Geochronology, geochemistry, and petrology of adakitic Pliocene-Quaternary volcanism in the Sebinkarahisar (Giresun) area, NE Turkey

Abstract

The Pliocene-Quaternary volcanics in NE Turkey are mainly hornblende-phyric trachyandesites having a narrow range of SiO2 from 61.88 to 63.00 wt.% and exhibiting adakitic signatures with their Na2O (3.67-4.27 wt.%), Al2O3 (16.19-16.80 wt.%), Y (14.1-16.5 ppm) contents and K2O/Na2O (0.87-1.12), Sr/Y (44.24-54.90), and La/Yb (36.80-43.88) ratios. Plagioclases as the main mineral phases show a wide range of compositions, and weak normal and reverse zoning. Hornblendes are generally edenite and pargasite (Mg#: 0.39-0.74). Clinopyroxenes are augite (Mg#: 0.58-0.76). Biotites have Mg# ranging from 0.45 to 0.66. The textural and compositional variations indicate disequilibrium crystallization possibly arising from magma mixing. The U-Pb zircon dating of the adakitic volcanics yielded 3.4-1.9 Ma. The studied rocks display moderate light rare earth element /heavy rare earth element ratios and enrichment in the lithophile element and depletion in high field strength element, implying that the parental magmas were derived from mantle sources previously enriched by slab-derived fluids and/or subducted sediments. The crystallization temperature and pressure estimations based on the clinopyroxene thermobarometry range from 1144 to 1186 degrees C and from 3.92 to 7.97 kbar, respectively. Hornblende thermobarometry, oxygen fugacity, and hygrometer calculations yielded results as 908-993 degrees C at a pressure of 2.87-5.22 kbar, water content of 4.4-8.4 wt.%, and relative oxygen fugacity (Delta NNO log units) of -0.6 to 0.9, respectively. Biotite thermobarometry suggests relatively higher oxygen fugacity conditions (10(-13.33) to 10(-17.60)) at temperatures of 676-819 degrees C and at pressures from 1.15 to 1.76 kbar. In the light of the obtained data and modelling, it can be concluded that the magmas of the adakitic volcanics were derived from enriched mantle source through relatively higher partial melting and experienced magma mixing with melts at the crustal level. Additionally, the fractional crystallization and assimilation-fractional crystallization processes may have played an important role during the evolution of the studied volcanics.