Dear Ben, Antonio and all other respondents,

I take this opportunity to thank all of you for your interest in the topic. Your suggestions have given me a lot of clarity on how to approach the problem.

I wish all of you Merry Christmas and a very Happy New Year, 2015.

Santanu 

----- Original Message -----
From: "Ben Harte" <[log in to unmask]>
To: "Metamorphic Studies Group" <[log in to unmask]>, [log in to unmask]
Cc: [log in to unmask]
Sent: Sunday, December 21, 2014 6:31:48 PM
Subject: Re: [geo-metamorphism] Rock Nomenclature

Dear Santanu and other respondents,

Thinking about a name or names for your rock, I feel it is important  
to distinguish between two aspects:
1) Giving a rock a straightforward descriptive (non-genetic) name;
2) Assigning a name indicating a metamorphic facies.

'Amphibolite' and 'eclogite' have clear meanings in the first sense,  
in that they indicate a specific mineral content, but they are also  
used in the second sense to indicate facies based on their particular  
mineral assemblages.
As Santanu points out, the names 'amphibolite' and 'eclogite' cannot  
be used in this case because the rock does not have the correct  
mineral content for either name.

BUT this does not preclude the rock being assigned either to the  
amphibolite facies or to the eclogite facies - whichever is the most  
appropriate on the basis of mineral assemblage, mineral compositions,  
and whatever mineral assemblages occur in rocks closely associated  
with it in the field. Many of the rocks in the Scottish Highlands, New  
England and the European Alps, belong to the amphibolite facies but  
are not amphibolites.

Actually assigning a facies to this particular rock is clearly  
problematic as Santanu recognises and it would seem to be complicated  
by the fact that it is to some extent overprinted or polymetamorphic.  
Perhaps it would be more useful to define a P-T path for the rock  
rather than a specific metamorphic facies.

HOWEVER, GIVING THE ROCK A DESCRIPTIVE NAME SHOULD BE STRAIGHTFORWARD.
It may be simply named as a garnet-hornblende-epidote-phengite "rock",  
where the word "rock" is replaced  by either: "schist" or "gneiss" or  
"granofels", according to the principal texture/structure of the rock.  
This follows the guidelines in the IUGS Subcommission on the  
Systematics of Metamorphic Rocks (SCMR) referred to by Pablo Diego  
Gonzalez. The prefix minerals included in the rock name should of  
course reflect the principal minerals in the rock.

regards,

Ben Harte


Quoting Santanu Kumar Bhowmik <[log in to unmask]> on Wed, 17  
Dec 2014 20:42:40 +0530:

> Dear Colleagues,
>
> I am seeking your suggestions on the nomenclature of a metamorphic  
> rock with the assemblage  
> garnet+hornblende+epidote+phengite+rutile+quartz. This assemblage  
> which became stable during peak metamorphism yielded P-T estimate of  
> ~ 15-16 kbar,~ 590-610 °C. If this assemblage is evaluated with  
> metamorphic rocks that are related to metamorphism during subduction  
> initiation, the closest rock name that comes to my mind is  
> garnetiferous amphibolite. However, in the strictest sense, the  
> rock, in the absence of plagioclase cannot be an amphibolite, if we  
> use amphibolite in facies sense. At the same time, the estimated P-T  
> range lies in the eclogite facies, also supported in part by  
> complete elimination of plagioclase. However, with omphacitic  
> clinopyroxene being absent, the rock is also not an eclogite in the  
> strictest sense. One additional information in this regard is that  
> Na in the protolith is completely partitioned in Hbl, which shows  
> substantial presence of Na in the M4 site, apart from its general  
> occurrence in the A site.
>
> Your comments and observations are welcome.
>
> With regards,
>
> Santanu
> -------------------------------------------------------
> Dr. Santanu Kumar Bhowmik
> Professor
> Department of Geology & Geophysics
> Indian Institute of Technology
> Kharagpur-721 302
> India
> Mobile:+919434041791
> Telephone:+91-3222-283390(O), 283391, 279766(R)
>
>



Ben Harte (Prof)
Grant Institute of Earth Sciences,
School of GeoSciences, University of Edinburgh,
Edinburgh EH9 3FE.
Tel: [44] (0)131 651 7220; Fax: [44] (0)131 668 3184

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