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- MakaleAçık Erişim2023Scopus SJRQ1Ordovician S-type granites in the western Sakarya Zone, NW Turkey; linkage to a back-arc setting
Karsli, Orhan; Sengun, Firat; Aydin, Faruk; Moghadam, Hadi S.; Dokuz, Abdurrahman; Griffin, William L.; Saka, Simge Oguz; Zack, Thomas
Turkey's Early Paleozoic tectonic evolution has received much attention in recent years. Early Paleozoic intrusive rocks are mainly distributed across Europe, Turkey (Istanbul-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 So center dot gut area (Bilecik, northwest Turkey). Our Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb analyses on zircons suggest that the So center dot gut 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 So center dot gut metagranites, with ages of >500 Ma to-2.2 Ga. The inherited Cadomian cores give concordant ages of 588 and 572 Ma. The So center dot gut 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 epsilon Hf(t) values of +3.6 to-35.4, whereas inherited zircon cores have epsilon Hf(t) values of +33.9 to-26.1. Our data, in conjunction with previous studies, suggest that the So center dot gut 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.
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İncele - MakaleAçık Erişim2022Scopus SJRQ1Late Jurassic Paleotethyan oceanic slab break-off revealed by Sr-Nd-Hf isotopes of Na-rich adakitic granites from northwestern Turkey
Karsli, Orhan; Şengün, Fırat; Santos, José Francisco; Uysal, İbrahim; Dokuz, Abdurrahman; Aydin, Faruk; Kandemir, Raif
The Mesozoic geodynamic evolution of the Paleotethys Ocean is critical but a poorly known piece of the geological history of the Sakarya Zone, north Turkey. Here, we describe new integrated research of zircon U-Pb geochronology, bulk-rock major-trace element and Sr–Nd isotopes as well as Hf isotope of zircons from the granitic rocks of the Demirköy and Kızıldamlar plutons located in Bilecik area along the western Sakarya Zone, northwestern Turkey. Zircon U–Pb ages obtained by LA-ICP-MS reveal that the granitic magma intruded into the basement rocks during the Late Jurassic (ca. 162–157 Ma). The granites are tholeiitic and show slightly metaluminous to peraluminous geochemical affinities. The granite samples are adakitic, having relatively high Na2O (6.35–7.58 wt%) and Sr (172–580 ppm), and low K2O (0.39–1.13 wt%) and Y (3.2–5.3 ppm), and thus have high Sr/Y (70–250). The samples exhibit enrichment of light rare earth elements (REE) and large ion lithophile elements (LILE) and depletion in heavy REE and the high field strength elements (HFSE; e.g., Nb, Ti, Ta). The samples possess low 87Sr/86Sr(t) values of 0.70313 to 0.70372, and εNd(t) of −0.66 to +9.24. Zircon grains exhibit positive εHf (t) values of 5.0 to 12.8 and depleted mantle model ages of Hf (TDM1 = 0.24 to 0.68 Ga). These geochemical features and Sr–Nd–Hf radiogenic isotope systematics reveal that the adakitic rocks were likely formed by low degree melting (<10%) of the eclogitic part of an oceanic slab and subsequent melt fractionation and crustal contamination. An ongoing subduction scenario cannot explain the formation of the investigated adakitic rocks. We suggest that the adakitic plutons were generated through slab break-off after the final closure of the Paleotethys Ocean throughout the Late Jurassic (ca. 162–157 Ma). Considering the previous data, the continental back-arc basin system, leading to the opening of Neotethys Ocean at the southern margin of the continent, has been put forward to decipher the Late Jurassic tectonic evolution of the Paleotethys Ocean and the western Sakarya Zone.
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