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Subject:

Re: epidote symplectite

From:

Eric Essene <[log in to unmask]>

Reply-To:

Metamorphic Studies Group <[log in to unmask]>

Date:

Wed, 15 Oct 2003 20:49:48 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (49 lines)

Keith,
     It is difficult to make epidote-quartz by decompression of
plagioclase-bearing rocks.  Epidote could form by isobaric cooling
but also requires oxidation unless it is forming from hematite.
eric


>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
>***************************************
>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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