ABSTRACT

Some granitic rocks and basaltic dikes of the Minnesota River Valley were dated by Rb-Sr and ⁴⁰Ar-³⁹Ar methods. The obtained Rb-Sr mineral isochron ages are as follows: Morton gneiss 2.55 ± 0.06 (1σ)b.y. and its biotite ages 2.38b.y., Sacred Heart granite 2.37 ± 0.05 (1σ)b.y., Montevideo gneiss 1.82 ± 0.09 (1σ)b.y. Each age may be corresponding to the respective geological event. Whole rock samples of the Morton and Montevideo gneisses, however, do not define a Rb-Sr isochron age. From ⁴⁰Ar-³⁹Ar dating results, a mild geological disturbance which might have occurred about 1.0b.y. ago was inferred. These results imply a complex history of the gneisses and a metamorphism in this region.