Petrology of the Mesoarchean Rio Maria suite and the discrimination of sanukitoid series

Abstract

The rocks of 2.87Ga Rio Maria sanukitoid suite of the eastern Amazonian craton include granodiorites, intermediate rocks, layered rocks and mafic enclaves. Their REE patterns and the behavior of Rb, Ba, Sr, and Y allowed the distinction of a granodioritic (granodiorite and intermediate rocks) and a monzonitic (mafic enclaves) sanukitoid series. Petrogenetic modeling indicated that the granodiorites and intermediate rocks are not related by fractional crystallization. The internal evolution of the intermediate rocks were leaded by fractionation of amphibole+biotite±apatite, whereas the granodiorites evolved by fractionation of plagioclase+amphibole±biotite. The layered rocks were probably derived from the granodiorite magma by an accumulation of 50% of amphibole (dark layer) and of 30% of amphibole±plagioclase (gray layer). The petrogenesis of the Rio Maria suite required melting of a modified mantle extensively metasomatized by addition of about 30% TTG-like melt to generate the granodiorite (11% of melt) and intermediate magmas (14% of melt), and ~20% TTG-like melt in the case of mafic enclave magma (9% of melt).Modeling and geochemical data, particularly the behavior of Sr and Y, suggest that mafic enclave and granodiorite magmas were originated at different depths and should have mingled during their ascent and final emplacement. The modal and geochemical differences observed between the granodioritic and monzonitic sanukitoid series of Rio Maria are apparently a general feature of the Archean sanukitoids. This indicates the existence of at least two distinct sanukitoid series and suggests that the nature of the sanukitoid series is strongly dependent of the pressure of magma generation.Our results indicate that the sanukitoid magmas were originated in a two stage process. The envisaged model admits an active subduction tectonic setting in the Rio Maria terrane in between 2.98 and 2.92Ga when the TTG magmas responsible by the mantle metasomatism were generated (first stage). At ~2.87Ga, a tectonothermal event, possibly related to slab-break-off or due to the action of a mantle plume, induced the partial melting of the metasomatized mantle and generated the Rio Maria sanukitoid magmas (second stage).