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    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.
    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 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.
    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 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.
    SHRIMP U-Pb zircon dating of the Wulian complex Defining the boundary between the North and South China Cratons in the Sulu Orogenic Belt,China [查看] Jian-BoZhou、SimonA.Wilde、Guo-ChunZhao、Xing-ZhouZhang、Chang-QingZheng、WeiJin、HongCheng
    Neoproterozoic metasedimentary and metavolcanic rocks of the Wulian Group are associated with gneissic granitoids at the northern margin of the Sulu UHP Belt,adjacent to the Wulian-Yantai Fault,and are collectively referred to as the Wulian complex,SHRIMP U-Pb dating of detrital zircon from the metasedimentary rocks of the Wulian Group indicates a range in age from 3478 to 625Ma.Based on zircons＜10% discordant,they can be broadly divided into four populations:＞3000Ma，with the oldest age being 3339Ma;2700~2400Ma,with a peak at 2436Ma;2200-1800Ma,with a peak at ~2156Ma;and at ~770Ma.These results indicate that deposition of the Wulian Group was after ~770Ma and that some detrital material most likely came from Neoproterozoic sources in the South China Craton(SCC).SHRIMP U-Pb zircon dating of a metavolcanic rock defines a weighted mean 206Pb/238U age of 761±8Ma，which is consistent with other rocks of South China affinity and can be correlated with syn-rift magmatism during the breakup of the supercontinent Rodinia.The Wulian Group,together with Neoproterozoic granites of the Wulian complex,constrains the location of the Triassic suture zone betwwen the North China and South China Cratons,Based on our new data,the suturemust lie north of the Wulian complex and is marked by the Baichihe-Yantai Fault.
    SHRIMP U–Pb zircon dating of the Neoproterozoic Penglai Group and Archean gneisses from the Jiaobei Terrane,North China, and their tectonic implications [查看] Jian-BoZhou、SimonA.Wilde、Guo-ChunZhao、Chang-QingZheng、WeiJin、Xing-ZhouZhang、HongCheng
    Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541±5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga.The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866±4Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at ∼1865 Ma.
    SHRIMP U–Pb zircon ages of granitoid rocks in the Luliang Complex Implications for the accretion and evolution of the Trans-North China Orogen [查看] GuochunZhaoa、SimonA.Wilde、MinSun、SanzhongLi、XupingLi、JianZhang
    The lvliang Complex is situated in the central segment of the Trans-North China Orogen (TNCO), a continent-continent collisional belt along which the discrete Eastern and Western Blocks amalgamated to form the basement of the North China Craton. The complex consists of supracrustal and granitoid rocks, of which the latter can be subdivided into pre-tectonic TTG gneisses, including the Yunzhongshan, Guojiazhuang and Chijianling–Guandishan gneisses; syn-tectonic gneissic granites represented by the Huijiazhuang gneissic granite; and post-tectonic granites,including the Luyashan charnockite, Luchaogou porphyritic granite and Tangershang/Guandishan massive granite. The pre-tectonic TTG gneisses are mostly calc-alkaline and considered to have formed in a magmatic arc environment. SHRIMP U–Pb analyses reveal that the Yunzhongshan gneisses were emplaced at ∼2499±9 Ma, representing the earliest arc-related magmatic event in the Luliang Complex. This was followed by the intrusion of the Guojiazhuang gneisses at 2375±10 Ma. The most widespread arc-related magmatic event in the region was the emplacement of the Chijianling–Guandishan granitoid gneisses, of which the tonalitic, granodioritic and monzogranitic gneisses were emplaced at 2199±11 Ma,2180±7Ma and 2173±7 Ma, respectively. Metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207Pb/206Pb age of 1872±7 Ma, consistent with the metamorphic age range of 1880–1820 Ma defined by metamorphic zircons from various high-grade rocks in the TNCO. Thus, the evolution of the lvliang Complex involved emplacement of the Yunzhongshan granitoids at ∼2499 Ma, the Guajiazhuang granitoids at ∼2375 Ma and the Chijianling–Guandishan granitoids at 2199–2173 Ma, with the final collision between the Eastern and Western Blocks in this area occurring at 1872±7 Ma, the whole series of magmatic events lasting nearly 650 Ma. This suggests that the Trans-North China Orogen represents a long-lived magmatic arc. The new SHRIMP zircon data, combined with structural and petrological studies, also reveal the existence of both syn- and post-collisional granites in the Luliang Complex. The former is represented by the Huijiazhuang gneissic granite that was emplaced at 1832±11 Ma, whereas the latter include the 1815±5Ma Luyashan charnockite, 1807±10 Ma Luchaogou porphyritic granite,1798±11 Ma Guandishan massive granite and 1790±14 Ma Tangershan massive granite.
    Further SHRIMP Geochronology on the Early Cambrian of South China [查看] W.COMPSTON、ZICHAOZHANG、J.A.COOPER、GUOGANMA、R.J.F.JENKINS
    ABSTRACT. Zircon 206Pb/238U ages are reported for bentonites from two Cambrian sections in South China, the Meishucun and the Fandian. The analyzed zircons vary widely in age because of the presence of detrital and altered grains. Spot ages were corrected for calibration drift during analysis, and the age of reference zircon SL13 was calibrated as 580 Ma against other standards using SIMS. Use of mixture modeling was essential to resolve the different age components and to identify the volcanic ages. Two age groups occur within the Meishucun Bed 5 bentonite, of which the larger at 518.7±2.5 Ma (σ) is interpreted as a post depositional zircon growth event and the other at 539.4±2.9 Ma as the volcanic age. Bentonites from the Meishucun Bed 9 and its Fandian equivalent have two age groups, 526.5±1.1 Ma and 540.2±1.0 Ma. We interpret the younger of these as the tuff magmatic age and the older as detrital. Small shelly fossils indicate Meischucunian Bed 5 is an equivalent of the late Nemakit Daldynian, and its 539.4±2.9 Ma age both closely times the base of the Cambrian and is a maximum for the basal Tommotian. The 526.5±1.1 Ma for Bed 9 and its equivalents approximates the close of the Tommotian.
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