RE-OS AND OXYGEN SYSTEMATICS OF VARIABLY ALTERED ULTRAMAFIC ROCKS, NORTH CAROLINA

Abstract

This study focuses on the origin and modification of six ultramafic bodies located in the Blue Ridge Province of North Carolina. The bodies consist mainly of harzburgites and dunites with associated chromites. Some of the bodies are associated spatially and genetically with mafic lithologies while others are fault bounded. All the bodies in the study are characterized by variations in their initial Os isotopic compositions, assuming a formation age of 490 Ma (187Os/188Osinitial 0.1114 to 0.1360). Most of the initial 187Os/188Os ratios are chondritic to subchondritic and can be explained by Re depletion during a partial melting event prior to ophiolite formation. By contrast, some initial 187Os/188Os ratios, particularly for those bodies in the Tallulah Falls formation, are suprachondritic suggesting the addition of radiogenic Os during a melt percolation or melt/rock reaction event, most likely during the event that led to the formation of the bodies. Oxygen isotopic δ18O values of the bodies range from +4.85 to +7.60 which overlap with and extend above mantle estimates. The cause of the higher values remains unresolved, but serpentinization and contamination by large amounts of crustal material can be excluded. It is concluded that the six bodies in this study have a common history as the residues of mantle partial melting, with chemical compositions and isotopic systematics similar to Phanerozoic ophiolite peridotites associated with the same collisional event, as well as modern abyssal peridotites. Nevertheless, Os isotopic characteristics indicate different processes acted within the bodies despite their relatively close spatial association.