Authigenic potassium feldspar (K-feldspar) occurs throughout the North American midcontinent in late Proterozoic crystalline basement and in the overlying sequence of Paleozoic sedimentary strata often in proximity to the Great Unconformity. Age determinations for this authigenic K-feldspar range from Paleoproterozoic (ca. 1750 Ma) to Mississippian (ca. 360 Ma), commonly interpreted as emplaced by orogeny-driven brines. The accuracy of these formation ages is often complicated by occurrences of authigenic K-feldspar on primary igneous or detrital K-feldspar, where mixing of these components during analysis is unavoidable. In this study, we examined an occurrence of authigenic K-feldspar associated with a late fault that crosscuts Proterozoic crystalline basement rocks and the Reef Deposit, a Paleoproterozoic gold and copper occurrence in northern Wisconsin. The major fault that cuts the Reef Deposit was previously interpreted as associated with the ca. 1100 Ma Midcontinent Rift. However, analysis of K-feldspar microveins hosted within Precambrian quartz veins, devoid of primary K-feldspar, using in situ 40Ar/39Ar methods yielded an age consistent with faulting and fluid circulation at 499.7 ± 0.8 Ma. This age predates known Paleozoic Laurentian collisional tectonic events. We propose that these K-feldspar microveins reflect a continuation of rifting and related isostatic adjustments along the Iapetus margin, prolonging the duration of extension by ∼25 m.y. If correct, this occurrence of authigenic K-feldspar would represent the most distal observation of early Paleozoic deformation and fluid migration in the midcontinent of North America. Moreover, this work highlights the potential role of extensional tectonics in the formation and distribution of authigenic K-feldspar in midcontinent North America.
- In situ 40Ar/39Ar dating of authigenic potassium feldspar in midcontinent Laurentia
- Erik L. Haroldson - California State University, SacramentoBryan Wathen - University of Wisconsin–MadisonBrian R. Jicha - University of Wisconsin–MadisonBrad S. Singer - University of Wisconsin–Madison
- Geology Department
- GEOLOGICAL SOC AMER, INC; BOULDER
- 04/08/2026
- Austin Peay State University U.S. National Science Foundation University of Wisconsin- Madison
Funding was provided by Austin Peay State University. WiscAr Lab is supported by grants from the U.S. National Science Foundation and the University of Wisconsin- Madison. The authors are grateful to Greenlight Metals for allowing us to publish the historic magnetic map. E. Haroldson thanks Tom Quigley for assistance accessing samples. Kurt Burmeister helped improve an earlier version of the manuscript. Mark Schmitz and Simon Kelley are thanked for their constructive reviews. We also thank Christopher Spencer and Gregory Hoke for editorial handling.
- 99258309427101671; https://hdl.handle.net/20.500.12741/rep:14039; https://doi.org/10.1130/GES02896.1
- English
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