Alain VAUCHEZ, Maria Helena B.M. Hollanda, Patrick Monié, Marcos EGydio-Silva, Mathieu Mondou

The Araçuaí-Ribeira belt formed during the amalgamation of West Gondwana in the late Neoproterozoic. Its evolution encompasses a main tectono-metamorphic peak at 600-580 Ma and a minor one, associated with the final collision with the Western Congo at 540-530 Ma. This belt holds characteristics of a hot-orogen: high thermal gradient (>30 °C/km), pervasive partial melting of the middle crust, emplacement of huge volume of magmas resulting from partial melting of the lower crust and underlying mantle, and slow cooling after the peak of temperature. We report 21 new amphibole, biotite and muscovite 40Ar/39Ar ages, which complement data already published by our group. Altogether, these data support slow cooling (3-4 °C/Myr) during several tens of million years after the peak temperature (~800 °C at ~600 Ma), followed by faster cooling (>10 °C/Myr) after the final amalgamation of Western Gondwana. We estimate that ~30 Myr were needed to heat the middle crust to the peak of temperature and that anatectic and plutonic bodies remained in the magmatic state over at least 40 Myr. This protracted thermal evolution likely had major effects on the rheology of the middle crust and on the tectonic evolution of this orogen. For instance, correlation of U-Pb zircon crystallization ages and 40Ar/39Ar biotite cooling ages in the anatectic core of the orogen denotes a diachronic thermal evolution likely related to a 3D deformation characterized by successive upwelling of anatectic components, along a channel crosscutting pre-existent fabric (channel-flow type). This study also highlights that classical structural analysis techniques relying on changes in pressure or temperature conditions to identify the succession of deformation phases, cannot be used to decipher the tectonic evolution of hot, slowly cooling orogenic belts, where temperature varies weakly over tens of millipn years, allowing diachronic episodes of deformation to occur under almost similar pressure and temperature conditions.