On the Magnetization History of the Northern Irish Basalts

Abstract

Summary Thermal and alternating field demagnetization studies of 23 basaltic lavas of the Lower Tertiary igneous province of Northern Ireland (the Antrim Basalts) have revealed a very stable but composite magnetization: the resultant appears to consist of a dominating reversed component, thought to be the original one, plus a certain superimposed magnetization of normal polarity. The polarity together with various information about the opaque mineralogy suggest that the latter magnetization, which is the most stable one, has resulted from post-cooling (low temperature) oxidation. A separate component (normal or reversed) appears impossible to estimate with sufficient degree of accuracy but based on a comparison with some previous data it is tentatively suggested that the mean direction of the very stable net magnetization deviates by approximately 20 degrees of arc from that of the original field. On this assumption the magnetization directions derived from the Antrim Basalts would be of doubtful value for instance with regard to the study of any Tertiary continental drift in the North Atlantic region. The remagnetization hypothesis laid down has to be further tested by studying the magnetization of the laterite horizons within the formation which carry a significant amount of haematite formed within the period of lava eruption.