SPINEL COMPOSITIONS AS CLUES TO THE ORIGIN AND TECTONIC SIGNIFICANCE OF METADUNITE BODIES IN THE BLUE RIDGE BELT OF THE SOUTHERN APPALACHIAN OROGEN

More than 200 ultramafic bodies occur as isolated pods within pelitic schist and amphibolite in the eastern part (the Gossan Lead Thrust Block=GLTB) of the Blue Ridge Belt (BRB) of the Southern Appalachian Orogen. Important petrogenetic and petrotectonic issues remain unresolved for most bodies. Most BRB ultramafic bodies are metadunites and metaharzburgites, with mineral assemblages representing anhydrous upper amphibolite facies conditions, or chlorite (+ amphibole + talc) schists that reflect hydrated assemblages of the middle to lower amphibolite facies. The metamorphic textures and mineral assemblages represent probable Taconic, Acadian, and Alleghenian recrystallization events. Unknown are the nature of the protolithic bodies (ophiolite, alaska-type, A-type intrusion) and the reasons for the abundance of relatively homogeneous metadunite and metaharzburgite bodies. Eastern BRB rocks are considered to have oceanic affinities and recent discoveries of eclogite and retrograded eclogite associated with ultramafic rocks at the structural base of the GLTB support the view that these rocks mark a Taconic suture. A few of the bodies (e.g., the Buck Creek body) likely are fragments of oceanic crust (ophiolites). All may be. In the metadunites and metaharzburites, most chrome spinels have low Mg #s (< 40) and high Cr #s (> 80) characteristic of spinels of alpine metamorphic bodies and the lower parts of ophiolites. Variable Ti components (wt. %=0.02-1.95) suggest a dual history of depletion and enrichment consistent with petrologic data and spinel chemistries from Hess Deep and Oman Ophiolite rocks. We suggest that the bodies of metadunite (and some metaharzburgite) represent melt channels within the uppermost sub-MOR mantle that locally experienced melt induced enrichment (forming high TiO2 values). These bodies were structurally resistant to fragmentation during melange-forming processes at the Taconic convergent margin. Associated mafic rocks are preserved as amphibolites, whereas associated ultramafic rocks may have been serpentinized, hydrothermally altered, metamorphosed, and fragmented to the point that the only remaining remnants are various chlorite schists.