The Lower Silurian Clear Springs Volcanic Suite: Sword Mountain olivine melilitite (433+/-3 MA) and Hanging Rock diatreme, Washington County, Maryland

Smith, Robert C., II (retired), Pennsylvania Geological Survey, 3240 Schoolhouse Road, Middletown, PA 17057, Foland, Kenneth A., Department of Geological Sciences, Ohio State University, 125 South Oval Mall, Columbus, OH 43210, and Nickelsen, Richard P., 432 Pheasant Ridge Drive, Lewisburg, PA 17837.

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 [—the Hanging Rock Diatreme/Tuff (HRD/T)—] 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 [HRD/T] 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 [HRD/T] and 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% TiO 2, 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.  [Some possible implications include: 1) some NW- and SE-dipping Alleghanian thrust faults might be reactivated Silurian normal faults, 2) the Bloomsburg, Wills Creek, and Tonoloway Formations in the Appalachian basin might include rift facies, 3) these units might include sabka zinc-lead and red bed copper deposits in the Ridge and Valley, and 4) there might be Silurian extensional structures beneath the Allegheny Plateau which might not be detectable in younger sediments.]

Although not studied herein, the Beemerville, New Jersey, nepheline syenite beneath the Lower Silurian Shawangunk Formation appears to be related.

Poster paper presented at the combined Annual Meetings of the Northeastern (39th) and Southeastern (53rd) Sections of the Geological Society of America, Tysons Corner, VA, March 25, 2004.


Smith, R. C., II, Foland, K. A, and Nickelsen, R. P., 2004, The Lower Silurian Clear Springs Volcanic Suite: Sword Mountain Olivine Melilitite (433+/-3 MA) and Hanging Rock Diatreme, Washington County, Maryland: Geological Society of America Abstracts with Programs, v. 36, no. 2, p. 71.