THE EARLY PALEOZOIC EVOLUTION OF THE SOUTHEASTERN LAURENTIAN MARGIN: SUBDUCTION INITIATION, BACKARC EXTENSION, AND THE EMPLACEMENT OF GRANITOIDS

The majority of modern convergent margins include retreating subduction zones and typically involve the emplacement of juvenile mafic materials in a backarc setting. Recognition of backarcs in ancient orogenic belts is often obscured by tectonothermal over printing during periods of contraction and/or terminal continental collision. The ~460 Ma Hillabee Greenstone and Pumpkinvinve Creek Formation, bimodal volcanic sequences that include primitive juvenile basalts and rhyodacites, of the southernmost Appalachian Blue Ridge are vestiges of an ancient backarc system formed along the Alabama promontory, an upper plate rifted margin at the southeasternmost fringe of Laurentia. At ~490 Ma, emplacement of the Elkahatchee Quartz Diorite is the first recognized evidence of subduction related magmatism, followed by TTG suite granitoid emplacement concomitant with backarc volcanism. There is paucity of evidence of orogenic uplift and significant deformation during 490-460 Ma granitoid emplacement along the Alabama promontory, a period associated with the Taconic orogeny. Whereas most Taconic models are associated with accretion and collision of a volcanic arc(s), the margin of the Alabama promontory apparently remained as a retreating subduction zone following subduction initiation at ~500 Ma until at least ~430 Ma, the youngest recognized period of granitoid emplacement. This paper presents a tectonic model for the initiation of subduction along the Alabama promontory, backarc extension, and granitoid emplacement prior to final accretion during the terminal Appalachian collision.