Carbonate-bearing eruptives between the Great Karas Mountains and the Bremen igneous complex, South West Africa
Date
1975-12
Authors
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT:Minor carbonate-bearing bodies of igneous origin are widely distributed in the Karasberg district, South West Africa. The area in which they occur consists of granites and gneisses of the Namaqualand Metamorphic Complex overlain
by relatively flat strata of the Nama and Karoo Groups
and intruded by plutonic and hypabyssal rocks of various ages.
The latter include a single post-Karoo carbonatite.
The largest concentration of carbonate-bearing eruptives is on the farm Garub 266 in the Great Karas Mountains, but they extend approximately 100 km westwards as far as the Bremen Alkaline Complex. The Karas Mountains are now believed to be the result of a series of thrust-faults which may perhaps be associated with the intrusion of plutonic complexes of the Kuboos-Tatasberg-Bremen-Haruchas lineament, to which the Garub eruptives may also be related.
The Garub-type pipes, dykes and sills are composed of
alkaline-ultrabasic carbonate-bearing breccia, lamprophyric carbonate rock and tuffisite. They intrude rocks of the Namaqualand Metamorphic Complex and the Kuibis, Schwarzrand and lower Fish River Formations of the Nama Group, and are considered to be subvolcanic.
The bodies contain between 10 and 20 per cent co2 and about 25 per cent Si02 and are obviously not typical carbonatite. Biotite, pyroxene, amphibole and ilmenite form both phenocrysts and fine-grained ccy.stals in a groundmass of ankerite. Minute ankeritised lath-shaped crystals (either melilite or feldspar originally}, are almost invariably present in the lamprophyric carbona~e rock. Interstitial
quartz and feldspar occur sporadically, whereas inclusions of wall-rock, where present, are usually abraded and rounded. Fenitisation has been observed at two localities, where quartz and feldspar in the wall-rock have been replaced by soda amphibole.
Fluidisation appears to provide a satisfactory
mechanism for the emplacement of these bodies. Strong evidence in favour of this interpretation are the intrusive contacts, abraded and rounded inclusions, nondilational veins in the wall-rock, accretionary pisolites, upward and downward movement of inclusions in the bodies, carbonated inclusions and matrices . and the absence of contact or pyrometamorphic effects.
Chemically the carbonate rocks bear similarities to
kimberlite and olivine-melilitite, but are most akin to
alnoite. Carbonatite, olivine-melilitite, kimberlite,
alnoite, damkjernite and the Garub rocks are all considered to have the same magmatic affinities.ยท It is tentatively suggested that the Garub suite is genetically related to an unexposed alkaline complex of the Fen type, and that the composition of the carbonate-bearing rocks approaches that of the parent magma of the plutonic complexes along the Kuboos lineament.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar
Description
Thesis (MSc)--Stellenbosch University, 1975.
Maps in multiple parts on last pages of pdf document.
Maps in multiple parts on last pages of pdf document.
Keywords
Carbonate rocks, Igneous rocks, Geology -- Namibia -- Great Karas Mountains