Low- and high-alumina komatiites of Goias, Central Brazil

摘要

Archean komatiites of Goias, central Brazil, have experienced deformation and low-grade metamorphism, but several outcrops preserve primary volcanic features. Samples from less deformed komatiites of four out of five greenstone belts (Crixas, Guarinos, Pilar de Goias, and Santa Rita) have been investigated for their geochemical properties. Komatiites from the Crixas greenstone belt have very low Al_2O_3/TiO_2, high CaO/Al_2O_3, and a hump-shaped rare earth element (REE) pattern. Those from the Guarinos and Pilar de Goias belts have similar REE patterns, characterized by a slight enrichment in LREE coupled with almost flat HREE, but differ in their inter-incompatible element ratios. Compared with those from Pilar de Goias and Guarinos, samples from the Santa Rita belt have fractionated REE patterns with LREE enrichment, as well as high Al_2O_3 contents, corresponding to Al-undepleted komatiites. Komatiites from Crixas have the lowest (La/Sm)_N, (La/Yb)_N, and Zr/Zr~* ratios compared with their equivalents from the other belts, which suggests their source was relatively depleted in LREE and high field strength elements (HFSE), probably due to the retention of garnet in the residue. Komatiites from the Guarinos, Pilar de Goias, and Santa Rita greenstone belts are enriched in incompatible elements, which can be attributed to either low-degree partial melting at high pressures or a source previously enriched in incompatible elements. Some of the studied komatiites belong to Al- and HREE-depleted and others to the Aland HREE-undepleted types. The depleted komatiites probably derived by melting at depths greater than 200 km, the undepleted at less than 200 km. Therefore, the komatiites of the four belts may have been derived from either one single mantle plume with different melting depths or sources from distinct plumes.