Coeval plutonism and metamorphism in a latest Oligocene metamorphic core complex in northwest Turkey

A metamorphic core complex of latest Oligocene age crops out in the Kazdag mountain range in northwest Turkey. The footwall of the core complex consis ts of gneiss, amphibolite and marble metamorphosed at 5 +/- 1 kbar and 640 degrees +/- 50 degreesC. The average muscovite and biotite Rb/Sr ages from the gneisses are 19 Ma and 22 Ma, respectively, and imply high temperature metamorphism during latest Oligocene times. The hangingwall is made up of a n unmetamorphosed Lower Tertiary oceanic accretionary melange with Upper Cr etaceous eclogite lenses. The hangingwall and footwall are separated by an extensional ductile shear zone, two kilometres thick. Mylonites and underly ing high-grade metamorphic rocks show a N-trending mineral lineation with t he structural fabrics indicating down-dip, top-to-the-north shear sense. Th e shear zone, the accretionary melange and the high-grade metamorphic rocks are cut by an undeformed granitoid with a 21 Ma Rb/Sr biotite age, analyti cally indistinguishable from the Rb/Sr biotite ages in the surrounding foot wall gneisses. The estimated pressure of the metamorphism, and that of the granitoid emplacement, indicate that the high-grade metamorphic rocks were rapidly exhumed at similar to 24 Ma from a depth of similar to 14 km to sim ilar to7 km by activity along the shear zone. The subsequent exhumation of the metamorphic rocks to the surface occurred during Pliocene-Quaternary ti mes in a transpressive ridge between two overstepping fault segments of the North Anatolian Fault zone. The high-grade metamorphic rocks of the Kazdag range are surrounded by voluminous calc-alkaline volcanic and plutonic roc ks of Late Oligocene-Early Miocene age, which formed above the northward-di pping Hellenic subduction zone. The magmatic are setting of the core comple x and stratigraphic evidence for subdued topography in northwest Turkey pri or to the onset of extension suggest that the latest Oligocene regional ext ension was primarily related to the roll-back of the subduction zone rather than to the gravitational collapse.