Geochemistry of part of the Santa Catarina granulitic complex, southern Brazil: a model of differentiation from high alumina basalt

Part of the Santa Catarina Granulitic Complex consists of a comagmatic calc-alkaline basic to acid sequence and pyroxenites. Subordinate anorthosites, trondhjemites, amphibolites and metasediments are also present. The calc-alkaline sequence is formed by norites, enderbites, and charno-enderbites. The composition of the acid- intermediate granulites is similar to Archaean high-Al tonalites and granodiorites, while the basic granulites correspond to high-Al tholeiites. The Barra Velha pyroxenites have augitic compositions and local cumulate textures. Geochemical data suggest that the Archaean calc-alkaline sequence was formed by crystal fractionation of high-Al tholeiites, with plagioclase, olivine or orthopyroxene and clinopyroxene as the main fractionating phases. The Barra Velha pyroxenites appear to correspond to pyroxene cumulates, formed early during differentiation of high-Al basalt. Rocks with very high Al contents are probably plagioclase cumulates and high-Fe rocks are ferromagnesian mineral cumulates. These observations show that subduction of cold, hydrated ocean crust followed by its dehydration, invasion of the overlying mantle prism by the released fluids and partial melting of mantle peridotite to yield mafic magmas could have occurred during the late Archaean, when the more common process was direct melting of the hot, anhydrous subducted slab to yield tonalites and related magmas.