Publication: The evolution of a mid-crustal thermal aureole at Cerro Toro, Sierra de Famatina, NW Argentina
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Publication Date
2014
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Elsevier Science B.V., Amsterdam
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Abstract
A sheeted tonalite complex at Cerro Toro in western Sierra de Famatina, Argentina, was emplaced in the middle crust during an early phase of the Ordovician Famatinian orogeny. The intrusion produced a large-scale thermal aureole that provoked significant melting of the host rocks and hybridization of the anatectic melts with the tonalitic magmas. Three zones were distinguished in a cross-section: (1) an external zone consisting of metatexitic gneisses, amphibolites and minor tonalites, (2) an intermediate zone formed by screens of highly melted gneisses and amphibolites lying between the tonalite sheets: stoped blocks of gneisses within the tonalite sheets, small bodies of metagabbro and hybrid rocks are common in this zone, (3) an internal zone formed almost exclusively of massive tonalite. Incongruent melting of biotite in gneisses of the intermediate zone produced abundant cordierite and garnet; anatectic granitoids developed in the innermost part at 750–800ºC and ca. 5kbar. High water activities within this zone eventually promoted significant melting of plagioclase + quartz in the gneisses. Pooling of anatectic magmas resulted in allochthonous bodies of variable size either concordant or discordant to the regional structural grain, and part were retained in or close to the residuum. Allochthonous granitoids did not reach isotopic equilibrium with the source protolith probably due to fast extraction of the melts. The Cerro Toro contact aureole shows that assimilation of metasedimentary rocks through partial melting can be an important mechanism during emplacement of tonalitic magmas at middle crust levels. Thus the chemistry of cordilleran magma can be modified as they rise through the crust.