BASALT-TRACHYANDESITE-TRACHYTE CLASTS WITHIN ORDOVICIAN DEEP-MARINE CONGLOMERATES IN THE MARATHON FOLD-THRUST BELT, WEST TEXAS: IMPLICATIONS FOR ANOROGENIC MAGMATISM ALONG THE SOUTHERN LAURENTIAN MARGIN

Lower Paleozoic strata in the Marathon fold-thrust belt at the western end of the Ouachita orogen contain turbidites, debris-flow deposits and olistostromes that were derived from the Laurentian shelf and accumulated in deeper water along the craton margin. Palinspastic restoration suggests these strata were originally located up to 200 km SE of their present outcrop prior to late Paleozoic thrusting. Some mass-flow deposits contain generally sparse igneous clasts, which provide a fragmentary record of Paleozoic (or older) magmatism along the now buried southern margin of the Laurentian craton.

We report initial results of geochemical and geochronological studies on vesicular volcanic cobbles and boulders within redeposited limestone conglomerates in the Lower to Middle Ordovician Marathon and Fort Peña Formations in the NW part of the Marathon fold-thrust belt. The volcanic clasts show extensive low-T alteration, with common secondary silica and carbonate, and here we focus on immobile trace elements to characterize original igneous compositions. Sixteen clasts analyzed to date show a compositional range from basalt, through trachyandesite, to trachyte. Basalts contain pseudomorphs of plagioclase and olivine phenocrysts in a hyalopilitic to intersertal groundmass; swallow-tail plagioclase microlites in some samples indicate aqueous quenching of lava. More felsic clasts contain partly replaced alkali feldspar phenocrysts in a groundmass of randomly arranged to flow-aligned feldspar microlites. Most samples define coherent trends on variation diagrams using immobile elements (e.g., Zr, Sc, Y, Nb), implying a linked petrogenetic history. The analyses fall in continental rift/intraplate settings on standard discrimination diagrams for mafic and felsic rocks. Two basalt boulders from the Marathon and Fort Peña Formations yielded zircon grains with a wide range of U-Pb SHRIMP ages (~1960-670 Ma), recording assimilation of older crust during evolution of the basaltic magmas. The available data thus constrain the age of the volcanic event(s) that produced the boulders between 670 Ma and Middle Ordovician. We infer that the boulders were eroded from one or more rift-related volcanic terranes that formed prior to or during opening of the Iapetus Ocean and are now hidden beneath extensive younger cover.