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Detrital zircon geochronology and rift-related magmatism : central Mackenzie Mountains, Northwest Territories Leslie, Christopher Dean


Neoproterozoic to Cretaceous age strata in the Mackenzie Mountains of the northern Canadian Cordillera record many geological events that affected the western margin of Ancestral North America. Two of these events that are the focus of this study are; (1) the development of a thick long-lived passive margin sequence of sedimentary rocks; and (2) continental rifting of this passive margin in Ordovician time that was accompanied with alkaline volcanism of the Marmot Formation. Sedimentary units from throughout the Mackenzie Mountains stratigraphy contain ubiquitous detrital zircons with U-Pb ages of 2800 – 2415 Ma and 2080 – 1700 Ma; these reflect a component of sediment derived from basement sources of western Laurentia. More surprising is the abundance of “anomalous” detrital zircons ages (e.g., 1800 – 1000 Ma) in these units that cannot be linked to known local sources. The most likely source for detrital zircons of these ages are felsic igneous rocks now exposed in southern and eastern North America. We therefore speculate that in the Neoproterozoic sediments were dispersed from these distal sources across the North American Craton. Reworking and recycling of these older sediments is evident in most of the younger investigated strata. In Carboniferous and younger strata there is also a major influence from sources exposed in the Canadian Arctic and possibly along Alaskan margin as evidenced by detrital zircons with Lower Paleozoic ages (e.g., 477 – 392 Ma). Continental extension of the western margin of Laurentia in Early to Middle Cambrian and again in Middle Ordovician time formed a rifted sedimentary basin termed the Misty Creek Embayment. Alkaline mafic magmatism associated with the Middle Ordovician rifting event (460 – 444 Ma) comprises massive mafic volcanic rocks (MFV and MFX suites), intrusive dykes and sills, volcaniclastic and epiclastic rocks and volcaniclastic filled diatremes (DVI suite). Petrographic studies together with whole rock, mineral chemistry and Nd isotopic studies indicate that the MFV and MFX suites were likely generated by small degrees of partial melting at the base of the subcontinental lithosphere with a minor asthenospheric input. The DVI parental magmas were also generated by small degrees of partial melting primarily in metasomatized subcontinental lithosphere.

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