EXAMINING THE ROLE OF EXTENSIONAL ACCRETIONARY OROGENESIS IN THE SOUTHERNMOST APPALACHIAN BLUE RIDGE: ISOTOPIC, GEOCHRONOLOGIC AND GEOCHEMICAL EVIDENCE

The role of extensional accretion, particularly in collisional orogenic belts, is often overlooked because of strong tectonic overprinting during the terminal contraction phase of orogenesis. Tectonic models of classic mountain belts including the Alps, Himalayas, and Appalachians were constructed relying heavily on features formed during closure of an intervening ocean. The NA and SA Cordillera, and many of the circum-Pacific orogenic belts, formed by accretionary processes always facing an ocean. Accretionary orogenesis is also variable in that there may be an advancing subduction boundary (cordillera-type accretionary orogen) or a retreating subduction boundary (extensional accretionary orogen), depending on the velocity of slab rollback relative to the velocity of the overriding plate. The presence of primitive basaltic rocks in a convergent margin setting is a diagnostic feature of an extensional accretionary orogen. The Pumpkinvine Creek Formation (PCF) and likely equivalent Hillabee Greenstone (HG) are suites of compositionally bimodal metavolcanic rocks found in the most inboard position of any accreted terranes in the southernmost Appalachians. The PCF and HG are geochemically most similar to rocks associated with backarc basin volcanism. New zircon age data of the PCF constrain the crystallization age to ~460 Ma. Detrital zircon suites from meta-sandstones interlayered with the volcanic rocks yielded a majority of ages between 1017 and 1190 Ma suggesting a Grenville-aged source. Nd isotopic values suggest that the PCF basalts formed incorporating little to no Laurentian crust. The geochemical, geochronolgical, and isotopic observations suggest that the PCF formed proximal to a Grenville-aged sediment source, but outboard of or along a highly extended continental margin. Nd isotopes of 490-430 Ma granitoids proximal to and currently outboard of the PCF/HG suggest interaction with an isotopically evolved crust, likely requiring this entire portion of the Blue Ridge to be native to Laurentia. Instead of employing complex tectonic models to emplace the units in this study to their current structural positions, an extensional accretionary orogen can provide the necessary crustal thinning to produce primitive mafic rocks and still provide enough continental crust as a source for the isotopically evolved granitoids.