Trace element geochemistry of primary mantle minerals in spinel-peridotites from polygenetic MOR-SSZ suites of SW Turkey: constraints from an LA-ICP-MS study and implications for mantle metasomatism

Ophiolites exposed across the western Tauride Belt in SW Turkey represent tectonically emplaced fragments of oceanic lithosphere incorporated into continental margin following the closure of the Neotethys Ocean during the Late Cretaceous. The mantle sections of the ophiolites contain peridotites with diverse suites of geochemical signatures indicative of residual origin by melt depletion in both mid-ocean ridge (MOR) and supra-subduction zone (SSZ) settings. This study uses a laser-ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS) for in situ measurements of trace elements in primary mantle phases in order to identify the upper mantle petrogenetic processes effective during variable stage of melt extraction in these discrete tectonic settings and to discriminate between the effects of reaction with chemically distinct mantle melts migrating through the solid residues. Trace element signatures in pyroxenes suggest small-length scales of compositional variations which may be interpreted to be a result of post-melting petrogenetic processes. Relative distribution of rare earth elements and Li between coexisting orthopyroxene-clinopyroxene pairs in the peridotites suggests compositional disequilibrium in sub-solidus conditions, which possibly reflects differential effects of diffusive exchange during melting and melt transport or interaction with subduction melts/fluids. On the basis of Ga abundances and Ga-Ti-Fe+3# [Fe+3/(Fe+3+Cr+Al)] relationships of chrome-spinels it is documented that the peridotites have experienced the combined effects of partial melting and variable extent of melt-solid interaction. The MOR peridotites have spinels with geochemical signatures indicative of melt-depleted residual origin with subsequent incompatible element enrichment through melt impregnation, while the Ga-Ti-Fe+3# relationships of chrome-spinels in SSZ peridotites indicate that these highly depleted peridotites are not simple melt residues, but have been subject to significant compositional modification by interaction with subduction related melts/fluids. The observed compositional variations, which are related to long-term tectonic reorganisation of oceanic lithosphere, provide evidence for a time integrated evolution from a mid-ocean ridge to a supra-subduction zone setting and may be a possible analogue to explain the coexistence of geochemically diverse MORSSZ suites in other Tethyan ophiolites. Copyright (C) 2011 John Wiley & Sons, Ltd.