ORIGIN OF THE SABINE BLOCK: A PROMONTORY OF NORTH AMERICA

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 dated zircons from granite xenoliths and from pyroclastic rocks from the Prairie Creek Lamproite pipe of Arkansas that penetrates this block. Zircons from the granites show U-Pb ages ranging 1600–1800 Ma, quite different from adjacent Grenville or Gondwana-affiliated blocks and also contrasting with the Yucatan Block on the conjugate side of the Gulf of Mexico. This 1600–1800 Ma 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. This implies either that the Grenville is missing from this part of the margin or that it does not fully overprint the Mazatal. It also indicates incomplete overprinting by Granite-Rhyolite magmatism. The ages also correlate with parts of the Amazon Craton, NE Greenland, Baltica and Cathaysia (SE China), 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 lie to the south and not run through the Ouachita Foldbelt. The anomalous age of the granites rules out a simple extension of Laurentian crust south of the Ouachita Foldbelt.