Three distinct intrusive phases have been identified in the Pool Creek map area (95C/5), southeast Yukon, during geological mapping as part the Central Foreland NATMAP project: a nepheline syenite and associated dykes (Pool Creek syenite), a biotite syenite, and an intrusive breccia. All intrusive units are exposed in the core of a large anticline in the transition zone between Early Paleozoic MacDonald Platform carbonates to the southeast and Selwyn Basin basinal shales to the northwest. U-Pb isotopic dating of zircons from the different intrusive phases resulted in a cluster of ages of 640–650 Ma for the Pool Creek syenite and associated dykes, and an age of 51.8 ± 0.2 Ma for the biotite syenite. An age could not be assigned to the intrusive breccia on the basis of the analytical data. Similar-age Neoproterozoic igneous rocks are rare in the North American Cordillera. The Pool Creek syenite may be related to prolonged syn-Winderemere age rifting on the western margin of Laurentia. The superposition of Eocene and Neoproterozoic intrusive activity is rare and cannot be readily explained, but may be related to focused magmatism along the Liard Line, which is interpreted as a Neoproterozoic transfer fault structure.

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