Critical evidence bearing on the breakup history of the supercontinent Rodinia near the end of the Proterozoic comes from widespread Cryogenian–Cambrian intraplate igneous assemblages present along the margins of cratonic blocks released during Rodinia fragmentation and now distributed around the globe. This magmatism occurred over a long time span (780–540 Ma) prior to and during final stages of Rodinia breakup along the eastern, western, and northern margins of the Laurentia craton, which forms the centerpiece of Rodinia in many reconstructions. Whether similar protracted magmatism occurred prior to the rift-drift transition along the southern Laurentian margin has remained uncertain because of deep burial beneath younger strata. We present geochemical and geochronological data from volcanic clasts within shelf-derived Ordovician turbidites and debris-flow deposits now exposed in allochthonous thrust slices in the Marathon uplift, west Texas (USA), that document one or more episodes of intraplate magmatism extending back at least to 706 Ma along this part of the ancient margin. These data raise the possibility that Laurentia may have been completely encircled by intraplate igneous activity prior to Rodinia breakup, with implications for the driving forces leading to supercontinent fragmentation and factors controlling the sites of ocean-basin formation during that process.