Zircon Ages of Precambrian Granitic Rocks, Northeastern Wisconsin

Abstract

U-Pb isotopic analyses of cogenetic zircon fractions from four granitic bodies in northeastern Wisconsin yielded the following concordia intercept ages: Amberg Granite and Newingham Granodiorite, ; Hoskin Lake Granite, ; Dunbar Gneiss, Emplacement of granitic rocks in this area therefore appears to have occurred during a time interval extending over and not exceeding a few tens of millions of years, presumably as part of the Penokean orogeny. Moreover, the sequence of ages agrees with geologic evidence, with the possible exception of the problematical Dunbar Gneiss. Slight differences in slopes and intercepts of the chords defined by the zircon data indicate that no single simple mechanism can explain the observed isotopic disturbance patterns.