Mineralogical and geochemical characteristics of altered volcanic ashes (tonsteins and K-bentonites) from the latest Permian coal-bearing strata of western Guizhou Province, southwestern China

Abstract

Although the geochemical and mineralogical characteristics and provenance of Permian-Triassic boundary (PTB) widespread volcanic ashes from southern China have been well investigated, the volcanic ashes just prior to the end-Permian mass extinction (EPME) have rarely been studied. This paper presents the mineralogical and geochemical compositions of volcanic ashes from the latest Permian coal-bearing strata of southwest China. Based on the relative abundances of kaolinite and mixed-layer illite/smectite (I/S), the volcanic ashes in the present study can be divided into tonsteins and K-bentonites, which are dominated by kaolinite and I/S, respectively. The I/S in K-bentonites formed during burial diagenesis in a non-marine environment, indicated by their low contents of sulfur and low ratios of Sr/Ba. Based on the general geothermal gradient and vitrinite reflectance of coals, the I/S was deduced to have formed under a temperature less than 100 °C. Collectively, the occurrence of vermicular kaolinite, as well as high-temperature quartz and euhedral primary zircon identified in tonstein samples indicate an intermediate-felsic pyroclastic origin for the volcanic ashes, while rounded quartz and mica grains both in tonsteins and K-bentonites reflect the products of erosion from sediment source regions. The negative Eu anomalies, rare earth elements (REY) distribution patterns, low contents of Sc, V, Cr, Co, Ni and Zn, and Al2O3/TiO2 values, suggest that the tonsteins and K-bentonites are related to felsic volcanic eruptions. The relationship between Al2O3/TiO2 vs. Nb/Yb and Al2O3/TiO2 vs. Zr/TiO2 ratios indicate that the tonsteins and K-bentonites were derived from at least two felsic sources: (1) some distal volcanic arc(s) and (2) the top of the Kangdian Upland exposing the alkaline-felsic volcanic rocks formed during the waning activity of the Emeishan plume and the basement of the Kangdian Upland exposing older felsic rocks.