PETROLOGIA, GEOQUÍMICA E GEOCRONOLOGIA DOS DIQUES MÁFICOS DO COMPLEXO METAMÓRFICO BONFIM SETENTRIONAL (QUADRILÁTERO FERRÍFERO) E SUAS IMPLICAÇÕES NA EVOLUÇÃO CRUSTAL DO CRATON DO SÃO FRANCISCO MERIDIONAL.

Abstract

Four generations of mafic dykes outcrop in the Neoarchean sialic crust of the Quadrilatero Ferrifero, especially in the Northern Bonfim Metamorphic Complex. Geological data of these mafic dykes (field relations, petrology, geochemistry and geochronology) indicate distinct magmatic episodes related to different stages of tectonic evolution of the Southern Sao Francisco Craton terranes. Paraopeba and Candeias Amphibolite have Neoarchean Sm-Nd (TDM) ages, but their K-Ar isotopic system was reset at the end of the Paleoproterozoic. The igneous hornblende K-Ar apparent ages indicate that emplacement of the Conceicao de Itagua Metadiabase is related to Proterozoic. These four generations of mafic dykes show geochemical affmity to withinplate tholeiite basalts, and their chemical composition is compatible with subalkaline basalts (Paraopeba Amphibolite), andesites (Candeias Amphibolite) and alkaline/trachyandesite basalt (Conceicao de Itagua Metadiabase). However, the chemical composition of the Santa Cruz Diabase ranges from alkaline basalt to dacite. The REE pattern of the Neoarchean mafic dykes is less fractionated than the pattern of the Conceicao de Itagua Metadiabase. Moreover, the Neoarchean mafic dykes are Ti-depleted and show negati ve Ba and positive Nb and Y anomalies. The geochemical criteria indicate that the magma of the Paraopeba and Candeias Amphibolite evolved from a Zr/Ti ratio (N-MORB) mantle source depleted. Probably, because of their high K/P and Sr/Rb ratios, the magmas of the Paraopeba Amphibolite suffered continental contamination. As indicated by their Zr/Y ratios, Conceicao de Itagua Metadiabase and Santa Cruz Diabase come from aTi-enriched mantle source (P-MORB). The crystallization and extraction of Ti-rich phases from this original magma led to the generation of the more evolved membcrs of these mafic dykes. Because of the preservation of their volcanic/sub-volcanic textures, the Santa Cruz Diabase must be younger and are probably related to a magmatic event occurred during the Post-Paleozoic tectonic evolution of the brazilian platform. Finally, the Northern Bonfim Metamorphic Complex mafic dykes are correlated to tectono-magmatic events occurred in the Southern Sao Francisco Craton from the Neoarchean times.