Mineral chemistry, thermodynamic evolution, and crystallization history of late miocene volcanic rocks around posof (Ardahan, Türkiye)

This study aimed to constrain the crystallization and emplacement conditions of magmas that produced the Late Miocene volcanic rocks exposed at the junction of the Erzurum-Kars Plateau and the Eastern Pontides in the Northeastern Anatolia Region through a series of thermobarometric analyses. The volcanic succession unconformably overlies Oligocene mudstone, siltstone, and sandstone alternations, with limestone and cherty limestone at higher levels. Petrographically, they consist of fine- to coarse-grained basalt, basaltic andesite, and andesite. Mineral compositions include plagioclase (An16-77), clinopyroxene (Wo39-45En43-50Fs11-12), orthopyroxene (Wo3-4En73-82Fs14-24), olivine (Fo63-86), hornblende (Mg#=0.62-0.79), and Fe-Ti oxides (Usp0.05-1.44, Ilm94.10-95.53). The groundmass comprises microlites of these minerals and volcanic glass. The dominant textures are microlitic porphyritic, hyalo-microlitic, and microlitic porphyritic, while glomeroporphyritic, intergranular, pilotaxitic, subophitic, flow, and vesicular textures are also observed. Widely present disequilibrium features, such as zoning, sieve, and resorption, are common in plagioclase and clinopyroxene. Thermobarometric calculations indicate crystallization temperatures of 880-1230°C, pressures of 0.73-8.71 kbar, oxygen fugacities between (-10.94) and (-8.92), and water contents of 4.60-6.32 wt.%. These results suggest that the volcanic rocks crystallized under volatile-rich, oxidizing conditions across a broad depth range (~2-32 km), at relatively high temperatures.