The geology, structure and petrology of the alkaline intrusions of Kalkfeld and Okorusu and the invaded Damara rocks

Abstract

Abstract provided by author:

The origin of the acid lavas and oversaturated alkaline plutons of late-Karroo age is sought primarily in the assimilation of Salem granite by the invading basaltic Karroo magma. The development of small local reservoirs of alkaline undersaturated magma in the crust is ascribed to the reaction of the thus generated acid magma with Damara limestone. It is concluded that the generation of alkaline undersaturated magma is restricted to areas of regional negative diastrophism. Variations in the relative proportions of potash, soda, alumina and ferric iron components of the alkaline intrusives are ascribed to the differential crystallisation of an already undersaturated magma. Damara limestone assimilated in slight excess of the amount necessary for the desilication of the intrusive granite magma and held in solution by the CO2, and volatile enriched residual magmatic fluids, ultimately became concentrated in the central parts of the vents to form the plugs of intrusive carbonatite

It is believed that the salient characteristics displayed by alkaline under-saturated intrusives and their associated carbonatites can more readily be accounted for by a modified version of the Daly-Shand hypothesis than by any one of the newly propounded theories

An analysis of the geological history since the deposition of the Damara sediments reveals that just as the Salem granite is intruded mainly into the area of maximum crustal sinking of the Damara orogen so also is the major group of late-Karroo alkaline intrusives confined to this area. A second minor group of alkaline intrusives is found to the north-east. Early basic intrusives are associated with the first mentioned group and under-saturated alkaline rocks and carbonatites largely with the second group