Ordovician S-type granites in the western Sakarya Zone, NW Turkey; linkage to a back-arc setting

Abstract

Turkey's Early Paleozoic tectonic evolution has received much attention in recent years. Early Paleozoic intrusive rocks are mainly distributed across Europe, Turkey (İstanbul-Zonguldak Zone, Strandja Zone, and Tauride Block), and Iran. However, the Early Paleozoic rocks have not yet been identified in north Turkey's Sakarya Zone, and thus the Early Paleozoic paleogeography of the Sakarya Zone remains enigmatic. Here, we provide new field observations, petrography, geochemistry, and geochronology of the western Sakarya Zone to identify the nature, origin, and tectonic setting of the Ordovician magmatic rocks cropping out in the Söğüt area (Bilecik, northwest Turkey). Our Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U–Pb analyses on zircons suggest that the Söğüt metagranites were emplaced at ∼484–461 Ma (except two zircons with ages of 490 and 494 Ma). There are abundant inherited zircon cores in the Söğüt metagranites, with ages of >500 Ma to ∼2.2 Ga. The inherited Cadomian cores give concordant ages of 588 and 572 Ma. The Söğüt metagranites are felsic and have high SiO2 (72–76 wt%) and K2O (4.1–5.3 wt%) contents, with high K2O/Na2O ratios (1.2–1.9), and low Mg# (17–35) values. The samples are Ca-poor (CaO = 0.52–0.68 wt%) and show peraluminous [A/CNK = molar Al2O3/(K2O + CaO + Na2O) = 1.15–1.28] and high-K calc-alkaline geochemical characters, with high modal content of muscovite (∼10%), consistent with S-type compositions. The samples have low REE concentrations, with negative Eu anomalies. Ordovician zircon rims show εΗf(t) values of +3.6 to −35.4, whereas inherited zircon cores have εΗf(t) values of +33.9 to −26.1. Our data, in conjunction with previous studies, suggest that the Söğüt metagranites were generated by fluid-absent muscovite-dehydration melting of metasediments accumulated in a back-arc setting. This back-arc basin was characterized by a denuded continental crust with abundant pelite-dominated sediments and minor input of juvenile mafic melts. Our results, combined with previously published data and paleogeographic reconstructions, allow us to conclude that the Ordovician S-type granites were formed by progressive back-arc extension and crustal thinning prompted by the rollback of the southward subducting oceanic-lithosphere slab. This led to the opening of the Rheic Ocean on the northern side of Gondwana in response to the final closure and termination of the Tornquist Ocean as a branch of the Iapetus Ocean. We argue that the Ordovician S-type granites from the northern side of Gondwana could signal significant crustal reworking away from any clear continental collision zone.