Structure of a hypovolcanic acid complex inferred from magnetic susceptibility anisotropy measurements: the Western Red Hills granites (Skye, Scotland, Thulean Igneous Province)

Abstract

The geochemistry and the injection mechanism of hypovolcanic ring dykes have been extensively studied, but such is not the case for their internal fabric. The Tertiary Western Red Hills epigranites of the Isle of Skye are a classic example of such intrusions. Using anisotropy of magnetic susceptibility measurements, we present the first structural data of their internal magmatic fabric. The magnetic foliations, equated with the magmatic flow planes, have strikes which roughly follow the walls of the different intrusions. They dip steeply toward the convex wall of each intrusion. The lineations, or maximal magnetic susceptibility axes, generally have shallow plunges, except in the latest granite intrusion. These structures appear to be related to the compressional deformation of each intrusion toward the end of its crystallization. This shortening would be a consequence of a radial and compressive stress field acting after each injection of magma. This radial stress field is interpreted as the effect of high magma pressures originating from the acid magma chamber underlying the ring-dyke complex at a shallow depth.