Chapter 7. Metamorphic Rocks

Metamorphism "to change form"

Metamorphism occurs when rocks are subjected to temperatures and pressures different from those under which they formed. Metamorphism also usually requires presence of a fluid – water.

Changes in: mineralogy - texture

Agents of Metamorphism include:
    heat - provides energy to drive chemical reactions
    pressure and differential stress
    pressure from burial (lithostatic stress)

differential stress from tectonics
    compressional - shortening
    tensional - pulling apart

chemically active fluids
    water or other fluids are required to speed up chemical reactions

Grade of metamorphism (low, medium, high)

Metamorphism occurs
    1. During mountain building.

Regional Metamorphism -- large volumes of rock are deeply buried and exposed to high temperatures and pressure.
2. When rock is near or touching magma.
Contact Metamorphism - heat from magma "bakes" the surrounding rocks
3. Along fault zones
Cataclastic Metamorphism rocks in the fault zone are broken and altered by fluids in the fault zone (mylonites)
4. In meteorite impacts
Shock Metamorphism may change mineral polymorphs
    quartz Þ stishovite
How Metamorphism Alters Rocks

1. Textural Changes
    texture - size, shape and distribution of minerals

    Rock becomes denser - porosity is decreased
    Mineral grains may become aligned (foliation)
    Mineral grains may recrystallize - mineral grains may become larger
    platy or needle-like minerals may have
    preferred orientation - perpendicular to force

    New Minerals may form in response to changes in Pressure and Temperature

foliation - occurs when minerals and/or structural features are in a preferred, parallel alignment

results from non-uniform stress Types of foliation
    Rock or Slaty Cleavage -
platy minerals (clays, micas) align parallel - rock can be easily split along alignment


Schistosity
    alignment of mica crystals which have grown larger due to higher grade metamorphism.
    gives rock a "scaly" appearance

Gneissic Texture

during high-grade metamorphism minerals may segregate into layers giving rock a banded appearance
Not all metamorphic rocks are foliated

Non-foliated Textures
    may be a result of presence of only one mineral in the rock - as in Marble

2. Mineralogical Changes

Minerals may recrystallize – same mineral, new shape
New minerals form which are stable under the new pressure and temperature conditions.
rock usually has same composition except for loss of water and carbon dioxide

clays change to micas (platy phylosilicates) then to rod-shaped (amphibole) or equant minerals (andalusite, garnet, etc.)

volatile species (H2O and CO2) may be driven off by heat and metamorphic reactions

CaCO3 + SiO2 = CaSiO3 + CO2
water and fluids released in metamorphism may produce hydrothermal ore deposits

hydrothermal – hot water

common type of ore deposit

Common Metamorphic Rocks (Table 7.1)

Slate (parent – shale)

very fine-grained foliated rock composed of small mica flakes
splits at an angle to bedding planes (shale splits along bedding planes)
used for paving stone, shingles, pool tables
low-grade metamorphic rock


Phyllite (parent shale)

fine to medium grained foliated rock - muscovite or chlorite
glossy sheen distinguishes from slate
Schist (parent shale)
coarse-grained intermediate-grade strongly foliated metamorphic rock
>50% platy and rod-like minerals (micas [muscovite, biotite], and amphibole)
accessory minerals (chlorite, talc, garnet, staurolite)
Gneiss
coarse-grained high-grade metamorphic rock - contain mostly elongated and granular minerals (not platy)

alternating light and dark layers - light (feldspars), dark (ferro-magnesian)


Non-foliated rocks

Marble

metamorphosed limestone
    commonly used building stone – sculpture
    small (microcrystalline) equigranular stone good for carving
    much marble weakly foliated stripes of accessory minerals
    not as good for carving

    Marble susceptible to acid rain

Quartzite - metamorphosed sandstone
 

Contact Metamorphism

magma heats surrounding country rock – contact aureole or halo
halo may extend only a few cms or for 100’s meters
depending on width of intruding body
rule of thumb - about half width of intruding body
hornfels – non-foliated fine-grained contact metamorphic rock Migmatite – partially melted metamorphic rock

Metamorphism along fault zones - cataclastic metamorphism

fault breccia – minerals broken/ground up by fault movement
    water percolating along fault aids metamorphism
    ductile deformation/metamorphism

higher temperature – minerals flow - rather than break

Regional Metamorphism – Figure 7.22

occurs when large volumes of rock are deeply buried during major tectonic events (mountain building) Zones of Regional Metamorphism

subduction produces paired belts of blueschist – greenschist

blueschist marks subduction zone - low Temperature, high Pressure

greenschist marks high T, low P

belts in California, Japan, New Zealand, etc.