AGE AND TECTONIC IMPLICATIONS OF THE ELKAHATCHEE QUARTZ DIORITE, EASTERN BLUE RIDGE PROVINCE, SOUTHERN APPALACHIANS, USA

The eastern Blue Ridge province (EBR) of the Alabama-Georgia Appalachian orogen (AGAO) contains a thick sequence of metaclastic rocks (Ashland-Wedowee belt) and intercalated mafic flows and sills(?). Metaclastic rcoks are dominantly metapelites, with minor graywacke and quartzite. These rocks are commonly interpreted to have been deposited along the rifting continental margin of SE Laurentia during the break-up of Rodinia, although no basement is recognized. Metamorphism is in the amphibolite facies. A suite of plutonic rocks intrusive into EBR metasedimentary units are critical time markers in unraveling the metamorphic/deformational history of the AGAO, yet their age relations remain uncertain because of the past reliance on discordant, multi-grain U-Pb zircon analyses and whole rock Rb-Sr data. Although all data suggest the plutons are Paleozoic, estimates range from ~500 Ma to 370 Ma. Consequently, it remains unclear to what extent they mark the times of Taconian vs. Acadian vs. Alleghanian metamorphism and deformation. In this study we report initial results from U-Pb zircon analyses using the SHRIMP RG ion probe for the Elkahatchee quartz diorite, the largest batholith in the EBR. Previous age estimates based on discordant multi-grain U-Pb analyses were ~500 Ma, and the Elkahatchee has been proposed to be a supra-subduction zone pluton. Such an age places severe restrictions on the tectonic history of the AGAO, particularly the nature of the Taconic orogeny, i.e., an age of 500 Ma requires initiating subduction earlier in the AGAO than in New England Appalachians. New U-Pb data from 2 samples of the Elkahatchee batholith suggest the batholith is likely a complex, diachronous map unit with components as young as 370 Ma. Proterozoic (Grenville) and older Paleozoic xenocrystic zircons suggest an s-type origin for some parts of the batholith, and that previously reported compositional diversity may in part also reflect a temporal diversity. These data clearly distinguish these parts of the Elkhatchee from other documented Ordovician metaplutonic rocks, to which we add the Mulberry gneiss (~450 Ma) in the Georgia EBR. Pre-Ordovician (pre-Taconic) subduction in the AGAO, therefore, remains problematic.