Ai. Okay et M. Satir, Coeval plutonism and metamorphism in a latest Oligocene metamorphic core complex in northwest Turkey, GEOL MAG, 137(5), 2000, pp. 495-516
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.