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    Early history of the eastern Sibao Orogen (South China) during the assembly of Rodinia New mica 40Ar39Ar dating andSHRIMP U–Pb detrital zircon provenance constraints [查看] Zheng-XiangLi、Jo-AnneWartho、SandraOcchipinti、Chuan-LinZhang、Xian-HuaLi、JianWang、ChaominBao
    The Sibao Orogen in South China is one of the poorest known Grenville-aged orogenic belts through which the Neoproterozoic supercontinent Rodinia assembled. We report here the first UV laser spot 40Ar/39Ar mica and SHRIMP U–Pb zircon ages from a rare Grenville-aged metamorphic complex, the Tianli Schists, in the eastern Sibao Orogen. Our U–Pb zircon provenance ages indicate that the protolith of the Tianli Schists was a clastic sedimentary succession most likely derived from the Yangtze Block.The depositional age of the protolith is younger than 1530 Ma, as constrained by the youngest detrital zircon grains, but is older than 1040 Ma as constrained by the oldest 40Ar/39Ar muscovite ages. The Yangtze Block provenance for the Tianli Schists suggests that the Sibaoan ophiolitic complexes in northeastern Jiangxi, the ca. 970 Ma Xiwan adakitic granite intrusions, and the ca. 900 Ma(?) Xiwan blueschists, all to the northwest of the study region, were likely formed during the closure of a back-arc basin along the margin of the Yangtze Block. Our in situ UV laser 40Ar/39Ar results from S1 and S2 muscovites suggest that the Tianli Schists underwent metamorphism and deformation at 1042±7Ma to 1015±4 Ma, the oldest known metamorphic event in the easternSibao Orogen. Muscovite/biotite cooling ages of ca. 968±4 and 942±8Ma are recorded by deformed and recrystallised muscovite and biotite, respectively, indicating tectonic reactivation before 900 Ma, during the later stages of the Sibao Orogeny. Together with previous results from the western Sibao Orogen, our work suggests that the closure of the ocean between the Yangtze and Cathaysia Blocks during the assembly of Rodinia was diachronous: ≥1000 Ma at the western Sibao Orogen and ca. 900 Ma at the eastern Sibao Orogen.
    Age and duration of the Emeishan flood volcanism, SW China Geochemistry and SHRIMP zircon U– Pb dating of silicic ignimbrites, post-vo lcanic Xuanwei Formation and clay tuff at the Chaotian section [查看] BinHe、Yi-GangXu、Xiao-LongHuang、Zhen-YuLuo、Yu-RuoShi、Qi-JunYang、Song-YueYu
    The age and duration of the Emeishan basalts (SW China) remain poorly constrained largely due to the severe thermotectonic overprinting of the Ar–Ar system and failure to obtain zircon separates from erupted basalts. In an attempt to solve this problem, geochemical analyses and SHRIMP zircon U–Pb dating have been carried out on rare felsic ignimbrite in the uppermost of the Emeishan lava succession, the Xuanwei Formation which immediately overlies the Emeishan basalts and a clay tuff at the Middle–Late Permian boundary at the Chaotian section. Clastic rocks of the lowermost Xuanwei Formation (Group 1) in eastern Emeishan large igneous province (LIP) have a geochemical affinity to the Emeishan felsic volcanic rocks, whereas the overlying sediments (Group 2) are compositionally more akin to mafic components of the Emeishan traps. This is the reverse of volcanic sequence of the central Emeishan LIP where the felsic extrusives sit above predominant mafic rocks. It is likely that the clastic rocks are water-transported sediments resulting from erosion of the volcanic rocks in the center of the Emeishan LIP. This interpretation is further supported by the general agreement between the age of the lowermost Xuanwei Formation (257± 4 Ma; 260±5 Ma) and that of the silicic ignimbrite (263±4 Ma) and the clay tuff at the Middle–Late Permian boundary at Chaotian(260± 5 Ma). These ages, interpreted as the termination age of the Emeishan volcanism, are indistinguishable within error from the Middle–Late Permian boundary age (260.4± 0.4 Ma) and the main stage (259–262±3 Ma) of the Emeishan volcanism inferred from dating of mafic and alkaline intrusions in the Emeishan LIP. All these suggest that the emplacement of the Emeishan volcanism took place over a very short interval. Moreover, the temporal link and geochemical analyses suggest that the Chaotian clay at the Middle–Late Permian boundary was genetically related to the Emeishan silicic volcanism. This, together with the fact at that both the Emeishan basalts and the Chaotian clay rest on the Maokou Formation, leads us to infer that the Emeishan basalt was emplaced at the Middle–Late Permian boundary. In this sense, the Emeishan volcanism can be regarded as
    A cold Early Palaeozoic subduction zone in the North Qilian Mountains, NW China petrological and U-Pb geochronological constraints [查看] J.X.ZHANG、F.C.MENG、Y.S.WAN
    The north Qilian high-pressure (HP)/low-temperature (LT) metamorphic belt is composed mainly of blueschists, eclogites and greenschist facies rocks. It formed within an Early Palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved HP/LT metamorphic belts in China. Here we report new lawsonite-bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the North Qilian Mountains. Five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet, indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks. Peak pressure conditions estimated from lawsonite omphacite-phengitegarnet assemblages were 2.1–2.4 GPa at temperatures of 420–510℃, in or near the stability field of lawsonite eclogite, and implying formation under an apparent geothermal gradient of 6–8 ℃ km)1,consistent with metamorphism in a cold subduction zone. SHRIMP U-Pb dating of zircon from two lawsonite-bearing eclogitic metabasites yields ages of 489 ± 7 Ma and 477 ± 16 Ma, respectively. CL images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism. An age of 502 ± 16 Ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph-bearing eclogite, which is in agreement with the formation age of Early Ordovician for some ophiolite sequences in the North Qilian Mountains, and may be associated with a period of oceanic crust formation. The petrological and chronological data demonstrate the existence of a cold Early Palaeozoic subduction zone in the North Qilian Mountains.
    Zircon U-Pb SHRIMP ages of weakly to unmetamorphosed granitoids of the Yangtze basement outcrop in Dabieshan,central China [查看] HuaiminXue、ShuwenDong、PingJian
    The Qichun granitoids exposed in the Dabie Orogen of China are composed of two types of rocks: porphyritic monzogranite (with variable schistosity) and syenogranite (without schistosity). The two types show large differences in geochemical characteristics. The porphyritic monzogranite is characterized by high Al2O3 content (15.73%), relatively high CaO (2.46%) and Na2O contents (Na2O/K2O=1.27), strong depletion in HREE and strong fractionation between LREE and HREE ((La/Yb)N=46.8), similar to some high Al2O3 Archaean TTG gneisses.Conversely, the syenogranite is characterized by relatively low Al2O3 (14.05%) and CaO (0.82%) contents, and higher K2O than Na2O(Na2O/K2O=0.81). The degree of fractionation between LREE and HREE is minor. The U–Pb SHRIMP zircon age of the porphyritic monzogranite is 841±15 and 824±27 Ma for the syenogranite. These ages are similar to the protolith emplacement ages of granitic gneisses in the Dabie Orogenic Belt. The existence of weakly to unmetamorphosed granitoids in the Dabie Orogen shows that the granitoids were situated in the back part of the subducted plate during collision and subduction between the Yangtze and the North China cratons, and may represent outcrops of the Yangtze basement.
    Zircon SHRIMP geochronology and geochemistry of TTG rocks in Sushui Complex from Zhongtiao Mountains with its geological implications [查看] TIANWei、LIUShuwen、LIUChaohui、YUShengqiang、LIQiugen、WANGYueran
    The tonalite-trondhjemite-granodiorite(TTG)rocks in Sushui Complex of Zhongtiao Block can be divided into two series according to their zircon U-Pb ages and geochemical characteristics:one is subduction-related(SR)and the other collision-related(CR).The SR TTG rocks,together with other late Archean island arc magmatism,were developed as a result of oceanic subduction betwween the Eastern and Western blocks in Late Archean;while the CR TTG rocks formed in a thickened crustal envionment,which was responding to the collision between the Eastern and Western blocks in Paleproterozoic.All these features support a model that the Zhongtiao Block is a part of the Trans-North China Orogen in the middle of the North China Craton
    What Happened in the Trans-North China Orogen in the Period 2560-1850 Ma [查看] GuochunZHAO、LIUShuwen、MinSUN、LISanzhong、SimonWILDE
    Abstract: The Trans-North China Orogen (TNCO) was a Paleoproterozic continent-continentcollisional belt along which the Eastern and Western Blocks amalgamated to form a coherent North China Craton (NCC). Recent geological, structural, geochemical and isotopic data show that the orogen was a continental margin or Japan-type arc along the western margin of the Eastern Block, which was separated from the Western Block by an old ocean, with eastward-directed subduction of the oceanic lithosphere beneath the western margin of the Eastern Block. At 2550-2520 Ma, the deep subduction caused partial melting of the medium-lower crust, producing copious granitoid magma that was intruded into the upper levels of the crust to form granitoid plutons in the low- to medium-grade granite-greenstone terranes. At 2530-2520 Ma, subduction of the oceanic lithosphere caused partial melting of the mantle wedge, which led to underplating of mafic magma in the lower crust and widespread mafic and minor felsic volcanism in the arc, forming part of the greenstone assemblages. Extension driven by widespread mafic to felsic volcanism led to the development of back-arc and/or intm-arc basins in the orogen. At 2520-2475 Ma, the subduction caused further partial melting of the lower crust to form large amounts of tonalitic-trondhjedtic-granodioritic (TTG) magmatism. At this time following further extension of back-arc basins, episodic granitoid magmatism occurred, resulting in the emplacement of 2360 Ma, -2250 Ma 2110-21760 Ma and -2050 Ma granites in the orogen.Contemporary volcano-sedimentary rocks developed in the back-arc or intra-arc basins. At 2150-1920Ma, the orogen underwent several extensional events, possibly due to subduction of an oceanic ridge,leading to emplacement of mafic dykes that were subsequently metamorphosed to amphibolites and medium- to high-pressure mafic granulites. At 1880-1820 Ma, the ocean between the Eastern and Western Blocks was completely consumed by subduction, and the closing of the ocean led to the continent-arc-continent collision, which caused large-scale thrusting and isoclinal folds and transported some of the rocks into the lower crustal levels or upper mantle to form granulites or eclogites. Peak metamorphism was followed by exhumatioduplift, resulting in widespread development of asymmetric folds and symplectic textures in the rocks.
    U-Pb zircon SHRIMP ages, geochemical and Sr-Nd-Pb isotopic compositions of intrusive rocks from the Longshan–Tianshui area in the southeast corner of the Qilian orogenic belt, China Constraints on petrogenesis and t [查看] Hong-FeiZhang、Ben-RenZhang、NigelHarris、LiZhang、Yue-LongChen、Neng-SongChen、Zhi-DanZhao
    The Longshan-Tianshui area is located in the southeast corner of the Qilian orogenic belt. Three intrusions (the Yanjiadian diorite, the Caochuanpu and the Guanshan granites) in this area provide insights into regional tectono-magmatic events, petrogenesis and tectonic evolution. Based on U–Pb zircon SHRIMP dating, the magma emplacement ages of the Yanjiadian diorite, the Caochuanpu and the Guanshan granites are 441G10, 434G10 and 229G7 Ma, respectively. The Yanjiadian diorites have geochemical characteristics similar to island-arctype rocks, but their formation resulted from northward subduction of the Huluhe (Erlanping) back-arc basin. The Caochuanpu granites with K2O/Na2O＜0.9, Y of ＜6.9 ppm, Yb of ＜0.39 ppm, Sr/Y of 36–97 and (La/Yb)N of 34.8–80.0 exhibit adakitic geochemical signatures. Combined with Pb–Sr–Nd isotope data, the magma for the Caochuanpu granites was suggested to be derived from partial melting of previously underplated Neoproterozoic mafic rocks in the Qinling (or Longshan–Tianshui) island-arc lower crust. The partial melting resulted from island-arc lower crustal thickening due to northward subduction of the Qinling ocean and closure of the Huluhe back-arc. The Guanshan granites with (La/Yb)N of 17.5–42.4, Sr/Y of 15.8–50.5, Y of 7.8–15.7 ppm and Yb of 0.64–1.31 ppm also display some adakitic geochemical signatures. However, the geochemical and Pb–Sr–Nd isotopic compositions of the Guanshan granites are quite distinct from those of the Caochuanpu granites. It is proposed that magma for the Guanshan granites was derived from partial melting of subducted South Qinling–West Qinling continental crust during Triassic continental collision between the North China and the South China plates. The result provides an example that the adakitic rocks can be generated from partial melting of subducted continental crust. The tectono-magmatic events and petrogenesis of intrusive rocks in the Longshan–Tianshui area are in agreement with those of the North Qinling tectonic unit in the Qinling orogenic belt. Therefore, the Qilian orogenic belt corresponds to the North Qinling tectonic unit in a regional tectonic framework. Both the Qilian and the Qinling belts had a similar tectonic evolution.
    Ultrahigh-pressure and Retrograde Metamorphic Ages forPaleozoic Protolith of Paragneiss in the Main Drill Hole of the Chinese Continental Scientific Drilling Project (CCSD-MH), SW Sulu UHP Terrane [查看] LIUFulai、XUZhiqin、XUEHuaimin、ZHOUKaifu
    Abstract: Laser Raman spectroscopy and cathodoluminescence (CL) images show that most zircon crystals separated from paragneiss in the main drill hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) at Maobei, southwestern Sulu terrane, contain low-pressure mineral-bearing detrital cores, coesite-bearing mantles and quartz-bearing or mineral inclusion-free rims. SHRIMP UPb dating on these zoned zircons yield three discrete and meaningful age groups. The detrital cores yield a large age span from 659 to 313 Ma, indicating the protolith age for the analyzed paragneiss is Paleozoic rather than Proterozoic. The coesite-bearing mantles yield a weighted mean age of 228 ± 5Ma for the UHP event. The quartz-bearing outmost rims yield a weighted mean age of 213 ± 6 Ma for the retrogressive event related to the regional amphibolite facies metamorphism in the Sulu UHP terrane. Combined with previous SHRIMP U-Pb dating results from orthogneiss in CCSD-MH, it is suggested that both Neoproterozoic granitic protolith and Paleozoic sedimentary rocks were subducted to mantle depths in the Late Triassic. About 15 million years later, the Sulu UHP metamorphic rocks were exhumed to mid-crustal levels and overprinted by an amphibolite-facies retrogressive metamorphism. The exhumation rate deduced from the SHRIMP data and metamorphic P-T conditions is about 6.7 km/Ma. Such a fast exhumation suggests that the Sulu UHP paragneiss and orthogneiss returned towards the surface as a dominant part of a buoyant sliver, caused as a consequence of slab breakoff.
    Structural and geochronological constraints on the tectonic evollution of the Dulong-Song Chay tectonic dome in Yunnan province, SW China [查看] Dan-PingYan、Mei-FuZhou、ChristinaYanWang、BinXia
    The Dulong-Song Chay tectonic dome lies on the border of China (SE Yunnan Province) and northern Vietnam, and consists of two tectonic and lithologic units: a core complex and a cover sequence, separated by an extensional detachment fault. These two units are overlain unconformably by Late Triassic strata. The core complex is composed of gneiss, schist and amphibolite. SHRIMP zircon U–Pb dating results for the orthogneiss yield an age of 799G10 Ma, which is considered to be the crystallization age of its igneous protolith formed in an arc-related environment. A granitic intrusion within the core complex occurred with an age of 43–402 Ma, which probably formed during partial closure of Paleotethys. Within the core complex, metamorphic grades change sharply from upper greenschist-low amphibolite facies in the core to low greenschist facies in the cover sequence. There are two arrays of foliation within the core complex, detachment fault and the cover sequence: S1 and S2. The pervasive S1 is the axial plane of intrafolial S0 folds. D1 deformation related to this foliation is characterized by extensional structures.The strata were structurally thinned or selectively removed along the detachment faults, indicating exhumation of the Dulong-Song Chay tectonic dome. The major extension occurred at 237 Ma, determined by SHRIMP zircon U–Pb and 39Ar/40Ar isotopic dating techniques. Regionally,simultaneous tectonic extension was associated with pre-Indosinian collision between the South China and Indochina Blocks. The S2 foliation appears as the axial plane of NW-striking S1 buckling folds formed during a compressional regime of D2. D2 is associated with collision between the South China and Indochina Blocks along the Jinshajiang-Ailao Shan suture zone, and represents the Indosinian deformation. The Dulong granites intruded the Dulong-Song Chay dome at 144G2, 140G2 and 116G10 Ma based on 39Ar/40Ar measurement on muscovite and biotite.The dome was later overprinted by a conjugate strike-slip fault and related thrust fault, which formed a vortex structure, contemporaneously with late Cenozoic sinistral movement on the Ailao Shan-Red River fault.
    SHRIMP zircon U-Pb dating for volcanic rocks of the Dasi Formation in southeast Hubei Province, middle-lower reaches of the Yangtze River and its implications [查看] XIEGuiqing、MAOJingwen、LIRuiling、ZHOUShaodong、YEHuishou、YANQuanren、ZHANGZusong
    Abstract The Jinniu Basin in southeast Hubei,located at the westernmost part of middle-lower valley of the Yangtze River, is one of the important volcanic basins in East China. Volcanic rocks in the Jinniu Basin are distributed mainly in the Majiashan Formation, the Lingxiang Formation and the Dasi Formation, consisting of rhyolite, basalt and basaltic andesite, (trachy)-basalt and basaltic trachy-andesite and (trachy)-andesite and (trachy)-dacite and rhyolite respectively, in which the Dasi volcanism is volumetrically dominant and widespread. The Dasi volcanic rocks were selected for SHRIMP zircon U-Pb dating to confirm the timing of volcanism. The results indicate that there exist a large amount of magmatic zircons characterized by high U and Th contents in the volcanic rocks. The concordia ages for 13 points are 128 ± 1Ma (MSWD = 3.0). On account of the shape of zircons and Th/U ratios, this age is considered to represent the crystallization time of the Dasi volcanism.The volcanic rocks in the Dasi, Majiashan and Lingxiang Formations share similar trace element and REE partition patterns as well as Sr-Nd isotopic compositions. In combination with the regional geology,it is proposed that the southeast Hubei volcanic rocks were formed mainly during the Early Cretaceous, just like other volcanic basins in middle-lower Yangtze valley. A lithospheric extension is also suggested for tectonic regime in this region in the Cretaceous Period.
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