METAMORPHIC REACTION-MINERAL EVOLUTION SERIES OF MAFIC GARNET GRANULITES IN THE MANQINGGOU REGION
Liu Fulai (Institute of Geology, CAGS, Beijing 100037)
Manqinggou region is one of the most important areas where the Archean metamorphic complexes composed of grey gneiss with mafic granulite and minor Khondalite are well exposed. Mafic garnet granulite is important due to the preservation of multistage reaction textures. On the basis of petrographic evidences, the mineral assemblages in mafic garnet granulite can be divided into three generations: The first stage mineral assemblage is characterized by Opx+Cpx±Hb+Pl±Qz as inclusions in garnet porphyroblasts and Gt+Opx+Cpx±Hb+Pl+Qz as matrix. They are formed in the peak facies of granulite metamorphism (M 1) at t =770～860℃, p = 0.90 ～1.04 GPa, the characteristics of the second stage mineral assemblage are the vermicular symplectites of orthopyroxene, hornblende, plagioclase and/or magnetite surrounding garnet porphyroblasts. The corona texture represents a rapid decompression reaction during the post-peak retrograde metamorphism(M 2) at t =740～820℃, p = 0.50 ～0.70 GPa. Fined-grained Hb(yellowish green)+ Pl(medium-acid), representing the third stage mineral assemblage, occurred around the rim of the garnet and pyroxene. They were formed at t =500～550℃, p =0.31～0.41 GPa during the late retrograde metamorphism(M 3). The pT path of the garnet mafic granulites is constructed by using the geothermometer - barometers data and metamorphic reaction experiments. The near-isothermal decompression(ITD) path from M 1 to M 2 represents a rapid uplifting of crust, then follows the normal decompressional cooling path(M 2→M 3) and finally the studied region becomes stable.