Thermal conditions during deformation of partially molten crust from TitaniQ geothermometry: rheological implications for the anatectic domain of the Araçuaí belt, Eastern Brazil

Abstract

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 constrain 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 geothermometer (TitaniQ) 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 800 °C, suggesting that quartz start crystallizing at a minimum temperatures ≥800 °C. These results, combined with the bulk-rock composition of isolated leucosomes allow to estimate a minimum of ∼30% melt in the anatectic leucossomes and a corresponding viscosity of ∼109–110 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 that the strength of the middle crust was severely weaken by extensive partial melting turning it prone to gravity-driven channel flow and lateral extrusion.