Submarine ‘crystal tuffs’: their origin using a Lower Devonian example from southeastern Australia

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Abstract
Summary. The submarine crystal-rich volcaniclastics of the Lower Devonian Merrions Tuff contain concentrations of angular to euhedral volcanic quartz, plagioclase and orthoclase. Lithic fragments, largely altered vitriclasts, are minor components and the average matrix content is 37.6%. Associated dacite/andesite and rhyodacitic lavas have average groundmass contents of 64.2%. Rare shards in the graded, pelitic tops of the thick volcaniclastic sedimentation units suggest that the mode of fragmentation originally was by explosive magmatic eruptions. The sedimentology of the volcaniclastics suggests subsequent redeposition by cold-state mass-flow processes. The volcaniclastics form an isotopically coherent suite and so redeposition must have occurred essentially contemporaneously with eruption.The high crystal fragment concentration in these volcaniclastics is higher than lavas and ignimbrites and suggests some process whereby the groundmass fraction of the erupting magma is selectively removed, so concentrating the crystal fraction. The crystal-rich character of the crystal-tuffs is not simply due to explosive eruption. Several primary and secondary factors/processes could have interacted to ultimately produce the crystal-rich character, these being: (i) eruption of highly crystallized magmas (≤ 65% phenocrysts), (ii) concentration of crystals in primary eruption columns, (iii) concentration of crystals in any resulting pyroclastic flows, fine vitric ashes being elutriated out and being carried away in accompanying, trailing ash clouds, (iv) concentration of crystals in secondary eruption columns generated by the flow of hot pyroclastic flows into the ocean, and (v) concentration of crystals by the elutriation of fines into the trailing fine sediment cloud accompanying submarine mass flows resulting from the slumping of volcaniclastic aggregates from shallow marine/subaerial settings.