The geology and structure of the Bushveld Complex metamorphic aureole in the Olifants River area.

Abstract

The contact metamorphic aureole of the Rustenburg Layered Suite of the Bushveld Complex extends to a depth of over 5 km into the underlying mainly argillaceous Pretoria Group. When compared to other parts of the metamorphic aureole, the Olifants River area is unique in that it is characterised by a high degree of syn-Bushveld Complex deformation and very coarse grained pelitic assemblages. This is believed to have resulted from a combination of greater magma thickness, a deeper emplacement depth and a high degree of subsidence related deformation that was focused along the Thabazimbi-Murchison Lineament. This area also contains a laterally extensive and deformed quartz-feldspar porphyry sill, the Roodekrans Complex that is shown to represent a hypabyssal equivalent of the volcanic Rooiberg Group. There are three main metamorphic zones. A wide andalusite zone dominated by staurolite, garnet and cordierite assemblages. This is followed by a narrow fibrolite zone without staurolite, and a wide inner aureole of migmatite. The migmatite zone is characterised by garnet-cordierite-aluminosilicate assemblages with corundum, spinel and orthopyroxene assemblages at the highest grades. Metamorphic pressure and temperature estimates indicate pressures of between 3 kb and 4 kb in the lower part of the andalusite zone at temperatures of approximately 550°C. Porphyroblast-matrix relationships reveal a close link between deformation and metamorphism resulting in a spectrum of textural relationships developed as a result of inhomogeneous strain. Porphyroblasts in low strain domains preserve textures of “static type" growth whereas syntectonic textures are found in foliated rocks. Pre-tectonic porphyroblasts in many foliated domains indicate that deformation outlasted porphyroblast growth and increased in intensity and extent with time. Retrograde porphyroblasts are post-tectonic. Evidence is presented for both rotation and non-rotation of porphyroblasts in relation to geographical coordinates during extensional top-to-south, down-dip shear in the floor. The unique structural setting in this area triggered the growth of large diapiric structures in the floor of the Rustenburg Layered Suite that are preserved as periclinal folds on the margin and within the northeastern Bushveld Complex. Extreme gravitational loading and heating of the floor by a thickness of up to 8 km of mafic magma resulted in the generation of evenly spaced, up to 7 km diameter wall-rock diapirs that penetrated the overlying magma chamber. Diapiric deformation is restricted to rocks above a decollement zone that is developed along competency contrasts and corresponds approximately with the 550 °C peak metamorphic isotherm. Strongly lineated, boudinaged and foliated rocks are developed in the interpericlinal domains between adjacent periclines. Migmatites in these domains are characterised by conjugate extensional ductile shears and associated asymmetrical boudinage suggesting bulk deformation by pure shear processes. The extension lineation was produced by lateral extension along flow lines directed toward dome culminations. Each of the four diapiric periclines is cut by a different erosional section enabling reconstruction of a typical diapir geometry. At the highest structural levels, periclines have bulbous shapes with overturned limb geometries forming overhangs. The surrounding layered igneous rocks are locally deformed into a series of outward verging folds that define a broad rim syncline. Deformation within the pericline cores is represented by constrictional deformation that produced radial curtain-type folds with steeply plunging lineations and concentrically orientated folds in the outer shell. Diapirism is closely linked to magma emplacement mechanisms. Floor folds in the country rocks were initiated in the interfinger areas of a fingered intrusion. With further magma additions and the coalescence of intrusion fingers into a single sheet, interfinger folds matured into large diapiric periclines which rose to the upper levels of the magma chamber. Strain rates estimated from strain analyses, pericline geometry and model cooling calculations are in the order of 10-14 S-1, corresponding to diapiric uplift rates of 0.6 cm/yr. Diapirism is broadly compatible with a N-S extension in the Olifants River area during emplacement of the Rustenburg Layered Suite. On a regional scale, this is indicated by existence of a major EW dyke swarm that coincides with the long axis of the Bushveld Complex. The accommodation of the Bushveld Complex into the Kaapvaal Craton was facilitated by a combination of craton-wide extension that accompanied plume related magmatic underplating, and loading of the Bushveld Complex. Isostatic adjustment in response to Bushveld Complex subsidence resulted in further development of large basement domes around the perimeter of the Bushveld Complex.