Magma Evolution in the Primitive, Intra-oceanic Tonga Arc: Petrogenesis of Basaltic Andesites at Tofua Volcano

J. T. Caulfield*, S. P. Turner, I. E. M. Smith, L. B. Cooper, G. A. Jenner

*Corresponding author for this work

    Research output: Contribution to journalArticle

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    Abstract

    Tofua volcano is situated midway along the Tonga oceanic arc and has undergone two phases of ignimbrite-forming activity. The eruptive products are almost entirely basaltic andesites (52 center dot 5-57 wt % SiO2) with the exception of a volumetrically minor pre-caldera dacite. The suite displays a strong tholeiitic trend with K2O <1 wt %. Phenocryst assemblages typically comprise plagioclase + clinopyroxene +/- orthopyroxene with microlites of Ti-magnetite. Olivine (Fo(83-88)) is rare and believed to be dominantly antecrystic. An increase in the extent and frequency of reverse zoning in phenocrysts, sieve-textured plagioclase and the occurrence of antecrystic phases in post-caldera lavas record a shift to dynamic conditions, allowing the interaction of magma batches that were previously distinct. Pyroxene thermobarometry suggests crystallization at 950-1200 degrees C and 0 center dot 8-1 center dot 8 kbar. Volatile measurements of glassy melt inclusions indicate a maximum H2O content of 4 center dot 16 wt % H2O, and CO2-H2O saturation curves indicate that crystallization occurred at two levels, at depths of 4-5 center dot 5 km and 1 center dot 5-2 center dot 5 km. Major and trace element models suggest that the compositions of the majority of the samples represent a differentiation trend whereby the dacite was produced by 65% fractional crystallization of the most primitive basaltic andesite. Trace element models suggest that the sub-arc mantle source is the residuum of depleted Indian mid-ocean ridge basalt mantle (IDMM-1% melt), whereas radiogenic isotope data imply addition of 0 center dot 2% average Tongan sediment melt and a fluid component derived from the subducted altered Pacific oceanic crust. A horizontal array on the U-Th equiline diagram and Ra excesses of up to 500% suggest fluid addition to the mantle wedge within the last few thousand years. Time-integrated (Ra-226/Th-230) vs Sr/Th and Ba/Th fractionation models imply differentiation timescales of up to 4500 years for the dacitic magma compositions at Tofua.

    Original languageEnglish
    Pages (from-to)1197-1230
    Number of pages34
    JournalJournal of petrology
    Volume53
    Issue number6
    DOIs
    Publication statusPublished - Jun 2012

    Keywords

    • Tonga arc
    • basaltic andesites
    • antecryst
    • fractionation
    • fluid addition
    • timescales
    • KERMADEC ISLAND-ARC
    • U-SERIES DISEQUILIBRIA
    • NATURAL SILICATE LIQUIDS
    • MOUNT-ST-HELENS
    • TRACE-ELEMENTS
    • SUBDUCTION ZONES
    • NEW-ZEALAND
    • SW PACIFIC
    • LESSER ANTILLES
    • PARTITION-COEFFICIENTS

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