Carbon and sulphur isotopic composition of Vardar ophiolite of North Macedonia: implications for volatiles cycling in subduction zones.

The North Macedonia is part of the Dinaric-Hellenic belt, an Alpine collisional belt extending from Slovenia to Greece. It resulted from the Mesozoic-Cenozoic convergence between Eurasia and Adria, ophiolite obduction, and continental collision after the closure of the Tethys Ocean. In this work we investigated the subvolcanic and volcanic rocks of the Vardar ophiolites of the North Macedonia, which represent the remnants of the Mesozoic Tethyan oceanic lithosphere formed in supra-subduction zone tectonic setting. Samples were collected at Lipkovo and Demir Kapija localities, in the northern and southern part of North Macedonia, respectively. Based on whole-rock major and trace element composition, two main groups of rocks can be distinguished: i) Group 1 rocks, which are subalkaline basalts showing backarc affinity and ii) Group 2 rocks, which are calc-alkaline basalts showing arc affinity. Petrogenetic modelling, based on trace and REE, indicates that Group 1 mantle sources were affected by limited metasomatic processes by slab-released components, in particular aqueous fluids and sediment melts, whereas the Group 2 mantle sources were strongly metasomatized by sediment melts and/or adakitic melts. In addition to this, the isotopic ratios of volatiles such as carbon (C) and sulphur (S) were also investigated to better constrain the nature and composition of the slab-components responsible for the metasomatism. In fact, volatiles are commonly transferred into the mantle from subduction of oceanic lithosphere and overlying sediments, whose C and S isotopic composition is well distinguishable from that of the mantle. The Group 1 rocks exhibit C-enriched and S-depleted signatures slightly different from those typical for the mantle, indicating a minor involvement of melts from the subducting sediments in the backarc basin settings. On the contrary, the C-depleted and S-enriched isotopic signatures of the Group 2 rocks suggest a major involvement of melts derived from the subducting sediments rich in organic matter and sulphate phases. Therefore, both geochemical and isotopic data of the North Macedonia ophiolites indicate that the sub-arc mantle sources are more affected by slab-released fluids than those of the backarc basin, which are more distal from the trench. This approach may be useful to better constrain the composition of the metasomatic agents, as well as to understand the origin and the fate of volatiles near subduction zones.