Idades Th-U-PbT de monazita e geotermobarometria de rochas metapelíticas de alto grau do sistema orogênico Itabuna-Salvador-Curaçá, BA, Brasil: um estudo em microssonda eletrônica

Abstract

Electron microprobe Th-U-PbT dating of monazite was performed on aluminous gneiss samples from the northern Jacurici and southern Ilhéus areas of the Itabuna-Salvador-Curaçá Orogenic System. Studied samples are spinel-sillimanite-cordierite-garnet gneiss and corundum-spinel-sillimanite-garnet gneiss that equilibrated during a high temperature metamorphism at T-P conditions of about 835°C/6.9 kbar and 890°C/7.4 kbar, respectively. Microprobe ages of monazite in equilibrium with these mineral associations are interpreted as true crystallization ages and date granulite facies metamorphism. Both samples gave ages of about 2.07 and 2.08 (± 0.02) Ga, in agreement with the known geochronological pattern in this system. The Ilhéus sample contains a younger, contrasting monazite generation (e.g., with distinct REE patterns and Th/Gd ratios), dated at ca. 1.97 (± 0.04) Ga. Based on the occurrence of the corundum-sillimanite-quartz association in the sample and information from the literature, we suggest that this age may be related to a regional decompression event and/or a thermal overprint due to late intrusive magmatism (1.9 - 2.0 Ga) in the orogenic system.