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Intersecting Isograds at
Whetstone Lake
Dugald Carmichael (1970)
Journal of Petrology, 11, 147-181.
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Pelitic and calcareous rocks
Wide range of chemical compositions
Textures indicate 1 metamorphic episode
Lowest grade rocks in the area are already
in the garnet zone
metagabbro
metasediments
E. pluton is pre-met (has sl. phengite S1 || regional S & across contact)
Central pluton is syn-met
Components = K2O-Al2O3-MgO-FeO-SiO2-H2O
How many?
5 (H2O is mobile)
 nice AKFM diagram (proj. from Q and Mu)
Expect 5-phase assemblages (6 phases = univariant if
eqm), but will see only 3 in projection
Extra components:
Phase-makers TiO2 & P2O5 can be ignored as add
1 component and only 1 phase (Il & Ap)
Partitioners- instead of new phase will act as solute
and make solid solutions
5 reactions occur in the area:
1. Chl + Ms + Grt = St + Bt + Qtz + H2O
2. Chl + Ms + St + Qtz = Ky + Bt + H2O
3. Ky = Sill
4. St + Ms + Qtz = Sill + Grt + Bt + H2O
5. Bt + Cal + Qtz = Ca-amphibole + Kfs +
CO2 + H2O
These can be documented texturally across each isograd
1. Chl + Ms + Grt = St + Bt + Qtz + H2O
f = 6, so univariant in KFMAS-H
Below the isograd
Above the isograd
What isograd would you call this?
What minerals appear and disappear? At A, B, C, D, E?
Within the "St-Bi" zone get a Continuous Reaction:
Chl + Ms = St + Bt + Qtz + H2O
f = 5 (divariant in KFMAS-H)
2. Chl + Ms + St + Qtz = Ky + Bt + H2O
f = 6, so univariant in KFMAS-H
Below the isograd
Above the isograd
3. Ky = Sill
Polymorphic transformation- univariant in Al2SiO5
Above the isograd
4. St + Ms + Qtz = Sill + Grt + Bt + H2O
f = 6, so univariant in KFMAS-H
5. Bt + Cal + Qtz = Ca-amphibole + Kfs +
CO2 + H2O
This reaction is not a pelitic one, but occurs
in more calcic rocks
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