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Chapter 28: Metamorphism of Pelitic Sediments

Chapter 28: Metapelites. The mineralogy of pelitic sediments is dominated by fine Al-K-rich phyllosilicates, such as clays (montmorillonite, kaolinite, or smectite), fine white micas (sericite, paragonite, or phengite) and chlorite, all of which may occur as detrital or authigenic grainsThe phyllos

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Chapter 28: Metamorphism of Pelitic Sediments

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    1. Chapter 28: Metamorphism of Pelitic Sediments Mudstones and shales: very fine grained mature clastic sediments derived from continental crust Characteristically accumulate in distal portions of a wedge of sediment off the continental shelf/slope Grade into coarser graywackes and sandy sediments toward the continental source Although begin as humble mud, metapelites represent a distinguished family of metamorphic rocks, because the clays are very sensitive to variations in temperature and pressure, undergoing extensive changes in mineralogy during progressive metamorphism They also tend to be beautiful brilliant mica schists with porphyroblasts of garnet, staurolite, kyanite, etc. Metapelites are the rocks of the classic studies in the Scottish Highlands in which the Barrovian sequence of isograds was developed They also tend to be beautiful brilliant mica schists with porphyroblasts of garnet, staurolite, kyanite, etc. Metapelites are the rocks of the classic studies in the Scottish Highlands in which the Barrovian sequence of isograds was developed

    2. Chapter 28: Metapelites The mineralogy of pelitic sediments is dominated by fine Al-K-rich phyllosilicates, such as clays (montmorillonite, kaolinite, or smectite), fine white micas (sericite, paragonite, or phengite) and chlorite, all of which may occur as detrital or authigenic grains The phyllosilicates may compose more than 50% of the original sediment Fine quartz constitutes another 10-30% Other common constituents include feldspars (albite and K-feldspar), iron oxides and hydroxides, zeolites, carbonates, sulfides, and organic matter

    3. Chapter 28: Metapelites Distinguishing chemical characteristics: high Al2O3 and K2O, and low CaO Reflect the high clay and mica content of the original sediment and lead to the dominance of muscovite and quartz throughout most of the range of metamorphism High proportion of micas ® common development of foliated rocks, such as slates, phyllites, and mica schists The chemical composition of pelites can be represented by the system K2O-FeO-MgO-Al2O3-SiO2-H2O (“KFMASH”) If we treat H2O as mobile, the petrogenesis of pelites is represented well in AKF and A(K)FM diagrams

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    16. Chapter 28: Metapelites Alternative Fig 28-11 in which discuss phases lost and gained at A - B - C - D E is indifferent to the reaction, but gains St later due to a continuous reactionAlternative Fig 28-11 in which discuss phases lost and gained at A - B - C - D E is indifferent to the reaction, but gains St later due to a continuous reaction

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    26. Reactions are not balanced and commonly leave out Ms and Qtz, which are considered to be present in excess.Reactions are not balanced and commonly leave out Ms and Qtz, which are considered to be present in excess.

    27. Chapter 28: Metapelites Reactions are not balanced and commonly leave out Ms and Qtz, which are considered to be present in excess.Reactions are not balanced and commonly leave out Ms and Qtz, which are considered to be present in excess.

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