PALEOMAGNETIC OVERPRINTING AS EVIDENCE OF NEO-ACADIAN TECTONISM IN NORTHERN APPALACHIAN PERI-GONDWANAN TERRANES

Primary Neoproterozoic magnetization in Lynn-Mattapan volcanic rocks surrounding Boston, MA establishes a peri-Gondwanan position for Avalonia off Africa at ca. 595 Ma, but other stable magnetic components in these units provide information on post-Avalonian tectonic history as well. A south-southeast to south directed and gently downward pointing component (C), for example, was found not only in the volcanic suite, but also in the Dedham Granite (ca. 610 Ma) and the Squantum Member of the Roxbury Conglomerate (maximum age 593 Ma). The same direction was earlier described in epidote veinlets, chilled margins of basaltic dikes and zones of hydrothermal alteration in mafic plutonic rocks northwest of Boston now dated at ca. 427 Ma. Affected rocks range from late Neoproterozoic through Early Silurian in age, so that the C component must represent a younger magnetic overprint. Mafic dikes at Hingham and Nahant, MA contain only the C component, suggesting that these intrusions and associated fluid migration led to chemical remagnetization in the older rocks. Virtual geomagnetic poles calculated from C directions vary from 28° to 42° in paleopole latitude and generally track the Late Devonian to Carboniferous segments of the North American polar wander curve of Van der Voo (1990).

Similar magnetic overprinting is present in peri-Gondwanan terranes of Maritime Canada. The B component of Harbour Main metabasalts (ca. 606 Ma, Newfoundland Avalon Zone), the B component of the Fourchu Group (ca. 575 Ma, Mira terrane of Cape Breton Island) and the intermediate temperature component of siltstones in the Bourinot Group (ca. 505 Ma, Bras d'Or terrane, CBI) all represent directions like component C in the Boston area. Paleopoles based on these directions resemble poles from Late Devonian to Lower Carboniferous rift-related red beds and volcanic rocks of Nova Scotia and New Brunswick. These were accompanied by dextral transcurrent faulting and voluminous intrusions, including the 361-356 Ma Pleasant Hills pluton (NS) emplaced synkinematically into the Cobequid fault zone. This tectonic activity is synchronous with granulite facies metamorphism and ductile deformation including dextral shear that first defined "Neo-Acadian" events in central Massachusetts, and must be integrated into evolving plate tectonic explanations.