Chronology of the Precambrian metamorphic series in the Alxa area, Inner Mongolia

Abstract：The formation ages of the early Precambrian metamorphic series in the Alxa area, Inner Mongolia, have preliminarily been determined using the zircon SHRIMP U-Pb and single-zircon stepwise evaporation methods. The Yiken Us Formation-complex in the lower part of the Debusige Group-complex formed ~2700 Ma ago and underwent two phases of metamorphism in the late Neoarchean and at the end of the Paleoproterozoic. The oldest (3.5 Ga) detrital zircons in the area have been found in the formation-complex, suggesting that older crustal material probably exists in the area. The Bayan Ul Formation-complex formed in the period of 2500-2264 Ma and is considered to be of Paleoproterozoic age. SHRIMP ages of 1367 and 1617 Ma have been obtained for detrital zircons in the Alxa Group-complex, which means that the possibility that the Alxa Group-complex might belong to the Mesoproterozoic cannot be ruled out. Isotope chronological studies indicate that the Precambrian metamorphic series in the area consists of several metamorphic series of different ages and geneses, which have their own compositional characteristics, so they should be subdivided into the Neoarchean Debusige Group-complex, Paleoproterozoic Bayan Ul Formation-complex and Paleo- and Mesoproterozoic Alxa Group-complex.