Paper No. 1
Presentation Time: 9:00 AM


BROWN, Laurie, Department of Geosciences, Univ of Massachusetts, 233 Morrill Science Center, 611 N. Pleasant Street, Amherst, MA 01003-9297, THOMPSON, Margaret D., Geosciences Department, Wellesley College, Wellesley, MA 02481, GRUNOW, Anne, Byrd Polar Research Center, Ohio State University, 108 Scott hall, 1090 Carmack Road, Columbus, OH 43210, BARR, Sandra M., Department of Earth and Environmental Science, Acadia University, Wolfville, NS B4P2R6, Canada and WHITE, Chris E., Nova Scotia Department of Natural Resources, P.O. Box 698, Halifax, NS B3J2T9, Canada,

Magmatic arcs that form Avalonia originated in the Cryogenian-Ediacaran interval of earth history somewhere on the margin of Rodinia and/or Gondwana. Avalonia joined eastern Laurentia during the mid-Paleozoic as Iapetus closed to its north and the Rheic Ocean opened in its wake. Final Rheic closure in the Carboniferous left Avalonia embedded in Pangaea. Positions of “West Avalonia” before it reached North America reflect imperfect paleomagnetic studies from SE New England, New Brunswick, Nova Scotia, and Newfoundland.

Questions plaguing many of these previous studies include lack of reliable radiometric ages for the units studied, a less-than-strong handle on the structural complications in these ancient terranes, and identification of the carrier and timing of the magnetic remanence. The latter is important as many published studies touted hematite as the carrier, but whether it is primary or secondary is not always clear. Detailed re-mapping and improved geochronology in the Mira terrane of Cape Breton Island, Nova Scotia have prompted us to re-investigate Neoproterozoic to early Paleozoic rocks in that part of Avalonia. A total of 36 sites were collected in volcanic and sedimentary units of the Mira terrane, with known ages of Ediacaran to Devonian. Work in the 620 Ma rhyolites of the East Bay Hills yields a preliminary, tilt-corrected, dual polarity pole of 70°N, 203°E, not far removed from poles of the Lynn-Mattapan volcanics (595 Ma) in the Boston Basin. Sedimentary rocks and gabbros of the ~560 Ma Main à Dieu Group in eastern Cape Breton Island (called Fourchu Group in previous paleomagnetic studies) give a similar untilted direction. Paleolatitudes of ~40° from all these studies put Avalonia well displaced from Laurentia during this time interval. Red sediments (sandstones and siltstones) of early Cambrian age in the Mira terrane have in situ directions yielding shallow VGPs with a mean pole position close to that of the 460 Ma Dunn Point volcanic rocks (2°N, 130°E). As this direction is removed from the Laurentian APWP, Avalonia was still outboard of North America in middle Ordovician time. Additional scattered in situ and tilt-corrected directions, not readily attributed to established pole positions, are seen in many sites, and make precise pole determinations difficult.