P–T– XCO2 –bulk rock composition modeling of garnet decomposition in amphibolite and mafic granulite: tectono-metamorphic insights into the Permian–Triassic orogeny on the eastern margin of the Korean Peninsula

Garnet amphibolites with relics of mafic granulite occur within the Precambrian Odaesan Complex and the Paleozoic Taebaeksan Basin on the Korean Peninsula. Atoll garnets and symplectites after garnet are locally developed in the mafic rocks. In order to constrain the metamorphic–metasomatic garnet decomposition, we undertook mineral equilibria modeling using pressure (P)–temperature (T)–fluid composition [XCO2; CO₂/(CO₂ + H₂O)]–bulk–rock composition (X_BC) pseudosections that were calculated in the Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–TiO₂–H₂O ± MnO ± Fe₂O₃ ± CO₂ system. The integrated thermodynamic pseudosection modeling successfully reproduced the variations in metamorphic mineral assemblages and the reaction textures produced by metasomatism during the exhumation of the mafic rocks. The peak P–T–XCO2 conditions of the mafic granulite and amphibolite were 9.5–11.0 kbar/670–720 °C at XCO2 = 0.4–0.5 and 7.0–9.0 kbar/580–630 °C at XCO2 = 0.4, respectively. Subsequently, during retrogressive metamorphism at 3.5–6.0 kbar and 470–530 °C in the presence of H₂O-dominated fluids, the symplectites and atoll garnets were formed by preferential dissolution and/or resorption. Furthermore, fluid-assisted local variations in rock composition (involving Al³⁺, Fe²⁺, and Ca²⁺) also affected the formation of the metasomatic reaction textures around the garnet porphyroblasts. The peak metamorphism with change of XCO2 occurred during the early Triassic (c. 250 Ma), as inferred from U–Pb zircon age data. Taking into account previous metamorphic and provenance studies on the tectonic environment of the eastern margin of the Korean Peninsula, the sub-isothermal decompressional P–T–XCO2–X_BC path of the amphibolites and mafic granulites resulted from the amalgamation of microcontinents and the Sino–Korean Craton at a trench–arc setting during the Permian–Triassic orogeny.