Palaeomagnetism of the Skinner Cove Formation of Western Newfoundland and the birth of the Iapetus Ocean

Abstract

The 550 Ma Skinner Cove Formation is a structural unit within the Humber Arm Allochthon of western Newfoundland, occurring as an alkali volcanic suite affected only by zeolite facies metamorphism. At 10 sites from its flows and dykes a magnetite-borne, stable characteristic 'A' remanence direction (tilt-corrected mean D=144°, I=31.5°; α₉₅=10.8°, k=21.1) is recognized and shown to be a primary thermal remanence by an intraformational conglomerate test. The palaeolatitude calculated for the Skinner Cove Formation from its ten 'A' site virtual geomagnetic poles is 18.6°S ±9°. -- An original relation between the Skinner Cove Formation and the Iapetan margin of Laurentia is suggested by several lines of evidence: As a structural slice it occupies a lower, less transported position in the Humber Arm Allochthon, implying an original adjacency to underlying slices of Laurentian margin sediments. The Skinner Cove volcanics have a within plate trace element geochemistry with enriched light rare earth elements (LREE), shared with other alkali volcanics of the northeastern Appalachians Humber Zone. Also, the ~550 Ma Skinner Cove volcanics are of similar age to alkali magmatic activity of Laurentia's Iapetan margin, marked by the ~554 Ma Tibbit Hill metavolcanics of Quebec and the ~555 Ma Lady Slipper Pluton of west Newfoundland. Hence, the 18.6°S palaeolatitude of the Skinner Cove Formation constrains Laurentia to an equatorial position at ~550 Ma. -- Comparison with other palaeomagnetically determined high southerly palaeolatitudes for ~570 Ma implies a large rapid northward drift of ~34 cm/yr for Laurentia during the latest Neoproterozoic. The start of Laurentia's rapid northward movement at ~570 Ma may mark the beginning of Iapetus sea-floor spreading, consistent with Laurentian geological data if thermally-delayed subsidence of the Laurentian margin is assumed. Further well-constrained palaeomagnetic results of ~580 ~550 Ma age from Laurentia and especially its suspected conjugate margins Amazonia and Rio de la Plata are required to test this proposed palaeogeography and to define the birth of the Iapetus Ocean precisely.