The South Harris gravity anomaly

Abstract

The results of a gravity survey of South Harris show that a residual gravity anomaly of + 220 g.u. occurs towards the southeast of the anorthosite-gabbro-tonalite igneous suite, with a minor low of +150 g.u. over the anorthosite outcrop and a minor high of + 170 g.u. over the main gabbro outcrop. The distribution of the maximum positive anomaly and the minor anomalies is interpreted as due to interbanding of gabbro and anorthosite. The tonalites do not produce a local anomaly and are interpreted as extending to only limited depth. North of the igneous suite the residual gravity anomaly falls to -10 g.u. over a hidden granite within migmatitic gneisses. It is suggested that the igneous suite was formed when a large body of gabbroic-anorthositic magma was injected into older continental crust, causing melting and the formation of contemporaneous tonalites, with complex fractionation producing gabbro and anorthosite layering. The sequence of events is interpreted as relating to an igneous episode that was a prelude to continental rifting which predated later phases of metamorphic/orogenic activity.