扬子地台西缘康定群的再认识:来自地球化学和年代学证据

New Understanding on Kangding Group on Western Margin of Yangtze Block: Evidence from Geochemistry and Chronology

康定群(康定杂岩)在扬子地台前寒武纪地质研究中具有非常重要的意义,但对其具体的形成时代存在不同认识。本文选择位于德昌茨达—大六槽之间康定群咱里组中斜长角闪岩、斜长片麻岩和花岗质片麻岩进行了较为系统的岩石学和地球化学研究。斜长角闪岩的主、微量元素主要具有N-MORB特征,一个斜长角闪岩样品的地球化学特征介于E-MORB和OIB之间,而斜长片麻岩地球化学具有岛弧岩浆特征。斜长角闪岩的εNd(t)为+3.65^+12.65,具有亏损地幔岩浆特征,综合分析认为其形成于弧后盆地环境,与盐边群类似。斜长角闪岩和斜长片麻岩的离子探针(SHRIMPⅡ)定年结果表明,康定群玄武岩的形成时代约为830Ma,为新元古代,同时经历了约700Ma的变质作用。本文研究不支持扬子地台西缘存在新太古代—古元古代的古老结晶基底,同时研究表明扬子地台西缘新元古代为岛弧环境。

Kangding Group （also named as the Kangding complex） is of significance for Precambrian research in the Yangtze block, but its age is still in dispute. This paper studied the petrology and geochemistry of amphibolite, plagioclase gneiss and granitic gneiss from the Zanli formation of Kangding Group in Cida abd Daliucao of Dechang county, western Sichuan. The amphibolite is consistent with N-MORB in major elements and trace elements, while one amphibolite is similar with E-MORB or OIB, and plagioclase gneiss are of the characteristics of island arc volcanics. The ENd（t） of amphibolite ranges from ＋3.65 to ＋12.65, with characteristics of depleted mantle. So the basic volcanics in the Zanli Formation erupted in the back arc environment. Zircon SHRIMP U-Pb dating from amphibolite and plagioclase gneiss yielded ages of 830Ma and - 700Ma respectively, which indicate they （the basalts） erupted in Neoproterozoic and underwent the metamorphism of amphibolite phase soon after their emplacement. The result in this paper doesn＇t agree with the idea that the Kangding Group formed in Neoarchean or Paleoproterozoic, and meanwhile put forward new evidence that the western margin of the Yangtze block was the active island arc in Neoproterozoic.