Magma degassing assisting hybridization and crystal alteration: evidence from plutonic rocks

Abstract

Magma hybridization, the mixing and mingling of compositionally distinct magmas, is commonly interpreted as involving only silicate melts and crystals. Yet magmas are multiphase systems comprising silicate melts, crystals, and exsolved fluid phases, suggesting that fluids may also play a role in hybridization. Through an integrated study of field relationships, microstructures, and the chemistry of major and accessory phases in hybrid rocks from the early Palaeozoic Archibarca pluton (Puna Plateau, NW Argentina), we identify microstructural and geochemical evidence for the presence and involvement of an exsolved fluid phase during magma mingling. We find that these diorites record a transition from magmatic to hydrothermal conditions (~800 to 500°C), driven by F-CO32−-SO42− aqueous fluids exsolved from granitic magma, that trigger an autometasomatic hornblendization reaction in which biotite is replaced by hornblende and titanite, plus other phases such as quartz, zircon and apatite, and minerals typical of hydrothermal alteration such as calcite, baryte and rutile, in response to increasing fO2 and aSiO2. This process promotes mobility of medium- to heavy-REE and HFSE, producing spoon-shaped REE patterns in hornblende and titanite, REE-poor fluorapatite, and light-REE-enriched altered zircon. Due to this alteration, commonly used thermobarometers and chemometers yield conditions outside the true magmatic conditions, reflecting disturbed rather than pristine conditions, hampering the use of petrochronologic tools as reliable indicators of magmatic crystallization timing and conditions. Simultaneously, these fluids enrich surrounding rocks in incompatible elements, fostering local geochemical hybridization. At the meso- and microscales, the hornblende-titanite-rich structures provide direct evidence of fluid migration through crystal mushes, recording both chemical and mechanical signatures of reactive transport. Features such as hornblende-titanite selvedges, patchy replacement textures, quartz ocelli, and hornblendite dykes serve as diagnostic markers of fluid-crystal-melt interaction. These structures could serve as key indicators of fluid transfer and autometasomatism in other calc-alkaline magmatic mingled systems and for reconstructing volatile evolution and element redistribution during pluton assembly.