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