Palaeomagnetic Studies in the British Caledonides-I Igneous Rocks of the Builth Wells-Llandridnod Wells Ordovician Inlier, Radnorshire, Wales

Comprehensive sampling of intrusive and extrusive volcanic rocks in Radnorshire, Wales (52.1° N, 3.3° W) leads to two new estimates of Ordovician palaeomagnetic field directions. The mean direction of magnetization for the Builth Volcanic Series of Upper Llanvirn age is ̀ = 177°, Ī = + 39° (α95 = 8.9°) based on 15 sites. In minor basic intrusions of early Ashgillian age two mutually antiparallel groups of three sites yield a mean direction irrespective of polarity of ̀ = 174°, Ī = + 55° (α95 = 11.2°) which is regarded as a best estimate although results from five other statistically significant sites have to be explained as transitional during polarity change or disregarded as anomalous. The polarity transition recorded in the intrusives is likely to be R→N as inferred from the probable sequence of intrusion. All results are presented after magnetic cleaning and correction for the regional westward tilt of the inlier. Supporting thermal demagnetization data are presented. Small scale folding in the vicinity of the Welfield intrusion is shown to have originated as part of the intrusion process, and the data are compatible with the postulate that the Welfield intrusion is laccolithic. Pole positions from the volcanics and intrusives are respectively 359° E, 16° S (dψ = 6°, dχ = 11°) and 2° E, 2° S (dψ = 11°, dχ = 16°). The results are significantly different at the 95 per cent confidence level but discussion of this difference is deferred to Paper VI.