Paleozoic Metamorphic History of Strandja Massif, NW Turkey

The Strandja Massif is a NW-SE trending, extensive polymetamorphic belt (300 km length and 100 km width) cropping out in the NW Turkey and SE Bulgaria. The last metamorphism is of upper greenschist- to lower amphibolite-facies and occurred during the Late Jurassic-Early Cretaceous. The massif constitutes a Paleozoic metamorphic basement cut by intrusions of various ages and overlain by a Mesozoic metasedimentary cover. The Mesozoic cover displays monotonous deformation and metamorphic history whereas the Paleozoic basement shows complex structural fabric and metamorphism. Age and metamorphic conditions of the Late Jurassic- Early Cretaceous metamorphism is well-constrained while the timing and P-T conditions of earlier metamorphism are hardly known. In this study, we deal with the timing and metamorphic conditions of the earlier metamorphism.

For this purpose, we studied the northwest part of the Strandja Massif where basement rocks are widely exposed. The basement rocks consist of biotite gneiss, metagranite, biotite garnet gneiss, amphibolite, quartzo-feldspathic schist, and metaperidotite. The most suitable rock type for the determination of P-T conditions and timing of the Paleozoic metamorphism is biotite garnet gneiss.

The biotite garnet gneiss is characterized by the development of mm- to 5 cm- thick leucosomes. The biotite garnet gneiss contains mineral assemblages involving biotite, garnet, staurolite, plagioclase, quartz, muscovite, and chlorite. Idioblastic garnet porphyroblasts, up to 1 cm across, contain inclusion- trails that define a curved internal foliation. Matrix foliation displays continuity with the internal foliation in the garnet, pointing to the syntectonic nature of the porphyroblast. A weak foliation which is defined by the parallel alignment of mainly muscovite crosscuts the main foliation. Based on these textural characteristics we infer that the migmatization and main foliation were developed probably at upper amphibolite facies conditions during the Paleozoic, and the relatively late weak foliation at upper greenschist- lower amphibolite- facies conditions during Late Jurassic-Early Cretaceous.

To constrain the timing and metamorphic conditions of the earlier and later metamorphism we separated monazite and rutile from biotite garnet gneiss and titanite from the amphibolite. Electron microprobe work and U-Pb dating on monazite, rutile, and titanite are in progress.