pb zircon dates on these igneous rocks provide direct evidence for the orientation and location of the Neoproterozoic-Paleozoic western Laurentian rift margin in the northern U.S. Cordillera. Dating delimits two discrete magmatic pulses at ca. 665-650 Ma and 500-485 Ma at the western and eastern ends, respectively, of this belt. Together with the nearby 685 Ma volcanic rocks of the Edwardsburg Formation, there is a 200 Ma history of recurrent extensional magmatic pulses along the belt. A similar history of recurrent uplift is reflected in the stratigraphic record of the associated miogeoclinal and cratonal platform basins, suggesting that the Big Creek-Beaverhead belt originated as a border fault during continental rift events. The magmatic belt is paired with the recurrently emergent Lemhi Arch and narrow miogeoclinal facies belts and it lies inboard of a northwest-striking narrow zone of thinned continental crust. These features define a northeast-extending upper-plate extensional system between southeast Washington and southeast Idaho that formed a segment of the Neoproterozoic-Paleozoic miogeocline. This segment was flanked on the north by the St. Mary-Moyie transform zone (south of a narrow southern Canadian upper-plate margin) and on the south by the Snake River transfer zone (north of a broad Great Basin lower-plate margin). These are the central segments of a zigzagshaped Cordilleran rift system of alternating northwest-striking extensional zones offset by northeast-striking transfers and transforms. The data substantiate polyphase rift and continental separation events that included (1) pre-and syn-Windermere rifting, (2) Windermere

  5. Large-scale displacement along the Altyn Tagh Fault (North Tibet) since its Eocene initiation: Insight from detrital zircon U-Pb geochronology and subsurface data

    Cheng, Feng; Jolivet, Marc; Fu, Suotang; Zhang, Changhao; Zhang, Qiquan; Guo, Zhaojie


    Marking the northern boundary of the Tibetan plateau, the Altyn Tagh fault plays a crucial role in accommodating the Cenozoic crustal deformation affecting the plateau. However, its initiation time and amount of offset are still controversial despite being key information for the understanding of Tibet evolution. In this study, we present 1122 single LA-ICP-MS detrital zircon U-Pb ages obtained from 11 Mesozoic to Cenozoic sandstone samples, collected along two sections in the northwestern Qaidam basin (Eboliang and Huatugou). These data are combined with new 3D seismic reflection profiles to demonstrate that: (1) from the Paleocene to early Eocene, the Eboliang section was approximately located near the present position of Anxi, 360 ± 40 km southwest from its current location along the Altyn Tagh fault, and sediments were mainly derived from the Altyn Tagh Range. At the same period, the Huatugou section was approximately located near the present position of Tula, ca. 360 km southwest from its current location along the Altyn Tagh fault, and the Eastern Kunlun Range represented a significant sediment source. (2) Left-lateral strike-slip movement along the Altyn Tagh fault initiated during the early-middle Eocene, resulting in northeastward displacement of the two sections. (3) By early Miocene, the intensive deformation within the Altyn Tagh Range and northwestern Qaidam basin strongly modified the drainage system, preventing the materials derived from the Altyn Tagh Range to reach the Eboliang and the Huatugou sections. The post-Oligocene clastic material in the western Qaidam basin is generally derived from local sources and recycling of the deformed Paleocene to Oligocene strata. From these data, we suggest enhanced tectonic activity within the Altyn Tagh Range and northwestern Qaidam basin since Miocene time, and propose an early-middle Eocene initiation of left-lateral strike-slip faulting leading to a 360 ± 40 km offset along the Altyn Tagh fault.

  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

    Ersoy, E. Yalçın; Akal, Cüneyt; Genç, Ş. Can; Candan, Osman; Palmer, Martin R.; Prelević, Dejan; Uysal, İbrahim; Mertz-Kraus, Regina


    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. Early Paleozoic intracontinental orogeny in the Yunkai domain, South China Block: New insights from field observations, zircon U-Pb geochronological and geochemical investigations

    Yan, Chaolei; Shu, Liangshu; Michel, Faure; Chen, Yan; Li, Cheng


    Debate on whether the Early Paleozoic tectono-magmatic event in South China is related to a subduction-collision or an intracontinental orogen has been lasted for decades within the geoscience community. This study deals with LA-ICP-MS zircon U-Pb ages, whole-rock chemistry, rare earth elements, trace elements and Hf isotopes from granitoid samples collected in the Yunkai domain in order to better constrain the Early Paleozoic tectonic evolution of the South China Block. The weighted mean 206Pb/238U ages for eight samples range from 426 Ma to 443 Ma, representing the crystallization ages of the magma. Fourteen samples were analyzed for geochemistry, all of which are characterized by a peraluminous signature with A/CNK values greater than 1.0. The REE geochemistry reveals enrichment in light rare earth element. LREE/HREE values range from 2.81 to 30.36 and (La/Yb)N vary from 1.23 to 55.14 (mean of 14 analyses is 14.69). All the samples exhibit distinct negative Ba, Sr and Nb anomalies and enrichment in Rb, Th, U and Pb. Hf isotopic analyses indicate negative εHf (t) values mainly ranging from - 3 to - 12, corresponding to two model age distributing from 1637 Ma to 2208 Ma. The geochemical analyses indicate that the Silurian granitic magmas in the Yunkai domain were derived from partial melting of crustal materials with little or no input of mantle source. These new data support the intracontinental subduction model already proposed to account for the Early Paleozoic tectonic, metamorphic and magmatic event of South China.

  8. Spatial and temporal evolution of Liassic to Paleocene arc activity in southern Peru unraveled by zircon U-Pb and Hf in-situ data on plutonic rocks

    Demouy, Sophie; Paquette, Jean-Louis; de Saint Blanquat, Michel; Benoit, Mathieu; Belousova, Elena A.; O'Reilly, Suzanne Y.; García, Fredy; Tejada, Luis C.; Gallegos, Ricardo; Sempere, Thierry


    Cordilleran-type batholiths are built by prolonged arc activity along active continental margins and provide detailed magmatic records of the subduction system evolution. They complement the stratigraphic record from the associated forearcs and backarcs. We performed in-situ U-Pb geochronology and Hf isotope measurements on zircon grains from a large set of plutonic rocks from the Coastal Batholith in southern Peru. This batholith emplaced into the Precambrian basement and the Mesozoic sedimentary cover. We identify two major periods of voluminous arc activity, during the Jurassic (200-175 Ma) and the Late Cretaceous-Paleocene (90-60 Ma). Jurassic arc magmatism mainly resulted in the emplacement of a dominantly mafic suite with ɛHf values ranging from - 9.5 to + 0.1. Published ages south of the Arequipa area suggest that the arc migrated southwestward out of the study area during the Middle Jurassic. After a magmatic gap of 85 Ma, arc activity abruptly resumed 90 Ma ago in Arequipa. Intrusive bodies emplaced into both basement and older Jurassic intrusions and strata. This activity culminated between 70 and 60 Ma with the emplacement of very large volumes of dominantly quartz-dioritic magmas. This last episode may be considered as a flare-up event, characterized by intense magmatic transfers into the crust and rapid relief creation. The Late Cretaceous-Paleocene initial ɛHf are shifted toward positive values (up to + 3.3 and + 2.6) compared to the Jurassic ones, indicating either a larger input of juvenile magmas, a lesser interaction with the ancient crust, or an increase of re-melting of young mantle-derived mafic lower crust. These magmatic fluxes with juvenile component are coeval with the onset of the crustal thickening at 90 Ma and represent a significant contribution to the formation of the continental crust in this area.

  9. U-Pb zircon geochronology of Paleoproterozoic plutons from the northern midcontinent, USA: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis

    Holm, D.K.; Van Schmus, W. R.; MacNeill, L.C.; Boerboom, Terrence; Schweitzer, D.; Schneider, D.


    We propose that the late Paleoproterozoic igneous and deformational history preserved in the northern midcontinent United States can be explained by a change in subduction-polarity from geon 18 south-dipping subduction during Penokean accretion to geon 17 north-dipping subduction as convergence continued after Penokean orogenesis. New U-Pb zircon ages indicate that late to post-Penokean magmatism occurred at ca. 1800, 1775, and 1750 Ma and generally migrated southeastward across the newly accreted Penokean terrane. We suggest that geon 17 Yavapai slab rollback caused continental arc magmatism to step southeastward between 1800 and 1750 Ma. As the slab steepened, reduced compressional stresses and magma-induced thermal weakening allowed for collapse of the overthickened portions of the Penokean crust. Postcollapse crustal stabilization (the 1750-1650 Ma Baraboo interval) was followed by geon 16 Mazatzal arc accretion further south. The 1900-1600 Ma tectonic history of the north-central United States, not surprisingly, records events related to the southward growth and tectonic development of the southern Laurentian margin. New and published 40Ar/ 39Ar mineral ages delineate the northern and western extent of geon 16 Mazatzal deformation. Interestingly, only little exhumed crust intruded by a small volume of shallow-level ca. 1750 Ma plutons (and associated rhyolites) was deformed significantly during geon 16. In contrast, more deeply exhumed crust and crust pervasively invaded by a large volume of post-Penokean magma (i.e., East-Central Minnesota Batholith) were largely unaffected by Mazatzal deformation and reheating. We suggest that posttectonic intrusions and crustal thinning were an important step in strengthening and stabilizing the crust in the southern Lake Superior region. ?? 2005 Geological Society of America.

  10. Geochemistry and zircon U-Pb geochronology of granitoids in the East Kunlun Orogenic Belt, northern Tibetan Plateau: origin and tectonic implications

    Zhou, Bo; Dong, Yunpeng; Zhang, Feifei; Yang, Zhao; Sun, Shengsi; He, Dengfeng


    The East Kunlun Orogenic Belt (EKOB) in the northern margin of the Tibet Plateau is characterized by widespread granitic plutons, which are keys to understanding the tectonic evolution of the EKOB. The Zhiyu pluton, newly recognized in the central part of the EKOB, mainly consists of monzogranites, biotite granites and quartz diorites. Their LA-ICPMS zircon U-Pb results show formation ages of 447 ± 1.6 Ma, 448 ± 2.5 Ma and 408 ± 1.8 Ma. The monzogranites and biotite granites are characterized by relatively high Sr (208-631 ppm), low Y (4.28-15.82 ppm) and Yb (0.44-1.59 ppm) contents, thus resulting in elevated Sr/Y (30-105) and (La/Yb)N (4-79) ratios, indicating geochemical features of adakitic rocks. These adakitic granites are medium- to high-K, calcic or calc-alkaline in composition, and display a weak peraluminous character. They have low MgO (0.57-1.84 wt.%, average 1.01 wt.%), Mg# (40-53, average 45), as well as low Cr (3.67-17.98 ppm, average 7.19 ppm) and Ni (2.59-9.30 ppm, average 4.71 ppm) contents. These rocks are enriched in LREE, and show negligible or variable positive Eu anomalies (Eu/Eu∗ = 0.61-3.80, average 1.45) and significant negative Nb and Ta anomalies. Majority of the zircon grains from these adakitic granitic rocks have positive εHf(t) values of 0.09-5.21 with two-stage model ages ranging from 1.1 Ga to 1.6 Ga. These features are compatible with those of adakitic rocks derived from a thickened lower crust in the garnet stability field. Their formation is mainly controlled by the process of crust thickening following the closure of the Qimantag Ocean. The younger quartz diorites belong to medium- to high-K, calc-alkalic or alkali-calcic and metaluminous series, and exhibit a relatively high MgO (2.23-5.18 wt.%) and Mg# (40-56, average 50.11), with significant LREE enrichment and negative Eu anomalies, as well as depletion of Nb, Ta. In addition, the quartz diorites have an enriched εHf(t) values ranging from -5.25 to -3.19. Combining

  11. An in situ zircon Hf isotopic,U-Pb age and trace element study of banded granulite xenolith from Hannuoba basalt:Tracking the early evolution of the lower crust in the North China craton

    ZHENG Jianping; LU Fengxiang; YU Chunmei; TANG Huayun


    Backscattered electron images, in situ Hf isotopes, U-Pb ages and trace elements of zircons in a banded granulite xenolith from Hannuoba basalt have been studied. The results show that the banded granulite is a sample derived from the early lower crust of the North China craton. It is difficult to explain the petrogenesis of the xenolith with a single process. Abundant information on several processes, however, is contained in the granulite. These processes include the addition of mantle material, crustal remelting, metamorphic differentiation and the delamination of early lower crust. About 80% of zircons studied yield ages of 1842±40 Ma, except few ages of 3097-2824 Ma and 2489-2447 Ma. The zircons with ages older than 2447 Ma have high εHf (up to +18.3) and high Hf model age (2.5-2.6 Ga), indicating that the primitive materials of the granulite were derived mainly from a depleted mantle source in late Archean. Most εHf of the zircons with early Proterozoic U-Pb age vary around zero, but two have high εHf up to +9.2-+10.2, indicating mantle contribution during the collision and assembly between the Eastern and Western blocks in the early Proterozoic that resulted in the amalgamation of the North China craton.

  12. Paleoproterozoic tectonic transition from collision to extension in the eastern Cathaysia Block, South China: Evidence from geochemistry, zircon U-Pb geochronology and Nd-Hf isotopes of a granite-charnockite suite in southwestern Zhejiang

    Zhao, Lei; Zhou, Xiwen; Zhai, Mingguo; Santosh, M.; Ma, Xudong; Shan, Houxiang; Cui, Xiahong


    The Badu complex and associated Paleoproterozoic granitoids are among the oldest known rocks in the Cathaysia Block in South China. The Paleoproterozoic units of the Badu complex are dominantly composed of metapelitic rocks and meta-greywackes. Here we report LA-SS-ICP-MS (laser ablation-split-stream inductively coupled plasma-mass spectrometry) zircon U-Pb data from a newly discovered garnet-bearing granite which show an emplacement age of 1929 ± 15 Ma and metamorphism at 1872 ± 34 Ma. We also report U-Pb ages of 1886 ± 16 Ma, 1858 ± 7 Ma, 1848 ± 11 Ma from a gneissic granodiorite, and two charnockites respectively. The garnet-bearing granite is peraluminous with A/CNK range from 1.1 to 1.3. The rock shows relatively high SiO2, K2O and Rb contents, and low total REE, Sr, CaO and ferromagnesian components, typical of leucogranites. The whole rock Nd two-stage model age(TDM2(Nd)) of this rock is ca. 2.7 Ga, zircon Hf crustal model ages(TDMC(Hf)) peak at about 2.7 Ga, and abundant inherited zircons occur with U-Pb ages in the range of 2044 to 2803 Ma. Evidences from zircon U-Pb age and Hf isotope compositions, whole rock Nd isotopes and whole rock major and trace elements suggest a metasedimentary protolith, and that the garnet-bearing granite (leucogranite) was derived by partial melting in a thickened crust at about 1.93 Ga. The gneissic granodiorite and charnockites show indistinguishable major and trace element features, as well as zircon Hf and whole rock Nd isotope compositions, indicating that they were generated from the same source rocks. The gneissic granodiorite and charnockites are ferroan, calc-alkalic and metaluminous with A/CNK range from 0.80 to 0.98. They display relatively low SiO2 contents and Ga/Al ratios, suggesting their A-type affinity. Their zircon TDMC(Hf) age-peak is 2.9 Ga and whole rock TDM2(Nd) ages range from 2.8 to 2.9 Ga. These high temperature rocks were generated possibly through the partial melting of ancient amphibolites (2

  13. Ductile-brittle deformation effects on crystal-chemistry and U-Pb ages of magmatic and metasomatic zircons from a dyke of the Finero Mafic Complex (Ivrea-Verbano Zone, Italian Alps)

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


    A detailed microstructural, geochemical and geochronological study was performed on zircon grains from plagioclase-rich dioritic dykes discordantly intruded within meta-diorites/gabbros forming the External Gabbro unit of the Finero Mafic Complex (Italian Alps). This unit is 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 dykes consist mainly of plagioclase (An18-24Ab79-82Or0.3-0.7) with subordinate amounts of biotite and spinel defining melanocratic layers. Zircon and corundum are common accessory phases. Both the dykes and the surrounding meta-diorites/gabbros show evidence of ductile deformation under amphibolite-facies conditions. Zircon grains/fragments (up to 2 mm in length) occur as porphyroclasts surrounded by fine-grained plagioclase within the leucocratic domains and may occur within the melanocratic layers completely or partially surrounded by biotite and spinels. Fractures are common within zircon, define networks and have associated displacements occasionally and/or they can be filled by secondary minerals. Cathodoluminescence (CL) images reveal that zircon grains from the leucocratic layers show relicts of primary magmatic (i.e. oscillatory and or sector) zoning generally related with the crystal shapes or crystallographic orientation, whereas those from the melanocratic domains do not. In both cases, zircon shows secondary CL features, i.e. mosaic-like textures, due to deformation. EBSD maps confirmed a profuse mosaic texture, resulting in an internal misorientation of ca. 10°, generally associated with fractures. Locally, zircon shows clear evidence of crystal-plastic deformation at the edges, with a gradual misorientations of up to 12°, suggesting an origin prior fragmentation. Trace elements and U-Pb analyses were carried out by LA-ICP-MS directly on petrographic thin sections. Such in situ investigations revealed a strong