Magnetic properties of Precambrian pillow lavas of the Mona Complex and a related dyke swarm, Anglesey, Wales

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Abstract
Four palaeomagnetic sites in Monian pillow lavas of the Gwna Group from Llanddwyn Island, Anglesey, yield statistically‐significant groups of magnetic directions after demagnetising in low fields. A palaeomagnetic pole of 57°E, 16° S is derived after dip correction assuming no plunge components, and is believed to apply to the time of formation of the pillow lavas (ca. 600 m.y.). By contrast to young pillow lavas these rocks have poor magnetic stability and low Köenigsberger ratios although petrologic features are comparable; this is believed to be due to replacement of the finer grain size fractions of the titanomagnetite. Five of the seven dolerite dykes cutting the Monian rocks here yield pole positions close to those of Upper Silurian to Lower Devonian rocks from elsewhere in Britain confirming that they are Palaeozoic in age.

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