Ultramafic rocks occur mainly as inclusions or xenoliths in Yishui magmatic and metamorphic complexes, without typical spinifex texture. Their chemical compositions are characterized by high MgO (22.23%～27.49%,>20%) and low SiO₂ (38.60%～49.40%), TiO₂ (0.11%～0.39%,<0.5%) and K₂O (0.02%～0.39%,<0.5%) content. As for other oxide compositions, total iron (FeO^T) values are in the range of 7.68%～11.14%, CaO content ranges 5.15%～12.86%, and Al₂O₃ content varies remarkably in the range of 1.56%～8.94%. The CaO/Al₂O₃ ratios of most samples are 0.61～1.36(≈1), with the exception of three samples, whose CaO/Al₂O₃ ratios are 7.70～8.20 (>1) and MgO/FeO^T ratios are 2.00～3.33. MnO(0.13%～0.20%) and Na₂O(0.12%～1.03%) content is low. The rocks can be roughly divided into two types, i.e., olivine-pyroxenite and spinel-hornblende-pyroxenite. The former containing olivine shows strong serpentinization, with a mineral assemblage of calcic clinopyroxene+olivine+minor orthopyroxene, followed by the altered mineral assemblage of serpentine±chromite+magnetite±hornblende±spinel. The latter containing no olivine has features of local talcization, and its mineral assemblage consists of (bronze) pyroxene + clinopyroxene (diopside)+spinel, followed by hornblende+magnetite±talc±serpentine. In general, the ultramafic rocks have fairly low REE content (∑REE) =11.45×10^-6～22.63×10^-6) with LREE/HREE ratios in the range of 1.64～4.40. The chondrite-normalized REE patterns of all samples show negative anomalies of Ce and Eu. Most samples show indistinct fractionation of LREE, LREE-HREE and HREE, with the exception of three olivine-containing samples showing relatively high LREE/HREE ratios (3.35～4.40). The trace elements in the ultramafic rocks show significantly different anomalies of incompatible elements Rb, Ba, U, Nb, Sr and Zr. Ba and Nb assume negative anomalies, and Rb and U as well as part of Sr display positive anomalies; in addition, negative Zr and Ti anomalies appear in olivine-pyroxenite, and there exist no Zr and Ti anomalies. SHRIMP zircon U-Pb dating of the ultramafic rock (YS0631) shows that the metamorphic zircon U-Pb ages are 2 560～2 605 Ma, with a crystal zircon age of 2 719 Ma. Hf isotopic composition of the sample has Hf(t) value of 8.2, indicating the depleted mantle model age of 2 680 Ma. In summary, the ultramafic rocks originated from the mantle, formed in the early Neoarchean, and were then subjected to anatexis and magmatic effects, thus experiencing metamorphic transformation.