It has been widely believed that a double layer structure (or three layer) of crystallization basement and folding basement occurs over the Pre-Sinian basement in the western margin of Yangtze block, with the Hekou Group as the transitional sequence between two basements, but its attribute and formation time have long been in dispute. Advanced study of Hekou Group would have important implications for understanding of the feature and evolution of the basement structure of the western margin of Yangtze block. Here we reported 13 groups of zircon SHRIMP U-Pb ages from the diabase that intruded into the Hekou Group, and obtained 1710-8 Ma concordant U-Pb age, which indicates that the Hekou Group formed earlier than that age. A large voluminous of isotopic evidence by previous studies shows that the Hekou Group should formed during 1817～1710 Ma, which suggests the Late Paleoproterozoic. The age of the diabase reported in this study, has confined the upper limited age of Hekou Group, and has great significance in further understanding of the tectonic patterns and evolutionary history of the western margin of the Yangtze terrane.%普遍认为,扬子地台西缘前震旦系基底存在结晶基底和褶皱基底的双层(或三层)结构,河口群作为结晶基底与褶皱基底之间的过渡层位,其归属及形成时代仍存在争议.对河口群的深入研究,有助于深化认识扬子地台西缘基底结构的特点及演化过程.通过对会理县河口地区侵入于河口群中辉绿岩体的锆石SHRIMP U-Pb测年,获得1710±8 Ma(MSWD=1.11)的锆石U-Pb年龄,表明河口群的形成时代要早于这个年龄.大量的同位素数据证明,河口群主要形成于1817～1710 Ma,为古元古代晚期.本文的研究成果,对认识整个扬子地台西缘的构造格架与演化历史具有重要的意义.
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