Magnetic fabrics in Archean granitoids, Northwestern Ontario: Isolation of accessory and matrix contributions by inspection of AMS data

Pre-tectonic and post-tectonic granitoid plutons in the same area share similar orientations for their AMS (anisotropy of magnetic susceptibility) foliation and lineation, and these are similar in orientation to the L–S fabrics of the country rock, which bears a vertical ENE–WSW foliation. The AMS fabrics in the granitoids are composite, blending contributions from accessory but highly susceptible magnetite with those from paramagnetic matrix silicates. We isolate magnetic sub-fabrics in the granitoids by a simple numerical approach that standardizes the mean tensor for susceptibility of specimens, thereby suppressing the skewing effect of high susceptibility accessories (i.e., magnetite). This approach approximately separates the contributions of “ferro”-magnetic and of paramagnetic minerals to AMS though not as completely as technical laboratory experiments.