At 10:45 AM 2000/08/03 -0500, Mike Holdaway wrote:
>...[good stuff about easy appearance of stable minerals and difficult
disappearance of unstable minerals with which I agree 100%]...
>...
>... I now suspect, as you guessed, Charlie, that the Picuris rocks are
>polymetamorphic... The earlier event was kyanite to sillimanite in the
>Al-rich quartzites. This took place at about 4.2 kbar, 525 deg C. so
>that staurolite-garnet-biotite-muscovite-graphite occurred without
>Al silicate in the nearby normal schists, but chloritoid-Alsilicate-
>hematite-ilmenite occurred without staurolite in the neighboring
>quartzites. Daniel et al. suggest that the widespread later event was
>an isothermal decompression related to crustal extension. It
>produced mainly andalusite which was locally developed especially near
>contacts between units at about 3.7 kbar, 525 deg C. ...
I wouldn't call this "polymetamorphic" - sounds like just a short segment of
the P-T path of a single cycle of heating and cooling.
>... Dugald, this would mean that even the Al-Mn schists
>you mentioned were still not very near the triple point, but were
>at higher T and perhaps lower P during the late stage metamorphism.
Mike, according to your paper, P-50A has Ky+Grt(sps39)+St+Mu+Bt+Qtz+... and
P-50B has And+Grt(sps39)+St+Mu+Bt+Qtz+..., but neither has Sil. They seem
to be from the same locality, "South of Honda Canyon". At "Honda Canyon"
(not more than 400m north, according to your map) are 6 occurrences of
Grt(sps4-9)+St+Mu+Bt+Qtz+-Pl. At "North Hondo Canyon" (not more than ~1 km
north) is PR-67Cl, with Sil+And+Grt(sps31)+Mu+Bt+Qtz+...
Accepting that stable minerals are easy to nucleate and grow, why is there
no Sil or And in P-50A (nor in any of 22 more remote occurrences of Ky only)
and no Sil in P-50B (nor in any of 12 more remote occurrences of Ky+And)?
My answer would be that the recorded part of your P-T path must in fact have
been very near the triple point (accepting Ky before Sil before And - on
Monday, Tuesday & Wednesday or so :-)). Accordingly, I still think your
Al,Mn-schist data support my two earlier contentions: (1) the
St+Mu+Chl=Als+Bt reaction is suppressed to the low-T side of the triple
point, and (2) the St+Mu=Grt+Als+Bt reaction is suppressed to the very near
vicinity of the triple point.
Cheers, Dugald
Dugald M Carmichael PhD PEng Phone/V-mail: 613-533-6182
Dept of Geological Sciences and Geological Engineering
Queen's University FAX: 613-533-6592
Kingston ON K7L3N6 E-mail: [log in to unmask]
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|