Geophysical constraints on structure and emplacement of Shap granite

Abstract

To understand Caledonian granite genesis in Britain it is essential that studies into the two-dimensional surface features of intrusions continue into the third dimension using various geophysical techniques. The characteristics of continental crust stabilised in the British Caledonian have been discussed elsewhere¹ and models for their evolution suggest a derivation direct from the mantle or by partial fusion of the oceanic crust. The Shap granite is no exception : geochronologically, geochemically and palaeomagnetically² it belongs to the Devonian group of Lake District granites, including the Eskdale, Skiddaw and Sub-Pennine Weardale intrusions. This suite yields the low initial Sr⁸⁷/Sr⁸⁶ ratios ( ∼ 0.706) (P. J. O'Connor, G. C. B. and M. D. Max, in preparation) which are characteristic of mantle fusion followed by limited crustal scavenging. The Shap intrusion occurs in the eastern Lake District and outcrops as an irregular oval measuring 2.8 km east–west and 2km north–south. It is a porphyritic adamellite³ dated 393 ± 11 Myr (ref. 4) and occurs at the junction between Ordovician Borrowdale volcanics and Silurian Brathay flags. The granite possesses a broad metasomatic aureole containing grossular and epidote. There has been debate concerning the subsurface shape of the intrusion, which may be lopolith⁵ or cone-shaped with depth. The gravity survey of the Lake District⁶ strongly suggests the latter, but modelling was based on only 14 stations along a single linear traverse of the granite. A stock-like body, extending to 8 km depth, gave a satisfactory model at that time. Here we report the combined observations of ground-based gravity and magnetic field surveys which apply to the structure of the intrusion and its aureole rocks.