Melting and mingling of phonolitic pumice deposits with intruding dykes: an example from the Otago Peninsula, New Zealand

Abstract

A mound of bedded phonolitic pumice lapilli ash was deposited on the marine shelf (modern Otago Peninsula, New Zealand) during a shallow subaqueous eruption in the Miocene. Basaltic dykes subsequently intruded the unconsolidated pumice deposit and formed an unusual peperite. Dyke–sediment interaction began with weak steam-driven mixing, accompanied by fluidisation at the contact zone that produced a metres-broad zone in which bulbous tongues (up to 3 m) of the dykes extended into, and locally engulfed, host sediment. Irregular, sinuous contacts of dykes with the pumice are marked by black glass that formed by partial melting, shearing and remoulding of the glassy pumice. Such melting and remoulding of host sediment is not a typical peperite feature. Temperature evolution, volume of water vapour, load pressure and time, as well as initial contact temperature, control the welding and remoulding of pumice and glass shards. A slow cooling rate of the dykes at the dyke contacts is inferred from remoulding textures, and is attributed to continuous feeding of the dyke. Seawater may have promoted softening and remoulding of the pumice because Na and K, if partially taken into the glass structure, can decrease the melting point of glass dramatically and reduce the viscosity of the shards. Remoulding may have occurred by continuous conversion of seawater in the peperite to steam, which is inferred to have enhanced transfer of Na and K from seawater to pumice glass.