ABSTRACT

The Late Permian Jiantianba reef complex is up to 250 m thick, 4 km wide and 50 km long. It is well exposed at Jiantianba, where the reef overlies black siliceous basinal limestones and is generally covered by algal-rich carbonates and, locally, by Triassic rocks. Multiple generations of cement are recognizable in the reef complex. Early cements in the complex include, in order of formation, fibrous calcite in multiple layers, isopachous fine-textured dolomite, radiaxial calcite, and bladed calcite, all of which are interpreted to be of marine origin. Broad calcite fans are rare and are also considered to be of marine origin. Later cements include, in order of deposition, coarsely crystalline dolomite, zoned calcite, and blocky calcite. These appear to be results of diagenesis in envi onments from shallow burial to burial over 3000 m deep. In addition to the two generations of dolomite cementation, two phases of replacement dolomitization can be differentiated, one during shallow burial and the other following deep burial cementation.

Fluid inclusions suggest moderately high temperatures of burial during precipitation of the coarse dolomite and blocky calcite cements, as indicated by homogenization temperatures (170 to 200°C) of two-phase fluid inclusions. Melting temperatures of frozen primary fluid inclusions (-38 to -20°C) indicate that parent solutions of the coarse dolomite and blocky calcite were brine waters.

Comparisons of magnesium and strontium data from the blocky calcite cement with data from modern marine aragonite suggest that precursors of the fibrous cement (with 1439 to 2439 ppm Mg and 121 to 135 ppm Sr) and radiaxial cement (with 1293 ppm Mg and 26 ppm Sr) were Mg-calcite or low Mg-calcite. Similarly, elevated Sr values in the blocky calcite indicate it did not form in meteoric water. Oxygen and carbon stable isotope compositions (¹³C = + 1.8 to + 4.19 PDB; ¹⁸O = -5.37 to -8.76 PDB) of cements and dolomites of the reef complex reflect influences of both marine water and elevated diagenetic temperatures.