MINERALOGY OF THE OXIDE MINERALS IN THE BUCK CREEK MAFIC-ULTRAMAFIC COMPLEX, NORTH CAROLINA

The mineralogy of oxides phases in rocks of the Buck Creek mafic-ultramafic complex in southwestern North Carolina (Berger et al., 2001; Peterson and Ryan, 2006) was the subject of study by students in the Earth Materials class. Students applied thier new mineralogy skills to the oxide minerals in the metatroctolite, metadunite, and a hydrated assemblage (edenite-margarite schist) between metatroctolite and metadunite. Coronal textures developed along plagioclase-olivine grain boundaries in the metatroctolite are the subject of extensive studies that define a complex history of metamorphism (Tenthorey et al., 1996; Lang et al., 2004).

Metadunite is composed of Mg-rich olivine and chromite. Unaltered chromite is brown or oqaque in PPL. The unaltered chromite is a Al (cr# = 100xCr/Cr+Al = 31-68) Mg (mg# = 100xMg/Mg+Fe) chromite. Progressive hydration alters the chromite to chromian clinochlore and lowers the Al content of the relict chromite (cr# = 42-96). Replacement of the chromite takes place along the (111) directions giving the grains a lattice appearance. A late-stage meshwork of serpentine alters the olivine and contains magnetite.

Metatroctolite contains complex coronal textures between olivine and plagioclase grains. A brown chromite (cr# = 4-41; mg# = 49-81) is included within the metatroctolite fabric and a Al-rich green spinel (pleonaste, cr# = 7, mg# = 74) is part of the symplectite intergrowths in the coronal texture. Green spinel also rims some of the brown chromite.

The edenite margarite schist is a bright green rock with pink grains of corundum and brown chromite. The chromite is Fe-rich (mg# = 17-21) with high Al contents (cr# = 31-48). A vein edenite margarite corundum occurs in a metadunite that contains green spinel (cr# = 4, mg# = 76).

The complex pattern of variation in oxide minerals in the Buck Creek complex mimics the history of crystallization and recrystallization in the corona assemblages.