Orthogonal AMS and SPO Fabrics in the MORB-like Rooi-Rand Dyke Swarm of South Africa and Swaziland

The cause of the ca. 185-175 Ma Karoo volcanism in southern Africa has been ascribed to the presence of a mantle plume centred on the Nuanetsi (now Mwenezi) Igneous Province, southern Zimbabwe. In the mantle plume model, this area is considered to represent a triple junction between the WNW-trending Okavango Dyke Swarm (ODS), the ENE-trending Sabi monocline and the N-S trending Lebombo monocline. The plume model predicts that magma flow in Karoo dykes of the Lebombo monocline should be away from the plume head and should be sub-horizontal in the distal regions. A brief study of the anisotropy of magnetic susceptibility (AMS) of 23 dykes in the MORB-like Rooi-Rand dyke swarm (RRDS) is presented. The AMS in the samples results from fine-grained, Ti-poor magnetite which in 20 dykes defines fabric sub-parallel to the dyke plane, consistent with the plume model. The magnetite defines a weakly anisotropic and dominantly oblate fabric. From a total of 10 dykes studied for plagioclase mineral shape preferred orientation (SPO), 8 have a dyke-parallel foliation most consistent with vertical magma flow. The plagioclase grains define a weakly anisotropic, oblate fabric, which is magmatic in origin. In 8 dykes this fabric is coaxial with the AMS fabric. However, in 40% of the dykes, the fabric defined by the SPO of opaque grains is non-coaxial with AMS and is at a high angle to the dyke plane and dips steeply. The non-coaxial AMS and SPO fabric, coupled with the orthogonal SPO fabrics suggests that late-stage lateral flow of relatively high viscosity magma has occurred. This results in a fabric which most workers would regard as “inverse” and/or non-magmatic, and, therefore, would misinterpret.