THE SNOWBIRD TECTONIC ZONE LIP: IMPLICATIONS FOR AN ABORTED RIFT ORIGIN

The Snowbird Tectonic Zone (STZ) is a 2800 km long geophysical lineament that bisects the western Churchill Province. Despite decades of work, the tectonic significance of the STZ remains controversial, owing to complex interleaving of Proterozoic and Archean tectonic components. Mafic intrusive rocks straddle the STZ along much of its length. Most of these have been interpreted as coeval with ca. 1.9 Ga convergent tectonics. However, direct igneous crystallization ages have remained elusive due to the rarity and small size of igneous zircon and due to a granulite-facies metamorphic overprint. New IN-SIMS micro zircon U-Pb data, zircon major and trace element geochemistry, Nd isotopic data, and bulk-rock geochemistry from mafic suites along the STZ in northern Saskatchewan (Chipman dike swarm) and southern Nunavut (Kazan dike swarm) demonstrate that crystallization was significantly older than ca. 1.9 Ga and coeval with extensional tectonics. U-Pb analyses from micro zircon grains with morphology and chemistry consistent with an igneous origin yield a ²⁰⁷Pb/²⁰⁶Pb age of 2113 +/- 13 Ma (MSWD: 0.71; n = 6; 3 grains from 2 dikes) for the Chipman swarm and 2116 +/- 12 Ma (MSWD: 0.50; n = 4; 2 grains from 1 dike) for the Kazan swarm. Both dike swarms contain nearly identical major and trace element geochemistry, are variably LREE enriched, have minor HFSE depletions and LILE enrichments, and range from mid-ocean ridge to continental arc fields on tectonic discrimination plots. Nd isotopic data (ε_Nd(Tc)) range from just below contemporaneous depleted mantle to below CHUR and have Neoarchean to Mesoarchean T_DM. These data indicate that the both swarms may have been of asthenospheric derivation; however, the magmas were also strongly influenced by interaction with preexisting metasomatized Archean lithosphere. The LIP (Large Igneous Province) record of the western Canadian Shield indicates that 2.1 Ga was a period of widespread extension reflecting opening of the Manikewan Ocean. Our data indicate that the STZ in northern Saskatchewan and southern Nunavut, at least in part, marks a narrow zone at least 800 km long, of ca. 2.1 Ga mafic magmatism, and may delineate an incipient rift that was repeatedly reactivated during the protracted assembly of the Nuna supercontinent.