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- W769718504 abstract "The lavas and pyroclastics of Ascension Island contain a suite ofcoarse grained igneous blocks which range in composition from olivine-gabbroto peralkaline-granite paralleling, but extending beyond thecompositional range of the volcanics. The lavas range from alkali-basaltthrough hawaiite, trachybasalt, trachyandesite and trachyte to comendite.True basalt is relatively rare and there is a scarcity of analyses with57 < Si02 < 63 wt %.No high pressure mineral assemblages and hence no possible mantlefragments have been found. Petrographic and isotopic data suggest thata suite of gabbros from Dark Slope Crater crystallised from a magma derivedfrom a MORB-like source. The remaining blocks and all the lavas evolvedfrom magmas derived from a less depleted source.The chemical variation seen in the lavas and blocks is best explainedby crystal fractionation mechanisms in a relatively shallow magma chamber.The gabbroic blocks exhibit cumulus textures suggesting that they formedby accumulation of settling crystals. The intermediate to acid blockscompare much more closely in composition with the evolved lavas and areprobably their slowly cooled equivalents. There is petrographic evidencethat partial melting of intermediate coarse grained material gave rise tomelts of granitic composition but these are not chemically equivalent tothe acid lavas and blocks. A pegmatoid body crystallised in situ and closedsystem crystal fractionation alone resulted in a very similar sequence ofmineral assemblages to the blocks and lavas and a peralkaline final liquid.High 87 S4/ 86 Sr ratios in the evolved lavas and blocks are attributable to contamination by a small quantity of highly radiogenic oceanic sediment.Comparison with other oceanic volcanoes suggests that these differentiation processes are much less important in determining the evolutionary path of the magma than its apparent starting composition." @default.
- W769718504 created "2016-06-24" @default.
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- W769718504 date "1982-01-01" @default.
- W769718504 modified "2023-09-23" @default.
- W769718504 title "Coarse-grained rocks of Ascension Island" @default.
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