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Metamorphic Fabric

Chapter 13A. Metamorphic Fabric. Solid-state Crystal Growth. Nucleation Crystallization of new phases Crystal growth Modification of existing grain boundaries. Nucleation. Homogeneous nucleation Heterogeneous nucleation. Homogeneous Nucleation.

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Metamorphic Fabric

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  1. Chapter 13A Metamorphic Fabric

  2. Solid-state Crystal Growth • Nucleation • Crystallization of new phases • Crystal growth • Modification of existing grain boundaries

  3. Nucleation • Homogeneous nucleation • Heterogeneous nucleation

  4. Homogeneous Nucleation • Formation of new minerals within another mineral • Free energy of new phase is greater than its surface energy • Rate of homogeneous nucleation is very slow

  5. Heterogeneous Nucleation • New minerals form along existing grain boundaries or other discontinuities • This mechanism is common

  6. Crystal Growth • Nucleation • Independent growth • Some contacts • Granoblastic texture

  7. Granoblastic Texture • Triple points • Straight grain boundaries • Interfacial angle controlled by surface energy of crystals in contact

  8. Interfacial Angles • Dihedral angles () • Controlled by facial energy • Sine relationship applies

  9. Stress Categories • Tension • Compression • Shear

  10. Hydrostatic Pressure • Defined as a uniform stress on a point regardless of direction • Hydrostatic pressure increases with depth in the earth • Its value equals gz

  11. Pressure Solution • Areas under high stress dissolve • Material moves to regions of low stress • Migration facilitated by an intergranular fluid • Driving mechanism is a chemical potential • Evidenced by growth into pressure shadows

  12. Formation of Porphyroblasts • Controlled by nucleation phenomena • Megacrysts have a high surface energy • If only a few nuclei may form • They may grow to a very large size

  13. Stress • Stress is measured by F/A • Units are Newtons/m2, MPa, bars, etc. •  is the symbol for stress •  = lim  F/ A as  A becomes infinitely small

  14. Directed Stress • Tectonism produces non-uniform stress • This causes: • Rock deformation • Preferred orientation of mineral grains • Development of large-scale structures

  15. Strain • Strain is the response to stress •  is the symbol for strain •  = lim l/lo as l approaches zero  l is the change in length in a line element l0 is the original length of the same line element

  16. Strain Measurements • Units of strain are given as a fraction of the initial dimension • Length strain • l = l/lo • Volume strain • v = V/Vo

  17. Timing of Porphyroblast Growth Pre-tectonic, Syn-tectonic, Post-tectonic

  18. Pre-tectonic Textures

  19. Post-tectonic Textures

  20. Types of Rock Behavior • Elastic (like a spring) • All deformation recoverable • Linear deformation with applied stress • Viscous (honey) • No deformation recoverable • No threshold for deformation

  21. Types of Rock Behavior • Plastic (clay) • Some deformation recoverable • Yield strength must be overcome • Brittle (halite) • Yields by fracturing • Generally elastic behavior prior to rupture

  22. Rheology • The study of the flow of materials • Strength describes the condition of materials when they fail • Soft materials begin to yield at their yield strength • Brittle materials will rupture at their fracture strength

  23. Linear Rheological Models • Hookean model elastic • Newtonian model viscous • Saint Venant model plastic

  24. Elastic Model

  25. Viscous Model

  26. Sliding Block Model

  27. Complex Rheological Models • Bingham model • Has both a yield strength (kb)and a viscosity (b) • Common behavior of natural materials

  28. Plastic Model

  29. Metamorphic Tectonites • Undulatory extinction • Wavy extinction in quartz • Bent twin planes in crystals • Deformation bands • Deformation lamellae

  30. Low Temperatures & High Strain Rates • Undulatory extinction • Kink bands • Deformation lamellae

  31. High Temperatures& Slow Strain Rates • Recovery and recrystallization occur • Sutured grain boundaries • Small new grains form

  32. Diffusive Flow • Thermally activated • Stress induced • Diffusive recrystallization • Sometimes called pressure solution

  33. Role of Fluids in Deformation • Hydraulic weakening of non-hydrous silicates • Prograde dehydration reduces ductility • High pore PH20 may cause rock to be brittle

  34. Anisotropic Fabric • Results from syntectonic flow under stress • Causes include: • Applied nonhydrostatic stress • Magnitude of strain • Strain rate • T & P

  35. Fabric Geometry • Very complicated • Non-homogeneous rock bodies • Linked chain of events • Unambiguous answers

  36. Grain Orientation • Foliation commonly parallel to axial plane of folds • Noted by orientation of platy minerals • Lineation commonly parallels hinge line • Given by alignment of elongate minerals

  37. Orientation Mechanism • Not easy to determine • Nucleation and growth? • Rotation of grains? • Pressure solution?

  38. Segregation Layering • Alternating bands of different minerals • Relict beds? • Mechanical transportation processes? • Easier to determine in lower grade rocks • Uncertain origin in higher grade rocks

  39. a. type of cleavage?b. how many S?

  40. c. texture?

  41. d. rock name?

  42. e. names? (1=L, 2=R)

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