Coeval TTG and sanukitoid magmatism during the Rhyacian tectonic evolution of the Juiz de Fora Complex (São Francisco Paleocontinent, SE-Brazil)

Abstract

Transitioning tectonic regimes from stagnant lid to plate tectonic models is still challenging. This tectonic transition appears to be diachronic in different old cratonic blocks worldwide. One key point to address tectonic models in early Earth is the appearance of the TTG-sanukitoid associations, which are interpreted as the products of melting oceanic slabs and melting of the hydrated mantle wedge in supra subduction zones. Although typical of the Archean, this association persists until the Paleoproterozoic in some cratonic remnants of South America. The Minas-Bahia Orogen (MBO) is one of the most extensive Paleoproterozoic belts of Brazil, cropping out in the São Francisco Craton but also as reworked thrust slices within surrounding Neoproterozoic belts. Here, we explore the southern part of MBO, bringing new geochemical, U-Pb zircon geochronology (LA-ICP-MS) and Nd and Sr isotopic data of the Juiz de Fora Complex that constitutes the external magmatic arc system of this Siderian to Orosirian orogen. Granodioritic to tonalitic orthogranulites with TTG geochemical signatures, crystallization ages of ca. 2.21 and 2.14 Ga, with positive εNd_i (+2.9 to + 5.9) and ⁸⁷Sr/⁸⁶Sr_i ratios between 0.7022 and 0.7045; Quartz-dioritic to granodioritic orthogranulites with sanukitoid composition, displaying crystallization ages between ca. 2.21 and 2.18 Ga, with near-chondritic εNd_i values of −2.43 to + 0.39, and ⁸⁷Sr/⁸⁶Sr_i ratios between 0.7076 and 0.7179. Integrating the new data with previously published analyses allows us to envisage a comprehensive tectonic evolution model starting in an intra-oceanic setting and progressively evolving to a continental magmatic arc environment along the Rhyacian period. Integrated geochronology data indicates coeval and continuous generation of TTG and sanukitoid magmas for 160 Myr. Initially, with more juvenile contribution (ca. 2.22–2.15 Ga), that evolves progressively to a more mature stage (2.15––2.06 Ga). Additionally, two groups of mafic rocks, with ages of ca. 2.16 (OIB-like) and ca. 2.14 Ga (E-MORB), fill the gap between these two stages and possibly reflect arc migration and upwelling of the asthenosphere. Finally, data suggests the operation of plate tectonic processes since the Rhyacian, within this accretionary segment of the MBO, that ultimately resulted in the building of the São Francisco Paleocontinent.