Carbonatitic magmatism and metamorphic processes in the Monashee complex, British Columbia : thermotectonic evolution of the western North American margin from the Neoproterozoic to the Paleogene

The Monashee complex, the deepest exposed structural level in the southern Omineca belt of the Canadian Cordillera, offers a rare window into the tectonic interface between cratonic North America and accreted terranes. This region hosts three Proterozoic-Paleozoic episodes of carbonatite magmatism, offering unique insights into the tectono-magmatic processes that shaped the Canadian Cordillera. Zircon U-Pb geochronology and trace element analysis date magmatic events. New carbonatite ages, approximately 500 and 360 Ma, correspond to regional extensional rifting events, suggesting a genetic link. Zircon Hf isotopes from carbonatites of 800 to 500 Ma suggest a consistent metasomatized mantle source, while those from about 360 Ma show multiple parental melts from depleted and enriched mantle and crustal sources. Mapping, stratigraphy, and geochemistry inform carbonatitic eruption processes. Five 360 Ma carbonatites in the Monashee complex are interpreted as pyroclastic density current deposits (PDC) based on their stratigraphic restriction, lack of intrusive features, and textural parallels to other PDC deposits. Their fine-grained matrix and high lithic-to-juvenile clast ratios suggest a phreatomagmatic origin. This volcanic interpretation within a marine succession directly pins the volcanic activity to the paleo-continental margin, correlating with Devonian-Mississippian alkaline magmatism during extensional back-arc tectonics. Pseudosection modeling, thermometry, and zircon U-Pb geochronology constrain the timing of metamorphic events and partial melting conditions in the deep crust. Reset and altered zircon from Monashee complex carbonatites date hydrothermal and tectonic events at ca. 163 and 130 Ma. These ages match metamorphism in the overlying Selkirk allochthon during accretion and suggest that the terranes were juxtaposed by the mid-Jurassic, earlier than previously thought. Leucosome from the Monashee complex records zircon U-Pb ages equilibrated with garnet, indicating peak metamorphism by 60 Ma, with continued partial melting and zircon crystallization during decompression until at least 50 Ma, ruling out exhumation models that rely on a competent and coherent basement, such as thrust systems. This refined tectonic history warrants a re-examination of the southern Cordillera’s tectonic evolution and architecture. More broadly, this dissertation introduces new field, petrographic, and geochemical approaches to interpreting deformed and metamorphosed carbonatites and highlights the potential of altered zircon in revealing overlooked tectono-metamorphic histories.