Petrography and geochemistry of tonsteins from the 4th Member of the Upper Triassic Xujiahe formation in southern Sichuan Province, China

Petrographic and geochemical studies on five layers of tonsteins from the 4th Member of the Upper Triassic Xujiahe formation (T(3)xj(4)) in the southern Sichuan Province of China are presented in this paper. The results suggest that these tonsteins were derived from synsedimentary acid volcanic ash fallout, which was subsequently altered into relatively pure kaolmitic rocks through hydrolysis and diagenesis in a peat-bog environment (for definition of tonsteins, refer to [Geol. Soc. Am., Spec. Pap. 285 (1993)]). The petrographic texture is mainly of dense and granular types, with a subordinate crystalline type. Each tonstein layer is characterized by its unique petrographic texture, mineral content, and assemblage of lithophile elements, which is related to the composition of its parent magma. The trace element geochemistry of these tonsteins is elaborated in this paper, which also suggests an origin as silicic volcanic ash fallout. Typically, these features are relatively consistent within the same tonstein layer over a broad lateral extent, and are characteristic of each specific tonstein layer. Therefore, some tonsteins in this region serve as distinct marker beds for correlation of coal seams within the framework of regional chronostratigraphy. (C) 2002 Elsevier Science B.V. All rights reserved.