The petrology of the Elephant's Head dyke and New Amalfi sheet

Abstract

The following is a detailed study of two allied intrusions of Karroo dolerite, situated in the Matatiele district of East Griqualand (South Africa). The one intrusion is a multiple dike of exceptional form and great length. It served as the main feeder of the second intrusion, a thick transgressive sheet. Two magmas contributed to the formation of the dike. A first intrusion of tholeiite magma was followed by the injection of a large volume of olivine-dolerite magma. The tholeiitic phase is absent in the sheet, which only contains the cooled representatives of the olivine-dolerite magma. A considerable degree of magmatic differentiation in the latter magma has led to the production of a great variety of rock types. In the dike such types are represented by picrites and olivine-dolerites, whereas the dolerites of the sheet range from olivine-bearing types to rocks, rich in soda and iron. Such variability is found to be consistent with processes of differentiation, involving gravitational settling of olivine and marked crystal-fractionation. The dike is roofed, and its behaviour recalls the characters of the Cleveland dike in northern England. Evidence is forwarded, suggesting that the dike possesses a floor as well as a roof. Both the tholeiite- and the olivine-dolerite magma were very active in their behaviour towards the associated sediments. The tholeiite magma mobilised and reacted with the Molteno sediments of the dike-walls. In the sheet a blcok of Burghersdorp sandstone was metasomatised by emanations derived from Olivine-dolerite magma. As a result a well-defined band of pale granophyric rocks was produced. Chemical and mineralogical data are given and the inferences drawn are applied to the general problem of the mode of intrusion and the differentiation of the Karroo magma.