The role of MORB-mantle source and continental crust in genesis of Tertiary volcanic rocks of Nash area in the east of Roudbar, North of Iran

The Nash''s Tertiary volcanic rocks in the Alborz Mountains in the east of Roudbar (North of Iran) show a compositional range from olivine basalt، basaltic andesite، pyroxene-andesite to andesite. From petrographical point of view، the diversity of xenoliths and xenocrysts are evidences for crustal contamination. The positive correlation of CaO/Al2O3 vs. MgO and Al2O3/CaO vs. SiO2 are signatures of differentiation of olivine and clinopyroxene in magmatic evolution. The positive correlations of K2O/P2O5 vs. SiO2 and La/Sm vs. K2O/P2O5 indicate the contamination of magma with continental crust. The incompatible elements patterns and their comparison with those of continental crust point to the contamination of volcanic rocks with lower and upper continental crust. The linear trends on Zr/Y vs. Nb/Y diagram، indicate two different sources for the studied rocks. The MORB-mantle source and the continental crust. The variations of Zr/Nb vs. Y/Nb and Rb/Y vs. Nb/Y display the crustal contamination of magmas which are derived from MORB-mantle source. Geochemical studies show that the studied rocks were originated from 15% melting of a MORB source in spinel facies in continental within-plate.