The record of magma chamber processes in plagioclase phenocrysts at Thera Volcano,Aegean Volcanic Arc,Greece |
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| Authors: | Karen Stamatelopoulou-Seymour Dimitrios Vlassopoulos Thomas H Pearce Craig Rice |
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| Institution: | (1) Department of Geology, Concordia University, H4B 1R6 Montréal, Québec, Canada;(2) Department of Geological Sciences, McGill University, H3A 2A7 Montréal, Québec, Canada;(3) Department of Gological Sciences, Queen's University, K7L 3N6 Kingston, Ontario, Canada |
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| Abstract: | Lavas and pyroclastic rocks throughout the volcanic stratigraphy of the Tertiary-Quaternary volcanic complex of Thera in the Aegean island arc display inhomogenous plagioclase populations and phenocryst resorption textures, interpreted as indicative of magma mixing. Plagioclase zoning characteristics studied by Nomarski and laser interferometry techniques establish three main categories of plagioclase: (i) inherited plagioclase (nucleated in endmember prior to initial mixing event) (ii) in situ plagioclase (nucleated in mixed or hybrid magma) and (iii) xenocrystic plagioclase. Nomarski contrast images and linearized compositional zoning profiles reveal striking differences between calcic and sodic plagioclases, depending on the composition of the lava in which they are hosted. These differences reflect the contrasting effects of changes in physical-chemical parameters in basic vis-a-vis more acidic melts during magma mixing and/or influx of new magma into the subvolcanic magma chamber, as well as the influence of magma chamber dynamics on plagioclase equilibration. Variations in bulk major and trace element abundances of Thera volcanic products reflect the dominant overprint of crystal fractionation, but decoupling between major and trace element fractionation models and variations in incompatible trace element distributions are all indicative of magma mixing processes, consistent with compositional and textural zoning in plagioclases. |
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