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    Zircon U-Pb SHRIMP ages of the Taiping(calc-alkaline)-Huangshan (alkaline) composite intrusionConstraints on Mesozoic lithospheric thinning of the southeastern Yangtze Craton,China [查看] XUEHuaiMin、WANGYingGeng、MAFang、WANGCheng、WANDeEn、ZUOYanLong
    The Taiping-Huangshan composite intrusion is a unique complex with characteristics changing from calc-alkaline (Taiping intrusion) to alkaline (Huangshan intrusion). Huangshan intrusion samples show a spectacular tetrad effect in their REE distribution patterns as well as non-CHARAC (charge-andradius- controlled) trace element behavior, indicating a highly evolved late-stage magma component. This composite intrusion provides a rare opportunity to investigate the variance of tectonic setting and lithospheric thinning of the southeastern Yangtze Craton in late Mesozoic era. Zircon SHRIMP U-Pb analyses yield an emplacement age of 140.6±1.2 Ma for the Taiping intrusion, and ages of 127.7±1.3, 125.7±1.4, 125.1±1.5, and 125.2±5.5 Ma for four samples from the Huangshan intrusion respectively. The ages for four different phases of the Huangshan intrusion agree within their small analytical errors,indicating that the emplacement was in a short time. The Taiping and Huangshan intrusions are intimately associated, but there is about 15 Ma interval between their intrusion, and the magma characters change from calc-alkaline to alkaline without transition. This probably corresponds to lithospheric thinning of the southeastern Yangtze Craton. This event possibly happened from about 141 Ma (the emplacement age of the Taiping intrusion), to 128 Ma (start of emplacement of the Huangshan intrusion).The thinning mechanism is dominantly delamination.
    SHRIMP zircon U-Pb ages of eclogite and orthogneiss from Sulu ultrahigh-pressure zone in Yangkou area, eastern China [查看] A.Katsube、Y.Hayasaka、M.Santosh、SanzhongLi、K.Terada
    We report SHRIMP U–Pb age of zircons in four samples of eclogite and one sample of orthogneiss from Sulu ultrahigh-pressure (UHP) zone in Yangkou area, eastern China. UHP rocks are distributed along the Sulu orogenic belt suturing North China Block with South China Block. In Yangkou area, UHP unit is well exposed for about 200 m along Yangkou beach section and consists mainly of blocks or lenses of ultramafic rocks and eclogite together with para- and orthogneiss which are highly sheared partly. Zircon grains examined in this study from eclogite show oscillatory zoning and overgrowth texture in CL images, and most of the grains have high Th/U ratio ranging from 0.8 to 2.1 indicating an igneous origin. The weighted mean 206Pb/238U ages of zircons from the four samples range from 690 to 734 Ma. These ages can be correlated to the magmatic stage of the protoliths. In rare cases, zircon grains possess a narrow rim with very low Th/U ratio (b0.02). EPMA U–Th total Pb dating of such rim yields younger ages that range from 240 to 405 Ma marking the metamorphic stage.On the other hand, zircons from the orthogneiss show irregular shape and zoning with inclusion-rich core and inclusion-free rim. These grains of zircon yield U–Pb discordia intercept ages of 226±63 Ma and 714±110 Ma (MSWD 0.78). Bulk of the areas of the rims rim of the zircons demonstrate younger 206Pb/238U ages close to the upper intercept, with low Th/U ratio (b0.20) indicating their metamorphic origin. In contrast, the cores show older 206Pb/238U ages close to lower intercept and high Th/U ratio of (0.14–5.25) indicating their igneous origin. The upper intercept age is also commonly noted in zircons from eclogite. Our results suggest a bimodal igneous activity along this zone during the Neoproterozoic, probably related to the rifting of the Rodinia supercontinent.
    SHRIMP zircon U-Pb age constraints on Neoproterozoic Quruqtagh diamictites in NW China [查看] BeiXua、ShuhaiXiao、HaiboZou、YanChen、Zheng-XiangLi、BiaoSong、DunyiLiu、ChuanmingZhoug、XunlaiYuang
    The Neoproterozoic Quruqtagh Group in the Tarim Block, NW China, contains multiple diamictites in the Bayisi, Altungol, Tereeken, and Hankalchough formations. These diamictites may represent three or possibly four discrete glaciations, although evidence for a glacial origin of the Bayisi and Altungol diamictite is ambiguous. To constrain their age and duration, we dated three volcanic beds (V1, V2, and V3) in the Quruqtagh Group using the SHRIMP (sensitive high-resolution ion microprobe) zircon U–Pb method.Volcanic bed V1 near the base of the Bayisi diamictite yields a 740±7Ma age, volcanic bed V2 near the top of the Bayisi Formation gives a 725±10Ma age, and volcanic bed V3 between the Tereeken and Hankalchough diamictites yields a 615±6Ma age. V1 and V2 have overlapping ages, and together these dates suggest that the Bayisi diamictite was deposited at around 730 Ma. The Tereeken and Altungol diamictites were deposited between 725±10Ma and 615±6 Ma, and the Hankalchough diamictite between 615±6Ma and ∼542Ma (i.e., the Neoproterozoic–Cambrian transition). These dates and previously published chemostratigraphic data are consistent with (but doe not require) the correlation of the Tereeken and Hankalchough diamictites with the 635Ma Nantuo and 582Ma Gaskiers glaciations, respectively.However, the new dates are inconsistent with a single and globally synchronous Sturtian glaciation that occurred in the pre-Nantuo Neoproterozoic Era. Instead, currently available data necessitate that either multiple glaciations occurred, or a globally diachronous glacial event developed during a protracted period between ∼750Ma and ∼650 Ma.
    Reclassification of the Meso- and NeoproterozoicChronostratigraphy of North China by SHRIMP Zircon Ages [查看] GAOLinzhi、ZHANGChuanheng、LIUPengju、TANGFeng、SONGBiao、DINGXiaozhong
    High-quality zircon U-Pb ages acquried from Meso-and Neoproterozoic strata in North China in recent years has provided a hig-resolution chronostratigraphic famework for dating.A basis of this hig-level chronostratigraphic system provides the foundation for a global Precambrian study and stratigraphic correlation and so recent geological studies have focused attention on systemic SHRIMP zircon dating.A chronology of Meso-and Neoproterozoic strata and the time of origin of the overlying Changcheng System is given on the basis of new SHRIMP zircon dating from the Qianxi Complex and diabase of the Chuanlinggou Formation.A new tectonostratigraphy for a Neoproterozoic chronostratigraphic framework in the southeastern margin of the North CHina continent is underpinned by the new SHRIMP zircon dating of Neoproterozoic mafic magma diabase in the Jiao-LIao-Xu-Huai Sub-Province.
    Mesozoic Multi-phase Magmatism and Gold Mineralization in the Early Precambrian North China Craton, Eastern Hebei Province, China SHRIMP Zircon U-Pb Evidence [查看] LAICEHNGMIAO、YUNMIANQIU、WEIMINGFAN、FUQINZHANG
    The early Precambrian North China craton(NCC) in eastern Hebei Province(also known as the Jidong area) was intruded by granitic batholiths and plutons spatially associated with gold deposits.No consensus has been reached regarding timing and tectonic setting of the gold deposits,chiefly due to the lack of reliable geochronological data.The gold deposits in the district are localized by NE-striking faults within granite plutons and/or nearby Archean amphibolites,as well as in Proterozoic sedimentary rocks.Gold mineralizations in the area are characterized by quartz(±albite)vein-and sulfide-disseminated styles:both types of ores have relatively low sulfide contents(＜10 vol%) and similar sulfide mineral assemblages dominated by pyrite+chalcopryrite+pyrrhotite+galena+sphalerite.locally with molybdenite,tellurides,and bismuthinoids.The alterations around the gold lodes include K-feldspathization,sericitization,silicification,chloritization,and sulfidation,Previous fluid inclusion data from all the deposits of the district show that the mineralizing fluids are characterized by relatively high salinities(3 to 17wt% NaCl equiv.)H2O-CO2±CH4，N2 solutions,with CO2 contents in the inclusions ranging from 5 to 40 mol%.Fluid inclusion homogenization temperatures are between 240 and 400℃,and estimates of the trapping presures vary significantly from 0.5 to 3.7Kba.Stable isotope(O,H,S,C,and Pb)data from these deposits indicate a major magmatic component in the mineralizing fluids and the ore-forming materials,with a partial contribution by Archean host rocks,suggesting that these deposits are basically intrusion-related. SHRIMP zircon U-Pb geochronology of the gold-hosting granitic intrusions,in combination with previous Ar-Ar and Rb-Sr dates on hydrothermal minerals)e.g.sericite),indicates that there was no Archean gold mineralization,but instead suggests that at least three episodes of granitic magmatism and associated gold mineralization took place druing the Mesozoic,The first episode occurred in the Late Triassic at ~222Ma,with emplacement of the Dushan granite batholith(223±2Ma).Aanjia granite porphyry(222±4Ma),and the Baizhangzi granite(222±3Ma）.This episode of magmatism and gold mineralization was coeval with collision of the NCC with the South Mongolian block in the north along the Solonker suture and with the Yangtze craton in the south along the Dabie-Sulu suture.The second episode took place in the Early Jurassic,with emplacement of the Qingshankou granite(199±2)Ma,and the third episode occurred in the Middle Jurassic,with intrusion of the Yuerya(~175Ma)and Niuxinshan(172±2）Ma and granites. These two events are about 25 and 45 Ma later than the coliision of the NCC with other continental blocks in a post-collisional environment.
    Late Jurassic-Early Cretaceous Plutonism in the Northern Part of the Precambrian North China Craton： SHRIMP Zircon U-Pb Dating of Diorites and Granites from the Yunmengshan Geopark, Beijing [查看] SHIYuruo、ZHAOXitao、MAYinsheng、HUDaogong、LIUQisheng、WUZhenhan、ZHAOYuanyi、LIUDunyi
    Abstract：The Yunmengshan Geopark in northern Beijing is located within the Yanshan range．It contains the Yunm engshan batholith．which is dominated by two plutons：the Yunmengshan gneissic granite and the Shicheng gneissic diorite．Four samples of the Yunmengshan gneissic granite give SHRIMP zircon U—Pb ages from 145 to 141 Ma。whereas four samples of the Shicheng gneissic diorite have ages from 159 Ma to 151 M a．Dikes that cut the Yunmengshan diorite record SHRIMP zircon U-Pb are of 162±2 and 156+4 Ma．The cumulative plots of zircons from the diorites show a peak age of 155 Ma．without inherited zircon cores．and the peak age of 142 Ma for granite is interpreted as the emplacement age of the Yunm engshan granitic pluton，whose igneous zircons contain inherited zircon cores．The data presented here show that there were two pulses of m agm atism ：early diorites。followed c13 Ma later by true granites．which incorporated material from an older continental crust．
    Combined U-Pb, hafnium and oxygen isotope analysis of zircons from meta-igneous rocks in the southern North China Craton reveal multiple events in the Late Mesoarchean-Early Neoarchean [查看] DunyiLiu、SimonA.Wilde、YushengWan、ShiyanWang、JohnW.Valley、NorikoKita、ChunyanDong、HangqiangXie、ChangxiuYang、YixinZhang、LinzhiGao
    We report the results of a comprehensive isotopic investigation of zircons from metamorphosed tonalites and amphibolites from Lushan in the far south of the Trans-North China Orogen of the North China Craton that reveal the oldest rocks yet dated from the orogen. A detailed investigation of the internal structures of zircons, combined with targeted SIMS U–Pb and oxygen isotope analyses and ICP-MS Hf zircon analyses, has resulted in the discovery of previously unknown geological events in the North China Craton. Magmatic zircon from the tonalite and amphibolite was found to be essentially coeval with a small spread of ages from 2829±18Ma to 2832±11 Ma in the tonalites and from2838±35 Ma to 2845±23Ma in amphibolite. Hafnium isotope data suggest derivation of both tonalite and amphibolite from a depleted mantle source with minor crustal contamination. Oxygen isotope ratios in zircons range from 5.1 to 7.3‰ VSMOW, with high δ18O results recorded only from metamorphic domains. The earliest stage of metamorphic zircon growth, present in all four samples analyzed in this study, occurred between 2772±17/22 in the tonalites and 2776±20 and 2792±12 Ma in the amphibolites. A younger metamorphic event at 2638±61 in the tonalite and between 2671±13 and 2651±13 Ma in the amphibolites is recognized in zircons from three of the four samples investigated. This is the first time that metamorphic events at either of these times have been recorded in the North China Craton. Importantly, there is no evidence for the 1.8 Ga high-grade metamorphic event that is manifest in the northern and central parts of the Trans-North China Orogen.
    Amalgamation between the Yangtze and Cathaysia Blocks in South ChinaConstraints from SHRIMP U-Pb zircon ages, geochemistry and Nd-Hf isotopes of the Shuangxiwu volcanic rocks [查看] Xian-HuaLi、Wu-XianLi、Zheng-XiangLi、Ching-HuaLo、JianWange、Mei-FangYe、Yue-HengYang
    South China was formed through the amalgamation of the Yangtze Block with the Cathaysia Block, but the timing of this amalgamation is controversial, ranging from Mesoproterozoic to Mesozoic. We report here SHRIMP U–Pb zircon ages, geochemistry and Nd–Hf isotopes of the Shuangxiwu Group volcanic rocks from the southeastern Yangtze Block. These rocks were strongly deformed, metamorphosed to greenschist-facies, intruded by 849±7Ma dolerites, and unconformably overlain by Neoproterozoic rift successions of no older than ca. 820 Ma. The Beiwu and Zhangcun volcanic rocks from the middle and uppermost Shuangxiwu Group were dated at 926±15Ma and 891±12 Ma, respectively. All the studied rocks are characterized by highly positive Nd(T) (5.4–8.7) and Hf(T) (11.0–15.3) values. The Pingshui basaltic and andesitic rocks from the lower Shuangxiwu Group, whichwere previously dated at ca. 970 Ma, are high in Al2O3 (15–20%) but low in MgO (<8%), and are characterized by enrichments in Th and LREE but depletions in Nb, Ta, Zr, Hf and Ti, broadly similar to high-Al basaltic rocks in many volcanic arcs. The Beiwu andesitic to rhyolitic rocks have higherMgOthan the experimental melts of basaltic rocks, and their Al2O3 content decreases with increasing SiO2, similar to the regional coeval tonalites and granodiorites,suggesting their formation by crystal fractionation of basaltic parent magma. The Zhangcun volcanic rocks are high in SiO2 (mostly >69%), low in MgO (0.35–1.2%), and have nearly constant Al2O3 contents of 14–15%and relatively uniform trace element concentrations. Theywere generatedby remelting of juvenile mafic to intermediate arc rocks. Overall, the Shuangxiwu Group volcanic rocks and associated intrusive tonalites and granodiorites constitute a typical calc-alkaline magmatic assemblage of a 970–890Maactive continental margin. These results and the 849±7Ma zircon U–Pb age for the undeformed doleritic dikes intruding the Shuangxiwu Group suggest that the tectonic regime of the study region transformed from plate convergence to intracontinental rifting in the time period between ca. 890Ma and ca. 850 Ma.Previously reported 1.04–0.94Ga metamorphic and deformation ages from the nearby Tianli Schists and evidence for the final closure of the back-arc basin at ca. 880Ma (ophilitic obduction at Xiwan), further suggest that the amalgamation between the Yangtze and Cathaysia Blocks, likely through “soft docking”at the eastern segment of the Sibao orogen, was completed at ca. 880Ma or soon after.
    Zircon U-Pb ages, Hf and O isotopes constrain the crustal architecture of the ultrahigh-pressure Dabie orogen in China [查看] Zi-FuZhao、Yong-FeiZheng、Chun-ShengWei、Fu-KunChen、XiaomingLiu、Fu-YuanWu
    The crustal structure of the Dabie orogen was reconstructed by a combined study of U–Pb ages, Hf and O isotope compositions of zircons from granitic gneiss from North Dabie, the largest lithotectonic unit in the orogen. The results were deciphered from metamorphic history to protolith origin with respect to continental subduction and exhumation. Zircon U–Pb dating provides consistent ages of 751±7 Ma for protolith crystallization, and two group ages of 213±4 to 245±17 Ma and 126±4 to 131±36 Ma for regional metamorphism. Majority of zircon Hf isotope analyses displays negative εHf(t) values of −5.1 to −2.9 with crust Hf model ages of 1.84 to 1.99 Ga, indicating protolith origin from reworking of middle Paleoproterozoic crust. The remaining analyses exhibit positive εHf(t) values of 5.3 to 14.5 with mantle Hf model ages of 0.74 to 1.11 Ga, suggesting prompt reworking of Late Mesoproterozoic to Early Neoproterozoic juvenile crust. Zircon O isotope analyses yield δ18O values of −3.26 to 2.79‰, indicating differential involvement of meteoric water in protolith magma by remelting of hydrothermally altered low δ18O rocks. North Dabie shares the same age of Neoproterozoic low δ18O protolith with Central Dabie experiencing the Triassic UHP metamorphism, but it was significantly reworked at Early Cretaceous in association with contemporaneous magma emplacement.The Rodinia breakup at about 750 Ma would lead to not only the reworking of juvenile crust in an active rift zone for bimodal protolith of Central Dabie, but also reworking of ancient crust in an arc-continent collision zone for the North Dabie protolith. The spatial difference in the metamorphic age (Triassic vs. Cretaceous) between the northern and southern parts of North Dabie suggests intra-crustal detachment during the continental subduction. Furthermore, the Dabie orogen would have a three-layer structure prior to the Early Cretaceous magmatism: Central Dabie in the upper, North Dabie in the middle, and the source region of Cretaceous magmas in the lower.
    Zircon SHRIMP U-Pb ages of the Gangdese Batholith and implications for Neotethyan subduction in southern Tibet [查看] Da-RenWen、DunyiLiu、Sun-LinChung、Mei-FeiChu、JianqingJi、QiZhang、BiaoSong、Tung-YiLee、Meng-WangYeh、Ching-HuaLo
    The Trans-Himalayan magmatism, which occurred extensively in the Lhasa terrane of southern Tibet, has long been related to the Neotethyan subduction before the India–Asia collision. To better delineate the magmatic duration, we report a geochronological study with 25 SHRIMP zircon U–Pb ages from the Gangdese Batholith that represents the largest Trans-Himalayan plutonic complex. The results suggest two distinct stages of plutonism in the Late Cretaceous (ca. 103–80 Ma) and early Paleogene (ca. 65–46 Ma),respectively. Our new data confirm if not refine the notion that a Gangdese magmatic gap or quiescent period existed between ca. 80 and 70 Ma. It is furthermore identified that the early stage ended with adakitic intrusion and the latter stage is marked by a peak activity at ca. 50 Ma.We attribute the cessation of the early stage, and following magmatic gap, to a flattening of the northward Neotethyan subduction, and the initiation of the latter stage to rollback of the subducted slab. The proposed scenarios can also account for the southward migration and intensification of Cretaceous to Paleogene volcanism in the Lhasa terrane that demonstrates a coeval, eruptive “flare-up” event around 50 Ma, interpreted as the result of detaching the Neotethyan oceanic slab from the adherent, more buoyant Indian continental lithosphere owing to the India–Asia collision. Our model is, moreover, in general accord with sedimentary and structural geologic records from southern Tibet where subduction-related orogenesis appears to have evolved through time before India started colliding Asia.
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