METAMORPHIC ZONES OF THE FANGSHAN AREA IN BEIJING
北京房山地区的变质带

The strata affect by metamorphism include Middle to Late Proterozoic, Carboniferous, Permian, Trias-sic and Jurassic rocks in the studying area, constituting a part of the Xishan folding belt in Beijing. They have subjected to two stages of metamorphism of different nature during the Yanshanian orogenesis. The first metamorphic stage was of regional low-temperature dynamic nature. It resulted in the basicly regional chloritoid-chlorite metamorphic zone occurring in a NNE direction of the Xishan region with a length about 50 km and width, 10 km or so. The second stage was principally a contact heat flow metamorphism, owing to the intrusion of Early Cretaceous granodi-orite, superposed on the previous chloritoidchlorite zone, probably under a progressively declining stress but increasing temperature condition. It was characterized by four successive metamorphic zones to biotite, andalusite, alman-dite-staurolite and sillimanite-muscovite, with gradual shrink in their respective extent, and also the width, towards from far to the intrusion, but superposed on the earlier ones. The formation to these zones were coeval with different stages of the folding and the emplacement of the granodiorite. Such a type of metamorphism may be considered of a superposition-shrink pattern.