西藏东部和南部变质岩中的白云母及其岩石学意义

变质岩研究工作中一个重要课题是如何确定它的P-T条件,而关键的问题是搞清楚不同变质带的压力类型(或相系)。许多研究工作指出,变质岩中自云母-绿鳞石固溶体成分以及白云母晶胞参数中的b_0和c_0均随环境条件的影响而改变,可以用来作为变质作用温度或压力的指示剂。

MUSCOVITE OF METAMORPHIC ROCKS IN EAST AND SOUTH XIZANG AND ITS PETROLOGICAL SIGNIFICANCE

In this paper, the authors deal with nine samples of muscovite collected from Xizang, China. The X-ray powder diffraction pattern of the muscovite indicates that its b0 value varies with the metamorphic pressure, i.e. the higher the pressure is, the larger the b0 value, is, thereby it can serve as an indicator of metamorphic pressure. However, d002 decreases with the increase of pressure. Obviously, the high pressure condition favours the formation of muscovite with small basal spacing and large horizontal spacing. Likewies, the mineral composition of muscovite can be also correlated to a certain extent with the metamorphic pressure. Especially, MgO in the muscovite may be taken as a good indicator of metamorphic pressure. The authors suggest that two types of original rocks series, i.e. metapelites and metabasites, can be recognized. Therefore, diagrams MgO vs. RM and Si vs. Mg of muscovite-phengite series of greenschist-glaucophane schist facies were constructed by the present authors and are presented in this paper. Two areas, medium pressure facies series areas and high pressure facies series areas, are distinguished herein. The latter can be further subdivided into Area I which contains glaucophane assemblages and Area II which contains no glaucophane assemblages. The samples from Yarlung Zangbo Jiang metamorphic belt in Xizang fall into the field of area II.