区域变质作用中的流体

区域变质条件下流体的流动有 4种标志 :( 1)细脉 ;( 2 )岩石学 ;( 3 )稳定同位素 ;( 4 )常量元素的交代作用。不同级别的区域变质作用中 ,流体影响着岩石的变质反应和变形 ;在高级变质的情况下甚至有熔体出现。在超高压变质条件下 ,流体量比地壳范围区域变质要少得多 ,从大别山超高压变质带的资料可知 ,流体的演化有明显的阶段性 ,局部曾发现熔融包裹体。水流体的介入 ,引起岩石的退变质和元素地球化学变异 ,是超高压变质岩抬升、进入中下地壳的产物。新近的实验岩石学成果说明 ,多硅白云母、角闪石等含羟基的矿物 ,在俯冲达 10 0km以下依然稳定 ,而一些花岗岩体系在超高压的条件下产生的超临界流体 ,乃是花岗岩、片麻岩只能部分保留超高压矿物组合的原因。

THE ROLE OF FLUID FLOW DURING REGIONAL METAMORPHISM

There are 4 lines of evidences for fluid flow during the regional metamorphism , respectively:(1)veins;(2)petrology;(3)stable isotopes;(4)metasomatism of major elements. Mobile fluids play an essential role in rock deformation and metamorphic reactions during the various grades of metamorphism. In some of the high-grade metamorphism even partial melting can occur. During the ultrahigh-pressure metamorphism fluids are scarce, however, the study of the Dabieshan UHP metamorphic belt had noticed that the evolution of the metamorphic fluids was obviously periodical. Melt inclusions within UHP metamorphic minerals have been reported . The influx of aqueous fluid had caused a series of retrogressive processes and the geochemical variation of major elements, which clearly depicts the uplifting of the UHP metamorphic rocks into the mid-lower crust. Recent results of experimental petrology show that the (OH)-bearing minerals , such as phengite and amphiboles are stable even deep-subducted to more than -100 km. The experiment of granite system reveals the presence of supercritical fluids in the UHP metamorphic conditions. Their evolution provides an explanation for the incomplete preservation of the UHP metamorphic assemblages in the deep-subducted granites and gneisses.