Highly altered tephra layers (K-bentonites) of mid-Ordovician age are widespread throughout the mid-continent region of the United States and the Appalachian Taconic foreland basin. The numerous ash layers, their wide areal extent, and their large volume provide evidence that extensive volcanic activity accompanied Taconic orogenesis. The mineralogical and geochemical characteristics of these widespread Ordovician tephra layers can be used to constrain the nature of their source region, and thereby help test recent hypotheses that Gondwana collided with Laurentia in mid-Ordovician time. Initial 143Nd/144Nd and 87Sr/86Sr ratios of two K-bentonites that occur traced throughout the midcontinent are 0.51183--0.51187 and 0.7101--0.7120, respectively. Isotopic ratios for K-bentonites within the Taconic basin are 0.51178--0.51201 and 0.70564--0.70905. The isotopic composition of the tephra layers and the ubiquitous presence of Proterozoic zircon xenocrysts within the K-bentonites is consistent with an origin by crustal anatexis. The cooling history of the source is constrained by 842--996 Ma 40Ar/39Ar apparent ages of hornblende xenocrysts contained within two Taconic basin K-bentonites. A possible South American source region of the K-bentonites is the Arequipa massif of Peru. This region contains early Paleozoic arc-related rocks, many of which contain mid-Proterozoic zircon xenocrysts, and Early Proterozoic gneisses which were affected by Grenville-age granulite grade metamorphism. Gneisses within the Adiron-dack Highlands of the Grenville Province have similar &egr;Nd(450 Ma) values and hornblende cooling ages to the K-bentonites, consistent with this region as a source. Caradocian K-bentonites in Sweden have Sr isotopic ratios similar to the North American beds. If the Swedish and North American K-bentonites were produced from the same source, then Baltica and Laurentia were proximal in Ordovician time precluding a Gondwana-Laurentia link. If Caradocian K-bentonites with North American characteristics are found in western South America, support for a Laurentia-Gondwana link would be established. If such K-bentonites cannot be found, considerable doubt would be cast on this model. ¿ American Geophysical Union 1996