冀西北麻粒岩相带富铝片麻岩的退变结构及其变质反应性质

Retrogressive textures and metamorphic reaction features of Al-rich gneisses in the granulite facies belt from northwestern Hebei province

冀西北麻粒岩相带中的富铝片麻岩（夹层）， 主要由石榴（紫苏） 黑云斜长片麻岩和夕线石榴黑云二长（钾长） 片麻岩组成。根据矿物相转变结构及其演化的详细研究， 确定富铝片麻岩存在石榴石与石英、钾长石、夕线石反应生成斜方辉石、堇青石、铝直闪石和黑云母等一系列复杂的变质反应。根据拓扑学、热力学及温压条件的研究结果， 并结合前人的实验资料， 确定富铝片麻岩所发生的这些反应均为特征的以减压为主的反应， 且发生于变质作用近等温减压阶段（Ｍ３）， 相应的变质温压条件： Ｔ＝ ７４０℃～８２０℃， Ｐ＝ ０．４１～０．６８ＧＰａ， 减压幅度达０．４０～０．５０ＧＰａ。研究表明该类反应及相应的矿物相转变不仅受控于温压条件，而且与体系局部水流体介入及其不均一性存在密切的成因关系。

The Al rich gneisses, as interlayers existing in the granulite facies belt from northwestern Hebei province, are predominantly comprised of garnet orthropyroxene biotite plagioclase gneiss and sillimanite garnet biotite two feldspar (potash feldspar) gneiss According to the characteristics of mineral phase transitional textures, a series of complicated metamorphic reactions, such as garnet, sillimanite, quartz and potash feldspar assemblage transforming to orthopyroxene, cordierite, gedrite and biotite, are recognized in the Al rich gneisses By the data of topology, thermodynamics and PT condition, together with previous experimental data, a series of reactions in Al rich gneisses belong to the typical decompression reactions, and occur at the near isothermal decompression stage (M 3) ( T =740℃~820℃, P=0 41~0 68 GPa), and the extent of decompression is up to 0 40~0 50GPa These typical metamorphic reactions are not only controlled by PT condition, but also genetically associated with no heterogeneous water fluid in the system This result may have an important significance for an in depth study of the genetic mechanism and metamorphic dynamic process of the Al rich gneisses in granulite facies belt from Northwestern Hebei province