Ig Met Pet Lecture 22: Metamorphism of Pelitic Rocks

Pelites (+ examples)

Fine-grained, clay-rich sedimentary rocks, including clay, shale, and mudstone

What are pelitic rocks high and low in chemically?

High in Al, K and fluids, with low Ca

The AFK Diagram

A pelitic version of the ACF diagram

The AFM Projection

A triangular diagram which best represents pelitic mineral assemblages, projected from a pyramid

Which is used to represent stable mineral assemblages in metapelites, the AFK or the AFM?

The AFM projection

What do shaded triangles represent in the AFM?

A mineral assemblage of three, which each corner representing one mineral composition

What do dots represent on the AFM diagram?

Phases with fixed compositions

What do solid black lines show on the AFM diagram?

Compositions of solid-solution minerals

What do subparallel lines do on the AFM?

Connect pairs of coexisting minerals

What are the two types of reactions which are apparent in plot?

Tie line flip and terminal reactions

What is a tie-line flip reaction?

A + B = C + D, due to a change in PT conditions

What is a terminal reaction?

E = F + G + H

What PT conditions are most pelitic rocks metamorphosed at?

pressures < 14 kbars, and temps < 800°C

Which polymorphic reaction is used as an indicator of PT space?

Kyanite-andalusite-sillimanite

If water is present, what is the low end where rocks will melt?

625-650°C

Are high temp metapelites or metabasalts more common?

Metabasalts, as they have a higher melting temperature and are more likely to withstand formation

What is the difference between Barrovian and Buchan metamorphism for metapelites?

Barrovian is most common. Buchan is lower P, and contains all the same minerals with the exception of andalusite instead of kyanite and cordierite

What is the petrogenic grid for metapelites called?

The KFMASH system

For medium pressure metapelites, what is the grade of each facies?

Greenschist is low, amphibolite is medium, and granulite is high

What are the Barrovian Zones with increasing grade for metapelites?

Chlorite, biotite, garnet, staurolite, kyanite, sillimanite

The Chlorite Zone

Low-grade pelitic rocks with little metamorphic texture, which have gone through low temp changes

The Biotite Zone

With increased heating, chlorite, k-spar, and pyrophyllite transition to biotite

The Garnet Zone

A number of reactions may produce garnet, depending on composition of reactants

The Staurolite, Kyanite, and Sillimanite Zones

Staurolite and Kyanite form, then Sillimanite replaces Kyanite when temps get too high

What happens in the granulite facies of metapelites?

Muscovite is lost through reaction

What does the loss of muscovite necessitate?

A projection from K-feldspar on the AMF diagram

What zone may appear if metapelites are dry or very dry?

The orthopyroxene zone

Leucosomes

Light colored zones in migmatites

Melanosomes

Dark-colored zones in migmatites

What are the pelitic zones in greenschist during the barrovian series?

Chlorite and Biotite

What are the pelitic zones in amphibolite during the barrovian series?

Garnet, staurolite, kyanite, silimanite

What are the pelitic zones in granulite during the barrovian series?

Sillimanite and k-spar