PALEOMAGNETIC POSSIBILITIES (AND PERILS) IN RECONSTRUCTING LATE NEOPROTEROZOIC AVALONIAN PALEOGEOGRAPHY

With Laurentia poised to break from Neoproterozoic Rodinia, the stage was set for the development of lithotectonic elements that would later arrive in the northern Appalachians as exotic terranes. Early reconstructions of this paleogeography rely on geologic arguments for positioning Avalonia (and then-undifferentiated Ganderia) along the Amazonian and West African cratons of western Gondwana. Establishing these relationships paleomagnetically, however, continues to prove challenging.

Recent sampling undertaken to take advantage of advances in mapping and U-Pb zircon geochronology in the Avalonian Mira terrane, Cape Breton Island, for example, typically reveals weak, scattered magnetization directions leading to site means with unacceptably large uncertainties. Results from sites within a single geologic unit, furthermore, seldom agree with each other. Hematite, often a secondary magnetic mineral, is the predominant carrier in both sedimentary and volcanic samples, yet because tilt corrections are quite similar across the region, it is difficult to determine whether the magnetization is pre- or post-tilting.

Statistically rigorous, precisely dated paleopoles obtained from Avalonia in SE New England are perhaps even more frustrating. Results from Ediacaran Lynn-Mattapan volcanic rocks (597-593 Ma) and from latest Cambrian Nahant intrusive rocks (490-488 Ma) place Avalonia near the West African margin of Gondwana during its migration from mid- to high southerly latitudes. In contrast, metasedimentary basement units pre-dating 610-585 Ma Avalonian arc activity in this region contain 1.3-1.0 Ga (“Grenvillian”) detrital zircons unlikely to derive from West African sources. A position bordering Baltica provides a provenance option for such detrital zircon suites, and Avalonian magmatic arcs have lately been shown here at ca. 615 Ma, though without supporting Avalonian paleopoles of that age. Amazonian sources are also possible if Avalonia formed in the Arica embayment of South America, the second position consistent with the 595 Ma pole. But other thorny questions arise from either of these alternatives. From the Arica niche, how did Avalonia make its way to the high-latitude African margin implied by faunal as well as paleomagnetic data? From Baltica, how did Avalonia find a path to Laurentia?