Hello to all in this group from someone who joined recently, and who is more
of a structuralist, with a growing interest in metamorphic petrology. I
think you may be able to help me on a question that arises from some of my
recent work in a granite-gneiss complex that crops out in the Late Archean
Abitibi granite-greenstone belt, Ontario. I will briefly synthesize.
The amphibolite-grade tonalite gneiss and a large tonalite-granodiorite
batholith that make up a large, refolded structural dome have a common
foliation within them that likely formed as a result of flattening during
the doming. In the gneiss, it is clearly a solid state tectonite fabric, and
in the batholith it is a magmatic state fabric. That foliation was folded by
regional-scale folds as the batholith was crystallizing and fractionating.
Within that doming-related (earlier) foliation, in the gneiss and in the
plutons, there is a very common occurrence of mm-scale epidote-quartz
symplectites. They occur in grain contact with plag and qtz and also (but
less often) with Kf, hbl, mt and titanite. Following the advice of an
esteemed member of the metamorphic community, I am interpreting the
symplectites to represent a solid state retrograde feature.
I also interpret they indicate a fairly rapid decompression, related to
exhumation in the hinge zones of the regional scale refolds of the dome.
My question is, do you know of other similar examples? I have looked through
the literature fairly thoroughly (I think) and I have not found
documentation of the same textures in similar rocks. This is clearly not a
standard hydrothermal greenschist grade overprint such as is found in many
Archean plutons.
Any hints where I might find such textures and such an assemblage
documented?
Thanks to all who may respond. keith
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Keith Benn, Associate professor
Department of Earth Sciences, University of Ottawa
140 Pasteur Street, Ottawa, ON K1N6N5, Canada
office tel: +1-613-562-5800 x6858
lab tel: +1-613-562-5800 x6833
dept fax: +1-613-562-5192
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