Geochemical Correlation of Pyroclastic Sheets of Scafell and Langdale Calderas, N.W. England

Several caldera volcanoes lie within the ~6 km thick Ordovician Borrowdale Volcanic Group of north-west England, which is a remnant of a Palaeozoic subaerial continental arc. Scafell caldera, >140 km2, is best known and intensively studied because it is a rare example, worldwide, where exhumation and glacial dissection reveals the entire succession of pre-caldera, caldera-fill and post-caldera lake succession along with the caldera floor faults, vents and domes. Several calc-alkaline high-K silicic ignimbrites and andesite pyroclastic units are associated with Scafell caldera and nearby Langdale caldera. Correlation of individual ignimbrites from the caldera fills to distal outflow sheets is hampered by their large number (>18), by hydrothermal alteration, Acadian cleavage, thrusting, and regional prehnite-pumpellyite metamorphism. Therefore, most outflow ignimbrites have not yet been traced to their source areas. Building upon previous work on the calderas and to the south of those (around Coniston), targeted field-mapping has been successfully used in combination with a range of mostly ‘immobile’ trace-element ratios (e.g. Nb/Y, Zr/TiO2, Th/Nb, Th/Y, V/Y) to resolve local successions and to confirm the presence of the Scafell caldera tuffs outflow sheets as far as 11 km south from Scafell caldera. The Coniston Fells succession is significantly revised, with new thickness estimates and the discovery of three high-level intrusions, the Oxendale-, Wetherlam-, and Glassy Crag Dacites, each geochemically distinct from Scafell units. A welded outflow sheet from Langdale caldera is identified ~14 km southwest of the caldera for the first time. Moreover, the Lincomb Tarns Formations in the central Lake District differs geochemically from the unit mapped as this in the Coniston area, for which a new name (Foul Scrow Tuff Member) is proposed. Whole-rock trace-element geochemistry combined with detailed fieldwork proves to be an effective tool for correlating ancient ignimbrites even where there has been locally limited tectonic deformation and alteration.