Petrogenesis, geochronology and thermochronology of Oligocene to Miocene Western Anatolia granitoid plutons in Turkey

Abstract

Western Anatolia (WA) hosts numerous I-and S-type plutons with variable compositions. This paper presents new comprehensive geology, petrography, bulk-rock and mineral chemistry, SrNd isotope, zircon UPb, biotite and hornblende Ar/Ar and first apatite UTh / He age data for seven plutons (Baklan, Evciler, Eybek, Ilıca-Şamlı, Kozak, Uludağ, Yenice) in WA. Western Anatolia granitoid plutons (WAGPs) are mainly granodiorite, with minor monzonite, diorite, quartz monzonite and granite. The dominant minerals are K-feldspar (usually orthoclase and rarely microcline), plagioclase (albite, oligoclase), amphibole (hornblende), biotite, quartz, and accessory minerals are titanite, zircon, apatite, and opaques. The crystallization temperatures are relatively uniform (640-740 °C) but pressures range from those typical of the upper crust to those typical of the mid crust. The plutons are characterized by moderate to high SiO2 (58.1–73.7 wt%), K2O (2.1–6.9 wt%), and relatively low P2O5 (0.03–0.60 wt%) and most are I-type and metaluminous, except for the Uludağ pluton that is S-type. They are enriched in large ion lithophile elements (LILEs e.g. U, Th, Rb, and K), light rare earth elements (LREEs) and Pb, and depleted in heavy rare earth elements (HREEs), high field strength elements (HFSE, e.g. Nb, Ti) and Sr, Ba and P relative to other LILEs. The plutons display small Eu anomalies (Eu/Eu*: 0.17–1.15) due to probaly suppression of feldspar fractionation/or partial melting of lower crust. They have relatively high initial 87Sr/86Sr = 0.705775–0.708357 and low initial 143Nd/144Nd = 0.512290–0.512581. The LA-ICP-MS 206Pb238U age analyses of zircon in addition to 40Ar/39Ar dating of hornblendes and biotites suggest that the plutons were emplaced between 29.4 Ma and 19.8 Ma and cooled with a relatively high rate (∼363 °C/Ma). These ages correlate with the collision between the Sakarya Zone and Anatolide–Tauride Platform in Rupelian to Burdigalian time. The (UTh)/He apatite ages range from 19.8 Ma to 7.6 Ma, for the WA granitoid plutons and indicates that the crustal terrains hosting the seven plutons had complex exhumation histories within their arc environment. Consequently, these ages constrain the crystallization, cooling and exumation times of the Oligocene to Miocene plutons in a post-collisional setting on the northern margin of Gondwana.