How extensive are subsolidus grain-shape changes in cooling granites?

R. H. Vernon*, S. R. Paterson

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

23 Citations (Scopus)

Abstract

The hypothesis that granites (sensu lato) commonly undergo extensive grain-shape changes ("recrystallization") during cooling is not supported by microstructural evidence, especially the abundance of non-truncated zoning patterns in mineral grains. Regional metamorphic felsic gneisses that have undergone extensive neocrystallization/recrystallization at amphibolite facies conditions (which some have suggested are equivalent to those in slowly cooling granites) typically lose igneous microstructures, including oscillatory zoning, and are characterized by polygonal to irregularly xenoblastic grains of quartz and feldspar, as well as by rounded inclusions of quartz in feldspar and vice versa. In contrast, granites typically show oscillatory zoning (especially in plagioclase), elongate, euhedral to subhedral feldspar grains, and euhedral plagioclase inclusions in quartz and K-feldspar. The contrast between these two situations is probably the result of felsic gneisses undergoing chemical reactions and regional deformation, especially in the presence of water released pervasively from dehydration reactions (which are strong driving forces for the production of new grains), whereas the minerals in cooling granites are chemically compatible, without strong deformation, and so do not react with each other or recrystallize. Therefore, massive granites without solid-state foliations generally do not undergo grain-shape changes, owing to inadequate driving forces, as well as to decreasing temperature. An important implication of this conclusion is that internal structures formed during chamber construction and magmatic flow in granite plutons are not likely to be removed by subsolidus intergranular microstructural changes.

Original languageEnglish
Pages (from-to)42-50
Number of pages9
JournalLithos
Volume105
Issue number1-2
DOIs
Publication statusPublished - Sep 2008

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