Geochemistry and petrology of Upper Triassic basic volcanic rocks in the Central Alborz region, south of Pole Sefid

The study area, situated 50 km north of Ghaemshahr, structurally belongs to the Central Alborz tectonic zone. The volcanic rocks of the area, including basalt, andesite, trachyandesite, and dolerite are the products of fractional crystallization and some contamination processes. Clinopyroxene, olivine, and plagioclase are the main minerals of these rocks. Porphyritic to mega-porphyritic texture with microlithic, glomeroporphyritic, and amygdaloidal matrix are observed in these rocks. Plagioclase, hornblende, and pyroxene can be considered the main mineral phases of these rocks. The rocks are enriched in LIL and LREE elements and depleted in HFS elements. The ratios of trace elements and rare earth elements in the studied region indicate that the basalts were formed by melting a garnet bearing peridotite at great depths and pressures. Negative Ce anomaly, Nb negative anomaly, Pb positive anomaly the role of continental lithosphere in contaminating the magmatic sources of volcanic rock in the study area. The Pb/Ce ratio similar to ocean island basalt (OIB) sources, induces OIB sources, the asthenospheric mantle, in the initial formation of basaltic magma. it can be concluded that the volcanic rocks of the area have formed in an intra-plate environment in an extensional regime.