Trachyte-phonolite transition at Dunedin Volcano: Fingerprints of magma plumbing system maturity and mush evolution

Abstract

Phonolite-trachyte associations are a common feature of alkaline volcanoes in intraplate settings, and their coexistence challenges closed-system magmatic differentiation scenarios. Here we have investigated the mineralogical and petrochemical features of dikes, lavas, pyroclastic deposits, and comagmatic crystal-rich enclaves outcropping at Dunedin Volcano (Otago region, southern New Zealand). These alkaline magmatic products show both highly and mildly alkaline affinities, trending towards phonolitic and trachytic end-members, respectively. Intermediate rocks are phonotephrites + tephriphonolites (highly alkaline series) and mugearites + benmoreites (mildly alkaline series) with a phenocryst assemblage of clinopyroxene + plagioclase ± amphibole formed at low to mid-crustal levels (i.e., ~29–16 km). Phonolites are porphyritic rocks characterized by alkali feldspar ± amphibole ± clinopyroxene. Their whole-rock compositions are highly enriched in incompatible elements, with variable Ba + Sr contents. A weak negative to slightly positive Eu anomaly is also associated with 87Sr/86Sr ratios of 0.7028–0.7031, which are comparable to those of parental magmas. Geochemical models indicate that phonolites originate as interstitial melts that are generated via abundant alkali feldspar crystallization from a shallow crystalline mush (i.e., ~14–5 km). Strong melt differentiation and extraction is testified by crystal-rich enclaves, as remnants of the mush region. On the other hand, trachytes are phenocryst-poor products strongly depleted in Ba + Sr and with a marked negative Eu anomaly. Trachytes are characterized by 87Sr/86Sr ratios of 0.7040–0.7060, which are different from intermediate rocks and phonolites, and trend towards crustal isotopic compositions. Integrated mass balance, trace element, and energy-constrained modeling confirm that trachytes originate from mildly alkaline magmas interacting with the country rock during feldspar fractionation. We interpret the transition from trachyte to phonolite formation and eruption resulting from the maturation of the plumbing system through accumulation, cooling, and degassing of both highly and mildly alkaline magmas.