TESTING THE CHEMICAL FINGERPRINT OF AMPHIBOLITES FROM THE CENTRAL BLUE RIDGE CARTOOGECHAYE AND MARS HILL TERRANES, NC BLUE RIDGE

Previous work indicates that Central Blue Ridge mafic and ultramafic rocks from the Buck Creek, Carroll Knob and Lake Chatuge complexes in southwestern North Carolina are distinct from those to the north, in the Webster-Addie-Willets, NC region, in terms of field relations, mineralogy, and major and trace element geochemistry. New amphibolite samples were collected to better evaluate this apparent distinction. Amphibolites from a broad region in the Central Blue Ridge, extending northward from the Trimont Ridge area in the Cartoogechaye terrane to the Mars Hill area, have similar mineral assemblages and consistent major and trace element whole rock chemistry. South of the east-west-trending, Middle Proterozoic, Trimont Ridge complex, amphibolites are more similar to those at Buck Creek.

Major element compositions for these samples are similar to amphibolites previously analyzed from the Webster-Addie-Willets region, and are distinct from amphibolites near the Buck Creek complex (BC has higher Mg, Al and lower Ti). Mineral chemistry results indicate fairly consistent prograde metamorphic assemblages (Hbld+plag+/-Gt+/-ilmenite) . Quartz and apatite are present in the more Si-rich amphibolites, and one sample includes hypersthene, suggesting peak metamorphic conditions that edge into the granulite facies. By contrast, garnet is absent in Buck Creek amphibolites, and titanite is abundant in most samples. Epidote+chlorite retrograde assemblages are similar regionally, and are consistent with regional lower amphibolite to greenschist facies conditions.

REE compositions are similar in pattern and concentration to mafic rocks from the Mars Hill Terrane (Ownby et al, 2004) and from the Webster-Addie region (Soraruf et al, 2002). Most samples are weakly to moderately LREE –enriched with chondrite-normalized concentrations between 10-100. REE data from the Buck Creek, Carroll Knob and Lake Chatuge complexes show distinctly lower REE concentrations and are generally LREE depleted.

A mafic sample from the Trimont Ridge complex is distinctly different from all of the other mafic rocks in this study, with a steep REE pattern and Eu anomaly, similar to Western Blue Ridge data of Carrigan et al. (2003).