Superimposed Recent, Permo-Carboniferous and Ordovician palaeomagnetic remanence in the Builth Volcanic Series, Wales

The Builth Volcanic Series of Llanvirnian age in Llanelwedd Quarries, mid-Wales, carries three components of natural remanent magnetisation. Component P, regarded as primary, is a thermochemical remanence directed at D = 181.7°, I = +54.5°, α 95 = 4.4° relative to bedding. Component S is a secondary component with in situ D = 178.7°, I = −6.7°, α 95 = 5.4° and is believed to be a low-temperature chemical remanence (CRM) of Permo-Carboniferous age. Component R is directed close to the present geomagnetic field and is believed to be a recent viscous remanence (VRM). The results are of interest for three reasons. First, they are an unusually good example of multi-component NRM analysis, the three components being so clearly discriminated by thermal demagnetisation because they have almost completely separate blocking temperature ranges. Second, they provide evidence of a Permo-Carboniferous event (possibly a mild thermal or hydrothermal pulse promoting CRM acquisition) some 40 km north of the Hercynian orogenic front. Third, they illustrate very clearly the importance of detailed demagnetisation: this work revises the pole position for these rocks by ∼ 10° and removes an obstacle to the palaeomagnetic recognition of the ∼ 1000 km wide Iapetus Ocean cutting Britain in Ordovician time.