Abstract
Monogenetic lava fields are a hallmark of intraplate volcanism but can be internally highly variable. This thesis investigates a suite of heterogeneous basalts in Maniototo region in the northwestern portion of the intraplate Dunedin Volcanic Group in New Zealand. These basalts are separated into an alkaline group (Tunaheketake, Taieri River, and three dykes) and a transitional to subalkaline group (herein “Transitional” and comprising Haughton Hill, Waipiata, Kokonga). Two new 39Ar40Ar dates from different facies show that the Waipiata flow, which is the largest in the area, erupted at 11.14± 0.16 Ma. In the context of a prior date of Haughton Hill (10.9 ± 0.3 Ma), it appears that the Transitional group erupted within relatively rapid succession. The lava types range from basanite (Tunaheketake, Dyke 2, Dyke 3) to alkali basalt (Taieri River, Dyke 1) and basalt (Haughton Hill, Waipiata, Kokonga). Major element geochemistry indicates that all lavas are relatively primitive, unfractionated and enriched relative to primitive mantle. However, there are clear differences between the groups; the Alkaline group has systematically higher concentrations of all trace elements compared to the Transitional group. The Alkaline group contains negative K, Ti and Hf anomalies which are absent within the transitional group. For all lavas, the 87Sr/86Sri ranges between 0.70291-0.70477, 143Nd/144Nd = 0.51285- 0.51293i, 206Pb/204Pb = 19.21-19.96, 208Pb/204Pb = 19.21-19.96 and 207Pb/204Pb is 15.64-15.67. Haughton Hill has anomalous 87Sr/86Sr (0.704608-0.704768) due to carbonate alteration of glass by post-magmatic fluids. The Alkaline group plots within the Otago mantle array, while the Transitional group forms a gradient from within the Otago mantle array to outside it, which highlights discrete clusters present within the Pb isotopes. The chemical and isotopic differences between the two groups is partially controlled by the degree of melting, with the Alkaline group forming through smaller degrees of melting. However, the primary control on the diversity between the two groups is differences in melt source composition. Melting likely occurred within the lithospheric mantle and crustal contamination, if present, is minor. Trace element chemistry indicates that both groups’ source contains a minor Ti-phase-bearing pyroxenite, which is absent within the wider Dunedin Volcanic group. Three different potential melt sources were modelled using non-model batch melting. Elemental ratios and non-model melt modelling indicate that the Alkaline group can be modelled to be derived from a combination of metasomatized lithospheric peridotite, hydrous and anhydrous veins and pyroxenite. Melt modelling was unable to confirm the source of the Transitional group, the melt sources probably include a less metasomatically enriched lithospheric peridotite and small quantities of metasomatic veins and pyroxenite. Two potential melting processes are proposed: two phase melting and polybaric melting but currently it is not possible to eliminate one. Two phase melting comprises a smaller early phase of melting within the enriched lithospheric mantle and rapid eruption forming the Alkaline group, followed by a second phase of melting within the now less enriched mantle, forming the Transitional group. Whereas polybaric melting consists of melting of metasomatic veins, pyroxenite, and peridotite within a melting column. The variation between the two groups is caused by differences in the degree of melting, size of the melting column and variation in the enrichment of the peridotite. The previously little-studied Maniototo basalts therefore demonstrate the variety and complexity of both monogenetic volcanism and the lithospheric mantle and are important with the context of the Dunedin Volcanic Group and globally.