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    Bentonised silicic pyroclastic fall deposits at the base of the Palaeogene Skye Lava Field which possess welded ignimbrite like fabrics

    Drake, S.M. and Beard, Andy (2013) Bentonised silicic pyroclastic fall deposits at the base of the Palaeogene Skye Lava Field which possess welded ignimbrite like fabrics. In: Volcanic and Magmatic Studies Group Annual Meeting, 7th-9th January 2013, School of Earth Sciences, University of Bristol, Bristol, UK. (Unpublished)

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    Abstract

    Silicic pyroclastic rocks deposited prior to basaltic lava fields within the British-Irish Palaeogene Igneous Province (BIPIP) are unrecorded. We report on the first such occurrence from beneath the lowermost Skye Lava Field at An Carnach, Strathaird Peninsula, Isle of Skye, NW Scotland. Here contemporaneous basic and silicic eruptions are recorded by former silicic ash-fall deposits which contain chlorite mica stacks. The presence of these stacks likely indicate basaltic ash settled out in a marine water column (Merriman and Peacor 1999). The silicic-ash fall deposits were subsequently altered to K-bentonite by either intrusion of a later adjacent tholeiitic sill, or by the weight of the overlying lava pile. The alteration of the protolith to K-bentonite has resulted in a remarkable eutaxitic like fabric which is defined by a strong planar fabric which has frequently been deflected around heterolithic lapilli. This fabric was produced in the cold state since it lacks recognised hot state characteristics (Branney and Sparks 1990). In the field the K-bentonite strongly resembles a silicic welded ignimbrite. Classification to K-bentonite was only possible using an electron microprobe since the Al and Si peaks were much higher and lower respectively than those expected in a silicic welded ignimbrite. The ability to distinguish between ‘ancient’ bentonite and welded ignimbrite in the field is therefore called into question and may have implications for volume considerations of silicic welded ignimbrite in some volcanic terrains, and the paucity of silicic air-fall deposits in others.

    Metadata

    Item Type: Conference or Workshop Item (Poster)
    School: Birkbeck Schools and Departments > School of Science > Earth and Planetary Sciences
    Depositing User: Sarah Hall
    Date Deposited: 10 May 2016 13:56
    Last Modified: 10 May 2016 15:40
    URI: http://eprints.bbk.ac.uk/id/eprint/15130

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