Variscan amphibolite-facies rocks from the Kurtoglu metamorphic complex (Gumushane area, Eastern Pontides, Turkey)

The Kurtoglu metamorphic complex, that forms part of the pre-Liassic basement of the Sakarya zone in northern Turkey, consists of at least two tectonic units. Blueschist-facies rocks of unknown metamorphic age in the southern part of the complex are tectonically overlain by Variscan low-pressure high-temperature metamorphic rocks. The latter comprise mica schists and fine-grained gneisses, cut by metaleucogranitic dikes, as well as migmatitic biotite gneisses and subordinate amphibolite intercalations. Structural data indicate that metamorphism and penetrative deformation occurred after dyke intrusion. Peak metamorphic conditions of the mica schists, fine-grained gneisses and metaleucogranites are estimated to similar to 650 degrees C and similar to 0.4 GPa, based on phase relationships in the system NCKFMASH, Fe-Mg partitioning between garnet and biotite as well as garnet-aluminosilicatequartz- plagioclase (GASP) and garnet-plagioclase-biotite- quartz ( GBPQ) barometry. Peak temperatures of the migmatitic biotite gneisses and amphibolite intercalations are not well constrained but might have been significantly higher (690-740 degrees C), as suggested from hornblende-plagioclase thermometry. Ar-40-(39) Ar incremental dating on muscovite and biotite fractions from the mica schists and fine-grained gneisses yielded plateau ages of similar to 323 Ma. Significantly older model ages of similar to 329 and similar to 337 Ma were obtained on muscovite fractions from two metaleu-cogranite samples. These fractions contain both relict igneous and newly formed metamorphic muscovite.