THE COALIFICATION OF TRIASSIC COALS IN CHUXIONG BASIN, YUNNAN PROVINCE, CHINA

The Chuxiong basin is one of the foreland basin located in Yunnan province developed near the southwestern margin of the Yangtze craton during Mesozoic time.. The west side of this basin was affected by the southern part of the Panxi rift process. The coalification process is analyzed in terms of vitrinite reflectance measurements, and current thoughts of tectonic and thermal evolution history of this basin. The coal measures in this basin were coalified twice; the earlier one at the foreland basin stage during Late Triassic to Early Jurassic age and the later one at the rift stage during Late Cretaceous to Paleocene age. The earlier coalification was mainly the result of deep burial with coal rank varies from lignite on the southeast side of the basin to high volatile bituminous coal with a maximum vitrinite reflectance of 1.0% on the northwest side. The superimposed second coalification process was the result of plutonic metamorphism and tectonic event occurred during the rift stage. The coalification trend continued its northwestward increase from bituminous coal to anthracite with maximum vitrinite reflectance reach and exceeded 3.5% on the northwestern margin of the basin.