QuiG elastic modeling reveals HT/LP metamorphic overprint in a garnet-clinopyroxene amphibolite: an example from Western Gondwana

Abstract

We report unprecedented Quartz-in-Garnet (QuiG) elastic modeling data for a garnet-clinopyroxene amphibolite sample overprinted by high-temperature metamorphism that corresponds to a Neoarchean (ca.2.65 Ga) oceanic crust metamorphosed during the Neoproterozoic (ca. 600 Ma) in the Borborema Province, NE Brazil. The data obtained from twenty quartz inclusions entrapped in the garnet indicate residual pressure (P_inc) values between ‐0.47(5) and ‐0.33 (5) GPa, suggesting a strongly tensile stress field. One population was identified based on P_inc values with an average value of ‐0.39(2) GPa, suggesting an elastic reset of the host-inclusion system during high-temperature and low-pressure metamorphism after high-pressure conditions. Calculated isomeke using Equation of State (EoS) for almandine garnet indicates a pressure of 0.60(1) GPa at 725 °C with a P_inc value of ‐0.39(2) GPa, consistent with the stability of orthopyroxene from previous thermodynamic modeling. These findings suggest metamorphic conditions consistent with granulite facies, as evidenced by the breakdown of high-pressure mineral assemblages and the formation of orthopyroxene, alongside the regional occurrences of kyanite-bearing granulites, mylonitic migmatites, and sillimanite-bearing metapelites. Moreover, the volumetric response of quartz inclusions to pressure and temperature changes during exhumation to Earth's surface suggests the development of tensile stresses, likely driven by a thermal influx during the exhumation of this segment of Western Gondwana. Therefore, the integration of chemical and physical methods for intensely retrograded metamorphic rocks allows an understanding of complex geodynamic processes, providing new insights into the exhumation of deeply burial rocks during the Neoproterozoic in NE Brazil.