Central sagging of a giant mafic intrusion: the Ediacaran Sept Îles Layered Intrusion (Québec, Canada)

Abstract

Anisotropy of magnetic susceptibility data validated by image analysis were acquired on the Sept Îles Layered Intrusion. Most data come from the mafic Layered Series of the intrusion, although the overlying anorthositic Upper Border Series and felsic Upper Series were also investigated. In the Layered Series, Type 1 magnetic fabric defined by gently to moderately dipping foliations and horizontal to moderately plunging lineations dominates over Type 2 fabric characterized by steeply dipping magnetic foliations and lineations of variable plunge, although steep on average. The inverse relationship holds true for the Upper Border Series and only Type 1 fabric occurs in the Upper Series. Several observations, mainly centripetal disposition of the igneous layering, and inclination of the mean Type 1 foliation and lineation towards the deepest point of the intrusion, near its centre, demonstrate post-cumulus, central sagging of the Layered Series. The Type 2 fabric recorded the effect of synmagmatic discontinuities, possibly shear zones, in the Layered Series, its origin being speculative in the Upper Border Series. The mean Type 1 lineations of the Layered Series and the Upper Series are discordant, suggesting that the latter was slightly affected by the sagging of the underlying cumulates, possibly owing to its rapid waning.Supplementary material: Analytical methods (Appendix 1) and equal-area projections of the anisotropy of magnetic susceptibility principal axes (Appendix 2) are available at https://doi.org/10.6084/m9.figshare.c.5057499.v1