The diorites of Yzerfontein, Darling, Cape Province

ENGLISH ABSTRACT: The recent suggestion of the existence of a basaltic magma in the South-west Cape Province shortly before the intrusion of the Cape granites is corroborated by the occurrence of a gabbroic body of pre-granite age at Yzerfontein. A detailed study of the primary rhythmic banding and igneous lamination of this body indicates its origi nal form as being sheet-like or Iaccolithic, and the differentiation appears to have been due to a combination of fractional crystallisation and gravitative crystal settling on to a sub-horizontal floor . The crystal settling process was probably later a rrested by the viscosi ty of the rest magma, due to its enrichment in potash and alumina. The Yzerfontein diorites proper represent a hybrid product resulting from the mixi ng, in dept h, of the marginal contaminated facies of the Darling granite with gabbroic material. The present uniformity of the diorite is ascribed to the emplacement of the hybrid magma to a higher level. Gabbroic xenocrysts a re widely distributed in the diorites, these single crystals almost invariably being surrounded by rims of material later in the reaction series. Amongst the mafic minerals a uniform orientation relation is found to exist between the cores and rims of such reaction products. Two different types of granite aplite and pegmatite, namely potash-rich and soda-rich varieties, have invaded both the diorites and gabbros along joints corresponding in direction to the joint system of the Darling granite pluton, while swarms of veins and dykes containing late hydrothermal minerals, which probably represent the final stages of the magmatic history of the granites, follow similar directions.