SUBDUCTION INITIATION AND METAMORPHIC SOLE FORMATION IN TETHYAN OPHIOLITES, TURKEY: EVIDENCE FOR TIMING OF PLATE BOUNDARY METAMORPHISM AND MAGMATISM

One of the crucial issues in Tethyan ophiolites is to better understand spatial and temporal relationships between ophiolites and their underlying metamorphic soles. The ophiolites in Taurides are the best candidates as we observe the primary intra-oceanic decoupling surface along which volcanics and associated sediments from the upper levels of the down-going plate were metamorphosed up to garnet amphibolite facies and accreted to the base of the hanging-wall plate, and in turn intruded by post-metamorphic mafic dykes. The metamorphic soles are characterized by high-temperature ductily deformed isoclinal folds and internal imbricate thrusts, suggesting that they do not display an original structural geometry due to distruption during emplacement process. Geochemistry of the amphibolites show that the protoliths of the metamorphic soles are more akin to alkaline within plate basalts, different from the ophiolite lavas, suggesting that subduction did not initiated along a ridge axis. Geochemistry of the ophiolitic crustal rocks (dykes and gabbros) clearly indicates their subduction-related origin. Zircons, rutiles and titanites extracted from the amphibolites in the metamorphic sole as well as the crustal sequences (gabbro, dyke and plagiogranite) yielded identical U-Pb ages (92-93 Ma) in 1σ analytical uncertainty, confirming that metamorphism of the sole rocks was occurring simultaneously with the formation of the ophiolites crustal sequence. Identical U-Pb ages of different mineral phases with different closure temperatures from the ophiolites and metamorphic sole rocks suggest that both the oceanic crust and metamorphic sole cooled very rapidly and should be interpreted as the crystallization age. The metamorphic sole amphibolites yielded constant and well-constrained ⁴⁰Ar/³⁹Ar plateau ages, suggesting that they did not experience any re-heating event over 550^oC after their initial formation. Genesis of SSZ-type oceanic crust, metamorphic sole and post-metamorphic dyke emplacement could be explained by subduction initiation and roll-back processes during the Late Cretaceous based on petrological, geochronolgical and structural data obtained from the Tethyan ophiolites. This work was funded by TÜBİTAK Project 113Y412 and NNSFC Project 91755213.