An equatorial Laurentia at 550 Ma confirmed by Grenvillian inherited zircons dated by LAM ICP-MS in the Skinner Cove volcanics of western Newfoundland: implications for inertial interchange true polar wander

Volcanics of the Skinner Cove Formation of western Newfoundland carry a primary remanence acquired at 550 Ma at a paleolatitude of 19degreesS. There has been doubt that this represents the latitude of the Laurentian margin at 550 Ma, because the Skinner Cove Formation is allochthonous. We present new evidence from inherited zircons in the volcanics that should remove this doubt. Zircon crystals extracted from an ankaramite flow and a trachyte flow were dated individually by measuring U-Th-Pb isotopes for similar to30 mum x 30 mum areas using a laser ablation microprobe (LAM) linked to an inductively coupled plasma mass spectrometer (ICP-MS). Most of the zircons from the ankaramite are concordant and yield a 550 +/- 5 Ma date indistinguishable from the 550 + 3/-2 Ma date previously reported using multi-grain thermal ionisation mass spectrometry. About half of the zircons from the trachyte are also concordant yielding an overlapping date of 556 +/- 5 Ma. The other half clustered at similar to 1000 and 1500-1600 Ma, which are characteristic ages of the Grenvillian basement exposed nearby in the Long Range Inlier. These zircon xenocrysts were very likely picked up as the Skinner Cove magma ascended through Grenvillian basement of the Laurentian margin. There can now be little doubt that the similar to19degreesS Skinner Cove paleolatitude represents Laurentia's southern margin at 550 Ma. This makes it unlikely that Laurentia changed from south polar at similar to523 Ma to equatorial at similar to508 Ma due to a rapid similar to90degrees change in the Earth's rotation axis. Such an inertial interchange true polar wander (IITPW) event has been proposed as a trigger for the Cambrian faunal explosion. An IITPW event may have occurred in the Vendian, mostly between 577 and 565 Ma, judging by paleomagnetic results from Laurentia. However, it is more likely that the paleomagnetic results suggesting high latitude for Laurentia at 577 Ma or low latitude at 565 Ma are flawed. We suggest that Laurentia remained at low latitude throughout the Vendian (from the similar to600 Ma start of near-equatorial glaciations). This is supported by magmatic evidence suggesting that Laurentia's incipient Iapetus margin was stationary over a mantle plume throughout the Vendian. Laurentia at low latitude throughout the Vendian also helps provide the predominance of low latitude continents needed to account for the Vendian's near-equatorial glaciations, according to the snowball Earth hypothesis. (C) 2003 Elsevier B.V. All rights reserved.