2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 30
Presentation Time: 8:00 AM-12:00 PM


THOMPSON, M.D., Department of Geosciences, Wellesley College, Wellesley, MA 02481 and GRUNOW, A.M., Byrd Polar Research Center, Ohio State Univ, Columbus, OH 43210, mthompson@wellesley.edu

Paleomagnetic investigations have long suggested that West Avalonian terranes of Newfoundland, Nova Scotia, New Brunswick and southeastern New England originated adjacent to northern Gondwana, but the exact position has never been resolved. In the absence of well dated Neoproterozoic paleomagnetic poles from West Avalonian terranes, reconstructions have generally placed them against the Amazonian portion of the Gondwanan margin. Geological data supporting such interpretations include Nd basement isotopic signatures and detrital zircon suites containing1.0-1.3 Ga "Grenvillian" components that could have derived from South American source belts. The latter approach, always ambiguous because belts of similar ages also occur in Laurentia and Baltica, has become increasingly problematic with the recent recognition of billion year old granitoids in the central Saharan belt in Africa. Integrated paleomagnetic and geochonologic results from arc-related magmatic and sedimentary rocks in the Boston portion of the Southeastern New England Avalon Zone permit a more direct assessment of terminal Neoproterozoic paleogeography.

Paleomagnetic samples were collected at nine sites located north and south of the Boston Basin from ca. 595 Ma basalt, andesite, rhyodacite and rhyolite representing the Mattapan and Lynn volcanic complexes. Results from these rocks yield normal and reversed polarity directions and a positive fold test. Paleomagnetic samples were also collected in red siltstone from the Squantum glaciomarine sequence (maximum age 595 Ma) in Quincy, MA. The Squantum data, after structural corrections, yield reversed polarity directions broadly similar to those from Lynn-Mattapan rocks. Lastly, ca. 610 Ma Dedham Granite underlying the volcanic rocks yields normal polarity results that resemble the structurally corrected Lynn-Mattapan directions.

The similarity of the paleomagnetic results from rocks of different lithology, different polarity and different structural corrections leads us to believe that we have obtained a primary magnetization for the 595-610 Ma time period. Our results suggest that Avalon was at mid-latitudes during the late Neoproterozoic, but it can be positioned adjacent to northern Africa or western Amazonia depending on the choice of north or south latitude.