Geophysical investigation of the Beinn an Dubhaich Granite, Skye

Abstract

AbstractThere has been a long-standing controversy concerning the form of the Beinn an Dubhaich Granite, which intrudes Cambro-Ordovician limestones and contains enclosures of limestone at outcrop. We have undertaken a three-part geophysical investigation with the aim of resolving some issues raised by previous workers. Our most significant finding results from the measurement of a gravity profile across Beinn an Dubhaich. It reveals a negative anomaly of more than 2 mGal over the granite outcrop. Our preferred model to fit this anomaly is a steep-sided granite stock extending down to about 1 km depth, which implies that the limestone enclosures are roof pendants. In a supplementary study, we measured magnetic profiles across dykes in some of the limestone enclosures in order to determine the depth to the contact with the underlying granite. However, in the most clear-cut cases we could only determine a minimum depth of 20 m, and were unable to constrain the maximum depth. In hindsight, we suggest that microgravity surveying would be a better method for determining the thicknesses of the limestone roof pendants. In the third part of the investigation, we measured a tight grid of magnetic profiles to the northeast of the granite outcrop to test a previous suggestion that dyke-like lobes of granite intrude pre-existing fractures in the overlying limestones. We located several large magnetic anomalies associated with outcropping doleritic dykes and concluded that these had not been correctly identified in the earlier work. Consequently, the ground magnetic profiles acquired to date provide no valid evidence to support the idea of an interfingering contact between the limestones and the underlying granite.