麻豆传媒

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M. Sc.听 Thesis

Petrochemistry and Origin of Altered Permian Basalts in the Sverdrup Basin, Arctic Canada

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This thesis presents new geochemical and stable-isotope data on hydrothermally altered Permian basalts of the Esayoo Formation and describes their role in the formation of the Sverdrup Basin in Arctic Canada.

Basaltic magmatism, coincident with late Paleozoic rifting, consists mainly of subaerial basaltic flows of the Late Carboniferous Audhild and the Permian Esayoo formations on northern Ellesmere and Axel Heiberg islands. North of Greely Fiord, Ellesmere Island, the Esayoo Formation attains a maximum thickness of 300 m, but rapidly thins westward, where pillow lavas, epiclastic basalt conglomerates, and associated marine sediments indicate proximity to an ancient shoreline.

Elemental mobility studies incorporating discriminant function analysis demonstrate that modifications in Fe₂O₃, K₂O, CaO, CO₂, Li, Ba, Rb, and Cu contents accompanied hydrothermal alteration. Significant enrichments in the s18O of the Esayoo basalts requires interaction with a high 18O hydrothermal fluid. The favoured hydrothermal model proposes that formation waters formed in the subsiding Sverdrup Basin by isotopic equilibration between Artinskian seawater and high -18O marine sedimentary rocks at 240^oC. A minor contribution to the elevated 18O values of the fluid may have originated by seawater evaporation in a closed basin.

Familiar discriminant diagrams involving relatively immobile trace elements and the composition of relict clinopyroxenes reveal within-plate, alkaline to transitional basalt affinities. Rare-earth-element ratios and compatible trace elements concentrations suggest that variable degrees of partial melting (between 5 and 20%) of an enriched garnet lherzolite from an asthenospheric source, and combined olivine-clinopyroxene crystal fractionation, respectively, can account for observed compositional variations. Trace-element data do not indicate significant lower-crustal contamination.

Small degrees of partial melting of a garnet lherzolite source generated primitive alkaline basalts of the Audhild Formation in response to small values of lithospheric stretching during Carboniferous rifting. Following a brief period of thermal relaxation, the alkaline to transitional Esayoo basalts were produced from a similar mantle source (minus the Audhild melt fraction). Slow conductive cooling throughout the Mesozoic caused accretion and coupling of this residual asthenospheric mantle to the thickening lower lithosphere. An elevated geotherm inherited from the initial Carboniferous rifting episode may account for the voluminous, tholeiitic lavas of the Early Cretaceous.

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Supervisor: Gunter Muecke

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