Magmatic evolution and magma mixing of Quaternary adakites at Solander and Little Solander Islands, New Zealand

Subduction-related Quaternary volcanic rocks from Solander and Little Solander Islands (46°34′S, 166°53′E), south of mainland New Zealand, are amphibole- and plagioclase-phyric trachyandesites-andesites. The Solander and Little Solander Island trachyandesites-andesites have a narrow range of SiO. ₂ (60·26-62·19 and 58·20 wt respectively), and high incompatible element concentrations (e.g. Sr 931-1265 ppm, Ba 619-769 ppm and Sr 2200-2269 ppm, Ba 733-798 ppm respectively). They have geochemical affinities with modern adakites (e.g. high Sr/Y ∼66-105 and ∼132-146, and depleted Y ∼11-16 ppm and 16-17 ppm, respectively). Isotopically similar porphyritic quench-textured enclaves reflect mixing with intermediate (basaltic andesite) magmas with high incompatible element concentrations. The presence of porphyritic and equigranular enclaves, disequilibrium phenocryst textures and compositions, and cumulate nodules is consistent with the Solander sample suite having evolved in an open crustal magma storage system through combined crystal fractionation and mafic magma recharge. Mixing with incoming batches of hotter and more mafic magma is marked by the appearance of oscillatory- and simple-zoned amphibole and sieve-textured plagioclase phenocrysts with An-rich rim overgrowths. High XMg clinopyroxene crystals in the Little Solander trachyandesites indicate the late influx of more mafic compositions. Although fractionation is clearly an important process in producing diversity in the Solander magmas, mafic magma recharge is suggested to be responsible for the enrichment in incompatible element concentrations observed in the more evolved Solander and Little Solander Island rocks. The concave-up rare earth element patterns of the Solander and Little Solander trachyandesites and andesites are consistent with significant amphibole fractionation of hydrous magmas at crustal depths outside the garnet stability field.