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Lecture NINE Metamorphism of pelitic rocks (Metapelites – Part II)

Lecture NINE Metamorphism of pelitic rocks (Metapelites – Part II). -3- Low Pressures metamorphism (Buchan Zonal scheme). 2- Buchan Zonal Scheme (LP metamorphism).

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Lecture NINE Metamorphism of pelitic rocks (Metapelites – Part II)

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  1. Lecture NINE Metamorphism of pelitic rocks(Metapelites – Part II)

  2. -3-Low Pressures metamorphism (Buchan Zonal scheme)

  3. 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:

  4. 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).

  5. 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.

  6. 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

  7. 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

  8. 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

  9. -4-High Temperature metamorphism

  10. 4- High temperature metapelites Vein-type Agmatitic • 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. Nebulitic Philibitic Stromatic

  11. 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:

  12. 2- upper sillimanite zone

  13. 2- Cordierite-garnet-K-feldspar zone • At higher grade, pelitic rocks develop assemblages 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  K-feldspar + cordierite garnet + melt

  14. 3- Ultra–high grade zone • Higher 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)

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