Very densely welded, rheomorphic ignimbrites of homogeneous intermediate calc-alkaline composition from the English Lake District

Within a largely concealed, caldera-related volcaniclastic succession of th e Ordovician Borrowdale Volcanic Group in the western Lake District, two th ick (100-350 m) ignimbrites within the Fleming Hall Formation exhibit a num ber of features that in combination make them unusual deposits. They are bo th homogeneous with comparatively low-SiO2 (63%) bulk composition, contain only a moderate crystal content, are generally poor in lithic clasts, show uniformly very dense welding (yielding parataxitic to massive vitrophyric t exture) throughout and lack associated fall-out or surge deposits. Ignimbri tes of comparable bulk composition in this geological setting are usually p art of zoned sheets and/or frequently very crystal-rich. Large-scale, unzon ed densely welded ignimbrites are usually rhyodacitic to rhyolitic. By cont rast, ignimbrites of intermediate composition that display dense welding ar e relatively small deposits that form by agglutination of hot, plastic spat ter. It is postulated that the Fleming Hall ignimbrites were derived from low co lumn height, low explosivity eruptions that conserved heat and minimized en trainment of accidental lithic clasts and the formation of fine ash. The ve ry dense welding and lack of bubble-wall shard vitroclastic textures indica te that pyroclasts were hot and relatively dry, probably occurring as mildl y vesicular (scoriaceous) fragments which welded or fused together during a ggradational deposition rather than by post-depositional compactional loadi ng. There is little variation in the degree of matrix or melt crystallizati on throughout the two ignimbrites, despite the fact that high temperatures must have been maintained for many years following deposition. Both display Virtually ubiquitous development of micropoikilitic glass devitrification texture, which suggests that the viscosity of the supercooled dacitic melt was sufficiently high, probably due to initial degassing, to inhibit signif icant melt crystallization after deposition. The eruption of the Fleming Hall magmas was probably initiated by the rise or injection of hotter, more basic, magma, and not by overpressurization du e to volatile exsolution resulting from cooling and crystallization. Founde ring of the chamber roof caused forcible and rapid eruption of the magma, p robably along a series of volcanotectonic faults rather than a central vent , and probably flooded the resultant caldera depression. It is predicted th at this type of eruption will not have produced a widely dispersed deposit, the bulk of which may have been largely contained within its own caldera.