Geochemical and geochronological constraints on origin of the Sawlava ophiolite (NW Iran): Evidence for oceanic mantle evolution beneath Iran-Iraq border

The Sawlava ophiolite is situated on the Iran–Iraq border within the Walash-Kermanshah ophiolitic belt and represents remnants of the Cenozoic southern Neo-Tethys Ocean, which was located between the Arabian plate and the Sanandaj–Sirjan zone. It contains a complete ophiolitic sequence started from harzburgites with foliated/discordant dunites, gabbroic bodies, a dyke complex, and pillow as well as massive basaltic lavas. The whole-rock and mineral chemistry of the Sawlava peridotites suggest that they are a mantle residuum after ca. 20–30% partial melting and melt extraction in the suprasubduction zone (SSZ) setting. The volcanic rocks generally range from island arc tholeiite to calc-alkaline affinities. Whole-rock chemistry indicates that volcanic units have compositions akin to depleted- and enriched-types mid-ocean ridge basalts (E-MORB). Layered- and isotropic gabbros show geochemical affinity similar to the normal mid-ocean ridge basalt (N-MORB) and E-MORB compositions, respectively. Their llight rare earth elements (LREE) observed in both N-MORB-like and E-MORB-like rocks, define two main basic geochemical types (LREE-enriched and LREE-depleted), suggesting several magmatic pulses from different mantle compositions that reveal mantle evolution and heterogeneity across the study area. Basaltic lavas weakly depleted in LREE show relatively flat REE patterns and (La/Yb)N values of 0.77 to 1.24, whereas LREE-enriched rocks have a (La/Yb)N mean values of 1.7, and lack an Eu anomaly. These subgroups of basaltic rocks show different extrusion ages – Paleocene–Eocene (59–50 Ma) and Oligocene (29 Ma), respectively. Petrogenetic modeling shows that mafic N- and E-MORB type rocks of Sawlava, which have (La/Yb)N = 0.97 and 1.7 and (Dy/Yb)N = 1.13 and 1.1, respectively, formed due to partial melting of a mantle source less enriched in LREE than an garnet lherzolite ((La/Yb)N = 0.88 and (Dy/Yb)N = 0.94) source or, alternatively, from very low degree (<2%) partial melting of a spinel-bearing DMM. The La/Yb–Dy/Yb systematics of these rocks are interpreted to be compatible with ~2 to 10% and ~ 20% of partial melting of a garnet bearing spinel lherzolite source for the N- and E-MORB type rocks, respectively. The geochemical data and results reveal that magmatic-originated rocks from both Paleocene-Eocene and Oligocene lavas seem to be less heterogeneous compared with the mantle sources of the Walash-Kermanshah ophiolites, likely due to different slab-fluid components and variations of partial melting degrees. The studied volcanic rocks therefore possibly record chemical heterogeneity due to magmatic activity at an oceanic spreading center alongside slab rollback and back-arc extension at the Eurasian continental margin.