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

[摘要] ENGLISH ABSTRACT: The close spatial and temporal relationship of shear-zones and magmas is commonly interpreted toindicate positive feedback between magma migration, granitic emplacement and shear-zone-associateddeformation. Emplacement geometries and structural fabrics are however rarely preserved, hampering thestudy of shear-zones and granitic magmas interactions. This study focuses on an area around the PofadderShear Zone (PSZ) in Namibia and South Africa as a case study for granitic bodies, mainly as pegmatite sillsand 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 inNamaqualand, interpreted to have accommodated late-stage lateral escape of the Namaqua MetamorphicComplex in response to southward indentation of the Kaapvaal Craton around 1030 – 1080 Ma. In thisstudy it is shown that the shear-core records an asymmetrical strain variation across the PSZ. This isindicated by pervasively banded ultramylonites, mylonites and the significant development of pervasivephyllonites 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 pegmatitesdocuments 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 atdifferent times of shear-zone development. The pegmatites are emplaced into earlier ductile to laterbrittle-retrograde fabrics that accompanied the ca. 45 Ma shear-zone exhumation. Pegmatitesconcentrated along the northern PSZ-margin are interpreted to be controlled by anisotropies developedaxial planar to large km-scale and parasitic folds during the initial, predominately strike-slip stages of shearzonedeformation that occurred as early as 1005 ± 5 Ma. Within the PSZ core, pegmatite emplacement iscontrolled 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 significantpegmatite development around the PSZ is the Skimmelberg Pegmatite Stockwork (SPS) which forms anextensive interconnecting network of concurrent, foliation-parallel sills and thick (> 50 m) discordant dykeswithin the southern footwall of the PSZ. The dykes intrude as late as 958 ± 5 Ma into feather-shaped N-Sextensional fractures (mode I) that developed due to episodic stick-slip at the boundary between the PSZcore and footwall rocks during periods of late-stage transpression. The SPS forms a steeply dipping fracturenetwork that not only creates space needed for emplacement but effectively acts as a conduit for magmatransport along the margin of the PSZ. The large extensional fractures of SPS create the necessary hydraulicgradients to tap the magma source of a regional trending pegmatite belt and form a sheeted complexadjacent to the PSZ. Therefore, this study documents how, during the progressive exhumation of a largescaletranscurrent 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 controlledsites along the shear-zone margins.

[发布机构] Stellenbosch University