Coexisting Clinpyroxene/Spinel and Amphibole/Spinel Symplectites in Metatroctolites from the Buck Creek Ultramafic Body, North Carolina Blue Ridge, USA
Document Type
Article
Publication Date
1-2004
Digital Object Identifier (DOI)
https://doi.org/10.2138/am-2004-0104
Abstract
Buck Creek metatroctolites preserve evidence of three stages in the geologic history of the region: a relict igneous stage; a high-pressure, granulite-facies metamorphic stage; and a hydrous alteration stage. During high-pressure granulite-facies metamorphism, adjacent olivine and plagioclase reacted to produce complex coronitic textures. Relict olivine is surrounded by radial, columnar orthopyroxene, and plagioclase is rimmed by fine-grained symplectite composed of separate grains of clinopyroxene/spinel, tschermakitic amphibole/spinel symplectite and minor clinopyroxene/sapphirine symplectite. Examination of textural relationships in back-scattered electron images and X-ray element maps of the symplectites demonstrates that amphibole/spinel and clinopyroxene/spinel symplectites grew simultaneously at high temperature to replace plagioclase in these rocks. Clinopyroxene/sapphirine symplectite grew in the latest stages of plagioclase replacement in some samples.
A successful model for the main corona-forming reaction in sample BC7, 1.00 Pl + 1.72 Ol + 0.17 H2O = 0.59 Di + 0.17 Amp + 0.68 Spl + 0.79 Opx, results only if amphibole is included. The model predicts relative volumes of product clinopyroxene, amphibole and spinel that are generally consistent with the proportions in imaged symplectites. Proportions of clinopyroxene-and amphibole-hosted symplectite appear to have been governed by local mass-balance of Na and Al in plagioclase and/or availability of H2O. The most successful model for formation of the inner clinopyroxene/sapphirine corona, 1.00 Pl + 1.47 Ol + 0.17 H2O = 0.59 Di + 0.17 Amp + 0.30 Spr (10 ox.) + 0.57 Opx, also includes amphibole, which is present in association with clinopyroxene/sapphirine symplectite. Sufficient H2O for formation of symplectic amphibole must have been present in the intergranular fluid of relict igneous troctolites or have infiltrated during metamorphism at high P-T conditions.
Was this content written or created while at USF?
Yes
Citation / Publisher Attribution
American Mineralogist, v. 89, issue 1, p. 20-30
Scholar Commons Citation
Lang, Helen M.; Watcher, Aletha J.; Peterson, Virginia L.; and Ryan, Jeffrey G., "Coexisting Clinpyroxene/Spinel and Amphibole/Spinel Symplectites in Metatroctolites from the Buck Creek Ultramafic Body, North Carolina Blue Ridge, USA" (2004). Geology Faculty Publications. 130.
https://digitalcommons.usf.edu/gly_facpub/130
Full Text URL
https://doi.org/10.2138/am-2004-0104
