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Petrogenesis of the Zoned Laacher See Tephra

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Type of Publication:
Wörner, Gerhard; SCHMINCKE, H. -U.
Journal of Petrology
The late Quaternary Laacher See phonolitic tephra deposit (East Eifel, W. Germany) is mineral-ogically and chemically zoned from highly evolved, volatile-rich and crystal-poor at its base towards a mafic, crystal-rich phonolite at the top (Wörner & Schmincke, 1984). This zonation is interpreted as the result of a continuous eruption from a zoned magma column. Major and trace element evidence shows that the last erupted mafic ULST (Upper Laacher See Tephra) phonolite can be derived from a basanite parent magma via fractional crystallization of 30 per cent clinopyroxene, 24 per cent amphibole, 4 per cent phlogopite, 3.8 per cent magnetite, 2.5–3.0 per cent olivine and 1 per cent apatite, leaving a derivative of 30 per cent evolved magma.Starting from the mafic (ULST) phonolite as a parent, the zoned sequence is postulated to have been formed by progressive fractional crystallization of the observed phenocryst phases. This model was tested by a series of 7 step-by-step mass balance fractionation calculations. Abundance, modal composition and relative variations of calculated fractionated phases agree well with the observed phenocryst abundances: sanidine followed by plagioclase and minor amounts of mafic phases are to be fractionated to give the observed zoned sequence.The most evolved phonolite, however, cannot be generated by subtraction of phenocrysts from the underlying phonolite. Processes such as liquid-state differentiation may therefore have chemically modified the upper part (cupola) of the Laacher See magma column subsequent to crystal fractionation.The erupted phonolite magma (5.3 km3) was calculated to have started with a volume of 56 km3 of parental basanite magma which fractionated to form 16.6 km3 of mafic phonolite. This magma further differentiated to give a 5.3 km3 zoned (erupted) phonolite column. The non-erupted volume of 50 km3 is postulated to form a cooling cumulate body below the present day Laacher See volcano.The Laacher See magma system represents a complex end-member type of a highly evolved small volume composition ally zoned magma chamber with steep major and trace element gradients, the uppermost volatile rich magma layer resembling the stable roof part of rhyolitic chambers.


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