U-PB ZIRCON GEOCHRONOLOGY OF NEOACADIAN AND EARLY ALLEGHENIAN PLUTONIC ROCKS IN THE ALABAMA EASTERN BLUE RIDGE, SOUTHERN APPALACHIAN MOUNTAINS

The Alabama Eastern Blue Ridge (EBR) of the Southern Appalachian Mountains hosts a variety of felsic plutonic rocks, which intrude multiply deformed Neoproterozoic to Ordovician (?) metasedimentary rocks. These plutons are commonly thought to have been emplaced during major Southern Appalachian tectonomagmatic events (Taconic, Neoacadian, and Alleghanian orogenies); however, few of the plutons have been dated by modern geochronologic techniques. Consequently, timescales of magmatism and associated deformation in this sector of the Southern Appalachian orogen are poorly understood.

Here, we report new whole rock geochemistry and U-Pb zircon SHRIMP-RG ages for 6 Neoacadian to Early Alleghenian plutons. These data allow us to investigate timescales of plutonism, geochemical changes through time, and possible relationships between magmatism, metamorphism and Neoacadian ductile crustal flow. We find that Early Neoacadian plutonism included the emplacement of biotite-muscovite granites and granodiorites which include the Rockford Granite (376.6 +/- 2.8 Ma) and the Bluff Springs Granite (363.8 +/- 3.6 Ma). These plutons are in general strongly deformed, contain ubiquitous inherited zircon cores, and display geochemical characteristics consistent with shallow-pressure partial melting of pre-existing continental crust. By contrast, Early Alleghenian plutons are deformed to undeformed, and consist of low-K tonalites and trondhjemites (Almond trondhjemites and Blakes Ferry pluton) with geochemical characteristics suggestive of deep-crustal partial melting of a garnet- + amphibole-bearing source. Two samples of the Almond trondhjemite (Wedowee pluton and Almond pluton) yielded ages of 334.3 +/- 3.0 Ma and 340.5 +/- 2.7 Ma, respectively. Another sample yielded complex zircon age populations that range from 350 to 330 Ma. The Blakes Ferry pluton also yielded complex results with Grenville-age cores (ca. 1000-1080 Ma), and rim ages ranging from ca 350 to 330 Ma (peaks ages at 345.8 +/- 2.1 Ma and 336.6 +/- 2.4 Ma). We propose that the transition from Neoacadian shallow-pressure partial melting (ca. 378-364 Ma) to Early Alleghanian deep-crustal partial melting (ca. 341-330 Ma) reflects thickening of the EBR during Neoacadian deformation.