Geochemical characteristics of peridotites from Guleman and Kizildag ophiolites, SE Turkey: Implications for a SSZ setting

Abstract

Ophiolites from SE Turkey has been long recognized as SSZ-type and interpreted as remnants of the Neotethys oceanic crust. In this study we provide constraints from geochemistry of peridotite in Kizildağ and Guleman ophiolites. The studied peridotites are all harzburgites with modal clinopyroxene contents from 2.6 to 4.8. Porphyroclastic textures along with granulation and kinking structures are commonly found. From depletion trends observed from mineral chemistry, the Kizildağ harzburgites are as depleted as the most depleted abyssal peridotites, while the Guleman harzburgites are even more depleted, in a degree similar to those fore-arc peridotites from the Izu-Bonin-Mariana (IBM) arc system. Trace element modelling not only suggests an approximately 14% melting of a refractory source that has already undergone around 10% melting, but also show enrichment in REEs and LILEs. This 2-stage melting scenario is consistent with the modern petrogenetic modal for boninites, hence we interpret these harzburgites as near pure melting residues after 2 stages of partial melting, with the latter possibly related to the generation of boninites in a fore-arc system. Their Re-Os isotope ratios range from 0.1168 to 0.1271 for ¹⁸⁷Os/¹⁸⁸Os and from 0.0581 to 0.4819 for ¹⁸⁷Re/¹⁸⁸Os, the latter indicating recent Re enrichment. Among them, the least unradiogenic ¹⁸⁷Os/¹⁸⁸Os ratio (0.1168) from one Kizildağ peridotite with low ¹⁸⁷Re/¹⁸⁸Os of 0.0816, and refractory PGE pattern (Pd/IrN ＜ 1) yields T_RD age of 1.62 Ga. On the other hand, the most radiogenic ¹⁸⁷Os/¹⁸⁸Os ratio (0.1271) from Guleman peridotites with even lower ¹⁸⁷Re/¹⁸⁸Os of 0.0581 and refractory PGE pattern (Pd/IrN ＜ 1), are similar with Os ratios of upper mantle. Compiling with available Os data of Guleman peridotites (Wang, K.-L., unpublished data), these peridotites yield T_RD model age clusters of ~300 Ma, 650-700, ~800 Ma and 1600 Ma that may record major tectonic events associated with the geodynamic evolution of the Neo-Tethyan and Proto-Tethyan oceans, and before the opening of Proto-Tethys.