Evolution of andesite magma systems; Egmont Volcano, New Zealand

R. B. Stewart, A. V. Zernack, M. B. Turner, R. C. Price, I. E. M. Smith, S. J. Cronin

    Research output: Contribution to journalMeeting abstract


    A major issue in andesite magmas genesis is explaining disequilibrium crystal, matrix glass and whole rock compositions. Taranaki/Egmont is a high-K andesite volcano in the western North, Island, New Zealand, with a 200, 000 year eruption record. Thirteen recently identified and dated pre-7 ka debris avalanche deposits record the magmatic evolution of the Taranaki volcanic system. Clast compositions show a gradual enrichment in K (sub 2) O and LILE with time to high-K andesites in the Holocene. Pre-100 ka magmas include relatively primitive basalts and basaltic andesites and mineral chemistry indicates crystallisation within the lower crust or mantle. Modal rock compositions become more silicic in younger units, and the appearance of late-stage low-pressure mineral phases (high-Ti hornblende, biotite and Fe-rich orthopyroxene), suggests an increase in more evolved magmas with time. Six compositionally distinct Holocene magma batches erupted on 1500-2000 year timescales, synchronous with variations in eruptive frequency in which the largest volume (>0.5 km (super 3) ) events erupt the most evolved magmas. We suggest that andesite magmas were generated within a lower crustal 'hot zone' [1]. Matrix glasses in both xenoliths and lavas/tephras are mostly dacitic to rhyolitic in composition and, in younger lavas have a high K (sub 2) O content. These glasses may represent some of the partial melts from the 'hot zone'[2]. The disequilibrium observed in the andesites is due to the mixing of these diverse components. A complex and dispersed magma assembly and storage system developed in the upper crust where the magmas were further modified by fractional crystallisation and magma mixing and mingling [2].
    Original languageEnglish
    Pages (from-to)1942
    Number of pages1
    JournalMineralogical Magazine
    Issue number3
    Publication statusPublished - 2011
    EventGoldschmidt Conference (21st : 2011) - Prague, Czech Republic
    Duration: 14 Aug 201119 Aug 2011


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