Abstract
D.Phil.
The tectonic history of the Kheis Terrane and its relationship with the Namaqua-Natal
Metamorphic Province (NNMP) along the western margin of the Kaapvaal Craton were
the focus of this study. Major issues addressed in this study are the origin and timing of
formation of the Kheis Terrane and the recognition and definition of terrane boundaries in
the area. Results of detailed measured sections across the Kheis Terrane, heavy mineral
provenance studies, 40Ar/39Ar analyses of metamorphic muscovite, U-Pb SHRIMP dating
of detrital zircon grains from 12 samples from the Kheis- and Kakamas Terranes and one
igneous body from the Kakamas Terrane are presented.
A new stratigraphic unit, the Keis Supergroup, comprising the Olifantshoek-,
Groblershoop- and Wilgenhoutsdrif Groups, is defined. The base of the Keis Supergroup
is taken at the basal conglomerate of the Neylan Formation. The Mapedi- and Lucknow
Formations, previously considered part of the Olifantshoek Group, are now incorporated
into the underlying Transvaal Supergroup. The Dabep Fault was found not to represent a
terrane boundary. Rather, the Blackridge Thrust represents the boundary between the
rocks of the Kheis Terrane and the Kaapvaal Craton.
Provenance studies indicate that the rocks of the Keis Supergroup were deposited along a
passive continental margin on the western side of the Kaapvaal-Zimbabwe Craton with
the detritus derived from a cratonic interior. Detrital zircon grains from the rocks of the
Keis Supergroup of the Kheis Terrane all gave similar detrital zircon age populations of
~1800Ma to ~2300Ma and ~2500Ma to ~2700Ma. The Kaapvaal Craton most probably
never acted as a major source area for the rocks of the Keis Supergroup because of the
lack of Paleo- to Mesoarchean zircon populations in the Keis Supergroup. Most of the
detrital zircon grains incorporated into the Keis Supergroup were derived from the
Magondi- and Limpopo Belts and the Zimbabwe Craton to the northeast of the Keis
basin.
The rock of the Kakamas Terrane was derived from a totally different source area with
ages of ~1100Ma to ~1500Ma and ~1700Ma to ~1900Ma which were derived from the
Richtersveld- and Bushmanland Terranes as well as the ~1166Ma old granitic gneisses ofthe Kakamas Terrane. Therefore the rocks of the Kheis- and Kakamas Terranes were
separated from each other during their deposition.
Detrital zircon populations from the Sprigg Formation indicate that it this unit was
deposited after the amalgamation of the Kheis- and Kakamas Terranes and therefore does
not belong to the Areachap Group.
Results provide clear evidence for a tectonic model characterised by the presence of at
least two Wilson cycles that affeected the western margin of the Kaapvaal Craton in the
interval between the extrusion of the Hartley lavas at 1.93Ga and the collision with the
Richtersveld tectonic domain at ~1.13Ga.
According to the revised plate tectonic model for the western margin of the Kaapvaal-
Zimbabwe Craton, the Neylan Formation represents the initiation of the first Wilson
Cycle, with rifting at ~1927Ma ago, on the western margin of the Kaapvaal-Zimbabwe
Craton. The metasedimentary rocks of the Olifantshoek Group were deposited in a
braided river environment which gradually changed into a shallow marine environment
towards the top of the Olifantshoek Group in the Top Dog Formation. The
metasedimentary rocks of the Groblershoop Group were deposited in a shallow, passive
or trailing continental margin on the western side of the Kaapvaal-Zimbabwe Craton.
The rocks of the Wilgenhoutsdrif Group overlie the Groblershoop Group unconformably.
This unconformity is related to crustal warping as a volcanic arc, represented by the
metavolcanics of the Areachap Group, approached the Kaapvaal-Zimbabwe Craton from
the west. The rocks of the Keis Supergroup were deformed into the Kheis Terrane during
the collision of the Kaapvaal-Zimbabwe Craton, Areachap Arc and the Kgalagadi Terrane
to form the Kaapvaal-Zimbabwe-Kgalagadi Craton. This event took place sometime
between 1290Ma, the age of deformed granites in the Kheis Terrane and 1172Ma, the
initiation of rifting represented by the Koras Group. This is supported by 40Ar/39Ar
analyses of metamorphic muscovite from the Kheis Terrane that did not provide any
evidence for a ~1.8Ga old Kheis orogeny (an age commonly suggested in the past for this
orogeny). This collisional event resulted in the deformation of the rocks of the Keis
Supergroup into the Kheis Terrane sometime between 1290Ma and 1172Ma.The second Wilson cycle was initiated during rifting along the Koras-Sinclair-Ghanzi rift
on the Kaapvaal-Zimbabwe-Kgalagadi Craton at ~1172Ma. It was followed soon after by
the initiation of subduction underneath the Richtersveld cratonic fragment at ~1166Ma
after which the rocks of the Korannaland Group were deposited. The closure of the
oceanic basin between the Kaapvaal-Zimbabwe-Kgalagadi Craton and the Richtersveld
cratonic fragment occurred about 50Ma later (~1113Ma, the age of neomorphic
muscovite in the metasedimentary rocks of the Kakamas Terrane) and resulted in the
large open folds characterising the Kheis terrane and NNMP.
Detrital zircon populations in the Sprigg Formation show that this formation does not
belong to the Areachap Group and that it was deposited after the closure of the oceanic
basin between the Kaapvaal-Zimbabwe-Kgalagadi Craton and the Richtersveld cratonic
fragment at ~1113Ma.
The Areachap Group can be extended towards the north and into Botswana along the
Kalahari line where it forms the boundary between the Kaapvaal-Zimbabwe Craton to its
east and the Kgalagadi Terrane to its west. The Areachap Terrane is thus related to the
collision of the Kaapvaal-Zimbabwe Craton and Kgalagadi Terrane and was deformed a
second time during the oblique collision of the Richtersveld cratonic fragment with the
combined Kaapvaal-Zimbabwe-Kgalagadi Craton. The extension of the Areachap Group
to the north along the Kalahari line opens up new exploration prospects for Coppertontype
massive sulphide deposits underneath the Kalahari sand.