Palaeomagnetism of ordovician ignimbrites from Killary Harbour, Eire

Abstract

Twenty-five samples have been collected from four bands of highly oxidized mid-Ordovician (post-Arenig) ignimbrite exposed in County Mayo, Eire. The natural remanent magnetization (NRM) of three of the bands was significantly misaligned with the present Earth's field. Sixty-nine specimens were then subjected to detailed demagnetization in alternating fields to 795 peak Oe and temperatures to 670°C. The results demonstrated that the NRM of the four bands is dominated by a very stable component with blocking temperature in the 400–550°C range. A positive fold test indicates that this magnetization was acquired prior to the folding of the Mweelrea syncline in the Ordovician or earliest Silurian. Assuming a geocentric axial dipole field of “reversed” polarity and allowing for possible incomplete averaging of the secular variation or other field assymmetries, a geographic north pole in the Pacific Ocean (9°N, 146°W, dp = 12°, dm = 22°) was calculated from the resultant of the four band mean directions relative to bedding planes, after treatment of all samples to 540 Oe. This result is compatible with the few published Ordovician poles relative to Europe.