GEOCHEMISTRY AND TECTONIC SIGNIFICANCE OF VOLCANIC CLASTS CONTAINED IN LOWER, MIDDLE AND UPPER ORDOVICIAN OFF-SHELF SEDIMENT GRAVITY-FLOW DEPOSITS IN THE MARATHON FOLD-THRUST BELT, WEST TEXAS

Ordovician off-shelf sediment gravity-flow deposits in the Marathon fold-thrust belt, West Texas, contain volcanic cobbles and boulders up to 70 cm in length, intermixed with more common carbonate debris derived from the Laurentian shelf margin. These strata were thrust over the shelf during late Paleozoic Ouachita orogenesis and originally were deposited as much as 200 km to the SE; the volcanic clasts contained within them provide a window into one or more now buried volcanic terrains along the ancient continental margin. We have found volcanic clasts within the Lower Ordovician (Floian) Marathon Fm., the Middle Ordovician (Darriwilian) Ft. Peña Fm., and the Upper Ordovician (Sandbian) Woods Hollow Fm., representing a time interval of as much as ~24 Ma. Zircons interpreted as xenocrysts have been separated from basaltic clasts in the Marathon and Ft. Peña Fms. and yield ²⁰⁷Pb/²⁰⁶Pb ages of ~1960 to 670 Ma, bracketing the volcanism between Neoproterozoic and Ordovician.

The volcanic clasts are unmetamorphosed but show variable low-T alteration. To date, 22 clasts from the Ft. Peña Fm. have been analyzed for major and trace elements and show a compositional range from alkali basalt through trachyandesite to trachyte on the Zr/TiO₂ vs Nb/Y classification diagram. Strong correlations between trace elements resistant to alteration (e.g., Zr, Nb, Sc) suggest the sample suite is related by crystal-liquid fractionation from a basaltic parent magma. Trace-element contents of basaltic and mafic trachyandesitic clasts are similar to ocean-island basalts and plot in fields for within-plate, alkaline basalts on standard discrimination diagrams. Trace-element contents of the more fractionated trachyandesite and trachyte clasts indicate affinities to rift- or hotspot-related A-type felsic rocks. We infer that the Ft. Peña clasts were derived from a volcanic succession erupted during intraplate magmatism that preceded or accompanied early Paleozoic opening of the southern Iapetus Ocean. Geochemical and geochronological studies are in progress on a larger suite of volcanic clasts from all three of the formations mentioned above to determine if the clasts were derived from a single major volcanic terrain or from several different sources recording protracted intraplate magmatism along the craton margin.