Significance of anisotropy of magnetic susceptibility fabrics in Proterozoic mafic dykes, Hopedale Block, Labrador

Abstract

Anisotropy of magnetic susceptibility fabrics have been studied in a suite of mafic early Proterozoic (Kikkertavak) dykes and two mid-Proterozoic (Kokkorvik) sills in the Hopedale Block of Labrador. Two sets of en-echelon dyke arrays from the Kikkertavak Swarm are considered to represent two linear dyke intrusions at depth. Three main fabrics were revealed in the Kikkertavak dykes. In the first, the maximum susceptibility axis is sub-horizontal or gently inclined and parallel to the dyke walls and is interpreted as the primary magma flow direction. A second fabric affects only dykes along strike from a much younger dyke and is interpreted as the result of local stress anisotropy. The third fabric with significantly lower susceptibility is only locally developed in the southeast, near the mid-Proterozoic Kanairiktok Shear Zone and is also interpreted as the result of local superimposed tectonic stress. The Kokkorvik sheets preserve a single flow-induced fabric, sub-parallel to the measured crack propagation lineation, indicating that the magma flow was parallel to the crack propagation direction. The gently inclined magma flow direction inferred for the Kikkertavak dykes is believed to reflect later-stage magmatic emplacement when consolidation had already commenced. At this stage, the buoyancy contrast between magma and host rock had been reduced to the point when the initially vertical magma flow was no longer sustainable and was replaced by lateral flow.