Geology and Rb-Sr age of Precambrian W Puritan Quartz Monzonite, northern Michigan

The Puritan Quartz Monzonite, in the western part of the Upper Peninsula of Michigan, composes a batholith at least 50 kilometres long and as much as 20 km wide on the south side of the Gogebic iron range. It is associated with subaqueous metavolcanic rocks assigned to the Precambrian W Ramsey Formation and with a complex gneissic unit of the same age. The quartz monzonite has a Rb-Sr isochron age of 2,710±140 million years and an initial ⁸⁷Sr/⁸⁶Sr of 0.7015±0.0017. The scatter of data points in excess of analytical error is interpreted as indicating that minor cataclasis and retrograde metamorphism produced slight open-system behavior in Precambrian X (circa 1,800 m.y.) time. The Puritan Quartz Monzonite and associated volcanic rocks are equated with similar Precambrian W greenstone-granite complexes in northern Minnesota, and clearly are a part of the Precambrian W greenstone terrane, as defined previously for the Lake Superior region.