MID-OCEAN RIDGE AND SUPRA-SUBDUCTION ZONE PETROLOGICAL CHARACTERISTICS FROM THE NEHBANDAN OPHIOLITIC COMPLEX, EASTERN IRAN

The Nehbandan ophiolitic complex (NOC) which marks a portion of Neo-Tethyan suture in Middle- East has been located to the east of Iran between Lut and Afghan continental blocks. Chemical characteristics of olivine and pyroxene in cumulate pyroxenite are comparable with those of ophiolitic counterparts formed in SSZ geodynamic setting. Moreover, mineral chemistry of mafic cumulates suggest two distinct tectonic setting for them (MOR and SSZ settings). HFSE and LREE enrichments are remarkable geochemical features of mantle tectonites from the NOC which have been interpreted as a result of various processes relating to ophiolite evolution in SSZ setting and melt-rock interaction. Based on theoretical partial melting modeling, the mantle peridotites due to LREE enrichments are not consistent with standard partial melting models.REE modeling shows that normal MORBs originated from non-modal batch partial melting of a depleted MORB-type mantle (DMM) source and the cpx-harzburgites correlate with the complementary mantle residua. The significant LREE/MREE enrichment observed in cpx- harzburgites suggests these rocks represent residual MORB mantle enriched in LREE by fluids released from the subducting slab in a SSZ setting. REE modeling shows that depleted harzburgites have REE compositions similar to those of residual SSZ mantle rocks. We propose a new model for the tectonomagmatic evolution of the Sistan Ocean. The Sistan Ocean existed at least from the Early Cretaceous between the Lut and Afghan continental blocks and the closure of this oceanic basin most likely occurred in post-Albian times and was accompanied by the development of an intra-oceanic arc associated with an east-verging subduction. By the Maastrichtian– Paleocene times, the intra-oceanic arc activity had completely ceased, and both MORB and SSZ-type lithospheric sections were incorporated into the NOC accretionary prism.