The Precambrian basement beneath much of Illinois, Michigan, southwestern Ontario, Indiana, western Ohio, western Kentucky, and southeastern Missouri (including exposures in the S t Francois Mountains) is known as the eastern granite-rhyolite prov ince. This crust is characterized by undeformed and mostly unmetamorphosed highsilica rhyolite to dacite with epizonal granite of equivalent composition having zircon UPb ages of 1,470 ± 30 Ma. The Precambrian subcrop beneath parts of southwestern Mis souri, southern Kansas, northern Arkansas, most of Oklahoma (including small areas of outcrop), and the panhandle of Texas is known as the southern granite-rhyolite province. This crust is also characterized by undeformed and mostly unmetamorphosed high-silica rhyolite to dacite with epizonal granite, but zircon U-Pb ages from this province are about 100 m.y. younger, 1,370 ± 30 Ma. We have obtained additional zircon U-Pb ages and whole-rock Sm-Nd data for many drill-hole samples and a few outcrop samples from these provinces to (1) define their geographic limits more accurately, and (2) use them as a means of defining the age of the crust which presumably underlies these two provinces and from which the granite and rhyolite magmas were derived.These granite-rhyolite provinces were previously thought to have formed from anatectic melting of preexisting Paleoproterozoic crust, which is a southward and eastward extension of the 1,600 to 1,800 Ma Central Plains orogen. However, recent Sm-Nd stud ies, including additional data reported here, indicate that some of the rhyolite and gran ite from both the eastern granite-rhyolite province and the southern granite-rhyolite province have Nd crustal residence ages indicating magma derivation from juvenile, mantle-derived sources that are only slightly older than the granites and rhyolites them selves. As a result, a relatively sharp boundary can be defined that separates Mesoproterozoic granites and rhyolites (regardless of age) having Nd crustal residence ages indicative of Paleoproterozoic crustal sources (TDM > 1,550 Ma) from ones having younger crustal sources (TDM < 1,550 Ma). This boundary extends northeasterly from southeastern Oklahoma through southeastern Missouri, southern Illinois, central Indi ana, southeastern Michigan, and into southwestern Ontario; its southwestern extension is not defined, but it probably parallels, and may be close to, the northwestern boundary of the Llano province in Texas. The underlying crust northwest of boundary line is inferred to be Paleoproterozoic, whereas the underlying crust southeast of this bound- Van Schmus, W. R., Bickford, M. E., and Turek, A., 1996, Proterozoic geology of the east-central Midcontinent basement, in van der Pluijm, B. A., and Catacosinos, P. A., eds., Basement and Basins of Eastern North America: Boulder, Colorado, Geological Society of America Special Paper 308. 7 on May 15, 2015 specialpapers.gsapubs.org Downloaded from 8 W. R. Van Schmus and Othersary is inferred to be Mesoproterozoic. We interpret this boundary ...