A stratigraphic re-evaluation of the Upper Borrowdale Volcanic Group, English Lake District: with a focus on geochemical techniques for ignimbrite correlation

Caldera-forming eruptions are amongst the most catastrophic events to affect the Earth’s surface, with the potential to reshape entire landscapes. A barrier to understanding calderaforming eruptions typically arises from inaccessibility of key volcanological features such as eruption conduits, caldera floor fractures and caldera-fill deposits, which are buried at most modern calderas. One of the best sources for understanding how calderas erupt and subside, lies in the investigation of exhumed, deeply dissected calderas, where such key features are visible.

Numerous large silicic ignimbrites occur within the Borrowdale Volcanic Group of NW England. However, despite recent geological mapping, good exposure, tectonic uplift and deep glacial dissection, most ignimbrites have not yet been traced to their source calderas: a problem being the large number of similar-looking ignimbrites coupled with intense volcanotectonic faulting causing frequent discontinuities and repetition of units. Correlation based on field criteria alone is precluded by the absence of distinct marker units or paleosol layers. Mineral and major element geochemistry is also precluded by caldera-related hydrothermal alteration. Therefore, this investigation has tested the utility of using immobile trace-elements to (1) characterise and distinguish between ignimbrite sheets; (2) correlate them for long distances; (3) help estimate the eruption volumes, and (4) help identify source calderas.

The ignimbrites were characterised using element: element plots, where Nb, Th, Y, and Zr proved most effective. Even when discriminant fields overlapped, some ignimbrites could be distinguished by considering stratigraphic relations, compositional zoning, and mineralogical information (e.g. presence of garnet). In total, 27 units, from 8 sites, have been successfully distinguished, resulting in major stratigraphical revisions and discovery of four potential caldera sources base on key caldera-forming characteristics. The correlation method has thus proved successful, and is a valuable approach to disentangle pyroclastic stratigraphy in ancient terrains, allowing the unique benefits of exhumation to be more widely utilised.