北京密云群西段灰色片麻岩的地球化学特征和成因

GEOCHEMICAL CHARACTERISTICS AND PETROGENESIS OF GRAY GNEISSES FROM WESTERN PORTION OF LATE-ARCHAEAN MIYUN GROUP, BEIJING

The gray gneisses from the western portion of Late-Archaean Miyun Group, northern Beijing, consist of four types of rock ,, i. e. A , B, C and D, of which type B can be divided into three subtypes , i. e. B1 , B2, and B3. Type A , and B1 are the metamorphosed dacitesodium rhyolite or volcanic-sedimentary rocks. B2 belongs to metamorphosed tonalites and adamel-lites, and B3 to metamorphosed neosomes. The SiO2 contents of types A and B account for 60. 93%~69. 55%. Type C with 67. 69%~75. 24% of SiO2 comes from the metamorphosed trondhjemitic-granitic dykes. Type D has only one sample, which comes from the metamorphosed arkose sandstone. It is suggested that the original material of types A ,B1 and B2 comes from partial melting of different degree of polygenetic quartz-eclogite,garnet-amphibolite and amphibolite with REE enrichment, those of B3 and C was derived from the partial melting of A and B1 subsequent to the regional metamorphism probably around 2. 5 Ga, and that those of D was obtained from the highly .immatured terrigenous detritus. It can be summed up that the majority of the gray gneiss from the western portion of Late-Archaean Miyun Group is of the supracrustal rocks.