SOLDIER'S DELIGHT AND THE BALTIMORE MAFIC COMPLEX: TECTONIC CLUES FROM OSMIUM ISOTOPES

A noteworthy band of ultramafic rocks extends the length of the Appalachians, accreted to the Laurentian margin during the Ordovician Taconic orogeny. In some cases sufficient lithologies are preserved to justify naming them ophiolites (i.e. Thetford Mines), but mostly these ultramafic bodies are composed of altered dunites or harzburgites with fault-bounded contacts. These more ambiguous bodies may represent true oceanic mantle, marginal-arc lithosphere, or ultramafic cumulates of mafic melts.

A cluster of ultramafic lithologies extends from southeastern Pennsylvania to northernmost Virginia, the largest of which are the Soldier's Delight and MD-PA Stateline serpentine barrens. The bodies range from chromitites, Cr-spinel bearing dunites/harzburgites that preserve some anhydrous silicates, to extensively altered serpentinites and magnetite-bearing rodingites (Rockville quarry). These ultramafic rocks wrap around the Baltimore Mafic Complex (BMC; gabbroic lithologies associated with an arc) in map pattern. They have been tentatively linked genetically to the BMC, but faulting may have juxtaposed unrelated ultramafic and mafic sequences.

Osmium 187/188 isotope ratios and Os and Re concentrations have been determined for spinel separates to constrain the tectonic provenance of these ultramafites. Cr-rich spinel is an ideal phase for retaining initial ¹⁸⁷Os/¹⁸⁸Os despite intense hydration and metamorphism due to its low Re/Os ratios and high Os contents.

All five samples analyzed to date have ¹⁸⁷Re/¹⁸⁸Os < 0.1, indicating insignificant radiogenic ingrowth since the Ordovician. Three samples from the Soldier's Delight area range in Os from 29 to 61 ppb and have γOs of +4 to +7 (where γOs is % isotopic deviation at 490 Ma from a chondritic reservoir). Interestingly, a sample from Woods Mine (MD-PA state line) is significantly more radiogenic (141 ppb Os, gOs = +13.5) while a sample from the Rockville quarry, near the VA-MD line, is less radiogenic (12 ppb Os, gOs < -5) despite being rodinized.

The two locations containing radiogenic samples support the involvement of crustal material in the source, consistent with old inherited zircons found in the BMC. The subchondritic Rockville sample requires the involvement of pre-Taconic depleted material, either continental or oceanic lithospheric mantle.