Long-lived stratovolcanoes display a thick volcanic apron surrounding the edifice. This sedimentary succession incorporates the majority of the deposits from both growth and destruction phases of a volcanic massif. The ring plain of Taranaki Volcano (>200 ka) is composed of volcaniclastic mass-flow deposits that are exceptionally well-exposed along its coastal-cliff shoreline, at 20 to 30 km distance from the edifice. Overall, the volcaniclastic deposits in the southern and south-western sector record three growth phases (65 to 34 ka) which can be investigated due to access and stratigraphic control of the ring-plain section. Each cyclic growth phase is represented by a sequence of mass-flow deposits. Lithostratigraphic units or repeated packages with similar properties were identified in order to understand the depositional sequences. The mass-flow units within these growth phases can be described by three criteria subdivided into nine distinct sedimentological textural types (for example, massive, graded, etc.), five different lithological types (for example, lithic-dominated, polylithological, etc.) and three main physiographic facies types (sheet, overbank and channel). The mass-flow deposits can then be further categorized through a classification scheme by assigning these three criteria. Widely distributed lithic-dominated hyperconcentrated-flow deposits were recognized, which are thought to be directly or indirectly associated with eruption-fed events (remobilized from 'block and ash' flows) providing evidence for eruptive activity occurring on a 4 to 10 kyr cycle. Therefore, this study proposes classification criteria for mass-flow deposits in volcanic ring-plains using a developed three-part coding system. The study also aims to clarify the order of sedimentary and volcanic events by establishing a stratigraphic model for the investigated time-period offering a better understanding for future research.
Many stratovolcanoes are characterized 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.
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