Evolução crustal arqueana do bloco Porteirinha, Orógeno Araçuaí (SE, Brasil)

Abstract

The Porteirinha Block (BP) is a basement inlier in the Araçuaí orogen that provides an important record of the origin and evolution of São Francisco-Congo Paleocontinent. The BP records the generation of Paleoarchean TTG, Mesoarchean anatexis, and Paleoproterozoic crustal reworking. The BP is located in the southeast of Brazil, between the Itacambira and Espinosa (MG) towns, as a result of a tectonic involvement of the continental margins of São Francisco Congo Paleocontinent (PSFC) during the collisional processes of Gondwana amalgamation at the Neoproterozoic. The PB basement unit is the Porteirinha Complex (PC), which consists of TTG orthogneisses, migmatites, amphibolites, metaultramafics rocks and Neoarchean granitoids. This thesis discuss the Archean evolution of the TTG orthogneisses and migmatites from PC based on field, geochronological and whole-rock geochemistry data. Zircon U-Pb ages (LA-ICP-MS) show a complex and polycyclic evolution of these rocks. Concordant ages were obtained for crystallization of TTG magma at ca. 3.4 Ga and the geochemical signature supports the idea of TTG generation by partial melting of hydrated mafic crust under garnet-amphibolite facies conditions. Zircon U-Pb ages also suggest crustal reworking events at 3.1 Ga. Metatexite and diatexite domains related to partial melting of TTG orthogneisses are individualized. Field structures and petrographical features reveal a transition from orthogneisses to domains related to partial melting, marked by a progressive syn-anatectic deformation that develops textural, mineralogical and geochemical changes, resulting in an network of metatexite-diatexite transitions in the field. The data obtained point to isochemical processes during crustal anatexis, suggesting that the PC migmatites as products of incomplete segregation with varying degrees of melt-residdum segregation in typical hydrated partial melting conditions. The geochronological data indicate that the processes of crustal anatexis and melt generation occurred between 2.9 and 2.8 Ga (LA-ICP-MS, U-Pb in zircon) and are linked to crustal reworking and heat generation events recognized regionally in this period. The new data presented in this work allow the establishment of correlations between the Archean and Siderian-Orosirian units of the BP and those that occur regionally, particularly with the Gavião block, where phases and events of crustal evolution seem to be shared.