Granitic melt transport and emplacement along transcurrent shear zones : case study of the Pofadder Shear Zone in South Africa and Namibia

Lambert, Christopher William (2013-12)

Thesis (MSc)--Stellenbosch University, 2013.

Thesis

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.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/85682
This item appears in the following collections: