Hi All,
Dougald Carmichael has made three suggestions regarding the Picuris rocks,
two of which appear reasonable to me. Here are Dougald's suggestions:
(1) The Picuris rocks are not polymetamorphic but simply represent a
clockwise P-T path with isobaric heating followed by isothermal
decompression. I looked over the Daniel et al. (1992) abstract again, and I
can see no reason why it couldn't all be part of the same event. In fact
their Gibbs method analysis suggests that it probably was all a single
event, and that would be consistent with my observations. (I stopped
working, and thinking, about these rocks in the early 1990's, and while I
was interested in their work, I didn't follow up on it. As a result, I
haven't integrated their work into my thinking very well.) It is also
possible that some of the sillimanite was formed during the decompression.
However, much of the sillimanite appears to be better foliated than the
andalusite.
(2) These rocks probably were in fact very close to the triple point. I
assume you are familiar with Grambling's (1981 Am. Min., p. 702-722) paper
where he thought he could see the triple point in the rocks of the Truchas
Range. This concept could be affected by the Daniel et al. (1992) result.
I am only familiar with the Picuris Range. John Goodge and I had a more
recent paper (Holdaway and Goodge, Am. Min., 1990, p. 1043-1058) which has
quite a bit more on the combinations of Al silicates in the Picuris and how
it varies with rock type. (I suspect the rock pressure vs. fluid pressure
concept could also be negated by Daniel et al's work). If you combine the
1978 and 1990 papers by approximate count, I get:
Ky 26 specimens
Ky + And 14 specimens
Ky + Sil 15 specimens
Ky + Sil + And 3 specimens
Sil 3 specimens
Sil + And 7 specimens
And 6 specimens
To put it another way, 58 specimens have Kya, 28 have Sil, and 30 have And.
Out of 74 specimens only the 14 Ky + And specimens have the possibility of a
P-T path at T below the triple point. I don't doubt that the specimens were
close to the triple point, as my earlier analysis suggests (actually about
20 deg above it), but there are several possible reasons why this apparently
random mix of assemblages could occur without being right on, or at T below,
the triple point: (1) SMALL local variations in P and T from specimen to
specimen; (2) a specimen didn't overstep the equilibrium enough OR the
change in P was too fast for nucleation of a phase or two; (3) one or two
phases could have totally reacted away in some specimens due perhaps to
greater access to fluids; (4) the breakdown of staurolite occurred in the
sillimanite and/or andalusite field for some specimens such as some of the
Al-Mn-rich schists; (5) fluid was present only locally at particular stages
along the P-T path (also see Goodge and Holdaway, J. Pet., 1995, p.
1229-1250). I think this last point may be the most important, and explains
why many specimens contain only kyanite or kyanite and sillimanite.
(3) Some of the Mn-rich rocks could have formed at T below the triple point.
For the reasons given above I don't see any reason why any of the rocks
might have experienced a peak-T below the triple point T. The enrichment in
Mn because of more oxidizing conditions did lead to a substantial lowering
of the staurolite breakdown (Holdaway, 1978, p. 1413), but I suspect that
the Al-Mn rich rocks did not experience T lower than any other rocks in the
region. And keep in mind that there is plenty of sillimanite. I prefer to
think that the 14 rocks that have kyanite and andalusite and the Al-Mn rocks
that have kyanite or andalusite actually passed through a sliver of the
sillimanite field either too rapidly, without fluid present, or while
staurolite was still stable, depending on the specimen.
If you are still reading I'll buy you a coke....
Mike Holdaway
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