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Subject:

Amphibole textures and composition

From:

James Cleverley <[log in to unmask]>

Reply-To:

James Cleverley <[log in to unmask]>

Date:

Fri, 18 Feb 2000 13:56:09 +0000

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Hi Manuel and everyone,

In reply to Manuel’s question concerning amphibole textures and
relationships - Yes this is interesting - not least of all because it is
very familiar to me.  I am working on two intrusive suites from the
Antarctic Peninsular, in particular comparing the dioritic - qtz-dioritic
units from both.  The mineralogy of the rocks is typified by plagioclase
(An80 to An50), amphibole (Mg-hbl - Act), biotite, quartz and k-feldspar.

The amphibole compositions in my samples all show a very similar story :-

a) Na enriched amphiboles, not pargasite (hbl in fact) but on the border
(Na 0.5pfu).  These amphiboles are a product of the replacement of a
pyroxene (cpx) phase w/ small ~2% Jadeite component.  In rocks with only
where the fluid flow was limited relict cpx can be seen being replaced my
Mg-hbl.

b) Mg-hbl as direct crystallisation from melt - Most common composition

and a spread of compositions between hbl to actinolite, crossing from melt
to sub-solidus dominated processes. 

The progressive increase in Si to go from hbl to act is a function of
decreasing temperature and/or pressure (e.g. hbl geothermometer).  So as
you can see we look at very similar amphibole compositional ranges.

If the problem is 'how to get paragasite a lower grades?' - well we can see
from the mineralogy of your rocks that cpx disappears at the lower grades
and pargasite appears - so locally replacement of the cpx gives rise to
pargasite stability and else where to mg-hbl at the same time.  What is
pargasite - a Na enriched hbl - OK with a bit of Si charge balance etc. etc. 

If the problem is how to get the assemblage act-hbl-prg in equilibrium?
Why does patchy zonation mean equilibrium, I would have assumed the
opposite??  What you are probably seeing is a locally controlled supply of
certain components with no thin section/hand specimen scale
re-equilibration.  I have seen lots of evidence for sub-microscopic
fractures (may be now annealed) controlling the distribution of amphibole
compositions.  Patches can also often be developed where different silicate
phases are in contact with the amphibole grain boundary above or below the
thin section plane.

Do you have any estimates to how much fluid has flowed through these rocks?
 Is it different in the rocks with cpx present relative to the prg rocks?

Any comments??

Cheers

James Cleverley

Fluid-Rock Interaction Group
School of Earth Science
University of Leeds
Leeds
LS2 9JT

0113 2335257
http://earth.leeds.ac.uk/~jsc/


>Hello to everyone:
>
>I'm Manuel and new in the list. I'm a PhD student working on regional
>metamorphism and variscan deformation in the Iberian Massif. Part of my
>investigation deals with prograde metamorphism of an amphibolitic unit.
>Amphiboles from the highest zone (with Cpx) are Mg-Hornblende. As we move
>towards lower grade zones, amphibole composition widens towards both
>actinolite and pargasite, which seems quite extrange. Amphibole crystals
>from this zone appear with a patchy zonation (Act-Hbl-Prg) which suggest
>they're in equilibrium. Is it possible to get this type of texture and
>Pargasite composition with decrease in metamorphic grade? I've been looking
>in the literature and haven't found a case like this. Does anyone know about
>similar cases around the world? I will appreciate some help on this matter.



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