BARIUM METASOMATISM IN ARCHEAN ROCKS: EVIDENCE FROM MAFIC XENOLITHS WITHIN THE MESOARCHEAN GRANITIC ROCKS OF THE BEARTOOTH MOUNTAINS OF THE NORTHERN WYOMING PROVINCE

Xenoliths included within 2.8 Ga granitic rocks from the eastern Beartooth Mountains of Montana and Wyoming show significant Ba metasomatism by hydrous fluids under greenschist facies conditions. The volumetrically dominant rock types include granodioritic mylonitic gneisses and mildly deformed granitic gneisses. Abundant and diverse xenoliths vary in their 1) lithology, including amphibolites, biotite schists, other felsic plutonic compositions and quartzites, 2) metamorphic grade, exhibiting local evidence of granulite to greenschist facies conditions, and 3) size, measuring from the centimeters to tens of meters scale. The granitic and granodioritic gneisses have variable Ba contents, with some exceeding 3500 ppm Ba.

In particular, one mafic xenolith included in a granitic gneiss records evidence of infiltration of a Ba-rich hydrous fluid under greenschist facies conditions partially converting the original amphibolite facies assemblage. The original amphibolite facies assemblage is magnesiohornblende + plagioclase (An₃₅) + biotite + quartz + titanite + zircon. Hornblende-plagioclase thermobarometry indicates peak conditions of ~650°C at 5-6 kbar partially overprinted by a later, cooler fluid-rich metamorphism at ~450°C. Submillimeter scale regions altered by the fluid are characterized by a change in texture denoted by vermicular epidote, actinolite rims on magnesiohornblende cores, epidote on allanite cores and complexly zoned Ba-rich K-feldspars. Ba-rich K-feldspar locally have interior regions with lesser celsian component (BaAl₂Si₂O₈) of Cn₇ flanked by two more enriched zones of Cn₂₁. Wispy exsolution lamellae are observed in the central zone of the K-feldspars. Within the metasomatized zones, plagioclase is complexly zoned primarily mimicking grain boundaries.

The Ba-rich fluid likely derived from the surrounding granitic rocks, which are irregularly enriched in barium across the area. The most likely ultimate source of the Ba is from subducted sediment. The fluids evolved from the Ba-rich sediment were likely incorporated in the fluids associated with formation of the 2.8 Ga granitic rocks, which were themselves derived from melting of 2.8 Ga tonalitic middle crust.