ANOMALOUS ASPECTS OF WEST AVALONIA IN SOUTHEASTERN NEW ENGLAND, USA

West Avalonia is the sum of terranes stretching from Newfoundland to southeastern New England, but proliferating data of many types—as illustrated in Boston and vicinity, MA—reveal significant disparities within this clan. In the Boston area, arc-related plutonic and volcanic rocks that are hallmarks of Avalonian lithostratigraphy reflect a single magmatic interval between ~610-584 Ma, but not > 670 Ma episodes documented in Newfoundland and Nova Scotia. In addition, the 30+ Ma gap between 595-584 Ma Roxbury Conglomerate at the top of the arc complex and overlying Cambridge "argillite" casts doubt on customary correlation with Newfoundland's Conception Group in which the youngest volcanic ash is ~566 Ma. This gap and an even larger one in New Brunswick's Caledonia terrane raise uncertainty about the nature of the transition to Cambrian platform conditions.

Major SE New England plutons (~610-606 Ma Dedham Granite, ~ 606 Ma Milford Granite, ~604 Ma Fall River Granite) most closely correspond in age to bodies in the Antigonish and Cobequid highlands, Nova Scotia, but SE New England's low positive to low negative ε_Nd values generally reflect more evolved crustal sources than in these or any other West Avalonian terrane. New England granitoids and unconformably overlying ~596 Ma Lynn-Mattapan volcanics also yield the lowest West Avalonian δ¹⁸O values. Ediacaran ¹⁸O depletion has been linked with ~550 Ma rifting of West Avalonia from Gondwana, though a ~490 Ma paleopole from intrusive rocks at Nahant, MA suggests that SE New England, at least, remained near Gondwana at that time.

Opinion is divided with respect to West Avalonia's original position on the Gondwanan margin. Mesoproterozoic components in detrital zircon assemblages are commonly taken as indicators of paleogeographic proximity to Amazonia. Age distributions lately reported for detrital zircons from the Assabet and Char sequences in the West African Taoudini Basin, however, reveal density maxima that overlap 983, 1025, 1469 and ~2038 Ma peaks and corroborating TIMS dates from the > 912 Ma Westboro Fm north of Boston. Intriguingly, unusually high Sm/Nd ratios in both the Milford and Fall River granites lead to respective T_DMvalues of 2237 and 2006 Ma consistent with paleomagnetic reconstructions placing West Avalonia off West Africa at both ~595 and ~490 Ma.