The diorites of Yzerfontein, Darling, Cape Province
Date
1951-12
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
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.
AFRIKAANSE OPSOMMING: Geen Afrikaanse opsomming beskikbaar nie
AFRIKAANSE OPSOMMING: Geen Afrikaanse opsomming beskikbaar nie
Description
Thesis (MSc)--Stellenbosch University, 1951.
This paper was awarded the Corstorphine Medal and the first prize of the Geological Society of South Africa for the year 1951.
This paper was awarded the Corstorphine Medal and the first prize of the Geological Society of South Africa for the year 1951.
Keywords
Diorite -- South Africa -- Yzerfontein, Geology -- South Africa -- Yzerfontein, Igneous rocks -- South Africa -- Yzerfontein, UCTD