The Role of Tholeiitic Magmatism in the English Lake District: Evidence from Dykes in Eskdale

R. Macdonald, D. Millward, B. Beddoe-Stephens and J. Laybourn-Parry
Department of Environmental Science, University of Lancaster, Lancaster LA1 4YQ
British Geological Survey, Windsor Court, Windsor Terrace, Newcastle upon Tyne NE2 4HB
British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA
Department of Biological Sciences, University of Lancaster, Lancaster LA1 4YQ

Abstract: Mafic dykes occur in close association with, and both cut and are cut by, the Eskdale granite in the south-western Lake District. The dykes range compositionally from magnesian basalt to andesite and are divided into two groups: (1) high-Fe-Ti rocks of tholeiitic affinity forming most of the dykes and (2) a lower-Fe-Ti group, comparable in composition to the lavas of the Borrowdale Volcanic Group. The dykes extend the range of tholeiitic magmatism in the Lakes into late Ordovician, and possibly Silurian times, and indicate that published plate tectonic models partly based on the distribution of magma types are perhaps over-simplified. The Eskdale dykes form one end of a spectrum of Lake District compositions from tholeiitic to calc-alkaline. All the magma types may have shared a common mantle source, their final composition reflecting residence times in the crust or LIL-enriched mantle.

Keywords: Tholeiitic magmatism • English Lake District • Eskdale

Mineralogical Magazine; September 1988 v. 52; no. 367; p. 459-472; DOI: 10.1180/minmag.1988.052.367.04
© 1988, The Mineralogical Society
Mineralogical Society (