A 3-billion-year history of magmatism, metamorphism, and metasomatism recorded by granulite-facies xenoliths from central Montana, USA

Abstract

Lower crustal xenoliths from the Missouri Breaks diatremes and Bearpaw Mountains volcanic field in Montana record a multi-billion-year geologic history lasting from the Neoarchean to the Cenozoic. Unusual kyanite-scapolite-bearing mafic granulites equilibrated at approximately 1.8 GPa and 890 °C and 2.3 GPa and 1000 °C (67 and 85 km depth) and have compositions pointing to their origin as arc cumulates, while metapelitic granulites record peak conditions of 1.3 GPa and 775 °C (48 km depth). Rutile from both mafic granulites and metapelites have U-Pb dates that document the eruption of the host rocks at ca. 46 Ma (Big Slide in the Missouri Breaks) and ca. 51 Ma (Robinson Ranch in the Bearpaw Mountains). Detrital igneous zircon in metapelites date back to the Archean, and metamorphic zircon and monazite record a major event beginning at 1800 Ma. Both zircon and monazite from a metapelite from Robinson Ranch also document an earlier metamorphic event at 2200–2000 Ma, likely related to burial/metamorphism in a rift setting. Metapelites from Big Slide show a clear transition from detrital igneous zircon accumulation to metamorphic zircon and monazite growth around 1800 Ma, recording arc magmatism and subsequent continent-continent collision during the Great Falls orogeny, supporting suggestions that the Great Falls tectonic zone is a suture between the Wyoming craton and Medicine Hat block. U-Th-Pb and trace-element depth profiles of zircon and monazite record metasomatism of the lower crust during the Laramide orogeny at ~60 Ma, bolstering recent research pointing to Farallon slab fluid infiltration during the orogeny.