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    Geochemical and mantle-like isotopic (Nd, Sr) composition of the Baklan Granite from the Muratdagi Region (Banaz, Usak), western Turkey
    (Pergamon-Elsevier Science Ltd, 2008) Aydogan, M. Selman; Coban, Hakan; Bozcu, Mustafa; Akinci, Oemer
    The (late syn)-post-collisional magmatic activities of western and northwestern Anatolia are characterized by intrusion of a great number of granitoids. Amongst them, Baklan Granite, located in the southern part of the Muratdagi Region from the Menderes Massif (Banaz, Usak), has peculiar chemical and isotopic characteristics. The Baklan rocks are made up by K-feldspar, plagioclase, quartz, biotite and hornblende, with accessory apatite, titanite and magnetite, and include mafic microgranular enclaves (MME). Chemically, the Baklan intrusion is of sub-alkaline character, belongs to the high-K, calc-alkaline series and displays features of I-type affinity. It is typically metaluminous to mildly peraluminous, and classified predominantly as granodiorite in composition. The spider and REE patterns show that the rocks are fractionated and have small negative Eu anomalies (Eu/Eu* = 0.62-0.86), with the depiction of Nb, Ti, P and, to a lesser extent, Ba and Sr. The pluton was dated by the K-Ar method on the whole-rock, yielded ages between 17.8 +/- 0.7 and 19.4 +/- 0.9 Ma (Early Miocene). The intrusion possesses primitive low initial Sr-87/Sr-16 ratios (0.70331-0.70452) and negative epsilon(Nd(t)) values (-5.0 to -5.6). The chemical contrast between evolved Baklan rocks (SiO2, 62-71 wt.%; Cr, 7-27 ppm; Ni, 5-11 ppm; Mg#, 45-51) and more primitive clinopyroxene-bearing monzonitic enclaves (SiO2, 54-59 wt.%; Cr, 20-310 ppm; Ni, 10-70 ppm; Mg#, 50-61) signifies that there is no co-genetic link between host granite and enclaves. The chemical and isotopic characteristics of the Baklan intrusion argue for an important role of a juvenile component, such as underplated mantle-derived basalt, in the generation of the granitoids. Crustal contamination has not contributed significantly to their origin. However, with respect to those of the Baklan intrusion, the generation of the (late syn)- post-collisional intrusions with higher Nd(t) values from the western Anatolia require a much higher amount of juvenil component in their source domains. (c) 2007 Elsevier Ltd. All rights reserved.
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    Melting of the juvenile lower crust in a far-field response to roll-back of the southern Neotethyan oceanic lithosphere: the Oligocene adakitic dacites, NE Turkey
    (Elsevier, 2020) Karsli, Orhan; Caran, Semsettin; Coban, Hakan; Sengun, Firat; Tekkanat, Osman; Andersen, Tom
    Late Cenozoic tectono-thermal events and associated magmatism in the Sakarya Zone (SZ) are still contested. Although documented in the western part of the SZ, thus far, no magmatic activity has been identified in the eastern part of the Oligocene SZ. Here, we report a newly identified Oligocene magmatism to interpret the genesis with tectonic setting and gain new insight into the geological evolution of the eastern SZ. We present extensive geochemical, bulk-rock Sr-Nd and zircon Hf isotope, and zircon U-Pb chronological analyses for the Tepebasi dacites in the Artvin area, NE Turkey. Zircon UPb dating analyses revealed a dacite formation age of similar to 29.8 +/- 0.3 Ma. Geochemically, with a K2O/Na2O ratio of 0.5 to 0.6, they are composed of rocks of a medium-K calc-alkaline adakitic affinity. The samples are further characterized by low Y (6-7 ppm), and high Sr (362-588 ppm) and Sr/Y ratios (58-98), with low Mg# (41-45) values, demonstrating a close affinity with the crustal source of adakitic rocks. They have slightly radiogenic isotope concentrations (Sr-87/Sr-86(t) = 0.70460-0.70544, epsilon(Nd)(t) = +1.7 to +2.0), and single-stage Nd model ages of T-DM1 = 0.61-0.63 Ga, as well as uniform and positive epsilon(Hf) (t) of 8.2-10.5, with young Hf depleted mantle ages (T-DM1 = 0.31-0.41 Ga). These isotopic features, in combination with the geochemical signature, preclude a mantle origin. Instead, they most likely originated from a juvenile mafic lower crustal material by low degree partial melting (<%5) rather than through partial fusion of a subducting slab or thickened lower crust. Trace element modeling reveals that the mafic juvenile lower crust is composed of <10% garnet-bearing amphibolite. Furthermore, trace element compositions imply that adakitic melts formed in an extensional setting without delamination of a thick mafic lower continental crust. We conclude that the Oligocene adakitic magmatism originated in an intracontinental setting, which was subjected to far-field extensional forces induced by roll-back of south Neotethyan oceanic lithosphere just before its detachment in the collision zone. We believe that hot asthenospheric upwelling due to the far-field extension induced by the roll-back of the southern branch of the Neotethyan oceanic lithosphere triggered adakitic magmatism. The heat induced by the upwelling of the asthenosphere likely led to the heat-fluxed melting of juvenile mafic crustal material in such an extensional tectonic setting during the Oligocene epoch in the eastern SZ. (C) 2020 Elsevier B.V. All rights reserved.