Birthplace of the São Francisco craton, Brazil : evidence from 3.60-3.64 Ga gneisses of the Mairi gneiss complex.

Abstract

Records of Earth's primitive crust are scarce. Eoarchean (older than 3.6 Ga) banded mafic to felsic gneisses have been discovered in the São Francisco Craton, Brazil, pushing back by over 100 million years the oldest gneisses known to date in South America (3.5 Ga). Zircon U-Pb data yield rock ages from 3598 Ma to 3642 Ma with a few ca. 3.65 to 3.69 Ga grains suggesting even older rocks in the area. Zircon grains show significantly negative to nearly chondritic initial εHf values and two-stage model ages from 3.82 Ga to 4.33 Ga, which may indicate the existence of a recycled Hadean to early Eoarchean crust in the region. The felsic gneisses are chemically similar to the low-pressure Tonalite-Trondhjemite-Granodiorite association whereas the mafic gneisses have geochemical signatures that resemble within-plate basaltic andesite to andesite of Iceland (icelandites). The results are relevant to constrain the composition of Earth's first continental crust.