Geology and Age of the Wolf River Batholith, Wisconsin

Abstract

The Wolf River Batholith is a large anorogenic plutonic complex in central and northeastern Wisconsin that is similar lithologically to the rapakivi massifs of Finland. Quartz monzonite, the most abundant rock type present, can be subdivided into three major lithologic units, one of which commonly has well-developed rapakivi texture. Other rock types include syenite, granite, quartz and feldspar porphyries, monzonite, and anorthosite; the anorthosite exists as totally isolated masses within the quartz monzonite and may or may not be genetically related to the main complex. Whole-rock Rb-Sr data yield an age of 1,468 ± 34 m.y., with initial Sr 87 /Sr 86 = 0.7048 ± 0.0017 (λ (Rb 87 ) = 1.39 × 10 −11 yr −1 ). U-Pb data from cogenetic zircon fractions define a concordia intercept age of 1,510 ± 15 m.y. for traditionally used values of the U 235 and U 238 half-lives or an age of 1,485 ± 15 m.y. based on recently redetermined values. Rb-Sr data also show that the syenite complex of the Wausau area is approximately, if not exactly, the same age as the Wolf River Batholith. The Wolf River Batholith is similar in age to other granites in southeastern Missouri and the southwestern United States and is apparently a representative of widespread anorogenic igneous activity that existed throughout the southern part of the North American continent 1,400 to 1,500 m.y. ago.