Two-stage bimodal volcanism in a Late Cretaceous arc/back-arc setting, NE Turkey: Constraints from volcano-stratigraphy, zircon U-Pb and 40Ar/39Ar geochronology and whole-rock elemental and Sr-Nd-Pb isotope geochemistry

The eastern Blacksea magmatic arc (EBMA) in the eastern Sakarya Zone (ESZ) provides an excellent opportunity to investigate birth of an extensional intra-arc and back-arc settings in the Late Cretaceous over the Early Cretaceous northern passive margin of the Neotethys Ocean. Volcano-stratigraphy clearly shows that the Late Cretaceous volcanic activity of the EBMA occurred in two major phases. Bimodality, characterized by mafic/ basaltic rocks at the base and felsic/silicic types on top of it, is a typical feature of the lower (LVS) and upper (UVS) volcanic successions in the Giresun region of the ESZ. U-Pb and Ar-Ar ages support the volcanic suc-cession as two-stage (LVS: ca. 92-85 and UVS: ca. 83-67 Ma) bimodal volcanism. Both the volcanic successions are represented by similar rock types consisting of tholeiitic to calc-alkaline basalt-basaltic andesites and calc-alkaline to shoshonitic dacite-rhyolites. Basaltic (M1-and M2-series) and felsic/silicic (F1-and F2-series) sam-ples of the LVS and UVS have an arc-like signature with enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted high field strength elements (HFSEs). Also, the felsic/silicic samples of the F1-and F2-series show prominent negative Sr and Eu anomalies (Eu/Eu* = 0.4 to 0.9), suggesting that plagioclase fractionation played a key role on the evolution of both felsic series. Bimodal rock series in two phases have a wide range of 87Sr/86Sr(i) (0.7048-0.7075) and 143Nd/144Nd(i) (0.5123-0.5127) ratios with var-iable epsilon Nd(i) values of-3.8 to +3.0. 206Pb/204Pb(i), 207Pb/204Pb(i) and 208Pb/204Pb(i) isotope ratios of the Giresun volcanic rocks vary in the range of 17.97-18.52, 15.55-15.65 and 37.53-38.56, respectively. Geochemical and isotopic data suggest that the parental magma of the M1-basaltic rocks were probably derived from a shallow (spinel-bearing) mantle metasomatized by slab/sediment-derived fluids. In contrast, the M2-basalts seem to have been originated from a deeper mantle source (spinel-garnet transition zone) enriched by slab/sediment-derived fluids and hydrous melts (bulk sediment) metasomatism with some contributions of lower/upper crustal materials. The least evolved basaltic samples in two phases are consistent with moderate (-10-15%) to high degree (-20-30%) partial melting of the metasomatized mantle. The silicic melts of the F1 -and F2-rocks series, on the other hand, were likely derived from melting of lower crustal materials consisting of meta-basalts/andesites and lesser amount of meta-sediments. Subsequently, these melts experienced FC +/- AFC and mixing processes during their ascent and emplacement to generate high-silica (rhyolitic) melts. Our data, combined with previous studies, suggest that two-stage bimodal volcanic rocks of the Late Cretaceous in the ESZ were formed in the transition from an extensional continental intra-arc to a back-arc setting during the north-ward subduction of the northern branch of Neotethys Ocean.