GEOLOGICAL-STRUCTURAL FEATURES OF THF FANSHI GROUP IN THE WESTERN PART OF MOUNT WUTAI

The area where the amphibolite-facies rocks of the Fanshi Group are distributed in Mount Wutai is an Archean granite-greenstone terrane.By means of the petrological-structuralmapping of the western part of Mount Wutai,the Fanshi Group is divided into three petrological-structural units The metavolcanic rocks of the Fanshi Group reflect an evolution from the tholeiite series toealc- alkaline series.The granitic rocks （with an isotopic age of over 2500 Ma） in the area of distribution of the Fanshi Formation fall into two evolutionary series: the tonalite-trondhjemite series and the talc-alkaline series.The infracruatal rocks consist of gray gneisses （T.T.G.） suite and the supracrustal rocks are made up of the Fansi Group of the greenstone belt,with the supracrustal rocks overlying unconformably the infraerustal rocks （Xitian）.The granites and part of granodiorite were emplaced as a diapir.The magmatie diapirism and subhor- izontal shearing combined to form oval granite domes and linear synformal basins. In the synformal basins there are recumbent folds and subhorizontal ductile faults formed by gravity sliding.The late-stage regional horizontal compression caused the domes and basins to be refolded and flattened and brought about large- scale overlapping deformation,thus forming the present linear structural belt and nappe structure.