Sources of K-bentonites across the Ordovician-Silurian transition in South China: Implications for tectonic activities on the northern and southern margins of the South China Block

Abstract

During the Ordovician-Silurian transition, continuous large-scale explosive volcanism occurred around the world, including in South China, which reflected intense global plate movement in this transition. The tectonic activities leading to volcanism in the South China Block during this transition, including the northern and southern margins, remain controversial. K-bentonite altered from volcanic ash maintains reliable records of local and global tectonic events. Accordingly, research on the sources of K-bentonite layers in South China across this transition can provide implications for disputes about tectonic activities in the South China Block. This study presents a detailed geochemical analysis of the sources of K-bentonite layers in the Wufeng Formation (Upper Ordovician Katian stage) and Longmaxi Formation (Lower Silurian Rhuddanian stage) in three sections and one well in South China. Both bulk and zircon geochemical features indicate that the K-bentonite layers in South China during this transition had two sources, implying that volcanism originated from two different locations with two different geodynamic settings in or near the South China Block. K-bentonite from a within-plate (anorogenic) tectonic setting implies that within-plate extension on the northern margin of the South China Block was initiated in the Late Ordovician. K-bentonite from an arc-related (orogenic) tectonic setting implies that subduction-collision process occurred on the southern margin of the South China Block during this transition.