Friends of the tripe point,
Mike - thanks for your comments, especially regarding Mt. Moosilauke.
A good source of information about that locality is Rumble (1973,
GSA Bull, v.84, 2423-2430). Doug describes the paragenesis andalusite
---> sillimanite ---> andalusite, which is consistent with my
observations. Note that the hypothesis of a thrust at the base of the
Mt. Moosilauke septum is based on the disparity of P-T histories
between those rocks and those underlying. The Littleton to the west
of Mt Moosilauke (lower plate) all underwent loading during prograde
metamorphism, whereas the Moosilauke rocks were isobaric (more or
less) heating. It turns out that there is one report of andalusite in
the belt west of Moosilauke in the Salmon Hole Brook Syncline by
Florence et al (1993, AJS, 293, 939-979). In this sample, the
paragenesis is andalusite ---> kyanite.
Mike - I also like your P-T conditions. They seem to fit
better with low-P grids than previous estimates. But these conditions
would also seem to force the triple point to be at higher pressure
than your 1971 experiments, if Mt. Moosilauke really is isobaric.
That is, you can't get the paragenesis andalusite ---> sillimanite
along an isobaric path at P of 4.3-4.9 kbar if the triple point is at
3.8 kbar. Anyone have any thoughts on this?
Cheers,
Frank
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Frank Spear
Professor and Chair
Department of Earth and Environmental Sciences, JRSC 1C25
Rensselaer Polytechnic Institute
Troy, New York 12180
Phone (office): 518-276-6103
(lab) : 518-276-4899
Fax: 518-276-6680
http://www.rpi.edu/dept/geo/spear/spear.html
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