THE SABINE AND COAHUILA TERRANES AT THE SOUTHERN MARGIN OF LAURENTIA

Gravity models of the Sabine and Coahuila terranes along southern Laurentia (Ouachita-Marathon orogen) show a full thickness of continental crust; however, continental affinities of the crust remain uncertain. A deformed forearc complex separates Sabine from the Alabama-Oklahoma transform margin of Laurentia in the corner of the Ouachita embayment, but continent-continent collision is probable on both west (Fort Worth foreland basin) and east (Black Warrior foreland basin). Similarly, Coahuila is within the Marathon embayment of the Laurentian margin. Previously published data from Sabine and Coahuila suggest Gondwanan microcontinental terranes and associated continental-margin arcs, beneath which Laurentian oceanic crust was subducted during Ouachita-Marathon orogenesis.

Synorogenic sandstones in the Sabine foreland (Arkoma basin) contain distinctive detrital-zircon-age groups: (1) dominantly 1250–950 Ma (Grenville) and >2900–1250 Ma, characteristic of Laurentian craton (and parts of Gondwana); (2) 800–600 Ma, interpreted to be from Sabine Gondwanan basement; (3) 542–524 Ma, possibly from Iapetan synrift igneous rocks; and (4) 520–330 Ma, possibly mixed from Sabine Gondwanan and recycled Appalachian sources. The Sabine forearc succession includes tuff beds with reported crystallization ages of 328.5±2.7 to 320.7±2.5 Ma; the tuffs also contain older zircons (2890–410 Ma), which have been interpreted as detrital. Alternatively, contrasts with other Ouachita detrital-zircon populations suggest that the older zircons may be crustal xenocrysts from Sabine arc magmatism.

Coahuila arc-magmatic rocks include granite (331±4 Ma), dacite (268±6 Ma), and granitoid plutons (286 and 236–220 Ma). Granitoid boulders represent Coahuila Gondwanan basement (1232±7 to 1214±2 Ma) and Pan-African basement (580±4 Ma). In the Coahuila foreland (Marathon thrust belt), synorogenic boulder beds contain rhyolite (371±12 Ma with Proterozoic zircon xenocrysts). Synorogenic sandstones contain zircons of various ages compatible with Laurentian sources, as well as other zircon-age groups: (1) 329–323 Ma, suggesting a Coahuila arc; (2) 423–394 Ma, interpreted to be from Gondwanan (Yucatan) basement; and (3) 861–719, 1575–1515, 2106–2031 Ma, suggesting non-Laurentian (probably Gondwanan) sources.