Thesis (MSc (Earth Sciences))--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: The Central Zone (CZ) of the Damara belt in central Namibia is underlain by
voluminous Pan-African granites and is host to numerous pegmatite occurrences,
some of which have economic importance and have been mined extensively. This
study discusses the occurrence, geometry, relative timing and emplacement
mechanisms for the Usakos pegmatite field, located between the towns of Karibib
and Usakos and within the core of the regional-scale Kranzberg syncline.
Lithological mapping of the Kuiseb Formation in the core of the Kranzberg syncline
identified four litho-units that form an up to 800 m thick succession of
metaturbidites describing an overall coarsening upward trend. This coarsening
upwards trend suggests sedimentation of the formation’s upper parts may have
occurred during crustal convergence and basin closure between the Kalahari and
Congo Cratons, rather than during continued spreading as previously thought.
The Kranzberg syncline is a regional-scale NW verging, NE-SW trending, strongly
non-cylindrical structure that consists of a moderately SE dipping, normal NW limb
and a steep- to overturned SE limb. First- and lower-order folds show relatively
consistent E - SE plunges at moderate angles and stretching lineations and
boudinage of competent layers point to a fold-parallel stretch during folding.
Folding is associated with a moderate- to steep SE dipping transecting foliation that
shows a consistent anticlockwise rotation with respect to the axial plane of the fold.
The transecting cleavage and a component of non-coaxial shear along the
overturned limb suggest that folding was accomopanied by a dextral component of
shear thought to be related to the SW-directed extrusion of the adjacent Usakos
dome during regional NW-SE directed shortening. It is further suggested that the
Kranzberg syncline evolved within the overall regional pattern of regional dome and
syncline structures in the sCZ, and not as a forced fold in response to the formation
of neighbouring dome structures.
Based on cross-cutting relationships and deformation, four main generations of
bedding-concordant sills and bedding-discordant pegmatite dykes were identified.
Along the normal limb, shallowly-dipping sills dominate, highlighting the
significance of bedding anisotropies for sheet propagation. Along the overturned
limb, interconnected dyke and sill geometries co-exist. Here, pegmatite
emplacement appears to have been influenced by (1) the regional strain, (2)
differing wall rock rheologies; (3) the orientation of pre-existing anisotropies; and
(4) driving melt pressures.
Dykes within the Usakos pegmatite field formed within dilational sites, at high
angles to the regional stretch, whereas sills formed at high angles to the regional
shortening strain and in contractional sites. Where driving pressures for melt ascent
were high enough, an interconnectivity of dykes and sills and subsequent melt
transfer from contractional into dilational sites is developed. Where melt pressures
dropped below a critical value pegmatites were arrested, thus preserving the ascent
pathways of the melts. These complex intersecting melt pathways are developed
throughout the Kranzberg syncline. This suggests the existence of fairly stable melt
networks in the continental crust. This geometrical complexity also accounts for the
stockwork-like structures observed in pegmatite fields. / AFRIKAANSE OPSOMMING: Die Sentrale Sone (CZ) van die Damara gordel in sentrale Namibië is onder lê deur
volumineuse Pan-Afrikaanse graniete en speel gasheer vir talle pegmatiet
voorkomste, waarvan party van ekonomiese belang is en is ekstensief ontgin.
Hierdie studie bespreek die voorkoms, geometrie, relatiewe tydsberekening en
inplasing meganismes vir die Usakos pegmatiet gebied, wat tussen die stede van
Karibib en Usakos en wat binne die kern van die regionale-skaal Kranzberg sinklien
geleë is.
Litologiese kartering van die Kuiseb Formasie in die kern van die Kranzberg sinklien
het vier lito-eenhede geidentifiseer. Hierdie eenhede, wat saam tot 'n 800 m dik
opeenvolging van metaturbidiete vorm, beskryf ‘n algemene opwaartse
vergrowwing neiging. Hierdie tendens dui aan dat sedimentasie van die Formasie
se boonste dele tydens die aardkorst konvergensie en kom sluiting tussen die
Kalahari en die Kongo kratons voorgekom het, eerder as in 'n
oseaanvloerverbreiding omgewing soos voorheen gedink was.
Die Kranzberg sinklien is 'n regionale-skaal struktuur met ‘n NW vergensie, ‘n NOSW
koersing, wat sterk nie-silindries is en wat uit 'n matige SO helling, normale NW
flank en 'n steil-tot omgeslaande SO flank bestaan. Eerste-en laer-orde plooie
vertoon relatief konsekwent matige O - SO duikings en strek lineasies en boudinage
van kompetent lae dui 'n plooi parallel strek tydens plooiing aan. Plooiing is
geassosieer met 'n ongeveer aksiale planêre, matig- tot steil SO helling foliasie wat
omstandig waargeneem word om 'n konsekwente antikloksgewyse rotasie met
betrekking tot die aksiale vlak van die plooi te hê. Hierdie antikloksgewyse rotasie is
‘n bewyse vir 'n komponent van nie-koaksiale regse skuifskeur deur die
omgekeerde flank en dui dit ook aan dat 'n regse komponent van skeer gedurend of
na plooiing plaasgevind het. Daar is gedink dat die regse komponent van skeur in
verband met die laterale, SW-gerig extrusie van die aangrensende Usakos koepel
gedurende plaaslike NW-SO verkorting ontwikkel het. Dit is verder voorgestel dat
die Kranzberg sinklien binne die totale patroon van plaaslike koepel en sinklien
strukture in die sCZ geontwikkel het, en nie as 'n gedwonge plooi in reaksie op die
formasie van die naburige koepel strukture (bv. Usakos koepel).
Gebaseer op kruis-sny verhoudings en deformasie, was vier generasies van
gelaagdheid-konkordant plate en gelaagdheid-diskordant pegmatiet dyke
geïdentifiseer. In die normale flank, vlak-helling plate oorheers, wat die
belangerikheid van die laagvlak-anisotropiese op plaat voortplanting beklemtoon.
In die steil, omgekeerde flank, bestaan onderlinge verbinde dyk en plaat geometrië
gelyktydig. Hier is pegmatiet inplasing blykbaar beïnvloed deur (1) die regionale
span; (2) verskillende wandgesteentes reologië; (3) die oriëntasies van anisotropie
(ie. gelaagdheid ); en (4) smeltsel druk.
Dyke in die Usakos pegmatiet gebied het binne dilatasionele liggings, teen hoë hoeke
aan die regionale strek gevorm, terwyl plate teen hoë hoeke aan die plaaslike
verkorting span en in kontraksionele liggings gevorm het. Waar smeltsel druk hoog
genoeg was, is 'n onderlinge verbinding van dyke en plate, en die daaropvolgende
smeltsel oordrag van kontraksionele liggings na dilatasionele liggings behou. In
teenstelling, waar smeltsel druk onder 'n kritieke waarde geval het, word die
pegmatiete geblokeer, en dus kan die behoude smeltsel styging paaie waargeneem
word. Hierdie snyende smeltsel geometrië, in beide kontraksionele en dilatasionele
liggings dui aan dat redelik stabiele smeltsel netwerke in die kontinentale kors kan
bestaan en verder kan en verklaar die algemene stokwerk-agtige strukture wat in
pegmatiet velde van ander mid-korstige omgewings waargeneem word.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/6743 |
| Date | 03 1900 |
| Creators | Owen, Geoffrey J. |
| Contributors | Kisters, Alex, University of Stellenbosch. Faculty of Science. Dept. of Earth Sciences. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 168 p. : ill., maps |
| Rights | University of Stellenbosch |
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