Experimental anatexis, fluorine geochemistry and lead-isotope constraints on granite petrogenesis in the Seridó Belt, Borborema Province, northeastern Brazil

Abstract

The Neoproterozoic Seridó Belt of northeastern Brazil is characterized by low-pressure, high-temperature metamorphism, large transcurrent shear zones and voluminous plutonic activity. The belt consists of a thick sequence of Neoproterozoic metasedimentary rocks overlying Paleoproterozoic metaigneous basement rocks. Both units were intruded by plutonic rocks related to two main magmatic episodes: large Neoproterozoic high-K calc-alkaline bimodal plutons and small intrusions of peraluminous pegmatitic granites and zoned pegmatites of Cambrian age. In this study, we partially melted samples of the potential protoliths (orthogneiss, paragneiss, mica schist and micaceous quartzite) as analogs for the sources of the magmas belonging to the felsic intrusions. Short-duration (24h) experiments assessed the composition of disequilibrium melts in both H2O-fluxed and fluid-absent conditions. Experiments with added water at 0.4 and 1.0GPa and 725°C produced strongly peraluminous and Ca-rich rhyolitic glass; these compositions are not observed in the natural rocks. In contrast, fluid-absent melting at 0.6GPa and 930°C produced mildly peraluminous rhyolitic glass with major-element compositions similar to those of the felsic intrusions in the Seridó Belt. The elevated F content of the glasses approximates that of the Neoproterozoic granites but is higher than that of the Cambrian pegmatitic granites. Mass balance calculations modeled the fractionation from initially enriched melts into exsolving fluids during the late-stage crystallization of the pegmatitic granites. The Pb-isotope compositions of the Seridó Belt felsic rocks show substantial variation from non-radiogenic mantle-like to high-radiogenic signatures. The basement gneisses and the metasedimentary belt rocks can be considered as the principal lead sources for several pegmatitic granite intrusions and zoned pegmatites. However, the high-radiogenic Neoproterozoic Acari granite and Cambrian pegmatitic granites are derived from an unknown lithologic unit and/or to the dissolution of zircon and monazite in high-temperature anatectic melts. These results place constraints on the crustal origin of most of the SiO2-rich plutons in the region.