THE LOWER SILURIAN CLEAR SPRINGS VOLCANIC SUITE: SWORD MOUNTAIN OLIVINE MELILITITE (433+/-3 MA) AND HANGING ROCK DIATREME, WASHINGTON COUNTY, MARYLAND

     The Sword Mountain Olivine Melilitite (OM) on Sword Mountain, previously mapped as a Triassic dike, is locally overlain by a lithified saprolite derived therefrom.  Polished slabs of samples of the OM itself suggest welded lapilli tuff.  A smectite-rich tuff occurs in an ~ 1-ha perched wetland along strike to the north.

     Field mapping indicates that the OM is ~100 m thick, 2.1 km long, and stratabound between the main portion of the Silurian Tuscarora Formation above and the Hanging Rock Diatreme beneath.  The diatreme is variably diluted by Tuscarora sand and is underlain by the thin, lower selvage of the Tuscarora.  The Ordovician Juniata Formation is present beneath the Tuscarora from Pennsylvania southward until it thins just as thin volcaniclastics, tuff, and the OM are progressively approached.  This thinning and eventual absence near the OM may be the result of non-deposition associated with volcanic doming.

     The implied Lower Silurian age was confirmed with detailed incremental-heating ³⁹Ar/⁴⁰Ar measurements on OM fine-grained groundmass phlogopite (median 2.8% TiO₂, 3.6% BaO, and 6.8% F) which yielded concordant spectra with plateaus at 432.6 ± 1.1 Ma.  Rb-Sr analyses of the groundmass phlogopite and whole rock aliquots yield an indistinguishable, concordant age of 434 ± 3 Ma.  For the OM whole rock, calculated initial ratios for ⁸⁷Sr/⁸⁶Sr of 0.70370 and ¹⁴³Nd/¹⁴⁴Nd of 0.51233, or e_Nd[433 Ma]=5.0, indicate a mantle origin with not more than minor crustal interaction.  The OM gives a model T_Nd age of 0.73 Ga consistent with a depleted mantle source.

     Based on the presence of OM itself and its composition, a continental, initial rifting environment that may have much to do with the development of the Appalachian Basin is suggested.  Although not studied herein, the Beemerville, New Jersey, nepheline syenite beneath the Lower Silurian Shawangunk Formation appears to be related.