The prevalence of plagioclase antecrysts and xenocrysts in andesite magma, exemplified by lavas of the Tongariro volcanic complex, New Zealand

Abstract

Beneath subduction zone volcanoes, the parental magmas are thoroughly blended during temporary storage in the crust via repeated heating and mixing from new inputs of magma. To obtain a clearer picture of magma assembly, we employed a sub-crystal-scale Sr-isotope analysis, coupled with textural and geochemical data, on plagioclase phenocrysts in basaltic andesite and andesite erupted from the Tongariro volcanic complex, New Zealand. Deposits from two neighbouring vents were examined: (1) 0.3–3.4 ka lava flows of the polygenetic Red Crater volcano and (2) 26 ka pyroclastics of the monogenetic Pukeonake scoria cone. Despite the differences in eruptive styles and history of the two volcanoes, the plagioclase growth histories are similar. One common phenocryst type comprises of a resorbed core zone (variously, ~ An50–70) surrounded by a sieve-textured mantle/rim of higher An (~ 80), Fe and Mg contents. Other phenocrysts have resorbed calcic cores (~ An70–90). In situ 87Sr/86Sr analyses reveal a wide isotopic range (~ 0.7044–0.7060 for pre-1.8 ka Red Crater; ~ 0.7044–7057 for post-1.8 ka Red Crater; ~ 0.7047–0.7055 for Pukeonake). Most of the values are higher than that of the host bulk rock (87Sr/86Sr ~ 0.7049–0.7051 for pre-1.8 ka Red Crater; 0.7045–0.7046 for post-1.8 ka Red Crater; 0.7048 for Pukeonake). The variation within some individual phenocrysts is comparable to that of the entire population. The higher 87Sr/86Sr ratios are related relic phenocryst cores, indicating that their parental source was more radiogenic than the host magma. In contrast, their sieve-textured mantle/rim zones have lower 87Sr/86Sr ratios (~ 0.7045–0.7049), similar that of their host rock. These features record magma mixing. Less than 10% of phenocryst interiors are cognate (autocrysts) with the host rock based on their high 87Sr/86Sr ratio. Some phenocrysts are likely to be xenocrysts derived from the disintegration of deep crustal meta-igneous rocks. However, based on textures consistent with growth from a melt, the majority are antecrysts from intrusive forerunners and/or crystal mush zones, entrained during eruption or storage. Overall, the andesites are a product of crystal-poor magmas of mantle or lower crustal origin carrying phenocrysts from mostly crustal sources. The magma system likely comprised of a series of semi-isolated melt pods or crystal mush zones in a larger network of intrusions of various ages and states of solidification, possibly hosted in country rock of various compositions. Resorption of the sieve-textured mantle zones on the phenocrysts and growth of a normally zoned outermost rim represents a final period of fractional crystallization in the newly homogenised melt following the ascent and intrusion of mafic magma. Hence, the final tipping point to eruption may have involved crystallization-induced volatile exsolution and degassing.