Investigation of the Mid-Age (65–34 ka) Period of Taranaki Volcano, New Zealand: Indications for the Effect of Volcano Growth

Abstract

ABSTRACT Many stratovolcanoes are characterised by cycles of edifice growth interrupted by collapse events. The long-term record of the evolution of such magmatic systems is mainly preserved in the deposits of the volcanic apron surrounding the active cone. Taranaki Volcano in New Zealand provides an unusually detailed example of these processes due to excellent coastal ring-plain and young cone exposures. In this study, we investigate the magmatic system of this volcano through three consecutive growth phases by sampling a detailed, stratigraphically controlled selection of volcanic clasts from volcaniclastic mass-flow deposits in the medial ring-plain. The clasts from three growth phases (GP1, 65–55 ka; GP2, 55–40 ka; GP3, 40–34 ka) differ in bulk composition and form geochemically distinct trends on variation diagrams. These trends can be modelled by mainly dacitic melt mixing with gabbroic and ultramafic xenolith compositions representing the plutonic assemblages beneath the edifice. Within short-term growth cycles (104 years), the geochemical differences between lower and upper sequences of GP units indicate that closer to an edifice collapse, both whole-rock major and trace element compositions display more evolved and scattered trends compared to post-collapse stages. Considering the long-term magmatic evolution of Taranaki Volcano, it is apparent that the pre-collapse compositions are more evolved than bulk rock compositions of the growth phases, indicating active upper-crustal reservoir conditions in pre-collapse states. Furthermore, the volume losses caused by sector collapses prior to GP2 and GP3 could decrease the pressure in the upper-crustal reservoir. Overall, the data obtained from the mid-age Taranaki volcanic system elucidate the mid- to upper-crustal magmatic processes and reservoir conditions throughout growth cycles. Further, it demonstrates the top-down control of volcanic edifice load change on the magmatic plumbing system expressed by the evolvement of whole-rock compositions towards the end of a growth cycle.