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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 (Kangding Complex) is of great significance in the study of Precambrian geology in the Yangtze platform. However, there are different understandings on the specific formation times. In this paper, we selected systematically petrology and geochemistry of amphibolite, plagioclase gneiss and granitic gneiss from Zangli Group, Kangding Group, located between the Tertiary and Tidal Flat in the Dachang. The main and trace elements of the amphibolite mainly have N-MORB characteristics. The geochemical characteristics of one amphibolite are between E-MORB and OIB, while the plagiogneiss geochemistry has the characteristics of island arc magma feature. The εNd (t) of plagioclase amphibolite is between +3.65 and +12.65 and has the characteristics of depleted mantle magma. According to the comprehensive analysis, the εNd (t) is formed in the backarc basin environment, which is similar to that of the Yanbian Group. The results of SHRIMP II dating of amphibolites and plagioclase gneiss show that the formation age of the Kangdingqu basalts is about 830 Ma, Neoproterozoic and metamorphic about 700 Ma at the same time. This study does not support the existence of the Neo-Archean-Paleoproterozoic ancient crystalline basement in the western margin of the Yangtze platform, and at the same time shows that the Neoproterozoic in the western margin of the Yangtze platform is an island arc environment.