Metamorphic imprint of ridge subduction on the Neo-Tethyan ophiolites from the Saka Unit (Central Pontides, northern Turkey)

The Intra-Pontide suture zone is the northernmost ophiolite suture zone exposed in the Anatolian peninsula. It consists of several variably deformed and metamorphosed tectonic units derived from the Neo-Tethyan Intra- Pontide oceanic (IPO) basin, and its continental margins that are currently represented by the Istanbul- Zonguldak and the Sakarya terranes. Recent data suggests that the IPO was a wide supra-subduction oceanic basin whose closure began in the uppermost Early Jurassic by a north dipping intra-oceanic subduction that divided the IPO into two different oceanic areas, namely IPO1 and IPO2. The Saka Unit is a small tectonic unit exposed in the eastern portion of the Intra-Pontide suture zone in the Central Pontides, which is interpreted as a tectonic mélange produced during the intra-oceanic subduction that led to the closure of IPO1. The unit was affected by polyphase deformation and epidote-amphibolite facies metamorphism peak conditions experienced during the uppermost Middle Jurassic. Metamorphic studies and thermodynamic investigations constrain the peak pressure conditions to T = 650-700 °C and P = 0.9-1.3 GPa. These conditions reveal a 'hot' and anomalous geothermal gradient (15-22 °C/km) with respect to those of the other ophiolite-bearing units of the Intra- Pontide suture zone involved in the subduction. This anomalous gradient was herein attributed to the thermal pulse produced by the subduction of the IPO1 mid-oceanic ridge. The final re-equilibration occurred under greenschist facies conditions during the Early Cretaceous exhumation, when the oceanic lithosphere of the IPO was completely consumed and the continental collision between the Istanbul-Zonguldak and Sakarya terranes occurred.