THE SUBMARINE DEVONIAN BASALTS OF THE ANCESTRAL APPALACHIANS

Ancient ancestries may include lineages that ended long ago, perhaps without leaving direct descendants. Lithologies formed in an evolving orogeny may be lost or obscured, though they may still be recorded in casts of foreland basins. Polymict conglomerates of the Pennsylvanian Pottsville formation in the Greater Black Warrior basin (GBWB) of Alabama contain a remarkable assortment of clasts, which generally seem compatible to lithologies of the Valley and Ridge and Blue Ridge Provinces. However, pebbles of unmetamorphosed to spilitic basalts (reported in an Auburn Univ. thesis by Moore, 2012) are an exception as these have no exposed, direct comparison in the southern Appalachians. In thin section these basalt pebbles comprise intergrown acicular, plumose plagioclase and augite phenocrysts, commonly with hopper textures. A suite of whole rock basalt samples prepared from the Hendrix Core (at a drilling depth of 1104’) for ⁴⁰Ar/³⁹Ar analysis in the ANIMAL facility of Auburn University (with a CO₂ laser) yield spectra that are mutually consistent and dramatically discordant: all of 22 separate basalt release spectra begin with ages greater than 500 Ma that monotonically decrease to ages less than CA. 250 Ma (substantially younger than their minimum depositional age), and have apparent Ca/K ratios that rise consistently throughout the experiments. These results are typical of ³⁹Ar_K recoil redistribution from plagioclase (that degasses early) into augite (that dominates the final step heating steps and fusion ages). Thus, the best estimate of age is obtained simply by reintegrating the total gas age of each basalt fragment. The total gas ages range from 375-430 Ma, with approximately half of the ages within 395±5 Ma, a result we interpret to represent the crystallization age of these basalts as sampled. Such basalts are typical products of submarine volcanism in zones of extension in general, and seafloor spreading in particular. We also note that age modes of ca. 395 Ma are not typical of detrital muscovite populations sampled from the GBWB, including samples thought to have a southern Appalachian source (Moore, 2012, and Uddin et al., 2016, Jour. Sed. Geol.). We suggest that ca. 395 Ma may have been a time of local extension for the southern Appalachians with sufficient magnitude to form sea-floor basalts.