Age of the Early Precambrian Rocks of the Saganaga Lake – Northern Light Lake Area, Ontario–Minnesota

Abstract

Whole-rock Rb–Sr isochron and mineral ages from the Saganaga Lake – Northern Light Lake area on the Ontario–Minnesota boundary indicate a major orogeny at 2700 m.y. ago. The sequence of events reconstructed in the geologic history started with the eruption of a volcanic pile of flows and pyroclastic material. During metamorphism and folding the basalt flows were converted to amphibolite and an interlayered series to the Northern Light Gneiss, which is now principally trondhjemite with lesser amounts of amphibolite and metarhyolite. The greenstones and the Northern Light Gneiss were intruded by the Saganaga Granite (tonalite), and the tonalite and the Northern Light Gneiss, in turn, were intruded by a syenodioritic phase of the lcarus pluton.The whole-rock isochron ages for the Northern Light Gneiss, the Saganaga tonalite, and the lcarus pluton are 2740, 2710, and 2690 m.y., respectively. Large uncertainties are attached to these values because of the unfavorable Rb/Sr ratios, but the mineral data that include Rb–Sr, K–Ar, and U–Pb analyses, together with geologic observations, permit dating of the Saganaga tonalite at 2700 ± 50 m.y.Mineral ages from this study and some published by the Geological Survey of Canada cluster in the range from 2650 to 2600 m.y. ago and may indicate h low-grade event or an epeirogenetic effect that dates the time of uncovering and stabilization of the region. Mineral ages in the range from 2600 to 2400 m.y. and at approximately 1800 m.y. ago may be partially reset and probably reflect local conditions of faulting and shearing associated with hydrothermal alteration, later low-grade metamorphism, and possibly recent weathering. Similarly, faults and sheared zones probably were effective in localizing the intrusion of dikes that occurred at various times between 2700 and 1100 m.y. ago.