Magma Degassing Assisting Hybridization and Crystal Alteration: Evidence from Plutonic Rocks
Year: 2026
Type: article
Abstract: Magma hybridization, the mixing and mingling of compositionally distinct magmas, is commonly interpreted as involving silicate melts and crystals, yet magmas are multiphase systems comprising also exsolved fluid phases, that 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 dioritic rocks from the early Paleozoic Archibarca pluton (Puna Plateau, NW Argentina), we identify microstructural and geochemical evidence for the presence and involvement of an exsolved fluid phase during magma hybridization. We find that hybridized diorites record a transition from magmatic to hydrothermal conditions (~800 to 500 °C), driven by F-CO3 2− SO42− aqueous fluids exsolved from granitic magma, which 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 a(SiO 2). This process promotes mobility of medium- to heavy-rare earth element (REE) and high-field-strength elements, 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 microstructures resulting from hornblendization 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 hybridized calc-alkaline magmatic bodies and for reconstructing volatile evolution and element redistribution during pluton assembly.
Source: Journal of Petrology
Language: English
Subfield: Geophysics
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: hybrid
Estimated APC paid: $4,353
OpenAlex ID: W7166845330