26-5 Amphibole Control on Melt and Trace-Element Evolution in Arc Plutonic Systems: A View from the Jurassic Bonanza Arc, Vancouver Island, Canada

Session: Subduction Zones and Their Volcanic Arcs: Initiation and Evolution, Structure, Metamorphism, Magmatism

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Abstract:

Amphibole crystallization has been proposed as a key process in generating the hydrous, calc-alkaline magmas characteristic of arc settings. Its crystallization produces the largest increase in SiO₂ and decrease in FeO in coexisting melts, making it a critical step in generating the large volumes of evolved arc magmas (dacite to rhyolite, and their plutonic equivalents) that form the continental crust. However, the extent to which amphibole-derived ‘mush’ liquids contribute to the erupted and non-erupted melts at arcs remains poorly constrained. Cumulates of olivine (ortho)pyroxenite, olivine hornblendite, hornblende norite, or hornblende gabbro exposed within the Jurassic Bonanza arc (Vancouver Island, Canada) provide an in-situ record of early melts saturated in amphibole. Cumulus olivine, orthopyroxene, ± plagioclase is common; and clinopyroxene is nearly absent, with the exception of some remnant cores in interstitial amphiboles. Trace element chemistry indicates at least three amphibole populations in olivine hornblendites, and suggests reaction with pyroxene(s) and formation before plagioclase. Calculated amphibole equilibrium melt compositions overlap with most major elements from bulk rock chemistry, with the exception of Si. Amphibole equilibrium melts have markedly higher LREE/HREE than what is observed in bulk rock chemistry, with the exception of some volumetrically minor felsic bodies (e.g., granodiorite dikes). These results demonstrate that hydrous ultramafic and mafic arc magmas hosted mush liquids enriched in strongly incompatible elements, and thus can help further our understanding of amphibole crystallization on critical metal trends (i.e., Cu, LREEs) in arc crust.

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