Magmatic Mn-rich garnets in volcanic settings: Age and longevity of the magmatic plumbing system of the Miocene Ramadas volcanism (NW Argentina)

Abstract

The Miocene “Corte Blanco Tuff” rhyolite deposit is the product of a large volume and high intensity Plinian erup-tion from the solitary and monogenetic Ramadas Volcanic Centre (Central Andes, Province of Salta, NWArgentina). The“Corte Blanco Tuff”consists of vitreous tube pumices with rare euhedral sub-millimetricMn-garnet phenocrysts, typically hosting inclusions of U-phases as zircon and monazite. Here, we present newtextural, major and trace elemental analyses of garnet, zircon and glass that, combined with in situ U-(Th)-Pbzircon and monazitedating, are used to reconstruct thethermobaricenvironment offormation,age andlongevityof the magmatic plumbing system of the Ramadas magma. The results indicate to a crystallization path of aperaluminous rhyolitic melt at shallow crustal levels (≤6 km), as sequentially tracked by the initial nucleationof zircon (780 °C at 9.16 Ma) and garnet (above or at ca. 700 °C), to thefinal monazite growth (660–670 °C, at8.70 Ma) in a water-saturated (H2O=3–5 wt%) environment, shortly before the eruption started. These data(1) define for thefirst time the primary magmatic origin of Mn-garnet in a rhyolitic volcanic setting; (2) providenew partition coefficients of rare earth elements (REE) between natural garnet, zircon and rhyolitic melts; and(3) permit reconstruction of the magmatic processes that resulted in the Ramadas eruption. On a wider scale,our results document the spatio-temporal (P-Tconditions, timing and longevity) time scales involved in the pet-rogenesis of a shallow peraluminous water-saturated rhyolitic magmatic plumbing system that is able to gener-ate the conditions for extremely explosive Plinian eruptions.