The Guleman ophiolitic chromitites (SE Turkey) and their link to a compositionally evolving mantle source during subduction initiation
Ibrahim Uysal, Samet Saka, Argyrios Kapsiotis, Recep Melih Akmaz, Hans Michael Seitz
article (Uluslararası, Hakemli) | Yazar(lar): Ibrahim Uysal, Samet Saka, Argyrios Kapsiotis, Recep Melih Akmaz, Hans Michael Seitz | Yıl: 2018 | Dergi/Kurum: ORE GEOLOGY REVIEWS | İndeks: WOS.SCI | DOI: 10.1016/j.oregeorev.2017.12.017 | Anahtar Kelimeler: Chromian spinel, Trace elements, Platinum-group elements, Ophiol
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#: 0.610.69) 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 mu m), 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.