麻豆传媒

B. Sc. Honours Thesis

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This paper is a geochemical and geochronological study of two rift-related Mesozoic basaltic dykes. The Shelburne Dyke of Nova Scotia and the Messejana Dyke of Spain are extensive vertical sheets of intrusive rock that are over 100 kilometres in length, average 100 metres in thickness, and strike northeast-southwest at a latitude of 44路N. An outcrop of the Shelburne Dyke crops out in Little Harbour, where it intrudes metawackes and metapelites of the Cambro-Ordovician Meguma Group. The Messejana Dyke crops out in Cabezuela de Valle and intrudes a body of sillimanite-bearing, two-mica granite belonging to the Tournaisian and Lower Visean Volcanic-Siliceous Complex. The two dykes contain plagioclase and pyroxene, with minor biotite, olivine, and opaque minerals, and have subophitic to ophitic textures. Chemical classification plots show that the Shelburne and Messenjana Dykes are tholeiites and, according to the CIPW normative tetrahedron, they are quartz normative. Major elements such as Fe and Mg show little or no effects of chemical variation, whereas many compatible and incompatible trace elements show fractional crystallization. The removal of Ni and Cr from the melt in olivine and chrome spinel causes passive enrichment of other trace elements such as Sr, Ba, and Rb. The REE patterns for the two dykes show low La/YbN (2.9-3.7) values and small Eu/Eu* (0.89-1.08) anomalies. Tectono-magnetic discriminator diagrams suggest the Shelburne and Messejana Dykes are within-plate basalts transitional to ocean floor basalts, however, some discriminator diagrams suggest the involvement of a subduction-derived component. The ⁴⁰Ar/³⁹Ar dating method shows that the intrusion of the Shelburne and Messejana Dykes straddles the Triassic-Jurassic boundary (208 Ma). Three models to explain the relationship between the two dykes are: one-dyke model, two parallel dyke model, and two dyke perpendicular model. The simplest model that fits all the analytical data is the two parallel dykes model, i.e. the Shelburne and Messejana Dykes were two contemporaneous dykes on opposite sides of the principal rift that formed the North Atlantic Ocean.

Keywords: Shelburne, Messejana, tholeiite, rift, within-plate basalts, ocean floor basalts,
⁴⁰Ar/³⁹Ar, Triassic-Jurassic, Atlantic Ocean
Pages: 113
Supervisor: Barrie Clarke