Dating and reconstruction of protoliths of the Laojunmiao Group and its indications for basement in eastern Junggar, Xinjiang

The Laojunmiao Group which is exposed on the northeastern margin of the Runggar basin consists of metamorphic complex of high greenschist facies to amphibolite facies. Its formation age and tectonic setting has been controversial for a long time. Zircon U-Pb dating of K-feldspar mylonite schist yielded a number of ages which are mostly concentrated on 500~544Ma with, a few belonging to early Neoproterozoic(740~920Ma).According to the youngest peak age(~520Ma), the age of Laojunmiao Group is redefined as early Cambrian.The lithogeochemical characteristics of Laojunmiao Group show that it has the characteristics of the continental crust, the protoliths formation is similar to a set of flysch formation and its tectonic environment should belong to the continental margin sea.As the problem in regard of the basement properties of funggar basin has not been conclusive yet, the results of this study in combination with, regional data can prove the existence of Precambrian basement in Eastern Junggar.