Abstract:

The petrologic,geochemical characteristics and origin of the Lower Ordovician dolomite in Yubei area were investigated through the analysis of the cores,thin-sections,SEM,cathode luminescence,trace element,C-O isotope and microthermometry of fluid inclusions.The following results were obtained.Matrix dolomites are of two types:fine crystalline,euhedral-subhedral dolomite and fine to coarse crystalline,anhedral dolomite with saddle fillings.The origin of dolomite is closely related to crystal texture(nonplanar anhedral,planar euhedral/subhedral)instead of crystal size.The fine crystalline,euhedral-subhedral dolomite has high Sr and low Fe-Mn content,and C and O isotope composition similar to that of seawater in Early Ordovician.So this type of dolomite was overall the product of low-temperature dolomitization associated with the Ordovician seawater during the early stage of shallow burial.In contrast,the fine to coarse crystalline,anhedral dolomite has low Sr,relatively high Fe and Mn content and more negative O isotope value.And its fluid inclusion microthermometry result suggests that it has high homogenization temperature and salinity.Combined with the regional tectonic background,it is believed that this type of dolomite was primarily the product of high-temperature dolomitization during late stage of shallow burial to moderate burial,and was locally transformed by hydrothermal fluids from marine-sourced saline water in formation.Moreover,there are two stages of saddle dolomite fillings:one is coeval with the matrix dolomite of hydrothermal dolomitization origin and the other may be associated with the thermal fluid post-magmatic activity during the Permian.

Key words: petrologic characteristics, geochemistry, dolomite, Lower Ordovician, Yubei area, Tarim Basin