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

• Published in: Geophysical Journal International
• Main Authors: Hodych, Joseph P., Buchan, Kenneth L.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Royal Astronomical Society 1998
• Subjects: Dunn Point; Newfoundland
• Online Access: https://research.library.mun.ca/6201/; https://research.library.mun.ca/6201/1/Palaeomagnetism.of.the.ca.440.Ma.Cape.pdf; https://research.library.mun.ca/6201/3/Palaeomagnetism.of.the.ca.440.Ma.Cape.pdf; https://doi.org/10.1046/j.1365-246X.1998.00263.x

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, a95=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.