Palaeomagnetic study of the Canisp Porphyry, NW Scotland: an Early Silurian palaeomagnetic pole from the Laurentian Foreland

Abstract

Synopsis The Canisp Porphyry is a member of the acid alkaline suite in the Northern Highlands of Scotland. It was emplaced between 460 and 430 Ma into Torridonian and Cambrian–Ordovician sediments of the NW foreland to the Caledonian Orogen where it is cut by the Sole Thrust. The interior of the body has a magnetism dominated by multidomain magnetite that partially preserves a primary ‘A’ remanence with NE direction. A distributed range of easterly positive directed (‘B’) components with some reversed equivalents is also widely represented and Early Devonian (‘C’) overprints are more sporadically present. The chilled margin against Torridonian sandstones preserves a clear record of the ‘A’ component accompanied by diminishing partial overprinting of primary Torridonian Components within 6 cm of the contact. The ‘A’ component (mean D/I=23/ −36°, α 95 =4.4°) yields a pole position at 342°E, 11S° comparable to magnetizations observed in Caledonian alkaline plutons within the Northern Highlands. It demonstrates that these are post tectonic magnetizations representative of the Laurentian Foreland in Late Ordovician to Early Silurian times. The collective data define a rotation of this region during Early Silurian times at c. 440–425 Ma. Poles from the Canisp Porphyry and alkaline igneous bodies within the adjoining orogenic belt correlate with contemporaneous North American poles on the pre-drift configuration and confirm the integrity of the Early Silurian palaeomagnetic record from the North Atlantic continents. The ‘B’ components in the porphyry (mean D/I=58/31°) are similar to minority components in other intrusions and to overprinted magnetizations observed in the Torridon Group sediments. The latter are therefore inferred to be Caledonian acquisitions imparted by fluid flow resulting from loading of the foreland during orogenesis.