Tholeiitic diabase, hornblende andesite, and olivine diabase dikes cut Precambrian gneissic rocks in the Minnesota River Valley near Granite Falls, Minnesota. K-Ar ages on hornblende from the dikes show that the tholeiitic diabase dikes were intruded more than 2000 m.y. ago and that the hornblende andesite dikes were intruded approximately 1800 m.y. ago.

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