Abstract: The characteristics of the komatiitic basalts, quartz-keratophyre and rhyolitic tuff formed in the subducted stages of the oceanic crust as well as the two kinds of basic rocks formed in the post-orogenic extensional stage in the development of the (middle-)western Jiangnan orogen are reported in the paper. The kamatiitic basalts are considered to be the high-MgO basalts formed in island-arc settings. Their high Ni and Cr contents suggest that they were the primary basalts derived from the sub-arc mantle.Their high MgO contents indicate that the basaltic magmas were extracted from its mantle source under relatively dry conditions.The two subtypes of the mafic rocks formed in the post-orogenic extensional stage might be derived from the partial melting of the asthenospheric and lithospheric mantles， respectively， and show contrasting geochemical features. U-Pb zircon dating data for samples of the mafic and acidic rocks as well as sandstones hosted in the Neoproterozoic basement strata in the western Jiangnan orogen reveal that deposition of the terrigenous fragments in the island-arc area took place between 872 Ma and 835 Ma. The magmatism with arc signatures occurred from 878 Ma to 822 Ma. Therefore, we believe that there existed coeval arc magmatism and sedimentation along the southeastern margin of the Yangtze Block during the Neoproterozoic.