Michael B. Parsons

Petrographic and geochemical analyses of bulk bentonites, volcanic minerals, and unaltered glass inclusions in quartz phenocrysts reveal the composition and tectonic setting of Late Cretaceous explosive volcanism in the Canadian High Arctic. The bentonitic clays represent altered volcanic ashes intercalated with Kanguk Formation mudstones on Axel Heiberg and Ellesmere Islands. Preliminary 40Ar/39Ar dating of sanidine phenocrysts yields a Late Cenomanian to Middle Campanian age for the Kanguk Formation. Grain-size distributions and ash dispersal suggest high energy Plinian-style eruptions. Primary mineral assemblages (quartz, sanidine, ilmenite, zircon, apatite, and acmite) indicate that the parent magmas of the Kanguk bentonites had oversaturated, marginally peralkaline compositions. Microprobe analyses of glass inclusions within quartz phenocrysts confirm such parent melt compositions, and permit evaluations of element mobilities during bentonite formation. Appropriate modelling of elemental fluxes allows recalculation of original ash compositions from bulk bentonite analyses, and indicates a potential for geochemical correlation between individual units or groups of units. Detailed major and trace element geochemistry on both primary phenocrysts and bulk bentonites, in conjunction with discriminant diagrams, reveals a within-plate tectonic setting for the parent volcanism, and dominance of peralkaline rhyolite compositions. The Kanguk bentonites may represent late-stage magmatic activity along the Sverdrup Rim, or they may be the products of extra-basinal volcanism. Late Cretaceous to Tertiary peralkaline volcanism occurred elsewhere along the north coast of Ellesmere Island and northwestern Greenland; however, no exact match to the age and composition of the bentonites exists to date.

Keywords: bentonite, volcanic ash, Sverdrup Basin, Plinian eruptions, glass inclusions,
peralkaline rhyolites, element mobility, tectonomagmatic discrimination, geochemical
correlation, Arctic volcanism
Pages: 131
Supervisor: Gunter Muecke