Felsic lavas or rheomorphic ignimbrites: is there a chemical distinction? |
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| Authors: | Linda A Kirstein Chris J Hawkesworth Frances G Garland |
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| Institution: | Faculteit der Aardwetenschappen, Vrije Universiteit, 1085 De Boelelaan, 1081 HV Amsterdam, The Netherlands,
Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK,
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| Abstract: | In the environment of felsic magma generation associated with continental flood basalt (CFB) provinces there is a close association between magma composition, phenocryst assemblage, temperature and eruption mechanism. In this paper we propose that by examining the chemical composition and, in particular the high field strength element (Zr and Nb) contents of the high silica rocks, we can identify those that contained halogen-rich volatiles and which degassed at different levels (deep versus shallow). The degassing depth has a direct influence on the type of eruption, with shallow degassing promoting explosive rather than effusive behaviour and consequentially the former is more likely to give rise to ignimbrites. Thus, we can infer likely eruption mechanisms, as high concentrations of F and Cl dramatically decrease magma viscosities and should favour deep degassing and hence promote lava effusion rather than explosive pyroclastic eruption. This hypothesis is tested by constraining the other possible variables which have an effect on the mode of eruption, and by examining the nature and composition of melt and fluid inclusions in quartz from CFB-associated felsic volcanic rocks. |
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