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Publication - Dr Frances Cooper

    Early Miocene large-volume ignimbrites of the Oxaya Formation, Central Andes

    Citation

    Van Zalinge, ME, Sparks, RSJ, Cooper, FJ & Condon, DJ, 2016, ‘Early Miocene large-volume ignimbrites of the Oxaya Formation, Central Andes’. Journal of the Geological Society, vol 173., pp. 716-733

    Abstract

    During the early Miocene ignimbrite flare-up, significant parts of the Central Andes (17–20°S) were covered by large-volume
    ignimbrites. High-precision 206Pb/238U zircon dates constrain the flare-up in northern Chile at c. 18°S to a 3 myr period, starting with the deposition of the Poconchile ignimbrite at 22.736 ± 0.021 Ma. Of four main pulses,
    the two largest occurred at 21.924 ± 0.017 and 19.711 ± 0.036 Ma, when the >1000 km3 in volume Cardones and Oxaya ignimbrites erupted, respectively. The ignimbrites are high-SiO2
    rhyolites and show significant heterogeneities in crystal content,
    mineral proportions and trace-element compositions. The
    zoned Oxaya ignimbrite implies incremental
    extraction of a crystal-poor magma overlying a crystal-rich magma. In
    contrast,
    petrological and textural heterogeneities in pumice
    clasts are spread throughout the Cardones ignimbrite and we propose
    magma
    mixing caused by destabilization of multiple magma
    bodies within a magmatic mush system. Distal and medial deposits of the
    Cardones ignimbrite, with a maximum welded
    thickness of at least 1000 m, entirely covered the western flank of the
    Central
    Andes, which implies infill of a significant
    topographic relief. Both compaction and welding resulted in a maximum
    thickness
    reduction of around 30% for the Cardones
    ignimbrite.

    Full details in the University publications repository