Paleomagnetism of the 550 Ma Skinner Cove volcanics of western Newfoundland and the opening of the Iapetus Ocean

The Skinner Cove Formation of western Newfoundland is an alkali volcanic suite transported in the Ordovician onto Laurentiaʹs Iapetus Ocean margin. It has previously yielded a U–Pb zircon age of 550.5+3−2 Ma, has no penetrative deformation and has only been affected by zeolite facies metamorphism. At 10 sites, flows and dykes of the Skinner Cove Formation exhibit a stable characteristic `Aʹ remanence carried by magnetite. This `Aʹ remanence is shown to be primary by an intraformational conglomerate test using 20 trachybasalt clasts (whereas 11 alkali basalt clasts were overprinted, probably because they originally contained titanomagnetite from which rutile exsolved soon after burial by overlying flows). After tilt-correction, the mean `Aʹ remanence direction is southeast and down (D=144°, I=32°; α95=11°, k=21, N=10 sites). The paleolatitude calculated from the ten `Aʹ site virtual geomagnetic poles is 19°S ± 9°, which likely represents the paleolatitude of Laurentiaʹs Iapetan margin at ∼550 Ma. Evidence that the Skinner Cove Formation originated at this margin includes its structural position within the Humber Arm Allochthon, its within-plate trace element geochemistry, and its similarity in age to other alkali magmatism that more certainly represents Laurentiaʹs Iapetan margin. Comparison with other Laurentian paleomagnetic data implies that Laurentia drifted very rapidly northward from the south polar region to the equator between ∼570 and 550 Ma. We suggest that the start of this rapid northward drift at ∼570 Ma marked the onset of Iapetus sea-floor spreading between Laurentia and West Gondwana. Further well-constrained paleomagnetic data of ∼570 to ∼550 Ma age are needed from Laurentia to test its high rate of northward drift, and from West Gondwanan cratons to test that they remained relatively stationary.