The petrology of two granite-slate contacts at Cape Town, South Africa

Abstract

Summary The porphyritic biotite-granite of the Cape Peninsula is intrusive into the tightly folded pre-Devonian Malmesbury Series which consists of psammitic hornfelses with subordinate politic intercalations. At two localities within the Cape Town municipal boundary, namely Sea Point and Kloof Quarry, the contact aureole is well exposed and includes a zone of spectacular migmatites. With the aid of 14 new chemical analyses it is shown that the contact phenomena probably arose in four main stages:— (i) Intrusion and differentiation of a contaminated biotite-granite magma, (ii) Permeation and softening of the Malmesbury hornfelses by alkaline emanations from the granite, (iii) Penetration of the hornfelses by granitic material with the formation of migmatites. (iv) Felspathization of the granite and migmatite zone by late potassic solutions which gave rise to potash pegmatites and large crystals of perthitic micro-cline. The evidence for the formation of an Mg-Fe front outside the migmatite zone is inconclusive.