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 journalArticlepeer-review

23 Citations (Scopus)

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 (52·5-57 wt % SiO 2) with the exception of a volumetrically minor pre-caldera dacite. The suite displays a strong tholeiitic trend with K 2O <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°C and 0·8-1·8 kbar. Volatile measurements of glassy melt inclusions indicate a maximum H 2O content of 4·16 wt % H 2O, and CO 2-H 2O saturation curves indicate that crystallization occurred at two levels, at depths of 4-5·5 km and 1·5-2·5 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 0·2% 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 ( 226Ra/ 230Th) 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

Bibliographical note

Corrigendum exists for this article and can be found in Journal of Petrology, volume 56, issue 3, 1 March 2015, pp. 641–644, doi:10.1093/petrology/egv009

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