Constraints on assembly of Tongariro and Ruapehu andesite magmas based on Sr-isotope compositions of plagioclase and groundmass

Abstract

Sub-crystal scale 87Sr/86Sr analysis of plagioclase phenocrysts and groundmass in andesites reveal cryptic disequilibrium that is not observable in bulk rock-based studies, shedding light on magma assembly. Isotopic, chemical and textural features demonstrate that plagioclase phenocrysts in continental arc andesites from Tongariro and Ruapehu volcanoes are a heterogenous population of various cognate and non-cognate origins. The studied rocks span a wide age range (~270 ka) and have different vent locations but their plagioclase share many characteristics. Most phenocrysts comprise of a relic core surrounded by one or more mantling overgrowths of various textures demonstrating a multi-stage growth during crustal transit. The total compositional range found in a single thin section is wide (~An45–90). Some zoned phenocrysts have a comparable wide range. Inter-crystal 87Sr/86Sr diversity is considerable with differences up to 0.7044–0.7058 within a hand-sized sample. Individual phenocrysts can be isotopically zoned with rim-core differences up to 0.0005, and contrasting isotopic zonation can occur in phenocrysts of the same sample. Overall, more than half the phenocrysts are non-cognate as indicated by their isotopic differences. Phenocrysts with relic calcic (>An85) interiors and low 87Sr/86Sr ratio (<0.7050) are probably antecrysts and xenocrysts from deep mafic cumulates and/or forerunner intrusions. Other phenocrysts have relic sodic interiors (<An55) with high 87Sr/86Sr ratios (~0.7055) compared to the accompanying groundmass, and were probably entrained from country rock and/or differentiated crystal mush in the crust. Phenocrysts most likely cognate because of their concordant composition and isotopic ratio with the groundmass, exhibit oscillatory zoning and lack relic cores, and others display decompression-induced replacement of calcic plagioclase with more sodic compositions. These textures reflect the dynamic processes of magmatic ascent and temporary storage. In some andesites the groundmass (host magma) has a greater mantle-like isotopic affinity than that of the phenocrysts, while in other andesites the reverse is true. This reflects the degree of mingling between the host magma and crustal melts during ascent. The cryptic isotopic disequilibrium revealed highlights the problem of petrogenetic modelling and mineral thermobarometry that assess equilibrium among the phases on elemental abundance. Such concordance could be coincidental rather than co-magmatic.