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《Geochimica》 2016-02
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Trace-element geochemical characteristics of different textural types of Lower Paleozoic dolomites in the Tazhong-Bamai area

HUANG Qing-yu;LIU Wei;SHI Shu-yuan;WANG Kun;MENG Xiang-hao;Research Institution of Petroleum Exploration and Development,Petrochina;State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology;  
The main objective of this paper is to investigate the origin of matrix dolomites and void-filling dolomite cements with different textures in the Lower Paleozoic carbonates in the Tarim Basin, based on the core observation, thin-section analysis, cathodoluminescence, trace-elements and electron microprobe analysis. The results show that micrite dolomite interbedded with evaporitic rocks has high Sr and Na contents but low Fe and Mn contents, suggesting that this type of dolomites was formed in a supersaline seawater enverionment. The Na, Fe and Mn contents of grain dolomites are similar to those of micrite dolomites. The Sr contents, however, are lower than those of other types of dolomites. The reason may be associated with microbial mineralization. Because the grain dolomites are usually overlaid by evaporites, their origin can be interpreted to be of reflux dolomitization during the penecontemporaneous stage. Fine crystalline, enhedral/subhedral dolomites with planar crystal boundaries have relatively low Sr and Na contents and slightly increasing Fe and Mn contents. These features imply that the dolomites were formed at lower temperature during the shallow burial period. Medium to coarse crystalline dolomites with nonplanar crystalline boundaries have the lowest Sr and Na contents and obviously increasing Fe and Mn contents. The co-variation of Fe and Mn, however, is of no positivity and the absolute contents of those trace-elements are lower than those of typical burial dolomites from other basins, showing that the dolomitization may occur at relatively high temperature in a medium burial rather than deep burial environment. Void/fracture-filling saddle dolomite cements have low Sr contents but high Fe, Mn, Ba and Zn contents, suggesting the effect of hydrothermal fluids. The formation mechanisms of saddle dolomite cements in different areas are various due to the diverse sources of hot fluids and varying tectonic activities. Different textural types of dolomites have different trace element compositions, thus the texture features can be used to distinguish the origins of different dolomites.
【Fund】: 国家科技重大专项(2011ZX05004);; 中国石油天然气股份有限公司重大科技专项(2014E-3201)
【CateGory Index】: P588.245;P595
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