On the Relationship between “Fronts” of Regional Metamorphism and “Fronts” of Granitization

Extract

Two important papers, relating to “fronts” of metamorphism, appeared in the Bull. Soc. Géol. France during the war years, and may, in consequence, have escaped the notice of many petrologists. The contents of both papers are so closely bound up with the important subject of granitization as to render their review desirable. In the first of these papers, Perrin and Roubault (1941) describe an apparently unconformable boundary between a Triassic conglomerate and an underlying schist. The Triassic conglomerate consists of red quartzite pebbles in a sandy matrix, whilst the schist is a normal low-grade sericite-chlorite-schist of a green colour. Detailed examination of the contact, however, shows that the relations between the two rock types are by no means so straightforward as a cursory examination suggests. Although the contact between the conglomerate and the schist is generally sharp, a close scrutiny establishes the following local but remarkable apparent anomalies: (1) “Xenoliths” of the conglomerate, and isolated pebbles from it, occur within the schist. (2) “Veins” and pods of the schist occur within the Triassic conglomerate. (3) Gradational contacts are sometimes found between the schist and the Triassic sandstone, the schist facies dying out within the space of a few centimetres. On the basis of these observations Perrin and Roubault reach the inevitable conclusion that the metamorphism which gave rise to the schist actually post-dated the deposition of the conglomerate.