Experimental Melting of Cordierite Gneiss and the Petrogenesis of Syntranscurrent Peraluminous Granites in Southern Brazil

Abstract

To understand the petrogenesis of peraluminous granites syntectonic INTRODUCTION to the Dorsal de Cangucu Transcurrent Shear Zone in the Sul-rioThe last two decades have seen an increasing interest in grandense Shield, Brazil, melting experiments were performed on processes of melt production from mantle or crustal one of the potential protoliths, a cordierite-bearing semi-pelitic sources, extraction, transport, accumulation and storage metasedimentary gneiss (PE-1). Experiments were conducted at in the crust (Clemens & Vielzeuf, 1987; Brown et al., pressures of 5, 10 and 15 kbar, at temperatures of 700–900°C, 1995b). As well as the interest in magmatic processes, and under fluid-absent and 5% H2O-present conditions. The the role played by structures, such as shear zones, in experiments show that fluid-absent melting begins at near-solidus collecting and transporting melts from their original conditions, around 700°C, promoted by participation of retrogressive sources to midand upper-crustal environments, has phengitic muscovite in the reaction Mus + Kf ± Qz = melt ± been an important field of study (Clemens & Mawer, Fe–Ti oxide ± Als, producing a very small amount of melt ( 800°C) produced S-type granitic melts promoted by participation rains are important issues in the study of migmatites of biotite or cordierite in the reactions Bio + Pl + Crd + Qz = (Brown et al., 1995a; Thompson & Connolly, 1995), as Px + Fe–Ti oxide + melt at 5 kbar, and Bio + Pl + Crd is the possibility of segregation of this melt to produce ± Qz = Grt + Als ± Kf + melt at 10 and 15 kbar, both plutonic bodies (Van der Molen & Paterson, 1979; Brown, producing a high amount of melt (10–63% by volume). The melt 1994; Vigneresse et al., 1996). compositions obtained at 900°C and 15 kbar under fluid-absent The experimental petrological study of reactions to conditions, promoted by biotite or cordierite breakdown, are similar elucidate the processes that generate melts has conto the syntectonic granites. However, it is unlikely that the granites tributed to the understanding of the petrogenesis of were formed at this pressure (corresponding to a depth of melting of granitoids, and volcanic and metamorphic rocks, and >54 km). has allowed the testing of different geological models, and the delineation of physical and chemical constraints. Tuttle & Bowen (1958) were among the first to perform experiments to understand the evolution of natural