PRODUCTION OF JURASSIC RHYOLITE BY ANATEXIS OF THE LOWER CRUST OF PATAGONIA

The mid-Jurassic Marifil and Chon-Aike volcanic rocks of eastern Patag onia are part of one of the largest silicic igneous provinces known. R b-Sr geochronology indicates eruptive ages of 175-190 Ma for the Marif il complex (mostly Toarcian-Aalenian), The majority of the rocks are i sotopically uniform, with initial Sr-87/Sr-86 = 0.7067 +/- 0.0003 and epsilon Nd-t, = -4 +/- 2. Primary magmas of andesitic composition were generated by partial melting of mafic ''Grenvillian'' lower crust, in dentified by depleted-mantle model ages of 1150-1600 Ma. Lower crustal pyroxene-granulite xenoliths can be modelled as residual Jurassic sou rce, although they may alternatively be co-genetic cumulates, The daci te-rhyolite suite formed by crystal-liquid fractionation processes fro m the primary andesites: involving multistage crystallization and re-m elting during magma ascent, with the later stages of evolution being e xplicable 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.