Late Paleoproterozoic Climate, Tectonics, and Metamorphism in the Southern Lake Superior Region and Proto–North America: Evidence from Baraboo Interval Quartzites

Abstract

Red, supermature quartzites of the Baraboo interval of the Lake Superior region contain detrital zircon that ranges in age from 1782 to 1712 Ma. Deposition clearly occurred after the geon 18 Penokean orogeny. These late Paleoproterozoic sedimentary rocks consist largely of quartz, kaolinite or pyrophyllite, and hematite; detrital feldspar and muscovite are rare or absent. Their Chemical Index of Alteration ranges from 96.8 to 98.6, among the most chemically mature clastic sediments in the geological record. The quartzites are underlain by mature, feldspar‐free paleosols, accounting for the absence of feldspar in the overlying sediments and indicating the presence of first‐cycle quartzose detritus. Such physical and chemical characteristics imply that late Paleoproterozoic deposition in the Lake Superior region occurred in a stable tectonic setting with subdued topographic relief in a warm, humid climate. Folding and low‐grade metamorphism of the quartzites is thought to reflect ∼1630 Ma foreland deformation related to the Mazatzal orogeny. Younger hydrothermal alteration is widespread in the Baraboo and Sioux quartzites and, based on 40Ar/39Ar dating of low‐temperature minerals, is attributed to the migration of fluids along permeable channels in response to the thermal effects of magmatism associated with the 1465 Ma Wolf River batholith. Much of the Paleo‐ and Mesoproterozoic crust of Proto–North America may have been affected by areally extensive, but stratigraphically restricted, hydrothermal alteration related to the influence of geon 14 transcontinental A‐type granitic magmatism.