THE FEATURES OF LATE ARCHEAN METAMORPHIC COMPLEX IN THE MOUNT LANGSHAN AREA,INNER MONGOLIA

There occurs a suite of amphibolite-facies metamorphic complex in the main peak of Mount Langshan. It is mainly composed of hornblende-plagioclase gneiss, biotite-plagioclase gneiss, plagioclase amphibolite, amphibolite, biotite granulitite and biotite-quartz schist, locally intercalated with itabirite. Its protolith consists mainly of intermediate-basic volcanic rocks and at least underwent two phases of migmatization. It is notably different from the Middle Proterozoic Langshan Group on the northern and southern slopes of Mount Langshan. The biotite-plagioclase gneiss has been measured to have a Sm-Nd isochron age of 2735 Ma, which proves it to be of Late Archean age. It has many similarities to the greenstone belt in respect to the sequence of metamorphic complex, petrochemistry and trace element characters. It shows higher contents of Cu, Pb, Zn and A. The stratabound polymetallic deposits in the Langshan Group were just formed in the taphrogenic trough developing on the crystalline basement composed of this kind of complex.