"ARC" MAGMATISM IN THE PEMBINE-WAUSAU TERRANE: A RESULT OF RIFTING CAUSED BY OBLIQUE COLLISION DURING THE PENOKEAN OROGENY

The Pembine-Wausau terrane is a Paleoproterozoic terrane in Wisconsin and Minnesota that is separated from the Superior Craton to the north by the easterly trending Niagara Fault. The terrane comprises mafic and felsic volcanic rock, granite plutons, minor metasedimentary rock and, south of the Niagara Fault, a probable ophiolite. These rocks are thought to have been deformed during the c. 1870-1830 Ma Penokean orogeny marking accretion of the arc terrane to the Superior Craton.

The northeast part of the Pembine-Wausau terrane, in northern Wisconsin, USA, comprises mafic and minor felsic volcanic rocks and syn-volcanic granites thought to be 1890-1860 Ma in age. However, the ages of these rocks are not well established owing to a lack of precise, reliable ages. New in situ U-Pb geochronology of zircon from two volcanic rock samples from the Quinnesec Formation, and tonalite samples from two plutons (Dunbar Gneiss and Newingham Tonalite) that intruded the volcanic rocks, indicates that the magmatism is confined to c. 1845 Ma. Therefore, these rocks are younger than the onset of uplift and erosion marking the Penokean orogeny. A second population of c. 1780 Ma zircons in two samples is indistinguishable from a c. 1775 Ma population of monazites in rhyolite sample. These populations are interpreted to reflect the widespread greenschist- to amphibolite-facies metamorphism in the Pembine-Wasau terrane.

Our ages imply that the first metamorphic overprint in the terrane is related to the c. 1780-1750 Ma Yavapai event, rather than the Penokean orogeny. Our results call into question the notion of a collision at c. 1880 Ma between the Pembine-Wausau terrane and the Superior Craton. The new data are consistent with magmatism in the northeast part of the terrane recording rifting within an oblique collision that started at c. 1860 Ma between the Marshfield terrane farther south and the Superior Craton.