GARNET EXSOLUTION IN MEGACRYSTIC CLINOPYROXENE: EVIDENCE OF ULTRAHIGH-PRESSURE METAMORPHISM IN THE MARYLAND PIEDMONT?

Relict chromian diopsides from the Hunting Hill ultramafic body in the Maryland Piedmont contain exsolution lamellae of grossular-uvarovite garnet. This distinctive texture, reported from known ultrahigh-pressure (UHP) localities, suggests the possibility that the Hunting Hill body also reached these extreme conditions. Results of Cr-in-Cpx barometry (Nimis and Taylor, CMP 2000) show that the Hunting Hill body reached pressures of ca. 4–5 GPa. Parallels between exsolved chromian diopsides from Hunting Hill and similar reported examples from coesite- and diamond-bearing UHP terranes support this pressure estimate. Reintegrated precursor clinopyroxene suggests shallower mantle conditions consistent with an oceanic geotherm. The Hunting Hill body and other ultramafic fragments occur as tectonic blocks in the Piedmont Province, and are related to Ordovician accretion of the Mather Gorge Formation onto Laurentia during the Taconic Orogeny. Garnet-clinopyroxene exsolution in the Hunting Hill body points to more extreme metamorphic conditions during the Taconic than have been previously suggested, and may be evidence for continental subduction during Appalachian orogenesis.