Magnetic susceptibility anisotropy: A new petrofabric tool in migmatites

Abstract

The rheological behavior of the lower to middle continental crust during orogenic deformation is poorly understood. Migmatite, common at this crustal level, remains difficult to analyze from a structural viewpoint because of pervasive recrystallization. We present new anisotropy of magnetic susceptibility (AMS) data from 217 specimens of migmatites of the Superior Province in Minnesota with the aim of investigating its potential as a petrofabric tool. Quarry exposures provide the ideal sampling playground for such a study. The migmatites are coarse grained, are granitic to tonalitic in composition, and display a centimeter‐scale compositional layering. The layering and the regional foliation are subhorizontal to shallowly dipping to the NE. Mineral lineations are difficult to observe due to recrystallization but trend around 70°N with gentle plunges. Magnetic properties are carried mainly by multidomain (MD) primary magnetite grains ranging in size from 20 to 200 μm. The magnetic susceptibilities are generally high, around (5–10) × 10 −3 SI. The degree of magnetic anisotropy is large, between 1.2 and 1.3. The principal axes of the AMS ellipsoids coincide with those of the finite strain, and the degree of anisotropy broadly correlates with the mineral fabric intensity. This study is the first detailed investigation of magnetic fabrics in migmatites. These preliminary results suggest that the AMS technique is an easy and powerful tool for the petrofabric analysis of migmatites.