Mineralogical and geochemical characteristics of K-bentonites from the Late Ordovician to the Early Silurian in South China and their geological significance

摘要

There are numerous K-bentonites interlayered with black shales from the Late Ordovician-Early Silurian that are widely distributed in South China. In this paper, we carried out mineralogical and geochemical investigations on K-bentonite samples collected from 6 sections in South China. The petrological features of thin sections, X-ray diffraction data, and major element results show that in addition to clay minerals, which are dominated by illite/smectite mixed layer and illite, the K-bentonites also contain quartz, microcline, albite, pyrite, and zircon. Zircon U-Pb dating for two K-bentonite beds (WXP-BT2 and LBP-BT1) by the LA-ICP-MS method yielded two weighted mean Pb-206/U-238 ages of 443.5 +/- 1.9 and 440.4 +/- 5.6 Ma, respectively. The concentrations of rare earth elements (REEs) range from 141.28 to 854.44 ppm, chondrite-normalized REE patterns display a negative Eu anomaly and an enhanced enrichment in light REE. Samples plot in the fields of trachyte, trachyandesite, rhyodacite (dacite), and andesite in a plot of Nb/Y against Zr/TiO2 suggest that the K-bentonites are most probably derived from felsic magmas with subalkaline to alkaline affinities. The discrimination diagrams (Y-Nb, Y + Ta-Rb, Y + Nb-Rb) show that the tectonic setting of the source volcanoes ranges from a volcanic arc to within-plate setting. The widely distributed bentonites suggest intensified volcanic ash eruptions in the early Late Ordovician, which released huge amounts of volcanic ash. The SO2 emission and the weathering of the volcanic ashes probably resulted in global climate cooling and indirectly caused the Hirnantian glaciation and the mass extinction at the end of the Ordovician Period.