The effects of partial melting, melt–mantle interaction and fractionation on ophiolite generation: Constraints from the late Cretaceous Pozantı-Karsantı ophiolite, southern Turkey

Abstract

The Pozantı-Karsantı ophiolite consists of a mantle unit and overlying crustal rocks. The mantle peridotite is composed of harzburgitic to dunitic rocks that are depleted in Al2O3 and CaO compared to primitive mantle. The low whole-rock Al and Ca values are consistent with the high Cr# [= 100 × Cr/(Cr + Al)] values of spinel that range from 44 to 70. These spinels generally have low TiO2 (< 0.06 wt.%) concentrations, although spinels with high-Cr compositions in certain samples exhibit enrichments of up to 0.16 wt.% TiO2. The concentrations of chondrite-normalized rare earth elements (REEs) indicate depletion toward heavy to middle REEs. However, the entire mantle peridotite samples exhibit marked enrichment in light REEs and large-ion lithophile elements (LILEs) compared to middle REEs. The heavy REE patterns of some peridotite samples are comparable to the calculated melting curves representing various degrees of melting and are modeled by ~ 24% to 30% melting in the spinel stability field. However, some samples are more depleted in the middle REEs than heavy REEs. The patterns of heavy to middle REEs do not follow the melting lines produced by various degrees of melting in the spinel stability field. The heavy REE composition of these peridotite samples suggests that partial melting began in the garnet stability field and continued into the spinel stability field; they represent ~ 22% to 26% melting under various pressure conditions. High anorthite content of plagioclase as well as Nb-depleted composition of mafic cumulates and isotropic gabbros indicate that the crustal rocks of the Pozantı-Karsantı ophiolite formed from a melt that has been produced by melting of depleted source. This melt is thought to have enriched in light REE and fluid mobile elements that were released from the subducting slab during the subduction of Neotethyan ocean. The whole-rock light REE and LILE enrichments and the higher TiO2 concentrations of the high-Cr# spinels in some mantle peridotite samples cannot be explained by simple melt extraction in a mid-ocean ridge (MOR) environment and require melting and enrichment processes in a supra-subduction zone tectonic setting. However, these features of the peridotites can be explained by the interaction of light REEs and TiO2-rich melts and fluids with peridotites that were depleted during melting events in an MOR environment. This interaction, took place in a supra-subduction zone environment, may have increased the light REE concentrations in the peridotites and caused the re-equilibration of the Ti-depleted spinel to produce Ti-richer spinel and therefore explains the geochemical signatures of the Pozantı-Karsantı peridotites.