中文摘要：

扬子西缘新元古代澄江组底部发育一套玄武岩,其形成时代一直未能准确限定.本文在野外地质调查的基础上,对云南武定罗茨地区澄江组底部玄武岩进行了较为系统的岩石学和同位素年代学研究.结果表明,最年轻一组锆石206Pb/238U年龄加权平均值为804±6 Ma （MSWD=0.40）,可以代表澄江组玄武岩的形成时代,从而进一步确定澄江组底界年龄为800 Ma左右.武定罗茨地区澄江组底部玄武岩显示低SiO2,高（K2O＋Na2O）、TiO2和Al2O3的特点,具有大陆裂谷碱性玄武岩的性质.结合区域相关研究资料,认为扬子西缘可能存在一个自～800 Ma持续至725 Ma的幕式双峰式岩浆岩带,而澄江组底部玄武岩和苏雄组火山岩均为～800 Ma双峰式岩浆活动的产物,并推测该双峰式岩浆岩带可能形成于大陆裂谷环境.此外,玄武岩中捕获的锆石年龄信息指示扬子西缘可能存在2.5Ga、1.8～1.6 Ga及1.0 Ga等数期重要的区域构造-热事件.

英文摘要：

The formation age of basalts from the lowermost Neoproterozoic Chengjiang Formation in the western Yangtze Block has not been determined yet. In this paper, field investigations and detailed petrologic and geochronologic researches were carried out on basalts of the lowermost Chengjiang Formation from the Luoei area, Wuding, and the results show that the formation age of basalts from the lowermost Chengjiang Formation is 804 ± 6 Ma, which further confirms that the lower age limit of the Chengjiang Formation should be about 800 Ma. Petrologic studies show that basalts from the lowermost Chengjiang Formation have lower SiO2 content and high K2O ＋ Na2O and TiO2 content, which indicates that their formation should be attributed to alkaline basalts developed in the continental rift. Combined with regional relevant research data, it is suggested that there possi- bly existed an episodic bimodal magmatic rock belt that lasted from ～800 Ma to 725 Ma along the western Yangtze Block. It is thus held that basalts of the Chengjiang Formation as well as volcanic rocks of the Suxiong Forma- tion were formed by 800 Ma bimodal magmatic activity, and that this bimodal magmatic rock belt might have been developed in the continental rift. Moreover, the ages of captured zircons from the basalts indicate that there were multi-stage regional tectonic-thermal events along the western Yangtze Block, which took place at 2.5 Ga, 1.8～1.6 Ga and 1.0 Ga.