Volcanic sources and diagenetic alteration of Permian-Triassic boundary K-bentonites in Guizhou Province, South China

摘要

Terrestrial Permian-Triassic boundary (PTB) sequences in South China usually contain one to three ash beds (K-bentonites), which provide evidence of a possible trigger for the terrestrial mass extinction and are useful as a distinct marker for correlation of the PTB boundary interval from terrestrial to marine areas. Here, we investigate the clay mineralogy and geochemistry of ash beds in the Zhejue and Jiucaichong sections, representing paludal and lacustrine environments, respectively, in order to better understand the source(s) and preservation of these ash beds. The two Zhejue K-bentonites exhibit similar compositions that are distinctly different from that of the Jiucaichong K-bentonite. The former contain mainly clay minerals with minor quartz, anatase, and lepidocrocite, and the clay minerals in these layers are mainly R1 and R3 mixed-layer illite/smectite (VS) and well-crystallized to poorly-crystallized kaolinite, whereas the latter consists mainly of R3 I/S with minor quartz, feldspars, and anatase. The former have higher CIA values (92.2 and 93.8) than the latter (79.4), indicating more intense chemical weathering that may have been related to deposition in a paludal setting with high organic acid concentrations in pore fluids. These K-bentonites also exhibit differences in major-element chemistry (MgO and K2O contents are higher at Jiucaichong due to lesser weathering intensity and, possibly, early diagenetic uptake from pore fluids), REE chemistry (Jiucaichong exhibits a larger Eu anomaly), and Nd isotopic compositions (Zhejue K-bentonites have Nd-143/Nd-144 ratios of 0.512436 and 0.512400, higher than the 0.512190 ratio for Jiucaichong). Although some of these compositional differences may be related to the dissimilar chemical environments in which diagenesis occurred (i.e., paludal vs lacustrine), the differences in Eu anomalies and Nd isotopic compositions imply heterogeneous volcanic ash sources. The source of these K-bentonites was probably a conti