Petrographic and geochemical characteristics of olivine basalts from east of Nehbandan and Chahchocho (Sistan suture zone, eastern Iran)

Abstract

Olivine basalts from east of Nehbandan and Chahchocho regions belong to sodic alkaline and transitional magma series. Compared to alkaline olivine basalts (AOB), the transitional olivine basalts (TOB) have higher MgO, SiO2, Ni, Cr, Ba, Th, Pb, and U, and lower TiO2, FeOt, REE, and HFSE. High MgO, Mg#, Ni, and Cr (13.8 wt%, 77, 531 and 860 ppm, respectively) of the TOBs indicate that their chemical composition is close to a primary magma in equilibrium with mantle peridotites, whereas, the same values of the AOBs (8.3 wt%, 59, 155 and 176 ppm, respectively) are not quite close to a presumed primary magma. Based on petrographic and geochemical criteria, the TOBs have undergone both crystal fractionation and crustal assimilation, but the AOBs just show evidence of fractional crystallization. The enrichment of all rocks in the LREEs compared to the HREEs, LILEs relative to the HFSEs, together with the REE and multi-element patterns close to those of OIB may be indicative of an enriched asthenospheric-lithospheric mantle source. Non-modal batch melting models show that the AOBs are generated by ~7% partial melting of an asthenospheric mantle source at garnet lherzolite stability field. Furthermore, the TOBs are formed either by 7-15% partial melting of a metasomatized lithospheric mantle, or they are products of partial melting of an asthenospheric-lithospheric mantle source at 50% garnet-50% spinel lherzolite stability field.

Keywords

• olivine basalt
• alkaline magmatism
• transitional magmatism
• enriched mantle
• nehbandan
• iran