孔兹岩系内棱柱状夕线石的成因

THE ORIGIN OF PRISMATIC SILLIMANITE IN THE KHONDALITE SERIES

对我国鄂西及黑龙江省东部孔兹岩系的地质观察发现,含棱柱状夕线石的高铝夕线片岩类与含红柱石的云母片岩类岩石在区域上共生。系统的岩相学研究表明,这些夕线石系由叶蜡石直接转变所成。原岩恢复以及对变质作用过程中可能发生的变质反应的理论探讨和原始沉积物理想组分计算的结果都支持上述结论。认为孔兹岩系中的棱柱状夕线石和红柱石系低角闪岩相条件下在不同的原岩中经不同的变质反应形成。它们的形成主要受原岩的成分控制。这种由叶蜡石直接分解形成的夕线石,其初始形成温度较低,不能作为划分高角闪岩相的标志。

Detailed field investigations revealed that prismatic sillimanite-bearing high-A1 rocks are intercalated with andalusite-bearing mica schists and gneisses in the khondalite series, western Hubei and eastern Heilongjiang provinces of China. Petrographic examination under the microscope showed that this kind of sillimanite is formed directly from the decomposition of pyrophyllite. Protolith restoration, theoretical analysis for the reactions involved in the process of metamorphism, and calculations all support the above conclusion. The author treated the prismatic sillimanite and andalusite in the khondalite series as the products formed under the same condition of low amphibolite facies and believed that the formation of these minerals is controlled mainly by their protoliths. Hence, sillimanite of such origin was formed under relatively low temperature conditions, and can not be treated as an index mineral of high amphibolite facies.