Petrogenesis, eruption and emplacement of the volcanic rocks at Otapahi, Otago Peninsula.
Baxter, Rachael Joy May
Intraplate volcanism is a globally common phenomenon that remains poorly understood. It formed the Dunedin Volcano at 16-10 Ma, with the oldest known exposures located at Otapahi, on Otago Peninsula. Investigation of the Otapahi area has revealed the physical volcanic and magmatic processes active at the onset of this volcano's growth. Paleomagnetic study of a sample sourced from a mingled dike complex comprising multiple, compositionally unrelated dikes, tightens a previously published 39Ar/40Ar date of 16.0 ±0.4 Ma to 15.97-16.26 Ma, or 16.303 - 16.40 Ma. At Otapahi submarine eruptions initially produced eruption-fed aqueous density currents, the deposits of which were cut by later diatreme-forming eruptions. The first eruptions are likely to have been initiated by mafic recharge and by mixing with evolved magma in a shallow reservoir. Diatreme formation was penecontemporaneous with dike emplacement. Within compositionally varied dikes two magmatic lineages are preserved, basanite to phonolite, and alkali basalt to trachyte. Differentiation processes were not the only driver of this variation; mixing of multiple magmas generated many of the observed compositions. The wide compositional range of magmas emplaced near-simultaneously can be attributed to the destabilisation and reorganisation of an early, weakly interconnected trans-crustal magma storage zone that contained multiple magma pockets at various points of evolution, many of which mixed to form chemically complex magmas. Multiple dikes contain abundant crystal cargoes derived from a mushy reservoir in the lower crust. Mush-dominated reservoirs have been implicated in other settings as facilitating long-lived magmatic systems and may have played this role for the Dunedin Volcano, as well as for other long-lived intraplate systems.
Advisor: White, James; Brenna, Marco
Degree Name: Master of Science
Degree Discipline: Geology Department
Publisher: University of Otago
Keywords: Intraplate volcanism; Dunedin Volcano; magma mixing; magma mingling; alkaline volcanism; diatreme; volcanic geochemistry; submarine eruption
Research Type: Thesis