Late Eocene–Early Oligocene two-mica granites in NW Turkey (the Uludağ Massif): Water-fluxed melting products of a mafic metagreywacke

This paper deals with the petrogenesis of two-mica granites within the amphibolite-facies metamorphic rocks in the Uludağ Massif (NW Turkey). The two-mica granites are represented by three different types (i) a ~ 17 km long and ~ 1.5 km-wide strongly foliated and lineated metagranite, (ii) a relatively younger circular granite, ~ 11 km by ~ 6.5 km, free of any penetrative fabric, and (iii) up to 5 m thick dikes of unfoliated granite-porphyries. Mineral constituents include quartz, plagioclase, microcline, biotite, muscovite and minor apatite, zircon, ilmenite and monazite. U–Pb zircon dating indicates that the metagranite was emplaced at 35 ± 1 Ma (2σ, latest Eocene) and the unfoliated granite at 28 ± 1 Ma (2σ) (Early-Late Oligocene boundary). Overall, the two-mica granites are characterized by (i) a restricted bulk composition (SiO₂ ~ 70–76 wt.%), (ii) near-linear to linear inter-element variations, (iii) high concentrations of Al₂O₃, Sr, Ba, LREEs, and low abundances of Rb, Y, Sc and HREEs, comparable with adakitic rocks, (iv) variably pronounced concave-upward shape of HREEs and absent to feeble Eu anomaly, (v) relatively high Na₂O, CaO and Sr, and lower K₂O and Rb contents at identical silica contents relative to the classical S-type granites. ⁸⁷Sr/⁸⁶Sr_(i) and εNd_(i) values range from 0.7052 to 0.7080, and 0.7 to − 3.6, respectively. The ²⁰⁶Pb/²⁰⁴Pb_(i) values range from 18.23 to 18.69, ²⁰⁷Pb/²⁰⁴Pb_(i) from 15.63 to 15.70 and ²⁰⁸Pb/²⁰⁴Pb_(i) from 38.48 to 38.77. ⁸⁷Sr/⁸⁶Sr_(i) and εNd_(i), and εNd_(i) and ²⁰⁷Pb/²⁰⁴Pb_(i) values are negatively correlated. Isotopic variation and negative correlations can best be explained by a heterogeneous source, including an older crustal component and a more juvenile basic component. Magma temperatures are estimated as 754–790 °C based on zircon and monazite saturation. All these features such as (i) relatively Na-, Ca- and Sr-rich compositions, (ii) isotopically heterogeneous source with inferred residual phases such as garnet and hornblende, (iii) occurrence in a ductile strike-slip zone, and (iv) relatively low magma temperatures can be accounted for by water-fluxed melting of a metagreywacke source with substantial mafic component at middle to lower crustal depths.