Refining the Paleoproterozoic tectonothermal history of the Penokean Orogen: New U-Pb age constraints from the Pembine-Wausau terrane, Wisconsin, USA

Abstract

An enduring problem in the assembly of Laurentia is uncertainty about the nature and timing of magmatism, deformation, and metamorphism in the Paleoproterozoic Wisconsin magmatic terranes, which have been variously interpreted as an intra-oceanic arc, foredeep or continental back-arc. Resolving these competing models is difficult due in part to a lack of a robust time-frame for magmatism in the terranes. The northeast part of the terranes in northern Wisconsin (USA) comprise mafic and felsic volcanic rocks and syn-volcanic granites thought to have been emplaced and metamorphosed during the 1890−1830 Ma Penokean orogeny. New in situ U-Pb geochronology of igneous zircon from the volcanic rocks (Beecher Formation), and from two tonalitic plutons (the Dunbar Gneiss and Newingham Tonalite) intruding the volcanic rocks, yielded crystallization ages ranging from 1847 ± 10 Ma to 1842 ± 7 Ma (95% confidence). Thus, these rocks record a magmatic episode that is synchronous with bimodal volcanism in the Wausau domain and Marshfield terrane farther south. Our results, integrated with published data into a time-space diagram, highlight two bimodal magmatic cycles, the first at 1890−1860 Ma and the second at 1845−1830 Ma, developed on extended crust of the Superior Craton. The magmatic episodes are broadly synchronous with volcanogenic massive sulfide mineralization and deposition of Lake Superior banded iron formations. Our data and interpretation are consistent with the Penokean orogeny marking west Pacific-style accretionary orogenesis involving lithospheric extension of the continental margin, punctuated by transient crustal shortening that was accommodated by folding and thrusting of the arc-back-arc system. The model explains the shared magmatic history of the Pembine-Wausau and Marshfield terranes. Our study also reveals an overprinting metamorphic event recorded by reset zircon and new monazite growth dated at 1775 ± 10 Ma suggesting that the main metamorphic event in the terranes is related to the Yavapai-interval accretion rather than the Penokean orogeny.