Evolution of Neoproterozoic ophiolites from the southern Brasiliano Orogen revealed by zircon U-Pb-Hf isotopes and geochemistry

Abstract

U-Pb and Lu-Hf isotopic analyses of zircon from albitites, southern Brasiliano Orogen, indicate Neoproterozoic ages for two ophiolites. The difficulty of dating ophiolites is overcome by finding zircon in albitites contained in serpentinite. Two key areas were selected for the study, the Cerro Mantiqueiras and the Ibaré ophiolites, both in the juvenile São Gabriel terrane, Rio Grande do Sul, Brazil. Two distinct U-Pb SHRIMP ages are registered in the Cerro Mantiqueiras ophiolite zircons, respectively 923±3Ma in the cores and 786±13Ma in the rims. In the Ibaré ophiolite, the dating of zircon grains resulted in a single age of 892±3Ma. Distinctive depleted mantle Hf isotope signatures characterize the zircons; in the Cerro Mantiqueiras sample, εHf=+8 to +13 and Hf TDM=0.91–1.23Ga, whereas the Ibaré sample displays εHf=+13 to +15 and Hf TDM=0.75–0.93Ga. Ophiolite accretion in the São Gabriel terrane of the southern Brasiliano Orogen occurred in the Tonian, which establishes the age of a significant event in the tectonic evolution of the continent. The Hf data indicate that the ophiolites were derived from juvenile depleted mantle. This first direct dating of ophiolites sets new limits to the evolution of the Brasiliano Orogen.