摘要
晋冀蒙三省交界的怀安-大同一带基性麻粒岩有保存完好的变质反应结构。依据岩相学特征,可在基性麻粒岩中区分出四个世代的矿物组合:①峰前阶段(M1)以包裹在石榴石或紫苏辉石中的Hb+Pl±Qz、Cpx+Hb+Pl±Qz组合为代表,T为600℃~700℃,P为0.70~0.82GPa,相应的aH2O值为0.50~0.62;②峰期阶段(M2)以三联点结构的基质矿物为特征,组合为Opx+Cpx+Hb+Pl±Qz、Gt+Opx+Cpx+Hb+Pl±Qz,T为750℃~850℃,P为0.84~1.05GPa,相应的水活度降至0.39~0.52;③峰后近等温减压阶段(M3)以石榴石普遍转变为Opx+Pl±Mt及角闪石转变为Opx+Cpx后成合晶为特征,T为700℃~780℃,P为0.50~0.70GPa,相应的aH2O值降至0.08~0.11;④至降温阶段(M4),以无水矿物石榴石、辉石退变为Hb+Pl组合为代表,T为530℃~630℃,P为0.30~0.42GPa,aH2O值再次升至0.37~0.40。研究表明,矿物组合演化和所发生的(脱水)变质反应不仅受PT条件的控制。
Huaian Datong region on the bounder of Hebei Shanxi Inner Mongolia is one of the most important area where the Archean granulite complex is relatively completed exposed. The granulite complex is mainly composed of basic granulites and grey gneisses. The multistage reaction textures were preserved in the basic hornblende pyroxene granulite and garnet hornblende pyroxene granulite. They provide information and indicators of P T condition and water activity evolution during metamorphsm, and let us speculate on the corrective PTt path of the terrane. Mineral assemblages and metamorphic reactions in the hornblend pyroxene granulite and garnet hornblend pyroxene granulite can be divided into four generations on the basis of petrographic and genetic mineralogic evidences. The early mineral assemblages are characterized by Hb+Pl±Qz, Cpx+Hb+Pl±Qz as relict minerals included in garnet and hypersthene porphyroblasts. They formed in the pre peak stage at T =600℃ ̄700℃, P = 0.72  ̄ 0.82 GPa and a H 2O = 0.50  ̄ 0.62 ; The peak stage mineral assemblages consist of Opx+Cpx+Hb+Pl±Qz and Gt+Opx+Cpx+Hb+Pl±Qz. They juncted with equal angle triple texture and formed at T =750℃ ̄850℃, P =0.84 ̄1.05GPa, and the water activity ( a H 2O ) reduced to 0.39  ̄ 0.52 . During the early retrograde isothermal decompression stage, the symplectitic of orthopyroxene+plagioclase+magnetite surrounded garnet porphyroblasts, and the corona texture represents a decompression reaction, while Hb transformed to Opx and Cpx in this stage. The mineral assemblages Opx+Cpx+Pl±Qz and Opx+Pl±Qz formed at T =700℃ ̄780℃, P =0.50 ̄0.70GPa, and a H 2O = 0.08  ̄ 0.11 . During the last stage of metamorphism, fine grained vermicular symplectites Hb+Pl±Qz occured around anhydrous mineral garnet and orthopyroxene. They developed at temperature about 530℃ ̄630℃ and pressure 0.30 ̄0.42GPa. After all, the border conditions of assemblages and dehydration reactions are not only controlled by P and T , but also closely relative to water activity evolution.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
1997年第1期27-43,共17页
Acta Petrologica Sinica
基金
博士后基金和自然科学基金
关键词
麻粒岩
变质反应序列
水活度
Basic granulite
Metamorphic reaction
Water activity
Shanxi Province
Huaian Datong