THE AYLI DAĞ OPHIOLITE SEQUENCE (CENTRAL-NORTHERN TURKEY): A FRAGMENT OF MIDDLE JURASSIC OCEANIC LITHOSPHERE WITHIN THE INTRA-PONTIDE SUTURE ZONE

M.Cemal Göncüoglu, Michele Marroni, Kaan Sayit, U.Kagan Tekin, Giuseppe Ottria, Luca Pandolfi, Alessandro Elero

Abstract


The Ayli Dağ ophiolites occur as an independent tectonic unit within the Intra-Pontide suture zone, central-northern Turkey. They crop out, together withthe other units of the Intra-Pontide suture zone, at the top of the Late Cretaceous-Middle Paleocene foredeep sediments of the Sakarya Terrane. The Ayli Dağophiolites sequence include in its lower part a mantle sequence consisting of not less than 2-3 km-thick peridotites, topped by 500-600 m-thick layered gabbroswith alternating, dm- to m-thick layers of spinel-bearing dunites, melatroctolites, troctolites, ol-gabbros and leucogabbros. The gabbro sequence is overlainby a sheeted dyke complex, that shows a transition to 100-200 m-thick massive basaltic lava flows followed by 600-800 m-thick massive and pillow lavasand breccias alternating with ophiolite-bearing arenites and cherts.Geochemical evaluation of the mafic lavas and dykes reveals three distinct chemical groups that reflect melt generation within an intra-oceanic subductionsystem. Among these, the first group shows island-arc tholeiite (IAT)-like features, showing very-depleted characteristics (very high Zr/Nb, low Zr/Y and Nb/Y)ratios coupled with light rare-earth (LREE)-depleted chondrite-normalized patterns. The second group is more akin to back-arc basin basalts (BABB); it displaysnormal mid-ocean ridge basalt (N-MORB)-like high-field strength element (HFSE) distribution except for depletion in Nb, and flat REE patterns. The third groupis somewhat similar to the second one, displaying BABB-like characteristics, but it is more enriched in terms of absolute trace element abundances.The radiolarian cherts sampled from the top of the pillow lavas yielded less-diverse but characteristic radiolarian assemblages indicating the middleBathonian to early Callovian ages.The Ayli Dağ Ophiolite is the first finding of back-arc type oceanic lithosphere in the Intra-Pontide suture zone. Together with previous data obtainedfrom basalts in the mélange from the Intra-Pontide suture zone, this finding represents a proof that an intra-oceanic subduction within the Intra-Pontide oceanicbasin occurred in the Middle Jurassic time.

Keywords


Ophiolites; geochemistry; Radiolaria; back-arc basin; Intra-Pontide suture zone; Turkey

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DOI: https://doi.org/10.4454/ofioliti.v37i2.407