Magnetic properties of oceanic pillow basalts from Macquarie Island

Abstract

WE summarise here the results of a combined palaeomagnetic, hysteresis, thermomagnetic, and electron microprobe analysis of late Tertiary oceanic pillow basalts from Macquarie Island, which indicate that pillow basalts from a significant depth in the oceanic lithosphere have suffered burial metamorphism during the crustal accretion process. This metamorphism results in an intensity of natural remanent magnetisation (NRM) of ∼10⁻³ gauss, a substantial component of viscous remanent magnetisation (VRM), and an order of magnitude decrease in the Koenigsberger ratio relative to unaltered pillow basalts. Since a pillow lava layer with such a low NRM intensity is insufficient to account for the observed amplitude of marine magnetic anomalies, we conclude that in the case of Macquarie Island the underlying basaltic and doleritic dyke complex must also contribute to the observed anomalies.