SNOWBIRD TECTONIC ZONE: PROBLEM CHILD OF NUNA

One of the most enduring controversies in Precambrian reconstructions concerns the assembly of Nuna’s central core: the Churchill Province of Arctic Canada. The Rae and Hearne cratons of Churchill are separated by the 2000 km long Snowbird Tectonic Zone. First documented exactly 30 years ago, Snowbird Tectonic Zone was originally defined as a Paleoproterozoic suture, but discovery of Neoarchean high grade rocks along its length led to an alternative model in which Rae and Hearne were sutured by 2.55 Ga. This latter model, also the foundation for Neoarchean supercontinent Kenorland, has major implications for Nuna, not least of which is repudiation of a substantive 2.5-1.9 Ga Manikewan Ocean between Hearne and Superior. Here we trace from outcrop to continent scale the evidence for a Paleoproterozoic Snowbird Tectonic Zone and present the first direct dating of its famous eclogites. Lu-Hf garnet ages on peak M1 grt₁-cpx₁-ru-qtz assemblages in three new locales of eclogite and garnet pyroxenite indicate that conditions in excess of 16 kbar were achieved by ca. 1.91 Ga, with subsequent rapid isothermal decompression to ca. 10 kbar by 1.9 Ga. U-Pb age dating of submarine sedimentary and volcanic rocks caught between southeastern Rae and Hearne yields maximum depositional ages of 2.1-1.94 Ga, indicating open marine conditions persisted between the two cratons until shortly prior to Snowbird Tectonic Zone orogenesis. These constraints, disparate paleolatitudes ca. 2.2 Ga, along with evidence that trans-Rae dyke swarms (2.27 Ga Orpheus) and orogens (1.94-1.93 Ga Taltson) do not continue into Hearne, collectively constrain Paleoproterozoic assembly of Churchill Province, disallow a Neoarchean supercontinent, and reinforce the importance of understanding cratonic margin sequences when building reconstructions, a key lesson from Ian Dalziel’s amazing legacy of research.