REE geochemistry of komatiites and associated rocks from Piumhi, southeastern Brazil

Abstract

Komatiitic rocks with spinifex textures have recently been found in two areas in Brazil: (1) in the vicinity of Piumhi, State of Minas Gerais and (2) in the Goias Central Massif. This adds to the notion of a world-wide occurrence of komatiites in most, if not all, Precambrian shields. This report documents major element compositions and rare earth abundances in komatiites and associated rocks from the Piumhi Massif, of probable Archean age. The komatiites are found to have flat heavy REE and somewhat enriched light REE distribution patterns. Basaltic komatiites have near-chondritic Al 2O 3/TiO 2 ratios. However, their low CaO/Al 2O 3 ratios (∼ 0.6) suggest that these rocks might have lost CaO by alteration, or might have undergone clinopyroxene fractionation. Modal calculations indicate that the LREE enrichment more likely reflects mantle source characteristics, and is also consistent with the LREE-enriched pattern in peridotic komatiite. Komatiites and associated rock types including tholeiites, minor andesites, sedimentary sequences and granitic intrusions in the Piumhi Massif are entirely comparable with those of typical Archean greenstone—granite terrains. However, the preliminary results of SmNd isotopic analyses suggest that they were emplaced in the Proterozoic, ∼ 1.84 Ga ago (Appendix).