Geochemical and geochronological relationships between granitoid plutons of the Biga Peninsula, NW Turkey

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Black, Karen Naomi

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The Aegean Sea is considered to be a classic back-arc basin. Back-arc basins may develop by active processes including retreat of the overriding plate or upwelling from the subducting slab. Alternatively, back-arc basins may develop as passive responses to regional tensional stresses. The Biga Peninsula of western Turkey provides an opportunity to explore and test these models. The Biga region is characterized by granitoid plutons of Cretaceous to Miocene age that may provide insight into the nature of extension. This study focuses on understanding the evolution of three of these plutons, the Kozak, Eybek, and Kestanbolu. Geochemical and geochronological data and cathodoluminescence (CL) images of the rocks and zircons were acquired. The first in situ (in thin section) ion microprobe U-Pb ages of zircon, and the first zircon ages ever reported from the Kozak and Eybek plutons are presented. Zircon ages range from 36.5±6.6 Ma to 17.1±0.7 Ma (238U/206Pb, ±1) with two ages from a single grain of 280±18 Ma and 259±14 Ma. Samples from the Kozak and Eybek plutons are magnesian, calc-alkalic, and metaluminous, whereas the Kestanbolu rocks are magnesian, alkali-calcic, and metaluminous with one ferroan sample. The Rb vs. (Y+Nb) diagram suggests the Kozak and Kestanbolu plutons have a volcanic arc source, whereas the Eybek pluton records a within plate setting. CL imagery documents magma mixing, brittle deformation, and fluid- rock interactions based upon cracked plagioclase cores, cross-cutting microcracks, and fluid reaction textures of myrmekite and red rims on alkali feldspar. The plutons were generated following the collision of the Sakarya continent with the Anatolide-Tauride block. Geochemical data suggest the Kozak and Kestanbolu granitoids were generated by fluid flux melting from dehydration of the subducting slab of the Anatolide-Tauride block. The Kestanbolu granitoid intruded into the Vardar Suture north of this collision, whereas the Eybek pluton was created within the lithosphere during exhumation of the Kazdağ Massif. The Eocene - Oligocene zircon ages indicate emplacement and initial crystallization of the plutons. Early Miocene ages indicate ongoing extension in the region at this time and are consistent with earlier interpretations that subduction slab roll-back along the Hellenic arc formed the extensional environment in the region at this time.



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