AVALONIAN PERSPECTIVES ON NEOPROTEROZOIC PALEOGEOGRAPHY: EVIDENCE FROM SM-ND ISOTOPE GEOCHEMISTRY AND DETRITAL ZIRCON GEOCHRONOLOGY IN SE NEW ENGLAND

Gaps and limitations in the Cryogenian-Ediacaran paleomagnetic database mean that paleogeographic relationships between northern Appalachian Avalonian terranes and particular cratons comprising their supercontinent neighbors in this interval rest largely on inferences drawn from geologic aspects of the terranes themselves. Commonly argued linkages with the Amazonian portion of Rodinia and later Gondwana rest largely on interpretations of detrital zircon ages and Sm-Nd isotopic compositions from Avalonian terranes in Atlantic Canada. New Sm-Nd isotopic analyses for 610-600 Ma arc-related granitoid rocks in southeastern New England along with LA-ICPMS and CA-TIMS analyses of detrital zircons in 912-606 Ma quartzite of the Westboro Formation north of Boston, MA suggest another possibility for at least this part of Avalonia.

Eight samples from Dedham, Milford, Fall River, Esmond and Westwood granites yield epsilon _Nd (600) values from -1.2 to +2.2 and depleted mantle model ages ranging from 2.2 Ga to 1.0 Ga. The younger model ages, reflecting largely juvenile magmatic components, are typical of previously reported values from Avalonian terranes in Newfoundland and Cape Breton Island. The oldest model ages (from three samples with unusually high Sm/Nd and ¹⁴⁷Sm/¹⁴⁴Nd ratios) can be produced by mixing juvenile mantle extractions with 10-15 % Baltic crust. The isotopic composition of Baltic crust in these calculations is proxied by Svecofennian metasedimentary rocks with detrital zircon suites comparable to those obtained from the Westboro Formation. An Avalonia-Baltica connection is likewise consistent with available paleomagnetic data placing both areas at moderate south paleolatitude between ca. 615 Ma and ca. 575 Ma during Ediacaran deposition of their respective Squantum-Gaskiers and Varanger glacial deposits.

Baltic affinity is also possible for sequences in the mainland Nova Scotia segment of Avalonia. Low epsilon _Nd values in ca. 620 Ma felsic volcanic and plutonic rocks of this area resemble values in SE New England granitoids, and detrital zircons from quartzite in the 997-629 Ma Gamble Brook Formation show ages similar to the distribution in the Westboro profile.