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δ~(30)Si,δ~(18)O and elements geochemistry on the bedded siliceous rocks and cherts in dolostones from Cambrian strata,Tarim Basin

CHEN Yong-quan1,2,JIANG Shao-yong2,ZHOU Xin-yuan1,YANG Wen-jing1 and HAN Li-jun1 1.Research Institute of Exploration and Development,Tarim Oilfield Company,Petro China,Korla 841000,China;2.State Key Laboatory for Mineral Deposits Research,Department of Earth Sciences,Nanjing University,Nanjing 210093,China  
Bedded siliceous rocks and cherts-bearing dolostone are widely distributed in Cambrian strata in Tarim Basin.The bedded siliceous rocks occur mainly as deep water sediments in the eastern Tarim Basin,and have been sampled from the Yingdong 2 well.The chert-bearing dolostones show a wide distribution in Cambrian dolostones of the whole Tarim Basin.In Al-Fe-Mn diagram,data for the bedded siliceous rocks fall within the normal seawater sedimentary field.In the crust-normalized trace element spider diagram,the bedded siliceous rocks show relatively smooth right-inclined curves,indicating sedimentary origin.Their silicon and oxygen isotope compositions are 0.9‰-1.4‰,and 21.4‰-24.4‰,respectively,coincident with those of the sedimentary siliceous rocks.The cherts in dolostone demonstrate higher U content in comparison with the bedded siliceous rocks.The stelliform cherts in Luoxi slope show high content of trace elements and low oxygen (15.8‰) and silicon (1.7‰) isotope compositions,as a result of high reaction temperature;on the contrary,the original texture remained cherts in the western platform demonstrate low trace and rare earth elements concentrations and high oxygen (21.6‰-27.0‰) and silicon (1.0‰-3.8‰) isotope compositions,resulting from relatively low reaction temperature.Compared to other types of siliceous rocks,the cherts show relatively high δ30Si value,maybe as a result of the selectivity of H430SiO4 during the silicatization process.Based on the homogeneous temperatures of fluid inclusions in quartz in Gucheng 4 well and the δ18O values of cherts,the δ18O value of silicatized fluid is calculated to be 9.1‰.Therefore,We can calculate the temperatures of silicatized fluid in western basin,and the temperatures range in 101.3-158.5 ℃.The temperatures show a decreasing trend from east to west in western platform,indicating that the hydrothermal fluids might have come from some eastern faults of the western platform.
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