ANCIENT CRUST OF THE MARS HILL TERRANE, NORTH CAROLINA-TENNESSEE: CONSTRAINTS ON ORIGIN FROM INITIAL GEOCHEMICAL AND GEOCHRONOLOGIC STUDIES

The Mars Hill Terrane (MHT) is a distinctive region of controversial origin characterized by diverse lithologies interspersed on cm to km scale that have experienced granulite facies metamorphism (e.g., Merschat, 1977; Monrad, 1985; Bartholomew & Lewis, 1988; Raymond & Johnson, 1994; Stewart et al., 1997; Trupe, 2001). The MHT crops out in a belt trending northeast between the Eastern and Western Blue Ridge Provinces, extending at least from Roan Mountain to Mars Hill and perhaps much farther in both directions. It includes a diversity of gneisses of granitic composition; mafic rocks; mildly to strongly aluminous metasedimentary rocks; sparse ultramafic rocks; and calc silicate rocks of uncertain protolith. Our initial elemental data set substantiates the extreme diversity observed in the field, with SiO₂ ranging from 45-76 wt%, low and high-K granitoids, and highly peraluminous metasedimentary compositions. Mafic rocks are moderately enriched in K and incompatible trace elements and have LREE-enriched REE patterns. Ion probe zircon geochronology documents 1.6-1.8 Ga crystallization ages for some felsic igneous protoliths, detrital zircons in metasedimentary rock up to 1.8+ Ga, and thick 1.0 Ga metamorphic rims (Carrigan et al., 2001). Nd T_DM ages for felsic gneisses and metasedimentary rocks are near 2.0 Ga; calculated model ages of mafic rocks are also old (>1.5 Ga), but they are less meaningful because of their higher present day ¹⁴³Nd/¹⁴⁴Nd and Sm/Nd. The MHT contrasts strikingly with the relatively homogeneous, 1.1-1.2 Ga granitic basement of the adjacent southern Appalachians (Carrigan et al., 2000; 2001), which has initial e_Nd values of -1 to +2 (in contrast, MHT felsic and metasedimentary gneisses had e_Nd of -5 to -8 during Grenville time). The closest crust of similar age is near the Great Lakes; other potentially correlative terranes are dispersed on other continents. The MHT may be a far-traveled fragment emplaced during Grenville, or perhaps Appalachian, orogeny.