A mafic xenolith included within 2.8 Ga granitic rocks from the eastern Beartooth Mountains of Montana, USA shows evidence of significant Ba metasomatism related to an influx of hydrous fluids under lower amphibolite-facies conditions, and is indicated by the development of “hyalophane”. The volumetrically dominant Mesoarchean host rocks include granodioritic-granitic-tonalitic gneisses that have variable and high barium (Ba) contents, with some rocks exceeding 3500 ppm Ba. The mafic xenolith retains some textural features inherited from its igneous tonalitic protolith, but it was affected by multi-stage metamorphic overprints. An initial amphibolite-facies overprint generated a distinctive texture, mineral assemblage, and mineral chemistry, but these features are locally replaced by mineral assemblages and chemistry that result from infiltration of lower-temperature Ba-rich hydrous fluids. The original amphibolite facies assemblage is magnesio-hornblende + plagioclase (An35) + biotite + quartz + titanite + allanite + zircon. Thermobarometry indicates peak conditions of ~660 °C and ~7 kbar partially overprinted by a later, fluid-rich metamorphism at ~550 °C. Submillimeter-scale regions modified by the invasive fluid are characterized by a change in texture denoted by vermicular epidote, epidote overgrowths on allanite cores, myrmekitic plagioclase (~An20), and complexly zoned "hyalophane". "Hyalophane" grains locally have interior regions with a low celsian component (Cn,BaAl2Si2O8) of ~Cn7 flanked by more enriched zones of up to Cn27. Very fine exsolution lamellae of albite-hair perthite are observed in the central zone of the low-Cn “hyalophane”. The Ba-rich hydrous fluid is considered to have been derived from the surrounding granitic rocks. Ultimately, the most likely source of the Ba is subducted sediments in a Mesoarchean subduction zone where fluids generated from Ba-enriched sediments were incorporated into fluids associated with formation of the 2.8 Ga granitic rocks. Upon crystallization of the granitic rocks, a Ba-enriched metasomatic fluid was released to develop “hyalophane” in the mafic xenolith.