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Metamorphic Textures 2 - Regional

Metamorphic Textures 2 - Regional. T his part contains several color reproductions of Figures and Tables in your text. I will pass over them with a quick comment on their importance without reading their captions. You can do that at home. Textures of Regional Metamorphism.

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Metamorphic Textures 2 - Regional

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  1. Metamorphic Textures 2 - Regional This part contains several color reproductions of Figures and Tables in your text. I will pass over them with a quick comment on their importance without reading their captions. You can do that at home.

  2. Textures of Regional Metamorphism • Dynamothermal (crystallization under dynamic conditions) • Orogeny- long-term mountain-building • May comprise several Tectonic Events • May have several Deformational Phases • May have an accompanying Metamorphic Cycles with one or more Reaction Events

  3. Textures of Regional Metamorphism • Tectonite- a deformed rock with a texture that records the deformation • Fabric- the complete spatial and geometric configuration of textural elements • Foliation- planar textural element • Lineation- linear textural element

  4. Example 4 • Progressive Syntectonic Metamorphism of a volcanic graywacke.

  5. Phase 1: Both plag and Kfs in isotropic matrix • Feldspars altered to fine sericite and secondary Ca-Al silicates • Sericite is a fine grained mica, either muscovite or Illite. Sericite is a common alteration mineral of orthoclase or plagioclase feldspars Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

  6. Phase 2: Pervasive foliation develops due mostly to shear • Grain size reduction • Porphyroclasts common and rounded • Matrix recrystallized and new minerals form (Qtz, Ep, Sericite, Ab, Chl) • Chl & Ser enhance foliation Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

  7. Phase 3: Fine-grained schist with larger crystals- no relict textures • Good muscovite and biotite define schistosity • Some metamorphic differentiation to layering • Qtz and Ab are polygonal mosaic in mica-free layers Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

  8. Phase 4: Good schist with coarser grains • More enhanced segregation into layers • Plag no longer Ab- accepts more Ca at higher T • Garnet is a new isograd mineral Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

  9. Fig 23-21 Types of foliations a. Compositional layering b. Preferred orientation of platy minerals c. Shape of deformed grains d. Grain size variation e. Preferred orientation of platy minerals in a matrix without preferred orientation others f. Preferred orientation of lenticular mineral aggregates g. Preferred orientation of fractures h. Combinations of the above Figure 23-21.Types of fabric elements that may define a foliation. From Turner and Weiss (1963) and Passchier and Trouw (1996).

  10. Figure 23-22.A morphological (non-genetic) classification of foliations. After Powell (1979) Tectonophys., 58, 21-34; Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag; and Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

  11. Figure 23-22. (continued)

  12. Analysis of Deformed Rocks Figure 23-43.Graphical analysis of the relationships between deformation (D), metamorphism (M), mineral growth, and textures in the rock illustrated in Figure 23-42. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  13. Crenulation: A slaty cleavage or schistocity that becomes microfolded Quartz grains commonly dissolve by pressure solution from the steep limbs and precipitate in the hinge Figure 23-24a.Symmetrical crenulation cleavagesin amphibole-quartz-rich schist. Note concentration of quartz in hinge areas. From Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag.

  14. Figure 23-27. Proposed mechanisms for the development of foliations • Mechanical rotation. b. Preferred growth normal to compression. c. Grains with advantageous orientation grow whereas those with poor orientation do not (or dissolve). d. Minerals change shape by ductile deformation. e. Pressure solution. f. A combination of a and e. g. Constrained growth between platy minerals. h. Mimetic growth following an existing foliation. Mimetic – growth during non-shear conditions follows old orientation

  15. Figure 23-28. Development of foliation by simple shear and pure shear (flattening) a. Beginning with randomly oriented planar or linear b. Beginning with equi-dimensional crystals c. Beginning with pre-existing foliation Shaded figures represent an initial sphere and the resulting strain ellipsoid Figure 23-28.Development of foliation by simple shear and pure shear (flattening). After Passchier and Trouw (1996) Microtectonics. Springer-Verlag.

  16. Development of an axial-planar cleavage in folded metasediments. Circular images are microscopic views showing that the axial-planar cleavage is a crenulation cleavage, and is developed preferentially in the micaceous layers. From Gilluly, Waters and Woodford (1959) Principles of Geology, W.H. Freeman; and Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.

  17. Figure 23-3. Plastic deformation of a crystal lattice (experiencing dextral shear) by the migration of an edge dislocation (as viewed down the axis of the dislocation).

  18. Figure 23-37.Si characteristics of clearly pre-, syn-, and post-kinematic crystals as proposed by Zwart (1962). a. Progressively flattened Si from core to rim. b. Progressively more intense folding of Si from core to rim. c. Spiraled Si due to rotation of the matrix or the porphyroblast during growth. After Zwart (1962) Geol. Rundschau, 52, 38-65.

  19. Syn-kinematic crystals Figure 23-38.Spiral Si train in garnet, Connemara, Ireland. Magnification ~20X. From Yardley et al. (1990) Atlas of Metamorphic Rocks and their Textures. Longmans.

  20. Figure 23-19. Mantled porphyroclasts and “mica fish” as sense-of-shear indicators. After Passchier and Simpson (1986) Porphyroclast systems as kinematic indicators. J. Struct. Geol., 8, 831-843.

  21. Figure 23-18. Augen Gneiss. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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