A STRATEGY FOR PALEOMAGNETIC INVESTIGATION OF INTRUSIVE COMPLEXES IN THE NORTHERN APPALACHIANS

Recent investigations integrating high-precision U-Pb zircon ages with paleomagnetic determinations in the vicinity of Boston Massachusetts have clarified the paleogeographic position and tectonic journey of this portion of Avalonia during Neoproterozoic through Cambrian time. Magnetic overprints in 595 Ma and 490 Ma rocks in this area further suggest that Avalonia accreted to Laurentia before Late Silurian time, but there is as yet no well dated Silurian paleopole from this terrane linking it paleogeographically with Laurentia or with neighboring portions of Ganderia. A number of Silurian silicic intrusive complexes, including Cape Ann (426 Ma), Vinalhaven (419Ma), Isle Au Haut, Swans Island, Cadillac Mountain (420 Ma) and Pleasant Bay, in both Avalonia and Ganderia may be appropriate for paleomagnetic resolution of these linkages because they contain structures permitting a range of field tests. These intrusions preserve stratigraphic sections of interlayered silicic, hybrid and mafic rocks replete with paleo-up indicators such as magmatic pipes, magmatic flame structures, trough structures and schlieren structures associated with magmatic enclaves, thus providing the opportunity to perform tilt corrections. In several of these intrusions the stratigraphic sections form basinal structures appropriate for the fold test. The Vinalhaven and Cadillac Mountain intrusive complexes contain magmatic breccias, as well as large (>100m) xenolithic blocks of country rocks appropriate for the conglomerate test. Finally, the Vinalhaven, Cadillac Mountain and Isle Au Haut intrusions are spatially associated with tilted volcanic rocks of comparable age providing the opportunity for consistency tests. Although the thermal history of these intrusions may result in failure of the field tests, any primary magnetic signatures obtained would fill a significant gap in the paleomagnetic records of Avalonia and Ganderia in southern New England.