Paper No. 24
Presentation Time: 2:45 PM
THE ORIGIN OF SANDSTONE XENOLITHS AT MOLE HILL, AN EOCENE VOLCANIC NECK NEAR HARRISONBURG, VA
KIRACOFE, Zachary A., Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, JOHNSON, Elizabeth A., Dept of Geology and Environmental Science, James Madison University, Harrisonburg, VA 22807 and HAYNES, John T., Dept of Geology and Environmental Science, James Madison University, 395 South High St, Harrisonburg, VA 22807, kiracoza@vt.edu
Mole Hill, located four miles west of Harrisonburg, VA, in the Shenandoah Valley, is a volcanic neck of Eocene age (48 Ma). The Mole Hill basalt exhibits moderate to poor columnar jointing in outcrop, and includes sandstone xenoliths and mantle-derived xenocrysts. Mole Hill is surrounded by limestone and dolostone of the Ordovician Beekmantown Formation and a thin contact zone of hydrothermal quartz. To constrain the origin of the sandstone xenoliths, petrographic and whole-rock geochemical analyses were performed on 23 samples from 14 regional sandstone units, 7 sandstone xenoliths from Mole Hill, and 8 samples of the Mole Hill basalt. Grain size distributions and sphericity were also determined for the framework grains of the sandstone xenoliths and 5 of the most similar sandstone units. The xenoliths are comprised mainly of monocrystalline quartz, commonly with undulose extinction. The matrix and grain boundaries of most xenoliths were partly to completely infiltrated with magma, but quartz overgrowths are observed on some quartz grains. The average xenolith grain size is 0.32 ± 0.12 mm (n=4) to 0.49 ± 0.27 mm (n=1) (longest axis), with an average long/short axis ratio of 1.98.
Compositionally, the xenoliths form mixing trends on chemical discrimination diagrams between the basalt end member and an unknown sandstone end member. Al2O3, La, Zn, and Ce concentrations are the best geochemical discriminators between possible sandstone units. Combining results from the three types of analysis shows that the Devonian Oriskany Sandstone and Silurian Tuscarora Formation are most similar to sandstone xenoliths. The Tuscarora Formation is exposed in the North Mountain thrust zone, both as a coherent unit and in breccias; the Tuscarora is a more indurated unit than the Oriskany and its sandstones are more likely to form xenoliths that preserve original textures when placed into an igneous environment. The structural evidence supports the hypothesis that the Eocene magma exploited the deep-seated thrust system to make its way to the surface, and strengthens the argument that the Mole Hill xenoliths are fragments of Tuscarora sandstone.
