Geology and geochronology of granitoid and metamorphic rocks of late Archean age in northwestern Wisconsin

Abstract

Granitoid rocks of the Puritan Quartz Monzonite and associated biotite gneiss and amphibolite in northwestern Wisconsin compose the southwestern part of the Puritan batholith of Late Archean age. They differ from rocks in the Michigan segment of the batholith in having been deformed by brittle-ductile deformation and partly recrystallized during shearing accompanying development of the midcontinent rift system of Keweenawan (Middle Proterozoic) age. Granitoid rocks ranging in composition from granite to tonalite are dominant in the Wisconsin part of the batholith. To the north of the Mineral Lake fault zone, they are massive to weakly foliated and dominantly of granite composition, whereas south of the fault zone they are more strongly foliated and mainly of tonalite composition. Massive granite, leucogranite, and granite pegmatite cut the dominant granitoid rocks. Intercalated with the granitoid rocks in small to large conformable bodies are biotite gneiss, amphibolite, and local tonalite gneiss. Metagabbro dikes of probable Early Proterozoic age as much as 15 m thick cut the Archean rocks. Rubidium-strontium whole-rock data indicate a Late Archean age for the granitoids and gneisses, but data points are scattered and do not define a single isochron. Zircon from two samples of tonalitic gneiss for uranium-thorium-Iead dating define a single chord on a concordia diagram, establishing an age of 2,735?16 m.y. The lower intercept age of 1,052?70 m.y. is in close agreement with rubidium-strontium and potassium-argon biotite ages from the gneisses. Two episodes of deformation and metamorphism are recorded in the Archean rocks. Deformation during the Late Archean produced a steep west-northwest-oriented foliation and gently plunging fold axes and was accompanied by low amphibolite-facies metamorphism of the bedded rocks. A younger deformation resulting from largely brittle fracture was accompanied by retrogressive metamorphism; this deformation is most evident adjacent to the Mineral Lake fault and took place during Keweenawan rifting about 1,050 m.y. ago. The Mineral Lake fault is one of several northwest-trending faults in the Lake Superior region that originated in the Late Archean and were reactivated intermittently during the Proterozoic, including Keweenawan time. The faults dominantly have right-lateral displacements. The Archean rocks of the Puritan batholith exposed in northwestern Wisconsin compose part of the greenstone-granite terrane, as defined in the Lake Superior region. These rocks were formed 2,7502,600 m.y. ago. The long dimension of the Puritan batholith as well as that of several batholiths in adjacent Minnesota are oriented sub parallel to the boundary between the greenstone-granite terrane and the older gneiss terrane, to the south. This conformity in trend is interpreted as indicating that the granite probably was emplaced after the two basement crustal segments had been joined.