Petrography and mineral chemistry of the Gölönü pluton (Kürtün-Gümüşhane-Türkiye)

The Eastern Pontides host numerous plutonic rock assemblages with diverse ages and composition. Among these, the Middle Eocene Gölönü Pluton, located in the Kürtün district of Gümüşhane, is distinguished by its unique petrographic and geochemical features. This study aims to determine the physicochemical parameters (temperature, pressure, and oxygen fugacity) associated with the crystallization and emplacement of the magma through an integrated evaluation of petrographic observations and mineral chemistry data. In addition, crystallization conditions were quantitatively constrained using mineral chemistry-based approaches. The pluton covers an area of approximately 14 km2 and exhibits a NE–SW trend. Modal mineralogical data indicate that the rocks are compositionally granodiorite and granite. Textural characteristics reveal fine- to medium-grained fabrics with locally developed porphyritic, poikilitic, and graphic textures. The principal mineral assemblage consists of plagioclase, K-feldspar, quartz, amphibole, biotite, and Fe-Ti oxides, whereas zircon, apatite, sphene, and allanite occur as accessory phases. Plagioclase compositions range from oligoclase to labradorite (An0.1–56), while K-feldspars are orthoclase in composition. Amphiboles belong to the calcic amphibole group and are classified as magnesioamfibole (Mg# = 0.78–0.85). Calculations based on amphibole and plagioclase mineral chemistry indicate crystallization temperatures ranging from 751 to 904 °C. Geobarometric estimates yield pressures of 0.67–2.23 kbar, corresponding to emplacement depths of approximately 3–7 km. Oxygen fugacity values range from approximately −13.0 to −10.6 log units, whereas the magma water content is estimated at 3.3–5.0 wt.%. These results suggest that the Gölönü Pluton represents a water-rich magmatic system that was emplaced at shallow crustal levels and underwent a complex, multi-stage magmatic evolution.