THE METAMORPHIC SIGNATURE OF CONFINED OROGENY AND COLLAPSE: AN EXAMPLE FROM THE NEOPROTEROZOIC ARAÇUAÍ BELT OF EASTERN BRAZIL

The Neoproterozoic Araçuaí belt of eastern Brazil developed in a unique tectonic setting: the orogen records the ca. 625 – 575 Ma closure of a Red Sea-type basin that had formed between the São Francisco and Congo cratons of West Gondwana. To the south this basin contained oceanic crust, so that closure involved subduction and arc magmatism prior to continental collision. To the north, the basin was entirely ensialic, so closure did not involve subduction, but high-grade metamorphism still occurred. Here we present the results of a metamorphic and structural traverse across the northern portion of the Araçuaí orogen in Brazil.

The highest metamorphic grades are found in the eastern sector of the belt, where partial melting of metapelitic gneisses occurred at about 700°C and 5 kbar during near-isothermal decompression, indicated by garnet breakdown to cordierite and plagioclase. This decompression probably records tectonic exhumation accompanying extensional collapse. The contact between pelitic migmatites and Transamazonian basement granulites on the eastern border of the orogen is abrupt and marked by a major dextral shear zone.

Greenschist and lower amphibolite facies metamorphism is preserved in metapelitic schists of the western part of the orogen. Metamorphism here attains cordierite-sillimanite grade (c. 600°C and 4 kbar) but with little or no migmatization. These schists crop out at progressively lower metamorphic grades towards the foreland in the west. Rocks at present-day exposure levels thus record similar pressures of peak metamorphism (i.e. P at Tmax) of about 4-5 kbar across the orogen, with higher temperatures recorded in the eastern region than in the western. We attribute this to greater crustal thickening on the eastern side (which had non-cratonic Paleoproterozoic basement) than on the western side (which had cratonic Archean basement), followed by more extensive collapse in the east. Our study emphasizes that the metamorphic signature in a closed ensialic basin depends, in part, on the character of the basement beneath the basin’s sedimentary fill.