Palaeomagnetism of the ca. 440 Ma Cape St Mary's sills of the Avalon Peninsula of Newfoundland: implications for Iapetus Ocean closure

Astronomy and Astrophysics – Astronomy

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Ordovician, Palaeogeography, Palaeolatitude, Palaeomagnetism, Silurian

Scientific paper

We report on the palaeomagnetism of the gabbroic Cape St Mary's sills of the Avalon Peninsula of Newfoundland, which have previously yielded a 441+/-2 Ma U-Pb baddeleyite age (latest Ordovician or earliest Silurian). At 12 of 19 sites, stepwise alternating-field or thermal demagnetization isolated a stable characteristic remanence carried by magnetite. This remanence is shown to pre-date Early Devonian folding of the sills. Although a baked-contact test was inconclusive, the positive fold test and the low grade of metamorphism of the sills (prehnite-pumpellyite facies) make it likely that the characteristic remanence is primary. The tilt-corrected site-mean characteristic remanence has a declination of 343° and an inclination of -51° (k=25, α95=9°), yielding a ~440 Ma palaeopole at 10°N, 140°E (dm=12°, dp=8°) for West (North American) Avalonia. The corresponding ~440 Ma palaeolatitude for the Avalon Peninsula is 32°S+/-8°. The only other West Avalonian palaeolatitude determination from rocks that could be of similar age is from the Dunn Point volcanics of Nova Scotia; their more southerly palaeolatitude of 41°S+/-5° suggests that they are significantly older than 440 Ma, a possibility that we recommend testing with U-Pb dating. Although no ~440 Ma palaeolatitude determinations are available for East Avalonia (parts of southern Britain and Ireland), interpolating between mid-Ordovician and mid-Silurian determinations gives an estimate of ~25°S. This is consistent with our Cape St Mary's result and, if the Iapetus Ocean closed orthogonally, with a narrow (~1000 km) Iapetus Ocean of approximately east-west orientation between Avalonia and Laurentia by 440 Ma.

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