Prehnite-forming reactions in scapolite-bearing calc-silicate granulites from Rundvågshetta, Lützow-Holm Complex, East Antarctica

Abstract

We report prehnite-forming reactions in scapolite-bearing calc-silicate granulites from the high-grade metamorphic terrane of Lützow-Holm Complex, East Antarctica. Based on textural observations, three possible reactions were identified: 1) Prehnite forming from andradite-grossular garnet, 2) Prehnite formation from anorthite, quartz, and calcite, and 3) Prehnite replacing meionite, quartz, and calcite. Phase equilibrium modeling indicates that prehnite formed under H₂O-rich conditions at T = 320-390 °C and X_CO2 < 0.004. The occurrence of Fe-rich prehnite reflects Al-Fe³⁺ partitioning that occurred during low-grade metamorphism and may serve as an indicator of oxygen fugacity conditions. The prehnite formation reactions exemplify the role of aqueous fluid infiltration during retrograde metamorphism and provide important constraints on changes in fluid conditions from fluid-absent to H₂O-rich.