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

Abstract

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 ~17km long and ~1.5km-wide strongly foliated and lineated metagranite, (ii) a relatively younger circular granite, ~11km by ~6.5km, free of any penetrative fabric, and (iii) up to 5m 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±1Ma (2σ, latest Eocene) and the unfoliated granite at 28±1Ma (2σ) (Early-Late Oligocene boundary). Overall, the two-mica granites are characterized by (i) a restricted bulk composition (SiO2 ~70–76wt.%), (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.