ENVIRONMENTS OF DOLOMITIZATION AND EVOLUTION OF CARBONATE DIAGENESIS IN THE TRIASSIC ZHENFENG MARGIN OF THE YANGTZE PLATFORM IN THE NANPANJIANG BASIN, SOUTH CHINA

The Yangtze Platform borders the Nanpanjiang Basin on its north and west. During the Early Triassic the platform evolved from low-relief ramp with oolitic margins to a steepening platform with a relatively flat-topped geometry with margin shoals and evaporitic interior facies.

At the Zhenfeng margin the precursor depositional facies include: oolitic grainstone to packstone, skeletal peloidal packstone, clotted microbialites and fenestral laminites. The Anshun strata range from undolomitized to partially dolomitized oolite and microbial facies to partially and completely dolomitized facies such as fenestral laminites. Dolomitization changes upward through the section with fenestral laminate facies being more pervasively dolomitized than the oolitic and skeletal packstone facies. The diagenetic evolution (paragenetic sequence includes: neomorphic alteration of aragonite, marine cementation, replacement dolomite, euherdral dolomites, saddle dolomites, calcite veins, stylolites, and late-stage fractures with calcite and oxide fill.

Previous data from the Yangtze Platform include dolomite showing δ¹⁸O values ranging from -7.7‰ to 0.75‰ (VPDB) and δ ¹³C values ranging from 0.77‰ to 4.0‰ (VPDB). Vein calcite values range from δ ¹⁸O -18.4‰ to -5.2‰ and δ ¹³C -6.1 to 3.4‰. ⁸⁷Sr/⁸⁶Sr values from dolomite ranges from 0.707677 to 0.708601 with the exception of elevated ⁸⁷Sr/⁸⁶Sr in three samples. Homogenization temperatures (Th) and freezing point depressions (Tm_ice) from primary fluid inclusion assemblages from dolomite crystals indicate entrapment of saline brines (9.5 to 16 wt. % NaCl) over temperatures of 80-185°C.

The preferential dolomitization of mud-rich platform interior facies and preservation of oolitic limestone facies at the platform margin points to interior derived dolomitizing fluids consistent with evaporative reflux. The range in δ ¹⁸O is consistent with enrichment by evaporative concentration of seawater, but also includes negative values consistent with high temperature fluids. ⁸⁷Sr/⁸⁶Sr values in dolomite are consistent with modified seawater including a radiogenic contribution of hydrothermal fluids. The geothermometric data, oxygen isotope values and radiogenic Sr contribution indicates that early dolomites were recrystallized at high temperatures by burial fluids.