扬子地台西缘结晶基底的时代

对扬子地台西缘结晶基底变质地层以及岩浆片麻岩中变质地层残片或包裹体中锆石SHRIMP U-Pb同位素年龄研究表明，在康定地区辉长–闪长质片麻岩中，表壳岩包体—糜棱岩化的长英质片岩和宝兴地区黑云斜长角闪岩的年龄分别为816±8.6 Ma和826±13 Ma，代表了它们的原岩——酸性火山岩和火山凝灰岩的形成年龄；泸定地区变质岩层中长英质糜棱岩和斜长角闪岩的年龄分别为816±9 Ma和818±8 Ma，代表了中性火山岩和基性火山岩的形成时代；茨达地区斜长角闪岩和角闪黑云斜长片麻岩的年龄分别为830±7 Ma和827±10 Ma，代表了其原岩火山–沉积建造的形成时代。以上数据表明，所谓的结晶基底都是新元古代的产物，在形成时间上与盐边群、盐井群等褶皱基底的相一致，只是变质程度上略有差异。因此该区并不存在古老（太古宙—古元古代）的结晶基底。

Age of Crystalline Basement in Western Margin of Yangtze Terrane

Based on the study of zircon SHRIMP U-Pb dating on metamorphic strata in crystalline basement,and supracrustal enclave in orthogneiss in western margin of Yangtze terrane, the ages of 816±8.6 Ma and 826±13 Ma for a mylonitic felsic schist and a biotite amphibolite in the gabbro-diorite gneisses in Kangding area were determined, which are considered as the eruption ages of felsic volcanics and volcanic tuff; The ages of 816±9 Ma and 818±8Ma for a felsic mylonite and an amphibolite in metamorphic strata of Luding area, respectively, were obtained which are also suggested to be the eruption ages of intermediate and basic volcanics; The ages of 830±7 Ma and 827±10 Ma were yieled for an amphibolite and a hornblende biotite plagioclase gneiss in Cida area, which indicate the timing of volcanic-sedimentary association. The above SHRIMP zircon U-Pb chronological results show that the so-called crystalline basement in the western Yangzi plateform is chiefly constructed by Neoproterozoic magmatic rocks corresponding to the Yanbian group and Yanjing group in age, and their difference is only expressed in the metamorphic grade. Therefore, our new chronological evidence do not support the existence of an Archean to Paleoproterozoic crystalline basement in the western margin of Yangtze terrane.