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COMPOSITIONAL VARIATIONS AS A RESULT OF PARTIAL MELTING AND MELT–PERIDOTITE INTERACTION IN AN UPPER MANTLE SECTION FROM THE ORTACA AREA, SOUTHWESTERN TURKEY

brahm Uysal^1,, Melanie Kaliwoda², Orhan Karsli³, Mahmud Tarkian⁴, M. Burhan Sadiklar⁵ and Chris J. Ottley⁶

¹ Department of Geological Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey
² Mineralogical State Collection Munich, LMU, D–80333, München, Germany
³ Department of Geological Engineering, Karadeniz Technical University, 29000, Gümühane, Turkey
⁴ Institute of Mineralogy and Petrology, University of Hamburg, D–20146 Hamburg, Germany
⁵ Department of Geological Engineering, Karadeniz Technical University, 61080, Trabzon, Turkey
⁶ Northern Centre for Isotopic and Elemental Tracing, Department of Earth Sciences, University of Durham, Durham DH1 3LE, U.K.

E-mail address: iuysal{at}ktu.edu.tr, miea003{at}mineralogie.uni-hamburg.de

The bulk composition (major and trace elements) and the composition of minerals in peridotites of the Ortaca area, southwestern Turkey, have been investigated to understand the processes controlling lithological and compositional variations in the dismembered mantle section, containing clinopyroxene-bearing harzburgite, harzburgite and dunite. The Ortaca peridotites vary from almost fertile clinopyroxene-bearing harzburgite (1.92–2.42 wt% Al₂O₃, 2.00–2.64% CaO) with a lower Cr# of the spinel phase (<24) to depleted harzburgite and dunite (0.17–1.60% Al₂O₃, 0.32–1.71% CaO) with a higher Cr# of the spinel phase (31–76). Systematic variations of major and trace elements of whole rocks and mineral phases are related to varying degrees of partial melting (up to 30%). The depleted harzburgite and dunite display LREE-enriched primitive-mantle-normalized patterns, suggesting mantle metasomatism, probably by fluid-bearing melt derived from the subducted slab, due to the melt–rock interaction process, whereas clinopyroxene-bearing harzburgite show HREE-enriched patterns. The Ortaca peridotites have low temperatures of equilibration relative to abyssal peridotites, because of H₂O-assisted diffusional equilibration in the suprasubduction zone. They are characterized by oxygen fugacities log FMQ between –1.61 and +0.74. Clearly, the clinopyroxene-bearing harzburgites define an increasing oxygen fugacity with almost constant Cr#, whereas the refractory harzburgite and dunite show a strong positive correlation between oxygen fugacity and Cr#. In view of the different measurements on minerals and whole rocks, and as a result of calculated temperatures and oxygen fugacities, we contend that the peridotites were subsequently affected by percolating hydrous boninitic melt, from which the high-Cr, low-Ti chromitites were formed, surrounded by a dunite envelope. It is likely that such an envelope of dunite resulted from interaction between melt and harzburgite, which caused the local removal of orthopyroxene within the mantle wedge above the suprasubduction zone.

Keywords: mantle peridotites, mineral compositions, depleted harzburgite, suprasubduction zone, boninitic magma, melt-rock interaction, southwestern Turkey.