TRACKING THE GROWTH OF A TRACHYTE LAVA DOME ON AKAROA VOLCANO, NEW ZEALAND WITH STRUCTURAL OBSERVATIONS, GEOCHEMISTRY AND CRYSTAL SIZE DISTRIBUTIONS

The magmatic evolution and emplacement history of an eroded lava dome in Devil’s Gap Scenic Reserve, Banks Peninsula, New Zealand are inferred from field observations, whole rock geochemistry and crystal size distributions. The dome lies on the flanks of the extinct and basaltic Akaroa Volcano and has a trachyte composition indicative of crystal fractionation in a shallow magma chamber. All lines of evidence show that the accessible areas of the dome can be grouped into four distinct lobes. Strike and dip measurements of macroscopic flow bands and joints show similar patterns within each lobe. The flow bands dip steeply and strike roughly parallel to the dome boundaries while joints dip steeply and strike perpendicular to the dome boundaries. These data are indicative of flow away from a common vent or fissure. Whole rock chemical analyses indicate that while all of the dome lavas have similar major element compositions, each of the four lobes can be distinguished based on trace element abundances. Sr and Ba concentrations provide the most distinct chemical signature for the lobes and indicate that lobe 3 has the lowest concentration and lobe 4 has the highest concentration, while lobes 1 and 2 have intermediate concentrations. Each lobe also has a distinct crystal size distribution that further constrain the emplacement sequence of the lobes. Preliminary analysis shows that lobe 1 has the smallest range of crystal sizes (80.1-319 μm) and has maximum population density at the smallest crystal size (80.1 μm), while lobe 2 has the largest range of crystal sizes (127-801 μm) and lobe 4 has maximum population density at the largest crystal size (201 μm). Based on these results lobe 1 is thought to have been extruded first, lobe 4 extruded last, and lobes 2 and 3 extruded intermediately. This sequence is reflected in the change in morphology of the lobes from massive, columnar jointed lavas in lobe 1 to spiny protrusions in lobe 4. The anomalous geochemical signature of lobe 3 may be due to an injection of less evolved magma. Further analysis of crystal size distributions and additional data from rare earth element geochemistry will help refine these interpretations.