Lithostratigraphy of a concealed caldera-related ignimbrite sequence within the Borrowdale Volcanic Group of west Cumbria

Abstract

SUMMARY A sequence of volcaniclastic rocks, more than 1200m thick, is described from beneath Permo-Triassic and Carboniferous rocks in the Sellafield area of west Cumbria. The rocks are locally weakly cleaved and are considered to be part of the Ordovician Borrowdale Volcanic Group which crops out a few kilometres to the east. Seven formations are defined from six of the deep boreholes drilled by UK NIREX Ltd. The rocks are predominantly high-grade welded ignimbrites. Abrupt lateral thickness changes and intraformational collapse mesobreccias characterize the sequence. The ignimbrites range from andesite to high-SiO 2 rhyolite. Individual formations show distinctive vertical zonation profiles. Compositions fall on a common magmatic trend, indicative of a similar fractionation history. There is no overall compositional trend with time through the succession, though the high-SiO 2 rhyolites were probably erupted early in the cycle. Deposition is considered to have been within an actively subsiding basin, probably a caldera complex. This is considered to have had an episodic history, with periods of caldera collapse following eruptions during which an evolving magma chamber was tapped.