2.17–2.10 Ga plutonic episodes in the Mineiro belt, São Francisco Craton, Brazil: U-Pb ages, geochemical constraints and tectonics

Abstract

Geologic information supported by in situ U-Pb zircon ages, and geochemistry coupled with zircon Hf and Nd-Sr isotopic analyses on orthogneisses, metagranitoids and metadiorites unravel the nature and tectonic significance of a calc-alkaline continental arc in the Mineiro belt – southern portion of the São Francisco Craton. The zircon U-Pb ages defined two plutonic episodes: 2174–2149Ma (metaquartz-diorite, metagranitoids and orthogneisses) and 2145–2100Ma (metadiorites, metagranitoids and orthogneisses) whereas several samples contain inherited zircons with concordant 207Pb/206Pb ages as old as 2785Ma. Our rocks show variable SiO2 contents (54.8–77.1wt.%), are metaluminous to peraluminous and mostly magnesian and variably enriched in LREE. The older rocks have La/YbN=2.24 to 11.87 whereas the metadiorites have La/YbN=9.60–13.82 and the youngest rocks in the range 7.79–91.27. Most samples show Nb and Ti negative anomalies that can be attributed to a subducted source whereas some yield marked Pb positive anomalies that highlight the important role of crustal contamination/assimilation. Dy/Dy* and coupled Dy/Yb values (from 0.7 to 0.5 and 1.4 to 2.0, respectively) indicate the metaigneous rocks derived from a LREE enriched MORB source in a subduction zone with mixed Paleoproterozoic and Archean material. This scenario is consistent with the U/Pb data from inherited nuclei and xenocrysts, the range of the zircon ɛHf(t) (from +8 to −10) and ɛNd(t) (from −7 to +0.3) values, as well as with the Lu-Hf TDM ages that vary between 2.3 and 3.2Ga. Our data and regional geologic–tectonic correlations are consistent with the onset of an ultimate collisional event in the Mineiro belt that followed a long-lived accretionary regime active in Proterozoic times, outboard the Archean core of the São Francisco Craton.