Associated middle to late Jurassic volcanism and extension in southern South America

Abstract

The extrusive products of a Middle to Late Jurassic volcanic event occur throughout a wide area of southern South America. These volcanic rocks are associated in time and space with a series of NNW-trending grabens. The extension that produced the grabens began perhaps in the latest Triassic and continued throughout most of the Jurassic. The Middle to Late Jurassic volcanic rocks represent the culminating event of this period of extension. The Jurassic volcanic rocks described here are dominantly rhyolites and basalts, but flows of intermediate composition are also present. Major element geochemistry on a suite of samples taken from a west-east transect near 44°S latitude shows that these rocks are not related directly to convergent arc volcanism along the margin of South America, but are the products of a separate tectonic/magmatic event that involved significant crustal anatexis. The extension and related volcanism directly preceded the opening of the Rocas Verdes marginal basin along the western margin of Chile and may have led to the initial separation of South America and South Africa. As such, the Middle-to-Late Jurassic extension and volcanism heralded the breakup of part of Gondwanaland.