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

Abstract

The 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, <x 95 = 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, a 95 = 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, a 95 = 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, <x 86 = 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, a 95 = 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.

Palaeomagnetism of the Borrowdale and Eycott volcanic groups, English Lake District: primary and secondary magnetization during a single late Ordovician polarity chron

“…The precise palaeofield inclination recorded by the lower members cannot be resolved because of uncertainties in the plunge components affecting the rocks around Ullswater, but a possible reduction is implied during emplacement of the lower part of the succession with I=~39°(equivalent to a palaeolatitude of 22°S) by results from the High Ireby and Lower Primary and secondary magnetic remanence, Lake District, UK 501 Ullswater formations. This value is close to the lower of several palaeolatitude estimates from (middle Llanvirn) Builth volcanic rocks in the Welsh Basin where results from younger mid-Caradoc volcanic rocks identify motion into higher southerly latitudes (Piper, 1995(Piper, , 1997a; Table 5). Subsequent progressive motion into higher southerly latitudes is implied in the Lake District data by systematic increase in palaeofield inclination to I = -51°(palaeolatitude 32°S) in higher formations of the BVG continuing to I = -62°(palaeolatitude 43°S) during the overprinting event.…”

“…Reversals with a slight predominance of reversed polarity are recognized during the Llanvirn and Llandeilo; their presence is recognized, for example, in the Welsh volcanic rocks of this age Piper, 1995). However, the uppermost part of the Llandeilo and the lower part of the Caradoc incorporating the Nemagraptus gracilis and Diplograptus multidens biozones were characterized by uniform normal polarity .…”

Tectonic rotation within the British paratectonic Caledonides and Early Palaeozoic location of the orogen

“…By Late Ordovician (mid-Caradoc, Soudleyan) times this palaeofield had rotated to a WNW -/ESE+ direction of predominant normal polarity. This is recorded by the Moel-y-Golfa intrusive andesite (declination/inclination (D/J) =294/ -SW) and the slightly younger Shelve (D/I=292/ -59') and Breidden (D/I=314/ -73') dolerites (Piper 1995). Remanence in the former unit is demonstrably primary and magnetizations in the dolerite suites predate late Caradocearly Ashgill regional folding.…”

“…In the Welsh Borderlands the Late Ordovician magnetizations are orthogonal (Piper 1995;Fig. 1) to the Late Ordovician fold o/p, overprint.…”

“…It commences with the Upper Ordovician results from the Lake District. Contemporaneous pole positions from the Welsh Basin (Channell et al 1992;Piper 1995 and references cited therein) are recalculated and plotted following adjustment of magnetic declinations for 55" of anticlockwise rotation. This eliminates the difference in poles from the two regions evident in earlier compilations Channeli et al 1992) and identifies a west to east trend in the Ordovician APW; the track incorporates primary magnetizations assigned to this interval at New York University on June 26, 2016 http://jgs.lyellcollection.org/ Downloaded from and is recognized in both the Wclsh and Lake District Caledonides.…”