A total of nearly 100 m_thick picritic lava succession rarely seen in the world was recognized in the Middle Devonian Beitashan Formation on the southern margin of the Altay Mountains. Serpentinized euhedral-subhedral olivine phenocrystals are ominant in the picrites. Electron microproble analyses of the olivine phenocrysts reveal their homogeneous composition, with Fo contents ranging from 79.8% to 81.8% and CaO contents being extremely low (0～0.06%), much lower than their values in typical arc picrites. Based on previous experiments on Ca partitioning between olivine and silicate melt, it can be inferred that such low_Ca and low_Mg olivine crystals might have been generated by crystallization of high-Si, low_Fe and low alkali basalt.