THE BANYUKOU FORMATION IN THE WUTAI MOUNTINS AREA

The quartzite of the Lanzhishan granite, which was once interpreted as the Banyukou Formation of the Wutai Group, has been redetermined as the Dashiling Formation of the Hutuo Group; therefore the zircon age（2560±9Ma） of the Lanzhishan granite is impossible to be the maximum age of the Wutai Group, The marble of the Banyukou Formation in the Kouquan area has a mineral assemblage of tremolite, dolomite and and phlogopite, and its metamorphic grade reaches upper greenschist facies. The overlying Jingangku Formation is metamorphosed to amphibolite facies. It appears that there is a discrepancy in metamorphic grade between these two formations. One of the possibilities is that the stratigraphic position of the Banyukou Formation should be redetermined,The phase diagram of the system MgO-CaO-SiO₂-H₂O-CO₂ shows that the temperature for disappearance of tremolite can be as high as 630℃ at Pr 5 kb and Pco₂ 1.5-3. 5kb;hence the marble is also metamorphosed to amphibolite facies, as is the case with the Jingangku Formation. It is concluded that the Banyukou Formation in the kouquan area still preserves its original strgraphic position.