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    Widespread transport of pyroclastic density currents from a Skye volcano: correlation of ignimbrite lithofacies and the evolution of the Palaeogene Skye Central Complex, NW Scotland

    Drake, Simon and Brown, A.D. and Beard, Andy and Downes, Hilary (2010) Widespread transport of pyroclastic density currents from a Skye volcano: correlation of ignimbrite lithofacies and the evolution of the Palaeogene Skye Central Complex, NW Scotland. In: Volcanic and Magmatic Studies Group Annual Meeting, 4th - 6th January 2010, Glasgow, UK. (Unpublished)

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    Abstract

    A re-investigation of the volcanic rocks of the Skye Central Complex (SCC), NW Scotland, provides evidence for the widespread transport (up to 20 km) of pyroclastic density currents (PDCs) across the Palaeogene landscape. Detailed mapping and logging of volcaniclastic rocks (previously interpreted as agglomerates, felsites and lavas [1]) within the SCC, has allowed us to identify several distinct silicic ignimbrite lithofacies: (1) lithic-rich massive lapilli tuff, locally with normal and reverse grading, and/or diffuse stratification; (2) poorly sorted massive lithic breccia; (3) fines-rich massive lapilli tuff containing abundant accretionary lapilli; (4) rheomorphic lapilli tuff; and (5) rare massive crystal tuffs. These lithofacies provide evidence for extensive silicic pyroclastic activity within the SCC, and can be correlated to recognise two distinct eruption phases in its evolution, which had not previously been constrained. At Fionn Choire on the NW margin of the Cuillin Hills on Skye, massive lapilli tuffs (unwelded, incipiently welded and intensely welded/rheomorphic), crystal tuffs and breccias crop out. These ignimbrites (and scattered exposures at Sgurr Thuilm, NNE of Glen Brittle) are cut by the Cuillin Centre gabbros and represent the first phase of silicic pyroclastic activity in the SCC. The massive crystal tuff, previously interpreted as porphyritic andesite lava [1], comprises ~50- 60% plagioclase crystals, typically 1-2 cm and rarely up to 5 cm across, and shows tractional stratification and inverse crystal grading. This massive crystal tuff also contains distinctive rounded lapilli of mechanically fractured granite. The same unit has been identified at Ben Suardal, some 20km to the SE of Fionn Choire, indicating widespread transport of this unusually crystal-rich PDC. The intensely welded lapilli tuff, previously interpreted as flow banded rhyolite [1], displays a eutaxitic/parataxitic fabric and is commonly rheomorphic. A second phase of silicic pyroclastic activity, post-dating the Cuillin and Western Red Hills intrusions is recognised, particularly in the Kilchrist area where a variety of ignimbrite lithofacies (types 1-4), forming a 200 m thick sequence, have been identified and correlated. A distinctive incipiently welded massive lapilli tuff can also be correlated with a sequence at Moll, some 10 km to the NNW, where it fills a palaeo-valley within the Northern Porphyritic Felsite (part of the Western Red Hills Centre). Our detailed new stratigraphy records a complex eruptive history. The low-grade ignimbrites were typically deposited from unsteady currents with variable vent mass flux, and the accretionary lapilli were probably produced within co-ignimbrite ash clouds. These eruptions were also interspersed with low eruption column boil-over events where heat was maintained sufficiently for intensely welded/rheomorphic high-grade ignimbrites to develop.

    Metadata

    Item Type: Conference or Workshop Item (Paper)
    School: Birkbeck Schools and Departments > School of Science > Earth and Planetary Sciences
    Depositing User: Sarah Hall
    Date Deposited: 12 May 2016 08:27
    Last Modified: 12 May 2016 08:27
    URI: http://eprints.bbk.ac.uk/id/eprint/15142

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