SERPENTINIZED HARZBURGITE CLASTS IN THE DIANA MILLS PLUTON, VIRGINIA: REMNANTS OF PALEOZOIC SERPENTINE MUD VOLCANISM?

The Silurian Diana Mills Pluton (~21 km²) is a metamorphosed mafic-ultramafic body in the Piedmont Province of central Virginia.  It occurs within the Mine Run Complex, a mélange zone that extends from central Virginia through Maryland.  Originally viewed as intrusive into adjacent metagreywackes, the mass has more recently been interpreted as tectonically emplaced, perhaps by gravity sliding. Rock types include metagabbro, hornblendite, and metaperidotite.  In addition, unusual serpentinites with a hint of orbicular texture occur at four localities.  These occurrences consist of abundant tan-weathering clasts in a dark green matrix.  Clasts range in length from <0.5 up to ~12 cm, and many are somewhat ellipsoidal; others are more angular and even rectangular. Some show a thin (<5 mm) shell of radially oriented serpentine, but others lack this feature.  Most clasts are dominated by serpentine or talc + magnetite. Matrix minerals are dominantly chlorite, tremolite,  and talc + magnetite ± serpentine.  Whole-rock analyses of 3 serpentine-rich clasts are similar (mg# ~0.9, ~46 wt% SiO₂, ~42% wt% MgO, <3 wt% CaO, <1 wt% Al₂O₃ on an anhydrous basis; up to 2000 ppm Ni; up to 2200 ppm Cr). These rocks represent serpentinized harzburgites, confirmed by normative mineralogy.  Matrix compositions (2 samples) are much richer in Al₂O₃ (11-20 wt%),  reflecting higher amounts of chlorite, but are also ultramafic (34-44 wt% SiO₂,  ≤1200 ppm Ni, ≤1500 ppm Cr). These rocks have been referred to as “orbicular serpentinite” in the past (including us: Garry & Owens, 1999, SE GSA abstract), and we tentatively interpreted them as metamorphosed orbicular ultramafic rocks.  However, the clasts do not display concentric mineralogical zoning, and some lack an obvious outer shell.  They superficially resemble examples of mantle xenoliths in alkali basalts, but an intraplate-setting is inappropriate here, as are the matrix compositions. Alternatively, these rocks bear some similarities to harzburgite clasts entrained in modern serpentinite mud volcanoes (e.g., Marianas and Izu-Bonin forearcs).  This interpretation is consistent with the convergent margin setting here, especially the location in a mélange zone.  The quasi-orbicular textures probably reflect metasomatic reaction zones between clasts and matrix during later metamorphism.