Basement domes in a greenschist facies context: Tectono-metamorphic evolution of the southern Quadrilátero Ferrífero, southeast Brazil

Abstract

New petrostructural data are presented from the southeastern Quadrilátero Ferrífero, a distinct polyorogenic domain of the São Francisco Craton characterized by granite-gneiss domes and keel structures. The Minas Supergroup of Siderian age that hosts a huge volume of ferriferous sediments was deposited on the flat and eroded surface of the Archean basement. The domes of Archean gneisses and granites represent semi-rigid extrusions around which the Minas Supergroup accumulated in deep synclines with common triple junctions. This study focuses first on the petro-structural context of low-pressure thermal aureoles observed towards the dome margins. Pressure-temperature estimates on these mineral assemblages indicate nearly static burial to ≥ 10 km depths of the Minas Supergroup cover, followed by decompression and heating during the intrusion of pegmatites and granitoids of late Rhyacian age in basement and cover. The Archean basement and the thermal aureoles were severely overprinted by synkinematic greenschist facies metamorphism portrayed by chloritoid, chlorite, pyrophyllite, kyanite, phengitic mica and secondary biotite. This deep greenschist facies metamorphism (T ca. 420°C and P = 0.4 to 0.45 GPa) is regionally observed in all units from the domes and the keels ranging in age from Archean to Ediacaran. It is concluded that this major tectono-metamorphic event relates to tectonic burial of the southern Quadrilatero Ferrifero below the west-verging nappes of the Araçuaí belt during the Early Paleozoic, as documented by 40Ar/39Ar ages of 490-485 Ma obtained on syn-kinematic metamorphic micas [Chauvet et al., 2001]. It is proposed that building up of the dome and keel tectonic style is first a consequence of the high-density contrast estimated around 0.5 g/cm3 between the crystalline basement and the Minas Supergroup, the final architecture being achieved during the Cambro-Ordovician stage of the E-W Late Neoproterozoic collision.