Gordon, Dave, and others,
If you consider the petrogenetic grid of Berman, the assemblage
chloritoid-sillimanite is quite all right. As Dave implies, there is plenty
of evidence that the chloritoid-sillimanite assemblage is stable in the
Picuris Mountains of New Mexico. Holdaway (1978, GSA Bulletin, p. 1404)
gives an explanation for the fact that chloritoid-sillimanite is possible,
but chloritoid-kyanite and chloritoid-sillimanite are much more common. In
a relatively pure aluminous, feruginous quartzite, chloritoid-sillimanite is
possible under conditions at T a little above the triple point and a
metamorphic fluid of pure water.
Along with the aluminous, feruginous quartzites in the Picuris Mountains
are staurolite- andalusite, staurolite-kyanite and staurolite-sillimanite
schists, but the most common schist is a simple
staurolite-muscovite-biotite-garnet-ilmenite-graphite-quartz schist. These
schists are compositionally less pure and are graphitic. A combination of
various bulk compositional factors and the fact that the fluid phase was
likely about 80% H2O, 10% CO2, 10%CH4 explains the fact that chloritoid has
broken down in the schists but not in the neighboring quartzites.
Like Dave, I suspect that the chlorite in such rocks is usually retrograde.
Mike Holdaway
|