Origin and evolution of a peraluminous silicic ignimbrite suite: The Violet Town Volcanics |
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| Authors: | John D Clemens Victor J Wall |
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| Institution: | (1) Earth Sciences Department, Monash University, 3168 Clayton, Victoria, Australia;(2) Present address: Chemistry Department, Arizona State University, 85287 Tempe, AZ, USA |
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| Abstract: | The Violet Town Volcanics are a 373 Ma old, comagmatic, S-type volcanic sequence mainly comprising crystal-rich intracaldera ignimbrites. Rock types vary from rhyolites to rhyodacites, all containing magmatic cordierite and garnet phenocrysts. Variation in the suite is primarily due to fractionation of early-crystallized quartz, plagioclase and biotite (plus minor accessory phases) in a high-level magma chamber prior to eruption. Early magmatic crystallization occurred at around 4 kb and 850° C with melt water contents between 2.8 and 4 wt.%. This high-temperature, markedly water-undersaturated, restite-poor, granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain, leaving a granulite facies residue.Table of Less Common Abbreviations Used
Pkb
pressure in kilobars
- T° C
temperature in degrees Celsius
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mole fraction of water in the fluid
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aH2O
activity of water
- Bi
biotite
- Cd
cordierite
- Gt
garnet
- Py
pyrope
- Gr
grossular
- Alm
almandine
- Sp
spessartine
- He
hercynite
- Ilm
ilmenite
- Kfs
potassium feldspar
- Opx
orthopyroxene
- Pl
plagioclase
- An
anorthite
- Q
quartz
- Sill
sillimanite
- Ap
apatite |
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| Keywords: | |
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