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Characteristics and origin of peperite involving coarse-grained host sediment


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Squire, RJ and McPhie, J 2002 , 'Characteristics and origin of peperite involving coarse-grained host sediment' , Journal of Volcanology and Geothermal Research, vol. 114, no. 1-2 , pp. 45-61 , doi: 10.1016/S0377-0273(01)00289-X.

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Peperite involving basalt and polymictic volcanic conglomerate occurs in the Pliocene Ba Volcanic Group at
Yaqara in northern Viti Levu, Fiji. Because the host sediment is coarse-grained and dominated by basalt clasts, the peperite could be easily overlooked and mistaken for another coarse volcaniclastic facies. However, the presence of groups of basalt clasts that show jigsaw-fit texture, fluidally shaped basalt clasts with complete glassy margins,
gradational contacts with adjacent sedimentary facies and the absence of stratification indicate that molten basalt
mingled with unconsolidated gravel. Using these criteria, we show that other superficially similar, coarse, polymictic
facies with fluidal basalt clasts are not peperite. Both blocky and fluidal basalt clasts occur together in the peperite.
The amoeboid basalt clasts in the fluidal peperite result from dismembering of ductile, low-viscosity, relatively hot
magma. At this stage, propagating magma lobes were probably insulated from direct contact with the wet sediment by
a vapour film. The angular, polyhedral basalt clasts in the blocky peperite indicate brittle disintegration of somewhat
cooler, higher-viscosity magma. The presence of jigsaw-fit texture and polyhedral clasts with glassy margins suggest
that quench fragmentation of the basalt was important in the formation of the blocky peperite. Although there is no
positive evidence for steam explosivity, the presence of steam could be recorded by small quartz-filled cavities that
occur within the host sediment. The co-existence of fluidal and blocky basalt clasts is interpreted to reflect successive ductile then brittle fragmentation of intruding magma. The change from fluidal to blocky peperite might have resulted from progressive cooling of the magma during intrusion, and also from the breakdown of fluidisation when the limited supply of fine sediment in the host gravel was exhausted.

Item Type: Article
Authors/Creators:Squire, RJ and McPhie, J
Keywords: basalt-gravel peperite; basaltic sill; polymictic volcanic conglomerate; submarine volcanic succession; Fiji; Pliocene
Journal or Publication Title: Journal of Volcanology and Geothermal Research
ISSN: 0377-0273
DOI / ID Number: 10.1016/S0377-0273(01)00289-X
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