Redefinition of the Alxa Group-complex (Precambrian metamorphic basement) in the Alxa area, Inner Mongolia

Abstract：The original Alxa Group-complex consists of metamorphic strata and deformed orthogneisses of different ages, geneses, tectonic styles and metamorphic grades；so it is not suitable for a lithostratigraphic unit and should be disintegrated and redefined. Based on recent studies, the original Alxa Group-complex may be divided into three metamorphic stratigraphic units and two deformed orthogneiss complexes. The oldest lithostratigraphic unit in the area is the Diebusige Group-complex, which consists of high-grade metamorphic supracrustal rocks and is the basement rock series in this area. Preliminary zircon SHRIMP U-Pb dating suggests that the Diebusige Group-complex formed at ~2700 Ma and is considered to be of Neoarchean age. The newly established Bayan Ul Formation-complex, which is principally composed of metavolcanic rocks, is a volcanic rock series in this area. Its newly obtained ages are 2264 and 2271 Ma, indicating a Paleoproterozoic age. The redefined Alxa Group-complex, which consists predominantly of meta-clastic rocks and meta-carbonate rocks, is a continental-margin sedimentary series and formed in the late Paleoproterozoic or still later. The Boluosutanmiao tonalitic-granitic gneiss complex is composed of early-stage tonalitic gneiss and late-stage granitic gneiss. The early-stage tonalitic gneiss formed at ~1.8 Ga and the late-stage granitic gneiss formed approximately during the Paleozoic. The Bijigetai granodioritic gneiss complex is mainly composed of hornblende-plagioclase gneiss and biotite-plagioclase gneiss and the preliminary isotopic dating indicates that the main stage of this complex occurred in the Jinningian at 0.8－1 Ga.