Introduction

In the 570 Ma since the beginning of the Phanerozoic, five great extinctions of organisms have been recorded, of which the one at the end of the Permian was the most serious. Thus, during the last decade, there has been a growing interest in study of the Permo-Triassic event (Asaro et al., 1982; Alekseev et al., 1983; Sun et al., 1984; Xu et al., 1985; Brandner et al., 1986; Oddone & Vannucii, 1986, 1988; Chai et al., 1987; Clark et al., 1986; He et al., 1987, 1988; Zhou et al., 1987a,b, 1988). Much work has been conducted in the fields of mineralogy, petrology, and isotopic and elemental geochemistry, and on microspherules and shock metamorphosed quartz. Even so, the nature of the mass extinction at the end of the Permian is still quite a controversial issue.

Authors have put forward various models to interpret the Permo-Triassic boundary (P/T) event, among which those favoring volcanic eruption and ones advocating extraterrestrial impact are dominant. Each model, of course, is supported by its own evidence.

Observed and experimentally derived facts that favor the volcanic-eruption model are:

1. Clays at the P/T boundary exhibit a residual rhyotaxitic structure and contain high-temperature beta-quartz, zircon, and apatite, which are common minerals in acidic intermediate volcanic rocks.

2. Multilayered clays present above and below the P/T boundary are similar to the boundary clay. Their elemental compositions resemble those of the volcanic ashes in the Triassic of South China.