Extension of a northern Minnesota Lamprophyre Province by geophysical studies

Abstract

Many small plutons of syenitic, lamprophyric, and granitic rocks occur in the western part of the lower Precambrian Vermilion district in northeastern Minnesota and in adjacent areas to the west that are not covered by glacial drift. They intrude the lower Precambrian metavolcanic and metasedimentary sequences and are not considered a part of either the Vermilion or the Giants Range batholiths. Many of these small intrusions are associated with positive gravity and magnetic anomalies. Similar anomalies also are discernible on geophysical maps in glacially covered areas in western Minnesota. One such anomaly, located near Fosston in Polk County was selected for interpretation. Analysis of the coincident gravity (20 mGal) and magnetic (1000 γ) anomalies shows the source to be an elongate body (1.9×4.8 km) in bedrock beneath the glacial drift (122 m thick). The model is that of a vertical prism extending from the bedrock surface to an unknown depth. The computed density and susceptibility contrast are approximately 3.1 g/cm3 and 0.0030, respectively. The source of the Fosston anomaly is interpreted to be quite similar to the small lamprophyre plutons of the Vermilion district, suggesting an extension of the lamprophyre province into western Minnesota along the 48th parallel.