Geochemistry and Geochronology of the Piratini Gneisses: 784 Ma (U-Pb SHRIMP) medium to high-K calc-alkaline magmatism in the SE Dom Feliciano Belt (RS, Brazil).

Abstract

Several occurrences of metamorphic xenoliths are found in Neoproterozoic granitoids in the Eastern Domain of the Dom Feliciano Belt (Rio Grande do Sul State, Brazil).  However, they still require detailed studies to investigate the past history and the nature of the crust of this domain. The present paper presents an investigation carried out in metamorphic xenoliths from the Piratini and Pinheiro Machado region, southeastern Sul-rio-grandense Shield, involving field works, optical microscopy, bulk-rock geochemistry and geochronology (zircon U-Pb SHRIMP). The studied gneissic association, named Piratini Gneisses, shows an intermediate to acid composition, predominantly granodioritic to tonalitic, medium to hig-K calc-alkaline, predominantly peraluminous, with a subduction-related magmatic arc signature. The U-Pb zircon ages obtained were 784 ± 4 Ma, representing the magmatism, and 664 Ma, representing the metamorphism. The new data allowed to interpret the protoliths as derived from an 800 Ma subduction-related continental arc magmatism, followed by a metamorphic event around 660 Ma.