Dados da Submissão

Título

THE 3.4 GA TTG PORTEIRINHA COMPLEX: A WINDOW INTO THE ARCHEAN CRUSTAL EVOLUTION OF SOUTH AMERICA

Texto do resumo

The Porteirinha Block (PB) is a fragment of the São Francisco-Congo paleocontinent (SFCP) that lies as basement inlier within the Araçuaí orogen, east Brazil. The oldest unit of PB is the Porteirinha Complex (PC) which records Paleoarchean orthogneisses and amphibolites affected by different grades of migmatization in the late Mesoarchean. Zircon U-Pb ages show a complex and polycyclic evolution for the the PC: 3.469±16 Ga crystallization of the magmatic protolith, 3.153±73 Ga metamorphic imprint and 2.952±14 Ga crustal anatexis. The 3.4Ga orthogneisses plots in the trondhjemites and tonalites fields and shows a typical geochemical signature of Archean TTGs, following the A/CNK, A/NK, and low K2O/Na2O with strong convergence to the sodic fields on discriminants diagrams, Sr/Y and La/Yb ratios that plots in the field of TTGs and Adakites and a complete distinguish from the potassic and post-Archean granitoids. The rocks exhibit low Nb/Ta and Zr/Sm ratios, low Ti and Y contents, as well as high Rb and K contents, suggesting that TTG orthogneisses of PC were generated by partial melting of hydrated mafic crust under garnet-amphibolite facies conditions. The low values of MgO, Mg# and Cr and Ni in these rocks are consistent with the idea of a partial melting without contamination by peridotite and mantle interaction, probably related to a progressively thickened lower crust or even in a flat slab subduction setting. The 3.1 Ga obtained age of metamorphic overprinting event was also identified by Silva et al., 2016 and can be linked to crustal reworking processes probably related to a short-lived accretionary orogen (Medeiros et al., 2017). This age also marks the generation of orthogneisses and migmatites with inherited zircons of ca. 3.4 Ga from Santa Izabel Complex - Gavião Block (GB), located on northern limit of PB. The available data support a continuity between the PB and the southern GB rocks, which share very similar petrogenetic scenarios: melting of mafic crust under conditions of crustal thickening. The view that the [northern half of] PB and the GB represented a single crustal segment during this period is suggestive. The 2.9 Ga ages for leucosomes marks the crustal anatexy of the 3.4 Ga TTG orthogneisses and can be linked to the high-K granites of the Rio Itacambiruçu Suite – RIS, crystallized between 2917.5±9.2 Ma (Pedra do Urubu pluton - PUp; Bersan et al., 2018) and 2657 ± 25 Ma (Barrocão pluton – Bap; Silva et al., 2016). The PUp has already been interpreted as product of reworking of ancient continental crust, and the endorses the PC as it source.

Palavras Chave

Porterinha Block; TTG; Archean Crustal Evolution; São Francisco Craton; Araçuaí orogen.