摘要
通过英云闪长质片麻岩变质结构的研究,把变质作用分为四个演化阶段,用三组分五个矿物相ACF体系P-T成因格子表示了这个演化过程。四个变质阶段矿物地质温度和压力计算结果构成了逆时针P-T-t轨迹。角闪石生成二辉石的平衡反应计算表明,主期变质水活度明显高于峰期变质水活度。水活度越高角闪石向二辉石转化温度越高,因此水活度是麻粒岩相变质作用的独立因素之一。
The major rock types of tonalitic gneisses which are located in Shang-ying to Malanyu area, Easter Hebei Province are dark-gray spot tonalitic gneisses and light-gray tonalitic gneisses. Investigation in rock textures shows that these rocks had undergone four stages of metamorphic evolution: the mineral association is (1) early stage, P1 +Amphl+ Q+i, (2) main stage, Pl + Opxl + Cpxl + Amph2 + Q ± Bi, (3) peak stage, Pl + Opx1 + Cpx2 + Q ± Ga,and (4) later stage, Pl + Amph3 + Mt + Q. This evolutional process is well reflected by genetic lattice diagram of five mineral phases with three components. The four evolutional stages of metamorphism constructed a P-T-t path with rising temperture and equal pressure at first, followed by increasing pressure with dropping temperture, and last, dropping tempreture and presure. The P-T-t trajectory togather with geological structure indicates that the metamorphism takes place in tectonic environment of ductile thrust. Water activities calculated through equilibrium Ca2Mg5Si8O22(OH)2 = 2CaMgSi2O6 +3MgSiO3+SiO2+H2O indicate that the water activities during main stage of metamorphism and peak stage of metamorphism are 0.53-0.62 and 0.11-0.21,respectively. These data are consistent with the experimental results given by W. M. Lamb et al (1988). This study indicates that the higher the water activities is, the higher the temperture of reaction for amphiboles changing into pyroxenes. Water activities control mineral associtions in certain P-T conditions under granulite facies, therefore, the water activities are considered to be an independent factors in granulite factier.
关键词
英云长岩
片麻岩
变质结构
P-T-t
tonalitic gneisses, metamorphic texture, P-T-t path, water activities,Eastern Hebei Province
