This study is a detailed investigation concerning the construction of granite plutons by the incremental emplacement of granitic sheets. The modem consensus is that sheeted plutons are often controlled by tectonic structures such shear zones. The Main Donegal Granite (MDG), NW Ireland forms the basis to this study. This pluton is the largest presently exposed member of the Caledonian Donegal Batholith (~405 Ma). Field evidence from this highly deformed pluton, attest to emplacement along the long-axis of a sinistral transcurrent shear zone. The presence of long and persistent xenolith "trains" within the pluton has been taken as evidence of an overall sheeted structure; however detailed maps have not been available to test this hypothesis. Two earlier members of the Donegal Batholith, the Ardara and Thorr plutons, whilst having their main outcrops outside the MDG, also occur as xenoliths within the main body. It can be demonstrated in a number of critical situations that these xenoliths are commonly more deformed than the host MDG facies. Furthermore the presence of original country rock contacts implies these xenoliths were originally in situ. These features imply that the shear zone was active prior to the emplacement of the MDG, with it controlling the emplacement of substantial parts of these earlier plutons. Further evidence from the study of parts of the petrographically similar and younger Trawenagh Bay Granite implies the sinistral shear zone was still operational after the majority of the MDG had crystallised. New, detailed (scale 1:250) and reconnaissance mapping of the MDG, reveals its hitherto unrecognised heterogeneity. At least seven major plutonic zones or packages have been identified. All these units have an NE -SW elongate form parallel to the long axis of the pluton and are often, but not always, separated by extensive "raft-trains" of country rock and older plutons. The major packages in the central regions of the pluton are often complex and are composed of three main granitoid phases, ranging in composition form early granodiorites and tonalites to latest porphyritic and to lesser extent equigranular, monzogranites. The early granodiorite and tonalite sheets are now only preserved as xenolithic rafts within the later monzogranites. The broad range in composition/chemistry together allied with field observations implies a complex intrusion history, with these granitoid packages representing sites of long-standing intrusion within the pluton. In contrast, towards the more marginal areas of the pluton there are large units of monzogranite which are characterised by general homogeneity, but in reality are believed to consist of relatively small compostionally similar sheets. On all scales, either meta-sediments, older plutonic material, or early MDG facies are found to lie along the boundaries of younger intrusive units. This implies the pluton is primarily sheeted in character and that the "raft-trains" are partially disrupted, in situ roof material which has been wedged apart during the intrusion of the sheets. The appearance of sheets within the field is dependent on the rheology of the material into which the granitic material was intruded into, i.e. to what extent has the host was crystallised. The degree of crystallisation in the host is related to how fast later sheets were being intruded, i.e. the rate of emplacement. The field relationships, in the central regions of the pluton, between the granodiorites tonalites and the later monzogranites, are interpreted as representing zones of episodic-to slow emplacement, where earlier phases had become essentially competent by the time later units were intruded (i.e. capable of fracture). These earlier phases may be preserved as angular rafts within later sheets. At moderate emplacement rates earlier sheets may still be crystallising but sufficiently viscous to prevent mixing, except at their immediate boundaries with transitional contacts developing. The more homogeneous zones are believed to be related to rapid emplacement with original contacts between pulses being destroyed at the level of emplacement due to homogenisation of pulses which had similar viscosities and hence allowed mixing. The emplacement of granitic melts within active shear zones can lead to the development of a self- perpetuating situation, where melts in a shear zone will enhance deformation rates and cause greater displacements subsequently allowing more melt to enter the shear zone promoting even greater displacement rates. This process is only halted when melts within the source regions are drained; hence the rate of pluton construction and appearance of sheets within plutons is ultimately related to how fast granitic melts are being generated within the source regions.

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.

Thesis (MSc)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The close spatial and temporal relationship of shear-zones and magmas is commonly interpreted to indicate positive feedback between magma migration, granitic emplacement and shear-zone-associated deformation. Emplacement geometries and structural fabrics are however rarely preserved, hampering the study of shear-zones and granitic magmas interactions. This study focuses on an area around the Pofadder Shear Zone (PSZ) in Namibia and South Africa as a case study for granitic bodies, mainly as pegmatite sills and dykes, and their spatial and temporal relationships to a crustal-scale shear-zone. The PSZ is a NW-SE trending, dextral, Mesoproterozoic-Neoproterozoic transpressional shear-zone in Namaqualand, interpreted to have accommodated late-stage lateral escape of the Namaqua Metamorphic Complex in response to southward indentation of the Kaapvaal Craton around 1030 – 1080 Ma. In this study it is shown that the shear-core records an asymmetrical strain variation across the PSZ. This is indicated by pervasively banded ultramylonites, mylonites and the significant development of pervasive phyllonites at the southern margin, defining the internal ductile to brittle-ductile fabrics of the shear, during a progressive deformational evolution. Mapping of the PSZ fabrics and associated pegmatites documents how pegmatites are emplaced in structurally distinctive sites within, and adjacent to the PSZ. New U-Pb monazite ages derived from this study, show how pegmatite emplacement has occurred at different times of shear-zone development. The pegmatites are emplaced into earlier ductile to later brittle-retrograde fabrics that accompanied the ca. 45 Ma shear-zone exhumation. Pegmatites concentrated along the northern PSZ-margin are interpreted to be controlled by anisotropies developed axial planar to large km-scale and parasitic folds during the initial, predominately strike-slip stages of shearzone deformation that occurred as early as 1005 ± 5 Ma. Within the PSZ core, pegmatite emplacement is controlled by the syn-kinematic development of (a) subvertical, mylonitic and phyllonitic foliations and (b) fracture permeabilities created by synthetic Riedel shears and dextral dilatant jogs. The most significant pegmatite development around the PSZ is the Skimmelberg Pegmatite Stockwork (SPS) which forms an extensive interconnecting network of concurrent, foliation-parallel sills and thick (> 50 m) discordant dykes within the southern footwall of the PSZ. The dykes intrude as late as 958 ± 5 Ma into feather-shaped N-S extensional fractures (mode I) that developed due to episodic stick-slip at the boundary between the PSZ core and footwall rocks during periods of late-stage transpression. The SPS forms a steeply dipping fracture network that not only creates space needed for emplacement but effectively acts as a conduit for magma transport along the margin of the PSZ. The large extensional fractures of SPS create the necessary hydraulic gradients to tap the magma source of a regional trending pegmatite belt and form a sheeted complex adjacent to the PSZ. Therefore, this study documents how, during the progressive exhumation of a largescale transcurrent shear-zone, magma emplacement is not only concentrated within the highly permeable, high-strain domains (cores) of shear-zones but may be concentrated in diachronous, structurally controlled sites along the shear-zone margins. / AFRIKAANSE OPSOMMING: Die noue ruimte-tydsverband tussen skuifskeure en magmas word algemeen geïnterpreteer as ‘n aanduiding van positiewe terugkoppeling tussen magma migrasie, graniet-inplasing en skuifskeurgeassosieerde vervorming. Geometrie en struktuurmaaksels van inplasings word egter selde bewaar en belemmer die studie van interaksies tussen skuifskeure en graniet-magmas. Die studie fokus op ‘n area rondom die Pofadder Skuifskeur (PSS) in Namibië en Suid-Afrika as ‘n gevallestudie vir graniet-liggame, hoofsaaklik as pegmatiet plate en gange, asook voorafgenoemde se ruimte-tydsverband met ‘n grootskaalse skuifskeur. Die PSS is ‘n NW-SO-waarts strekkende, regs-laterale, Mesoproterosoïse-Neoproterosoïse transpressieskuifskeur in Namakwaland, wat geïnterpreteer word om die latere-stadium laterale ontsnapping van die Namakwa Metamorfiese Kompleks te akkomodeer in reaksie op die suidwaartse indrukking van die Kaapvaal Kraton omstreeks 1030-1080 Ma. In hierdie studie word getoon dat asimmetriese vervormingsvariasie deur die skuifskeurkern aangeteken word. Hierdie word aangetoon deur gebande ultramilioniete, milioniete en die noemenswaardige ontwikkeling van filoniete wat die suidelike rand deurtrek en definiëer die interne plastiese- tot bros-plastiese maaksels van die skuifskeur gedurende ‘n progressiewe vervormingsevolusie. Kartering van die PSS maaksels en geassosieerde pegmatiete dokumenteer hoe pegmatiete, aangrensend en binne die PSS, in eiesoortige strukturele terreine binnedring. Nuwe U-Pb monasiet ouderdomme, afgelei vanuit hierdie studie, toon aan hoe inplasing plaasgevind het gedurende verskillende tye van skuifskeurontwikkeling. Pegmatiete het vroeëre plastiesetot latere bros-retrogressiewe maaksels binnegedring wat die herontbloting van die ca. 45 Ma skuifskeur meegaan. Pegmatiete, gekonsentreerd langs die noordelike rand van die PSS, word geïnterpreteer as beheer deur anisotrope wat parallel aan die asvlak van groot km-skaalse en ondergeskikte plooie ontwikkel gedurende die aanvanklike, hoofsaaklik strekkingwaarste, stadiums van skuifskeurontwikkeling wat so vroeg as 1005 ±5 Ma plaasgevind het. Binne die kern van die PSS word die inplasing van pegmatiete beheer deur die sinkenimatiese ontwikkeling van (a) subvertikale, milionitiese- en filonitiese foliasies en (b) breukdeurdringbaarheid wat gevorm is deur sintetiese riedelskuifskeure en regslaterale uitsettende “jogs”. Die mees noemenswaardige pegmatiet ontwikkeling rondom die PSS is die Skimmelberg Pegmatiet Stokwerk (SPS) wat ‘n intensiewe netwerk vorm van intergekonnekteerde konkurrente plate, parallel aan die foliasie, en dik (>50m) diskordante gange binne die suidelike vloer van die PSS. Die gange dring in so laat as 958 ± 5 Ma binne-in veervormige N-S uitbreidende breuke (modus1) wat ontwikkel het as gevolg van die episodiese hak-en-glip op die grens tussen die PSS kern- en vloergesteentes gedurende periodes van laat-stadium transpressie. Die SPS vorm ‘n styl hellende breuk-netwerk wat nie net spasie maak vir indringing nie, maar dien ook effektief as ‘n geleidingsweg vir die vervoer van magma langs die rand van die PSS. Die groot uitbreidende breuke van die SPS skep die nodige hidroliese gradiënt om die magma bron van ‘n regionale pegmatiet gordel te tap en vorm ‘n bladvormige kompleks aangrensend tot die PSS. Gevolglik dokumenteer die studie hoe, gedurende die progressiewe ontbloting van ‘n grootskaalse torsieskuifskeur, magma inplasing nie net gekonsentreer is binne die hoogs deurdringbare, hoogsvervormde areas (kerne) van skuifskeure nie, maar ook hoe magma kan konsentreer in diachroniese, struktuur beheerde gebiede teen die rande van skuifskeure.