TIMING OF PRIMITIVE ARC MAGMATISM AND TERRANE ACCRETION FROM COMBINED CA-ID-TIMS AND LA-ICP-MS U-Pb ZIRCON DATING OF THE POLARIS ALASKAN-TYPE INTRUSION IN THE CANADIAN CORDILLERA

Establishing the age and duration of ancient primitive arc magmatism in the North America Cordillera is important for a better understanding of subduction cycles under western North America and the geodynamic evolution of magmatic arcs. The development of subduction zone magma reservoirs is being investigated through a geochronological study of the Early Jurassic Polaris Alaskan-type ultramafic-mafic intrusion in northwest British Columbia. Polaris is an exceptionally exposed sill-like intrusion in the accreted, Late Paleozoic to Mesozoic Quesnel arc terrane. Polaris is irregularly zoned and consists of dunite (up to Fo₉₂ olivine), wehrlite, clinopyroxenite, and more evolved rock types (hornblendite, gabbro-diorite). U-Pb zircon dating of relatively fractionated rocks by LA-ICP-MS (~2800 spot analyses from 27 samples, all zircon annealed) yields results that span an apparently large age range (ca. 182-190 Ma) and identify an earlier, previously undocumented suite of Permian-Triassic gabbros at ca. 250 Ma along the southern and eastern margins of the intrusion. A subset of these samples was selected for single-crystal CA-ID-TIMS dating (n = 7 samples) based on LA-ICP-MS age spectra, SEM-CL imaging, and Raman spectroscopy. CA-ID-TIMS results distinguish two distinct magmatic phases: an early phase at ca. 187.5 Ma in the eastern part of the body, and a phase at ca. 186 Ma in the west that was emplaced and crystallized over a ~300,000-year span. Relative to the tightly constrained CA-ID-TIMS dates where the results from a given sample are identical within uncertainty, the more dispersed LA-ICP-MS dates are inferred to result from variable degrees of Pb loss, common Pb, incorporation of melt, fluid, or mineral inclusions, and altered zircon. These defects are effectively removed and mitigated by the CA-ID-TIMS technique. The ca. 186-187 Ma crystallization age of the Polaris Alaskan-type intrusion coincides with the Early Jurassic accretion of the Quesnel arc terrane to the pericratonic margin of Laurentia. This study highlights the importance of integrating both high-precision U-Pb zircon geochronology and spatially constrained analyses in assessing the relationship between primitive arc magmatism and the tectonomagmatic setting within the northern Cordillera.