Rock magnetic and structural investigation of the Moss Lake stock and local area : western Shebandowan belt

Abstract

The thesis area is located 120 km west of the City of Thunder Bay, Ontario, straddling the contact of the Quetico and Wawa subprovinces of the Archean Superior Province. Metasedimentary rocks of the Quetico Subprovince with basic and granitic intrusions occupy the northwestern portion of the study area and are in contact to the southeast with metamorphosed mafic and felsic metavolcanic rocks of the Wawa Subprovince. Tectonic compression has shortened the metasedimentary rock layers a minimum of 80%. Bedding dips steeply to the northwest and a later cleavage, developed by transpression, is sub-parallel to bedding. Graded beds young predominantly to the northwest. Some graded beds young to the southeast and may represent pre-cleavage folding. No large-scale folds are present. Metamorphic grade increases northwest from greenschist to amphibolite facies over a distance of about 10 km. Magnetite is the predominant magnetic component of the Moss Lake stock; hematite is present in trace amounts. Magnetic susceptibility of the stock is high ( 12,000 X10-6 SI), making the intrusion amenable to anisotropy of magnetic susceptibility (AMS) work. AMS study of the Moss Lake stock shows that individual directions of maximum susceptibility have been reoriented, in most cases sub-parallel to the local planar fabric strike of the Quetico Subprovince. Magnetic fabric parameters show that the rock magnetic fabric of the intrusion is deformed. Vestiges of original magmatic fabric are evidenced by prolate (constricted) magnetic fabric associated with the central long axis of the stock but magnetic fabric parameters confirm that the intrusion margin is more deformed than the interior. The predominant oblate (flattened) magnetic fabric of the Moss Lake stock is the product of northwestsoutheast tectonic compression. Alternating field and thermal demagnetization of oriented rock specimens confirm that the Moss Lake stock is deformed by tectonic compression. Separation of magnetite and hematite magnetic contributions, by blocking temperature, reveals that primary natural remanent magnetization (NRM) orientations from hematite are parallel to the local planar fabric strike of the Quetico Subprovince. Minor preservation of primary magmatic fabric is indicated by the mean principal component analysis (PCA) orientation for magnetite which corresponds closely in trend to the mean ~ e orientation for the region, obtained by previous investigators. Original Moss Lake stock magnetic fabric is overprinted by compression and shearing. Comparison of magnetic studies (AMS and NRM) of the Moss Lake stock to structural data of country rocks argues in favor of a common tectonic control.