MAGMATISM ASSOCIATED WITH COLLISIONAL TECTONICS: CORRELATION WITH PRE-COLLISION DEPOCENTERS AND GEOMETRY OF RIFTED MARGINS OF THE CENTRAL APPALACHIAN OROGEN

The spatial and temporal distributions of igneous rocks provide significant constraint on the geodynamic and thermal record of collisional tectonics. In the central Appalachian orogen, both published and new ion probe U/Pb igneous rock zircon ages provide a window into the thermal and temporal evolution of mid Ordovician Taconic orogeny. Ion probe U/Pb results screened for inheritance and lead loss, yield five episodes of igneous activity: (1) arc stage: 470-488 Ma, (2) syntectonic: 472-460 Ma, (3) stitching plutons: 446-441 Ma, (4) extension-related plutons: 438-426 Ma, and (5) post-tectonic plutons: 409-362 Ma. These events suggest that non-arc igneous activities define a complete orogenic cycle (collision, thickening, extension), although early Silurian extensional structures have not yet been identified. The igneous events are also characterized by spatial compositional and strontium isotopic diversity, e.g., Sri of 0.7075 appears to separate source compositions of the Pennsylvania embayment granitoids from those south of the Virginia transform. We also suggest that 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 late Precambrian and Cambrian ages 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 within the orogenic cycle, or are perhaps related to Neoacadian orogeny. 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 source that is not related to the basement rocks of Grenville age (but of Amazonian affinities based on lead isotope data), 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.