The structure and metamorphism of the Pewsey Vale area North - East of Williamstown, S.A.

Abstract

The structure and petrology of Upper Precambrian and Cambrian rocks have been studied in detail, in an area 38 miles north - east of Adelaide, South Australia. The rocks occur within a broad zone of high grade metamorphism on the eastern side of the Mt. Lofty Ranges. The Upper Precambrian succession consists predominantly of pelitic and semi - pelitic schists, quartzites, calc - silicate rocks and calc - schists, and the Cambrian sequence of quartzo - feldspathic schists, migmatites, granite gneiss, calc - silicate rocks and minor pelitic schists and quartzites. The rocks have reached the sillimanite grade of metamorphism and the metamorphism is of the low pressure - intermediate type. Dolerites, pegmatites, minor granodiorites and granites intrude the meta - sediments. Mineralogical and structural relationships of the granite gneiss, indicate that it has been formed by recrystalliaation of the quartzo - feldspathic schists. Small scale metamorphic differentiation, appears to have accompanied the recrystallization. The migmatites are believed to have been formed by metamorphic differentiation rather than by anatexis. Three phases of deformation are recognised in the Upper Precambrian rocks and two in the Cambrian. The second deformation recorded in the Upper Precambrian rocks does not appear in the Cambrian rocks. Each deformation has been accompanied by the formation of foliation. In the Proterozoic rocks deformed by the second and third phases of folding, the foliation is a crenulation cleavage. The deformations in both the Upper Proterozoic and Cambrian sequences are considered to be related. Petrofabric studies of quartz, scapolite and biotite are related to the respective macroscopic structures. An analysis of the chronology of crystallisation and deformation of these rocks indicates that crystallisation continued during and after each phase of deformation. Faulting commenced either prior to or during meta - morphism. Intense metasomatic activity followed a later phase of faulting resulting in the widespread development of albitites and in some cases talc ore bodies. The albitites formed in the fault zone were subsequently brecciated by further movement and later healed by the introduction of more metasoinatic fluid.