Petrogenesis of the Webster-Addie ultramafic body, Jackson County, North Carolina

The Webster-Addie ultramafic body is situated within a rather continuous belt of ultramafic bodies in the Blue Ridge Province of North Carolina. The foliation of this elliptical ultramafic body is concordant with that of the enclosing country rocks. Petrofabric orientations of olivine crystallographic axes are related to this foliation, which indicates deformation of the ultramafic body along with the country rocks, and also constrains emplacement of the body to pre- or syndeformation. Mineral textures, pyroxene geothermometry (700-960°C, using the Di-En solvus) , and MgO + FeO_T vs. AI₂O₃ for orthopyroxenes suggest that the present mineral assemblages were not produced by the thermal dehydration of serpentinite. Mineral assemblages, though, do indicate regional metamorphism of middle to upper amphibolite fades (approximately 600-675°C at 4-6 kb). The fluid accompanying metamorphism contained an appreciable amount of CO₂. Variation in accessory chromite chemistry from high Fe and Cr to high Mg and Al is similar to trends exhibited by chromite from other metamorphosed ultramafics. Platinum group elements and strontium isotope abundances suggest a depleted mantle protolith for the Webster-Addie body. Platinum group elements and chromite compositions show some affinities with those from ophiolitic ultramafics. The Webster-Addie body may represent the basal portion of an obducted ophiolite sequence or an intruded ultramafic diapir.