GEOCHEMISTRY, PETROGENESIS, AND TECTONIC SETTING OF METABASALTS FROM THE SOUTHERNMOST APPALACHIAN PIEDMONT AND THE SUWANNEE TERRANE

We report major, trace, and REE analyses for 14 amphibolites/metabasalts from terranes across the Alabama Piedmont and from the buried Suwanee terrane to attempt characterizing magma sources and discriminating their various tectonic settings. The terranes, from northwest to southeast, are the eastern Blue Ridge (EBR), Inner Piedmont (i.e., Dadeville [DC] and Opelika Complexes [OC]) , Pine Mountain (PMB), and Uchee (UB) belts, and, from beneath Gulf Coastal Plain sediments, the Suwanee terrane. Attempts at using discriminant plots based on major and trace element abundancies did not reveal much useful information probably due to the primary chemistry of the igneous protoliths having been modified by amphibolite-facies metamorphism and possibly (?) sea-floor alteration; the Suwannee terrane samples, on the other hand, indicate only low-grade metamorphic affects. Spider diagrams for MORB-normalized LILE-HFSE's and for chondrite-normalized REE's, however, show only minor enrichments/depletions compared to basalts from various settings. Three distinct groupings of REE patterns are recognized. The PMB patterns have the highest REE abundances (200 to 30 x chondritic) and overlap fields reported for Iapetan-rift basalts of the western Blue Ridge, consistent with their occurrence in known Laurentian (Grenville) basement-cover units. The UB and DC have lower REE abundancies (70 to 18 x chondritic) and are similar to patterns previously reported for other UB amphibolites. The EBR and OC have relatively flat profiles and the lowest REE abundancies (16 to 8 x chondritic). MORB-normalized trace element plots support a continental-rift setting for the PMB and Suwannee terrane samples, and back-arc-basin to island arc for the EBR and OC. UB samples are enriched in LILE's (Rb, Ba, K, and Sr) and depleted in HFSE's (Zr, Ti, and Y) relative to amphibolites from the EBR, DC, and OC, perhaps pointing to a more complex origin, such as magmas derived from evolved and immature arcs. The OC amphibolites are more similar to those of the EBR rather the DC, which is consistent with recent lithologic, structural, and intrusive age findings that the OC may be incorrectly assigned to the Inner Piedmont. That these potential-EBR units may be juxtaposed against rocks of the PMB has some important implications for tectonic evolution of the southernmost exposed Appalachians.