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Mineralogical and Chemical Zonation of the Laacher See Tephra Sequence (East Eifel, W. Germany)

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Wörner, Gerhard; SCHMINCKE, H. -U.
Journal of Petrology
The late Quaternary Laacher See Tephra (LLST, MLST, ULST: Lower, Middle, Upper Laacher See Tephra) of the East Eifel volcanic field (W. Germany) is zoned from highly evolved, volatile-rich and crystal-poor phonolite at its base towards a mafic, crystal-rich phonolite at the top of the deposit. This is shown by phenocryst abundances, major and trace element chemistry of whole rocks, matrix-glass and some mineral compositions. MgO content in whole rocks ranges from 0.07 wt. per cent in lower LLST to 0.85 wt. per cent in ULST phonolite. Late mafic hybrid pyroclasts contain up to 7.0 wt. per cent MgO. Na2O shows a reverse trend from 11.7 wt. per cent in LLST to 5.1 per cent in ULST. Trace elements are 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 minimum concentrations at an intermediate stratigraphic level (MLST). All compositional gradients are smooth showing a major compositional interface between LLST/MLST and ULST. Twelve phenocryst phases occur: sanidine, plagioclase, hauyne, amphibole, clinopyroxene, sphene, apatite, Ti-magnetite, biotite, nepheline, cancrinite and zircon. The latter three are restricted to LLST phonolite. There is a steep gradient in mineral abundances from the LLST and MLST to the mafic ULST phonolite. Microprobe analyses define two phenocryst populations: (a) Fe, Na and Mn-rich evolved phenocrysts (hastingsite, aegirine-augite, Ab-rich plagioclase) which predominate within highly differentiated (LLST) phonolite; (b) Mg and Ti-rich and Fe, Na, Mn-poor phenocrysts, which are most abundant in ULST. This compositional zonation is interpreted as the result of continuous eruption from a zoned magma column; highly evolved early magma representing the upper part and mafic late phonolitic magma representing the lower part of a stratified magma chamber emplaced at shallow, crustal levels.


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