USING CRYSTAL SIZE DISTRIBUTIONS AND QUALITATIVE TEXTURAL ANALYSIS TO DEDUCE THE CRYSTALLIZATION HISTORIES OF TRACHYTIC DOMES AND DYKES IN THE AKAROA VOLCANIC COMPLEX, NEW ZEALAND

Study of silicic domes and dykes located in predominantly basaltic volcanic complexes is crucial to the understanding of late-stage volcanic plumbing systems. Crystal size distribution (CSD) analysis—when substantiated with qualitative petrographic observations and crystal damage assessment—can be used to construct models for the storage, ascent, and emplacement of such structures. Moreover, estimates of residence time for related feeder magmas can be calculated from CSD regression slopes. In the case of the Akaroa Volcanic Complex (AVC), an intraplate composite shield volcano active during the Miocene on New Zealand’s Banks Peninsula, CSDs and petrographic evidence for three trachytic structures—Mount Sinclair dome, Panama Rock dyke, and Devil’s Gap Senior dome—show that late-stage trachyte magma faced at least two different paths of ascent after fractionating from a hawaiite reservoir at depths of 10-15 km: (1) a direct and constant ascent, which is reflected in the linear CSD trends of Mount Sinclair dome and Panama Rock dyke, and (2) a stalled ascent, which is reflected in the downward inflections observed in Devil’s Gap Senior dome CSDs. The steeper CSD regression slopes for Panama Rock dyke and Mount Sinclair dome— -0.0236 and -0.0215, respectively—show that magma ascended relatively rapidly compared to Devil’s Gap Senior dome, which has an average CSD regression slope of -0.0127. The steeper regression slopes for Mount Sinclair dome and Panama Rock dyke also yield estimates of magma residence time over 4.5 times greater on average than those calculated for Devil’s Gap Senior dome—a disparity likely the result of Devil’s Gap Senior dome having crystallized from a larger body of magma. Furthermore, crystal damage assessments show that Devil’s Gap Senior dome likely breached the surface as an effusive lava dome, while Panama Rock dyke and Mount Sinclair dome solidified in the subsurface as shallow intrusions. In addition to increasing our understanding of the crystallization dynamics of the individual units considered in this study, the research methods proposed here will serve as an easily adaptable framework for future research on trachytic domes and dykes—both in the AVC and in modern analogues abroad.