USE AND MISUSE OF ANISOTROPY OF MAGNETIC SUSCEPTIBILITY DATA FOR INTERPRETING MAGMATIC DEFORMATION: A REVIEW OF CASE STUDIES

The anisotropy of magnetic susceptibility (AMS) is frequently used to determine the mineral preferred orientations in igneous rocks, including granitic plutons. Like other petrofabric methods (U-stage, X-ray diffraction, neutron diffraction) the AMS is a valuable tool for establishing the mineral fabric of a rock, which may be used to determine the strain and kinematic histories. The AMS technique is efficient, uses relatively large volumes of rock (and is therefore useful for coarse grained rocks like granites) and, even when rocks appear macroscopically isotropic, the AMS will generally provide reproducible magnetic fabrics. Therefore, the AMS can be used to map petrofabrics over large surface areas. However, measurement of the AMS is only the first step in its proper interpretation; it is often impossible to pass directly from the compilation of the AMS data to the interpretation of the rock fabric. We present an overview of our own well documented case studies that illustrate the significance of the AMS in granitic plutons, how the AMS may or may not be related to the mineral fabric of interest, and methods that may be used to test for normal and anomalous fabrics (by anomalous, we mean AMS that is not coaxial with the petrofabric of interest, in the sense of Rochette et al., 1992). The proper interpretation of AMS measurements requires that the susceptibility data be considered in light of the full data set, including structural geology of the granitoids and surrounding host rocks, mineralogy of the samples and, if necessary, additional magnetic measurements to verify the source of the susceptibility signal. Often, the influence of ferrimagnetic minerals (e.g. fine grained magnetite) on the AMS is underestimated, especially in weakly susceptible rocks that are sometimes interpreted to be "paramagnetic". These include (amongst others) High Field Analyses (HFA), Anisotropy of the Anhysteretic Remanent Magnetization (AARM), calculating theoretical AMS from u-stage measurements and numerical modeling of fabric overprinted by small strains. Each of these techniques provides useful insights as to the origin of the AMS and may help to avoid misinterpretations of the AMS.