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,1,2,3,4
1 Publication Division, American Chemical Society, 1155 16 Street N.W., Washington, D.C. 20036, USA
2 Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
3 Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA
4 Department of Geological Sciences, Queens University, KL7 3N6 Ontario, Canada
Authigenic K-feldspar occurs widely in the North American Midcontinent in an alteration profile developed on uppermost Precambrian rocks. In the study areas of west-central Wisconsin and southeast Missouri, the alteration profiles are kaolinitic and interpreted as former saprolite formed by weathering prior to deposition of Upper Cambrian sandstone. K-feldspar in granitic alteration profiles of west-central Wisconsin occurs either as coarse-grained red crystals and veins replacing kaolinite or as overgrowths and fine-grained crystals in pore space of partly altered granite. In a granitic profile of southeast Missouri, K-feldspar replaces primary feldspar and probably kaolinite. In a diabasic profile, K-feldspar is disseminated and forms veins in saprolite, and it replaces the outer 3–5 cm of core-stones.
Dating was conducted with the 40Ar/39Ar method using incremental heating. Observed spectral types are plateaus (6), near plateaus (3), undulatory (14), and stair-step (5). Undulatory patterns are attributed chiefly to recoil, and integrated ages of these are usually within the error of plateau ages where present, indicating a balanced enrichment/depletion of 39Ar due to recoil. Stair-step spectra are attributed to severe recoil.
Plateau 40Ar/39Ar ages of 446–427 Ma (Late Ordovician and Silurian) were obtained from coarse-grained K-feldspar replacing kaolinite in Wisconsin, suggesting a duration of the alteration episode of 
19 m.y. However, potentially accurate nonplateau age measurements (integrated and near-plateau ages) suggest that alteration continued for a considerable time thereafter, possibly to ca. 399–395 Ma (Early Devonian). Integrated 40Ar/39Ar ages of ca. 362 Ma (Late Devonian and/or Early Mississippian) were obtained from fine-grained K-feldspar. Plateau ages of ca. 450 (Late Ordovician) were obtained from K-feldspar of the diabase profile in Missouri; duration of K-feldspar alteration in this region was at least 16 m.y. and may have been as long as 52 m.y.
The Late Devonian–Early Mississippian K-feldspar formed at temperatures of 100 °C from saline fluid averaging 10% NaCl equivalent and having a 
18O value of 3.5
. This brine may have originated from the Michigan Basin, East-Central Iowa Basin, and/or the Illinois Basin, coincident with the Acadian orogeny.
The Middle Ordovician to Early Devonian dates of K-feldspar do not generally correspond in time with orogenic activity at the continental margin, and the mechanism(s) for forming the K-feldspar are uncertain. Fluid sources proposed for some or all of the K-feldspar include the Michigan Basin, Taconic mountains, Reelfoot Rift, Illinois Basin, and Midcontinent epeiric seas. Middle Ordovician K-feldspar of Missouri very likely formed at elevated temperatures, but evidence of fluid temperature is lacking for other pre–Late Devonian K-feldspar.
Key Words: authigenic K-feldspar • potassic alteration • saprolite • 40Ar/39Ar dating • fluid inclusions
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