Nature and genesis of potassic high Ba Sr granitoids associated with syn-convergent extension in NW Turkey

Abstract

Abstract The Cenozoic post collisional magmatism in NW Turkey produced several granitic emplacements during the Eocene and Late Oligocene-Early Miocene. Both Eocene and Late Oligocene-Early Miocene plutons are represented by high Ba Sr granitoids. We document here whole-rock chemical and Sr–Nd–Pb-O isotopic compositions of the Solarya plutonic association (SPA) with the age of 21 Ma (Lower Miocene) to evaluate the nature and genesis of potassic, high Ba Sr granites. SPA is one of the major plutons representing the Late Oligocene- Early Miocene high Ba Sr granitoids. The SPA consists of three coeval granitic members (K-Feldspar megacrystalline granodiorite, microgranite-granodiorite and haplogranite) with associated mafic microgranular enclaves (MME) and mafic dikes of gabbroic diorite to diorite in composition. Along the southern contact of the pluton, hypabyssal rocks having quartz diorite and granodiorite composition were injected into the ring fault system coeval with SPA. Hypabyssal rocks (HA) display similar mineralogical, geochemical and isotopic characteristics with SPA which indicate that HA and SPA are temporally and spatially related. K-Feldspar megacrystalline granodiorite, microgranite-granodiorite and haplogranite display high K-calcalkaline affinity whereas the mafic dikes and MME's present mid-alkaline and shoshonitic character. SPA presents Ba values between 710 and 1505 ppm and Sr values between 305 and 706 ppm. They display low Y, Yb and HREE contents and an insignificant negative Eu anomaly, which are typical geochemical signatures of high Ba Sr granitoids. SPA has initial 87Sr/86Sr values of 0.70702–0.70805 and 143Nd/144Nd values vary from 0.51235 to 0.51250. They display ƐNd values of −4.9 and −2.05. 207Pb/204Pb and 206Pb/204Pb isotopic values range between 15.68 and 15.73 and 18.75 to 18.88, respectively. Quartz and whole rock δ 18O isotopic values vary from 8 to 10.6. These similar Sr-Nd-Pb and O isotopic features of MME/mafic dikes and host granodiorites points out to a common igneous precursor for the magma source. All these major-trace element and isotopic compositions of SPA suggest an enriched mantle (EMII) or sub-continental lithospheric mantle source for their parental magma. Bulk mixing model between ƐNd and 87Sr/86Sr isotopic data suggests that, the crustal contribution to the enriched mantle source is about 30–45%. Regarding the Cenozoic tectonic history of NW Anatolia, the inferred mantle source and magma evolution suggest that the melt generation to produce high Ba Sr granitic magmatism was most probably the result of syn-convergent extension.