The evolution of a mid-crustal thermal aureole at Cerro Toro, Sierra de Famatina, NW Argentina

Abstract

A more than 12km wide sheeted tonalite complex in western Sierra de Famatina, NW Argentina, was emplaced at middle crust levels (ca. 5kbar), coeval with regional metamorphism during an early phase of the Ordovician Famatinian orogeny (ca. 480Ma). Advective heat from the tonalite complex caused a rise in the host regional temperatures (≤700°C) by a maximum of ca. 100°C, developing an aureole (~3km wide) parallel to the igneous contact. This was accompanied by significant melting (ca. 40%) of the host rocks that hybridized to a variable extent with the tonalitic magmas. Three metamorphic zones were distinguished in a cross-section through the aureole: (1) an external zone consisting of metatexitic gneisses, amphibolites and minor tonalites, (2) an intermediate zone formed by screens of highly melted gneisses, amphibolites and metagabbros lying between tonalite and newly formed leucogranitoid and hybrid rock sheets, and (3) an internal zone formed almost exclusively of massive tonalite and minor hybrid rocks. Incongruent melting of biotite in gneisses of the intermediate zone produced peritectic cordierite and garnet. Hybrids resulting from variable mixing of anatectic granitoids and tonalite magma developed in the innermost part of the aureole at 750–800°C. Increased water activity within this zone eventually promoted increased melting of plagioclase+quartz in the gneisses. Leucogranitoid magmas formed in part by extraction from the hybrid magmas led to heterogeneity of the Sr-isotope composition. The Cerro Toro contact aureole shows that assimilation of metasedimentary rocks through partial melting can play an important role during emplacement of tonalitic magmas at mid-crustal levels.