INSIGHTS INTO THE MAGMATIC EVOLUTION OF AKAROA VOLCANO FROM THE GEOCHEMISTRY OF VOLCANIC DEPOSITS IN OKAINS BAY, NEW ZEALAND
Forty-five volcanic rock samples were analyzed using x-ray fluorescence spectroscopy. These samples represent a suite of alkaline volcanics ranging from picrite basalt to trachyte. While SiO2 contents vary from 43 to 63 wt. % and more mafic compositions dominate, there is a prominent gap in intermediate compositions (53-63 wt % SiO2) that is also seen in previously published geochemical data from the volcano. Lava domes within this sample set from the surrounding area are marked by consistent enrichments or depletions in groups of trace element (e.g. Zr, Rb, and Nb) relative to the surrounding lavas. Stratigraphic sequences of lava flows display several alternating cycles between picrite basalt and hawaiite rather than an overall evolution towards higher SiO2.
Clear major and trace element correlations in Harker diagrams indicate a prominent role for fractional crystallization processes in the petrogenesis of these volcanic rocks. Cyclic fluctuations in whole rock geochemistry through the stratigraphy suggest these lavas represent the eruptive products of small, frequently replenishing batches of magma in the upper crust sourced from a larger magma reservoir at depth. This data, coupled with ongoing geomorphic study by other workers, implies a refined conceptual model for Akaroa Volcano characterized by small, shallow magma batches and multiple eruptive centers.