The Palaeogene Eskdalemuir dyke, Scotland: long-distance lateral transport of rhyolitic magma in a mixed-magma intrusion R. MACDONALD 1,2, *, B. BAGIN ´ SKI 1 , B. G. J. UPTON 3 , P. DZIERZ ˙ ANOWSKI 1 AND W. MARSHALL-ROBERTS 4 1 IGMP Faculty of Geology, University of Warsaw, al. Z ˙ wirki i Wigury 93, 02-089 Warsaw, Poland 2 Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK 3 School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK 4 22 Campbell Road, Longniddry, East Lothian EH32 0NP, UK [Received 21 March 2009; Accepted 13 May 2009] ABSTRACT The Palaeogene Eskdalemuir dyke, part of the Mull dyke swarm in the Southern Uplands of Scotland, is ~60 km long and up to 40 m thick. Its southern tip is 230 km from the inferred source on Mull. The dyke is composite, with tholeiitic basaltic margins and a vitreous central facies ranging from basaltic andesite to andesite in composition. Plagioclase and pyroxene phenocrysts and matrix crystals in the central facies show unusually large compositional ranges and complex textural relationships. Whole- rock major and trace-element abundances show linear variations against MgO content, consistent with the rocks in the central facies having formed by mixing of basalt and rhyolite magmas. The rhyolite can be closely matched by rocks from the Mull centre. The mafic and silicic magmas were intruded from a compositionally zoned chamber beneath Mull, perhaps during collapse of the Centre 1 caldera. The lower-viscosity basaltic magma was emplaced before, but lubricated the lateral propagation of, the silicic magma, which mixed with the partially solidified basalt, the proportion of rhyolite increasing towards the dyke centre. The Eskdalemuir dyke represents an unusual, perhaps unique, example of a rhyolite magma being emplaced >200 km from its inferred source. The supposed correlative of the Eskdalemuir dyke north of the Southern Uplands Fault, the Dalraith-Linburn dyke, is not comagmatic with it. KEYWORDS: composite dyke, magma mixing, lateral propagation of rhyolitic magma. Introduction THE British section of the Palaeogene Hebridean Igneous Province (formerly called the British Tertiary Igneous Province; Emeleus and Bell, 2005) consists of a series of igneous centres, exposed along the western coast of Scotland, in Northern Ireland and as several submerged bodies (Fig. 1a). The centres consist of lava piles, intrusive central complexes and sets of linear and radial dyke swarms. One of the most intensive of the linear swarms is associated with the Mull centre. In southern Scotland and northern England the swarm is dominated by a group of dykes of great length (~400 km) and persistence of strike direction. In that single intrusions may be separated from their neighbours by several kilometres, Geikie (1897) termed them the ‘‘Solitary dykes’’ (Fig. 1b). In between the Solitary dykes are several sets of shorter dykes, all trending approximately NWÀSE and consti- tuting part of the Mull swarm (Holmes and Harwood, 1929). If individual dykes do, in fact, persist the full distance from Mull to northeastern England, they are among the longest Phanerozoic dykes in the world. Our ongoing studies have revealed features of the Solitary dykes which bear on the mechanisms of emplacement: (1) some of the dykes are composite; in some cases, a silicic component preceded a mafic, in others the mafic component * E-mail: r.macdonald@lancaster.ac.uk DOI: 10.1180/minmag.2009.073.2.285 Mineralogical Magazine, April 2009, Vol. 73(2), pp. 285–300 # 2009 The Mineralogical Society