Geochemical evidence used to test alternative plate tectonic models for pre-Upper Jurassic (Palaeotethyan) units in the Central Pontides, N Turkey

Abstract

Ease-west trending, inferred Palaeotethyan subduction-accretion complexes form much of the pre-Upper Jurassic basement of the Central Pontides. A number of specific tectonic units are present that include variably metamorphosed basic igneous rocks, as dismembered ophiolites. coherent stratigraphical successions, ordered thrust sheets, and blocks in melange. About 200 samples from a c. 200 km long N-S traverse were analysed by a high-precision X-ray fluorescence technique, supplemented by electron microprobe analysis. The basic igneous rocks have not experienced significant chemical mobility after formation and are thus useful for discrimination of tectonic settings of formation using well known binary and ternary plots and mid-ocean ridge basalt (MORB)-normalized 'spidergrams'. From N to S the main results are that basalts of the Kure Complex were erupted in a subduction-influenced tectonic setting, favouring interpretation as a dismembered ophiolite formed in a marginal basin along the S Eurasian margin. Thick, mainly volcanic, successions of the Cangaldag Complex show a subduction-influenced calc-alkaline composition, above a basement varying from MORB/island are tholeiite type to high-Mg (boninitic) type, with an inferred origin as an oceanic are above forearc basement. Meta-basites from the adjacent Bayam Melange indicate a supra-subduction zone origin, possibly as a dismembered forearc ophiolite (possibly related to the Cangaldag). A large overriding thrust sheet of mainly ultramafic rocks, the Elekdag, is interpreted as a supra-subduction zone ophiolite, based on depleted chrome spinel composition (analysed by microprobe). Eclogitic blocks in structurally underlying melange are of mid-ocean ridge (MOR) type. Further south, a northward-dipping, inferred subduction-accretion complex, the Domuzdag-Saraycikdag Complex, includes MOR-type meta-basites in the north, and more 'enriched' compositions in the south, of within-plate origin. Lastly, the geochemistry of meta-basites in the south, within the Kargi Complex. suggests a plume-MOR to within-plate type eruptive setting, without continental crustal influence.