The geology and geochemistry of the Rooi Rand Dyke Swarm.
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Date
1996
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Abstract
The Jurassic Rooi Rand dolerite dyke swarm was emplaced sub-parallel to the Lebombo
Monocline during the initial stages of Gondwana breakup. The dykes extend northwards
from the southern Lebombo region in northern KwaZulu-Natal, into central Swaziland,
spanning a distance of approximately 200 kilometres with a width between 10 and 22
kilometres. Detailed mapping of a 600m-Iong section on the Pongolo River, established at
least eleven phases of intrusion. Each dyke age was systematically sampled and analysed for
whole-rock major, trace and rare earth element composition, as well as mineral chemistry.
Selected samples were analysed for stable isotopes.
In addition to notable intra-dyke chemical variations, there is also a high degree of inter-dyke
mineralogical and geochemical variation, each dyke age bearing distinct geochemical
characteristics. The apparent geochemical trend is not one of simple fractionation with time.
Dyke chemistries are closely linked to magma genesis and magma volumes with time.
Evolution of the magmas may be described in terms of varying degrees of partial melting and
fractional crystallization, with a small degree of crustal contamination.
Major, trace and rare earth element data indicate a lithospheric mantle source for the
majority of dyke phases, and an asthenospheric source for only two of the eleven ages.
Contrary to this, isotopic data (oxygen and radiogenic) indicate an enriched asthenospheric
source for all the dolerites. This suggests that all ages may have originally been derived
from the asthenosphere, with the majority of ages being intruded into the lithospheric mantle
to later undergo partial melting and fractional crystallization, with some contamination.
Previous studies assumed an asthenospheric source with depleted MORB-like rare earth
element profiles to be representative for the majority of Rooi Rand dolerites.
The Rooi Rand dolerites appear to display a geochemical link with the southern Sabie River
Basalt Formation, as well as the Lebombo rhyolites. Magmatic evolution of the dykes was
intimately linked to the initial rifting processes of lithospheric stretching and asthenospheric
upwelling, which in this case concluded in a classic failed rift situation.
Description
Thesis (M.Sc.)-University of Natal, 1996.
Keywords
Geology--KwaZulu-Natal., Geochemistry--KwaZulu-Natal., Intrusions (Geology)--KwaZulu-Natal., Pottery--KwaZulu-Natal., Theses--Geology.