Petrology, geochemistry and geochronology of volcanic clasts contained in Ordovician strata in the Marathon fold-thrust belt, West Texas

Abstract

This project examines ancient volcanic boulders derived from the Laurentian shelf and contained in Ordovician off-shelf sediment gravity-flow deposits in the Marathon fold-thrust belt, West Texas. Thirty-six volcanic clasts up to 70 cm across were analyzed from the Lower Ordovician Marathon Formation, Middle Ordovician Fort Pe?a Formation, and Upper Ordovician Woods Hollow Shale. The clasts are unmetamorphosed but show variable low-temperature alteration. Major and trace element geochemical analyses show a compositional range from alkali basalt through trachyandesite to trachyte. Trace element contents for both the mafic and felsic clasts indicate within-plate settings with an OIB-type source for the volcanism. Felsic clasts in the Fort Pe?a Formation yielded U-Pb zircon ages of ~706 Ma. These data indicate that the volcanic clasts were derived from one or more Neoproterozoic intrapate volcanic terranes, possibly rift-related, that developed near or along the southern Laurentian margin.