Abstract

Paleomagnetic measurements and U-Pb geochronology of alkalic igneous rocks intruding the Ediacaran Cambridge Argillite and the Lower Cambrian Weymouth Formation north of Boston at Nahant, Massachusetts yield a well-constrained paleopole. The oldest intrusive phases are a syenodiorite sill and a granite dike with 206Pb/238U crystallization ages of 490.19 ± 0.90 Ma and 489.03 ± 0.82 Ma, respectively. This sequence along with undated mafic sills penetrating both the argillite and Weymouth strata are cut by 488.53 ± 0.81 Ma gabbro locally associated with 488.48 ± 0.79 Ma syenite forming a steep-sided intrusive plug. Ten paleomagnetic sampling sites within the mafic sills and the gabbro contain a stable A magnetic component with a mean direction of D = 278.5°, I = -76.7° (A95 = 3.9°, N = 10). This is considered to be the primary magnetization direction because it passes both fold and reversals tests. The corresponding paleopole at 320°E, 34°N (A95 = 7.2°, N = 10) places the Southeastern New England Avalon Zone at a southerly paleolatitude of 65° slightly removed from the African margin of Gondwana and suggests that bimodal Nahant magmatism is linked with latest Cambrian-Early Ordovician rifting of West Avalonia to form the Rheic Ocean.

Stable B and C magnetic components in Nahant syenodiorite and syenite give rise to virtual geomagnetic poles that track Late Silurian through Carboniferous segments of the North American apparent polar wander path, indicating docking of West Avalonia by mid-Paleozoic time.

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