SHALLOW MAGMATIC RECHARGE IN AN INTRAPLATE VOLCANIC COMPLEX, AKAROA, BANKS PENINSULA, NEW ZEALAND

Lava flows of the intraplate Miocene Akaroa Volcanic Complex (AVC), Banks Peninsula, New Zealand, display a cyclical geochemical trend from picrite to benmoreite. When observed within a single stratigraphic section, flows reveal repeating patterns, or batches, of primitive to evolved magmas. Primitive flows are generally porphyritic, while the more evolved flows are consistently aphyric. Previous studies have led to a model for the AVC in which a deep reservoir (lithospheric detachment sourced) fed and replenished multiple shallow magma chambers, which then fractionated individually to produce several independently evolving magma batches. The purpose of this research is to test that model and extend it spatially across the eastern flanks of the AVC, as well as to characterize magma chamber dynamics using geochemistry and petrography.

Sixty-nine samples were taken from six stratigraphically controlled transects across the eastern AVC for XRF analysis. Based on rock type and composition, samples were separated into individual batches within their respective transects. Distinct geochemical variations were observed in samples ranging from 43-59 wt. % SiO₂, 0.5-7 wt. % MgO, 1-4 wt. % TiO₂, and 0-270ppm V. The distinction between batches were drawn where element concentrations varied significantly within stratigraphy, further supported by petrographic distinctions (plagioclase resorbtion and sieved cores, and skeletal textures).

By correlating the crystallinity and textures of each flow with the bulk-rock geochemistry, we argue that the shallow magma chambers underlying the AVC experienced cycles of magma evolution punctuated by magmatic recharge from depth. As each batch evolved, the phenocrysts would settle in the chamber forming a crystal mush while the residual evolved liquid erupted. With further magma replenishment this crystal mush was incorporated into the eruptives, resulting in the initial, most primitive flow(s) of each cycle to contain the greatest degree of crystallinity, along with resorbtion and sieve textures in the phenocrysts. The phenocrysts contained within these early batch flows are essentially antecrysts in that they are inherited from an earlier melt.