PALEOMAGNETISM OF THE CA440-MA CAPE ST-MARY SILLS OF THE AVALON PENINSULA OF NEWFOUNDLAND - IMPLICATIONS FOR IAPETUS OCEAN 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 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.