Age and origin of authigenic K-feldspar in uppermost Precambrian rocks in the North American Midcontinent

摘要

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.