Geology and Metamorphic Petrology of part of the Damara Orogen along the Lower Swarkop River ...

222 Pages·2014·6.19 MB·English

GEOLOGY AND METAMORPHIC PETROLOGY OF PART
OF THE DAMARA OROGEN ALONG
THE LOWER SWAKOP RIVER,
SOUTH WEST AFRICA

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ROGER Ey. JACOB
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Thesis submitted in fulfilment of the
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requirements for the degree of
Doctor of Philosophy in the
Faculty of Science
University of Cape Town
] 974
The copyri,ght of this thesis is held by the
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The copyright of this thesis vests in the author. No
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published without full acknowledgeement of the source.
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The thesis is to be used for private study or non-
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commercial research purposes only.

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Published by the Universit y of Cape Town (UCT) in terms
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of the non-exclusive license granted to UCT by the author.
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GEOLOGY AND METAMORPHIC PETROLOGY OF PART OF THE DAMARA OROGEN ALONG THE
LOWER SWAK.OP RIVER, SOUTH WEST AFRICA
ABSTRACT
The stratigraphic nomenclature of the Damara and Nosib Groups has been
revised and a purely lithostratigraphic terminology is here adopted. Clastic
sediments of the Etusis and Kha.n Formations of the Nosib Group were deposited
in shallow water above, and on the flanks of, the Abbabis swell and were
followed by elastic-carbonate shoreline sediments of the Rossing Formation of
the Damara Group. Deposition of glacial-marine sediments of the Chuos
Formation, which forms the base of the Khomas Subgroup, followed a period of
erosion. Further subsidence led to the accumulation in the westn of carbonate
and pelitic sediments of the Husab and vlit;;:ioort Formations, respectively, and
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mixed pelitic and siliceous carbonate sediments of the Tinkas Formation in the
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Two major phases of deformation (F1 and F2) gener ated northeast·-trending
isoclinal folds and associated foliations and were folleowed by local minor
folding in the southeast during F3. The F4 deformaption produced north
northeast- to northwest-trending folds and accentuaated dome structures
initiated during F2. C
High T/P regional metamorphism accompa nied F1 and F2 and outlasted the
latter. Mineral assemblages throughout thoef area are those of the amphibolite
facies, but there is an increase in metamorphic grade from medium stage in the

southeast to high stage in the northwesyt. In calc-granofelses in the south
eastern part of the area epidote has treacted with quartz to form grossularite/
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· andradite. The stability of theses minerals was largely controlled by the
composition of the fluid phase (eXrco2) and oxygen fugacity. The origin of
narrow para-amphibolite bands in the Tinkas Formation is ascribed to the
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reaction of incompatible biotite, calcite and quartz along the contacts of
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calcareous and pelitic layenrs. Forsterite appears to have formed after
diopside in siiiceous dolUomitic marbles. The An-content of plagioclase in
amphibolites increases with increasing metamorphic grade from An30-60 in the
southeast to An>ao in the northwest.
Andalusite, sillimanite, cordierite, staurolite and almandine are
present in medium-stage metapelites and cordierite, sillimanite, K-feldspar
and almandine in high-stage metamorphites. Isograds have been mapped in the
field and interpreted in terms of mineral reactions. With increasing grade
of metamorphism staurolite and andalusite disappeared from pelitic assemblages,
partial anatexis and migmatisation became widespread, muscovite and quartz
reacted to form sillimanite and K-feldspar, and biotite, sillimanite and
quartz reacted to form cordierite and K-feldspar (± almandine).
Metamorphic temperatures in the southeast and northwest are estimated
to have attained 650° C and 750° C respectively. Evidence provided by the
co-existence of andalusite-sillimanite, cordierite-staurolite and cordierite
garnet, and the final disappearance of staurolite just short of the anatexis
boundary as well as the breakdown of muscovite + quartz beyond the same
boundary, indicates that prevailing pressures were between 4 and 5 kb. This
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corresponds to a depth of burial of J4-J7 km and a geothermal gradient of
40-50° C per km.
The autochthonous and parautochthonous syntectonic rocks of the Red
Granite-Gneiss and Salem Granite suites are located on the northwestern side
of the anatexis boundary. The former granite-gneisses are in part products
of anatexis of Nosib metamorphites and in part reactivated mantled gneiss
domes. The Salem Granite Suite comprises three members occurring in synclinal
structures above the level of the Chuos Formation and originated by anatexis
of Khomas Subgroup rocks. Late- to post-tectonic intrusive granites are
connnon.
Homogeneous pegmatites of metamorphic or1g1n formed syntectonically and
are older than the zoned and layered pegmatites derived by fractionation from

the intrusive granites. n
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CONTENTS
I . INTRODUCTION
1.1. General
1.2. Previous work
1.3. Present investigation I
1.4. Acknowledginents 2
2. FIELD RELATIONSHIPS, STRATIGRAPHY AND LITHOLOGY 3
2.1. Classification and nomenclature 3
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2.2. Nosib Group 6
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2.2.J. Etusis Formation 6
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2.2.2. Khan Formation 12
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2.3. Damara Group 13
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2.3.1. Rossing Formation 13
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2.3.2. Chuos Formation 16
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2.3.3. Husab Formation 17
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2.3.4. Tinkas Formation 19
2.3.5. Witpoort Formation 21
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2.4. Igneous Rocks 22

2.4.1. Red Granite-Gneiss y 23
2.4.2. Salem Granite Suite it 24
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2.4.2.1. Non-porphryritic gneissic granite 25
2.4.2.2. Porphyreitic biotite granite 25
2.4.2.3. Leucovgranite 27
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2.4.3. Basic Intrusnives 28
2.4.4. BloedkoppiUe Granite 28
,2.4.5. Achas Granite 29
2.4.6. Gawib Granite 30
2.4.7. Alaskitic Pegmatitic Granite 31
2.4.8. Horebis Granite 31
2.4.9. Donkerhoek-Granite 32
2.4.10.Pegmatites 33
2.4.Jl.Dolerite 33
2.5. Superficial deposits 33
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3. STRUCTURE 34
3.1. Terminology and Methods 34
3.2. Structural Data 36
3.2.l. F1 structures 36
3.2.2. F2 structures 38
3.2.3. F3 structures 48
3.2.4. F4 structures 52
3.2.5. Faults 54
3.3. Discussion 55
4. PETROGRAPHY AND METAMORPHIC PETROLOGY 62

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4.1. Carbonate and siliceous carbonate associationws 62
4.1.1. Marbles o 62
4.1.2. Cale-silicate rocks T 65
4. 1.3. Quartz-microcline para-amphibo lites 71
4.J.4. Interpretation e 72
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4.2. Basic-and banded-gneiss. associatiaons 82
4.2.1. Amphibolites C 82
4.2.2. Origin 84
4.2.3. Banded gneisses of 86
4.2.4. Discussion 90
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4.3. Pelitic and psannnitict associations 92
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4.3.1. Medium stages 92
4.3.2. High stageer 97
4.3.2.1v. Metapsannnites 100
4.3n.2i.2. Metapelites 100
4U.3.2.3. Migmatites 103
4.3.3. Petrochemical considerations 104
4.3.3.1. Whole-rock chemistry 104
4.3.3.2. Co-existing cordierite and biotite 107
4.3.3.3. Co-existing cordierite and garnet 123
4.3.4. Interpretation of the mineral assemblages and
petrogenesis 124
4.4. Contact Metamorphism 135
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5. GRANITIC ROCKS 140
5.1. Petrography .140
5.l.l. Red Granite-Gneiss 140
5.1.2. Salem Granite Suite 142
5 .1. 2. l. Non-porphyritic gneissic granite 142
5.l.2.2. Porphyritic biotite granite 1·44
5.J.2.3. Leucogranite .145
5.1.3. Bloedkoppie Granite 145
5.1.4. Achas Granite 147
5.1.5. Gawib Granite n 147
5.1.6. Alaskitic Pegmatitic Granite 148
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5. 1.7. Horebis Granite 148
5.1.8. Donkerhoek Granite o 149
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5.2. Petrogenesis 150

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6. PEGMATITES a 154
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6.l. Classification 154

6.2. Homogeneous pegmatites f 155
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6.2.1. Bodies with constituents derived from the

surrounding rocks y 155
6.2.2. Bodies with consttituents derived from intrusive
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granites s 156
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6.3. Inhomogeneous pegmeatites 156
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6.3.J. Zoned pegmatites 156
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6.3.2. Layeredn pegmatites 160
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6.4. Origin 162
7. CONCLUSIONS 165
APPENDIX 169
REFERENCES 171
PLATES
CONTENTS OF POCKET AT END:
Geological map of an area east of the Khan-Swakop confluence,
South West Africa. Scale I : 100 000.
Simplified geological map showing major structures, structural
subareas and grid for sample localities. Scale I : 250 000.
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ABBREVIATIONS
A number of abbreviations have been used in the text. A list of these
is presented below.
millimetre(s) dol dolomite
llllil
cm centimetre(s) epid epidote
m metre(s) for forsterite
km kilometre(s) gar garnet
n refractive index gr grossularite

kb kilobar(s) graph graphinte
Ma million years hb hornwb le-nde
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T temperature Kf K-ieldspar
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f02 oxygen fugaci ty ky kyanite
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xco2 mole fraction C02 mcl p microcline
PH20 partial pressure H20 mause muscovite
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Pco2 partial pressure C02 phl phlogopite

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Pf fluid (gas) pressure o pi em piemontite
PT total pressure y plag plagioclase
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l\n distribution coeffiicient qtz quartz
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all allanite r sauss saussurite
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amph amphibole v scap scapolite
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An anorthite n serp serpentine
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and al andalusite sill sillimanite
ap apatite sph sphene
bi biotite staur staurolite
cc calcite ta talc
chond chondrodite tourm - tourmaline
clz clinozoisite tr tremolite
cord cordierite vesuv vesuvianite
di diopside woll -~ollastonite
zrn zircon
I. INTRODUCTION
I.I. General
The area investigated during this survey (Map I) is situated 75 km east
of Swakopmund and 190 km west of Windhoek, in the Namib Desert of South West
Africa. It covers approximately 2600 km2 and is bounded by the Tinkas Flats
in the southeast, the Tubas and Gawib Flats in the southwest and the Rio Tinto
Concession in the northwest. It lies between latitudes 22°251 and 22°56'S
and longitudes 14°52' and J5°41'E.
The morphology, climate, vegetation and drainage in this general region
have been adequately described by Smith (1965) and little fur ther need be
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added. The area is drained by two major rivers, the deeply incised Khan and
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Swakop Rivers. In the northwest and south, the Welwitschia and Gawib Flats
form part of the Namib Plain. The eastern part of theo area consists of
rugged, hilly terrain, and is readily accessible onlyT on foot. The major
portion of the area is occupied by the Namib Desert Park, a game reserve, and
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the remainder is virtually unpopulated.
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1.2. Previous work C

Early investigations by German wofrkers have been sunnnarised by Smith
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(1965) and none of these dealt specifically with the area mapped by the

author. The first comprehensive sytudies were carried out by Gevers (193la,
1934a, 1934b, 1935), who mapped at very large area in western South West Africa
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including the region dealt withs in this report at a scale of I : 100 000.
These maps were compiled withrout topographic control or aerial photographs
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and neither the geographical nor geological features match completely with the
present findings. Howevver, the maps proved useful as a basis for more
detailed investigationsn.i
Smith (1965) dUescribed the geology to the north. There is a small
overlap of mapping on Bloemhof 109 and Vlakteplaas JJO. More recently,
Nash (1971) studied the metamorphism in a small area northwest of the region
mapped by the writer.
1.3. Present investigation
The main purpose of this investigation was to study the metamorphism of
this part of the Damara Orogen and to relate it to the genesis of pegmatites
and radioactive mineralisation. To this end it was necessary to compile a
geological map and to undertake studies of the stratigraphy, structure and
granites. The work was performed at the instigation of, and with the
generous financial support of, the Atomic Energy Board, while the author was
attached to the Precambrian Research Unit of the University of Cape Town and
later to the Department of Geology at Rhodes University.
Fifteen months were spent on fieldwork during 1969 and 1970. Mapping
was carried out on aerial photographs (scale J : 36 000), from which data
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were transferred, by means of an optical pantograph and radial-line plotter,
onto I : SO 000 topographic maps. Laboratory investigations were carried
out at the University of Cape Town until April, 1971, and thereafter at
Rhodes University.
1.4. Acknowledgements
The.writer wishes to express his gratitude to several persons and
organisations for assistance given during the course of this project.
Firstly, to the late Professor John de Villiers, under whose supervision
most of the investigation was made, for arranging the project, for his
continued interest and for many valuable discussions. Dr. A. Kroner took
over the supervision in the final stages and edited variou s drafts. Sincere
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thanks are due to him for this onerous task and.for constructive comments and
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criticism.
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It is a pleasure to acknowledge the financial support of the Atomic
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Energy Board for the project and to thank Dr. J.W. von Backstrom for his

personal interest and help in the procurement oef chemical analyses and
radioactive-mineral identifications. Thankps are due to Mr. J. Berning and
Mr. R. Cooke of Rio Tinto Exploration Ltd.a, Swakopmund, for their hospitality
and.facilities extended to the writer while in the field.
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Professor L.H. Ahrens of the De partment of Geochemistry, University of
Cape Town, permitted the author to omfake use of ·electron microprobe facilities
under the guidance of Mr. J.P. W illis, whose help and patience is gratefully
acknowledged. Mr. Willis kindlyy processed the raw data. The cost of the
·analyses was de-frayed from a iRthodes University Council Research Grant. The
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National Institute for Metallurgy is thanked for the chemical analyses. The
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support received from thee Deputy Director, Mr. L.N.J. Engelbrecht, and the
staff of the Geologicavl Survey office in Windhoek is very much appreciated.
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A special voten of thanks is due to Professor H.V. Eales for making
available the facUilities of the Department of Geology at Rhodes University,
for critical reading of the manuscript and for taking considerable trouble in
arranging departmental duties so as to expedite the research. Thanks are
due, too, ·to Mr. D.S. Cawood, technician in the Department of Geology for
considerable co-operation, and to Mrs. A. Wicks for typing the manuscript.
Finally, I wish to express my appreciation to my wife Anamie for her
help during the course of the research.