Abstract
Palaeomagnetism of the ca. 440 Ma Cape St. Mary's sills of the Avalon Peninsula of Newfoundland: implications for Iapetus closure
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 date (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 predate 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 343o and an inclination of -51o (k=25, alpha 95 = 9o) yielding a
440 Ma palaeopole at 10oN, 140oE (dm=12o, dp=8o) for West (North
American) Avalonia. The corresponding 440 Ma palaeolatitude for the
Avalon Peninsula is 32oS ± 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 41oS ± 5 suggests that they are significantly older
than 440 Ma, a possibility which 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 ~25oS. 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.