The Late Cambrian volcanic rocks in Shangyanqing-Baiyan'aolinchang of Baoshan area within western Yunnan Province are poorly developed but distributed extensively. The volcanic rocks consist of early tholeiitic and alkali-basalt and late calc-alkali ba-salt. The geochemical characteristics of rocks show that early volcanic rocks formed in an extensional environment, whereas advanced volcanic rock was formed by subduction. The tectonic background of Baoshan terrane exhibited the transformation from extension to subduction in the late Cambrian, with the formation of the unconformity between Ordovician and Cambrian. This discovery pro-vides petrological evidence for the conversion of the eastern part of the northern margin of Gondwana from passive continental mar-gin to active continental margin.%滇西保山地区上岩箐—白岩凹林场一带的晚寒武世火山岩虽不甚发育,但分布广泛.早期为拉斑玄武岩、碱性玄武岩,晚期为钙碱性玄武岩.岩石地球化学特征表明,早期火山岩形成于伸展环境,晚期火山岩为消减作用产物.保山地体构造背景在晚寒武世由伸展环境向挤压环境转换,并形成寒武系与奥陶系之间的不整合,为东冈瓦纳大陆北缘由被动大陆边缘向主动大陆边缘转换提供了岩石学证据.
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