Palaeomagnetic studies of the Longmyndian and other British late Precambrian/early Palaeozoic rocks, and their regional tectonic implications

Abstract

The natural remanent magnetization (NRM) of the Longmyndian sediments is carried by hematite and was imposed post-tectonically in a palaeomagnetic field D = 54°, I = +80° (α 95 = 6°). Crosscutting dykes and their baked contacts have stable NRM D = 316°, I = -16° (α 95 = 10°). The corresponding palaeomagnetic poles are 60°N, 29°E and 19°N, 223°E respectively. These results are consistent with (but do not prove) the notion that deformation of the sediments and dyke intrusion both occurred during a period of rapid “apparent polar wander” during the late Proterozoic. After rotating the European and American plates into their pre-Wegenerian drift positions the pole from the dykes is consistent with late Proterozoic data from the whole region but the (older) pole from the Longmyndian sediments is not—a change which may reflect contemporary closure of the pre-Caledonide ocean. Uriconian and Malvernian rocks carry magnetizations which are probably post-tectonic; their in situ directions are not significantly different from either the Longmyndian NRM or the present geomagnetic field. Because at least some of these NRMs reside in multidomain magnetite of doubtful palaeomagnetic stability, and in the absence of further controls, the age and geological significance of these remanences is unknown. Some Monian metasediments from Lleyn are only partially stable palaeomagnetically, and even the more stable component is probably secondary and of Caledonian age. Arvonian metavolcanies from North Wales could carry a primary remanence, but the preliminary studies reported here are inadequate to define it with acceptable precision.