Metamorfose : die sleutel tot die herkenning van oer gebiede van bergbouing

Inaugural lecture--Department of Geology, Rand Afrikaans University, 19 May 1988In the last twenty years, the study of metamorphism has undergone a major shift of emphasis: from a descriptive to a more quantitative approach aimed at identifying the intensive thermodynamic variables (P, T and X) which controlled the process of metamorphism. The quantitative study of metamorphism comprises experimental phase-chemical studies, thermodynamic calculations and the study of metamorphic mineral assemblages by means of the electron microprobe. Consequently, the metamorphic petrologist today is capable of calculating accurately the distribution of temperature and pressure in the earth's crust with time within approximately 50°C and a few hundred bars. This makes it possible for him to examine, from a quantitative point of view, the continuous adjustment (that is, the submersion, heating and consequent elevation, erosion and cooling = PoT-time loops) of rock masses under changing physical conditions. Since the mineralogical and chemical composition, fabric and field-relations of metamorphic rock formations are a reflection of the underlying geological processes which played a role in the evolution of the earth's crust, metamorphism may thus be regarded as the key to identifying primordial areas of mountain formation. The chemographic analysis of phase-relations in metamorphic rock formations and the identification of continuous and discontinuous metamorphic reactions form the basis for the construction of petrogenetic diagrams by means of which different p-T loops may be deduced. These loops are then used to make deductions concerning the underlying geological processes. The study of metamorphism is, however, not simply an interesting geological exercise, but the information gained from this type of examination plays an important role in the establishment of models of exploration which are used by mining companies to locate ore deposits in complex metamorphic areas