The role of edifice load on andesitic magma generation by melt-mush mixing: an example from Taranaki Volcano, New Zealand

Abstract:

Andesitic stratovolcanoes are characterized by cycles of cone growth interspersed by sector or cone collapses. The long-term record of the evolution of their magmatic system is mainly preserved in the volcanic apron surrounding an active cone. As a back-arc composite volcano, Taranaki in northern New Zealand provides a detailed example of these processes due to excellent coastal ring-plain and young cone exposures. We present key findings on the magmatic system during three successive growth cycles of Taranaki from a stratigraphically controlled selection of pumiceous clasts deposited by volcaniclastic hyperconcentrated flows in the medial (25-30 km) ring-plain. The clasts from the three studied growth cycles differ in bulk composition and form distinct trends on elemental variation diagrams. Major element chemistry displays a stratigraphic pattern, which is expressed by the increase of the compositional ranges through the strata, i.e. whole-rock contents show both more primitive and more evolved compositions over time during edifice growths. Additionally, modelling results suggest that the bulk rock compositions are well reproduced by melt-mush mixing processes, with variable mixing ratios within each individual growth cycle. Further, modelling provides estimations for both melt and mush chemistry, the former displaying trachyandesitic and the latter mafic (<50wt% SiO₂) compositions. The modelling results suggest a scenario for mid-crustal andesitic magma generation, where rising intermediate melts interact with a primitive mush domain throughout repeated cycles of edifice growth. The results are further explained by the complex interaction of edifice loading and unloading with changes in plumbing system geometry and intermittent depressurization controlling the mid- to upper-crustal magmatic processes beneath Taranaki.