The Derwent-Hunter submarine volcano: The product of a long defunct subduction zone

Abstract

The Derwent-Hunter Volcano (31°S, 156°E) is one of a 1400-km-long chain of Oligocene-Miocene undersea volcanoes that transect the Tasman abyssal plain. It developed some 15–16 Ma ago and the rocks it contains form an abbreviated differentiation sequence that range in composition from olivine tholeiite to either mugearite or basaltic andesite. These rocks generally contain normative hypersthene, but do not contain significant quantities of either normative quartz or normative nepheline; and their trace-element abundances generally lie in the area of overlap between the subalkali and alkali rock series. The more primitive rocks contain spinels (36% Cr2O3), olivines (Fo81–83), plagioclases (An63–70) and less commonly augite, set in a glassy mesostasis. Some specimens contain xenocrysts of anorthitic plagioclase (An85–94) and calcic salite (En36 Fs12 Wo52).The volcano is part of the Tasmantid volcanic chain, and the latter is one of a number of parallel volcanic chains of Oligocene-Miocene age that arise in eastern Australia and the Tasman Sea. Both the onshore and offshore volcanoes contain rocks that belong to a transitional petrographic series that comprises tholeiitic, transalkalic and alkalic members. With one exception the ages of the volcanoes across the various chains are similar for the same latitudes. The exception is the easternmost chain (i.e. Lord Howe) where the ages of the individual volcanoes lag behind their counterparts at similar latitudes. When considering the origin of these parallel volcanic chains the fundamental problems are: 1.(a) What processes occurred within the asthenosphere to create a volcanic focus that extends for 1000 km in an east-west direction and has been active for circa 40 Ma?2.(b) Why are the ages of the Lord Howe Seamounts not synchronous with the other volcanic chains?3.(c) What processes and materials are required to generate the geochemically diverse and complex magmas needed to construct these intraplate volcanoes?It is proposed that these magmas evolved from a line of diapirs within the asphenosphere. The diapirs began forming at a depth of ≈ 650 km as the result of the heating and partial melting of pressure-transformed materials that had originally been subducted. After traversing the asthenosphere individual diapirs impinged on the base of the lithosphere where they reactivated pre-existing fractures and produced extensional rock failure. This resulted in the extraction of batches of magma from the diapirs; and the composition of each batch was substantially controlled by, the degree of partial melting, and the ambient pressure at the source of magma extraction. Intratelluric fractional crystallization, limited assimilation of crustal materials, high-level magmatic differentiation and the mixing of different batches of magma, all assisted in the evolution of the more differentiated magmas.The diapirs developed along a trend parallel to the long axis of a long defunct subduction zone. It is proposed that this subduction zone was linear along most of its length, but in the east it terminated in a section that was displaced northwards by a transform fault, and the materials subducted by this displaced segment triggered the string of diapirs that eventually produced the Lord Howe Seamount Chain.