Pumiceous rhyolitic peperite in ancient submarine volcanic successions

Abstract

Pumiceous peperite is associated with a rhyolitic sill that intruded wet, unconsolidated, submarine stratified pumice breccia in the Cambrian Mount Read Volcanics, Australia. Other examples of pumiceous peperite described in this paper occur at the pumiceous base of a rhyolitic lava and the margins of a rhyolitic cryptodome complex in the Miocene Green Tuff Belt, Japan. Intervals of pumiceous peperite are thin ( 6 15 m), laterally discontinuous, massive, poorly sorted and the clast-to-matrix ratio varies significantly over short distances. They are composed of feldsparphyric tube pumice clasts and domains of clastic sediment that include stratified pumice breccia, pumiceous siltstone and bioturbated mudstone. The sediment domains may be massive or may display relic depositional structures. Sediment adjacent to pumice clasts is silicified, possibly reflecting induration on contact with hot pumice. Pumiceous peperite has gradational contacts with in situ and clast-rotated pumiceous hyaloclastite and coherent pumiceous rhyolite. Gradational relationships between pumiceous peperite and coherent pumiceous rhyolite, pumice clasts with chilled margins and curviplanar surfaces, and abundant jigsaw-fit texture imply that quench fragmentation and autobrecciation were the dominant mechanisms of clast formation. The formation of pumiceous peperite is favoured by a combination of low confining pressure and delayed quenching allowing vesiculation. Pumiceous peperite may easily be misinterpreted or overlooked as it resembles other pumice-rich facies that are common in submarine volcanic successions. The correct identification of pumiceous peperite can provide evidence to constrain the timing of intrusive episodes in volcanic successions. ß

Magma–sediment interaction during the emplacement of syn-sedimentary silicic and mafic intrusions and lavas into and onto Triassic strata (Circum-Rhodope Belt, northern Greece)

“…Other processes such as fallout of juvenile pyroclasts into unconsolidated sediment, water-settling of juvenile pyroclasts contemporaneous with deposition of other sediments, resedimentation of volcaniclastic deposits by mass flows, and infiltration of sediment into volcaniclastic deposits can all produce mixtures of igneous clasts and sediment matrix that resemble peperite (cf. Branney & Suthren 1988;White et al 2000;Gifkins et al 2002).…”

“…The major characteristics of the studied peperites that are assumed to be key criteria (cf. Goto & McPhie 1998;Hunns & McPhie 1999;Gifkins et al 2002;Skilling et al 2002;Squire & McPhie 2002;Agnew et al 2004) for their interpretation with the sense that hot magma interacted with wet unconsolidated sediment are the following.…”

“…Localized silicification is represented by chert-like quartz grain aggregate along the rhyolite/sediment interface. The subtle, gradational colour change and local silicification of the sediment are interpreted as results of thermal modification of the sediment (baking) in contact with hot rhyolite (McPhie 1993;Hunns & McPhie 1999;Gifkins et al 2002).…”

“…Bleaching and silicification of the host sediment at the contact with the rhyolite clasts are inter- preted to reflect baking and compositional modification of the host sediment, as a result of heat and magmatic fluids rich in SiO 2 released from the rhyolite (cf. Hunns & McPhie 1999;Gifkins et al 2002). It is a reaction rim at the rhyolite clast/sediment matrix contact and is formed by interaction of the hot magma and wet sediment during the formation of peperite.…”

Architecture of Upper Cretaceous rhyodacitic hyaloclastite at the polymetallic Madneuli deposit, Lesser Caucasus, Georgia

“…Like lobe-Hyaloclastite flow at Noranda [51], rhyolitic lobes at Blahnukur are characterized by a massive, typically columnar-jointed glassy interior, flow-banded border zone, and in situ brecciated glassy selvage, which are similar to glass-like selvages described in the Madneuli open pit. There are other analogue examples from submarine settings, in submarine lava flow-dome complex, such as in Ponza in Italy [48], in the Early Devonian Ural volcanic rocks [52], pumiceous rhyolitic peperite, which is associated with a rhyolitic sill which intruded a wet, unconsolidated, submarine pumice breccia in the Cambrian Mount Read volcanic rocks in Australia [53] and an example of silicic intrusion-dominated volcanic center at Highway-Reward, Australia [17].…”