Records of 2.9 G.a crustal anatexis on South American crust - Source and melt segregation in the Porteirinha Complex, Araçuaí Orogen (SE, Brazil)

Abstract

Crustal anatexis is a key process in the evolution and differentiation of the crust. Many Archean terranes record a transition from typical sodic TTG magmatism to extensive calc-alkaline potassic magmatism, similar to modern ones. At SE Brazil, within the Araçuaí orogen, the Paleoarchean TTG orthogneisses (3.4 Ga) of the Porteirinha Complex (PC) are exposed as a basement inlier. These rocks document a evolution to a variety of migmatites during the Meso-Neoarchean (2.9 Ga). This study presents field data, microstructures, and geochemistry to investigate the petrogenetic processes related to the crustal anatexis of Paleoarchean TTG orthogneisses. The PC is the basement unit of the Porteirinha Block, an Archean crustal segment associated with the São Francisco-Congo paleocontinent. Field and petrographic structures reveal a gradual evolution from TTG orthogneiss to partially melted domains, characterized by distinct morphological changes. This progression includes the development of metatexite and diatexite, which reflects increasing degrees of partial melting and syn-anatectic deformation. As deformation progresses, it induces textural, mineralogical, and geochemical changes, with notable shifts in the composition of the rocks, culminating in a network of metatexite-diatexite transitions. Four main rock types are recognized in the PC: (i) orthogneisses; (ii) metatexites; (iii) diatexites; and (iv) amphibolite lenses. The REE content suggests isochemical processes during crustal anatexis, suggesting that PC migmatites may result from incomplete segregation with varying degrees of melt-residue separation. Typical water-fluxed melting reactions are recognized by Hbl + Plg + melt (Kfs-rich) phases and linked to P-T scenarios of amphibolite-granulite transition facies, marked by key-melting microstructures. Processes of crustal thickening, lithospheric delamination, and gravitational collapse are proposed as tectonic triggers for the melting of the orthogneiss protolith, generation of migmatites, and emplacement of Neoarchean BP granitoids.