The crust underlying the northern coast of the Gulf of Mexico, known as the Sabine block, is largely unknown as far as its origin and age are concerned. We date zircons in granite xenoliths and from pyroclastic rocks from the Prairie Creek lamproite pipe that penetrates the block. Zircons from the granites show U-Pb ages ranging from 1800 to 1600 Ma, quite different from adjacent Grenville or Gondwana-affiliated blocks. This age range correlates with rocks in the Yavapai-Mazatal terrane of North America, implying that the Sabine block could be a promontory to the continental core. The ages also correlate with parts of the Amazon craton, northeast Greenland, Baltica, and Cathaysia, raising the less likely possibility of an exotic origin. Regardless, the Sabine block must have been accreted to North America before 1.4 Ga, requiring the Iapetus Ocean suture to be to the south and not extend through the Ouachita fold belt.