Experimental melting of cordierite gneiss and the petrogenesis of syntranscurrent peraluminous granites in Southern Brazil

Abstract

To understand the petrogenesis of peraluminous granites syntectonic to the Dorsal de Cangugu Transcurrent Shear Zone in the Sul-rio-grandense Shield, Brazil, melting experiments were performed on one of the potential protoliths, a cordierite-bearing semi-pelitic metasedimentary gneiss (PE-1). Experiments were conducted at pressures of 5, 10 and 15 kbar, at temperatures of 700-900° C, and under fluid-absent and 5% H2O-present conditions. The experiments show that fluid-absent melting begins at near-solidus conditions, around 700° C, promoted by participation of retrogressive phengitic muscovite in the reaction Mus + Kf ± Qz = melt ± Fe-Ti oxide ± Als, producing a very small amount of melt (<9%) with widely ranging composition. All hypersolidus experiments (>800° C) produced S-type granitic melts promoted by participation of biotite or cordierite in the reactions Bio + Pl + Crd + Qz = Px + Fe-Ti oxide + melt at 5 kbar, and Bio + Pl + Crd ± Qz = Grt + Als ± Kf + melt at 10 and 15 kbar, both producing a high amount of melt (10-63% by volume). The melt compositions obtained at 900° C and 15 kbar under fluid-absent conditions, promoted by biotite or cordierite breakdown, are similar to the syntectonic granites. However, it is unlikely that the granites were formed at this pressure (corresponding to a depth of melting of ~ 54 km).