A SET OF PRIMARY DOLOMITE ROCKS FORMED IN LACUSTRINE MANTLE-ORINGINED HYDROTHERMAL ENVIRONMENT IN SANTANGHU BASIN, XINJIANG, NW CHINA

The NW-SE trending Santanghu Basin is located between Tianshan Mountains and Altai Mountains. The basin went into intralpate rift evolution under regional extension since late Carboniferous and it was a starved, deep lacustrine intracontinental rift basin mainly consisted of intraclastic deposits in early-mid Permian.

Here we report a set of dolomite rocks which are characterized by extreme thin laminated of 0.3 cm to 1 cm thick. Potassium ferricyanide and alizarin red staining results show iron dolomicrite and silty dolomite (grain size 0.01-0.05 mm) are dominant composition. Dolomite crystals are poor crystallized. Their shapes are either spherulitic or anhedral, at times subhedral. Potassium feldspar and analcite silt (less than 50%) are observed in dolomite rocks. Thin sections show some analcite occurs in leucithohedron shape associated with harbour-like corrosion rims. Either accompanying residual tuff or residual leucite is found via electron probe. This indicates the analcite silt is from analcime phonolite.

Sr content of dolomite rocks in study area reaches to 164×10^-6～537×10^-6, 379. 1×10^-6 on average, most Sr/Ba> 1, 4.05 averagely. δ¹³C PDB values are 4.8‰~11.4‰, 6.93‰ averagely. The average content of apatite is 2% and that of halite is 4.3%. Abundant pyrite (95.7% of the highest relative content) is observed. The crude oil produced from Lucaogou Formation is rich in β-carotane and gammacerane (GI=0.04~0.74). These indicate the environment of Lucaogou Period was under quiet, strong reducing and high salinity conditions.

The δ¹⁸O PDB values are between -5‰ to -21.1‰ (-11.9‰ on average) and radiogenic ⁸⁷Sr / ⁸⁶Sr values are 0.704570-0.705775, 0.705120 on average. Results of electron probe processed on iron dolomite with zonal texture show the highest content of FeO reaches to 31.734%, 12.314% on average.

Given the upper strata of Lucaogou formation is ultrabasic–basic volcanic rocks with huge thickness (thickest to 958m), we assume it was during mantle uplifting when ultrabasic magma diapired into lower crust leading to serpentinization which released abundant Mg²⁺ and Fe²⁺, consequently required Mg and Fe for dolomite precipitation was supplied. We primarily believe the dolomite studied here is a set of primary dolomite of exhalative mantle-hydrothermal-related formed in continental rift basin.