有关混合岩和混合岩化作用的一些问题——对半个世纪以来某些基本认识的回顾

本文首先论述了混合岩和混合岩化作用的一些基本特征(四大特征)。提出了混合岩和有关岩石的混合岩化程度的分类,即三分为部分混合岩化的岩石、混合岩和混合花岗质岩石三大类。论述了确定混合化岩石下界的五个综合标志及混合花岗质岩石的八大特征等,还讨论了部分混合岩化岩石的命名问题。最后从混合岩化作用的地质背景将其分为三类:区域性混合岩化作用、边缘混合岩化作用及断裂带混合岩化作用,并对各自的特征进行了讨论。

ON MIGMATITES AND MIGMATIZATION--HALF A CENTURY'S RECOLLECTION OF CERTAIN RELATED PROBLEMS

The collective term migmatites or migmatitic rocks includes various types of "mixed rocks" derived essentilly in situ from pre-existing rocks, mostly already metamorphosed, by anatexis (melting or partial melting) and/or metasomatism. The final products of migmatization are graniticrocks or granitiods which are usually known as migmatitic granitics or granitoids if consolidated in place, but are taken as intrusive rocks when the "migma" has been moved away, for various distances, from the site of its origination and crystallized elsewhere, these being roughly corresponding to the "para-autochtonous granites" as well as" intrusive magmatic granites" of H. H. Reads.Jt has been suggested that the migmatites and related rocks should be considered as an entity and grouped together under the collective term of "migmatitic series" (Cheng Yuqi, 1965). This series may be appropriately classified according to their increasing degree of migmatization into three groups: 1, migmatized (or partly migmatized) rocks; 2, migmatites; and 3, migmatitic granitics (granitoids). Preliminary comprehensive criteria based on both megascopic and microscopic characters of rocks concerned, with due gpnsideration of the geological setting of their formation, have been suggested in this paper to delimit these three categories and to differentiate the migmatized rock from the original one. The appearance in the rocks concerned of the first batch of quartzo-feldspathic spots, veinlets, or patches of metasoma-tic or anatectic origin and the often concomitant slight decrease in the par-centage of An in the newly formed plagloclose as compared with that of the same mineral in the "protoliths" are taken as first signs of their entry into the realm of migmatization, and this can be used to delimit the migmatized rock from the original one. The criteria for differentiating the migmatites from the partly migmatized rocks of metasomatic origin are the increase in the total amount of feldspar (plagioclase and/or K-feldspar) >5-10%, and the decrease in the percentage of An in the (newly formed) plagioclase in the migmatites as compared with those of the corresponding minerals in the protoliths, the abundance of micrdscopic replacing features such as sutured, checker-board, myrmekitic, metasomato-perthitic, metasomato-antiperthitic textures and clear-rim to the sericitized oligoclase in certain types of the rocks affected by migmatization, such as leptitic rocks and hornblende (bioti-te) -feldspar-gneisses, etc. The characteristics of the migmatitic granitics as compared with the migmatites are the practical elimination of the trace of original bedding of the sedimentogenous or volcano-sedimentogenous protoliths and also that of any type of foliation planes of the protolithic metamorphic rocks, and the concomitant appearance of massive structure as defined by more than two sets of joints in different orientations, the possible presence of faint zoning in the metasomatic granitic dikes as a result of crystallization of the migma, and above all, the almost complete reconstitution both mineralo-gically and petrographically. The absolute dominance of the microscopic replacing textures in the thin slides of the metasomatic migmatitic granitoids is very characteristic. It is equally striking that the textural relationship between the rock-forming minerals of the anatectic granitics as observed under the microscope are similar to the order of crystallization of the corresponding minerals in the "normal" magmatic granitoids. The key, or the basic principle, for the delimitation of the three groups of the migmatitic series and also their differentiation from the unmigmatized rocks is a series of comprehensive comparative study.