The palaeomagnetism of a Precambrian metamorphic boundary in the Loch Torridon area, NW Scotland

Abstract

A palaeomagnetic conglomerate test of Lewisian amphibolite clasts in basal Torridonian Group sediments is used to demonstrate that the magnetic record of the Lewisian complex predates Torridonian sedimentation; it appears to have survived with minimal overprinting since post-Laxfordian uplift and cooling. A traverse across a Precambrian tectono-metamorphic boundary on the north side of Loch Torridon identifies progressive replacement of a relict remanence as the zone reworked during the Laxfordian events is entered. The relict remanence (E steep positive) is predominant in the Scourian Terrane and probably dates from the Laxfordian LM1 metamorphism. It survives locally only in dyke interiors in the zone profoundly affected by LMl and LM2. A single coherent NW shallow remanence is identified on the south side of Loch Torridon and is linked to pervasive effects of the LM3 episode here. The degree of magnetic overprinting does not appear to be related to the extent of Laxfordian fabric development, but it is closely linked to dyke contacts probably because these zones permitted a more ready access of metamorphic fluids.