Mid-ocean ridge and supra-subduction geochemical signatures in spinel-peridotites from the Neotethyan ophiolites in SW Turkey: Implications for upper mantle melting processes

The Lycian and Antalya ophiolite complexes in SW Turkey represent fragments of oceanic lithosphere emplaced following the closure of the Neotethys Ocean during the Late Cretaceous. The peridotites from both of these ophiolites have compositions ranging from relatively undepleted lherzolites to highly depleted harzburgites and display a diverse suite of geochemical signatures indicative of both anhydrous, mid-ocean ridge (MOR)-type and hydrous, supra-subduction zone (SSZ)-type melting regimes. Whole-rock major and trace element systematics and mineral chemistry indicate that the MOR- and SSZ-type peridotites represent the residues from 5-9% and 13-25% of mantle melting, respectively, and display evidence for a multi-stage evolution of an oceanic lithosphere. Olivine-orthopyroxene-spinel equilibria indicate that the clinopyroxene-bearing, MOR- type peridotites are moderately reduced with their oxygen fugacity (fO(2)) ranging from -2.22 to -1.44 log units relative to the FMQ (fayalite-magnetite-quartz) buffer and are similar to abyssal peridotites, while more refractory, SSZ-type harzburgites and dunites (<4% modal clinopyroxene) are more oxidized with their higher oxygen fugacity (FMQ -0.72 to +1.02). The latter can readily be explained by interaction with oxidizing melts/fluids originating from a subducted slab in a SSZ environment. Precise determination of trace element compositions of residual clinopyroxenes by LA-ICPMS indicates that the SSZ peridotites are strongly depleted in Ti and HREE and enriched in Zr and LMREE, compared to the most depleted MOR- type peridotites. The results of quantitative model calculations show that moderate degrees of anhydrous mantle melting accounts for the composition of MOR peridotites, while SSZ peridotites are likely to have originated from hydrous, orthopyroxene-dominated remelting of previously depleted mantle. For the case of hydrous melting, it is inferred that flux of a slab-derived fluid component sustained further melting of MORB-depleted peridotite, resulting in higher degrees of depletions in the SSZ mantle than the anhydrous melting in the MOR setting. Interaction of SSZ-type melts with depleted peridotites enriched the source mantle in more incompatible trace elements, resulting in elevated Zr/Ti and LMREE/HREE ratios in peridotites. The occurrence of both MOR and SSZ styles of melting regimes indicates that these ophiolites contain mantle residues from discrete stages of oceanic lithosphere generation. (C) 2009 Elsevier B. V. All rights reserved.