Zircon U-Pb ages, Lu-Hf data, and whole-rock Sm-Nd data from xenoliths in the Eocene Grassrange diatremes of central Montana reveal the presence of Proterozoic-Archean crust at depth in the Great Falls tectonic zone. The data suggest that the Proterozoic granitoids are not part of the juvenile Little Belt magmatic arc but likely involved melting of Neoarchean crust after closure of the Little Belt ocean. Zircons in the Archean granites suggest their source(s) was part of the Medicine Hat block, not the northern Wyoming craton, and that the samples represent crustal material that was reworked during the Paleoproterozoic collision.

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