Origin and emplacement of a composite benmoreite-trachyte intrusion : Castle Rock, Lyttelton volcano (1991)
Type of ContentTheses / Dissertations
Degree NameMaster of Science
PublisherUniversity of Canterbury
Castle Rock and the Eastern Sheet are elongate intrusions associated with the Lyttelton Volcano, Banks Peninsula. These intrusions were emplaced about 9.7 Ma, late in the evolutionary history of the volcano, and are connected to feeder dikes of the Lyttelton radial swarm. Trachytes, Groups A and B, and benmoreites, low silica and high silica, are components of Castle Rock, whereas only Group A trachyte is found within the Eastern Sheet.
The flow foliation and cooling joints of the trachytes and benmoreites indicate the emplacement of bodies with domal cross - sectional shape in a single episode. Velocity gradients in the magmas during emplacement produced groundmass crenulations subsequent to the development of a strong preferred orientation. These crenulations enabled the growing· domes to continuously change their shapes, thus facilitating magma movement within. During the emplacement of Castle Rock magma flow was generally away from the magma source, however, restricted magma movement at the northern end during the trachyte intrusion produced magma movement back towards its origin. Overriding of the trachyte and benmoreite magmas at the advancing magma front did not occur during the emplacement of Castle Rock.
Major and trace element geochemistry, mineral chemistry and quantitative modelling indicate that the trachytes and benmoreites were produced by crystal fractionation from a single mafic parent magma. The mafic precursors of the trachytes and benmoreites were derived by partial melting of a depleted mantle source which was enriched in incompatible elements just prior to or during melting.
Multiple populations of feldspar and clinopyroxene phenocrysts within the Group B trachyte and high silica benmoreite and the presence of disequilibrium textures in many of these phenocrysts are interpreted as evidence of magma mixing. Quantitative modelling suggests that magma mixing was a minor process in the magma reservoir and petrographic evidence indicates that it was probably arrested prior to eruption. The Group A trachyte and low silica benmoreite show no evidence of mixing.
The trachytes and benmoreites were fractionated at shallow depths within the crust and erupted following a short residence time in the magma reservoir. Isotopic data suggest that minor amounts of continental crust were assimilated during the evolution of the trachytes and benmoreites. Quantitative modelling indicates that Torlesse Supergroup, or their metamorphosed equivalents, was the probable contaminant.
The trachytes and benmoreites were erupted from a compositionally zoned magma reservoir in which the magmas became less evolved with depth. A fissure, initiated and propagated by a magmatic stress regime developed around a central conduit, tapped the top of the magma reservoir and in time tapped progressively deeper levels. Magma withdrawal was arrested prior to complete emptying of the reservoir.
The emplacement of Castle Rock and the Eastern Sheet occurred at very shallow depths close to or at the surface on the northern flanks of the Lyttelton Volcano.
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