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SHRIMP Zircon U-Pb Dating of Gabbro and Granite from the Huashan Ophiolite,Qinling Orogenic Belt,China Neoproterozoic Suture on the Northern Margin of the Yangtze Craton [查看] SHIYuruo、LIUDunyi、ZHANGZongqing、MIAOLaicheng、ZHANGFuqin、XUEHongmei

Abstract: The recently identified Huashan ophiolitic melange was considered as the eastern part of the Mianliie suture in the Qinling orogenic belt. SHRIMP zircon U-Pb geochronology on gabbro from the Huashan ophiolite and granite intruding basic volcanic rocks indicates crystallization ages of 947±14 Ma and 876±17 Ma respectively. These ages do not support a recently proposed Hercynian Huashan Ocean, but rather favor that a Neoproterozoic suture assemblage (ophiolite) is incorporated into the younger (Phanerozoic) Qinling orogenic belt.

High-pressure granulite from Western Kunlun,northwestern China Its metamorphic evolution,zircon SHRIMP U-Pb ages and tectonic implication [查看] QUJunFeng、ZHANGLiFei、AIYongLiang、LÜZeng、WANGJianPing、ZHOUHui、WANGShiYan

High-pressure mafic granulites occurring as lenticular bodies within garnet-amphibolites in Kangxiwar Fault have been first reported in this paper. The P-T conditions of two metamorphic stages were obtained using calibrated geothermal barometers and ThermoCalc Program. The peak metamorphic condition of these high-pressure granulites is about 760―820℃, 1.0―1.2 GPa and the retrograde metamorphic condition is about 620―720℃, 0.7―0.8 GPa. The petrological studies show that they have a near-isobaric cooling P-T path which suggests that the Western Kunlun underwent initial crustal thickening, subsequent exhumation and cooling. The SHRIMP zircon U-Pb dating gives two groups of ages for high-pressure granulites. One is 177±6 Ma which is obtained from the rim of the zircon. We consider this age should be the metamorphic age. And the other is 456±30 Ma which is obtained from the core of the zircon and should be the protolith age. The formation of these high-pressure granulites in western Kunlun is closely correlated with the evolution of the Paleo-Tethys and has important implications for the research on Tethys and Paleo-Asian tectonic zone.

A New SHRIMP Age of the Xiamaling Formation in the North China Plate and Its Geological Significance [查看] GAOLinzhi、ZHANGChuanheng、SHIXiaoying、ZHOUHongrui、WANGZiqiang、SONGBiao

Setting up the hypostratotype of late Precambrian is the main aim of the research on the meso-and Neoproterozoic in North China.The chronostratigraphic position is the key in this study,However,many key horizons have not been calibrated with the high-quality isotopic ages.Using the reported new U-Pb age with the Sensitive High-Resolution Ion Microprobe(SHRIMP II),a zircon U-Pb age was obtained of the ash bed in the Xiamaling Formation in North China Plate,yielding a weighted mean 206Pb/238U age of 1368±12Ma.It is the first SHRIMP U-Pb age from the Ciamaling Formation in the North China Plate,and represents the depositing time of the middle part of the Xiamaling Formation.The zircon age plays an important role to understanding geological evolution of the North China Plate during Meso-and Neoproterozoic.

U-Pb Zircon Dating of the Granitic Conglomerates of the Hutuo GroupAffinities to the Wutai Granitoids and Significance to the Tectonic Evolution of the Trans-North China Orogen [查看] JimZHANG、GuochunZHAO、LISanzhong、MinSUN、LIUShuwen、XiaopingXIA、YanhongHE

The Wutai Complex associated with the adjacent Fuping and Hengshan Complexes represents the best and classical cross-section in the middle segment of the Trans-North China Orogen, generally divided into Eastern and Western Blocks. Unconformably overlying the Wutai and Fuping Complexes is the Hutuo Group considered as the youngest lithostratigraphic unit in the region and important both for interpreting Precambrian history as well as the overall evolution of the Trans-North China Orogen. Lack of knowledge about provenance of the sedimentary rocks in this group has hindered understanding of the depositional environments and tectonic significance. LA-ICP-MS was applied to obtain U-Pb zircon ages for the granitic pebbles, the lowest lithostratigraphic rock of the Hutuo Group, which, combined with previous lithostratigraphic, geochronological, structural and metamorphic data, provides new constraints on the sedimentary provenance and tectonic evolution of the region. The sequence of the Hutuo Group ranges upward from lower basal conglomerates and volcaniclastic rocks (Doucun Subgroup), through clastic sediments, slates, dolomites and marbles (Dongye Subgroup), to sandstones and conglomerates at the top (Guojiazhai Subgroup). Zircons from granitic pebbles preserved in the Doucun Subgroup basal conglomerates give weighted mean 207Pb/206Pb ages between 2517 Ma and 2566 Ma, which are the same as those for the late Archean Wutai Cranitoids, indicating that the pebbles were derived from the Wutai granitic intrusions (2566-2515Ma). Based on the new data and previous studies, combined with an igneous zircon crystallization age of 2087±9 Ma obtained for volcanics in the Hutuo Group, the Hutuo Group was deposited in a subduction-related retro-arc foreland basin environment that developed behind the Wutai arc during the eastward-directed subduction of the Western Block beneath the western margin of the Eastern Block. This basin underwent long-lived sedimentation and finally closed during the -1850 Ma collisional event that resulted in the final amalgamation of the North China Craton.

U-Pb Age and Hf Isotope Study of Detrital Zircons from the Wanzi Supracrustals Constraints on the Tectonic Setting and Evolution of the Fuping Complex, Trans-North China Orogen [查看] XiaopingXIA、MinSUN、GuochunZHAO、WUFuyuan、XUPing、JianZHANG、YanhongHEandZHANGJiheng

Located in the middle segment of the Trans-North China Orogen, the Fuping Complex is considered as a critical area in understanding the evolution history of the North China Craton (NCC). The complex is composed of various high-grade and multiply deformed rocks, including gray gneiss, basic granulite, amphibolite, fine-grained gneiss and marble, metamorphosed to upper amphibolite or granulite facies. It can be divided into four rock units: the Fuping TTG gneisses, Longquanguan augen gneisses, Wanzi supracrustals, and Nanying granitic gneisses, U-Pb age and Hf isotope compositions of about 200 detrital zircons from the Wanzi supracrustals of the Fuping Complex have been analyzed. The data on metamorphic zircon rims give ages of 1.82-1.84 Ga, corresponding to the final amalgamation event of the NCC, whereas the data for igneous zircon cores yield two age populations at -2.10 and -2.51 Ga, with some inherited ages scattering between 2.5 and 2.9 Ga. These results suggest that the Wanzi supracrustals were derived from the Fuping TTG gneisses (-2.5 Ga) and the Nanying granitic gneisses (2.0-2.1 Ga) and deposited between 2.10 and 1.84 Ga. All zircons with -2.51 Ga age have positive initial EW values from +1.4 to +10.9, suggesting an important crustal growth event at -2.5 Ga through the addition of juvenile materials from the mantle. The Hf isotope data for the detrital zircons further imply that the 2.8 Ga rocks are important components in the lower crust, which is consistent with a suggestion from Nd isotope data for the Eastern Block. The zircons of 2.10 Ga population have initial values of 4.9 to +6.1, interpreted as mixing of crustal remelt with minor juvenile material contribution at 2.1 Ga. These results are distinct from that for the Western Block, supporting that the Fuping Complex was emplaced in a tectonic active environment at the western margin of the Eastern Block.

SHRIMP zircon U-Pb dating for subduction-related granitic rocks in the northern part of east Junggar, Xinjiang [查看] ZHANGZhaochong、YANShenghao、CHENBailin、ZHOUGang、HEYongkang、CHAIFengmei、HELixin、WANYusheng

SHRIMP U-Pb zircon dating on the Xileketehalasu granodiorite porphyry and Kalasayi monodiorite porphyry that intrude middle Devonian Beitashan Formation at the north part of east Junggar region shows that they were formed at 381±6 Ma and 376±10 Ma respectively. They are interpreted as subduction-related granitic rocks, which is the first report that the isotopic ages for the granitic rocks range from 350 to 390 Ma. Another determined age for the Kalasayi monodiorite porphyry is 408±9 Ma,representing the age of underlain Lower Devonian volcanic rocks. Thus, the U-Pb dates suggest that the northeastward subduction of Junggar ocean from southwest occurred at 408 to 376 Ma (the real inter-val may be larger). Because the ore bearing porphyry intruded following the formation of the volcanic rocks of middle Devonian Beitashan Formation, their tec-tonic setting is similar to the Andes Mountains that hosts world-class porphyry copper deposits, and the researched area could be regarded as a potential area for prospecting large porphyry copper deposits.

Polyphase tectonothermal history recorded in granulitized gneisses from the North Qaidam HPUHP metamorphic terrane,Western Chinaevidence from zircon U-Pb geochronology [查看] ZhangJianxin、ChristopherG.Mattinson、MengFancong、WanYusheng、Dongkuoan

Magmatic Event at the End of the Archean in Eastern Hebei Province and Its Geological Implication [查看] GENGYuansheng、LIUFulaiandYANGChonghui

By using the SHRIMP U-Pb and single zircon stepwise evaporation methods, the authors have obtained some results for granitoids from eastern Hebei Province. The Yuhuzhai hyperthene tonalitic granite was formed 2550 Ma ago, the Qingyangshu gabbroic gneiss 2536 Ma, the Yinmahe granodioritic gneiss near Lücao, Lulong County, 2533 Ma, the gabbro-dioritic gneiss near Longwan, Qianxi County, 2518− 2515 Ma, the Qiuhuayu trondjemitic gneiss at Zunhua 2515 Ma, the Xiaoguanzhuang tonalitic gneiss at Zunhua 2495 Ma, and the Cuizhangzi gneiss in Qianxi County 2492 Ma. These geochronilogical data demonstrate that, though diverse in composition, type and origin, the granitic gneisses in eastern Hebei Province were emplaced and crystallized in a rather short period of magmatic activity. The formation of such a great amount of gneisses in this small time gap suggests that it was a critical crust accretion stage at the end of Ne oarchean. The fact that granitoids of various types occurred at the same time implies a large-scale underplating (mantle plume) activity, which was then responsible for the crust accretion.

Identification of Hercynian shoshonitic intrusive rocks in central Hainan Island and its geotectonic implications [查看] XIECaifu、ZHUJinchu、DINGShijiang、ZHANG

An identification has been made of some shoshonitic intrusive rocks in central Hainan Island recently. These rocks are K-rich (K2O=2.9%-5.1%, K2O/Na2O=0.95-2.12), distinctly enriched in LILE and LREE, strongly depleted in Nb, Ta, and moder-ately depleted in Sr and Ti, with (87Sr/86Sr)i = 0.70859-0.71425 andεNd(t)=(− 2.77―− 7.49). They were derived from an EMⅡ-type mantle source. The enrichment process is related to metasomatism of depleted mantle caused by a great amount of fluid-melt released from oceanic crust and terri-genous sediments at great depth (eclogite facies) during the subduction of the South China plate under the Indochina-South China Sea plate in the Carbon-iferous–Early Permian. A SHRIMP U-Pb zircon dating yields a crystallization age of 272±7 Ma for the sho-shonitic intrusions, which is coeval with the strongly peraluminous granites found in central Hainan Island. These two kinds of rocks generally possess syn-intrusion ductile deformation structures. Thus they are considered to have been generated during the early stage (syn-thrust phase) of a post-collisional event. The primary magma of shoshonitic rocks was produced at a depth > 80 km by decompression-dehydration melting of previously enriched litho-spheric mantle wedge, phlogopite-bearing garnet peridotite, which was in turn caused by the break-off of a descendent slab and upwelling of a hot as-thenosphere. The rising of melts was accompanied by crustal contamination and crystallization fractiona-tion (AFC). Combining with other related data, it is proposed that the southwards subduction and amal-gamation of the South China plate with the Indo-China-South China Sea plate took place at ca. 287－278 Ma, which was a part of the convergence proc-ess of the Pangea supercontinent. The suture zone was probably located along the line of Song Ma-Beibu Gulf-north margin of the Yunkai Moun-tains-Wuyi Mountains.