Avalonian perspectives on Neoproterozoic paleogeography: Evidence from Sm-Nd isotope geochemistry and detrital zircon geochronology in SE New England, USA

Abstract

Arc-related Northern Appalachian Avalonian terranes developed in the ocean surrounding Rodinia beginning around 760 Ma, so paleogeographic constraints on these terranes have implications for the margins of Rodinia itself in Neoproterozoic time. The first Sm-Nd isotopic data together with new detrital zircon ages from SE New England suggest that at least this segment of Avalonia contains rocks with contributions from crustal sources in Baltica, rather than Amazonia as commonly argued. Eight samples from 610 to 600 Ma Dedham, Dedham North, Milford, Fall River, Esmond, and Westwood granites in SE New England yield e 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 other Avalonian terranes in Atlantic Canada. The oldest model ages 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 younger than 912 Ma quartzite of the Westboro Formation intruded by the Dedham Granite north of Boston (Massachusetts). An Avalonia-Baltica connection is also 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.