Geochemical make-up of oceanic peridotites from NW Turkey and the multi-stage melting history of the Tethyan upper mantle

Abstract

We present the whole-rock and the mineral chemical data for upper mantle peridotites from the Harmanci{dotless}k region in NW Turkey and discuss their petrogenetic-tectonic origin. These peridotites are part of a Tethyan ophiolite belt occurring along the İzmir-Ankara-Ercincan suture zone in northern Turkey, and include depleted lherzolites and refractory harzburgites. The Al2O3 contents in orthopyroxene and clinopyroxene from the depleted lherzolite are high, and the Cr-number in the coexisting spinel is low falling within the abyssal field. However, the orthopyroxene and clinopyroxene in the harzburgites have lower Al2O3 contents for a given Cr-number of spinel, and plot within the lower end of the abyssal field. The whole-rock geochemical and the mineral chemistry data imply that the Harmanci{dotless}k peridotites formed by different degrees of partial melting (~%10-27) of the mantle. The depleted lherzolite samples have higher MREE and HREE abundances than the harzburgitic peridotites, showing convex-downward patterns. These peridotites represent up to ~16 % melting residue that formed during the initial seafloor spreading stage of the Northern Neotethys. On the other hand, the more refractory harzburgites represent residues after ~4-11 % hydrous partial melting of the previously depleted MOR mantle, which was metasomatized by slab-derived fluids during the early stages of subduction. The Harmanci{dotless}k peridotites, hence, represent the fragments of upper mantle rocks that formed during different stages of the tectonic evolution of the Tethyan oceanic lithosphere in Northern Neotethys. We infer that the multi-stage melting history of the Harmanci{dotless}k peridotites reflect the geochemically heterogeneous character of the Tethyan oceanic lithosphere currently exposed along the İzmir-Ankara-Erzincan suture zone.