Geology of the Kranzberg syncline and emplacement controls of the Usakos pegmatite field, Damara belt, Namibia

Owen, Geoffrey J. (2011-03)

Thesis (MSc (Earth Sciences))--University of Stellenbosch, 2011.

Thesis

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.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/6743
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