Petrogenesis of the Harsin–Sahneh serpentinized peridotites along the Zagros suture zone, western Iran: new evidence for mantle metasomatism due to oceanic slab flux

The Harsin–Sahneh serpentinized peridotites are widely exposed along the Zagros suture zone in the western region of Iran and are considered to represent remnants of Neo-Tethys oceanic lithosphere at the junction of the Arabian and Iran Plates. These rocks are characterized by low contents of SiO2 (38.8–43.5 wt%), Al2O3 (0.1–3.8 wt%), CaO (0.2–8.2 wt%) and TiO2 (< 1 wt%) and high MgO contents (31.1–46.0 wt%). Their enrichments of large ion lithophile elements and light rare earth elements, with high 87Sr/86Sr(i) values (0.7036–0.7109) and relatively high variations in their εNd(t) (–7.5 to +7.8) values, indicate that the Harsin–Sahneh peridotites were metasomatized by flux released from the oceanic subducting slab in an active margin. The chemical compositions and isotopic ratios of these rocks suggest that they were formed as residue of mid-oceanic ridge basalt in the lithosphere that was then subsequently re-melted and metasomatized in a supra-subduction zone system. The occurrence of both mid-oceanic ridge and supra-subduction zone-type peridotites suggests that the heterogeneity of the upper mantle may have occurred due to the different ratios of partial melting and melt–rock reaction processes in different tectonic settings within the Neo-Tethys realm. The Harsin–Sahneh peridotites provide a good explanation of multistage melt extraction as well as melt–rock and metasomatic reactions in the mantle sequence of the Zagros ophiolite complex.