The Southern Granite-Rhyolite (SGR) province, in the southern midcontinent of North America, represents a major pulse of crustal formation during the Mesoproterozoic. Basement rock units of this province are primarily restricted to the subsurface with the exception of exposures in the Arbuckle Mountatins and in Mayes County, Oklahoma. Felsic igneous rocks characterize the SGR province with published U-Pb ages ranging from 1400 to 1340 Ma. In addition to the crystallization ages, crustal residence ages of 1.98 to 1.49 Ga have been determined by previous Sm-Nd isotopic studies. Information presented in this study includes Sm-Nd isotopic data for 71 whole-rock samples. Whole-rock samples from drill cores, drill cuttings, and surface exposures were analyzed for Sm-Nd isotopes to determine the crustal residence age with respect to the depleted mantle model curve (TDM) and the eNd(t) values. Values of eNd(t) within the SGR province ranged from –1.2 to +4.5 with corresponding crustal residence ages of 1.95 to 1.34 Ga. This range of Sm-Nd data suggests variable contamination by the melting of older lithospheric material that was added to a presumably juvenile magma (epsilon=+4.5). Tectonic interpretations of previous and new isotopic data along with geochemical data within the province, focus on the development of the southern margin of the continent. These interpretations suggest episodes of crustal growth along an Andean-type continental margin to the south followed by felsic igneous activity possibly in a back-arc extensional setting.