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.