Plutonic Metamorphism and Its Products

Abstract

By plutonic metamorphism is meant the changes which are produced in rocks by great heat and uniform pressure. These changes necessarily take place in the kata-zone of Grubenmann, at depths wherein directed pressure becomes less and less pronounced, and finally becomes a practically negligible factor in metamorphism. The high temperature which is also a characteristic of this depth-zone is maintained by the natural increase of heat in depth due to the temperature gradient, and by magmatic heat. Geological study of deeply-eroded regions of the earth’s crust shows that the lower levels are almost everywhere penetrated by igneous intrusions on a far greater scale than the upper parts of the crust; and as these lower levels in general consist of the oldest rocks, it is the Archæan basement (the Grundgebirge of German and Scandinavian geologists) which most often shows the effects of plutonic metamorphism. As the regional intrusion of magma, especially magma of granitic composition, is a character of great depth, the problems of plutonic metamorphism are more or less closely connected with the problems of the soaking of rocks in magmatic emanations, their wholesale injection by igneous veins and sheets, and their final melting and incorporation within the magma.