NEW ZIRCON U-PB GEOCHRONOLOGY FROM THE DEVIL’S RIVER UPLIFT - INSIGHTS INTO THE NEOPROTEROZOIC AND EARLY PALEOZOIC EVOLUTION OF THE SOUTHERN MARGIN OF NORTH AMERICA

The Devils River uplift in SW Texas records supercontinent interactions along the southern Laurentian margin from Grenvillian orogenesis and assembly of Rodinia, to its fragmentation by rifting, and to the amalgamation of Pangaea. It was cored by a well (Shell No. 1 Stewart), penetrating Precambrian gneisses, Cambrian sandstone, Ordovician dolomite, and Cretaceous limestone. New zircon LA-ICP-MS data from a total of 10 samples elucidate the crystallization and depositional ages, as well as the detrital provenance of Precambrian and Cambrian rocks from the DRU. Zircons from four Precambrian crystalline basement samples (6000-9693’) yield uniform U-Pb crystallization ages of ~1230 Ma that are similar to ages for the Valley Spring Gneiss (Llano uplift) in central Texas, where they mark the cessation of arc magmatism within the Grenville orogenic belt. The ~1230 Ma igneous basement is overlain by L.-M. Cambrian metasedimentary rocks (~4000-6000’) with maximum depositional ages of ~533-545 Ma. The lower sandstone interval is dominated (>80%) by Neoproterozoic detrital zircons with two major distinct age clusters at 570-700 Ma and 780-820 Ma, supporting a two-stage Rodinia rift model and providing strong evidence for major Cryogenian intraplate magmatism along the southern margin of Rodinia. This magmatism is proposed to have thermally weakened the lithosphere, leading to rifting and eventually to Ediacaran-Cambrian break-up. Detrital zircon provenance signatures for L-M Cambrian sandstones show the absence of locally reworked 1230 Ma zircons and instead, a dominant detrital component at ~1070 Ma, likely derived from basement units exposed in Texas (Llano uplift, Franklin Mts.). In contrast, U. Cambrian sandstones lack Cryogenian detrital zircons and yield a similar detrital zircon signature to that of the Cuyania composite terrane, Sa. Pie de Palo, Argentina, incl. 500-550 Ma (Rodinia rifting), 1070 Ma (Grenville, Llano or Franklins), and 1380-1500 Ma (Granite Rhyolite Province). In summary, these new zircon U-Pb data from Devils River uplift in SW Texas show that it is part of the Grenville orogenic belt, characterized by 1230 Ma magmatism. Subsequent Cryogenian intraplate magmatism, evidenced by abundant detrital zircons, is associated with Rodinia rifting and creation of the southern Laurentian margin.