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    Early Cambrian S-type granites in the Sakarya Zone, NE Turkey: A record for transition from subduction to post-collisional extension deduced from U-Pb zircon age and Nd-Hf isotopes
    (Elsevier, 2022) Karsli, Orhan; Sengun, Firat; Dokuz, Abdurrahman; Aydin, Faruk; Kandemir, Raif; Kristoffersen, Magnus; Santos, Jose Francisco
    Cadomian realms have recently received much attention in the Alpine-Mediterranean orogenic belts, with discontinuous outcrops extending from the Alps to the Istanbul Zone, Menderes Massif, Taurides in Turkey and Iran as well. However, they have not been identified in the Sakarya Zone, northern Turkey to date. Here, we present, for the first time, zircon U-Pb geochronology, zircon Hf-isotopes and trace elements as well as whole-rock geochemistry and Nd-isotopes from the Karamese metagranitoid in the Yusufeli (Artvin) area, NE Turkey. The zircon U-Pb age data demonstrate that the Karamese metagranitoid intruded into the Precambrian basement units in the Early Cambrian (ca. 534-530 Ma). Zircon overgrowths in the Karamese metagranitoid interpreted as a result of metamorphic overprint yielded ages of 328 Ma. This age indicates that the Sakarya Zone experienced the Variscan metamorphic events at 328 Ma. The Early Cambrian metagranitoid shows that some rocks in the basement of the Sakarya Zone date back to Precambrian in age. The Karamese metagranitoid is an S-type granite, with high modal content of muscovite (similar to 10%). The samples show highly peraluminous [A/CNK = molar Al2O3/(CaO + K2O + Na2O) = 1.34-1.58] and medium-K calc-alkaline geochemical affinities. The Karamese metagranitoid displays low REE concentrations and a slight positive Eu anomaly and show slightly light REEs and LILEs enrichments, and HFSEs depletions. The samples have negative epsilon(Nd)(t) of -4.76 to -2.90 and epsilon(Hf) (t) of -4.75 to -1.08 values. These geochemical-isotopic characteristics reveal that the Karamase metagranitoid originated through fluid-absent muscovite dehydration melting of heterogeneous metasedimentary source rocks (mostly greywacke), with a minor input of juvenile mafic melt. We argue that the Early Paleozoic metagranitoids along the eastern Sakarya Zone is an expression of crustal thinning in the Cadomian orogenic belt in northern Gondwana, with the tectonic turnover from convergent margin to crustal extension possibly induced by break-off of Tornquist oceanic lithosphere which is a branch of Iapetus Ocean during Early Cambrian.
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    Tracking the timing of Neotethyan oceanic slab break-off: Geochronology and geochemistry of the quartz diorite porphyries, NE Turkey
    (Pergamon-Elsevier Science Ltd, 2020) Karsli, Orhan; Dokuz, Abdurrahman; Aydin, Faruk; Uysal, Ibrahim; Sengun, Firat; Kandemir, Raif; Santos, Jose Francisco
    The initiation of the break-off of the northern branch of the Neotethyan oceanic lithosphere is an important but poorly understood event in the geology of the Sakarya Zone (SZ) in northeastern Turkey. Although it is wellknown that Latest Cretaceous intrusives (-70 Ma) and early Eocene adakitic magmatic rocks are present in the eastern SZ, the outcrops of the early Eocene non-adakitic rocks are very limited, and their tectono-magmatic evolution has not been studied. We describe a small outcrop of non-adakitic quartz diorite porphyry in the Kov area of the Gumushane region in northeastern Turkey. The genesis of these porphyries is significant in evaluating the syn-to post-collision-related magmatism. The LA-ICP-MS zircon U-Pb dating revealed that the Kov quartz diorite porphyries (KQDP) formed at ca. 50 Ma, coeval with adakitic rocks, and-20 Myr later than the slab rollback-related intrusive rocks. The KQDPs are calc-alkaline in composition and enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Ti), with significant negative anomalies of Nb, Ta, and Ti but positive anomalies of Th, U, and Pb. Isotopic compositions of the samples show limited range of variation and slight enrichment of 87Sr/86Sr(t) (0.70489 to 0.70555), epsilon Nd(t) (-1.4 to -1.2) with TDM of 1.11 to 1.61 Ga. Pb isotopic ratios of the samples point to an enriched mantle source. They were likely crystallized from the melt that originated from an EM2-type spinel-facies subcontinental lithospheric mantle (SCLM), followed by the fractionation with insignificant crustal assimilation. The SCLM was metasomatically enriched, and the metasomatic agent was likely H2O-rich fluids rather than sediments released from subducting oceanic crust during the Late Cretaceous closure of the Neotethyan oceanic lithosphere. In conjunction with the geological background and previous data, we propose that the generation of the KQDPs resulted from a slab break-off event that caused ascending or infiltration of hot asthenosphere, triggering mantle melting. Such sporadic occurrences of the KQDPs, with coeval adakitic rocks in the SZ, are likely associated with the onset of extensional tectonics due to the earlier stage of slab break-off along the region during the early Eocene period.