STRUCTURE AND PETROLOGY OF WINDERMERE AGE INTRUSIONS IN THE NORTHERN ROCKY MOUNTAINS

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 three major igneous intrusive events. This project focuses on sills and dikes that represent a fourth intrusive event, and establishes their regional extent and relationship to late Proterozoic Windermere rift activity. These sills and dikes intruded Archean basement rock and the middle Proterozoic formations of the Belt-Purcell Supergroup. They are truncated at the western margin of the basin and are unconformably overlain by Cambrian Flathead Sandstone. Geochemical analysis is compatible with a single intrusive event. 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 co-linear with the basin axis, and form a regional bowl-shape. A preliminary U-Pb zircon age of 763 ±12 Ma date obtained from granophyre sampled near Rogers Pass, Montana, strongly correlates with the earlier argon dates of previous studies, which indicates the sills have not surpassed the reset temperature of the argon system. This date approximates the Gunbarrel magmatic event (780 Ma) and earliest Windermere rift activity. Recent dates of Windermere rocks in Idaho suggest a westerly shift of the rift axis to establish the eventual western margin of Laurentia. Windermere-related rocks occur in British Columbia, Washington, and Idaho, but are absent in Montana. The sills and dikes fall on the eastern margin of an en echelon pattern delineated by Windermere rock exposures along the margins of the Laurentian craton. The dikes and sills may represent earliest Windermere rift activity and comprise the only such rocks in Montana. Because of this relationship, this study refers to these sills and dikes as the Windermere intrusive group or Windermere sills and dikes.