Download Metamorphism of Pelitic rocks (Part-II)

Lecture NINE
Metamorphism of pelitic rocks
(Metapelites – Part II)
-3Low 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 + Naplagioclase (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 + Naplagioclase (albite) ± garnet ± staurolite
IV- Sillimanite zone zone
Sillimanite in this zone can occur due to the occurrence of the
polymorphic solid-solid reaction:
andalusite  sillimanite
but 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 zone
The 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
-4High Temperature metamorphism
4- High temperature metapelites
 At high-temperatures, above or
coeval to sillimanite zone,
metapelites undergo partial
melting, and the yielded rock is
known as Migmatites.
 The Migmatites are mixed
rocks predominantly schists
but with pads, veins or layers
of leucocratic material of
granitic
composition.
The
leucocratic (granitic) materials
are well known as leucosomes,
while the metamorphic parts
are known as mesosome
(resistite) and melansomes.
Agmatitic
Vein-type
Nebulitic
Philibitic
Stromatic
Migmatization processes
 migmatization processes could form as a result of :
A- closed system (no gains or loses during migmatization)
1- Aanatexis (partial melting) at higher temperature
2- Metamorphic differentiations at higher temperature
B- Open system
3- K-Na rich external fluid metasomatism
4- Injection of granitic materials to the schistose rocks
In the closed system migmatites three mineral zones
develop:
2- upper sillimanite zone
2- Cordierite-garnet-K-feldspar zone

At
higher
develop
grade,
pelitic
assemblages
rocks
with:
Cordierite + garnet + K-feldspar +
sillimanite + muscovite + Qtz

This mineral assemblage is typical
for
the
high
grade
pelitic
migmatites, and is often taken to
mark the beginning of the granulite
facies

The
assemblages
result
from
continuous reaction such as:
biotite + sillimanite + quartz  Kfeldspar + cordierite garnet + melt
 Higher
3- Ultra–high grade zone
grade granulite facies, metapelites with mineral
assemblage orthopyroxene + sillimanite can be formed as a
related to breakdown of common corderite-garnet assemblages
through the equilibrium:
Crd + Grt  Opx + Sill
 At
even higher temperatures, sillimanite + orthopyroxene
assemblage becomes not stable, and assemblage of sapphirine
+ quartz has been formed through the following reaction:
Sill + Opx
 sapphirine + quartz (at 850-1000 °C)