Mineral chemistry and petrology of mantle peridotites from the Guleman Ophiolite (SE Anatolia, Turkey): Evidence of a forearc setting

Abstract

The Guleman ophiolite situated in SE Anatolia, Turkey is regarded as a fragment of Late Cretaceous oceanic lithosphere, consisting a core of mantle rocks overlain by an ultramafic sequence, layered and isotropic gabbros, sheeted dykes structurally overlies the Lower Miocene Lice Formation and is depositionally overlain by sandstones and shales of the Upper Maashtrichtian-Lower Eocene Hazar complex and Middle Eocene Maden Complex (Righo de Righi and Cortesini, 1964; Erdogan, 1977; Perinçek, 1979; Özkaya, 1978; Perinçek, and Çelikdemir, 1979; Bingöl, 1986; Beyarslan and Bingöl, 2014). The mantle peridotites consist mainly of fresh and in place serpentinized harzburgites with local bands and lenses of dunite and large-sized chromitite pods. The harzburgites contain 70-80 modal % of olivine and 15-25 modal % of orthopyroxene. The minor phases are clinopyroxene (2-3 modal %) and chrome-spinel (2-3 modal %). They commonly display high-temperature deformation fabrics such as kink-bands in olivines. The main texture of the harzburgites is porphyroclastic texture, and occasionally mylonitic textures can be observed. Orthopyroxene and spinel are stretched in some samples. Chrome-spinel exhibits vermicular and xenomorphic, and rarely idiomorphic habits in peridotites. They contain 2-3% clinopyroxene as exsolution lammelles in orthopyroxene. The harzburgite and dunite have low CaO and Al2O3 abundances similar to Mariana fore-arc peridotite (Pearce et al. 1992). The average Cr-ratio = (Cr/(Cr + Al) atomic ratio) of Cr-spinels in harzburgites, and dunites is remarkably high (>0.63). The Fo content of olivine is between 90.9 to 92.3 in harzburgites and dunites. In the Mg# (Mg/(Mg+Fe2+)) versus Cr# in spinel diagram used to determine the degree of partial melting and the tectonical environment, the spinel plot in the forearc peridotites field and the degree of the partial melting is > 35% (Fig.1). Orthopyroxene and clinopyroxene lammelles from the Guleman harzburgites have low CaO, Al2O3 and TiO2 contents, resembling those of depleted harzburgites from modern fore-arcs and different from moderately depleted abyssal peridotites. Fore-arc peridotites are typically more depleted than abyssal peridotites, except for some lherzolites from the South Sandwich forearc (Arai and Ishimaru, 2008; Pearce et al. 2000). In fertile abyssal lherzolites, spinels with Cr#< 30 are dominant (Dick and Bullen, 1984), while spinel in fore-arc peridotites may cover a slightly wider range, 40 < Cr# < 80 (Arai, 1994). The unusually depleted nature of forearc peridotites requires unusual melting conditions: abnormally high temperature, volatile flux, or both. Higly depleted harzburgites, dunites and chromitites in the ophiolites form by melts of the mantle wedge overlying the new subduction zone (Shervais, 2001). These melts form in response to continued melting of previously depleted asthenosphere brought about by increasing flux of fluids and melts from the subducting slab (Shervais, 2001). According to Whattam and Stern (2011), most ophiolites are fragments of exhumed forearcs and that forearcs form during subduction initiation allows us to use ophiolites to explore how subduction zones form. Consequently, we propose that the Guleman peridotites form in a forearc setting during the subduction initiation that developed as a result of northward subduction of the southern branch of the Neo-Tethys in response to the convergence between Arabian and Anatolian plates