Ignimbrites of basaltic andesite and andesite compositions from Tanna,New Hebrides Arc |
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Authors: | Claude Robin Jean-Philippe Eissen Michel Monzier |
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Institution: | (1) ORSTOM, UR 1F, BP 76, Port Vila, Vanuatu, and Department of Geology, Mines and Water Resources of Vanuatu, F-29280 Plouzané, France, FR;(2) ORSTOM – UR-1F, B.P. 70, F-29280 Plouzané, France, FR |
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Abstract: | Tanna island is part of a large volcanic complex mainly subsided below sea-level. On-land, two series of hydroclastic deposits
and ignimbrites overlie the subaerial remains of a basal, mainly effusive volcano. The ‘Older’ Tanna Ignimbrite series (OTI),
Late Pliocene or Pleistocene in age, consists of ash flows and ash- and scoria-flow deposits associated with fallout tephra
layers, overlain by indurated pumice-flow deposits. Phreatomagmatic features are a constant characteristic of these tuffs.
The ‘younger’ Late Pleistocene pyroclastics, the Siwi sequence, show basal phreatomagmatic deposits overlain by two successive
flow units, each comprising a densely welded layer and a nonwelded ash-flow deposit. Whole-rock analyses of 17 juvenile clasts
from the two sequences (vitric blocks from the phreatomagmatic deposits, welded blocks, scoriaceous bombs and pumices from
the ignimbrites) show basaltic andesite and andesite compositions (SiO2=53–60%). In addition, 296 microprobe analyses of glasses in these clasts show a wide compositional range from 51 to 69% SiO2. Dominant compositions at ∼54, 56, 58.5 and 61–62% SiO2 characterize the glass from the OTI. Glass compositions in the lower – phreatomagmatic – deposits from the Siwi sequence
also show multimodal distribution, with peaks at SiO2=55, 57.5, 61–62 and 64% whereas the upper ignimbrite has a predominant composition at 61–62% SiO2. In both cases, mineralogical data and crystal fractionation models suggest that these compositions represent the magmatic
signature of a voluminous layered chamber, the compositional gradient of which is the result of fractional crystallization.
During two major eruptive stages, probably related to two caldera collapses, the OTI and Siwi ignimbrites represent large
outpourings from these magmatic reservoirs. The successive eruptive dynamics, from phreatomagmatic to Plinian, emphasize the
role of water in initiating the eruptions, without which the mafic and intermediate magmas probably would not have erupted.
Received: February 19, 1993/Accepted October 10, 1993 |
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Keywords: | : Island arc volcanism – New Hebrides – Ignimbrite – Phreatomagmatic deposits – Basaltic andesite – Glass composition – Crystal fractionation |
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