CHROMITE AND OTHER SPINELS FROM METAULTRAMAFIC ROCKS IN THE CARTOOGECHAYE TERRANE OF NORTH CAROLINA

The use of chromite (and other associated spinels) as a guide to the igneous heritage of metaultramafic rocks relies on the preservation of premetamorphic compositions in the spinels. Identifying these relict primary compositions is no small task in high grade, multiply recrystallized rocks like those of the southern Appalachian Blue Ridge Belt. Previous studies of Blue Ridge spinels identified high-Al spinels and spinel cores as the most likely igneous relicts. The most Al-rich spinels yet analyzed in Blue Ridge metaultramafic rocks occur in the large Buck Creek (BC) and Webster-Addie (WA) bodies of the Cartoogechaye terrane of western North Carolina.

Spinels have variable Al contents ranging from 37 - 53 wt. % Al2O3 in metawebsterite. Metadunite spinels range from <1 - 53 wt % Al2O3 reflecting varying degrees of Al loss associated with the formation of chlorite.

The highest Al contents of chromite from metadunite at BC suggest an origin in an oceanic MOR setting. Moderate Al contents in WA metadunites indicate either a MOR or suprasubduction zone (SSZ) origin for the rocks. The smaller bodies of metadunite at Balsam Gap (BG) and Dark Ridge (DR) contain chromites with lower Al contents, consistent with an origin in a SSZ setting. Variability of Al content in spinels from the metaultramafic rocks of the Cartoogechaye terrane is related to differences in host rock composition, original tectonic setting, and degree of recrystallization, a complex matrix that must be solved for application to petrotectonic problems.