Significance of magnetic and petrofabric in Karoo-feeder dykes, northern Lebombo

Abstract

The orientation of magnetic and petrofabric in 14 mafic dykes of the Northern Lebombo dyke swarm (NLDS) has been determined using the anisotropy of magnetic susceptibility (AMS) and mineral shape preferred orientation (SPO) in order to constrain the magma flow direction during dyke emplacement. The N–S striking NLDS intruded the N–S trending; 700 km long Lebombo faulted monocline between 182 and 178 Ma. The Lebombo faulted monocline, which forms the eastern-most sub-province of the Karoo large igneous province (LIP), is essentially a volcanic rifted margin with a protracted magmatic and tectonic history related to the break-up of southern Gondwana. It also forms the southern limb of the Karoo triple-rift system which has been hypothesised to have formed by a mantle plume centred on the triple junction locus. The other two arms of the rift which converge on the triple junction are the WNW trending Okavango dyke swarm (ODS) and NE trending Save-Limpopo dyke swarm (SLDS). In the NLDS it is found that the magnetic fabric, which is of magmatic origin in ~ 50% of the samples, is carried predominantly by stoichiometric magnetite. There is remarkable agreement between the orientations of the AMS and SPO fabrics and at least two dykes show coaxial AMS and mineral SPO fabric consistent with lateral magma flow from the north. This direction is supported by the imbrication of the foliations of the well-defined bulk AMS and plagioclase SPO fabric when viewed in a dyke co-ordinate system. Although steeply plunging maximum axes occur in the SPO fabric, their possible use as magma flow vectors is questioned because they do not necessarily mimic the elongation of plagioclase grains visible in orientated thin sections. The magnetic and petrofabric of the NLDS is interpreted to have developed during lateral magma flow from the locus of the Karoo triple junction.