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) degrees C, respectively; olivine - whole-rock (melt) equilibrium thermometry yields crystallization temperatures of 1212-1264 (+/- 27) degrees 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, Sr-87/Sr-86) 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 <1 % partial melting of the descending pyroxenite could create the Plio-Quaternary alkali basaltic magma of the Qorveh-Bijar. The geochemical evidence for lithospheric foundering, and hence drip magmatism, in the Qorveh-Bijar volcanic belt is supported by seismographic studies indicating thinned lithosphere beneath the study area.