Paleomagnetic and 40Ar/39Ar
Geochronologic Data from
Late Proterozoic Mafic Dikes and Sills,
Montana and Wyoming


By Stephen S. Harlan, John Wm. Geissman, and Lawrence W. Snee


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Swarms of Proterozoic mafic sheets are a common feature of many uplifts of Wyoming and western Montana. In the basement-cored uplifts of the Archean Wyoming province, they are represented by dikes of diverse trend, composition, and apparent age. Reported dike ages from the basement uplifts range from about 2.6 Ga to 0.75 Ga, with apparent age maxima at 2.56 Ga, 2.2­2.0 Ga, 1.45 Ga, 1.3(?) Ga, 1.2 Ga, and 0.75 Ga (Snyder and others, 1989; Baadsgaard and Mueller, 1973; Wooden and others, 1978). In western Montana, mafic intrusions are generally found as sills that intrude sedimentary strata of the Middle Proterozoic Belt Supergroup and have apparent ages of 1.45­1.38 Ga and 0.75 Ga. Despite their wide occurrence and the recognition that mafic dike and sill swarms may be sensitive recorders of fundamental geologic and geodynamic processes (Halls, 1982), correlation of dike and sills of similar age and geochemical compositions between uplifts of western Montana and Wyoming has been limited in scope and is problematic. Although dikes in some individual uplifts have been extensively studied, at present there is little detailed information regarding the regional distribution of sheets of particular age groups or their geochemical characteristics. Hence, the tectonic and geodynamic significance of many swarms is poorly understood.

It has long been recognized that the natural remanent magnetization (NRM) acquired by mafic dikes during cooling can be a reliable recorder of the geomagnetic field and can be geologically stable throughout billions of years. These properties make paleomagnetism an extremely powerful tool for correlating dikes of individual swarms over wide areas (Buchan and Halls, 1990). Recent advances in geochrono-logic dating methods, including the recognition that mafic dikes can sometimes be dated with great precision by the U-Pb technique and, in some cases, by the 40Ar/39Ar method (Hanes, 1988; Krogh and others, 1988; Heaman and LeCheminant, 1993), have improved understanding of the distribution and mechanisms of mafic dike emplacement (Buchan and others, 1993, 1994). As part of an ongoing study of the paleomagnetism and geochronology of several dike swarms in basement-cored uplifts of Wyoming and Montana, we report paleomagnetic and 40Ar/39Ar results from mafic dikes exposed in the Teton Range of northwestern Wyoming and the Beartooth uplift of Montana and Wyoming, and a 40Ar/39Ar date from a mafic sill in northwestern Montana (fig. 1). We conclude that these dikes and sills were probably emplaced during a single intrusive episode at about 780­770 Ma. Similarity of our results with paleomagnetic and geochronologic data from other well-dated mafic rocks from the northwestern United States and Canada indicate that 780-Ma mafic magmatism was regional in extent.


USGS Professional Papers

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