The geology and geochemistry of Akaroa volcano, Banks Peninsula, New Zealand

Abstract

This thesis presents a detailed geological, petrological and geochemical study of Akaroa Volcano, Banks Peninsula, New Zealand. The Akaroa Volcanic Group is defined as comprising all the volcanic products of central, flank and parasitic vent eruptions in the south-eastern two-thirds of Banks Peninsula, which collectively form Akaroa Volcano. Field mapping has shown that the lavas and pyroclastics of which Akaroa Volcano is constructed can be grouped into an Early Phase and a Main Phase. Early Phase rocks (?11-9 Ma) are restricted in outcrop to the inner shoreline of Akaroa Harbour. The oldest exposed basaltic lava flows of Akaroa Volcano are assigned to Early Phase I. Early Phase II comprises extensive trachyte tuffs, breccias, agglomerates, flows, sills, and a large dome, with minor basaltic tuffs, and appears to represent a major episode of eruption of trachytic lava marking the end of the construction of a proto-Akaroa Volcano. Weathered basaltic flows, tuffs, lahars, scoria cones and pyroclastic breccia of Early Phase III unconformably overlie rocks of Early Phase II. The contact between Early Phases II and III shows considerable relief indicating a period of erosion prior to eruption of Early Phase III flows and pyroclastics. A diverse stratigraphy and a significant portion of the early history of Akaroa Volcano remains buried beneath sea level. A period of prolonged weathering and erosion occurred prior to the eruption of Main Phase lava flows and pyroclastics. The main cone of Akaroa Volcano is constructed predominantly of hawaiite lava flows and pyroclastics and rare mugearite, benmoreite and trachyte lava flows of the Main Phase, erupted 9-8 Ma. Activity was hawaiian to mildly strombolian in character. Throughout its eruptive history, Akaroa Volcano was intruded by predominantly trachytic dikes of the Akaroa radial dike swarm, and five large trachyte domes. Dikes radiate from a broadly defined central zone south to south-east of Onawe Peninsula which coincides with the inferred location of the main conduit, and with the maxima of local bouguer and isostatic gravity anomalies. Analysis of the gravity anomaly surfaces indicates a substantial sub-surface intrusive complex containing> 615 km³ of intrusive material. Panama Rock trachyte dome can be seen to have been fed by a large dike of the radial dike swarm and a similar origin is inferred for the other intrusive trachyte domes. Akaroa Volcanic Group lavas have a mineralogy typical of alkaline volcanic associations, dominated by olivine, Ti-rich calcic clinopyroxene, titanomagnetite, plagioclase and apatite. Rare kaersutite megacrysts occur in evolved lavas, and per alkaline differentiates contain arfvedsonite and aenigmatite. Minor biotite and amphibole occur in coarse-grain basic lavas. Akaroa Volcanic Group lavas comprise a mildly to moderately (sodic) alkaline association, with a trend of moderate iron enrichment. Two end-member lineages are recognised: a dominant basalt-hawaiite-mugearite-benmoreite-trachyte lineage with ne-, hy- and qz-normative variants, and a basanite-nepheline hawaiitenepheline mugearite-nepheline benmoreite-phonolite lineage. Peralkaline differentiates are also recognised. The dominant lava type is hawaiite, rather than basalt, and most lavas have Mg numbers (100 X Mg²⁺ /Mg²⁺ +Fe²⁺) in the range 35-48, indicating that Akaroa Volcanic Group lavas do not represent primary magmas but have undergone significant high pressure fractionation. Geochemically, Akaroa Volcanic Group lavas form a comagmatic suite characterised by (i) A logarithmic decrease in MgO, TiO₂, Cr, Ni and V; (ii) A linear decrease in CaO and FeO; (iii) A linear increase in Na₂O, K₂O, Y, Nb, Rb, La, Ce, Nd, Ga, Pb, Th, and Ba; (iv) A complex variation in Al₂O₃; (v) A rapid increase in P₂O₅ and Sr followed by a rapid decrease; and (vi) An increase in REE abundances with increasing differentiation. These variations are consistent with evolution by fractional crystallization of olivine, clinopyroxene, titanomagnetite, plagioclase, apatite and possibly kaersutite. Lavas have linear, parallel, LREE-enriched REE patterns (CeN/YbN ≈ 7-9.5) indicative of magma generation by small degrees of partial melting of a garnet peridotite mantle source. Covariance of ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotope ratios is consistent with derivation of Akaroa Volcanic Group magmas from a time-integrated, LREE-depleted mantle source, whereas Sm/Nd and Rb/Sr trace element ratios indicate a LREE-enriched source. Mantle enrichment processes prior to, or associated with, the melting event and/or very small degrees of partial melting (< 1%) are postulated to account for this dichotomy. Qz-normative felsic lavas have high ⁸⁷Sr /⁸⁶Sr isotope ratios, and high-level crustal contamination appears to be an important process in the evolution of these lavas.