Swinburn Volcanic Complex, Central Otago, New Zealand
Swinburn Volcanic Complex is the remnant of a Miocene volcano, forming part of the Dunedin Volcanic Group (~ 24 - 9Ma), on the South Island of New Zealand. It is part of the Waipiata Volcanic Field, and its basalts are distinct from most others in that field. New analyses of Swinburn rocks, which have only very few mantle nodules, are compared with other new data from peridotite-bearing basanites collected from different volcanic rocks of Dunedin Volcanic Group. Swinburn basaltic rocks are geochemically and isotopically different than any other rock so far described from the intraplate Waipiata Volcanic Field or the Dunedin Volcanic Complex. I have identified geochemical and isotopic affinities (e.g. high 207Pb/204Pb trend together with lower trace element concentrations) of Swinburn rocks with those formed from Cretaceous basaltic magmas elsewhere in Zealandia and the Hikurangi Plateau. Percolation processes have: a) metasomatized the lithosphere, leaving strong geochemical imprints (e.g. carbonatite, OIB and HIMU-like), b) locally modified the thermal gradient c) produced metasomatic cumulates (amphibole-bearing veins and/or pyroxenites) which have major and trace-element compositions suitable for the mantle source that fed the Dunedin Volcanic Group. Within the footprint of this Group, Swinburn Volcanic Complex is an anomaly. Under the Swinburn Complex, percolating fluids interacted with one part of the lithospheric mantle that had been already metasomatized by Cretaceous melts. The combination of Cretaceous and Miocene percolation through the same mantle domain formed the distinctive geochemistry of Miocene Swinburn magmas. Petrographic and geochemical differences among samples from Swinburn also reveal that more than one batch of magma was involved in the formation of the complex. By combining field evidence with new geochemical data, two effusive eruptions or eruptive periods are inferred, separated an episode of magma intrusion. Reversed polarity of the magnetic field at the time of Swinburn effusion and intrusion activity was identified by paleomagnetic analysis. A magnetometer survey suggests the presence of several buried features. There are pegmatitic domains in the Swinburn basaltic rocks. Compaction of a crystal mush, together with the buoyant separation of residual melt rich in dissolved volatiles, is interpreted as the main mechanism driving the upward movement of Swinburn segregation domains and the formation of those segregation veins and domains. Results of this multidisciplinary analysis of the Swinburn Volcanic Complex, based on petrographic, mineralogical, geochemical and geophysical investigations are summarised below. 1) Older pyroclastic rocks, and the majority of the exposed vesicular lavas, represent the first activity of the complex, whereas younger pyroclastic products and a lava exposed in the western part of the complex (G7) formed during the last event. 2) The Swinburn body was emplaced during two effusive events by the intrusion of two batches of magma with similar composition. The higher amount of magnetic minerals forming the rocks of the Main body explains the inhomogeneous magnetic signal showed by my newly produced 3D magnetic map of the area. The association of magnetic profiles with cross sections unveiled several buried features such as a fault, possible thickness variations in the body, and places where Quarry and Main rocks overlap. 3) Compaction together with the buoyant rise of volatile-rich residual melt are interpreted to have been the main mechanisms driving upward movement of segregated residual liquids to form pegmatitic domains and veins. Compaction of the lower and central parts of the emplaced magma body caused dilatation and tearing in the upper crystallising front, which were filled by residual liquid to form segregation veins. 4) The concentration of major and trace elements is similar for plagioclases and clinopyroxenes crystallised near the top and bottom of the body but different for mineral crystallised near the mid-level of it. These differences reflect slower cooling of the interior of the body, furthest from basal and top cooling surfaces. The comparison of Swinburn features with those of volcanoes elsewhere worldwide sheds light on processes that regulate the origin of melts, and formation of segregation products during cooling of emplaced magmas. Following the main aim to determine the volcanic evolution and processes of the Swinburn Volcanic Complex and how it impacts on the wider volcanological perspective, this study demonstrates that even inside a broadly monogenetic volcanic field, individual volcanic complexes may be products of multiple eruptions. They can preserve significant local heterogeneities and magma source regions distinct from those of most volcanoes in the field.
Advisor: White, James D.L.; Scott, James M.
Degree Name: Doctor of Philosophy
Degree Discipline: Geology
Publisher: University of Otago
Keywords: Waipiata; Swinburn; Miocene; Magmatism; Geochemistry; Geophysics; Volcano; Zealandia
Research Type: Thesis