Geochemistry and geochronology of meta-igneous rocks from the Tokat Massif, north-central Turkey



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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 a heterogeneous mélange and associated metasedimentary rocks. 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 which developed into a marginal ocean basin or accretion-subduction units of the Paleo-Tethys. Structurally incorporated into the Tokat Massif is a Cretaceous ophiolitic mélange that formed from the closing of the Neo-Tethyan ocean basin, and it is proposed that the closing of the Paleo- and Neo-Tethys are both documented by imbricated thrust faults in the region. Metagabbros, metabasites, and serpentinites (n=38, 9 sample locations) were collected from the Tokat Group and metabasalts (n=8) were collected from a Cretaceous ophiolitic mélange. These rocks were studied to determine geologic source and age. All rocks from the Karakaya Complex are both oceanic rift-related and enriched from a deeper OIB plume source based on REE, trace element and clinopyroxene mineral chemistry. Metabasalts from the Cretaceous mélange have IAT affinites based on REE patterns and trace geochemical data. Karakaya gabbroic rocks were likely generated by mixing of plume-related and enriched magmas near the rifting Paleotethyan ocean crust. Metamorphism and deformation occurred during the northward subduction of the Paleotethyan ocean basin and accretion of the Karakaya units to the Laurasian continental margin. Small (<20 µm) zircon and baddeleyite grains from four Karakaya samples were dated in thin section using a Cameca 1270 ion microprobe at UCLA. The rocks yield a maximum ²³⁸U/²⁰⁶Pb crystallization age of 256±17 Ma (±1σ) and minimum metamorphic age of 173±9 Ma.