GEOCHEMISTRY AND GEOCHRONOLOGY OF OPHIOLITIC ROCKS IN THE TOKAT MASSIF, NORTH-CENTRAL TURKEY

Located in the Sakarya Zone of the eastern Pontides, the Tokat Massif is a Permian-Triassic metamorphic, volcanic, and sedimentary group of rocks cut by strands of the active strike-slip North Anatolian Shear Zone (NASZ). The assembly of the Tokat Massif is debated and is likely due to the region’s complex tectonic history. According to one interpretation, the massif is comprised of various imbricated piles of Pre-Tethyside and Tethyside units fragmented by the NASZ. Others distinguish three specific units that make up the subduction-accretion complex: the Tokat, Yesilirmak, and Akdagmadeni Groups. The Tokat group, is a pre-Jurassic metamorphic unit made up of the Turhal metamorphics and a Silurian to Triassic heterogeneous unit called the Devecidag melange. It has also been proposed that the Tokat Group is the easternmost extension of the Karakaya Complex, an extensive Permian-Triassic metamorphic unit within the Sakarya zone. The history of the Karakaya Complex is disputed and either represents oceanic rift deposits or accretion-subduction units of the Paleo-Tethys ocean basin. Also within the Tokat Massif is a Cretaceous ophiolitic mélange that formed from the closing of the Neo-Tethys, and it is proposed that the closing of the Paleo- and Neo-Tethys are both documented by imbricated thrust faults in the region.

Igneous and meta-ophiolitic samples were collected from the Tokat Group and analyzed for their major and trace elements. The data shows a compositional range that exhibit two environments: enriched MORB and OIB. Electron microprobe analyses of clinopyroxene grains from some of these rocks are consistent with a rift-related history. Small (<20 μm) zircon grains from four samples were dated in thin section using an ion microprobe. The rocks yield an average ²³⁸U/²⁰⁶Pb age of 209 ± 21 Ma. Cathodoluminescence and high-resolution backscattered electron images of some dated zircon grains show mineral overgrowths and mottled zoning, but Th/U ratios are typical of magmatic origin. The ophiolitic rocks in the Tokat massif do not have a simple history or single origin, but instead correspond heterogeneities in the Paleo-tethys ocean basin and subsequent subduction-accretion deformation and imbrication.