James A. Nott

PCIGR

BCGS

Architecture of subduction zone magma reservoirs and origin of primitive arc magmatism: Petrochronology and geochemistry of the Polaris Alaskan-type ultramafic-mafic intrusion

Magmatism at subduction zones has contributed significantly to the growth of continental crust throughout geologic time. Convergent margin ore deposits of nickel-copper-platinum group elements (Ni-Cu-PGE) hosted in ultramafic-mafic intrusions, rocks dominated by olivine-pyroxene-hornblende that crystallized from mantle-derived magmas, are gaining global importance as an economic resource, yet remain poorly understood and underexplored (Manor et al., 2016, 2017; Nixon et al., 2015). The Polaris complex exhibits many of the features typical to Uralian-Alaskan-type ultramafic-mafic intrusions and is one of the best exposed (mostly alpine) ultramafic-mafic bodies in the Canadian Cordillera, second in size only to the Tulameen complex. The North American Cordillera hosts a considerable number of zoned ultramafic-mafic bodies (e.g., Duke Island, Tulameen, Turnagain, Giant Mascot), variably endowed in Ni-Cu-PGE mineralization, that are interpreted to represent the frozen magmatic conduits of hydrous Mg-rich arc lavas. Contributing to the understanding of arc-related magmatic conduit systems is paramount to filling knowledge gaps which currently exist regarding the origin of the zoning and timing of mineralization within the chronology of intrusion assembly and overall relationship to the life cycle of a subduction zone. These gaps are particularly pronounced where subduction-related magmatism coincides with arc-continent collisional geodynamics.

In collaboration with the British Columbia Geological Survey (BCGS), this project combines extensive field mapping and sample collection to provide observational and analytical constraints for the development of emplacement mechanism models. Current research at UBC, following established high-precision analytical workflows at the Pacific Centre for Isotopic and Geochemical Research (PCIGR), integrates U-Pb geochronology and Lu-Hf isotope systematics of zircon to constrain petrological processes and develop a spatio-temporal (4D) emplacement framework for the Polaris intrusion as an arc magmatic conduit system. This zircon petrochronological framework provides the basis for evaluating variations in magma source components and differentiation processes through time, informed by comparison with compositionally analogous ultramafic-mafic systems described in the literature (e.g., Nixon et al., 2024). These results aid in the evaluation of primitive arc magmatism and the role of mid-crustal magmatic systems, situating the Polaris intrusion and coeval Ural-Alaskan-type bodies within tectonomagmatic models for the North American Cordillera through comparison with lesser exposed intrusions from Alaska to California and globally (e.g., the Urals). This work has also produced extensive complementary datasets in whole-rock lithochemistry and zircon major and trace element geochemistry, with detailed characterization by EPMA and Raman spectroscopy. These additional data will further characterize the compositional evolution and differentiation trends of hydrous primitive arc magmas, to be presented in forthcoming publications. As the age of the Early Jurassic Polaris intrusion coincides with the protracted breakup of the Pangea supercontinent and the onset of arc-continent collision and associated terrane accretion in the Cordillera, this research contributes to our understanding of Cordilleran tectonics and the architecture of transcrustal arc magmatic systems.