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    Metamorphism, anatexis, zircon ages and tectonic evolution of the Gongshan block in the northern Indochina continent-an eastern extension of the Lhasa Block [查看] ShuguangSong、YaolingNiu、ChunjingWei、JianqingJi、LiSu
    The Gongshan block near the Eastern Himalayan Syntaxis is a fault-bounded block at the northern tip of the triangle-shaped Indochina continent (NIC). Exposed in this block are late Paleozoic (Carboniferous to Permian) strata and a north–south belt of intermediate to felsic batholiths (i.e., Gaoligongshan magmatic belt). The contact between the Gaoligongshan batholiths and Carboniferous/Permian strata is characterized by a series of high-grade metamorphic gneisses with leucosome granite veins (i.e., the so-called “Gaoligong Group”). U-Pb SHRIMP and LA-ICP-MS dating of zircons indicate that these gneisses are actually metamorphosed Paleogene sediments containing inherited Archean to Cretaceous detrital zircons (from 2690 to 64 Ma) and have undergone medium- to high-pressure granulite-facies metamorphism at ~22 Ma.Leucosome and S-type granite of 22–53 Ma by anatexis are ubiquitous within high-grade metamorphic rocks in the southern part of the Gongshan block. An Early Paleozoic gneissic granite and granitoid intrusions of Jurassic, Cretaceous and Oligocene-Miocene ages are also recognized in NIC blocks. These ages suggest that the NIC differs distinctly from the Indian continent, the Greater and Lesser Himalaya zones, and the Yangtze Craton, but resembles the Lhasa Block in terms of Paleozoic to Mesozoic magmatism and detrital zircon ages.This offers an entirely new perspective on the tectonic evolution of the Gongshan block in particular and of the history of the Lhasa Block in the context of the India-Asia continental collision in general. Furthermore,the high-grade metamorphism in the NIC indicates a strong crustal thickening (vs. strike-slip shearing) event during much of the Eocene to the Oligocene (~53–22 Ma) that has brought the Paleogene sediments to depths of greater than 25 km. Continuous northward convergence/compression of the Indian Plate at the Eastern Himalayan Syntaxis may have led to the clockwise rotation, southeastward extrusion and extension of the southeastern part of the Indochina continent.
    Geochronology of middle Neoproterozoic volcanic deposits in Yangtze Craton interior of south China and its implications to tectonic settings [查看] WANGZhengJiang、WANGJian、DUANTaiZhong、XIEYuan、ZHUOJieWen、YANGPing
    Here we report new SHRIMP dating results of the crystal tuff in Ejiaao Formation of middle Banxi Group in northern Guizhou.The results indicate that the volcanic deposition occurred at ~780 Ma. We also suggests that the igneous activities across Yangtze Craton with diabase dyke swarms and basal volcanic rocks during middle Banxi time are indicative of the episodic,extensive and vigorous great bimodal igneous events during middle Neoproterozoic (825–720 Ma) in South China. The characteristics of the igneous series are contrary to the model claiming they are of island-arc origin, but suggest that they are the records of rifting process and may be related to the episodic plume activities leading to the break-up of Rodinia.
    Geochemistry of TTG and TTG-like gneisses from Lushan-Taihua complex in the southern North China Craton Implications for late Archean crustal accretion. [查看] Xiao-LongHuang、YaolingNiu、Yi-GangXu、Qi-JunYang、Jun-WeiZhong
    The Late Archean Taihua complex, mainly composed of amphibolite and TTG and TTG-like gneisses, is volumetrically most important metamorphic rock suites scattered along the southern margin of the North China Craton (NCC). Zircon SHRIMP U–Pb dating shows two episodes (2765±13 and 2723±9 Ma) of Archean magmatism in the Lushan area with distinct geochemical features. The early (2765±13 Ma) suite (TTG-like gneisses) has low-SiO2 (52.5–66.1 wt%), high-Mg# (0.47–0.68), low HREE (YbN = 3.0–5.4) and Y (8.07–13.9 ppm) with low to moderate (La/Yb)N (6.7–37.1) and Sr/Y (25.9–119.3). The younger (2723±9 Ma) suite (TTG gneisses) has high-SiO2 (63.5–74.3 wt%), low-Mg# (0.13–0.52), very low REE (YbN < 1.8) and Y (<4 ppm) with a wide range of (La/Yb)N (5.2–86.6), Sr/Y (71.4–949) and showing Eu/Eu* > 1 (1.20–2.43). Both suites show pronounced negative Nb–Ta and Ti anomalies on the primitive mantle-normalized spidergram. The TTG-like gneiss suite has similar bulk-rock Nd and zircon Hf model ages (∼3.0 Ga) with εNd(t) > 0 (0.26–1.46), and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet + clinopyroxene + rutile±amphibole, which favors subducted slab model for the late Archean TTG. The TTG gneiss suite has abundant relic zircons (2.95–2.80 Ga) with εNd(t)<0 (−1.31 to −0.23), which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet-amphibolite residue (garnet + amphibole±rutile). Significant high-pressure fractional crystallization (garnet±amphibole) and accumulation (plagioclase) are also required in the petrogenesis. The Lushan amphibolite with nearly flat primitive mantle-normalized trace-element pattern is interpreted to represent early ocean crust metamorphism. These observations suggest a possible model of late Archean crustal accretion from ocean crust to continental terrain in the southern North China Craton.
    Forming age and tectono-petrogenises of the Jiugequan ophiolite in the North Qilian Mountain- NW China [查看] XIAXiaoHong、SONGShuGuang
    The Jiugequan ophiolite is one of the representative ophiolite fragments in the Early Paleozoic orogenic belt of the North Qilian Mountain. It has been drawn much attention and extensively studied in recent years. In this study, ion microprobe (SHRIMP) U-Pb dating was carried out for zircons from isotropic gabbro from the Jiugequan ophiolite. Eighteen analyses yield a relatively consistent apparent 206Pb/238U ages from 480 to 508 Ma with a weighted mean age of 490±5 Ma (MSWD=1.06), which is believed to be the crystallization age of the gabbro and thus the forming age of the Jiugequan ophiolite. Major and trace element geochemical study indicates that the diabase-basalts from the Jiugequan ophiolite have N-MORB and E-MORB characteristics with some subduction-related signatures. The petrological, geochemical and chronological data enable us to conclude that the Jiugequan ophiolite is most likely to be formed at a spreading center of back-arc basin during the early Ordovician, while the ancient Qilian oceanic plate subducted northwards. The acquisition of forming age and determination of tectonic setting for Jiugequan ophiolite provide significant constraints on the evolution of intra-oceanic subduction system in the North Qilian orogenic belt during the Early Paleozoic era.
    Episodic growth of zircon in UHP orthogneisses from the North Dabie Terrane of east-central China,implications for crustal architecture of a collisional orogen [查看] Z.XIE、J.-F.CHEN、Y.-R.CUI
    Zircon U-Pb dating of three orthogneiss samples from the North Dabie terrane (NDT) is undertaken in order to reconstruct their formation and evolutionary histories, and also the crustal architecture of the Dabie orogen after Triassic subduction and exhumation. SHRIMP zircon U-Pb dating, in combination with back scattered electron (BSE) imaging and Laser Raman spectrometry, provides accurate identification of the core, mantle and rim structure for zircon growth during protolith formation and overgrowth during subduction ⁄ exhumation and post-collisional metamorphism. Concordant U-Pb ages of 760–730 Ma and high Th ⁄U ratios of >0.4 are obtained for relict oscillatory zoning fields of inherited cores that were not metamictized. These features suggest that these ages represent the time of magmatic protolith formation during the breakup of Rodinia. The overgrown mantle domains around the metamictized cores are clean with few mineral inclusions (e.g. quartz, garnet and apatite). Mantle domains have low Th ⁄U ratios of <0.1 and yielded U-Pb ages of 215–205 Ma, which are slightly younger than the known ages of peak ultrahigh-pressure (UHP) metamorphism, suggesting that overgrowth took place during initial exhumation. The ages are similar to the time of retrograde metamorphism of the UHP orthogneisses in the Central Dabie terrane (CDT). Overgrown rims are also clean, with a few mineral inclusions of apatite and quartz. They yield two groups of U-Pb ages, 138–137 Ma and 124–120 Ma. The former is considered to be the time of onset of orogenic extension and tectonic collapse, whereas the latter falls into the age range of widespread magmatism in the Dabie orogen, and is regarded as the time of extension climax that resulted in intensive anatexis of the crust. Whole-rock Sr-Nd isotope analyses of four orthogneisses show Nd(t) values of )1.2 to )15 and ISr values >0.719, similar to the values obtained from UHP orthogneisses in the CDT. It is concluded that, as with the CDT, the orthogneisses with episodic zircon growths from the NDT should also be a part of the exhumed slice following the continental deep subduction. However, the orthogneisses in this study were buried at a lower level in the orogenic crust compared with those of the CDT prior to the Cretaceous magmatism. Therefore, the orthogneisses from the NDT were affected by the Cretaceous magmatism whereas the CDT orthogneisses were not affected.
    Devonian A-type granitic magmatism on the northern margin of the North China craton,SHRIMP U-Pb dating and Hf isotopes of the Hongshan granite at chifeng,inner mongolia,China [查看] YuruoShi、DunyiLiu、LaichengMiao、FuqinZhang、PingJian、WeiZhang、KejunHou、JunyuXu
    We report first data for the Devonian Hongshan A-type granite along the northern margin of the Precambrian North China Craton. The granite is rich in silica (SiO2=76.07–77.62 wt.%) and alkalis (Na2O+K2O=8.45–8.78 wt.%), but poor in calcium (CaO=0.28–0.70 wt.%) and magnesium (MgO=0.08–0.20 wt.%). Chondritenormalized REE patterns for the samples fromtheHongshan granite show significant negative Eu anomalies (Eu/Eu*=0.13–0.19). They contain high Rb, Ga, Y, Nb, and Zn, but low Ba and Sr contents, and in the multi-element spider diagrams normalized to primitive mantle, they show a similar pattern to that of A-type granites, such as negative Ba, Nb, Ta, Sr, P and Ti anomalies. Geochemical plots also show an affinity with A-type granites and suggest a post-orogenic setting (A2 type granite). SHRIMP U–Pb zircon dating yielded a concordant weighted mean 206Pb/238Uage of 387±4 Ma,which defines a Devonianmagmatic event. The zircons have negative εHf(t)of −8.4 to−11.5 and give old Hf model ages of TDM2=2548–2820 Ma (mean 2667Ma). Most 176Hf/177Hf and εHf values display Hf-isotope evolutional line of the lower continental crust. These results suggest that the Hongshan granite originated from melting of Archaean lower crust beneath the North China Craton in a post-orogenic extensional setting during the Devonian.
    Ca.2.5 billion year old coeval ultramafic-mafic and syenitic dykes in Eastern Hebei Implications for cratonization of the North China Craton [查看] TieshengLi、MingguoZhai、PengPeng、LiangChen、JinghuiGuo
    A group of extremely rare coeval ultramafic–mafic and syenitic dykes was discovered in the Eastern Hebei region of the North China Craton. These dykes intrude the 3.8–2.55 Ga old Caozhuang complex. An olivine gabbro dyke and syenite dyke yield, respectively, SHRIMP zircon U–Pb ages of 2516±26Ma and 2504±11 Ma, interpreted as the magmatic crystallization age. Their zircons have single-stage Hf model ages 2677Ma and 2705 Ma. The olivine gabbros have Mg# values of 59–63, similar to high-magnesian tholeiitic basalt. They show relatively LREE-enriched patterns without Eu anomalies (La/YbN = 9.28–9.78, Gd/YbN = 2.8, δEu = 0.96–1.00), with enrichment in LILEs and depletion in HFSEs. They also contain high Cr and Ni, and have Zr/Hf and Nb/Ta ratios similar to those of the primitive mantle. The syenites are alkaline in composition, with 8.67–8.88 wt.% Na2O+K2O, and show high total REE contents (543–854 ppm) and strong LREE-enriched patterns with minor Eu negative anomalies (La/YbN = 50–101, Gd/YbN = 5.2–5.3,δEu = 0.80–0.82). Trace element diagrams show strong LILE-enrichment relative to HFSE. Zr/Hf and Nb/Ta ratios are 47–49 and 26–76, respectively, which are much higher than those of primitive mantle and higher than the average Nb/Ta ratio of post-Archean continental crust. The petrological and geochemical features indicate that these dykes were derived from a deep subcontinental lithospheric mantle source, which implies that the NCC probably had continental crust of considerable thickness at this time. Combined with evidence for ∼2.5 Ga granite intrusion and metamorphism in the Eastern Hebei region and adjacent areas, we propose that the NCC has been a present scale craton at the end of Archean.
    Baogutu Porphyry Cu-Mo-Au Deposit, West Junggar, Northwest China Petrology, Alteration, and Mineralization [查看] PINGSHEN、YUANCHAOSHEN、HONGDIPAN、JINGBINWANG、RUIZHANG、YUNXIAOZHANG
    Baogutu is the first porphyry Cu-Mo-Au deposit discovered in West Junggar, Xinjiang, China. It is part of the Central Asian orogenic belt. Baogutu is associated with a Carboniferous intrusive complex that was emplaced into lower Carboniferous volcano-sedimentary sequences of the Baogutu and Xibeikulasi Groups. The intrusive complex is made up of main stage equigranular to porphyritic diorites and quartz diorites, and minor late stage diorite porphyries. Intrusive activity occurred at about 325 Ma based on U-Pb (SHRIMP) analyses of zircons. The main stage diorites host the bulk of the Cu-Mo-Au mineralization at Baogutu. They have been overprinted by three alteration assemblages, including an early potassic (biotite) assemblage that occurs in the center of the deposit. A propylitic assemblage surrounds the potassic zone concentrically. Both of these alteration assemblages have been overprinted locally by phyllic alteration (quartz-sericite-pyrite), which is associated with the highest Cu-Mo grades. Mineralized and hydrothermally cemented breccias have disrupted the main stage diorites. The late stage diorites have undergone moderate potassic alteration and contain weak Cu mineralization.Matrix-rich breccias with very weak mineralization have cut the intrusive complex and disrupted the orebody. Re-Os dating of molybdenite indicates that mineralization occurred at about 310 Ma.
    Age, geochemistry, and tectonic implications of a late Paleozoic stitching pluton in the North Tian Shan suture zone, western China [查看] Bao-FuHan、Zhao-JieGuo、Zhi-ChengZhang、LeiZhang、Jia-FuChen、BiaoSong
    The Central Asian orogenic belt is the largest tectonic assembly of continental and oceanic terranes on Earth due to closure of the paleo–Asian Ocean in the Phanerozoic. Among major suture zones in the North Xinjiang region of western China, the North Tian Shan suture zone, because of collision between the Yili terrane in the south and the Junggar terrane in the north, contains the youngest ophiolitic rocks and may represent the terminal stage of development of the Central Asian orogenic belt in western China, but the timing of the suture zone remains poorly constrained. A sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb age of 316 ± 3 Ma (i.e., the beginning of the late Carboniferous) from the undeformed Sikeshu pluton, which crosscuts the suture zone, places a crucial upperage bound for the time of collision between the Yili and Junggar terranes. This event occurred later than, or nearly concurrent with, other accretion-collision events in the North Xinjiang region, implying that final terrane amalgamation was completed in the late Carboniferous. The Sikeshu pluton shares geochemical characteristics of the widespread late Carboniferous to Permian postcollisional A-type and I-type granitoids with depleted-mantle–like Sr-Nd isotopic signatures in the North Xinjiang region. They all occurred during a protracted (ca. 320–270 Ma) episode of postcollisional magmatism that may have been induced by basaltic under plating due to either slab breakoff or delamination of thickened mantle lithosphere beneath the Central Asian orogenic belt. The same postcollisional magmatism also generated Cu-Ni-sulfi de–bearing, mafi c-ultramafi c magmatic complexes complexes,adakites, and porphyry-type coppermolybdenum–bearing magmatic rocks in the North Xinjiang region.
    A Permian large igneous province in Tarim and Central Asian orogenic belt, NW China,Results of a ca.275 Ma mantle plume [查看] Chuan-LinZhang、Zheng-XiangLi、Xian-HuaLi、Yi-GangXu、GangZhou、Hai-MinYe
    New sensitive high-resolution ion microprobe(SHRIMP) U-Pb zircon ages, geochemical data, and a synthesis of existing stratigraphic, geochronologic, and geochemical results from the Tarim block and the Central Asian orogenic belt in northwestern China suggest the presence of a Per mian (ca. 275 Ma) large igneous province (the Bachu large igneous province). The large igneous province consists predominantly of coeval mafic rocks (basalts and mafic-ultramafic intrusions) having an aerial coverage of more than 600,000 km2, and its formation was accompanied by voluminous emplacement of A-type granites. This large igneous province,interpreted to be of mantle plume origin, is~15 m.y. older than the ca. 260 Ma Emeishan large igneous province in southwestern China and ~25 m.y. older than the 251 Ma Siberian Trap in Russia. Such a sudden fl air up of plume activity in the Permian may represent the early stage of the Pangean superplume event. The Permian plumes likely played a role in late Paleozoic rapid continental crustal growth in the Central Asian orogenic belt. In addition, there appear to be two types of mantle geochemical provinces (domains) in the region, a long-term enriched Tarim province and a subduction-metasomatized and depleted Central Asian orogenic belt province.
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