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The evolution of late-Hercynian granites and rhyolites documented by quartz – a review

Published online by Cambridge University Press:  01 March 2009

Axel Müller ,
Alfons M. van den Kerkhof ,
Hans-Jürgen Behr ,
Andreas Kronz  and
Monika Koch-Müller
Axel Müller
Affiliation:
Geological Survey of Norway, N-7491 Trondheim, Norway Email: Axel.Muller@ngu.no
Alfons M. van den Kerkhof
Affiliation:
Geowissenschaftliches Zentrum Göttingen, Goldschmidtstr. 3, D-37073 Göttingen, Germany
Hans-Jürgen Behr
Affiliation:
Geowissenschaftliches Zentrum Göttingen, Goldschmidtstr. 3, D-37073 Göttingen, Germany
Andreas Kronz
Affiliation:
Geowissenschaftliches Zentrum Göttingen, Goldschmidtstr. 3, D-37073 Göttingen, Germany
Monika Koch-Müller
Affiliation:
GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany
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Abstract

The potential of igneous quartz for providing a better understanding of magmatic processes is demonstrated by studying late-Hercynian rhyolites and granites from central and western Europe. Cathodoluminescence (CL) reveals growth patterns and alteration structures within igneous quartz reflecting the magma crystallisation history. The relatively stable and blue-dominant CL of zoned phenocrysts is principally related to variations in the Ti concentration, which is a function of the crystallisation temperature. The Al/Ti ratio of igneous quartz increases with progressive magma differentiation, as Ti is more compatible, compared to Al, Li, K, Ge, B, Fe, P during magma evolution. The red-dominant CL of the anhedral groundmass quartz in granite is unstable during electron bombardment and associated with OH- and H2O-bearing lattice defects. Thus, CL properties of quartz are different for rocks formed from H2O-poor and H2O-rich melts. Both groundmass and phenocrysts in granites are rich in alteration structures as a result of interaction with deuteric fluids during cooling, whereas phenocrysts in extrusive rocks do not usually contain such structures. The combined study of trace elements along with the analysis of quartz textures and melt inclusion inventories may reveal detailed PTX-paths of granite magmas. This study shows that quartz is a sensitive indicator for physico-chemical changes during the evolution of silicarich magmas. Common growth textures show a wide variety in quartz phenocrysts in rhyolites and some granites. This paper presents a classification of textures, which formed as a result of heterogeneous intra-granular lattice defects and impurities. The alternation of growth and resorption microtextures reflects stepwise adiabatic and non-adiabatic magma ascent, temporary storage of magma in reservoirs and mixing with more mafic, hotter magma. The anhedral groundmass quartz overgrowing early-magmatic phenocrysts in granites is free of growth zoning.

Keywords

adiabaticcathodoluminescencecrystal resorptiongrowth textureslate-Hercynian magmatismquartz
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 100 , Special Issue 1-2: Sixth Hutton Symposium on The Origin of Granites and Related Rocks , March 2009 , pp. 185 - 204
Copyright
Copyright © Royal Society of Edinburgh 2010

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