Attention is directed to those parts of the crust where periods of greatest mobility-coincided with the appearance of granitic material. Especial consideration is given to the deeper parts of mountain belts where thick sedimentary piles were laid down on an older crystalline basement. It is believed that the site of the geosynclinal trough was primarily determined by inward movement of parts of this basement. At later periods, during and after the principal deformation of the mountain belt, the structural evolution was accompanied by the emplacement of granitic rocks of diverse forms and kinds. This enquiry therefore has two main aspects—the one concerned with mobility connected with the appearance of granite in the basements, the other with the appearance of granite connected with mobility in the geosynclines.
Worldwide studies, initiated by Wegmann and others, have shown how the styles of deformation in a basement give information about its state. During orogeny, the basement acquires a new structure and locally becomes plastic. The development of this state is associated with the appearance of diffuse granitic material of a definite quality. The most granitized parts are found to be the most mobile, and certain characteristic patterns of deformation can be established. The structural changes at the contacts of old basements and the newer sediments are exemplified by Eskola’s gneiss domes of Finland and by comparable examples elsewhere.
Two problems now arise. The first is the relation of deformation at depth, best studied in the old orogenic belts, to that displayed in the less deeply eroded younger mountain chains. The second problem concerns the relations of the granitic rocks present at different levels in a deformed belt—that is, the Granite Series and its nature and validity. A survey shows that pre-orogenic sediments are often regionally metamorphosed, migmatized and granitized; the synorogenic sediments are less often so affected but are the places of granitic intrusion; the post-orogenic sediments are never migmatized and enclose small granites with a special mode of emplacement. The granitic rocks produced vary in form and composition with time and place. They pass from early diffuse granitization granites, dominantly sodic, to late circumscribed and intrusive granites, dominantly potassic—a variation related to the evolution of the orogenic belt.
As a consequence of this review, tentative proposals involving the derivation of granitizing material from the sima, its activation during orogeny, and its penetration into the fold belts are advanced. These operations are held to be all related to deep simatic movements.
