Neogene ultrabasic volcanic rocks in central Urumieh-Dohktar Magmatic Arc (NW Iran): melilitites and nephelinites in subduction setting

About 180 km SW Tehran (central-northern Iran) young (likely <1 Myr old) and small volume lava flows are emitted from volcanic chains close to Nowbaran town. These volcanoes belong to the so-called Urumieh-Dokthar Magmatic Arc (UDMA), running from NW to SE along the western margin of Iran. The volcanism of the UDMA is related to the NE-ward Neotethys Ocean subduction beneath the Iranian plate since Early Cretaceous time, evolved into Arabia-Iran continental collision during early Cenozoic. The lava flows consist of strongly ultrabasic magmas, represented by melilitites and nephelinites. Clinopyroxene is the most abundant microphenocryst, followed by olivine, calcite and biotite. Diopside is always twinned or shows normal zoning, and olivine is Mg-rich, with Fo content ~86. Primary calcite largely occurs as large plagues (often including opaque minerals and diopside); Mg-rich (Mg# ~0.87) and Tirich (TiO2 up to ~7.3 wt%) phlogopite-eastonite is also frequent. Cancrinite-sodalite group minerals are the main groundmass foids, together with nepheline and minor Ti-magnetite. The investigated rocks show anomalous mineral paragenesis and whole-rock chemical compositions compared to the other Cenozoic volcanic rocks occurring in Iran. SiO2 content is extremely low (down to 36.0 wt%), with CaO reaching contents as high as ~19.2 wt%, coupled with generally high MgO (~9.1-13.9 wt%). Alkalis range between ~2.2 and 6.2 wt%, with Na2O/K2O varying from ~0.9 to 8.5. Primitive mantle-normalized patterns show marked troughs at K, Rb and Pb and enrichment in Nb and Ta, resembling typical HIMU-OIB compositions. The presence of these compositions imply a carbonatitic component in the mantle source, which is uncommon in subduction-related settings, and certainly unique within the entire Cenozoic volcanic rocks of UDMA. Experimental petrology studies in the last decades have demonstrated that carbonated peridotite is able to produce melts characterized by extremely low content of silica but high amount of CaO and MgO (e.g. Presnall and Gudfinnsson, 2005; Hammouda and Keshav, 2015). It is therefore necessary to understand these peculiar rock compositions in order to constrain their mantle source, the role of carbonates and their origin in a subduction-related setting.