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

Ignimbrites of the Fleming Hall formation, of the Ordovician Borrowdale volcanic group, exhibit features that in combination make them unusual, their remarkable homogeneous thickness (> 250 m), their comparatively low SiO (sub 2) (63%) bulk composition, only moderate crystal content, very dense welding (giving parataxitic to massive vitrophyric texture), and lack of associated fall-out or surge deposits. Major, trace and REE analyses are reported. It is suggested that these ignimbrites were derived from low column height, low explosivity eruptions that conserved heat and minimized the entrainment of accidental lithic clasts and the formation of fine ash. The very dense welding and lack of bubble-wall shard vitroclastic textures indicate that pyroclasts were hot and relatively dry, probably occurring as mildly vesicular (scoriaceous) fragments which welded or fused together during aggradational deposition rather than by post-depositional compaction. The eurption of the Fleming Hall magmas was probably initiated by the rise or injection of hotter, more basic, magma, and not by over-pressurization due to volatile exsolution resulting from cooling and crystallization.