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Character and Origin of Cambrian Hydrothermal Dolomite Conglomeration in the Northwestern Margin of Tarim Basin

ZHANG Juntao1),HU Wenxuan2),WANG Xiaolin2),QIAN Yixiong1),WU Shixiang 1)1)Exploration & Development Research Institute,Sinopec,Beijing,100083;2)Department of Earth Science,Nanjing University,Nanjing,210093  
Hydrothermal conglomerated dolomite developed mainly in the Cambrian in the northwestern margin of the Tarim Basin and show distinct strata-bound feature.Hydrothermal conglomerated dolomite is hosted only in crystalline dolomite,the transitional zone from the top of the Mid-Cambrian to evaporate at the bottom of the Upper Cambrian to dolomite.Hydrothermal dolomite is characterized by saddle crystal morphology,curved crystal and wavy extinction in thin section under crossed-polarized light.Geochemical analysis shows that hydrothermal dolomite has higher Ca/Mg values and Sr contents,slightly higher Fe,Mn and Ba contents,and more depleted δ18O values(-10.1‰~-12.2‰)than those of matrix dolomite(-6.1‰~-7.8‰).The δ13C values of hydrothermal dolomite in different locations vary greatly,but are similar to those of related host dolomite.The homogeneous temperatures of primary inclusions in the dolomites range from 90℃ to 120℃.The field geometry,petrography and geochemistry of hydrothermal dolomites indicate that higher temperature at which hydrothermal dolomite deposited resulted from the depth of deep burial and temperature gradient of the earth.The fluids were not magmatic but formation water from the Cambrian dolomite,possibly with involvement of deep circular meteoric water.After long-term fluid-rock reaction,these fluids were armed with the same geochemical features.Driven by faulting,hydrothermal fluids migrate along the interlayers.During this process,anhydrite nodule in the dolomite is dissolved to form voluminous of dissolved porosity.With the temperature and pressure decreasing and a large amount of CO2 released,hydrothermal dolomite starts to precipitate.Study of the forming mechanism of hydrothermal dolomite of this kind has some significance for understanding of evolution of the carbonatic rock reservoir in the Tarim Basin.
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