Major element and Sm-Nd isotopic studies suggest that the mafic-ultra mafic rocks in the Yuanbacshan and Baotan, southern margin of the Yangtze Block were probably derived from a Al-depleted mantle source.and from the mixture of Al-depleted and Al-undepleted mantle sources, respectmely. A few of Baotan samples might have been contaminated by the crustal wall rocks. The Sm-Nd isotopic data of these samples yield a geologically nieaivngless pseudoisaehron, whose slope gives an age of about 2.2Ga sighficautly older than the Eormatoon age of the samples.The consistent Nd model ages(TDM) of the low-grade metamorphic sedimentary rock samples from the Sibao Group constrian a maximum age of about 1.8Ga for the Sibao sediments and the interlayered mafic-ultra mafic rocks. The oldest basement rocks, i. e. the Sibao Group and its equivalents, in the southern margin of the Yangtze Slock are dominantly composed of the unmature crustal recycled materials with crustal residence ages of f.8-1.9Ga. The Sm-Nd stotopic data obtained so far indicate that there is probably no early Proterozoic-late Archean basement rocks along, the Southern margin of the Yangtze Block.