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Age, geochemistry and tectonic setting of the Neoproterozoic (ca 830 Ma) gabbros on the southern margin of the North China Craton [查看] Xiao-LeiWan、Shao-YongJiang、Bao-ZhangDai、W.L.Griffin、Meng-NingDai、Yue-HengYang

Many studies have addressed the Paleozoic to Triassic collisional processes along the southern margins of the North China Craton (NCC), but little is known about the evolution of this margin during Precambrian time. This paper examines the geochronology and geochemistry of gabbros from the Luanchuan Group,which is located on the southern margin of the NCC. These gabbros are dated at ca 830 Ma by SHRIMP and LA–ICP-MS zircon U–Pb methods. They are characterized by high contents of TiO2 (2.21–4.45 wt%) and light-REEs (rare earth elements) ((La/Yb)N = 9.54–7.71). The gabbros have overall OIB (ocean island basalts)-like trace element patterns, without the positive Pb and Sr anomalies and negative Nb–Ta anomalies. In addition, they have low La/Nb (<1.5) and La/Ta (<30) ratios, indicating an origin in the asthenospheric mantle. The depletions of HFSEs (high field strength elements) (e.g. Zr–Hf and Ti) probably suggest that their source has been metasomatized by carbonates. The gabbros show negative εNd(t)(−1.5 to −3.0), suggesting crustal contamination or mixing with metasomatized lithospheric mantle. However, the low Th and U contents, constant incompatible-element ratios (e.g. Zr/Nb, La/Nb, La/Sm) and the absences of correlations of εNd(t) with MgO, Nb/Nb*, SiO2 or 1/Nd preclude significant crustal contamination. Alternatively, the wide range of Hf–isotope ratios in zircons (εHf(t) from −3.6 to +6.3) may imply the interaction between asthenosphere-derived melts and the metasomatized lithospheric mantle. Petrological modeling suggests that the gabbros may have been generated from the low-degree(2–3%) partial melting of lherzolite with 2% garnet at depths greater than 85 km. The gabbros may have been generated in a within-plate rift setting. The gabbros may best be correlated with the Neoproterozoic magmatic rocks in the NQB (North Qinling Belt), representing the relicts onto the southern margin of the NCC following the Neoproterozoic rifting between the NCC and the NQB. Therefore, the NCC and NQB may have been connected at ca 830 Ma. Moreover, the occurrence of the gabbros implies that the present southern margin of the NCC may have been mixed with late Mesoproterozoic to Neoproterozoic crustal materials from the NQB. The new findings imply that the southern margin of the NCC has been the locus of at least three extension-convergence cycles, and the reactivation of such tectonic margins may be more common in the geological record than previously recognized.

Petrogenesis of late Triassic post-collisional basaltic rocks of the Lancangjiang tectonic zone, southwest China, and tectonic implications for the evolution of the eastern Paleotethys Geochronological and geochemical [查看] YuejunWang、AimeiZhang、WeimingFan、ToupingPeng、FeifeiZhang、YanhuaZhang、XiawuBi

The Xiaodingxi and Manghuihe volcanic sequences represent key components of the Lancangjiang igneous zone in southwest China. Their petrogenesis provides important constraints on the tectonic evolution of the eastern Paleotethys ocean. The basaltic rocks from the Xiaodingxi and Manghuihe sequences yield SHRIMP zircon U–Pb weighted mean ages of 214±7 Ma and 210±22 Ma, respectively, which is 15–20 Ma younger than the ages of the syn-collisional granite magmatism (230–241 Ma). Samples from the volcanic sequences are dominated by alkaline basalts and basaltic andesites, and can be geochemically classified into two groups. Group 1 samples, mainly from the Xiaodingxi sequence and the lower part of the Manghuihe sequence, are characterized by low MgO(1.49–7.50 wt.%) and Zr/Nb (9.4–15.3), and high Al2O3 (15.95–18.39 wt.%). They are enriched in LILE and LREE contents and depleted in HFSE, and have 87Sr/86Sr(t) ratios of 0.705473–0.706972,εNd(t) of −1.47–0.75, and similar Pb isotopic compositions to the global average composition of pelagic sediments. In contrast, Group 2 samples from the middle–upper parts of the Manghuihe sequence have similar Al2O3 (16.62–18.23 wt.%) but higher MgO (8.08–11.74 wt.%) and Zr/Nb (15.9–23.9) than those of Group 1 samples. They exhibit relatively flat REE patterns, significantly negative Nb–Ta and Th–U anomalies and positive Sr anomalies. In comparison with Group 1, Group 2 samples show higher Cr, Ni contents and εNd(t) values (1.17–5.02), and lower 87Sr/86Sr(t) and Pb isotopic ratios (Δ8/4=43.2–59.8 andΔ7/4=11.8–19.8). The geochemical data suggest that Group 1 samples might be the differentiated product of primitive high MgO and low Al2O3 melts originating from a refractory modified mantle with the involvement of 5–10% recycled pelagic sediments. The parental magma for Group 2 samples may have been derived from a plagioclase-rich,garnet-free source comprising 80–85% fluid-metasomatized and 15–20% asthenospheric components. Based on all available data, a tectonic model involving eastward subduction in the Permian and collision in the Triassic can be proposed for the evolution of the eastern Paleotethys ocean. During the late Triassic, the progressive upwelling of the asthenospheric mantle, shortly after slab detachment, may have led to the melting of the metasomatized mantle wedge, resulting in the post-collisional Group 1 and Group 2 magmas.

Zircon SHRIMP U-Pb dating for gabbro at Chaotiehe in the Haicheng area- eastern Liaoning [查看] MIAOLaiCheng、ZHANGFuQin、LIUDunYi、SHIYuRuo、XIEHangQiang

Chaotiehe gabbroic intrusion in the eastern part of Liaoning Province was dated by the zircon SHRIMP U-Pb technique. The results gave an emplacement age of 126±4 Ma, indicating that the intrusion thus formed during the lithospheric thinning of the North China Craton (NCC) in the Early Cretaceous rather than in a rifting setting during the Paleoproterozoic as previously thought. The gabbroic intrusion contains abundant old zircons with Paleoproterozoic (2.10–2.46 Ga and ca. 1.87 Ga) and Neoproterozoic(747–969 Ma) ages. The Paleoproterozoic zircons were probably derived from NCC itself, whereas the Neoproterozoic ones were likely from materials of the Yangtze Craton that had previously been subducted beneath NCC. These geochronological dates are of important implications for understanding the Mesozoic crustal evolution of NCC.

SHRIMP U-Pb dating for a K-bentonite bed in the Tieling Formation,North China [查看] SUWenBo、LIHuaiKun、HUFFWD、ETTENSOHNFR、ZHANGShiHong、ZHOUHongYing、WANYuSheng

A SHRIMP U-Pb zircon age of 1437±21 Ma was obtained for a recently discovered K-bentonite bed in the Tieling Formation,situated northeast of Beijing at the boundary between Liaoning and Hebei provinces, on the northern margin of the North China Craton (NCC). The SHRIMP U-Pb age places Tieling Formation near the end of the Calymmian Period of the early Mesoproterozoic Era. In addition, a SHRIMP U-Pb zircon date of 1372±18 Ma was acquired for K-bentonite beds in the overlying,dark-shale-dominated Xiamaling Formation from the same location northeast of Beijing. This date assigns a similar Mid-Mesoproterozoic (Ectasian Period) age for the Xiamaling Formation, as have previously determined dates from other sections northwest of Beijing. These dates indicate that the Tieling and Xiamaling formations, as well as the related succession in the eastern part of the Yanshan Mountains, represented by the well-known Meso- to Neoproterozoic standard section in Jixian, can be correlated well with sections northwest of Beijing in the western part of the Yanshan Mountains. In other words, the boundary between the Calymmian and Ectasian periods in the northern parts of the NCC is marked by the unconformity between the Tieling and Xiamaling formations in the northern NCC. This boundary was previously ascribed to the “Qinyu Orogeny” and thought to be of Grenville age. In this regard, the conventional “Qinyu Orogeny” should now be regarded as a short-lived regional uplift during Early Mesoproterozoic time, rather than a result from the Grenvillian assembly of the NCC to the Rodinia Supercontinent (~1.0 Ga).

Progress and controversy in the study of HP-UHP metamorphic terranes in the West and Middle Central China Orogen [查看] LiuLiang、YangJiaxi、ChenDanling、WangChao、ZhangChengli、YangWenqiang、CaoYuting

ABSTRACT: During the past ten years, various types of HP-UHP metamorphic rocks have been discovered in the South Altyn Tagh, the North Qaidam and the North Qinling (秦岭) in the West and Middle Central China orogen. The UHP rocks, as lentoid bodies in regional gneisses, include eclogite (garnet-bearing pyroxenite), garnet peridotite and various pelitic or felsic gneisses. There are many records of minerals and microstructures of exsolution indicate the UHP metamorphism, such as coesite (or its pseudomorph), diamond, exsolution of clinopyroxene/amphibole/+rutile or rutile+quartz+apatite in garnet, exsolution of quartz in omphacite and exsolution of kyanite+spinel in precursor stishovite.The discovery of microstructure evidence for the presence of precursor stishovite in typical Alrich gneiss from the South Altyn Tagh reveals continental subduction and exhumation to and from a depth of more than 350 km. It is the petrological record of the deepest subduction and exhumation of continental rock in the world.The in situ zircon U-Pb dating using LA-ICPMS or SHRIMP methods shows that the metamorphic ages of the HP-UHP rocks in the South Altyn Tagh, the North Qaidam and the North Qinling are 475–509, 420–457, and 485–514 Ma,respectively. The metamorphic ages of HP-UHP rocks in the North Qaidam are 20–80 Ma younger than those in the South Altyn Tagh and the North Qinling, and the metamorphic ages do not systematically increase or decrease from the South Altyn Tagh through the North Qaidam to the North Qinling. The absence of time transgressive variety of the metamorphism in the three regions does not support the hypothesis that the HP-UHP rocks in these regions form the same HP-UHP metamorphic zone. And the HP-UHP rocks in these regions can not be simply correlated to the collision between the North China plate and the South China plate. At present,the study of the HP-UHP rocks in the West and Middle Central China orogen faces several key issues or challenges, such as: (1) the continental subduction to the mantle depth of stishovite stability field (>9 GPa) is occasional or universal; (2) the mechanism of exhumation for the continental rocks subducted to the depth of stishovite stability field (>300 km); (3) the tectonic setting and geodynamical mechanism of producing the HP-UHP metamorphic zones in the South Altyn Tagh, the North Qaidam and the North Qinling. Further studies aiming at these problems will make important progress not only in metamorphism of the HP-UHP rocks in the West and Middle Central China orogen, but also in continental deep subduction and exhumation in solid earth science. It will also contribute to the establishment of the theory of continental deep subduction.

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.

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.

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.

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