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

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 (更多