Paleomagnetic Dating of Brecciated Veins in Durness Group Carbonates, NW Scotland

A paleomagnetic study of brecciated veins in the Cambro-Ordovician Durness Group (NW Scotland), part of the Laurentian continental margin, was conducted to date the formation and determine the origin of the veins. The veins strike west-northwest and are found in close proximity to the Moine Thrust Zone (MTZ). The reddish veins range from 5-100 cm thick and contain brecciated fragments of the host Durness Group. Clasts of breccia cemented by coarse calcite suggest multiple brecciation events in some veins. Mean stable carbon and oxygen isotope values for both the host Durness and the breccia are consistent for Cambro-Ordovician marine carbonates and indicate alteration by meteoric fluids with a low water to rock ratio. The host Durness is a gray dolomite that contains authigenic iron oxides and a magnetization with northeasterly declinations and shallow inclinations that resides in magnetite. The pole is close to the Devonian part of the apparent polar wander path and the magnetization is interpreted as a chemical remanent magnetization (CRM). All the veins contain magnetizations that reside in hematite and are interpreted as CRMs. At several locations the veins contain a CRM2 with southerly declinations and moderate up inclinations with a few samples containing both normal and reversed components. In one area, the veins contain a CRM3 with NNE declinations and steep down inclinations that is not antipodal to CRM2. The poles suggest acquisition of the CRMs in the Triassic (CRM2) and the Jurassic (CRM3). The two CRMs could represent two separate brecciation and fluid flow events that precipitated authigenic hematite. The veins are associated with faults that trend west-northwest and cut the MTZ. The brecciation events are interpreted to be related to extension in the Mesozoic. This is consistent with the extensional history of the northern Atlantic margins, which was particularly active in the Triassic-Jurassic.