Origin of Granites

Abstract

Field features and petrographic criteria for granitic rocks of magmatic origin are listed, and a brief discussion is presented concerning the mode of emplacement and the differentiation of intrusive masses.Transitional facies of granitic rocks of metamorphic origin are described in detail. These facies include replacement dikes, replacement breccias, and metamorphic migmatites. Interpretations of mechanisms of the advance of granitization including geochemical migrations are discussed, not only from the detailed evidence shown by miniature features, but also with regard to the relationship of metamorphic granitic rocks to regional metamorphism. Some aplites,_ pegmatites, and_ quartz veins are explained as retrogressive features following granitization. Granitic rocks which exhibit both crystalloblastic textures and marked flow structure are interpreted as examples of rheomorphism, and the term neomagma instead of magma is used for the material that flowed.The use of large lantern-slide (3J X 4J inches) thin sections has proved to be most helpful in the study of critical features of either magmatic or metamorphic granitic rocks. In these large sections, microtextures and microstructures can be observed much better than in an ordinary thin section, or a number of thin sections, and an integrated overall picture can be obtained.