Ophiolitic chromitites from the Kızılyüksek area of the Pozantı-Karsantı ophiolite (Adana, southern Turkey): Implication for crystallization from a fractionated boninitic melt

Abstract

Ophiolitic rocks are widely distributed in Turkey. One type, the Pozantı-Karsantı ophiolite from southern Turkey, contains a large number of chromitite deposits located mostly in the mantle peridotites and close to the Moho transition zone dunite and cumulate dunites. Cr-spinel grains from the chromitites are represented by high Cr# [100×Cr/(Cr+Al)=68–81], and their Mg# [100×Mg/(Mg+Fe2+)] range from 54 to 71. Gallium and Co contents vary between 18 and 32ppm and 185–266ppm, respectively, and they show negative correlation with Cr#. A detailed optical investigation reveals that the Cr-spinel grains contain silicate, platinum-group mineral (PGM) and base metal sulfide (BMS) inclusions. Single phase inclusions of amphibole are the only hydrous silicate phases in the investigated chromitites, and they contain low TiO2 (<0.43wt.%). Olivine, with high Fo (~96) and NiO contents (0.48–0.68wt.%), and clinopyroxene, with low TiO2 (<0.1wt.%), Al2O3 (<2.84wt.%) and Na2O contents (<0.4wt.%) were also observed as primary silicate inclusions. Chromitites contain low concentrations of total platinum-group elements (PGE) ranging between 32 and 162ppb, with an average value of 93ppb. Primitive mantle-normalized PGE diagrams show almost flat to positive slopes from Os to Rh (RhN/OsN=0.99 to 8.5) and negative slope from Rh to Pt and Pd. All samples show marked positive Ru anomalies. Consistent with the geochemical data, Ru, Os, and Ir bearing PGE sulfide (laurite-erlichmanite solid solution series [(Ru, Os)S2–(Os, Ru)S2] phases) are the most common PGM detected in the investigated chromitite samples. They show a narrow range of Os-Ru substitution [Ru#; Ru/(Ru+Os)=0.72–0.97], indicating no erlichmanite in the PGM paragenesis. In addition to the most common PGM laurite, several osmium (Os, Ir), iridium (Ir, Os), irarsite, and one single grain of speryllite (PtAs2) were detected as magmatic inclusions in Cr-spinel. Three unknown PGE/PGE–BME (base metal element) phases were also detected in Cr-spinel grains with compositions that correspond to the chemical formulas of (Os, Ru, Ir, Rh, Fe, Pd)2S5, Ir(Rh,Pt,Ni,Cu)S3, and (Ir, Rh, Ru)2(Ni, Cu)S3, respectively. The high Cr# and low Ti content of Cr-spinel grains and amphibole inclusions with low Ti content as hydrous phases in Cr-spinel grains require a hydrous melt depleted in incompatible trace elements for the formation of investigated chromitites; therefore, we suggest a fore arc tectonic environment for the generation of Kızılyüksek chromitites. The presence of Os-Ir alloys and Ru-rich laurites implies that Cr-spinel crystallization took place at relatively high temperature (1100–1300°C) and low ƒ(S2) (between −1 and −3) conditions. Major and trace element compositional variations of Cr-spinel, wide variation of RhN/OsN ratios of the chromitites and depletion of Os in the chromitites compared to Ir and Ru may imply that Kızılyüksek chromitites crystallized from a variously fractionated boninitic melt.