Fluid-driven peralkalization of silicic peraluminous magmas: evidence from the McDermitt caldera (USA)

The influx of Na–F–rich fluids have the potential to shift magma compositions from peraluminous (molar [Na₂O + K₂O]/Al₂O₃ < 1) to peralkaline (molar [Na₂O + K₂O]/Al₂O₃ > 1) in silicic continental systems. The genesis of peralkaline rhyolites has long been debated, and their association with peraluminous magmas is commonly interpreted in terms of distinct magma series derived from different sources. Their contrasting compositions lead to divergent crystallization paths and mineral assemblages, often resulting in markedly different whole-rock geochemistry. At the McDermitt caldera (Nevada–Oregon, USA), both peraluminous and peralkaline rhyolites erupted between ca. 16.7 and 16.4 Ma in association with Yellowstone hot-spot activity. Peraluminous rhyolites are characterized by plagioclase, sanidine, and Fe-rich biotite (with quartz appearing in the most evolved units), whereas peralkaline rhyolites contain sanidine, quartz, and two coexisting amphibole populations (Ca-rich and Na–F-rich) within the same rocks. Despite these mineralogical differences, whole-rock compositions of the two magma types show no systematic contrasts, except for stronger negative Ba, Sr, P, Eu, and Ti anomalies in the peralkaline rhyolites, consistent with their more evolved mineral assemblages. These observations argue against distinct magma sources and instead suggest a progressive “peralkalinization” of a common parental magma, restricted to the most silicic units. Strong Mg–Fe–F zoning in biotite and the coexistence of chemically distinct amphiboles indicate a major shift in the chemical conditions of the shallow magmatic system. Previous studies have shown that Na–K–F–rich fluids can effectively modify peraluminous melts toward peralkaline compositions. Here, we use mineral chemistry, in situ and whole-crystal trace-element analyses of biotite and amphiboles, and major- and trace-element data from melt inclusions in quartz and feldspars to test and characterize this peralkalinization process. Our aim is to constrain the pre-eruptive and pre-crystallization conditions of these magmas and to assess the role of alkali–halogen-rich fluids in driving compositional evolution and metal enrichment. McDermitt thus represents a natural laboratory for investigating the magmatic–hydrothermal transition responsible for Li and other critical-metal endowments, and for evaluating whether such enrichments are primary magmatic features or are enhanced by late-stage fluid–melt interaction.