Mineral Phase Transformation and Water Fluid Evolution of Khondalite Series in the Border Area of Shanxi and Inner Mongolia,China

作     者：LIU Fulai Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 

出 版 物：《Continental Dynamics》 (大陆动力学(英文版))

年 卷 期：1999年第1期

页      面：62-69页

学科分类：0709[理学-地质学] 070901[理学-矿物学、岩石学、矿床学] 07[理学] 

主　　题：Metamorphic reaction water activity khondalite series Shanxi and Inner Mongolia 

摘      要：Typical Precambrian khondalite series is widely exposed in the border area of Shanxi and Inner Mongolia, and is composed primarily of sillimanite garnet biotite gneiss (SS), garnet pyroxene biotite gneiss (GS) and attendant migmatitic gneiss. According to the characteristics of multistage reaction texture and mineral phase transformation, the mineral assemblage of the khondalite series can be divided into three generations: ① peak stage (M 1) mineral assemblage, involving hydrous biotite, is composed of Sil+Gt+Bi+Kf±Pl+Qz in the SS and Gt+Opx+Bi+Kf+Pl+Qz in the GS, formed at T=750℃-850 ℃ and P=0.80-0.95 GPa. Water activity (a H 2O ) of this stage was about 0.25-0.37 (0.28±) in the SS and 0.38-0.53 (0.45±) in the GS; ② in the post peak retrograde near isothermal decompression stage (M 2), the khondalite series of the study region is characterized by dehydration reaction of biotite, partial melting of felsic minerals and decompression texture of garnet. Sillimanite garnet migmatitic gneiss (SMS) and garnet pyroxene migmatitic gneiss (GMS) were formed at this stage. The attendant anhydrous mineral assemblages are Sil+Gt+Crd+Kf±Pl+Qz in the SMS and Gt+Opx+Crd+Kf+Pl+Qz in the GMS, formed at T= 730 ℃ - 820 ℃ and P=0.50-0.65 GPa. However, the water activity (a H 2O ) of this stage was reduced to 0.08-0.12 ( 0.09± ) in the SMS and 0.13 -0.18 (0.15±) in the GMS, and ③ in the later cooling stage (M 3) of metamorphism, fine grained hydrous vermicular symplectite Bi+Kf replaced anhydrous garnet at T=500 ℃-560 ℃and P=0.32-0.44 GPa, indicating that the water activity (a H 2O ) of M 3 increased again. Research results suggest that the border conditions of mineral evolution and (dehydration) metamorphic reactions are not only controlled by PT conditions, but also genetically related to water activity evolution in the khondalite series of the study region.