Insights into orogenic processes from drab schists and minor intrusions : Southern São Francisco Craton, Brazil.

Abstract

Minor altered intrusions and drab retrogressed schists can easily be overlooked in geological studies but this contribution explores these rocks within the Archaean and Palaeoproterozoic southern São Francisco Craton (SSFC), Brazil using geological relationships and accessory mineral in situ analyses in the context of cratonic assembly. Three magmatic pulses are documented: i) Archaean and ii) Palaeoproterozoic felsic intrusions, both hosted by Archaean protoliths, and iii) Palaeoproterozoic felsic intrusions in Palaeoproterozoic supracrustal sequences. Archaean felsic intrusions confirm the Palaeoarchaean age of the mafic/ultramafic sequence of the Rio das Velhas Greenstone Belt and Rhyacian intrusions mark the collisional stage of the Mineiro Belt with the SFC at c. 2130 Ma. Greenstone belt schists show a wide distribution of rounded ‘soccer ball’ Archaean detrital and metamorphic zircon grains ranging in age from 3200 to 2750 Ma with an interpreted overprinting high-grade metamorphic event at c. 2700 to 2680 Ma. Most high-grade metamorphic rims have Th/U > 0.1, negative εHf(t) values and REE pattern consistent with eclogite/granulite metamorphic facies, reinforcing the hypothesis of a dehydrated-refractory crust formed during the stabilization of the SSFC, even though no such protoliths are preserved. This event links crustal thickening and partial melting of Archaean lower crust. Archaean rutile crystals from the greenstone belt schist were reset during the Palaeoproterozoic event but still preserve the early Archaean high-grade metamorphic signature. The presence of unstable ilmenite replaced by rutile in the schist, associated to felsic intrusions with the same age at c. 2130 Ma suggest high pressure, low temperature prograde metamorphism during the collisional stage of the Palaeoproterozoic orogen. Elongate and prismatic zircon grains from the Rhyacian intrusions have low εHf(t) signature and crystallised from partial melting of sedimentary protoliths. Accretionary events produced thicker and more differentiated crust by the end of Rhyacian time. Easily overlooked rocks in this study, when studied, have revealed a rich multi-event history of cratonic evolution.