Interpretation of the gravity and magnetic anomalies over the Mull Tertiary intrusive complex, NW Scotland

Abstract

A 50 mGal local positive gravity anomaly over the Mull intrusive complex is interpreted as caused by an underlying dense body of mafic (basic and/or ultrabasic) rocks extending to a depth of between 6.5 and 13 km and occupying a volume of between 2000 and 3600 km 3 , depending on density contrast. The granites produce smaller superimposed negative gravity anomalies, which indicate that the granites of the late Loch Bà centre extend to a depth of about 2 km and those of the early Glen More centre occupy a significantly smaller volume. The granites occupy between 5 and 9% of the total volume, consistent with a derivative origin. The complicated aeromagnetic anomalies have been simplified by applying the pseudogravimetric transformation. The most conspicuous positive pseudogravimetric anomaly occurs over the rocks of centres 1 and 2. This is of much smaller areal extent than the positive gravity anomaly, and it is attributed to highly magnetic rocks extending to a depth of about 2 km beneath centres 1 and 2, including cone sheets. The magnetization of the deeper parts of the complex is much weaker and has not been resolved. A partial ring of linear or arcuate negative magnetic anomalies occurs along the SE, S and W margins of the deep-seated intrusive complex and may mark an unexposed early ring dyke with strong reverse magnetization. It is estimated that up to a quarter of the volume occupied by the intrusive complex may have been provided by various types of forceful intrusion but that most of the upper crustal space has been made available by ring subsidence and other types of stoping. This is feasible because the magma was demonstrably lower in density than the Moine and Lewisian. The mantle-derived magma probably rose through the more ductile lower crust by diapirism, thereby causing a slight thickening of the underlying crust which may provide isostatic support for the excess mass of the intrusive complex and its associated topography.