Geochemistry and petrogenesis of Ghezelhesar Quaternary alkali olivine basalt flows (NE Hamedan, Iran)

The Ghezelhesar alkali olivine basalt flows as dark horizontal flows with about 20 m thickness covered the Quaternary deposits, 2 km southwest of Ghezelhesar village, 60 km northeast of Hamedan and in the Sanandaj-Sirjan geological and tectonic zone. The phenocrysts of these rocks are olivine and clinopyroxene often as euhedral crystals and common porphyritic, vitrophyritic and glomeroporphyric textures. The matrix of the rocks studied formed from volcanic glass, leucite, nepheline and microliths of pyroxene. The Ghezelhesar basalts, with continental within plate alkaline nature, are silica-under saturated and characterized by normative minerals of Diopside, Hematite, Anorthite, Albite, Orthoclase, ±Hypersthene, ±Olivine, ±Nepheline, and ±Quartz. REE normalized patterns show enrichment in LREEs rather than HREEs and fractionation between LREE and HREE is relatively high (LaN/YbN = 37.05–43.43). A negative anomaly in High Field Strength Elements (Nb, Ta, Zr, Ti, P, and Rb) and positive anomaly in Large Ion Lithophile Elements (i.e. Ba, Pb) and elements such as Th, La, Ce, Nd, and Sm are remarkable geochemical features of the rocks under study. These criteria along with parameters of their trace elements such as the ∆Nb indicate that the parent magma of the rocks under investigation derived from the low-grade degree partial melting of an enriched garnet-mantle, and the fractional crystallization has affected their chemical properties.