Magnetic susceptibility, magnetic mineralogy and magnetic fabrics in a late Archean granitoid-gneiss belt

Abstract

The Opatica belt is a late Archean granitoid-gneiss terrane situated within the northern Abitibi Subprovince, along the northern margin of the Abitibi greenstone belt, Quebec. This belt is composed of several suites of tonalite-granodiorite-granite composition, variably deformed under epidote-amphibolite to upper-amphibolite grade conditions. Specimens from 87 sampling sites within the Opatica belt show a wide range in variation of magnetic susceptibility ( K ); the large variations are due essentially to varying proportions of ferromagnetic trace minerals. Detailed study of the opaque mineralogy by reflected light petrography, electron microprobe analysis and SEM observations, as well as alternating field and thermal demagnetization experiments confirm that multi-domain, very pure magnetite is the dominant ferromagnetic phase which, along with ferrosilicates, contribute to the magnetic susceptibility. In some cases, hematite may also make a significant contribution to K . In rocks with low K , the susceptibility may be dominated by the ferrosilicates hornblende and biotite. Intensity of anisotropy of magnetic susceptibility (AMS) is also highly variable, and appears to be controlled by the mineralogy of the rock rather than by strain intensity or deformation state. The shapes of AMS ellipsoids are also controlled by mineralogy. Detailed comparisons are made of AMS fabrics with field measurements of magmatic and metamorphic foliations and mineral and stretching lineations, and with mineral lineations determined by image analysis of polished slabs. There is very good agreement between the average orientations of poles to structural foliations, and the average orientation of the K 3 principal axes of the susceptibility ellipsoids in all outcrops. In most cases, there is also good agreement between average structural lineations and the average K 1 axis. In quartz-monzodiorites, disagreement between the average lineation and average K 1 may be due to complex textural occurrences of magnetite, and the presence of abundant hematite. In gneisses, broad scatter of the orientations of K 1 for individual samples from an outcrop (quantified by statistical confidence regions about the principal axes) locally occurs within the foliation plane, and may be attributed to the partial overprinting of earlier fabrics by subsequent deformation.