Early Evolution of the Palaeogene Mull volcano: An integrated volcanological, Geochemical and Geochronological approach

The British Palaeogene Igneous Province (BPIP) has long been an area of research, both for British volcanology and in the context of the early stages of opening of the North Atlantic. Previous studies have investigated the petrogenesis of the Mull lavas (e.g. [5]) and the evolution of other igneous centres associated with the BPIP, such as on Arran [3],[4]. However, since the Mull Memoir was published by Bailey et. al. [1], little substantive research has been carried out on the early evolution of the Mull central complex, the focus of this study.

Through combined fieldwork, geochemical and geochronological research, this study has elucidated significant new information in understanding the early development of the central complex on the Isle of Mull.

Volcanological focussed fieldwork, combined with geochemical fingerprinting and petrological examination, has revealed a new model for the post-collapse volcanic infill sequence of the early caldera on Mull. The range of volcanic units within this sequence, from pillow basalts to massive lapilli tuffs and rhyolites, indicates during this period of evolution, the Mull volcano had varying eruption styles and showed a general trend towards higher silica magma compositions, with increasingly explosive eruptive episodes.

Major and trace element analysis, on a wide range of samples, has investigated processes including magma mixing and crustal contamination and links between sub-surface magma conduits and erupted deposits. Isotopic and elemental fingerprinting has revealed the nature of the basement rocks, through which magmas have risen and evolved. Most intrusive and extrusive units associated with the early Mull central complex show a significant upper crustal contamination signature, consistent with contamination by Moine metasediments. A contrasting trend is seen in the basaltic rocks at the base of the caldera infill sequence, which show less contamination than the units later in the stratigraphy, indicating a potentially shorter crustal residence time or a separately fractionating basaltic system to the basalts erupted during earlier events on Mull. Ongoing U-Pb geochronology has determined timescales of activity for the two earliest volcanic centres of the Mull central complex to temporally constrain these models.  

This study has resulted in an updated volcanological and petrological model of the early evolution of the Mull central complex. It has also expanded our understanding of the BPIP, through allowing comparison of timescales and volcanic evolution to other centres, such as Arran [3],[4] and Skye (e.g. [2]). More widely, this study has implications for our understanding of explosive volcanism, particularly in young and evolving environments.

References:

[1] Bailey E.B. et.al. (1924) “Tertiary and Post-tertiary Geology of Mull, Loch Aline and Oban”, British Geological Survey

[2] Drake S.M. et.al. (2022) Volcanica 5: 397–432

[3] Gooday R.J. et.al. (2018) Bulletin of Volcanology 80:70

[4] Gooday R.J. (2024) Lithos 488-489, 107789

[5] Kerr A.C. (1995) Journal of the Geological Society, London 152:975-978