NEOPROTEROZOIC (WINDERMERE) LARGE IGNEOUS PROVINCE IN THE BELT-PURCELL BASIN, MONTANA?

The Mesoproterozoic Belt-Purcell basin is a major geologic feature of the Northern Rocky Mountains. Mafic intrusions are abundant within the Belt-Purcell Supergroup, and provide an important record of the tectonic history of the basin. This project focuses on establishing the regional extent and origin of a particular set of widespread diabase sills in northwestern Montana that have intruded many units of the Belt-Purcell Supergroup. In one locality near Alberton, Montana, the sills and associated dikes cut the Garnet Range Formation, which locally comprises the youngest unit in the Belt Supergroup, and are truncated by the Cambrian Flathead Sandstone. These stratigraphic brackets suggest that the sills were emplaced during Neoproterozoic time, possibly coevally with the Windermere rifting event. This is supported by previous work which produced ages of 750 to 780 Ma for several of these sills using K/Ar (Mudge et al., 1982) and 40-Ar/39Ar (Harrison et al., 1986 and 1992, Harlan et al., 1997) dating methods. There is some question, however, of whether these sills are components of one mafic intrusive episode or represent several smaller events that are separated in time and resulted from differing tectonic processes. In order to establish emplacement history and relationships among the sills, this project is analyzing samples from many diabase sill outcrops in northwestern Montana. Geologic mapping, structural analyses, and analyses of major, trace, and rare earth element compositions are being used for genetic comparison of sills. In addition, U/Pb dating is in progress at the University of Wyoming to establish ages of sill crystallization. The U/Pb dates, sill structures, and geochemistry will provide a fingerprint for petrological comparison of the many younger Neoproterozoic sills in northwestern Montana. These data will be used to determine if other sills in Idaho, Wyoming, and Canada are genetically related and part of a Neoproterozoic large igneous province or represent several distinct events. Characterization of these sills will test reconstructions of Rodinia and Precambrian plate margins and may establish a tectonic relationship between these sills and the Neoproterozoic Windermere rifting event, which has not previously been recognized within the Belt-Purcell basin in Montana.