Relicts of high-temperature fabric in the Strandja Massif, NW Turkey

The Strandja Massif, exposed along northwestern Turkey is an NW striking polymetamorphic belt. The massif is mainly composed of a Neoproterozoic-Paleozoic metamorphic sedimentary complex intruded by plutons of various ages, all of which are covered by Mesozoic metasedimentary units. In previous studies Natal’in and co-authors have shown crosscutting relations between bedding or intrusive contacts were well documented all above-mentioned rocks reveal uniform penetrative foliation, which is observed in stratigraphic units of various ages. Isotopic dating shows that the massif has undergone at least two metamorphic events during the Mesozoic and Paleozoic. Greenschist to lower amphibolite facies metamorphism and intense deformation occurred in the Middle Jurassic to Early Cretaceous times as is evident from Ar-Ar and Rb-Sr studies. However, the formation of migmatites in some rocks assigned to the Paleozoic and the heterogeneous distribution of metamorphic rock types contrary to the more or less regular behavior of fabric require additional attention to the reconstruction of P-T conditions of deformations and their variations in time. The goal of this research is to improve our understanding of the P-T conditions during Paleozoic metamorphism and deformation. For this purpose, we studied the northwest part of the Strandja Massif which mainly consists of migmatitic biotite gneiss, metagranite, migmatitic biotite garnet gneiss, amphibolite, and quartzo-feldspathic schist.

The metamorphic conditions of the Paleozoic metamorphism are restricted by the following criteria: (1) The units that have undergone the Paleozoic metamorphism show evidence of migmatization. The temperature should exceed ~650 °C for the beginning of partial melting. (2) The occurrence of amphibolite indicates that the Paleozoic metamorphism was within the amphibolite facies conditions. (3) The founding of partially preserved kyanite restricts the pressure conditions of the Paleozoic metamorphism. By these criteria, the peak metamorphic conditions of the Paleozoic metamorphism are restricted to 650-720 °C and 6-12 kbar.

The Paleozoic metamorphism is accompanied by highly ductile deformation compatible with the metamorphic conditions. The Paleozoic deformation is characterized by mesoscale intrafolial folds, macroscale sheath folds, and migmatitic foliation. The intrafolial folds have foliation parallel axial planes and can only be recognized through their hinges since their limps are commonly detached. The presence of melt, due to migmatization, possibly has a crucial role on the rheological properties during this deformation.

The microstructural imprints of the Paleozoic metamorphism are investigated within the framework of this study. The chessboard subgrain pattern of quartz within the leucosomes of the migmatitic gneiss provides evidence that peak metamorphism occurred during one of the episodes of a prolog structural history of the Strandja Massif. The absence of this subgrain pattern out of leucosomes supports that the Paleozoic metamorphism did not advance into the granulite facies. The grain boundary migration (GBM) and subgrain rotation (SGR) recrystallizations and the growth of deformation myrmekites along the high-stress sites of feldspar also require higher temperature conditions than the compared with those that are seen in rocks that are assigned to the Mesozoic. These structures could be formed or represent relicts of Paleozoic metamorphism.