PYROXENE-RICH ROCKS OF THE WEBSTER-ADDIE COMPLEX, WESTERN NORTH CAROLINA

A distinctive feature of the Webster-Addie Complex (WAC) of western North Carolina is the presence of pyroxene-rich rocks (metawebsterite, metaclinopyroxenite, metaorthopyroxenite). The pyroxene-rich rocks occur as pods and discontinuous layers in a matrix of metadunite. Contacts between the pyroxene-rich rocks and metadunite are generally sharp, but local shearing incorporates pyroxene porphyroclasts into the metadunite.

All of the WAC pyroxene-rich rocks contain variable proportions of hydrous metamorphic phases (tremolite/edenite, chlorite, talc) along with olivine and Cr-Al spinel. Pyroxene grains recrystallized at subsolidus conditions as evidenced by exsolution textures, oriented rod-like inclusions of Cr-rich spinel, and temperatures based on coexisting pyroxenes of 825 - 910°C. Spinel also shows evidence of recrystallization with some grains zoned from more Al-rich cores to Fe- and Cr-rich rims. Olivine compositions show regular variation with those of pyroxenes, amphiboles, talc, and chlorite, indicative of metamorphic olivine recrystallization.

Little in the mineral chemistry of the WAC rocks reflects igneous crystallization. Cores of some Al-rich spinels may represent igneous compositions, but olivine and pyroxene equilibrated with hydrous minerals during metamorphic recrystallization. Nevertheless, clear compositional differences exist between clinopyroxene-rich rocks and metadunite/metaorthopyroxenite. Metawebsterites and metaclinopyroxenites have olivine % Fo and cpx/opx Mg#s of 79-85 and 87-89/80-82, respectively, whereas the range in metadunite/metaorthopyroxenite is 89-93 and 95/91-92. Bulk rock analyis of a metawebsterite gives a whole rock Mg# of 84 versus 90-91 in metadunite/metaorthopyroxenite. These compositional differences, along with the sharp contacts between rock units, likely are precursor igneous remnants and provide the best hope of seeing through the complex metamorphic history of the WAC to the igneous protoliths.