Petrological characteristics and geochemical compositions of the Neotethyan Mersin ophiolite (southern Turkey): Processes of melt depletion, refertilization, chromitite formation and oceanic crust generation

Abstract

The Mersin ophiolite of the Central Taurides in southern Turkey represents an incomplete section of ocean lithosphere consisting from bottom to top of chromitite-bearing mantle peridotites, cumulate dunites, clinopyroxenites, and gabbros with rare pillow lavas covered by bathypelagic sedimentary rocks. The tectonized peridotites are harzburgites with minor dunites. They are strongly melt-depleted peridotites, comprising chromian spinel (Cr-spinel) with medium to high Cr-numbers (Cr#) [100 × Cr/(Cr + Al) = 48–82] and low Al2O3 concentrations (0.05–2.2 wt%). They display concave chondrite-normalized rare earth element (REE) patterns, which we interpret as evidence for their refertilization by boninitic melts in a mantle wedge. Geochemical evidence suggests that the parental magmas of the high-Cr podiform chromitites (Cr#Sp = 71–83) were of boninitic composition. The chromitites host a platinum-group mineral (PGM) assemblage of laurite [Ru# = 64.0–90.7] and Os-Ir alloys, indicating crystallization within a range of low fS2 conditions (logfS2 ∼ −2 to −1.3) and a Tmax of ∼1200 °C.Mineral compositions and geochemical data of the basalts show tholeiitic to alkali compositions. The tholeiites show mildly light REE depleted patterns and high Th/Yb and La/Nb ratios indicate the presence of a sediment component, implying a subduction-related origin. Petrogenetic modeling indicates that the tholeiitic basalts resulted from 20% melting of a spinel-bearing asthenospheric mantle. The initial εNd values of 7.2 and 4.9 for two tholeiite samples indicate depleted asthenosphere and more enriched mantle sources probably due to the input of older sediment. A comparison with the data of other ophiolite complexes from southern Turkey suggests subduction-overprinted mantle sources as seen in immature island arcs. The trace element patterns of the alkali basalts are similar to OIB implying an origin unrelated to that of the ophiolite assemblage. The composition of the OIB can be modeled by mixing of magmas from low-degree (1–2%) melting of garnet-bearing lherzolite (GLS) with melt fractions from a spinel-bearing lherzolite (SLS) at variable proportions (GLS/SLS = 7/10–1/10).We propose the following tectono-magmatic model for the petrogenetic evolution of the Mersin ophiolite: i) oceanic ridge spreading generated MORB type magmas (not observed in our sample collection), (ii) melting of deep mantle sources beneath MORB type crust generated OIB-like basalts in Neo-Tethyan oceanic basin, (iii) subduction initiation and formation of an infant forearc basin formed island arc tholeiites and boninites.