Abstract

The Great Falls tectonic zone and the Vulcan structure both have been proposed as the site of a Paleoproterozoic suture between the Archean Hearne and Wyoming provinces. Both hypotheses remain viable because all Precambrian rocks composing the Vulcan structure and much of the Great Falls tectonic zone are buried beneath Phanerozoic cover. The primary exceptions to this are the mafic to felsic igneous and metamorphic rocks of the Little Belt Mountains (Montana), previously considered the northernmost exposures of the Wyoming province. New U-Pb zircon ages from the late kinematic Pinto diorite (207Pb/206Pb age: 1864 ± 5 Ma) and a gneissic unit intruded by the Pinto (207Pb/206Pb age: 1867 ± 6 Ma), however, confirm their Paleoproterozoic age. These rocks exhibit an overall calc-alkaline affinity and the depletion in high field strength elements typical of convergent margin environments. Whole-rock Sm-Nd data (initial epsilon of −1 to +4) and a lack of premagmatic zircons indicate that the magmas were principally derived from a depleted mantle source, not from older crust. These data suggest that at least some rocks within the Great Falls tectonic zone originated at a convergent margin that developed during the closure of an ocean basin along the northwestern margin of the Wyoming craton ca. 1.9 Ga.

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