RARE-EARTH ELEMENT SYSTEMATICS OF WEBSTER-ADDIE/WILLITS/BALSAM GAP MAFIC AND ULTRAMAFIC ROCKS

Rare earth element abundances were measured on samples collected during the 2001 REU summer program from mafic and ultramafic rock exposures in the Willits area (WI), and in and near the Webster-Addie (WA) and Balsam Gap (BG) ultramafic bodies of the eastern Blue Ridge province of NC. Samples were dissolved at the Univ. South Florida via an Na₂CO₃ fluxed fusion method, and analyzed for REE abundances by ICP-MS at the University of Boston.

Variably migmatitic amphibolites from Willits and Webster-Addie show LREE-enriched patterns with REE abundances ranging from 10-120x chondrites. Leucosome-melanosome splits from migmatitic amphibolite samples show complementary patterns, with HREE and Eu enriched in the leucosome, and LREE higher in the melanosome. Amphibolitized zones within the Webster-Addie ultramafic body show lower REE abundances overall (0.5 -10x chondrites) and concave-up REE patterns; while a fresh pyroxenite from the Balsam Gap ultramafic body shows a "U" shaped REE pattern at 0.5-1x chondrite levels. The WI-WA amphibolites display markedly different REE systematics than amphibolites of the Buck Creek mafic-ultramafic complex (BC). Berger and others (2001) found lower REE abundances in BC amphibolites, with consistently LREE depleted patterns, and suggested an origin from MORB-like protoliths. REE and bulk chemical systematics of WI-WA amphibolites auggest subduction-related protoliths which have been intensely reworked by metamorphism and partial melting. The BG pyroxenite shows similar REE abundances and patterns to pyroxenites from the Moore's Knob and Webster-Addie ultramafic bodies (see Berger et al. 2001), raising the possibility that these units have similar origins.

*This abstract is coauthored with 2001 REU Program, University of South Florida, Tampa Florida.