We have conducted measurements of a new whole rock major and trace elements for Hirna mafic lavas (MgO > 4.5 wt %) in southeastern Ethiopia plateau to assess the petrogenesis of the lavas in the area. The mafic rock of the area exhibit two types of lava series regarding major and trace element compositions: 1) Silica saturated basalt and 2) silica under-saturated melanephelinite lavas. The silica saturated basaltic lavas show tholeiitic character with SiO2 and Na2O + K2O ranges between 45.5 and 49.04 wt% and 3.3–4.07 wt% respectively. Melanephelinite lavas exhibits alkaline nature and characterized by SiO2 (34.89–35.02 wt%), Na2O + K2O (4.14–4.18 wt%) and high Fe2O3 (17.34–17.54 wt%), this lava is enriched in incompatible trace elements (e.g Sr = 806–1198.5 ppm; Zr = 323–423 ppm, Nb = 111–142.5 ppm and light rare earth elements (LREE)). The basalt lavas have relatively restricted highly incompatible trace element composition (e.g. Sr = 594–721 ppm; Zr = 145–172 ppm; Nb = 17–19.5 ppm and LREEs). Major and compatible trace element variation suggests that primary magmas for both lavas have undergone fractional crystallization involving olivine, pyroxene, and iron/titanium oxide ± plagioclase. The basalt samples in the area have moderate fractionated light rare earth element to heavy rare earth element ratio (LREE/HREE) (e.g. La/Yb = 9.6–10.56; La/Lu = 65–72.7) and middle rare earth element to heavy rare earth element ratio (MREE/HREE) (Sm/Yb = 3.47–3.79) whereas melanephelinite lavas show strong fractionated LREE/HREE (e.g. La/Lu = 322.5–3236) and MREE/HREE (e.g. Sm/Yb = 6.3–6.5). The trace element data of both lavas suggests their derivation from amphibole bearing garnet peredotite lithospheric mantle. Variation of the two lavas in the level of high incompatible trace element most likely suggests their derivation from a mantle source with a different source region enrichment history. Melanephelinite lavas exhibit very high enrichment of high field strength element (HFSE) (Zr, Nb, Hf, Ta and LREEs) and have elevated Zr/Hf ratio (44–45) (and overall high incompatible trace element enrichment) suggests that, this lava series might have originated from a more metasomatized mantle region.