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

北京新太古宙密云群的灰色片麻岩可分为A、B、C、D4个岩石类型,其中B型又可细分为B_1、B_2和B_3 3个亚型,A型的绝大部分岩石和B_1亚型岩石属变质的英安岩-钠质流纹岩或火山-沉积岩,B_2型为变质的英云闪长岩和二长花岗岩体。B_3为变质的新成脉体。C型为变质的奥长花岗岩质-花岗岩质脉岩。D型是变质的长石石英砂岩。A型和B_1、B_2亚型的物源来自多源的石英榴辉岩、石榴斜长角闪岩和REE富集的斜长角闪岩不同程度的部分熔融;B_3亚型和C型来自A型和B_1亚型的部分熔融;D型则来自高度不成熟的陆源碎屑。

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.