怀安-大同一带基性麻粒岩变质反应序列与水活度

The Metamorphic Reaction and Water Activity of Basic Granulite in the Datong Huaian Region

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.