Origin, petrogenesis and geodynamic implications of the early Eocene Altınpınar adakitic andesites in the eastern Sakarya Zone, northeastern Turkey

Early Cenozoic magmatism in the eastern Sakarya Zone (NE Turkey) provides an important constraint on the regional tectono-magmatic evolution of the region. Early Eocene syn-collisional adakitic rocks are observed as small stocks with outcropping areas commonly less than 10 km2. This study presents petrography, whole-rock geochemistry and Sr-Nd-Pb isotope data, as well as in-situ 40Ar/39Ar age constraints on one of these adakitic andesites in the Altınpınar area of Gümüşhane, and discusses source region, petrological processes and geodynamic setting prevailed during their genesis. Andesites commonly show microlitic porphyric and vitrophyric porphyric textures, and include significant amounts of mafic microgranular enclaves (MMEs). Plagioclase, hornblende, Fe-Ti oxides and minor pyroxene are the main mineral phases. In-situ 40Ar-39Ar amphibole dating constrains the cooling age of andesites into a time span from 52.8 ± 1.3–48.8 ± 1.9 Ma. Andesites are medium to high-K calc-alkaline and display most of the signatures typical of those of the adakites. They are characterized by moderate MgO (1.7–4.1 wt%), low Y (9−14 ppm), Yb (0.9–1.5 ppm), and HREE and high Sr (325−964 ppm) contents, and high Sr/Y (36–76) ratios. 87Sr/86Sr(t) (0.704948−0.705100) and 143Nd/144Nd(t) (0.512588−0.512628) ratios are in the isotopic range of the adakites. All these geochemical and isotopic data suggest that the parental magma of adakitic andesites has been produced by partial melting of oceanic basalts under amphibole-eclogite facies conditions during the breakoff of the northern Neotethyan oceanic slab.