At 12:45 PM 07/06/2005 +1000, you wrote:
>Eric
>We deliberately do not work with calc pelites, for this type of work, to
>avoid exactly the problems that you mention. Every time a differentiated
>crenulated cleavage forms (this occurs in most deformations and, indeed,
>shuts down porphyroblast growth -if you have any queries about that read
>Bell & Hayward, 1991 (JMG) and Bell et al 2004 (JSG) first and then fire
>away), there is significant volume loss due to the dissolution of non platy
>or linear mineral phases, which shifts the bulk composition from the time
>when the porphyroblasts grew.
Hi Tim,
What I write below is meant to be restricted to using phase diagram
sections for determining garnet-growth conditions.
These are very important points you make; it has caused some headaches for
me. However, dissolution-mobilization-regrowth of plagioclase can also be a
blessing in disguise in some cases, can't it. If the rate of plagioclase
growth = rate of plagioclase dissolution, and
Grt_porph_diameter/compositional_layering_width is high, then plagioclase
dissolution-regrowth may actually be required in order to make approaching
equilibrium a possibility (in uppermost greenschist-amphibolite facies at
least). Of course you are right about significant volume loss, if the
volume loss is measured with respect to a hypothetical chemical equilibrium
domain. The problem is knowing what that garnet equilibrium domain is.
Regarding steering clear of calc-pelites: do you define any pelite with
plagioclase a calc-pelite? If you steer clear of plagioclase-bearing
pelites, how is one to be certain that the reason the pelite does not
currently have plagioclase is because of the reasons you describe
(plagioclase component volume loss)? It seems that in rocks that many would
call a "normal pelite", plagioclase/albite actually is a phase present at
garnet nucleation T's at moderate-low crustal P's. I sometimes worry about
working with pelites that don't have plagioclase, for fear of the volume
loss you describe.
Cheers,
Doug
>We certainly take into account Mn. Our pseudosections are most commonly
>calculated in the NCMnKFMASH system.
>
>Cheers
>Tim
>
>Prof. T.H. Bell
>School Earth Sciences
>James Cook University
>Townsville
>Qld 4811
>Australia
>Work Phone +61-7-47814766
>Work Fax +61-7-47251501
>Home Phone+61-7-47732534
>Email [log in to unmask]
>http://www.es.jcu.edu.au/dept/Earth/research/samri/index.html
Doug Tinkham
Dept. of Geology and Geophysics
University of Calgary
2500 University Drive NW
Calgary, Alberta T2N-1N4 Canada
Ph:403-220-8209
e-mail: [log in to unmask]
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