Palaeomagnetism of Late Ordovician igneous intrusions from the northern Welsh Borderlands: implications to motion of Eastern Avalonia and regional rotations

Abstract

AbstractThe palaeomagnetism of a Late Ordovician dolerite suite widely distributed in the northern Welsh Borderlands has been investigated to evaluate (i) regional rotations associated with later deformations and (ii) Lower Palaeozoic latitudes of Eastern Avalonia. Only local effects of Acadian and Variscan overprinting are observed in this region and most bodies appear to preserve a primary cooling-related magnetization. Magnetic properties in the Breidden Dolerite are related to primary igneous differentiation and a dual polarity characteristic remanence (D/I = 333.1/−71.6°, 6 sites, α95 = 10.4°) predates Late Ordovician folding. Dolerites elsewhere in the Breidden Hills Inlier record the same reversal of remanence and the combined mean (D/I = 314.3/−72.8°) is rotated clockwise from the mean remanence (D/I = 291.5/−59.5°, 21 sites, α95 = 6.0°) in a similar dolerite suite within the adjoining Shelve Inlier which is also of dual polarity and predates Late Ordovician folding. The difference in magnetic declinations is similar to the rotation of Late Ordovician fold axes and fracture systems between the two inliers and is probably the resultant effect of local rotation, possibly about a constraining bend in the Severn Valley Fault System. The Moel-y-Golfa Andesite of Caradocian (Soudleyan) age has a remanence (D/I = 294.3/−50.4°, 13 samples, α95 = 2.8°) which study of an adjoining volcanic conglomerate shows to be of primary cooling-related origin. The equivalent palaeolatitude is ˜ 31° S in early Caradocian times. A similar estimate is derived from a probable diagenetic overprint in the contemporaneous Castle Hill Conglomerate at Montgomery. The Mynydd-Bryn Andesite in the Berwyn Inlier, of probable similar origin and age to Moel-y-Golfa, has a magnetization of opposite polarity but similar inclination (D/I − 239.1/47.4°, 14 samples, α86 = 5.5°). Declination differences between these volcanic outcrops are a possible signature of later block rotations between the Lower Palaeozoic inliers and identify the importance of terrane definition in analysis of Lower Palaeozoic palaeomagnetic results from the Caledonides. A dolerite sheet complex at Hendre in the Berwyn Hills has a younger remanence (D/I − 182.3/ − 3.0°, 20 samples, α95 = 7.8°), probably of mid-Carboniferous age.The collective Upper Ordovician results define a rapid movement of the Avalonian Plate, probably spanning no more than 10 Ma, from ˜ 30° S in early Caradocian times to ˜ 57° S prior to folding of the dolerites in late Caradocian to early Ashgill times. This motion into higher southerly palaeolatitudes marks a reversal of the motion of Avalonia into more shallow southerly palaeolatitudes between Arenig and Llanvirn times and shows that Ordovician closure of the Iapetus Ocean was more complex than recognized hitherto.