Authigenic K-feldspar of apparent low-temperature origin is widespread in the uppermost part of the two main Precambrian basement terranes of Ohio: the subsurface extension of the Grenville Province of Canada, and the anorogenic granite–rhyolite terrane of the Transcontinental Proterozoic Province of the United States. The authigenic K-feldspar occurs mainly as a replacement of both primary K-feldspar and albitic plagioclase in a variety of igneous and metamorphic rocks and can be identified by a salmon color in plane or ordinary light, moderate to low optic angle, monoclinic or triclinic symmetry, and Or-rich composition. Its origin has been ascribed to alkali exchange by potassic brines in early Paleozoic time.The K-feldspar authigenesis has changed whole-rock major-elemental compositions by increasing K2O, decreasing Na2O and CaO, and in general modifying SiO2 and Al2O3. These changes rule out using chemical parameters such as alkalis, alkaline earths, alumina, and silica as tectonic or petrologic discriminators on rocks affected by K-feldspar authigenesis. Of the 53 granitoid and felsic extrusive rocks in the basement of Ohio that we analyzed for major-element compositions, only 19 are unaffected by the authigenesis. Among the unaltered granitoids, 13 have major-element abundances and ratios indicative of orogenic paragenesis, and at least three of the granitoids have apparent anorogenic chemical characteristics. All but one of the orogenic granitoids occur east of the Grenville Front and within the Grenville Province. Anorogenic granitoids occur on both sides of the front, and they are probably part of the broad belt of anorogenic igneous rocks that extend from the southwestern United States to Labrador.