Lecture NINE Lecture NINE Metamorphism of pelitic Metamorphism of pelitic

rocksrocks (Metapelites – Part II)

-3-Low Pressures metamorphism

(Buchan Zonal scheme)

2 -Buchan Zonal Scheme (LP metamorphism)

- At lower pressure, such as contact metamorphism or shallow level regional metamorphism, where pressure <3.5 kbar, metamorphism of metapelites exhibit the Buchan Zonal Scheme

The principle characteristic features of Buchan zonal scheme are:

1- Cordierite is common and forms at relatively LT,

2- Kyanite does not occur, but andalusite may be present,

3- Garnet is less abundant or absence, and staurolite may be lacking

4- Migmatities are not developed until well above the sillimaninte zone

Sequence of metapelites metamorphic zones in the

Buchan type metamorphism include:

I- Biotite zone- Biotite zone is the lowest grade of Buchan series

-Metapelites are typically fine-grained Slates and contain mineral assemblage: biotite + chlorite + muscovite + quartz + Na-plagioclase (albite).

2- Cordierite zone- Cordierite appears as the first distinctive index mineral via the reaction:

Chl + Ms Crd + Bt + Qtz+ H2O

Cordierite occur as spots; therefore, the common rock is Spotted slates/schists and contain mineral assemblage: Cordierite + biotite + chlorite + muscovite + quartz + Na-plagioclase (albite) ± garnet.

III- Andalusite zone Andalusite can form in most pelites at low pressures as a result of

the discontinuous reactions:chlorite + muscovite + quartz cordierite + andalusite + biotite + H2O

Cordierite + muscovite + quartz biotite + andalusite + H2O

-Metapelites are typically medium grained schists and contain mineral assemblage: Andalusite + biotite + muscovite + quartz + Na-plagioclase (albite) ± garnet ± staurolite

IV- Sillimanite zone zoneSillimanite in this zone can occur due to the occurrence of the polymorphic solid-solid reaction:

andalusite sillimanitebut as with regional metamorphism the occurrence of muscovite + cordierite + quartz in this zone suggests that a separate reaction may occur:chlorite + muscovite + quartz cordierite + sillimanite + biotite + H2O

-Mineral assemblage include: Sill + Qtz + Bt + Pl ± Grt ± St ± Crd

V- Upper sillimanite zoneThe highest grade of contact metamorphism of pelites is characterized by the assemblage sillimanite + cordierite + biotite + K-feldspar + quartz + muscovite .This assemblage resulting from the reaction:Muscovite + quartz sillimanite + K-feldspar + H2O

-4-High Temperature metamorphism

4- High temperature metapelites At high-temperatures, At high-temperatures, above or above or

coeval to sillimanite zonecoeval to sillimanite zone, , metapelites undergo metapelites undergo partial partial meltingmelting, and the yielded rock is , and the yielded rock is known as known as MMiiggmmaattiitteess..

The The MMiiggmmaattiitteess are mixed are mixed rocks predominantly schists rocks predominantly schists but with pads, veins or layers but with pads, veins or layers of leucocratic material of of leucocratic material of granitic composition. The granitic composition. The leucocratic (granitic) materials leucocratic (granitic) materials are well known as are well known as leucosomesleucosomes, , while the metamorphic parts while the metamorphic parts are known as are known as mesosome mesosome (resistite)(resistite) and and melansomes.melansomes.

Stromatic

Agmatitic

Vein-type

Nebulitic

Philibitic

Migmatization processesMigmatization processes migmatization processes could form as a result of :migmatization processes could form as a result of :

A- A- closed systemclosed system (no gains or loses during migmatization)(no gains or loses during migmatization)

1- Aanatexis (partial melting) at higher temperature 1- Aanatexis (partial melting) at higher temperature

2- Metamorphic differentiations at higher temperature2- Metamorphic differentiations at higher temperature

B- B- Open systemOpen system

3- K-Na rich external fluid metasomatism3- K-Na rich external fluid metasomatism

4- Injection of granitic materials to the schistose rocks4- Injection of granitic materials to the schistose rocks

In the closed system migmatites three mineral zones In the closed system migmatites three mineral zones

develop:develop:

2- 2- upper sillimanite zoneupper sillimanite zone

2- 2- Cordierite-garnet-K-feldspar zoneCordierite-garnet-K-feldspar zone At higher grade, pelitic rocks At higher grade, pelitic rocks

develop assemblages with: develop assemblages with:

Cordierite + garnet + K-feldspar Cordierite + garnet + K-feldspar

+ sillimanite + muscovite + Qtz+ sillimanite + muscovite + Qtz

This mineral assemblage is This mineral assemblage is

typical for the high grade pelitic typical for the high grade pelitic

migmatites, and is often taken migmatites, and is often taken

to mark the beginning of the to mark the beginning of the

granulite faciesgranulite facies

The assemblages result from The assemblages result from

continuous reaction such as:continuous reaction such as:

biotite + sillimanite + quartz biotite + sillimanite + quartz

K-feldspar + cordierite garnet + K-feldspar + cordierite garnet +

meltmelt

3- 3- Ultra–high grade zoneUltra–high grade zone Higher grade granulite facies, metapelites with mineral Higher grade granulite facies, metapelites with mineral

assemblage assemblage orthopyroxene + sillimaniteorthopyroxene + sillimanite can be formed as a can be formed as a related to breakdown of common corderite-garnet related to breakdown of common corderite-garnet assemblages through the equilibrium:assemblages through the equilibrium:

Crd + Grt Crd + Grt Opx + Sill Opx + Sill At even higher temperatures, sillimanite + orthopyroxene At even higher temperatures, sillimanite + orthopyroxene

assemblage becomes not stable, and assemblage of assemblage becomes not stable, and assemblage of sapphirine + quartz has been formed through the following sapphirine + quartz has been formed through the following reaction: reaction:

Sill + Opx Sill + Opx sapphirine + quartz (at 850-1000 °C) sapphirine + quartz (at 850-1000 °C)