ORDOVICIAN TO DEVONIAN MAGMATISM ASSOCIATED WITH TACONIC COLLISIONAL TECTONICS IN THE CENTRAL APPALACHIANS: AN EXAMPLE OF AN OROGENIC CYCLE

The spatial and temporal distributions of igneous rocks provide significant limits on the geodynamic and thermal record of collisional tectonics. In the central Appalachian orogen, both published and new ion probe U/Pb zircon ages from igneous rocks provide a window into the thermal and temporal evolution of mid-Late Ordovician Taconic orogeny. Ion probe U/Pb results screened for inheritance and lead loss, yield five discrete episodes of igneous activity: (1) arc stage: 489-470 Ma (2) syntectonic: 472-455 Ma (3) terrane stitching plutons: 446-441 Ma (4) extension related plutons: 437-426 Ma and, (5) post tectonic (Acadian-Neoacadian) plutons: 409-362 Ma. These events suggest that non-arc related igneous activities define a complete orogenic cycle (collision, thickening, extension). Although Early Silurian extensional structures have not yet been identified in the region, this event maybe related to the Salinic event documented in the Maritime Appalachians and Newfoundland. The compositionally bimodal magmatic record associated with the suggested Silurian extension is probably the result of the presence/absence of synrift sedimentary and volcanic rocks of Neoproterozoic age, which are uniquely related to the outline of promontories and embayments of the Laurentian continental margin prior to collision. The entrapment of extension - related mafic magmas in thick sedimentary sections is likely to have provided the heat for the generation of temporally related felsic liquids. The areally small Devonian plutons in the central Appalachians probably reflect melting associated with uplift and decompression following the collision and extensional events. Our ion probe U/Pb results of ages associated with inheritance in arc - related igneous rocks are typically non-Grenville (~1700 Ma) and suggest formation of the arc on an outboard terrane with a different crustal history. Silurian and younger igneous rocks commonly yield inherited ages of ~1 Ga and probably represent derivation from or contamination by a Grenville lithosphere. However two plutons in Virginia yield inherited ages of ~1400 Ma. These ages serve as a reminder that exploring the subsurface through patterns of inheritance may play a crucial role in tectonic reconstructions.