Characteristics of saddle dolomite cements in the Cambrian-Ordovician carbonates in Tabei and Tazhong area of Tarim Basin

Detailed researches of Cambrian-Ordovician dolostones in Tabei and Tazhong areas of Tarim Basin demonstrated widespread presence of saddle dolomite cements, which commonly occur as cements (or infilling) in vugs and/or fractures. These dolomite crystals, generally 500 mu m up to 6 mm in size, are creamy white non-planar crystals with curved or lobate or stepped crystal faces. They are gappy and sweeping extinction under microscope, commonly associated with other hydrothermal minerals. The saddle dolomites have delta (13)C values from -2.446%c to 0.686%c PDB,and delta 5(18)O values from -9.101% to -5.117% PDB, and (87)Sr/(86)Sr ratios from 0.7086 to 0.7102. Fluid inclusion micro thermometry data of the saddle dolomites cements indicates homogenization temperatures (T(h)) of gas-liquid inclusions is about 121~159.5 degree C and salinities of dolomitization fluids is about 18.7%~23.1% (wt% NaCl equivalent) from which the saddle dolomites precipitated. These data suggest that the saddle dolomites could have forme