The anisotropy of magnetic susceptibility of some igneous and metamorphic rocks

The anisotropy of magnetic susceptibility of some basic igneous and metamorphic rocks has been found to be due to the preferred orientation of the long axes of grains of magnetite. The degree of anisotropy is in a few samples as great as 40 per cent but usually is less than 10 per cent. The variation in anisotropy is believed to be due to variation in the degree of preferred orientation, and in susceptibility due to grain size and chemical composition. Half of the susceptibility ellipsoids determined were found to approach an oblate shape while only a quarter of the remainder approached prolate shapes. The magnetic ellipsoids of a number of Tertiary lava flows, banded gabbros, dikes, cone sheets, a ring dike, and Precambrian basic gneiss have been determined. The principal susceptibility directions of most of the lavas were widely dispersed but in some flows were tightly grouped—the minima round the pole of the flow plane, the maxima normal to the flow direction in the flow plane, and the intermediates in the flow direction. Similar groupings were found with the banded gabbros but not with the other intrusives. Basic gneisses show a very marked concentration of maxima in the direction of the visible lineations and of minima normal to the observed foliation. The applicability of anisotropy measurements in structural petrology is discussed.