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The Rainy Lake region on the boundary between Ontario and Minnesota is a classical Precambrian area in which two periods of folding and igneous activity, Laurentian and Algoman, were recognized by Lawson. Early efforts to resolve the two orogenic periods on the basis of K-Ar and Rb-Sr age determinations on micas were not successful. More sophisticated whole-rock Rb-Sr and zircon and sphene U-Pb studies likewise have not been wholly successful, but the U-Pb data suggest that all the early Precambrian events, including the Laurentian and Algoman igneous activity, probably occurred within the interval from 2,700 to 2,750 m.y. ago.

Whole-rock Rb-Sr isochron studies, however, give younger ages as follows:

Coutchiching Series  2,615 ± 50 m.y. 
Keewatin Series  2,595 ± 45 m.y. 
Algoman granites   
Ontario  2,540 ± 90 m.y. 
Minnesota  2,680 ± 95 m.y. 
Coutchiching Series  2,615 ± 50 m.y. 
Keewatin Series  2,595 ± 45 m.y. 
Algoman granites   
Ontario  2,540 ± 90 m.y. 
Minnesota  2,680 ± 95 m.y. 

Mineral ages and a rock-mineral isochron clearly indicate that both the Rb-Sr and the K-Ar systems were affected by subsequent events, but it is not certain that the isochron ages date the time of specific events. It is more likely that the determined ages mark the end of periods of retrograde metamorphism, faulting, and shearing. The different apparent ages of stabilization of the Rb-Sr system probably resulted from differences in rock composition, in water content, and in local heat flow.

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