Abstract Mafic intrusions within the Mesoproterozoic Belt-Purcell basin of the Northern Rocky Mountains of Montana record important Mesoproterozoic and Neoproterozoic rifting episodes. Previous studies established four major Mesoproterozoic igneous intrusive events. This project focuses on sills and dikes that represent a fifth, Neoproterozoic intrusive event and establishes their regional extent and relationship to the Gunbarrel magmatic event and late Proterozoic Windermere rift activity. The sills and dikes of this study intruded Archean basement rock and the Mesoproterozoic formations of the Belt-Purcell Supergroup and are unconformably overlain by the Cambrian Flathead Sandstone. When restored on a palinspastic map, the thickest and deepest segments of the sills, along with dike swarms in western Montana and northern Wyoming, are colinear with the Belt-Purcell basin axis. The sills and dikes fall on the eastern margin of Windermere rock exposures along the margins of the Laurentian craton. The sills and dikes are tholeiitic diabase and have locally undergone low-grade metamorphism and alteration. Granophyre is associated with some sills, and Belt-Purcell xenoliths found in the granophyre show some signs of interaction with the magma. Geochemical analysis is compatible with a single intrusive event. A U-Pb zircon date of 777.5 ± 2.5 Ma obtained from granophyre in the Holland Lake sill supports argon dates of previous studies. Discordant U-Pb data from two other sills, exposed at Turah and near Rogers Pass, are consistent with this date, although it is also possible that these sills were emplaced at different times. The 777.5 ± 2.5 Ma date from the Holland Lake sill establishes the emplacement of this sill during the Gunbarrel magmatic event (ca. 780 Ma) and may reflect earliest Windermere rift activity.