Subduction signature of the Vardar ophiolite of North Macedonia: new constraints from geochemical and stable isotope data

Abstract

Volatiles such as carbon (C) and sulphur (S) are commonly transferred into the mantle from subduction of oceanic lithosphere and overlying sediments. C and S isotopic signatures of magmatic rocks could be used as proxies of the slab components involved in the petrogenesis of subduction-related ophiolites. Therefore, in this work we investigated the major and trace element composition, as well as the C and S elemental contents and isotopic ratios (¹³C/¹²C and
³⁴S/³²S) of subvolcanic and volcanic rocks of the Vardar ophiolites of North Macedonia, which represent the remnants of the Mesozoic Tethyan oceanic lithosphere formed in supra-subduction zone tectonic settings. The ophiolites were sampled 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, three groups of rocks can be distinguished: i) Group 1 rocks, which are subalkaline basalts having backarc affinity, ii) Group 2a and iii) Group 2b rocks, which are calc-alkaline basalts having arc affinity, with and without adakitic signatures, respectively. The qualitative petrogenetic models indicate that studied rocks formed by partial melting of mantle sources variably metasomatized by subduction-related components,
such as aqueous fluids, sediment melts, and adakitic melts. Accordingly, all the North Macedonia ophiolites are characterized by C and S signatures which deviate from those typical for mantle and Mid Ocean Ridge melts. The variably low δ13C values recorded by Group 1 and 2 rocks could be related to the different contributions of melts released by subducting sediments rich in organic matter. However, we cannot exclude that such C-enriched signature is the result of
isotopic fractionation during degassing process. In contrast, the enriched S isotopic signatures of the North Macedonia ophiolites suggest a major involvement of melts derived from the subducting sediments rich in sulphate phases. In particular, the calc-alkaline basalts of Group 2 rocks record more positive δ³⁴S values than the subalkaline basalts of Group 1 formed in backarc basin suggesting that the subarc mantle sources were more affected by slab-released fluids than those of the backarc basin, which were more distal from the trench.