Petrogenetic significance of almandine garnet in intermediate calc-alkaline magmas from the South Apuseni Mountains, (Romania): Constraints from mineral geochemistry and phase equilibria

We report new petrographic evidence, mineral-scale major and trace element composition, and thermodynamic constraints on the origin of garnet-bearing andesites and dacites from the South Apuseni Mountains, Romania. Garnets in these Mid-Miocene calc-alkaline magmas occur as rare phenocrysts hosting high-anorthite plagioclase, apatite, zircon, ilmenite, and magnetite inclusions. To assess the significance of these assemblages in the regional context, we evaluated plausible origins for garnet - ranging from primary magmatic crystallization to peritectic formation during crustal assimilation or accidental incorporation as xenocrysts. Electron microprobe and LA-ICP-MS analyses reveal homogeneous almandine-dominated solid solutions with relatively high CaO contents and strongly fractionated REE patterns (LREE-depleted, HREE-enriched), consistent with a magmatic origin. Ilmenite-magnetite thermometry and phase-equilibrium modelling constrain garnet stability to temperatures between 750 and 830 °C and pressures above 7 kbar, corresponding to lower-crustal conditions in the South Apuseni region. U Pb dating of garnet-hosted zircon inclusions (12 ± 3 Ma) matches the emplacement age of the host rock, confirming a magmatic origin. Taken together, the results of this study demonstrate that the Ca-rich almandine garnets sporadically found in some early South Apuseni dacites and andesites, likely crystallized from evolved or hybrid melts, through deep-seated fractional crystallization and survived due to rapid magma ascent towards the emplacement levels.