Abstract

Integrated geochronologic and paleomagnetic investigation of the arc-related Lynn and Mattapan volcanic complexes around Boston, Massachusetts, establishes a Neoproterozoic paleopole for the Southeastern New England Avalon Zone. Four new U-Pb zircon dates obtained using isotope dilution and thermal ionization mass spectrometry (ID-TIMS) methods range from 597.4 ± 1.5 Ma to 595.7 ± 1.6 Ma, in agreement with published Lynn-Mattapan results of ca. 596 Ma. The mean direction calculated from the A component of magnetization at nine Lynn-Mattapan sites has a declination (D) of 319.9° and an inclination (I) of 57.0° (α95 = 7.4°); the paleopole based on this mean direction is 206°E, 60°N. These results pass both reversals and fold tests and are regionally consistent among lithologies that include basalt, andesite, rhyodacite, and rhyolite, which represent both volcanic and subvolcanic phases. Most sites also contain stable B and C magnetic components with directions that suggest Ordovician to Permian remagnetizations. A broadly comparable A component direction with D = 109.0° and I = −63.4° (α95 = 9.4°, n = 11 samples) was obtained from one site in the Squantum Member of the Roxbury Conglomerate (maximum age of 593 Ma) overlying the volcanic sequence.

The Lynn-Mattapan results indicate that southeastern New England occupied a mid-latitude position at ca. 595 Ma, similar to those reported from Avalonian terranes in maritime Canada. The most likely peri-Gondwanan position for Avalonia consistent with these paleomagnetic data lies off the West African margin. Using poles from the East Sahara craton (Egypt) and the Rio de la Plata craton (Brazil) has permitted reconstruction of West Gondwana without recourse to Laurentian proxies.

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