Slow cooling and crystallization of the roots of the Neoproterozoic Araçuaí hot orogen (SE Brazil): Implications for rheology, strain distribution, and deformation analysis

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The Araçuaí-Ribeira belt formed during the amalgamation of West Gondwana in the late Neoproterozoic. Its evolution included a main tectonometamorphic peak at 600–580 Ma and a minor peak associated with the final collision with the Western Congo at 540–530 Ma. This belt has the characteristics of a hot orogen, including a high thermal gradient (>30 °C/km), pervasive partial melting of the middle crust, emplacement of large volumes of magmas resulting from partial melting of the lower crust and underlying mantle, and slow cooling after the peak temperature. We report 21 new amphibole, biotite and muscovite 40Ar/39Ar ages that complement previously published data. These data suggest slow cooling (3–5 °C/Myr) over 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 West Gondwana. We estimate that ~30 Myr was required to heat the middle crust to the peak temperature and that anatectic and plutonic bodies remained in the magmatic state for ≥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 example, the 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 3D deformation involving successive upwelling of anatectic components within a thrust unit crosscutting the pre-existing fabric (“channel flow-like”). This study also highlights that classical structural analysis techniques relying on changes in pressure or temperature conditions to identify the succession of deformation phases are not efficient at deciphering the tectonic evolution of hot, slowly cooling orogenic belts, where the temperature varies slowly over tens of million years, allowing diachronic episodes of deformation to occur under nearly similar pressure and temperature conditions.