Geoquímica das rochas meta-vulcânicas máficas do greenstone belt de Riacho de Santana, Bahia, Brasil

Abstract

The Riacho de Santana greenstone belt (GBRS) of Archaean age, located in the southwestern of Bahia State, Brazil, occurs as an unit in the regional crystalline basement rocks known as Gaviao Block unit. Three distinct lithostratigraphic units are identified in GBRS: (i) Lower Unit, comprising of komatiitic ultramafic rocks with thin intercalations of metabasalts, metagabbros, quartzites and metacherts, calc-silicates and aluminous schists; (ii) Intermediate Unit, comprising of fine-grained metabasalts and meta gabbros, intermediate to acid meta tuffs, aluminous schists, graphite schists; and (iii) Upper Unit, comprising of carbonate-silicate sequence. In this study, we report major, trace and earth rare element composition of the basic metavolcanic rocks belonging to the Intermediate Unit, and infer the likely source (s) for these metavolcanics. All of the samples may be classified as high-iron tholeiites. Flat HREE patterns and smaller LREE/HREE fractionation in these rocks suggest that these melts were generated at a shallow depth in the absence of garnet in the source mineralogy. Primitive-mantle normalized REE patterns (and Ba/Zr, Ba/Nb, Ti/Y, Nb/Y, Nb/Th, and Zr/Y ratios) suggest that the primary magmas of these metavolcanics were derived from an enriched mantle reservoir or a mixture of enriched and depleted mantle reservoir. The magma composition was significantly affected by either due to assimilation of granitic crustal material, or due to the presence of subducted crustal sediments and/or thin Archaean subcontinental lithosphere in the mantle source region. Future work on Sr, Pb, and Nd isotope systematics may reveal the effect of crustal assimilation