Petrogenesis and geochemical characteristics of Plio-Quaternary alkali basalts from the Qorveh–Bijar volcanic belt, Kurdistan Province, NW Iran

The Pliocene–Quaternary volcanic rocks which outcrop between Qorveh and Bijar are part of post-collisional within-plate volcanic activity in northern Iran. These mafic alkaline rocks form part of the northern arm of the Sanandaj–Sirjan (Hamedan–Tabriz) zone. Thermobarometry on equilibrium clinopyroxene – whole-rock pairs yields pressures and temperatures of 4–6 (±1.8) kbar and 1182–1213 (±27) °C, respectively; olivine – whole-rock (melt) equilibrium thermometry yields crystallization temperatures of 1212–1264 (±27) °C. Field relationships, including the presence of pyroxenitic xenoliths, and geochemical evidence (e.g. high FeO/MnO, and low CaO compared to lavas derived from peridotite sources) suggest a pyroxenitic mantle source for the studied rocks. Variation of trace elements and isotopic ratios (i.e. Ce/Pb, Ba/La, 87 Sr/ 86 Sr) indicate that this pyroxenite mantle source was generated by interaction between melted sediments of the subducted Neo-Tethys slab with ambient peridotitic lithospheric mantle. The resulting metasomatized lithosphere is denser and has a lower viscosity than the peridotitic mantle, and tectonic disturbance can cause it to fall into the depths of the mantle. The descending volatile-rich material starts to melt with increasing temperature. Modelling of rare earth element (REE) abundances suggests that