Origin of Eocene adakitic magmatism in northwest Turkey

Abstract

In this study, we focus on adakitic porphyries, which are rare and recently discovered in the northwest (NW) Turkey. We present a comprehensive dataset including their petrography, geochemistry, and 40Ar/39Ar geochronology to gain insight into the causes, timing, and possible melt sources of adakitic magmatism in the absence of active subduction. The studied adakitic porphyries are found as stocks, dikes, and sills associated with the Orhaneli plutonic complex, which intruded into ophiolitic and blueschist rocks of the Izmir–Ankara suture zone (IASZ). The porphyries yielded 40Ar/39Ar biotite ages of 53.70 ± 0.29 to 53.84 ± 0.16 Ma (early Eocene), indicating they were coeval with the Orhaneli pluton. They are represented by porphyritic microgranite and microgranodiorite and consist of quartz, plagioclase, K-feldspar, hornblende, and biotite. These adakitic porphyries are similar to those found worldwide, and display distinct geochemical properties such as high SiO2 and Sr at 63.80–69.43 wt% and 1129–1719 ppm, respectively; low Y and Yb at 6.2–12.8 ppm and 0.53–1.07 ppm, respectively; high ratios of Sr/Y and LaN/YbN at 169–2645 and 10–215, respectively; enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE); depletion in high-field-strength elements (HFSE); and insignificant or lack of Eu anomaly (Eu/Eu*) at 0.87–1.11. However, they differ from adakites by their potassic nature with high 87Sr/86Sr and 207Pbi/204Pbi, at 0.706249–0.706577 and 15.669–15.700, respectively, and low 143Nd/144Nd isotopic values (εNd −2.94 to 0.59), all of which correspond to continental adakites (C-adakites). Overall evaluation of major-trace elemental composition and isotopic data revealed that adakitic porphyries were generated by partial melting of mafic lower crustal rocks and heterogeneous lithospheric mantle and their interaction. Assimilation and fractional crystallization (AFC) played an important role during the evolution of these melts at shallow crustal levels. Collectively, the geochemical characteristics, timing, and nature of the adakitic porphyries and the geological background of the region indicate that the adakitic magmatism was not formed above an actively dehydrating subducted slab; rather, these characteristics are consistent with magmatism that is more typical of intraplate tectonic settings. We infer that upwelling of the asthenospheric mantle as a result of steepening and breaking of the subducted Tethyan oceanic slab or partial delamination of the base of the lithosphere would raise the geothermal gradient beneath the suture zone and increase heat flow to trigger the generation of K-rich C-adakite magmas in NW Turkey during the early Eocene.