Palaeomagnetism of the ca. 440 Ma Cape St Mary's sills of the Avalon Peninsula of Newfoundland: implications for Iapetus Ocean closure |
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| Authors: | Joseph P Hodych & Kenneth L Buchan |
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| Institution: | Department of Earth Sciences, Memorial University of Newfoundland, St. John's, NF, A1B 3X5, Canada. E-mail:;, Geological Survey of Canada, 601 Booth St., Ottawa, K1A 0E8, Canada |
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| Abstract: | 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, α 95=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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| Keywords: | Ordovician palaeogeography palaeolatitude palaeomagnetism Silurian |
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