北京密云麻粒岩相区变质作用演化及PTt轨迹的研究

THE EVOLUTION OF METAMORPHISM AND PTt PATHS OF GRANULITE FACITES TERRANE IN MIYUN, BEIJING

北京密云麻粒岩相区在早前寒武纪时期主要经历了两期变质作用。每期变质作用又可划分为三个连续的变质阶段，峰期阶段的Ｐ－Ｔ条件均达到了麻粒岩相．第一期变质作用约结束于２５００Ｍａ，具有与造山带相似的顺时针ＰＴｔ轨迹，峰期阶段Ｔ为８００～９００℃，Ｐ＝０．９８～１．０２ＧＰａ，这种轨迹主要与早期平卧褶皱和推覆构造使地壳大幅度增厚有密切关系。第二期变质作用约发生在１８００Ｍａ，ＰＴｔ轨迹为具近等压加热（ＩＢＨ）特点的顺时针轨迹，峰期阶段Ｔ为７６０～８１０℃，Ｐ为０．９６～１．０４ＧＰａ，这种轨迹与大陆地壳的拉伸作用有密切关系。

From the metamorphic and deformational characteristics of supercrustal, intrusive rocks and their relationships, the authors infer that the granulite factes terrane in Miyun of Beijing has experienced two periods of metamorphism. Three continuous stages are distinguished in the periods of metamorphism. Moreover, their peak metamorphism are of granulite factes.The first metamorphism mainly affected the supercrustal rocks and intrusive rocks (including ultrabasic, basic and intermediate-acidic intrusive rocks). The temperature of the first metamorphism ranged from 800℃ to 900℃ and the pressure from 0. 98GPa to 1. 02GPa in the peak stage. The PTt path of the first metamorphism is characterized by a clockwise loop similar to that of orogenic belt, which is mainly related to the crustal thickening resulted from the recumbent fold and nappy structure. The second metamorphism mainly affected the three types of the basic dykes. They cut across the supercrustal rocks and intrusions which were metamorphosed and deformed intensively during the first metamorphism. but show no featutres of folding or deformation in themselves. Therefore, it is reasonable to infer that these dykes were metamorphosed during the second cetamorphism, which may be responsi ble for the formation of varioous corona structures in the metamorphosed supercrustal rocks and intrusions. The temperature of the second metamorphism ranged from 760℃ to 810℃ and the pressure from 0.94GPa to 1.04GPa in the peak stage. The PTt path for the second metamorphism is also a clockwise loop characterized by near isobaric heating (JBH), which has a relation with continental extension. On the basis of radiometric dating, the first metamorphism did not come to an end until 2500Ma, and the second metamorphism was present at about 1800Ma.