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Gneiss
Geology for Engineers
Minerals and Rocks (II)
Metamorphic Rocks
Rocks may be altered through
• Temperature
• Pressure
• Deformation
• Fluid influx
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
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
Pressure-Temperature-Time
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
Gneiss
Metamorphic Grades
• Anchi-metamorphic
• Greenschist
• Amphibolite
• Granulite
• Eclogite
• Blueschist
• Increasing temperature and pressure
P-T fields of metamorphic grades
Index of Metamorphic Minerals
csmres.jmu.edu/
Index of Metamorphic Minerals
csmres.jmu.edu/
Isograd Map
Different Styles of Metamorphism
• Barrovian (regional) metamorphism
• Buchan (regional) metamorphism
• Contact metamorphism
• Shock metamorphism
• Dynamic metamorphism (e.g. shearing)
Contact Metamorphism
• 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.
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!
Tectonic Settings of Metamorphism
Metamorphic Rocks
• Sandstone can be metamorphosed to quartzite.
• Sutured quartz boundaries (changes mechanical properties of the rock)
Metamorphic Rocks
• A limestone protolith can be metamorphosed into a marble (this example is quite pure, i.e. white).
Metamorphic Rocks
• Mudstone (pelite)
Metamorphic Rocks
• Phyllite - is the metamorphic step beyond slate. New layers of mica minerals give phyllite a shiny, “wavy” appearance.
Metamorphic Rocks
• Schist - heavily foliated rock in which all the original clay minerals are fully transformed
Metamorphic Rocks
• Greenschist – minerals such as epidote, chlorite and actinolite
Metamorphic Rocks
• This is a blueschist. It was formed in a subduction zone in high pressure and relatively low temperature
• Blue mineral = glaucophane
Metamorphic Rocks
• Migmatite
Partial melting of protolith
Dark parts = restite
Light parts = leucosome
Metamorphic Rocks
• Garnet Gneiss
Metamorphic Rocks
• This is an eclogite. You can see the presence of garnet (pink) and pyroxene (green)
Metamorphic Rocks
• This image shows chert (red), which is a sedimentary rock being replaced by silica (SiO2) veins.
Metamorphic Rocks
• This shows a basalt (igneous rock) which has been metamorphosed into an amphibolite (i.e. pyroxene changed to amphibole)
Metamorphic Rocks
• This shows an olivine-rich igneous rock (dunite) which has been metamorphosed into an serpentinite