Palaeomagnetism of the Hypersthene-Gabbro Intrusion, Ardnamurchan, Scotland

Abstract

THE hypersthene-gabbro of Centre II, Ardnamurchan, is an arcuate intrusion of Eocene age with an annular width of about 0.5 mile and a diameter of outer circumference of 5 miles¹. It has funnel shaped layering the dip of which steepens systematically towards the centre from as low as 8° at the south margin to more than 60° near the younger ring intrusions forming the inner margins². The classic model used to explain the genesis of layered gabbroic intrusions is that of Skaergaard^3,4 where the layering is caused chiefly by the settling of crystals through a convecting magma. Layers formed in this way have a strong tendency to be saucer shaped with dips low at the periphery and flattening towards the centre. Hypotheses to account for funnel shaped layering can be separated into two principal groups: (1) dips are a primary feature formed during magmatic crystallization and consolidation^2,5; (2) dips are a secondary feature produced by post-consolidation tilting (ref. 6 and unpublished work of R. R. Skelhorn and R. W. D. Elwell).