西拉木伦断裂带双井微地块北缘混合岩的基本特征及其成因机制讨论

CHARACTERISTICS AND GENESIS OF MIGMATITES WITHIN THE NORTHERN SHUANGJING MICRO-PLATE IN THE WEST LAMULUN FAULT ZONE,INNER-MONGOLIA PROVINCE

西拉木伦断裂带双井微地块北部边缘出露有以条带状为主的混合岩,岩石中中色体-暗色体-浅色体明显分带,岩相学特征显示暗色体和浅色体经历了较高温的交代变质作用。对该混合岩浅色体的矿物空间分布分析显示浅色体重结晶现象显著,新生石英、长石聚集分布,是交代或变质分异的结果。质量平衡计算结果表明本区混合岩形成于开放体系,有物质的带进带出。对中色体、暗色体、浅色体进行单独的地球化学分析,结果显示暗色体与中色体元素组成具有相似性,而浅色体与中色体的元素组成差异较大。电子探针数据显示浅色体中斜长石的An牌号低于暗色体,中色体中角闪石压力计计算平均压力为0.81 GPa,钙质角闪石-斜长石相平衡温度计求得混合岩形成温度为584℃。综合分析认为该混合岩在成因上早期以岩浆注入为主导,晚期则以高温交代为主导。混合岩的岩浆注入成因可能意味着混合岩的形成与区域的岩浆活动有关。

Typical migmatites within the northern Shuangjing micro-plate in west Lamulun fault zone of Inner-mongolia, were characterized by obvious alternated bands of mesosome-melanosome-leucosome. Petrographic features showed that melanosome and leucosome in the migmatites experienced metasomatic metamorphism at a relatively high temperature. Study of mineral spatial distribution in the leucosome revealed that these minerals were subjected to obvious recrystallization, demonstrated by the concentrated distribution of newly formed quartz and feldspar, which were resulted from the metasomatism or metamorphic differentiation. The mass-balance calculation showed that the migmatites were formed in open-system, and involved in the addition and dispersion of components. Individual geochemistry studies of mesosome, melanosome and leucosome indicated that the component elements between mesosome and melanosome were similar and those between mesosome and leucosome were different. Electron probe analysis data revealed that the ordinal number of An of feldspar in leucosome was lower than that in melanosome. Geobarometer calculated from amphibole in the mesosome obtained an average pressure of 0.81 GPa and equilibria geothermometer by calcic amphibole-plagioclase obtained a forming temperature of 584℃. The research proved that the migmatites in the area were induced by magma injection in the early-stage, followed by mechanism of metasomatic reactions in the late stage. The magma injection mechanism indicated the formation of migmatites was related to the magma activity in the area.