15 Petrology and Tectonics of the Southern Appalachians

Thursday, 29 March 2007
1:20 PM - 5:20 PM

ALL ENSTATITE IS NOT CREATED EQUAL: A TALE OF ORTHOPYROXENE IN METADUNITES OF THE EASTERN BLUE RIDGE OF NORTH CAROLINA AND GEORGIA

Paper number: 15-4
Program: THEME SESSIONS
Time: 2:20 PM – 2:40 PM

Authors

Abstract

Two generations of enstatite (one a relict igneous phase with small amounts of Al and Cr plus inclusions of chromite and a second metamorphic phase with little nonquadrilateral components) occur in metadunites of the eastern Blue Ridge of North Carolina and Georgia. Relict igneous enstatite is found at Day Book, Dark Ridge, and Laurel Creek. Olivine-rich rocks at Day Book and Dark Ridge contain enough enstatite to make the rocks metaharzburgites, but most of the rocks with relict igneous enstatite are metadunites. Amounts of metamorphic enstatite are variable; host rocks vary form metadunite to metaperidotite to metaorthopyroxenite. Metamorphic enstatite is found in metadunites at a number of bodies in the Spruce Pine area of NC (Siena, Henson Creek, Newdale, Mine Creek, Woody) and to the south near Sylva, NC (Balsam Gap, and the Webster-Addie Complex) and Clayton, GA (Laurel Creek).

Relict igneous enstatite forms deformed, rounded grains included in a matrix of olivine. Inclusions of small rods of chromite (microns to 10's of microns) give the enstatite a light brown color, while very fine grained clinopyroxene exsolution lamella occur in some grains. Aluminum contents are 1-2.5 wt % Al2O3 and Cr contents are lower (0.2 to 0.5 wt % Cr2O3).

Metamorphic enstatite occurs as anhedral grains in a matrix of olivine and as large poikilitic grains in metaperidotite. Euhedral tremolite and cummingtonite, subhedral chlorite and anhedral olivine form inclusions in the anhedral poikilitic enstatite. Metamorphic enstatite is a pure Fe-Mg silicate with other components at or below microprobe detection limits.

The recognition of two varieties of orthopyroxene in the ultramafic rocks helps distinguish relict igneous rock types (dunite and harzburgite) from recrystallized metaperidotites.