Basalt is produced by partial melting of the mantle, and the early-crystallized olivine within it can reveal the composition and lithology of its mantle source. Previous studies have suggested that recycled materials, such as sediment and oceanic crust, are present in the source of potassic volcanics from Northeast China. Therefore, the source lithology of potassic volcanics can be identified through the compositions of the olivine phenocrysts that crystallize from them, which can also provide important insights into the deep cycling of those surface materials. Here, systematic sampling was conducted in the Wudalianchi region, a typical area of potassic volcanics, and olivine phenocrysts from 12 volcanoes (e.g., Xiaogushan, Molabushan, and so on) were selected for analysis. We found that these olivines show significantly compositional heterogeneity. Some olivines exhibit low Mn/Fe ratios and moderate Ni/(Mg/Fe) ratios, consistent with previous studies, which resemble those olivines crystallized from eclogite-derived melts. Other olivines show high Ni contents and high Fe/Mn ratios, consistent with those olivines crystallized from melts derived from reactive pyroxenites. This finding again confirms the diverse lithology of the mantle source of potassic volcanics from Northeast China. Further modeling suggests that the presence of eclogite (high-silica) is the key to explaining the high FeO/MnO ratios observed in the Wudalianchi potassic volcanics. During their ascent, the melts formed by partial melting of this recycled component undergo melt-peridotite interaction, and the composition of the finally crystallized olivines can account for the variation in Fe/Mn ratios observed in the olivine phenocrysts of the Wudalianchi volcanics.