The petrology of a large intra-oceanic silicic eruption: the Sandy Bay Tephra, Kermadec Arc, Southwest Pacific

The pumiceous pyroclastic deposits known as the Sandy Bay Tuff on Macauley Island in the Kermadec Group represent a medium-scale silicic eruption in an oceanic subduction setting. The Sandy Bay eruption occurred about 6310 yr BP, forming a large submarine caldera in the summit of the mainly submarine and essentially basaltic Macauley Volcano. The Sandy Bay magma contained <1 vol% phenocrysts of plagioclase, augite, hypersthene and accessory oxides together with crystals and fragments of basaltic origin and lithic fragments representing an earlier silicic eruptive episode. Although prolonged fractionation of basaltic magma to produce silicic daughter is a generic paradigm, it fails in detail to completely explain the observed compositions of the silicic rocks, the variety of compositions, the distribution of rock types, their aphyric nature or their volumes. It also encounters difficulties as a general explanation for silicic magmatism in the Kermadec Arc. An alternative, that silicic magmas represent anatexis of underplated basaltic arc crust, explains more of these details and can be modelled as the logical consequence of the thermal evolution of an oceanic arc. In addition, this model can provide an explanation for the onset of silicic magmatism at the current stage in the evolution of the arc. A precondition for anatectic melting in an oceanic arc is the development of a suitable source at the base of the arc crust; this takes of the order of 106 years. The onset of silicic magmatism creates an infertile residue and complementary silicic magmas. Separation of silicic magma from its source and its rise into the upper crust removes heat from the system. Continuation of silicic magmatism depends on replenishment of fertile arc material. An alternative is that generation of silicic magmas is limited to a window of opportunity at an adolescent stage in the evolution of an oceanic arc.