Abstract
Rb–Sr geochronologic studies have been carried out on Lower and Middle Precambrian rocks exposed in Marquette County, Michigan. These rocks were subjected to varying degrees of metamorphism during formation of the Republic metamorphic node. Samples of the Lower Precambrian granitic gneiss generally yield whole-rock model ages of 2500 to 2800 Ma for metamorphic grades ranging from chlorite through sillimanite zones, indicating relatively minor net flux of Rb and Sr on a scale of several tenths of a metre during the metamorphism. Mineral ages show partial to complete internal re-equilibration of the Rb–Sr systems about 1665 ± 50 Ma ago. Muscovite Rb–Sr ages from cross-cutting pegmatites are also about the same age. Data on minerals from metamorphosed Middle Precambrian metasediments of the Michigamme Formation are consistent with this age but are not definitive.Published structural studies indicate that the main deformation in the Republic node occurred essentially at the same time as the peak of the major mineral-forming metamorphic event. The age of 1665 Ma is inconsistent with the regional tectonics and chronology, which indicate that the major deformation and metamorphism in Upper Michigan occurred about 1900 Ma ago. However, the age of 1665 Ma does correspond to a time of more recent metamorphism prevalent throughout the western Great Lakes area, and we interpret the results from the Republic area as a general overprint younger than the main period of deformation and metamorphism.Two of the Lower Precambrian samples yield model whole-rock ages in excess of 3000 Ma. Detailed study of the rocks involved show that the apparently old ages are due to Sr and Rb redistribution on a larger scale during another event which occurred about 2100 to 2200 Ma ago.
Published Version
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