A lot of basic volcanic rocks occur discontinuously in a NNEstriking zone from Baisong to Panyong to the east of the Jinshajiang suture zone. Based on petrogeo chemical study and 40Ar/39Ar dating of the Panyong pillow basalts，the paper points out that these basic volcanic rocks formed from about 15％ partial melting of a weakly enriched MORBtype mantle．Using a stepbystep evaporation technique the authors obtained an 40Ar/39Ar plateau age of 231.6±6.7Ma from plagioclase porphyrocrysts. The pillow basalts represented a local ocean basin within the Zhongza micro continental block during the early stage of the Late Triassic．Its formation was both the result of the lasting subduction toward the west of the Jinshajiang ocean basin and the westward subduction of the GarzeLitang ocean basin, which led to formation of the Yidun arc belt.