First evidence for Neoproterozoic magmatism in the Quadrilátero Ferrífero of Minas Gerais, Brazil, and geotectonic implications

Abstract

A decimetre-thick, deeply weathered sill intrusive in the Itabira iron formation and exposed at the historical Veloso underground mine in Ouro Preto, Minas Gerais, contains three populations of magmatic zircon grains: (i) xenocrysts with Archaean Pb–Pb and Hf model ages; (ii) xenocrysts of Palaeoproterozoic crystallisation ages of ca. 2200 Ma, with superchondritic εHf2.2 Ga values between 1.5 and 11.6; (iii) autocrysts and overgrowths formed at ca. 620 Ma, with εHf0.62 Ga values between −17.6 and 11.5 (mean = 1.5 ± 6.9, n = 15). Furthermore, there is evidence for zircon age reset at ca. 565 Ma. The three magmatic zircon populations indicate that the sill emplaced during the Neoproterozoic and probed different magmatic sources at depth, comprising anatectically reworked Palaeoproterozoic and Archaean felsic rocks. Variable Hf-isotope compositions of zircon rims and whole-rock geochemical data additionally suggest a hybrid origin for the sill magma. Our results provide the first evidence that the southern edge of the Quadrilátero Ferrífero is underlain not only by Archaean, but also by juvenile Palaeoproterozoic rocks, similar to those exposed in the adjacent Mineiro belt, and that the latter became accreted to the São Francisco craton prior to lower crustal anatexis during an early extension at ca. 620 Ma.