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Geology for Engineers

Geology for Engineers. Minerals and Rocks (II). Gneiss. Metamorphic Rocks. Rocks may be altered through Temperature Pressure Deformation Fluid influx. Metamorphic Rocks. The original rock is termed a protolith

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Geology for Engineers

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  1. Geology for Engineers Minerals and Rocks (II) Gneiss

  2. Metamorphic Rocks Rocks may be altered through • Temperature • Pressure • Deformation • Fluid influx

  3. Metamorphic Rocks • The original rock is termed a protolith • The composition of the protolith and the conditions of metamorphism and deformation will largely determine the end structure and composition of the metamorphic rock

  4. Metamorphic Rocks • Protoliths may be sedimentary, igneous or even metamorphic • Metamorphism may result from a general increase (prograde), or decrease (retrograde) of temperature and pressure

  5. Pressure-Temperature-Time

  6. Metamorphic Rocks Metamorphism may be: • Dynamic (i.e. some deformation involved), e.g. gneiss • Static (i.e. the end rock does not have a preferred shape fabric) e.g. hornfels

  7. Gneiss

  8. Metamorphic Grades • Anchi-metamorphic • Greenschist • Amphibolite • Granulite • Eclogite • Blueschist • Increasing temperature and pressure

  9. P-T fields of metamorphic grades

  10. Index of Metamorphic Minerals csmres.jmu.edu/

  11. Index of Metamorphic Minerals csmres.jmu.edu/

  12. Isograd Map

  13. Different Styles of Metamorphism • Barrovian (regional) metamorphism • Buchan (regional) metamorphism • Contact metamorphism • Shock metamorphism • Dynamic metamorphism (e.g. shearing)

  14. ContactMetamorphism • This shows a basalt (igneous rock) intruding into a limestone. You can see that it has been metamorphosed into marble near the contact with the basalt.

  15. Mylonite • Intensely sheared rock. Note the recrystallization of feldspar (lightest crystals) and the ribbons of quartz (grey ) in a dark, biotite-rich matrix. • Difficult to determine the protolith!

  16. Tectonic Settings of Metamorphism

  17. Metamorphic Rocks • Sandstone can be metamorphosed to quartzite. • Sutured quartz boundaries (changes mechanical properties of the rock)

  18. Metamorphic Rocks • A limestone protolith can be metamorphosed into a marble (this example is quite pure, i.e. white).

  19. Metamorphic Rocks • Mudstone (pelite)

  20. Metamorphic Rocks • Phyllite - is the metamorphic step beyond slate. New layers of mica minerals give phyllite a shiny, “wavy” appearance.

  21. Metamorphic Rocks • Schist - heavily foliated rock in which all the original clay minerals are fully transformed

  22. Metamorphic Rocks • Greenschist – minerals such as epidote, chlorite and actinolite

  23. Metamorphic Rocks • This is a blueschist. It was formed in a subduction zone in high pressure and relatively low temperature • Blue mineral = glaucophane

  24. Metamorphic Rocks • Migmatite Partial melting of protolith Dark parts = restite Light parts = leucosome

  25. Metamorphic Rocks • Garnet Gneiss

  26. Metamorphic Rocks • This is an eclogite. You can see the presence of garnet (pink) and pyroxene (green)

  27. Metamorphic Rocks • This image shows chert (red), which is a sedimentary rock being replaced by silica (SiO2) veins.

  28. Metamorphic Rocks • This shows a basalt (igneous rock) which has been metamorphosed into an amphibolite (i.e. pyroxene changed to amphibole)

  29. Metamorphic Rocks • This shows an olivine-rich igneous rock (dunite) which has been metamorphosed into an serpentinite

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