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Magmatic, Metamorphic and Tectonic Processes in Ophiolite Genesis |
1 Department of Geology, Appalachian State University, Boone, NC 28608, USA raymondla{at}appstate.edu
2 Department of Geology, University of Georgia, Athens, GA 30602, USA
Resolution of the petrotectonic history of Blue Ridge ophiolites of the Southern Appalachian Orogen has remained enigmatic because of metamorphism and tectonic fragmentation of ultramafic bodies. Understanding of this history is confounded by the presence of five partial metamorphic overprints and by similar Ti enrichments in spinels from Blue Ridge and modern mid-ocean ridge basalt ultramafic rocks that result from different processes. Chrome spinels from oceanic ultramafic lithosphere show increases in Ti caused by metasomatism induced by passing mafic melts, which create both dunite melt channels within harzburgite wall rocks and associated troctolite impregnation zones. In the Blue Ridge Belt, the oldest metadunite mineral association generally lacks high-Ti spinel, whereas the higher Ti spinels are relatively low in Al and Mg and occur in three amphibolite- to greenschist-facies retrograde metamorphic associations that occur in deformed, metasomatized ultramafic bodies with high aspect ratios. Some spinel compositions in the oldest mineral association are similar to those from arc-suprasubduction zone ultramafic lithosphere. Together, available data are consistent with the hypothesis that: (1) the Blue Ridge ophiolites are fragmented, metamorphosed, very slow-spreading ridge, Xigaze-type ophiolites, consisting of mafic rocks, minor plutonic rocks, and a sublithospheric ultramafic tectonite base; (2) the metadunites represent sublithospheric melt channels and zones of high melt flux, perhaps formed in a suprasubduction zone setting; (3) pre-Taconic subduction may have been west-directed rather than east-directed. The Taconic orogenesis deformed, fragmented, and metamorphosed the ophiolites; and later Taconic, Acadian, and Alleghenian metamorphism hydrated the bodies, while associated deformation exaggerated their elongation.
