Published online by Cambridge University Press: 12 October 2009
In this chapter we deal mainly with volcanism in the Permo-Triassic boundary interval in South China, with brief reference to other parts of the Tethyan realm. In much of South China a level commonly regarded as the Permo-Triassic boundary is marked by the so-called ‘boundary clayrock’, which is a layer of montmorillonite or interstratified montmorillonite and illite several centimeters to a few decimeters thick. The boundary clayrock is marine because it yields conodonts, ostracodes, foraminifera, and glauconite, and because its boron content is everywhere much higher than 100 ppm. It is at the level of the boundary clayrock that the strongest phase of Permo-Triassic mass extinction is recorded and it is also at this level that anomalous concentrations of δ13C, iridium, and other elements have been reported.
Our ideas about volcanism in the Permo-Triassic boundary interval are based on the nature of the boundary clayrock, and on evidence from sphaerules in the boundary clayrock.
Nature of boundary clayrock
More than 30 occurrences of the boundary clayrock and other boundary rocks have been found to be of volcanic origin. The well-known boundary clayrock of the Meishan section, Changxing, Zhejiang Province, was first regarded as sedimentary. However, owing to discoveries in it of high-quartz (beta quartz, Plate 13.1, fig. 8) and other volcanic indicators (He et al., 1987) this layer has been shown to be hydrolyzates of volcanic ash. The equally famous boundary clayrock in the Shangsi section, Guangyuan, Sichuan Province (bed 21 of Yang et al., 1987, or bed 27b of Li et al., 1986) is montmorillonitized tuff (Li et al., 1986).