Jp. Hodych et Kl. Buchan, PALEOMAGNETISM OF THE CA440-MA CAPE ST-MARY SILLS OF THE AVALON PENINSULA OF NEWFOUNDLAND - IMPLICATIONS FOR IAPETUS OCEAN CLOSURE, Geophysical journal international, 135(1), 1998, pp. 155-164
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 Silu
rian). At 12 of 19 sites, stepwise alternating-field or thermal demagn
etization isolated a stable characteristic remanence carried by magnet
ite. This remanence is shown to pre-date Early Devonian folding of the
sills. Although a baked-contact test was inconclusive, the positive f
old test and the low grade of metamorphism of the sills (prehnite-pump
ellyite facies) make it likely that the characteristic remanence is pr
imary. The tilt-corrected site-mean characteristic remanence has a dec
lination of 343 degrees and an inclination of -51 degrees (k=25, alpha
(95)=9 degrees), yielding a similar to 440 Ma palaeopole at 10 degrees
N, 140 degrees E (dm=12 degrees, dp=8 degrees) for West (North Americ
an) Avalonia. The corresponding similar to 440 Ma palaeolatitude for t
he Avalon Peninsula is 32 degrees S+/-8 degrees. The only other West A
valonian palaeolatitude determination from rocks that could be of simi
lar age is from the Dunn Point volcanics of Nova Scotia; their more so
utherly palaeolatitude of 41 degrees S +/- 5 degrees suggests that the
y are significantly older than 440 Ma, a possibility that we recommend
testing with U-Pb dating. Although no similar to 440 Ma palaeolatitud
e determinations are available for East Avalonia (parts of southern Br
itain and Ireland), interpolating between mid-Ordovician and mid-Silur
ian determinations gives an estimate of similar to 25 degrees S. This
is consistent with our Cape St Mary's result and, if the Iapetus Ocean
closed orthogonally, with a narrow (similar to 1000 km) Iapetus Ocean
of approximately east-west orientation between Avalonia and Laurentia
by 440 Ma.