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

摘要

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 % 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°C and 0·8–1·8 kbar. Volatile measurements of glassy melt inclusions indicate a maximum H₂O content of 4·16 wt % H₂O, and CO₂–H₂O 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.