Petrography, mineral chemistry, geochemistry, geochronology, and petrogenesis of late miocene-quaternary volcanic rocks from the Posof (Ardahan, Turkey) region

Petrography, Mineral Chemistry, Geochemistry, Geochronology, and Petrogenesis of Late Miocene-Quaternary Volcanic Rocks from the Posof (Ardahan, Turkey) Region were investigated. Late Miocene volcanic rocks (8.6-7.3 Ma) range in composition from basalt to basaltic andesite and are represented by plagioclase+clinopyroxene+olivine+hornblende+opaque minerals. In contrast, the Pliocene-Quaternary volcanic rocks (5.2-0.73 Ma), which display adakite-like geochemical characteristics, are andesitic in composition and are characterized by plagioclase±hornblende±biotite±opaque minerals. The Late Miocene volcanic rocks have SiO2contents of 48.14-61.22 wt. %, Al2O3 of 14.81-16.86 wt.%, MgO of 3.01-8.13 wt.%, Mg# values of 34.9-51.3, Sr/Y ratios of 13.45-69.73, and La/Yb ratios of 3.91-39.23. The Pliocene-Quaternary volcanic rocks exhibit higher SiO2 (64.56-70.83 wt.%), Al2O3 (15.51-16.97 wt.%), lower MgO (0.25-1.34 wt.%), Mg# values of 10.68-33.20, and elevated Sr/Y (43.72-143.42) and La/Yb (43.45-76.68) ratios. Chondrite-normalized rare earth element patterns show moderately enriched LaN/LuN ratios (2.66-27.12) for the Late Miocene volcanic rocks, whereas the Pliocene-Quaternary volcanic rocks display strongly enriched patterns with LaN/LuN ratios of 29.36-75.58. Initial isotopic ratios of the Late Miocene volcanic rocks range from (87Sr/86Sr)i=0.703792-0.704395 and (143Nd/144Nd)i=0.512796-0.512881, whereas those of the Pliocene-Quaternary volcanic rocks vary between (87Sr/86Sr)i=0.704153-0.704506 and (143Nd/144Nd)i=0.512740-0.512809. In conclusion, the magmas forming the Late Miocene volcanic rocks were generated in an extensional tectonic setting by partial melting of a subduction-enriched mantle source related to lithospheric thinning and asthenospheric upwelling driven by delamination processes, together with melts derived from juvenile lower crust. In contrast, the Pliocene-Quaternary volcanic rocks were produced under similar geodynamic conditions by partial melting of juvenile lower crust, and subsequently evolved through magmatic processes such as fractional crystallization and magma mixing within the crust.