河北平山小觉地区阜平岩群中三种岩类深熔现象的岩相学特征

Petrographic Characteristics of Three Anatectically Transformed Metamorphic Rock Types of the Fuping Group--Complex of the Xiaojue Region, Pingshan, Hebei

深熔作用(Anatexis)引起了地壳内部固体物质的重溶与熔融和进一步的初始花岗质岩浆的形成。本文主要从岩相学角度对河北平山阜平岩群3种主要变质岩类中一部分岩石在薄片中所显示的初期深熔特征进行了报道,提出识别深熔作用发生发展初期的一些岩相学标志,为相应的理论探讨提供岩相学方面的依据。区域变质岩石初始深熔的共同特征之一是不同矿物粒度都较相应的同类变质岩石为粗。由于经历粒间流体作用的重溶、溶蚀乃至再度晶出的过程,长英质岩石中长石、石英等矿物改变为具有弯曲、波纹状外形的缝合线结构,而“铰链状结构”的形成则是两微斜长石颗粒之间出现钠交代的结果。深熔作用过程中微斜长石显示出不均匀的光性特征,形成具交代成因的条纹长石细脉和条痕;斜长石出现不规则长方形和补丁状的微斜长石反条纹长石,显示“糟化”或绢云母化,或存在不同程度的“暗化”区域,也可转变成为纤维状至细柱状夕线石,沿斜长石中的一些细小裂纹和遭受应变的解理还出现几乎不透明的物质,它们可能原由深熔作用所形成,可称之为“网格状结构”;黑云母可部分地发生分解,在适当的温压条件下则完全分解,形成原可能为熔体的物质,也可转变成为纤维状至细柱状夕线石;角闪石存在一定程度的“暗化”,并形成与斜长石中所见类似的“

The general sign of anatectic transformation of the metamorphic rocks is the overall increase in grain size of their mineral constituents. In slides the quartzo-feldspathic rocks are mostly characterized by the presence of re-sorbed feldspar and quartz with sutured and wavy outlines (i. e. the appearance of 'sutured texture') and also ' hinge texture' of Na-replacement origin at the contact of two microclines as a result of circulation of intergran-ular anatectic fluids. For individual minerals, microcline may exhibit heterogeneous optical properties and contain perthitic veins and streaks of replacement origin. Plagioclase may contain somewhat irregularly rectangular to patchy microclinic antiperthites, or exhibit very confusedly sericitized or somewhat 'darkened' areas. It may have been transformed to fibrolite and fine sillimanite or to an almost opaque substance along small fractures and strained cleavages, thus giving to a 'network texture' (P1. Ⅰ-3,8; Ⅲ -20). Biotite may be partly 'disintegrated' or completely transformed to what was probably a melt formed under high appropriate pressure and temperature conditions, or to fibrolite or fine sillimanite. Hornblende may be somewhat 'darkened'. Similar to plagio-clase, it may also exhibit a 'network texture' (P1. Ⅰ -3; P1. Ⅰ -10), or has been transformed to clinopyrox-ene. Most of the petrographic features mentioned above are those of the rocks that show characteristics of the initial or early stage of anatexis. Some of them have also been observed in the slides of the samples of the melting experiments of relevent metamorphic rocks without showing any sign of anatexis.