A geochemical investigation of the Pre-Cambian Knife Lake Series, northeastern Minnesota

Abstract

A geochemical study was made of the Precambrian eugeosynclinal graywackes and slates of the Knife Lake Series in northeastern Minnesota. Unlike some other graywacke sequences, these sedimentary rocks show no systematic secular variations in chemical or mineralogical composition within the stratigraphic sequence. The average composition of the Knife Lake graywackes is similar to the composition of other Precambrian graywackes. A comparison of the composition of the Precambrian graywackes with Paleozoic and Mesozoic graywackes indicates that the Precambrian graywackes are enriched in Fe, Mg and Ca relative to the younger graywackes. The average The/U ratios of the Knife Lake graywackes and slates are 4.5 and 4.8 respectively. Assuming that any variation in the The/U ratio is caused solely by radioactive decay since the time, about 3 by. ago, that the Knife Lake rocks were formed, the original Th/U ratios are calculated to have been 3.1 to 3.6. These values are well within the range 3.0 to 4.0 characteristic of Paleozoic and younger material. Because graywackes represent a comprehensive sample of their crustal source rocks, the Th/U ratio of newly formed crustal material may be presumed to have been relatively constant throughout all of geologic time.