北京房山地区的变质带

本区出露一套古生代至中生代的地层。由于中生代区域构造运动的影响及中酸性深成岩体的侵入,地层受到两次不同性质的变质作用的改造。早期在北京西山发生了区域低温动力变质作用,产生了区域硬绿泥石-绿泥石带;后期在房山地区发生了接触热流变质作用,在早期变质带的背景上,随着岩体的侵位,围绕岩体的一定范围,从早至晚依次产生了黑云母带—红柱石带—铁铝榴石十字石带—硅线石白云母带,它们的前者依次被后者叠加,揭示了前进变质带的发育,具叠置—收缩的双重特征。

METAMORPHIC ZONES OF THE FANGSHAN AREA IN BEIJING

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.