Part 7. Pelitic Rocks

The protoliths for pelitic rocks are shales (aluminous rocks), not sandstones (quartzofeldspathic rocks)
Here are the stability fields of common pelitic minerals:

the presence of andalusite ^VIAl^VAlSiO₅ implies low P metamorphism
kyanite ^VIAl^VIAlSiO₅ implies high P
sillimanite ^VIAl^IVAlSiO₅ implies high T. Sillimanite first grows as fibrolite, a fine-grained acicular form:

chlorite MgFeAlSiO(OH)_x implies low T
biotite K(Mg,Fe)AlSi₃O₁₀(OH)₂ implies moderate T
garnet implies moderate T
staurolite MgFeAlSiO(OH)_x implies moderate-high P
chloritoid MgFeAlSiO(OH)_x implies moderate-high P
cordierite MgFeAlSiO(OH)_x implies low P
paragonite NaAlSi₃O₁₀(OH)₂ implies high P

The A(K)FM diagram is used to describe the phase relations of pelites

1. To start, we assume excess SiO₂, H₂O, meaning that quartz/coesite and H₂O are stable in all mineral assemblages.
2. Then we assume that plagioclase is also stable and of an unknown, but unimportant, composition; by this we ignore the effects of Na and Ca.
3. This results in a tetrahedron of A=Al₂O₃, K=K₂O, F=FeO, M=MgO:
   
4. To render this diagram in easier-to-understand-and-draw 2D, we project the compositions in the tetrahedron from muscovite KAl₃Si₃O₁₀(OH)₂ onto the AFM plane
   
5. This results in a common pelitic assemblage such as st + gar + bio + mus + plg + qtz + H₂O (7 phases) plotting as a triangle. This is convenient, and possible only because we have assumed that i) qtz + H₂O are in excess, ii) plagioclase is present, and iii) muscovite of fixed composition is present.
   

Metamorphic Zones

This development of metamorphic mineral assemblages corresponds to this P-T path:

Below is an example showing the prograde metamorphism of pelites around the Kangmar Dome of Tibet.


View the AFM movies #1 and #2 of Worley and Powell showing how mineral compositions and stabilities can change with changing temperature.