The Lar igneous suite (LIS), in southeastern Iran, is part of post collisional alkaline magmatism in Sistan suture zone. Shonkinite and kersantite are the only two high-Mg, K-rich olivine bearing rocks in the LIS. We study major and some compatible trace elements in the Lar shonkinite and kersantite (LSK) olivines to define mantle mineralogy and metasomatic processes. Olivines in shonkinite have higher Fo (83-90), compared with those in kersantite (Fo76-80). Ca and Ni contents in the olivines are relatively low, whereas their Mn and Ti contents are high and variable, respectively. Low Ni contents exhibit olivine crystallization at igneous conditions from a magma originated by partial melting of an olivine-rich mantle source. Geochemical date reveal that magma evolution is responsible for high-Mn and low Fo contents in kersantitic olivines. In contrast, high Mn, Mn/Fe and Fo contents in shonkinitic olivines indicate an existence of Mn-rich and Ca-Si-poor metasomatic agents in the source. So, considering the Middle Oligocene-Miocene post-collision nature of the Lar igneous suite, melts or fluids derived from upwelling asthenosphere in the form of magnesitic-carbonatite melts, had great potential in metasomatism of subcontinental lithospheric mantle. This CO2 and K-rich liquid then reacts with peridotite to produce new mineral assemblages including low-Ca clinopyroxene, olivine and phlogopite. Partial melting of such metasomatized source region was responsible for producing the undersaturated, K-rich shonkinite and kersantite in the LIS.