Oligo-Miocene extrusive rocks of Razan-Avaj area (Urumieh-Dokhtar magmatic belt): evidence of OIB- like magmatism in supra-subduction zone setting

The Urmieh- Dokhtar magmatic belt (UDMB) lying parallel to the Zagros suture zone is resulted from the subduction of Neotethyan oceanic lithosphere beneath the southern margin of Eurasia. The studied volcanic rocks of the Razan- Avaj area are part of UDMB magmatism. These rocks are composed of olivine-basalts, basalts and occasional trachyandesites with alkaline nature. The interlayered stratigraphic relationships of volcanic horizons with sediments of Qom Formation implies Oligo-Miocene age. Major and trace element contents of volcanics and chemical modeling are indicative of significant role of olivine + clinopyroxene + plagioclase fractional crystallization (35-45%) in melt compositional trend. Bulk rock chemistry of theses rocks displays LREE/HREE enrichment and lack of HFSE negative anomaly similar to oceanic island basalts (OIB). It seems that after Eocene magmatic flare-up with obvious magmatic arc signatures in UDMB, during the Oligo-Miocene the origin of magmas has shifted to a deeper asthenospheric mantle. Probably, after the initial stages of the Arabian-Eurasian plate collision in the Late Eocene-Early Oligocene and the slab roll-back, an upwelling asthenospheric mantle is injected into the mantle wedge and is weakly affected by the subduction material, which is the source of Oligo-Miocene melts.