Structure and geochemistry of Tethyan ophiolites and their petrogenesis in subduction rollback systems

Suprasubduction zone (SSZ) ophiolites in orogenic belts represent oceanic crust generation in subduction rollback cycles during the closing stages of basins prior to terminal continental collisions. Mantle flow and slab rollback result in one or more episodes of arc splitting and basin opening, producing a collage of ‘protoarc and forearc oceanic lithosphere’ in suprasubduction zone settings. The Jurassic–Cretaceous SSZ Tethyan ophiolites in the eastern Mediterranean region (i.e. Mirdita, Pindos, Troodos, Kizildag, Oman) generally have Penrose-type oceanic crust and contain well-developed sheeted dike complexes indicative of magmatic extension beneath narrow rift zones during their seafloor spreading evolution. Igneous accretion of these SSZ Tethyan ophiolites involved upper plate extension and advanced melting of previously depleted asthenosphere in host basins, showing a progressive evolution from MORB-like to IAT (island arc tholeiite) to boninitic (extremely refractory) protoarc assemblages. However, there are some distinct differences in the geochemical evolution of these Tethyan ophiolites that appear to have resulted from variations in their subduction zone geodynamics. Whereas a major part of the Kizildag and Troodos lavas shows island arc affinity similar to their counterparts in the Pindos and Mirdita ophiolites, a significant component of the Oman lavas indicates MORB affinity and the majority of the Kizildag and Oman data plot within the mantle array between N-MORB and E-MORB on the Nb/Yb–Th/Yb discriminant diagram. Furthermore, the Troodos and Oman lavas do not show any particular Th-enrichment in their multi-element patterns, suggesting that fluid/melt input from subducted sediments was not that significant in generation of their magmas. Although all ophiolites exhibit geochemical features indicating increased subduction influence during the melt evolution of their younger extrusive sequences and dike intrusions, as evidenced by their negative ɛNd values, their overall characteristic trace-element patterns seem to have been strongly affected by the maturity of the subduction systems in which they developed.