The supra-subduction zone Pozanti–Karsanti ophiolite, southern Turkey: evidence for high-pressure crystal fractionation of ultramafic cumulates

The Pozanti–Karsanti ophiolite (PKO), forming one of the late Cretaceous discontinuous oceanic lithosphere remnants in the eastern Tauride belt in southern Turkey, is characterized by mantle tectonites, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes and volcanics. Well-preserved crustal cumulate rocks are mainly composed of dunite±chromite, wehrlite, olivine clinopyroxenite, clinopyroxenite, olivine websterite and low-Ti gabbro. The crystallization order in the plutonic section is olivine (Fo93–83)±chromian spinel [(Cr/Cr+Al)=79], clinopyroxene [(Mg/Mg+Fe)=93–87], orthopyroxene [(Mg/Mg+Fe)=90–83]. The mineral chemistry of the ultramafic cumulates forming the basal portion of the plutonic section of the Pozanti–Karsanti ophiolite (PKO) is not consistent with crystal–liquid fractionation of primitive mid-ocean ridge basalts at low pressures. The presence of highly magnesian clinopyroxene and orthopyroxene together with the absence of plagioclase, as early fractionating phases, indicate medium- to high-pressure (up to 10 kbar) crystal fractionation of primary basaltic melts. Mineralogical and geochemical data suggest that the ultramafic cumulates are distinct from rocks in mid-ocean ridge and back-arc basin ophiolites. The ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc/suprasubduction zone tectonic setting. It is believed that the arc crust was thick for the PKO during the late Cretaceous, with the existence of an active magma chamber situated at the base of the crust around pressure of 8–10 kbar for the high-pressure products and a related active shallower level magma chamber for the low-pressure products.