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.