NEW RECOGNITION ON THE STRATIGRAPHY AND STRUCTURE OF THE WUTAI GROUP IN THE WUTAI MOUNTAINS AREA

The Wutai Group is a Precambrian metamorphic series. Previously it（?）ad been long considered to be a composite monoclinal structure dipping in general to the northwest and its strata were older in the southeast and younger in the northwest. In recent years, through regional geological surveys, new advances have been made in the study of the above-mentioned problems. The authors have determined a new tectonic framework and a new succession of strata of the Wutai Group, according to the tectonic analysis, volcanic cycles, original rock formations, primary structures, ore-bearing character and data of isotopic age determination in connection with follow-up large-area geological mapping. In the Proterozoic, this group underwent more than three phases of deformation and metamorphism. The basic tectonic framework resulting after superimpositiou and compounding is marked by a large-scale composite syncline. The framework is essentially composed of five tight folds that are normal in the castern section and mostly overturned in the western section. The latest strata are preserved in the trough region of the composite syncline. There is a flysch formation at both the top and the base of the strata. The principal part is composed of three basic-acid volcanic cycles. on that basis five formations are distinguished, of which three volcanic formations are sabdivided into eight members according to the lithology and ore contents. In the paper the authors also discuss other important geological problems such as the upper boundary, age, metamorphic conglomerates and regional metamorphic features of the Wutai Group.