蔓箐沟一带石榴基性麻粒岩变质反应与矿物演化

METAMORPHIC REACTION-MINERAL EVOLUTION SERIES OF MAFIC GARNET GRANULITES IN THE MANQINGGOU REGION

蔓箐沟地区石榴基性麻粒岩发育多期变质反应结构。矿物组合演化可划分三个阶段：①峰期阶段（Ｍ１），以包裹于石榴石中的Ｏｐｘ＋Ｃｐｘ±Ｈｂ＋Ｐｌ±Ｑｚ和由Ｇｔ＋Ｏｐｘ＋Ｃｐｘ±Ｈｂ＋Ｐｌ±Ｑｚ组成基质矿物组合为代表；②退变早期近等温减压阶段（Ｍ２），形成蠕虫状后成合晶Ｏｐｘ＋Ｈｂ＋Ｐｌ±Ｍｔ；③退变晚期阶段（Ｍ３），则形成细粒、黄绿色Ｈｂ＋中酸性Ｐｌ的较低温组合。并揭示该区石榴基性麻粒岩从中下部地壳的麻粒岩相变质，到快速抬升至中浅部地壳发生的近等温减压及晚期降温退变的复杂ｐＴ演化历史。

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.