When an ‘inverse’ fabric is not inverse: an integrated AMS‐SPO study in MORB‐like dykes

Abstract

Terra Nova, 23, 49–55, 2011 Abstract Magma flow direction is recorded in the macroscopic mineral petrofabric and magnetic fabric of the MORB‐like Rooi Rand dyke swarm. The bulk magnetic fabric is sub‐parallel to the dyke plane, resulting from preferred orientation of grains during magma flow. This is referred to as type‐A fabric. A study of mineral shape preferred orientation reveals an additional fabric, which is orthogonal to the average dyke orientation in 30% of the data. This fabric is carried by both plagioclase and opaque grains and would typically be interpreted as ‘inverse’. It is referred to here as type‐B fabric. However, because it is carried by the shape preferred orientation of macroscopic grains and is in most cases not coaxial to the anisotropy of magnetic susceptibility fabric, it is interpreted as having been acquired during increased grain interaction during late‐stage magma flow associated with decreasing magma pressure.