Thermal conditions during deformation of partially molten crust from TitaniQ geothermometry: rheological implications for the anatectic domain of the Araçuaí belt, eastern Brazil
Abstract. During the Neoproterozoic orogeny, the middle crust of the Araçuaí belt underwent widespread partial melting. At the regional scale, this anatectic domain is characterized by a progressive rotation of the flow direction from south to north, suggesting a 3-D deformation of the anatectic middle crust. To better determine whether melt volumes present in the anatectic middle crust of the Araçuaí orogen were large enough to allow a combination of gravity-driven and convergence-driven deformation, we used the titanium-in-quartz (TitaniQ) geothermometer to estimate the crystallization temperatures of quartz grains in the anatectic rocks. When possible, we compared these estimates with thermobarometric estimates from traditional exchange geothermobarometers applied to neighboring migmatitic kinzigites. TitaniQ temperatures range from 750 to 900 °C, suggesting that quartz starts crystallizing at minimum temperatures of ≥ 800 °C. These results, combined with the bulk-rock chemical composition of diatexites, allows the estimation of a minimum of ~ 30% melt and a corresponding viscosity of ~ 109–1010 Pa s. Such a minimum melt content and low viscosity are in agreement with interconnected melt networks observed in the field. Considering that these characteristics are homogeneous over a wide area, this supports the finding that the strength of the middle crust was severely weakened by extensive partial melting, making it prone to gravity-driven flow and lateral extrusion.
