RELATIONSHIP OF LOW-FIELD AMS FABRICS WITH MAGMA FLOW IN A LAYERED GABBROIC SILL

Low-field anisotropy of magnetic susceptibility (AMS) fabrics of igneous rocks often coincide with the mineral fabric and are useful to map magma flow. However, the interpretation of magnetic fabrics in mafic rocks is not always straightforward. Complications arise from the ferromagnetic inclusions in mafic silicates and from single-domain magnetites.

The Insizwa sill, is a 30 km-diameter,1000 m-thick layered intrusion, part of the Karoo Province in South Africa. The peridotitic and gabbro-noritic rocks are undeformed and mineral fabrics demonstrably result from magma flow. An horizontal centimeter-scale, layering is visible in numerous outcrops. Plagioclase tabular crystals mark a layering-parallel foliation interpreted as the magma flow plane. Elongated crystals of plagioclase form a NW-SE trending mineral lineation interpreted as the magma flow direction.

Throughout the 78 stations of this study (699 specimen), magnetic susceptibilities (Km) range from 750 to 10000 x 10^-6 SI. The magnetic anisotropy (Pj) ranges from 3 to 8 %. Magnetic ellipsoids are neither prolate nor oblate (average Tj=0). AMS fabrics are dominated by multi-domain to pseudo-single domain magnetite. High-field magnetic experiments indicate that the paramagnetic contribution from the mafic silicates is less than 50 % for low susceptibility rock types. The anisotropy results from magnetite grain shape and not from interactions as shown by no significant increase in Pj with increasing Km.

The magnetic lineation (305°, 05°) is consistent throughout the sill at various scales and coincides with mineral lineations. In contrast, the magnetic foliation (125° NE 10°) is generally perpendicular to the mineral foliation and to the layering. Several explanations are possible as determined by high-field measurements of the magnetic fabric (HFAMS) which isolates the paramagnetic mafic silicates form the ferromagnetic magnetite grains.