PETROGENETIC SETTING OF ORDOVICIAN PLUTONISM, EASTERN BLUE RIDGE, ALABAMA

Protoliths of the Kowaliga and Zana orthogneisses intruded the Ordovician Wedowee- Emuckfaw Dahlonega back-arc basin (WEDB) of the eastern Blue Ridge. Previous studies’ (1980’s) multi-grain zircon and Rb-Sr whole-rock analyses suggested the intrusives were Middle Ordovician (ca. 460 Ma). Here we apply single grain, laser-ablation (LA) ICPMS U-Pb analyses of zircons to determine the earliest crystallization ages of these units. The high spatial resolution LA method allows for the avoidance of high-U overgrowths and xenocrystic components and increases the possibility of detecting Pb-loss that may obscure the true crystallization ages. Results indicate a minimum crystallization age range of ~485 to ~440 Ma (Ordovician) for units mapped as the Zana and Kowaliga. Negative initial whole-rock Ɛ_Hf indicate input from isotopically evolved crust, while T_{Nd DM} model ages suggest an ~1.2 Ga age for the source rocks of all plutons. Whole-rock Pb-Pb isochron ages cluster around ~880-860 Ma and suggest mixing of Pb from an older source (Grenville?) and a younger source, likely Ordovician depleted mantle. Trace element patterns show LILE enrichment (particularly Pb) relative to HREE and HFSEs, suggesting water played a significant role in the formation of these units. Pyrolite-normalized trace element analyses show depletion in barium relative to rubidium and thorium, depletion in strontium relative to praseodymium and neodymium, and titanium depletion relative to europium and dysprosium. CaO vs K₂O/Na₂O suggests the source may be tonalitic in composition and/or that metasomatism may have accompanied metamorphism of these units. REE patterns consistently display negative europium anomalies, suggesting fractionation of plagioclase, and a range of cerium anomalies (+ and -). Some samples show low (less than x10 chondrite) heavy REEs, indicative of garnet, or pyroxene/amphibole in the source. Combined with data from other Ordovician igneous rocks in the WEDB (e.g., ca. 470 Ma Hillabee Greenstone, 466-461 Ma Pumpkinvine Creek Formation), these data support models of back-arc bimodal volcanism and extension/crustal thinning as a feature of the Taconic orogeny in the southernmost Appalachians.