The Central Qiangtang Metamorphic Belt has long been controversial as to whether it represents the continuous evolution of the (Proto-) Paleo-Tethys Ocean since the Cambrian or the independent evolution of two oceanic basins. In this study, a rock assemblage with characteristics of seamount sedimentation was discovered in the northern part of the Central Qiangtang Metamorphic Belt, mainly composed of basalt, limestone, and siliceous rocks, none of which have undergone high-pressure metamorphism. Geochemical characteristics indicate that the basalts from this seamount assemblage have high total rare earth element (REE) content, show significant fractionation between light and heavy REEs, and are enriched in large ion lithophile elements (LILE), similar to typical ocean island basalt (OIB) characteristics. Chronological analysis of a tuff interlayer (about 10 cm thick) within the basalt shows that the zircon U-Pb ages range from 461 Ma to 499.7 Ma (after excluding points with discordance greater than 10%), with a weighted average age of 473.5 ± 2.3 Ma (MSWD = 1.9), which may represent the main eruption period of the underlying basalt. This study suggests that this rock assemblage is the product of in situ accretion during the subduction of the Longmu Co–Shuanghu Ocean between the South and North Qiangtang terranes, and that the Longmu Co–Shuanghu Ocean may have reached a considerable scale as early as the Middle Ordovician and continued to evolve until the Triassic.