Lead-isotope investigations in the Minnesota River valley; late-tectonic and posttectonic granites

An epizonal anorogenic granite has been found to have a Pb/Pb age of 1840 + or - 50 m.y. The Th/Pb age of the feldspar-whole rock pair is 1900 m.y., in satisfactory agreement with the Pb/Pb age; but the U/Pb age is greater and probably indicates that uranium has been leached from the rock. Another granite, the mesozonal, late- tectonic Precambrian Sacred Heart granite is found to have a Pb/Pb age of 2605 + or - 6 m.y. Both the Th/Pb and the U/Pb isochron ages are much greater than the Pb/Pb age. An older age was expected for the U-Pb system but to also find it for the Th-Pb system is surprising. There is also an extended discussion of (super 238) U/ (super 204) Pb ratios and their variation from the 'normal' value of >9.