Mineralogical and Chemical Zonation of the Laacher See Tephra Sequence (East Eifel, W. Germany) |
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| Authors: | WORNER G; SCHMINCKE H-U |
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| Institution: | Institut für Mineralogie, Ruhr-Universität Bochum Postfach 102148, D-4630 Bochum, Germany |
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| Abstract: | The late Quaternary Laacher See Tephra (LLST, MLST, ULST: Lower,Middle, Upper Laacher See Tephra) of the East Eifel volcanicfield (W. Germany) is zoned from highly evolved, volatile-richand crystal-poor phonolite at its base towards a mafic, crystal-richphonolite at the top of the deposit. This is shown by phenocrystabundances, major and trace element chemistry of whole rocks,matrix-glass and some mineral compositions. MgO content in wholerocks ranges from 0.07 wt. per cent in lower LLST to 0.85 wt.per cent in ULST phonolite. Late mafic hybrid pyroclasts containup to 7.0 wt. per cent MgO. Na2O shows a reverse trend from11.7 wt. per cent in LLST to 5.1 per cent in ULST. Trace elementsare divided into depleted (compatible) elements (e.g. Sr, Sc,Co), and enriched (incompatible) elements (e.g. Zn, Zr, Nb).‘Semi-compatible’ elements (Ta, Y) show minimumconcentrations at an intermediate stratigraphic level (MLST).All compositional gradients are smooth showing a major compositionalinterface between LLST/MLST and ULST. Twelve phenocryst phasesoccur: sanidine, plagioclase, hauyne, amphibole, clinopyroxene,sphene, apatite, Ti-magnetite, biotite, nepheline, cancriniteand zircon. The latter three are restricted to LLST phonolite.There is a steep gradient in mineral abundances from the LLSTand MLST to the mafic ULST phonolite. Microprobe analyses definetwo phenocryst populations: (a) Fe, Na and Mn-rich evolved phenocrysts(hastingsite, aegirine-augite, Ab-rich plagioclase) which predominatewithin highly differentiated (LLST) phonolite; (b) Mg and Ti-richand Fe, Na, Mn-poor phenocrysts, which are most abundant inULST. This compositional zonation is interpreted as the resultof continuous eruption from a zoned magma column; highly evolvedearly magma representing the upper part and mafic late phonoliticmagma representing the lower part of a stratified magma chamberemplaced at shallow, crustal levels. |
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