The evolution of the proterozoic gneisses and other metamorphites between Springbok and Vioolsdrif, South Africa
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Date
1988-01
Authors
Van Aswegen, Gerhard
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
This study covers aspects of the stratigraphy, structure and
metamorphism of part of the Proterozoic Namaqua Metamorphic
Complex in northern Namaqualand, between Vioolsdrif in the north, and the area around Springbok in the south. The investigation
formed part of the Namaqua Geotraverse, National Geodynamics
Project (Blignault et al., 1983). The study area is subdivided
into four thrust bounded tectonic domains. The Richtersveld Domain
in the north is seperated from the centrally situated Steinkopf
Domain by the Transition Zone. The Copper District occurs in the
south while the Geselskapbank Domain represents a small easterly
extension of the study area. The latter domain was not part of the
original geotraverse, but a brief study of the metamorphites
provided important insights into the tectonic development of the
region. The stratigraphy of this domain is not discussed. The
major Steinkopf fault juxtaposes the Nama Group (+/- 600 Ma) and
the Proterozoic rocks and forms a natural western boundary tor the
study area.
The Namaqua Metamorphic Complex is the product of two orogenies,
the Orange River orogeny (1730 - 2000 Ma) and the Namaqua orogeny
(1000 1300 Ma). The stratigraphy of the supracrustal rocks are
possibly related to the Orange River orogeny, while most of the
observable metamorphic and structural features are the products of
the Namaqua orogeny.
The supracrustal rocks of the Namaqua Metamorphic Complex are
considered to belong to a single Group, the Bushmanland Group,
subdivided into the Haib, Eenriet and Khurisberg Subgroups. The
Haib Subgroup in the study area consists of a lower felsic unit
(Tsams Formation), which straddles the boundary between the
Richtersveld Domain and the Transition Zone, and the more mafic
Nous Formation, the latter being confined to the Richtersveld
Domain. The Eenriet Subgroup comprises three spatially seperated
metasedimentary units, namely the GroothoeI< Formation (mainly mica
schists, confined to the Transition Zone), the Kabina Formation
(metaquartzite and schistose metapelite; which builds the Eenriet
Mountain Range) and the Besondermeid Formation (penetratively
retrogressed metapelites and minor dark metaquartzite which occurs
as a xenolitic enclave in the Steinkopf Domain). The Khurisberg
Subgroup comprises the supracrustal rocks of the Copper District,
i.e. white metaquartzite, high grade metapelites and minor
metabasites.
The supracrustal rocks are engulfed in granitic batholiths and cut
by smaller intrusive bodies. The most voluminous units are the
Vioolsdrif, Gladkop, Little Namaqualand and Spektakel Suites. The
Nariams metadolerite dykes, Koperberg Suite, Blesberg (pegmatite)
Suite and the Gannakouriep (dolerite) Suite complete the list ot
significant Intrusive units.
The Vioolsdrif Suite (granodiorite to leucogranite) is mainly
confined to the Richtersveld Domain and together with the Haib
Subgroup, forms the Orange River Igneous belt, the igneous
products of the Orange River orogeny.
The Gladkop Sulle constitutes the main lithological entity of the
steinkopf Domain. Three major units are distinguished, namely the Steinkopf Gneiss
(granodioritic to granitic), the Brandewynsbank Gneiss (granitic) and the Noenoemaasberg Gneiss (leucogranilic). Together they forma grey gneiss complex similar to basement complexes elsehwere. The gneisses exhibit well-developed banding
shown to be of secondary origin through processes of mechanical
flattening of primary heterogeneities coupled with metamorphic
and/or metatectic differentiation. By distinguishing primary trom
secondary features, it is shown that the precursors to the three
gneiss. units were homogeneous in terms of composit.ion, texture and
structure. The gneisses do not exhibit primary banding. Intrusive
relations are established on groundS of the recognition of
xenoliths and observation of contamination and assimilation at the
contacts. It is shown that the Steinkopf Gneiss' is intrusive into
metasediments of the Khurisberg and Eenriet Subgroups (i.e. it is
not a basement tor these supracrustal rocks), that the
Brandewynsbank Gneiss is intrusive into the Steinkopf Gneiss and
that the Noenoemaasberg Gneiss is intrusive into both the older
units. Both the Gladkop and Vioolsdrif Suites are intrusive into
the Eenriet Subgroup and are in turn intruded by the Little
Namaqualand Suite. Isotopic characteristics confirm the suggested
time equivalence while chemical, stratigraphic, structural and
metamorphic evidence point to a spatial separation of the two
suites during early Proterozoic times. The Konkyp Gneiss of the Transition Zone is correlated with the
Little Namaqualand Suite augen gneisses of the Copper District on
grounds of similar structural, textural and stratigraphic
relations. A slightly more silicic and iron rich compositions of
the Konkyp Gneiss relative to other augen gneisses in the
Geotraverse is interpreted to reflect inhomogeneities of the
source area. The augen of the Little Namaqualand Suite are best
interpreted as primary phenocrysts which are large due to growth
during very slow cooling. The Spektakel Suite includes the Concordia, Rietberg and
Kweekfontein Granite of the Copper District and the Eyams Granite
and Middelplaat dykes (normatively potassium-rich nepheline
syenite for most part) of the Steinkopf Domain. The Eyams and
Kweekfontein Granites are very similar in terms of their
migmatitic characters and modes of emplacement.
The structural development spans 1000 million years and the older
structures are largely obliterated by the main deformational
phases of the Namaqua tectogenesis. Pre-Namaqua deformation in the
Gladkop Suite is inferred and correlated with pre-Namaqua
northeasterly directed thrusting in the Richtersveld Domain. The
most important deformation phase involved early Namaqua, deep
seated thrusting which imparted a regionally penetrative toliation
to the augen gneisses and because of which lithological units were
subhorizontally displaced over distances as much as 100 km from
the northeast to the southwest. This was followed by a second
phase of thrusting (commonly referred to as the Skelmtontein
phase) which produced zonally developed refoliation in the augen
gneisses and in the gneisses of the Gladkop Suite and which
produced the foliation in the Concordia and Rietberg Granites. The
direction of tectonic transport during Skelmfontein thrusting was
essentially the same as tor the early thrusting. The Kweekfontein
and Eyams Granites are in part syn-tectonic with the Skelmfontein
thrusting. The present distribution of lithological units in the
Geotraverse is mainly the result of the two thrust phases. Late
structures include the open folds, the Ratelpoort shearing
(easterly trend), the Dabbiknik shearing (northwesterly trendl and
the development of steep structures.
The Richtersveld Domain comprises a core zone charcterized by greenschist facies mineral assemblages, the products of thermal
overprinting during the Namaqua orogeny, superimposed on the
kinematic fabrics of the Orange River orogeny. Outside the core
zone the metamorphic grade increases to lower amphibolite facies.
The Transition Zone is characterized by middle amphibolite facies
mineral assemblages with prominent retrogression, involving the
growth of andalusite at the expense of sillimanite, during late
thrusting. In the Konkyp area higher grade prograde features are
spatially associated with the Konkyp Gneiss. Mineral assemblages
and compositions indicate peak metamorphic conditions of 650D C
and less than 4 kb pressure for the Eenriet region. Penetrative
reworking during Skelmfontein deformation tranformed dolerite
dykes, which transect early Namaqua fabrics, to amphibolites in
the Steinkopf Domain. Geothermometry indicates temperatures of
around 700D C tor the central and southern parts of the Steinkopf
Domain during the late Namaqua metamorphism. The Copper District
is a granulite terrane. Peak metamorphic around 800D and 5 kb
pressure during Skelmfontein deformation is indicated. Metamorphic
recrystallization of minerals in bodies of Koperberg Suite
leucodiorite suggest elevated temperatures during emplacement of
these rocks. Temperatures around 5000 C indicated by
biotite-garnet compositions of metapelites penetratively deformed
in steep structures may reflect retrogression at a much later
stage. The Geselskapbank Domain contains exotic stratigraphic
units (including early Namaqua granulites) tectonically emplaced
during late thrusting. These granulites formed at a tempet'ature of
about 750D C. Retrogression similar to that experienced by the
rocks of the Transition Zone produced andalusite + chlorite in
cordierite-sillimanite-garnet rocks. Geothermometry and
geobarometry yield about 5000 C and less than 3 kb respectively,
consistent with the observed retrograde mineralogy.
The grey gneissses of the Gladkop Suite display maximum
migmatization correlated with the Orange River orogeny at the
northern extremities of Gladkop Suite outcrops, in the Transition
Zone. Most significant migmatization developed during early
Namaqua metamorphism and is located adjacent to the Copper
District in the south and spatially associated with the Konkyp
Gneiss in the north.
A tectonic model that fits the data includes high grade
metamorphism and migmatization of the Gladkop Suite during the
Orange River orogeny, culminating in the thrusting of the
Steinkopf Domain over the Richtersveld Domain in a northeasterly
direction. During the Namaqua orogeny the sense of thrusting
reversed and first caused the overriding of thrust sheets,
rendered hot and ductile due to high volumes of the Little
Namaqualand Suite magmas, over the Richtersveld Domain. Later the
Richtersveld Domain was undercut by foot wall propagation of
thrust faults. The Richtersveld Domain then started to override
the Steinkopf Domain along the Groothoek thrust, transporting a
superjacent augen gneiss sheet. Late Namaqua thrusting caused
rejuvenation of the Groothoek thrust and new movement along the
Skelmfontein thrust. The distribution of thrust sheets with high
volumes of still hot augen gneisses and recently added Spektakel
Suite magmas caused high grade metamorphism at the leading edge of
the Skelmfontein thrust sheet and retrograde metamorphism at the
trailing end.
Description
Keywords
Geology, Stratigraphic -- Precambrian, Gneiss -- South Africa, Metamorphism (Geology) -- South Africa, Metamorphic rocks, Thesis (Ph.D. (Geology))-- University of the Free State, 1988