Magnetic fabric and microstructural evidence for a tectono-thermal overprint of the early Proterozoic Murray pluton, central Ontario, Canada

Abstract

The 2.38 Ga old Murray granite pluton intruded the southern footwall of the Sudbury Igneous Complex and was subjected to repeated ductile deformation. Its NW and SE domains display differences in strain state, AMS signature and the nature and distribution of Fe-oxide minerals. The magnetic foliation is parallel to the visible foliation in the NW domain, but discordant to it in the SE domain. Microstructural data show that the magnetic fabric in the NW domain coincides with a foliation formed at high temperature, probably in response to thermal softening during the emplacement of the adjacent Sudbury Igneous Complex. This tectono-thermal overprint affected pre-existing shape fabrics and was facilitated by incipient melting of the granite near the intrusive contact with the Sudbury Igneous Complex. Rocks of the SE domain are only partially overprinted and two mineral subfabrics are preserved. Prolate susceptibility ellipsoids and clusters of maximum susceptibility directions at the intersections of the two planar fabrics suggest oblique superposition of strain increments. Increases in bulk magnetic susceptibility and magnetic anisotropy degree towards the intrusive contact with the Sudbury Igneous Complex can be explained by respective increases in the proportion of magnetite over ilmenite and strain intensity in the Murray pluton.