Metamorphic imprints on units of the southern Araçuaí belt, SE Brazil : the history of superimposed Transamazonian and Brasiliano orogenesis.

Abstract

The extensive Neoproterozoic Brasiliano belts that surround the São Francisco craton in southern Brazil are a consequence of tectonic processes that resulted in the formation of the supercontinent Gondwana in the Ediacaran to Cambrian. One of these Brasiliano-age orogens, the Araçuaí belt, overprints an earlier orogen developed during the so-called Transamazonian event at the transition between the Rhyacian and Orosirian periods. Archean and Paleoproterozoic basement units in the southern Araçuaí belt have been shown to have experienced both the Transamazonian and Brasiliano orogenies and have the potential to have preserved the P-T conditions of both events. This study investigates a series of granulite- and amphibolite-facies samples from the southern Araçuaí belt. Granulite-facies samples from the Acaiaca Complex and Pedra Dourada Granulite give monazite U-Pb ages in the range: 2015–1960 Ma. Both the granulite samples preserve two metamorphic textures. Coarse-grained garnet + biotite + orthopyroxene + K-feldspar + plagioclase + quartz + ilmenite + inferred melt and later, fine-grained garnet + biotite + hornblende + quartz that grows at the expense of orthopyroxene. Peak conditions for the earlier event were c. 750–800 °C and 6 kbar, whereas those of the later event are poorly constrained. A garnet-bearing amphibolite-facies sample from the Mantiqueira Complex contains monazite included within garnet, which gives U-Pb ages of c. 1940 Ma, whereas matrix monazite grains give ages of c. 590–540 Ma. Garnet core compositional zonation indicates P-T conditions of 6 kbar and 700 °C for the c. 1940 Ma metamorphism, while the matrix assemblage records a P-T path culminating in peak conditions of 8–9 kbar at 650 °C. A second amphibolite-facies sample was obtained from the Dom Silvério Group, whose depositional age post-dates the Transamazonian event. This sample gives two monazite U-Pb age populations, 597 ± 3 Ma and 547 ± 5 Ma. Garnet core zonation indicates a prograde P-T evolution with peak conditions of c. 8–9 kbar and 650 °C, although it is uncertain at this time what age population this event corresponds to. The results of this study provide better constraints on the chronology and metamorphic regimes of both the Transamazonian and Brasiliano events in the southern Araçuaí belt.