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    Zircon SHRIMP U-Pb ages and in-situ Hf isotopic analysis for the Mesozoic intrusions in South Taihang, North China craton Evidence for hybridizati on between mantle-derived magmas and crustal components [查看] B.Chen、W.Tian、B.M.Jahn、Z.C.Chen
    The North China craton (NCC) was stabilized in Paleo-Proterozoic times, and the eastern part of it was remobilized in the Mesozoic, as is indicated by the presence of voluminous Mesozoic magmas in eastern NCC. In particular, the lithospheric mantle beneath eastern NCC was significantly thinned during the intense Mesozoic tectono-magmatic activity, which makes the NCC different from most other ancient cratons. The Mesozoic magmas may thus provide crucial evidence regarding the processes that transformed the NCC from craton to a tectonically active region.The South Taihang magmatic complex lies in the western part of the Mesozoic magmatic zone in the NCC. The complex falls into three main plutonic rocks: the Hongshan syenite, the Xishu gabbroic diorite–monzonite and theWu'an monzonitic rocks. Zircon U-Pb dating reveals that the three plutonic rocks were emplaced contemporaneously at ∼132 Ma. In-situ Hf isotopic analyses for the dated zircons indicate that the Hongshan syenites show homogeneous ɛHf(t) values (−10 to −13)which are slightly lower than those (−9.0) for the enriched subcontinental lithosphericmantle (SCLM), as revealed by zircons frommafic rocks that originated frommelting of SCLM.This suggests that the parental magma to the syenites was likely derived from melting of the enriched SCLMand contaminated slightly by old lower continental crustal material during ascent. The Xishu and Wu'an gabbroic diorites to monzonitic rocks, however, show significantly varied Hf isotopic compositions, with ɛHf(t)=–9.2 to –16 and –15.5 to −24, respectively. The wide range of ɛHf(t) values for a single sample suggests that the Xishu and Wu'an plutons formed through a process of hybridization between enriched mantlederivedmafic magma and crustal components in variable proportions. This is supported by the textural and compositional disequilibrium of plagioclase phenocrysts fromthe two plutons. TheWu'an monzonitic rocks showlower ɛHf(t) values than the Xishu gabbroic diorite-monzonite, suggesting more incorporation of crustal components in the source. The sudden surge of magmatic activity in the South Taihang orogen took place as a result of lithospheric extension that caused partial fusion of the enriched portions of SCLM generating voluminous mafic magmas. Underplating of the mafic magmas in the lower crust consequently caused partial melting of the old basement rocks to produce granitic melts, which was followed by mixing between the mafic and granitic magmas.
    THE SIGNIFICANCE OF PALEOPROTEROZOIC ZIRCON IN CARBONATITE DIKES ASSOCIATED WITH THE BAYAN OBO REE-Nb-Fe [查看] YULONGLIU、IANS.WILLIAMS、JIANGFENGCHEN、YUSHENGWAN、WEIDONGSUN
    Sensitive high resolution secondary ion mass spectrometry (SHRIMP II), laser ablation inductively coupled plasma mass spectrometry (ICP-MS) and laser Raman spectroscopy have been used to determine the ages in zircons from carbonatite dikes associated with the Bayan Obo giant REE-Nb-Fe deposit, Inner Mongolia, China.Analyses of small amounts of zircon extracted from large samples of three carbonatite dikes yield late Palaeoproterozoic 207Pb/206Pb ages of 1894±27, 1944 ±20 and 1956±9 Ma. One sample also contained inherited grains with ages up to ~2.55 Ga.Zircon grains, even from a single rock, have a wide range of REE patterns, suggesting that they are probably inherited zircons trapped during magma genesis, not zircons crystallized from a single magma. None of the zircon grains has the extremely high Th/U characteristic of the Bayan Obo ore deposit or of the associated carbonatites. Further, mineral inclusions in the zircon identified by Raman spectroscopy are all silicate minerals (quartz and feldspars), consistent with crystallization of the zircon from silicate, not from carbonate, magmas. The Palaeoproterozoic zircons in the Bayan Obo carbonatite dikes did not crystallize from carbonatite magma at the time of dike emplacement, but were derived from wall rock contamination. The ages obtained from the zircons are consistent with ages measured on basement igneous rocks elsewhere in the Bayan Obo region.
    U-Pb SHRIMP zircon geochronology and T-t-d history of the Kampa Dome, southern Tibet [查看] M.C.Quigley、Y.Liangjun、Gregory、Corvino、M.Sandiford、C.J.L.Wilson、L.Xiaohan
    Structural, petrographic and geochronologic studies of the Kampa Dome provide insights into the tectonothermal evolution of orogenic crust exposed in the North Himalayan gneiss domes of southern Tibet. U–Pb ion microprobe dating of zircons from granite gneiss exposed at the deepest levels within the dome yields concordia 206Pb/238U age populations of 506±3 Ma and 527±6 Ma, with no evidence of new zircon growth during Himalayan orogenesis. However, the granite contains penetrative deformation fabrics that are also preserved in the overlying Paleozoic strata, implying that the Kampa granite is a Cambrian pluton that was strongly deformed and metamorphosed during Himalayan orogenesis.Zircons from deformed leucogranite sills that cross-cut Paleozoic metasedimentary rocks yield concordant Cambrian ages from oscillatory zoned cores and discordant ages ranging from ca. 491–32 Ma in metamict grains. Since these leucogranites clearly post-date the metasedimentary rocks they intrude, the zircons are interpreted as xenocrysts that are probably derived from the Kampa granite. The Kampa Dome formed via a series of progressive orogenic events including regional ~N–S contraction and related crustal thickening (D1), predominately top-to-N ductile shearing and crustal extension (D2), top-to-N brittle–ductile faulting and related folding on the north limb of the dome, localized top-to-S faulting on the southern limb of the dome, and crustal doming (D3), and continued N–S contraction, E–W extension and doming (D4). Structural and geochronologic variability amongst adjacent North Himalayan gneiss domes may reflect changes in the magnitude of crustal exhumation along the North Himalayan antiform, possibly relating to differences in the mid-crustal geometry of the exhuming fault systems.
    The oldest known rocks in south-western China SHRIMP U-Pb magmatic crystallisation age and detrital provenance [查看] MatthewR.Greentree、Zheng-XiangLi
    The Paleoproterozoic Dahongshan Group outcrops along the present day southwest margin of the Yangtze Craton. It is the oldest rock found in the region and forms the basement for the late Mesoproterozoic sedimentary successions. The Dahongshan Group is a sequence of volcanic and sedimentary rocks which have undergone structural deformation and metamorphism to upper greenschist–lower amphibolite facies. Some of the original sedimentary features, such as unimodal cross-bedding in quartzite and rounded conglomerate clasts, are still preserved, suggesting deposition in a fluvial environment. Abundant carbonate rocks, also found within the sedimentary sequence, indicate periods of shallow marine or lagoonal conditions. A weighted mean SHRIMP 206Pb/207Pb age of 1675 ± 8 Ma for a tuffaceous schist unit confirms the Paleoproterozoic formation age of the group. SHRIMP and LAM ICP-MS U–Pb analysis of detrital zircon grains from metasedimentary units indicate a late Archean to Paleoproterozoic (ca. 2780–1860 Ma) source region. The age spectrum of the detrital zircons is similar to that of the late Archean and early Paleoproterozoic basement rocks previously identified in the Yangtze Craton, but includes a population of Paleoproterozoic grains (ca. 2400–2100 Ma) which have no known source region in the exposed Yangtze Craton.
    SRHIMP U-Pb Zircon Dating of the Tula Granite Pluton on the South Side of the Altun Fault and Its Geological Implications [查看] WUSuoping、WUCailai、WANGMeiying、CHENQilong、JosephL.WOODEN
    The Tula A2-sutbtype 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±8.1Ma，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.
    SHRIMP zircon U–Pb geochronology, elemental, and Nd isotopic geochemistry of the Neoproterozoic mafic dykes in the Yanbian area, SW China [查看] Wei-GuangZhua、HongZhong、Xian-HuaLi、Hai-LinDeng、De-FengHe、Kong-WenWu、Zhong-JieBai
    Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb zircon ages, geochemical and Nd isotopic data are reported for two groups of Neoproterozoic mafic dykes in the Yanbian area of Sichuan Province, SW China. The Group 1 and Group 2 dykes were dated at 792±13Ma and 761±14 Ma, respectively. While Group 1 and Group 2 rocks are commonly high in Fe2O3 and TiO2 contents with similar MgO contents and Mg numbers, they exhibit distinct trace element features. Group 1 rocks with high εNd(T) values (+5.8 to +7.2) and total rare earth element (REE) contents (109–171 ppm), are characterized by LREE-enriched and “humped” trace-element patterns with slightly negative Nb–Ta anomaly, similar to typical alkaline basalts associated with continental rifts.Group 2 rocks have relatively lowtotal REE contents (50.4–103 ppm) with relatively flat REE patterns and can be further divided into two sub-types. Group 2a rocks showMORB-like trace element patterns depleted in highly incompatible elements such as Th, Nb, Ta, La and Ce, whereas the Group 2b rocks exhibit flatter REE patterns than those of the Group 2a. The εNd(T) values of the Group 2a and Group 2b rocks are +5.4 to +7.2 and +4.5 to +5.3, respectively.The Group 1 and Group 2 dykes display different magma crystallization trends. Group 1 rocks were crystallized from a high-Ti alkaline basaltic magma that was generated by low degrees of partial melting of OIB-like mantle source in the garnet-bearing field. In contrast, the parental magma of Group 2 rocks is similar to those parental to low-Ti tholeiitic basaltic melt generated by relatively higher degrees of partial melting of the asthenospheric mantle source in the spinel-stable field. Group 1 and Group 2a rocks show the intraplate basalts affinity. It is therefore suggested that they originated from an intra-continental rift regime, which was likely related to a mantle plume underneath the supercontinent Rodinia.
    SHRIMP zircon U-Pb dating from K-bentonite in the top of Ordovician of Wangjiawan Section, Yichang, Hubei, China [查看] HUYanHua、ZHOUJiBin、SONGBiao、LIWei1、SUNWeiDong
    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.
    SHRIMP Zircon U-Pb and Molybdenite Re-Os Datings of the Superlarge Donggou Porphyry Molybdenum Deposit in the East Qinling,China,and Its Geological Implications [查看] YEHuishou、MAOJIngwen、LIYongfeng、GuoBaojian、ZHANGChangqing、LIUJun、YANQuanren、LIUGuoyin
    Located in the eastern part of the East Qinling molybdenum belt,the Donggou deposit is a superlarge porphyry molybdenum deposit discovered in recent years.The authors performed highly precise dating of the mineralized porphyry and ores in the Donggou molybdenum deposit.A SHRIMP U-Pb zircon dating of the Donggou aluminous A-type granite-porphyry gave a rock-forming age of 112±1Ma,and the ICP-MS Re-Os analyses of molybdenite from the molybdenum deposit yielded Re-Os model ages ranging from 116.5±1.7to 115.5±1.7Ma for the deposit.The ages obtained by the two methods are quite close,suggesting that the rocks and ores formed approximately at the same time.The Donggou molybdenum deposit formed at least 20Ma later than the jinduicheng,Nanihu,Shangfanggou and Leimengou porphyry molybdenum deposits in the same molybdenum belt,implying that these deposits were formed in different tectonic settings.
    SHRIMP Zircon Age and Geochemical Constraints on the Origin of Lower Jurassic Volcanic Rocks from the Yeba Formation, Southern Gangdese, South Tibet [查看] DI-CHENGZHU、GUI-TANGPAN、SUN-LINCHUNG、ZHONG-LILIAO、LI-QUANWANG、ANDGUANG-MINGLI
    We present SHRIMP zircon dating, bulk-rock geochemical, and Sr-Nd-Pb isotopic results for Yeba volcanic rocks and a mafic dike from Southern Gangdese (SG), southern Tibet, in order to constrain their tectonic setting and origin. Yeba volcanic rocks span a continuous compositional range from basalt to dacite, although andesites are minor, and mafic and felsic rocks are volumetrically predominant. New SHRIMP zircon dating for a dacite coupled with previous SHRIMP zircon dating for a mafic dike and fossil constraints for the sedimentary sequence indicate that Yeba volcanic rocks were emplaced in the Early Jurassic (174–190 Ma). Yeba tholeiitic mafic rocks possess compositional diversity and are divided into three groups based on concentrations of MgO, Al2O3, and La. Mafic samples are all characterized by marked negative Nb, Ta, and Ti anomalies and positive εNd(T) values (+ 2.4 to + 4.5). Yeba calc-alkaline felsic rocks are characterized by coherent,concave-upward MREE patterns and negative anomalies in Nb, Ta, P, and Ti, with positive εNd(T) values (+ 0.3 to + 2.6). Sr-Nd-Pb isotopes overlap among the different groups of Yeba mafic rocks;Pb isotopic compositions in both mafic and felsic rocks are nearly identical. These features are consistent with a subduction-related origin, most likely in an arc built on thin, immature continental crust. Yeba volcanic rocks are interpreted as having been created by northward subduction of Neo-Tethyan oceanic crust in Early Jurassic time. Geochemical signatures and quantitative modeling indicate that fractional crystallization and crustal assimilation played insignificant roles in the generation of Yeba mafic magmas, and that their geochemical diversity was probably produced by variable degrees of partial melting from a common but heterogeneous mantle source, which had been metasomatized by variable contributions of sediments/fluids released from the subducted Neo-Tethyan oceanic crust. Yeba felsic rocks were probably generated by moderate degrees of partial melting of juvenile basaltic lower crust, which consists of dominant underplated magmas (similar to Yeba mafic rocks in composition) and variable contributions from ancient lower crust beneath the Gangdese Back-Arc fault uplift belt (GBAFUB).
    SHRIMP U-Pb zircon geochronology of Neoproterozoic Korla mafic dykes in the northern Tarim Block,NW China implications for the long-lasting breakup process of Rodinia [查看] WENBINZHU、ZHIYONGZHANG、LIANGSHUSHU、HUAFULU、JINBAOSU、WEIYANG
    Mafic dykes are observed in the Korla region along the northern Tarim Block, NW China.Our sensitive high-resolution ion microprobe U–Pb zircon ages, the first determined for these dykes, indicate that the mafic dykes were mainly formed at 650–630 Ma, and thus document the youngest known igneous activity associated with rifting in the Tarim Block during the Neoproterozoic. Combined with previous geochronological data, at least three pulses of magmatic activity, from c. 830 to 800 Ma, from c. 790 to 740 Ma and from c. 650 to 630 Ma, are recognized, which reveal that multiple episodes of rifting occurred within the Tarim Block, implying that the breakup of the Rodinia supercontinent in the Tarim Block may have been a long-lasting process.
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