VARIATION IN CHROMITE COMPOSITION WITH METAMORPHIC GRADE, AN EXAMPLE FROM METAULTRAMAFIC ROCKS FROM THE SPRUCE PINE DISTRICT, NORTH CAROLINA

Compositions of CrFe oxide phases in metaultramafic rocks from the Spruce Pine district of western North Carolina are sensitive recorders of metamorphic conditions in these rocks. Chromite is found in olivine-rich(>80% modal olivine) metadunites as accessory grains and as segregation in pods and layers. Chromite is often zoned from more Al-rich cores (5-15% Al2O3) to lower Al rims in contact with chromian clinochlore. The olivine-rich metadunites also contain enstatite and tremolite consistent with the upper Amphibolite Facies metamorphism of the associated country rocks. Metaultramafic rocks with moderate olivine contents (20-80 % modal olivine) contain low-Al chromite (0-3% Al2O3). Chromite transforms to chromian magnetite with decreasing modal olivine. The moderate olivine metaultramafic rocks also contain talc, magnesiocummingtonite, anthophyllite and magnesite along with the phases found in the olivine-rich metadunites. Replacement textures are common with tremolite being replaced by the low-Ca amphiboles and talc replacing all the low-Ca am phiboles. The chromian magnetites often are zoned from low-Cr core to Cr-free rims of magnetite. Low-olivine metaultramafic rocks contain little, if any, olivine and are composed mostly of talc, chlorite, low-Ca amphiboles and serpentine. Magnetite, perhaps with a few % Cr2O3 in the cores, is the oxide phase in the low-olivine metaultramafic rocks.

Spruce Pine metaultramafic rocks contain mineral assemblages consistent with recrystallization during retrograde cooling from upper Amphibolite (possibly Eclogite) to Greenschist Facies conditions. Recrystallization was incomplete because of the limited availability of fluids during the cooling. The CrFe oxide compositions record this progressive hydration and recrystallization during retrograde cooling. Given the metamorphic character most of the CrFe oxides it is doubtful much of their original igneous heritage is preserved in these phases. The Al-rich chromites offer the best chance of preserving original igneous compositions and these compositions suggest a suprasubduction zone setting for the emplacement of the ultramafic protoliths.