Petrological and Geochemical Characteristics of the Granulitic Terrain of BrejGes, Bahia, Brazil

Abstract

New petrological and geochemical characteristics of the Brejbes region, situated in the south of Bahia, Brazil are discussed. The region forms a part of the most important and extensive granulite facies terrain in Brazil of Archean/Paleoproterozoic age. Five groups of rock types all equilibrated in the granulite facies are identified in this region. They ars: i) supracrustal and related rocks, ii) undifferentiated granulites, iii) hornblende bearing enderbite-charnockites, iv) hornblende free enderbite-charnockites, v) charnockites. The first group appears to be the oldest in the region as they form enclaves in the 2.9 Ga old undifferentiated granulites. The third and fourth group are enderbite-charnockites, whose protoliths constitute two series of calc-alkaline rocks, one titanium poor (hornblende free) and another titanium rich (hornblende bearing). U/Pb zircon SHRIMP dates indicate ages of formation at 2.8 1 Ga (hornblende free) and 2.69 Ga (hornblende bearing) for the two groups. The fifth group of rocks have charnockitic affinity and are present in the center of the BrejBes Dome. These rocks also have calcalkaline affinity, but show petrographic and geochemical characteristics distinct from those of other groups. Preliminary geochronological investigations by zircon Pb-Pb evaporation method yielded 2.6 Ga and 2.0 Ga for the charnockites from the inner core of the Brejbes Dome. These age data suggest that the circular structure was formed by the re-fusion of the 2.6 Ga old deep crustal material generating younger charnockites at 2.0 Ga.