Rosie Cobbett is a project geologist in the bedrock mapping group at the Yukon Geological Survey and a PhD candidate at the Memorial University of Newfoundland. She holds a B.A.Sc. in geological engineering and an M.Sc. degree in structurally geology, both from the University of British Columbia. Cobbett spent several seasons working in the mineral exploration sector in Yukon before starting with the Yukon Geological Survey in 2012 where she has been conducting regional mapping for the survey for 7 years.
Early Ordovician Seamounts Preserved in Yukon: Implications for the Rift History of Western Laurentia
The breakup of supercontinent Rodinia and development of the western Laurentian margin is recorded by Neoproterozoic to mid-Paleozoic continental margin strata that are partially preserved in Yukon. Constraints for the timing of the breakup include the age of rift-related magmatism, tectonic subsidence analysis and regional unconformities, which together suggest the rift to drift transition occurred at ca. 500 Ma in the northern Canadian Cordillera. Igneous rocks occur sporadically in post-rift lower Paleozoic strata along the length of the Cordillera but their relationship to the evolution of the margin remains enigmatic.
Extensive exposures of mafic volcanic and plutonic rocks of the Menzie Creek Formation are interlayered with Lower Ordovician continental margin strata in central Yukon. The facies and distribution of the Menzie Creek Formation suggests they represent partially preserved seamounts. Zircon collected from two separate samples within the volcanic successions yielded unimodal U-Pb age peaks at ca. 484 Ma, which are interpreted as the age of eruption. Menzie Creek Formation rocks are alkali basalt with ocean island–like chemistry. The trace-element and isotope systematics of these rocks suggest the melt source is the subcontinental lithospheric mantle from a depth between 75 and 100 km.
The Menzie Creek Formation is spatially associated with the Faro deposits and our new U-Pb dates provide further constraints on their age. Latest Cambrian to Early Ordovician magmatism is also documented at the Matt Berry deposit (ca. 486 Ma) near Frances Lake, in the Vampire Formation of southeast Yukon (ca. 486 Ma), and in the Kechika Group of Cassiar Terrane, southwest of the Tintina fault. Together these occurrences suggest that a regional magmatic and metallogenic belt was developed along the continental margin in the latest Cambrian to Early Ordovician.