THE MAGMATIC RECORD OF THE TACONIC OROGENIC CYCLE: EVIDENCE FROM ION PROBE U/PB AGES OF ZIRCONS FROM THE CENTRAL APPALACHIAN OROGEN

The spatial and temporal distributions of igneous rocks provide significant constraint on the geodynamic 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 Ordovician Taconic orogeny. Ion probe U/Pb results (obtained at UCLA National Ion Probe Laboratory), screened for inheritance and lead loss, yield five discrete episodes of igneous activity: (1) Arc Stage: 476-488 Ma (2) Syntectonic: 472-450 Ma (3) Stitching plutons: 446-441 Ma (4) Extension related plutons: 434-423 Ma and, (5) Post-Tectonic plutons: 381-362 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. The bimodal magmatic record associated with the suggested Silurian extension is probably the result of the presence/absence of synrift sedimentary and volcanic rocks which are uniquely related to the outline of promontories and embayments of the continental margin prior to collision. The entrapment of mafic magmas in thick sedimentary sections is likely to have provided the heat for the generation of felsic liquids. The areally small Devonian plutons 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 a crust/source that is not related to the basement rocks of Grenville age exposed in the region. Silurian and younger igneous rocks commonly yield inheritance ages that are Grenville, although two plutons in Virginia yield ages of ~1400 Ma. These complex ages serve as a reminder that exploring the subsurface through patterns of inheritance may play a crucial role in tectonic reconstructions.