Palaeomagnetic dating of clastic dykes in Proterozoic basement, NW Scotland: evidence for syndepositional faulting during deposition of the Torridonian

The Lewisian basement rock in NW Scotland contains sandstone dykes that are interpreted to have formed during syndepositional faulting. Although the ages of some dykes are known, the time of formation for other dykes is unknown. To test if the dykes can be dated using palaeomagnetism, dykes were sampled at Clachtoll, where the timing of dyke emplacement is known (Stoer age, c. 1200 Ma). The results confirmed that the Clachtoll dykes contain a Stoer-age magnetization residing predominantly in haematite with a pole position at 29.1 degrees N, 220 degrees E. This dating approach was applied to dykes of uncertain age near Gairloch. These dykes contain two magnetizations with similar directions, one interpreted to reside in magnetite and another in haematite. Both pole positions (magnetite: 31.8 degrees S, 217.2 degrees E; haematite: 37.8 degrees S and 236.7 degrees E) are similar to the reference Torridon Group poles (c. 980 Ma). The magnetite magnetization is interpreted to be of detrital origin whereas the haematite component is considered to be a detrital and/or early chemical remanence. The magnetizations in both dyke sets are inferred to have been acquired during or soon after infilling of the dykes. The results from Clachtoll are consistent with previous studies which indicate that emplacement of elastic dykes and syndepositional faulting occurred during deposition of the Stoer Group. The results from the dykes near Gairloch indicate another younger period of syndepositional extensional(?) faulting during deposition of the Torridon Group.