Cooling and deformation history of the Çataldağ Metamorphic Core Complex (NW Turkey)

Abstract

The Çataldağ Metamorphic Core Complex (ÇMCC) consists of the Latest Eocene-Early Oligocene (34–32 Ma) Turfaldag gneiss-migmatite complex (TGMC) and an Early Miocene (21 Ma) Çataldağ synkinematic pluton (ÇSP) which were exhumed as a dome-shaped core complex in the footwall of the Çataldağ Detachment Fault Zone (ÇDFZ) in the Early Miocene. Microstructural features of the Çataldağ Metamorphic Core Complex (ÇMCC) and a two-feldspar geothermometry were studied to understand the cooling and deformation mechanisms. Microtectonic analyses of the TGMC and ÇSP show that the quartz, feldspar, and mica minerals underwent continuous deformation from ductile to brittle conditions during cooling of the ÇMCC. Two main deformation zones were determined according to temperature and strain intensity: a ductile deformation zone (DZ) within the central zones of the TGMC and ÇSP, and a mylonitic zone (MZ) within the peripheral zones of the TGMC and ÇSP (close to the ÇDFZ). Within the ductile zone, K-feldspar displays microcline twinning, myrmekite along the K-feldspar megacryst and flame-shaped perthite, and quartz displays chessboard extinction, grain boundary migration, and sub-grain rotation recrystallisation. These microstructures indicate that deformation temperature reached about 600 °C. Under high strain intensity along the ÇDFZ, the DZ microstructures turn to MZ microstructures. In the MZ, protomylonitic gneiss and mylonitic schists show distinct foliation and their K-feldspar and micas display C-S structures. The feldspars show bulging recrystallisation, a feldspar-fish structure, and domino-type microfractures, while the quartz displays ribbon structures, indicating ductile-to-brittle deformation with temperatures ranging from 500 °C to <250 °C. Two-feldspar geothermometry yields deformation temperatures in the DZ of 501–588 °C (an average of 544 °C for the ÇSP and an average of 517 °C for the TGMC) consistent with the temperature estimated by the microstructural analysis. The mylonitic zone has lower deformation temperature values of 430–557 °C (an average of 484 °C for ÇSP and an average of 436 °C for TGMC). Thermochronology data display TGMC was cooled slowly (<50 °C/my) throughout the Eo-Oligocene and rapidly (>500 °C/my) along the ÇDFZ in the Early Miocene (21 Ma). The shear zone-controlled ÇSP was emplaced into shallow levels of the Anatolian extending crust, deformed progressively (ductile-to-brittle) along the ÇDFZ, and cooled rapidly (>500 °C/my) in the Early Miocene (21 Ma). Together, the microstructure, two-feldspar geothermometry and thermochronology data indicate that the TGMC and ÇSP, which were emplaced into different crustal levels and in different periods, suffered continuous ductile-to-brittle deformation and were exhumed together along the ÇDFZ during the Early Miocene under the N-S extensional regime in western Anatolia.