ISOTOPE GEOCHEMISTRY AND GEOCHRONOLOGY OF SYN-DEPOSITIONAL VOLCANISM IN PORONGOS METAMORPHIC COMPLEX, SANTANA DA BOA VISTA ANTIFORM, DOM FELICIANO BELT, BRAZIL: ONSET OF AN 800 MA CONTINENTAL ARC

The Porongos Metamorphic Complex (PMC) is composed of supracrustal rocks, predominantly pelitic micaschist, quartzite and felsic metavolcanic rocks. It represents the remnants of a Neoproterozoic sedimentary basin developed in the western Gondwana, now located in the central section of Dom Feliciano Belt in southern Brazil. Isotope signature comparison and U-Pb ages shows that there are at least three main source-areas for the Porongos sediments: (i) one that can be related to Paleoproterozoic terrains; (ii) a younger component with Mesoproterozoic (1.5 - 1.2 Ga) ages; and (iii), a Neoproterozoic source with magmatic zircons (0.8 Ga), with relative age distribution mostly between 1.7 and 1.0 Ga, T-DM ages between 2.58 and 1.71 Ga, and younger ages of ca. 1.21 Ga, which can be related to 1.4 - 1.0 African signature sources. Ten concordant analysis obtained in zircon grains from a metarenite rock yield Pb-206/U-238 median age of 802 + 26/-33 Ma, which is related to metavolcanic samples from PMC and correlates with the Neoproterozoic Cerro Bori Continental Arc, indicating a syn-depositional volcanism in the sedimentary record. The depositional setting for these sequences can be understood as an evolution between a passive margin developed after 1.6 Ga and a terrain-collisional setting (ca. 570 Ma).