MELT-SHEAR ZONE RELATIONSHIPS DURING THE LIFESPAN OF A CONTINENTAL TRANSCURRENT SHEAR ZONE

The close spatial and temporal relationship of shear zones and magmas is commonly interpreted to indicate positive feedback between magma migration, emplacement and shear zone associated deformation. This study focuses on an area around the Proterozoic Pofadder Shear Zone (PSZ) in Namaqualand, as a case study for analogous interactions between the emplacement of granitic magmas and structural sites created during the progressive evolution of crustal-scale shear zones. Here granitic magmas in the form of pegmatites show distinct spatial and temporal variations across the shear-zone. Mapping of the shear-zone fabrics and pegmatites shows the pegmatites are emplaced in structurally distinct sites within and adjacent to the PSZ and, with newly acquired U-Pb monazite ages, shows emplacement has occurred at different times of shear zone development from earlier ductile to later, brittle retrograde fabrics that accompanied the ca. 45 Ma of shear zone exhumation.

Pegmatites concentrated along the northern PSZ-margin are interpreted to be controlled by steeply orientated anisotropies developed axial planar to large km-scale and parasitic shear zone folds during the initial, predominately strike-slip stages of deformation 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 through synthetic Riedel shears and dextral dilatant jogs. The most significant pegmatite development around the PSZ is the Skimmelberg Pegmatite Stockwork (SPS), 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 N-S extensional fractures (mode I) during periods of late-stage transpression. Here the SPS significantly forms a steeply dipping fracture-network which effectively creates a conduit for magma transport along the margin of the PSZ.

This study therefore largely supports a case for deformation assisted magma transport and illustrates how magma is not only concentrated within the high strain core of shears but may be focused in structurally controlled sites along shear zone margins, emplaced during different stages of shear zone evolution.