Production of Jurassic rhyolite by anatexis of the lower crust of Patagonia

Abstract

The mid-Jurassic Marifil and Chon-Aike volcanic rocks of eastern Patagonia are part of one of the largest silicic igneous provinces known. Rb Sr geochronology indicates eruptive ages of 175–190 Ma for the Marifil complex (mostly Toarcian-Aalenian). The majority of the rocks are isotopically uniform, with initial 87Sr/ 86Sr= 0.7067 ± 0.0003 and εNd t = −4 ± 2. Primary magmas of andesitic composition were generated by partial melting of mafic “Grenvillian” lower crust, indentified by depleted-mantle model ages of 1150–1600 Ma. Lower crustal pyroxene-granulite xenoliths can be modelled as residual Jurassic source, although they may alternatively be co-genetic cumulates. The dacite-rhyolite suite formed by crystal-liquid fractionation processes from the primary andesites: involving multistage crystallization and re-melting during magma ascent, with the later stages of evolution being explicable by fractional crystallization of plagioclase, amphibole and accessory minerals. The province represents large-scale lower crustal reworking associated with the unique tectonic and igneous environment of Gondwana break-up.