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    Eruption chemistry and ignimbrite deposition recorded by accretionary lapilli within pyroclastic density current deposits, Isle of Syke, NW Scotland

    Drake, Simon M. and Brown, D.J. and Beard, Andy (2011) Eruption chemistry and ignimbrite deposition recorded by accretionary lapilli within pyroclastic density current deposits, Isle of Syke, NW Scotland. In: Volcanic and Magmatic Studies Group Annual Meeting, 5th - 7th January 2011, Cambridge, UK. (Unpublished)

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    The origin and evolution of accretionary lapilli remains controversial. Ongoing field and petrographic studies have helped to refine models of formation but there is a paucity of data relating to important geochemical trends between cores and rims of the lapilli [1]. Here we use both field and geochemical data to propose a new model to account for the growth of accretionary lapilli within pyroclastic density currents (PDCs) and to trace their evolving eruption chemistry. At Kilchrist, Isle of Skye, two Palaeogene accretionary lapilli-bearing massive lapilli-tuff units (mLTacc), interbedded with massive breccia, have been mapped and logged in detail. The lower unit is cut by cooling joints suggesting cooling of the mass was contemporaneous. Both mLTacc units possess a dipping fabric allowing way up to be determined. Careful, methodical sampling of accretionary lapilli was conducted up section and subsequent EMP analysis undertaken. Distinct evolved trends from basaltic andesite at the core to dacite at the rim were noted in 75% of samples and these differences were also reflected in the changing mineralogy of the lapilli. Intimately associated changes in matrix chemistry were also noted, with the mLTacc evolving from basaltic andesite to andesite to dacite up section. These trends were noted in both units suggesting similar processes occurred within temporally distinct density currents. Our study concurs with and builds on recent work conducted in Tenerife [2] and suggests that proto-accretionary lapilli fell into PDCs, and ascended and rotated by aerodynamic lift into chemically distinct fresh magmatic pulses. Lapilli interaction, which would have retarded lift effect within such “dusty” PDCs, would have been minimal since the matrix is frequently >80 vol. % and other lapilli are largely absent. The magma chambers that fed these eruptions were complex, and the mLTacc record tapping of a chemically stratified chamber from the bottom up. Our model has significant implications for studies of PDC deposition, and supports the progressive aggradation of ignimbrites. Sampling of mLTacc in modern volcanic terrains to further test our model is ongoing.


    Item Type: Conference or Workshop Item (Paper)
    School: School of Science > Earth and Planetary Sciences
    Depositing User: Sarah Hall
    Date Deposited: 10 May 2016 16:10
    Last Modified: 15 Dec 2017 07:05


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