The Guleman ophiolitic chromitites (SE Turkey) and their link to a compositionally evolving mantle source during subduction initiation

• DOI: 10.1016/j.oregeorev.2017.12.017
• Publication Date: 2017-12-19T13:19:29Z
• Authors: Ibrahim Uysal; Argyrios Kapsiotis; R. Melih Akmaz; Samet Saka; Hans Michael Seitz
• Keywords: Chromian spinel; Trace elements; Ophiolite; Platinum-group elements; 01 natural sciences; Guleman; 0105 earth and related environmental sciences

Abstract Economically viable chrome ore bodies are hosted within a unit of pervasively serpentinized harzburgite in the Guleman mine of the homonymous ophiolite in SE Turkey. Chrome ores consist of m- to tens of m-scale tectonically imprinted chromitite bodies, showing semi-massive to massive texture and podiform to lenticular morphology. They are hosted in dunite envelopes of variable thickness and are composed of chromian spinel [Cr-spinel] with high Cr# [Cr/(Cr + Al) = 0.61–0.81] and Mg# [(Mg/Mg + Fe2+) = 0.56–0.72], and low TiO2 (0.06–0.33 wt%) content. Compositionally, they are divided into intermediate-Cr (CrSp#: 0.61–0.69) and high-Cr chromitites (Cr#Sp: 0.71–0.81). Chromian spinel from the intermediate-Cr chromitites is richer in Ni, Ti, Zn and Ga compared to that from the high-Cr chromitites. Both chromitite types show a general enrichment in IPGE (Os, Ir and Ru) over PPGE (Rh, Pt and Pd). The PGM assemblages of these chromitites are dominated by small (≤6 μm), idiomorphic inclusions of Ru-Os bisulfides and subordinate Os-Ir-Ru alloy in Cr-spinel. Bisulfides within the intermediate-Cr chromitites show a wider range of Os-for-Ru substitution than those hosted in the Cr-rich chromitites [Ru/(Ru + Os): 0.15–0.86 vs. 0.71–0.86, respectively]. Geochemical calculations and PGE-mineralogical data demonstrate that the parental melts of the investigated chromitites had variable Al2O3 (10.62–14.58 wt%) and S contents. This implies a range of magma compositions generated at different degrees of partial melting of a common depleted mantle source. The calculated range of compositions is not consistent with a typical mid-ocean ridge or mature suprasubduction zone setting, but is thought to be representative of the evolving composition of a deep-seated, heterogeneous mantle source during subduction initiation beneath a forearc basin.