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 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 titaniumin-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 ∼ 10 9 -10 10 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 gravitydriven flow and lateral extrusion.

The influence of partial melting and melt migration on the rheology of the continental crust

“…Temperature estimates using the TitaniQ (Titanium-in-quartz) geothermometer presented by Cavalcante et al (2014) suggest that the minimum temperature at which quartz grains crystallized, and therefore the minimum temperatures needed to trigger partial melting, was ~750 °C. Cavalcante et al (2014), using traditional exchange geothermobarometers in kinzigitic migmatites neighboring the Carlos Chagas unit, suggested that peak metamorphism occurred at 750 to 840 ± 50 °C at pressures ranging from 650 to 700 ± 100 MPa.…”

“…2a), or interconnected networks of melt, in which thick leucosomes (3-30 cm) are highly discordant to the foliation (Fig. 2c); these leucosome networks, together with the aforementioned migmatitic structures, suggest a minimum of 30% melt content in the migmatitic system (e.g., Cavalcante et al 2014;Sawyer 2008). In many outcrops of diatexites, the leucosomes may occupy up to 70% of the migmatite, suggesting amounts of melt considerably higher than 30%.…”

“…The belt underwent high temperature-low pressure (700 -800 °C, 600 MPa) metamorphism that triggered extensive partial melting in its eastern part (Oliveira et al 2000;Cavalcante et al 2013Cavalcante et al , 2014.…”

“…The central plutonic domain consists of tonalite and granodiorite bodies that were deformed in the magmatic state at ~580 Ma (U-Pb on zircon, Mondou et al 2012). The eastern part consists of a ~300 km long and 50 -100 km wide anatectic domain composed of anatexites and leucogranites known as the Carlos Chagas unit (Cavalcante et al 2013(Cavalcante et al , 2014. This unit is the result of widespread partial melting of middle crustal rocks at ~600-570 Ma.…”

“…The dataset described in the following sections represents a summary of the main results already published in Cavalcante et al (2013Cavalcante et al ( , 2014 and Viegas et al (2013Viegas et al ( , 2014.…”