Crustal homogenization revealed by U-Pb zircon ages and Hf isotope evidence from the Late Cretaceous granitoids of the Agaçören intrusive suite (Central Anatolia/Turkey)

Koksal S. ; Moller A. ; Goncuoglu M.C. ; Frei D. ; Gerdes A. (2011)

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Geochemical and isotopic evidence from the Agaçören Igneous Association in central Anatolia-Turkey indicates that this suite of calc-alkaline granitic rocks have undergone crustal homogenization during regional metamorphic and related magmatic events. Whole-rock chemical and Sr-Nd isotopic data of the granitoids reveal crustal affinity with an earlier subduction component. Zircons show inherited cores and subsequent magmatic overgrowths. The laser ablation ICP-MS 206Pb/ 238U zircon ages are determined as 84.1 ± 1.0 Ma for the biotite-muscovite granite, 82.3 + 0.8/-1.1 Ma for the hornblende-biotite granite, 79.1 + 2.1/-1.5 Ma for the granite porphyry dyke, 75.0 + 1.0/-1.0 Ma for the alkali feldspar dyke, and 73.6 ± 0.4 Ma for the monzonite. This is interpreted as continuous magma generation, possibly from heterogeneous sources, from ca. 84 to 74 Ma during the closure of the northern branch of the Neotethyan Ocean. The oldest granitoids (84-82 Ma) were probably formed due to crustal thickening after obduction of the MORB-type oceanic crust onto the Tauride-Anatolide microplate. The younger granitoids are interpreted to be related to the subsequent post-collisional extension after lithospheric delamination. Combination of the laser ablation ICP-MS zircon Lu-Hf isotope data with the U-Pb ages of inherited cores suggests that Cretaceous granitoids formed by melting of heterogeneous crustal protoliths, which results in significant variation in εHf (t) data (from -12.9 to +2.2). These protoliths were probably composed of reworked Early Proterozoic crust, minor juvenile Late Proterozoic magmatic components, and Paleozoic to pre-Late Cretaceous recycled crustal material. Moreover, the Late Cretaceous zircon domains of the different granitoids are characterized by a crustal signature, with a relatively restricted zircon εHf (t) data ranging from -4.1 to -8.8. This variation is only about twice the reproducibility (ca. ±1 εHf) of the data, but much smaller than the isotope variability of inherited zircons. Our preferred interpretation is effective isotopic homogenization of the heterogeneous central Anatolian crust during the Late Cretaceous high-grade metamorphic and magmatic events, a process that we propose to be relevant for other active continental margins. © 2011 Springer-Verlag.

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