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 Laurentia's Iapetus Ocean margin. It has previously yielded a U-Pb zircon age of 550.5(-2)(+3) 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 'A' remanence carried by magnetite. This 'A' 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 'A' remanence direction is southeast and down (D = 144 degrees, I = 32 degrees; alpha(95) = 11 degrees, k = 21, N = 10 sites). The paleolatitude calculated from the ten 'A' site virtual geomagnetic poles is 19 degrees S +/- 9 degrees, which likely represents the paleolatitude of Laurentia's Iapetan margin at similar to 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 Laurentia's Iapetan margin. Comparison with other Laurentian paleomagnetic data implies that Laurentia drifted very rapidly northward from the south polar region to the equator between similar to 570 and 550 Ma. We suggest that the start of this rapid northward drift at similar to 570 Ma marked the onset of Iapetus sea-floor spreading between Laurentia and West Gondwana. Further well-constrained paleomagnetic data of similar to 570 to similar 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. (C) 1998 Elsevier Science B.V. All rights reserved.