On Granitization and Associated Processes

Abstract

A review of the literature, and our own observations have led us to the conclusion that granitization, widely defined as the process by which solid rocks are converted to rocks of granitic character, has been an important process in the evolution of many subjacent masses of granite and allied rocks, such as granodiorite and diorite, in erogenic regions.Granitization of pre-existing rocks seems to involve two processes: (a) Metasomatism under the influence of permeating highly energized fluids—emanations—ahead of advancing magma; and (b) Mechanical penetration by magma. During the first process metasomatic exchanges take place between the minerals of the country-rock and its pore fluid: the reactions tend towards the achievement of equilibrium under the existing physico-chemical conditions. This metasomatism is selective, and promotes the convergence, in bulk and mineral composition, of rocks which were originally different, towards a common end-product. During the early stages conditions are essentially those of thermal metamorphism, but with rising energy level and increasing magmatic contributions the final conditions are dominantly “igneous”.The mechanical penetration of the altered rocks and their mixing with magma may follow the first process without a break. Alternatively, intrusion may occur before metasomatism is completed, or has even had appreciable effect. The mixing or intrusion stage is accompanied by reactions, which tend towards the attainment of equilibrium, under magmatic conditions, between the magma and the foreign material. In terms of bulk composition the products will be intermediate between that of the magma and that of the incorporated material.The metasomatism trends of the various examples that we have discussed are plotted together in Text-fig. 8. The diagram brings out the convergence of the curves from the normal sedimentary field to the rhyolite-diorite line, which is itself the locus of mixtures between metasomatized products and acid parental magmas.Granitic rocks may originate as the result of either or both these processes. That which predominates in any particular case depends on the supply of emanations and the local physico-chemical conditions. In the deeper parts of the granitic layer complete or selective fusion is to be expected, and the magmas so formed may rise and granitize the overlying rocks: firstly, by metasomatism, and at higher levels chiefly by mechanical penetration. At still higher levels, owing to loss of energy, rising magma is unable to cause extensive modification of the pre-existing rocks, and forms true intrusions.

Mineralogical and Structural Evolution of the Metamorphic Rocks

“…This, we believe, has often been the case; but exactly similar rocks (incipient migmatites) may also form under the influence of a percolating fluid magma of trondhjemitic composition. For discussion (in English) of opinion on granitization and the development of migmatites, the reader is referred especially to the papers of Eskola (1933), Barth (1936), Read (1939Read ( , 1943Read ( , 1944, MacGregor and Wilson (1939), and Grout (1941). Whether the products of these processes are strictly to be classified as metamorphic or in some other category, students of pre-Cambrian rocks unanimously recognize the existence of an intimate relationship between deep-seated regional (plutonic) metamorphism, migmatization, and the development of rocks of granitic character (whether granites or extreme products of granitization) on a vast scale in pre-Cambrian terranes.…”