The volcanic geology and geochemistry of central Banks Peninsula and relationships to Lyttelton and Akaroa volcanoes

Based on field-mapping and geochemistry of lavas outcropping between the two stratovolcanic centres of Lyttelton and Akaroa, a new Miocene volcanic stratigraphy for central Banks Peninsula has been established. Five lithologically and geochemically distinct phases of volcanism are recognised. These are the Lyttelton Volcanics (11 – 10 Ma), Mt Herbert Volcanics (9. 7 - 8.0 Ma), Akaroa Volcanic s (9.0 - 8.0 Ma). Church Volcanics (8.1 - 7.3 Ma) and Stoddart Volcanics (7.0 - 5.8 Ma). A model is proposed to account for the volcanic evolution of Banks Peninsula. The Lyttelton Volcanics range in composition from hawaiite to trachyte and consist of a mildly alkaline to transitional association of lava flows, pyroclastic deposits and high level intrusive rocks. Eruption style was dominantly of the mild Strombolean to Hawaiian-type and during the later stages of volcanic activity, a large breach formed in the south-east sector of the Lyttelton crater wall. The Mt Herbert Volcanics comprise a volcanic complex of mildly alkaline basalt plugs and lava flows, epiclastic and pyroclastic rocks that record the migration of volcanic activity from the Lyttelton centre to that of Akaroa. Field and geochemical relationships together with new K-Ar data, show that the Mt Herbert Volcanics can be divided into five formations. In order of eruption these are: - 1/ Kaituna Olivine-Hawaiites; 2/ Orton-Bradley Volcanic Suite; 3/ Port Levy Volcanic Suite: 4/ Castle Rock Hawaiites; 5/ Mt Herbert Hawaiites. Volcanic products associated with the Orton-Bradley Volcanic Suite infill the breach in the south-east crater wall of Lyttelton Volcano. There is evidence that a crater lake occupied the floor of Lyttelton Volcano during this infilling. Concomitant lacustrine deposition and Surtseyan volcanic activity is recorded by sub-aqueous pyroclastic deposits. Lavas of the Mt Herbert Volcanics were erupted from two main centres located respectively beneath the present position of Mt Herbert and in the vicinity of Port Levy. Eruption of lavas from the main vents was accompanied by the building of local Surtseyan and Strombolean tuff cones. These are recorded by base-surge and airfall deposits. The mineralogy and geochemistry of the Mt Herbert Volcanics indicate that magmas evolved in reservoirs near the base of the crust. Magmas of the Kaituna Olivine-Hawaiites have undergone selective crustal contamination. The Akaroa Volcanics in central Banks Peninsula comprise a mildly alkaline association of basalt to mugearite lava flows and high-level basaltic to trachytic intrusive rocks. Lavas reached the central area of Banks Peninsula during the final stages of Mt Herbert Volcanism. Geochemical data indicate that Akaroa magmas evolved mostly in reservoirs near the base of the crust. The Church Volcanics represent the most geochemically primitive rocks in central Banks Peninsula. A newly defined formation of basanitoid lava flows, dikes and plugs (Darra Basanitoids) is shown to be chemically related to alkali-basalt lava flows defined as the Church Bay Olivine-Basalts. The mineralogy and geochemistry of the Church Volcanics indicate that magmas were derived from different degrees of partial melting from a common mantle source. Magmas ascended directly through the crust without significant interruption. The Stoddart Volcanics comprise a volcanic group dominated by olivine-basalt to olivine-hawaiite lava flows and plugs, minor basanite and conglomerate units and rare olivine-hawaiite dikes. The Stoddart Volcanics are divided into two formations:- the Stoddart Point Olivine-Basalts and the Kaioruru Olivine-Hawaiites. Mineralogical and geochemical data indicate that Stoddart Volcanics were derived from different degrees of partial melting from a common mantle source. Some of these lavas underwent selective crustal contamination during ascent to the surface. Geochemical and isotopic characteristics of lavas from central Banks Peninsula indicate that magmas were derived from a common, light Rare-Earth-Element enriched mantle source. Tectonically, all the lavas of central Banks Peninsula are within-plate basalts and show no affinity with subduction-related volcanism. A within-plate stress field corresponding to the intersection of E - W trending faults of the Chatham Rise and NE - SW trending faults of the transform plate boundary through the South Island is considered to have triggered the Miocene volcanism recorded on Banks Peninsula. A model is proposed to account for the petrogenetic evolution of Banks Peninsula magmas during the Miocene