Provenance changes for mineral matter in the latest Permian coals from western Guizhou, southwestern China, relative to tectonic and volcanic activity in the Emeishan Large Igneous Province and Paleo-Tethys region

Abstract

Recent studies have found that the provenance for water-transported sediments in some coal seams in the Late Permian and Early Triassic sequences from SW China have changed from mafic rocks of Emeishan Large Igneous Province (ELIP) to intermediate-felsic and alkaline igneous rocks from the ELIP and Paleo-Tethys convergent margins. However, the mechanism of provenance changes for mineral matter in the late Permian coals from SW China remains unclear. This paper uses the detailed mineralogical and geochemical characteristics of the latest Permian coal-bearing strata from the Huopu coal deposit in SW China to identify the potential detrital provenances of mineral matter.The mineralogy of the Huopu coals is dominated by kaolinite, quartz, and calcite. Fracture-filling kaolinite, calcite, quartz, anatase, as well as closely coexisting fracture-filling kaolinite, quartz, and chamosite, were possibly formed by multiple-stage solution injections. The Al2O3/TiO2 values, REY distribution patterns (REY, rare earth elements and Y), as well as Nb/Yb vs. Al2O3/TiO2 and Zr/TiO2 vs. Al2O3/TiO2 diagrams all indicate that the roof and floor sediments of the coal seam were sourced mostly from the ELIP high-Ti basalts. In contrast, the inorganic matter of the coals was derived from the Kangdian tops exposing (1) the Proterozoic meta-granitic complex; (2) the ELIP’s volcanic complex including, in addition to basalts, alkaline and intermediate-felsic lavas and small intrusions; and (3) volcanic ashes from the Paleo-Tethys convergent margins. The latest Permian sedimentation in the Huopu coal deposit is suggested to have been controlled by tectonic activity along the Xiaojiang Fault that separates the inner ELIP from the Western Yangtze Cratonic Basin (WYCB). This activity is characterized by repeated uplifts of the inner ELIP associated with some downward movements in the WYCB and vice versa. The ELIP uplift and WYCB subsidence may have led to intensive erosion of the ELIP basalts and marine transgression producing the clastic sediments, while the ELIP’s downward movement associated with some WYCB uplift may have promoted peat deposition and erosion of the Kangdian tops exposing both the ELIP intermediate-felsic volcanic rocks and the older meta-granitic complexes. Both sedimentation regimes were evidently accompanied by volcanic activity on the Paleo-Tethys convergent margins, but preservation of the volcanic ash layers was possible only during more quiescent peat-accumulation periods. The reduced detrital supply in these periods would result in a higher proportion of the intermediate-felsic pyroclastic debris disseminated in the coals.