NEW PALEOMAGNETIC POLE FROM THE LATE CAMBRIAN NAHANT GABBRO, NAHANT, MASSACHUSETTS: IMPLICATIONS FOR DRIFT HISTORY OF THE SOUTHEASTERN NEW ENGLAND AVALON ZONE

A new paleomagnetic pole for the Southeastern New England Avalon Zone has been determined northeast of Boston at Nahant, MA where gabbro sills intrude the Lower Cambrian Weymouth Formation. Syenite associated with the gabbro at Saunders Ledge yields a ²⁰⁷Pb/²⁰⁶Pb zircon date of 491 ± 0.6 Ma (MSWD=0.64; average of four analyses), confirming a Late Cambrian age for this magmatic suite. A raft of older felsite lies within the gabbro at one location and numerous mafic dikes (with reported ⁴⁰Ar/³⁹Ar whole rock ages as young as 372 Ma) cut both the gabbro and Weymouth strata.

The new pole reflects results from 7 sites (45 samples) located at Johns Peril on the north side of peninsular Nahant, and East Point and Great Ledge on the southeast. Five of six gabbro sites and one mafic dike site yielded stable remanent magnetizations that are westerly directed and steeply upward pointing (reversed). The felsite raft yielded a low temperature component similar to the enclosing gabbro, but of normal polarity. We interpret this as a partial remagnetization of the felsite during emplacement of the gabbro. All of these directions have been corrected for open synclinal folding about an axis oriented N47E/3NE (based on structural control provided by the Weymouth Formation and a kinked zone in the gabbro itself), indicating that the observed magnetization pre-dates folding. The Nahant data are similar to tilt-corrected magnetic directions from tightly folded beds at one site in the Weymouth Formation south of Boston at Weymouth, MA The paleomagnetic pole from the Nahant Gabbro suggests that the Boston portion of the Southeastern New England Avalon Zone lay at high latitudes during Late Cambrian time, and was most likely located near the coast of northwest Africa. The alkalic, within-plate geochemical characteristics of the Nahant suite are interpreted in this paleogeographic context to reflect rifting of Avalon from Gondwana during the opening of the Rheic Ocean.

South directed, low inclinations measured in a NW trending diabase dike cutting the Nahant Gabbro at East Point resemble those obtained previously at Nantasket, MA for volcanic ash containing a 362 Ma zircon. All of these results match North American directions and suggest that the Southeastern New England Avalon Zone was part of North America by Late Devonian time.