metamorphic

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Terms: metamorphism, regional metamorphism, contact metamorphism, hydrothermal metamorphism, foliation, slaty, phyllitic, schistose, mica, garnet

What causes metamorphism?

Metamorphic rocks are created by solid-state recrystallization when pressure, heat, and/or chemically active fluids are introduced to the rock. The preexisting rock is not destroyed by weathering, as in the formation of sediment in sedimentary rocks, nor is it melted, as in the formation of igneous rocks. 

Type of metamorphism driving process occurrence
Contact or Thermal metamorphism introduction of heat surrounding igneous intrusive bodies (plutons)
Cataclastic metamorphism pressure (shear) shearing along faults
hydrothermal metamorphism introduction of chemically active fluids related to fluids released from plutons and metamorphic reactions
Regional metamorphism a little bit of everything Convergent Plate Boundaries

How are metamorphic rocks classified?

Like all rocks Metamorphic rocks are classified by their texture and composition (tables 1 and 2).   The texture refers to their mineral fabric and grain size. Metamorphic may or may not have a fabric, called foliation.   Foliation is the alignment of metamorphic minerals in response to pressure.  Rock that are contact metamorphosed lack foliation as do metamorphic rocks that have equant grains and lack platy or needle-like crystals. 

Table 1. Classification of Non-Foliated Metamorphic Rocks

TEXTURE

COMPOSITION
METAMORPHIC ROCK
Randomly-oriented crystals.
May be microscopic.
Talc and Green Amphibole
(white-green, soapy feel, H<2.5)
Soapstone
Serpentine
(various shades of green, waxy luster, commonly microscopic)
Serpentinite
Randomly-oriented crystals.
May be microscopic (low grade)

Original sedimentary bedding may be visible.
Variable
(quartz, feldspar, andalusite, mica, etc.)
Hornfels
Composed of granular crystals.
Calcite
(white-to-brown, H < 5.5)
Marble
Quartz
(white-to-brown, H > 5.5)
Quartzite

 

Table 2. Classification of Foliated Metamorphic Rocks
TEXTURE
FOLIATION
COMPOSITION
METAMORPHIC ROCK
microscopic
Slaty Rock Cleavage
(breaks into smooth, flat fragments and has a dull luster)
microscopic clay minerals and incipient micaceous minerals
Slate
Phyllitic Foliation
(breaks into smooth, flat or wavy fragments and has a shiny luster)
microscopic micaceous minerals
Phyllite
macroscopic
Schistose Foliation
(composed predominantly of tabular or needle-like minerals. Rock cleavage is fair to good.)
Variable
Contains one or more of the following minerals: biotite, chlorite, muscovite, talc, garnet hornblende, quartz, feldspar, etc.
Schist
(Examples: biotite schist, chlorite schist, talc schist, muscovite garnet schist, mica schist, hornblende schist, etc.)

Gneissic Foliation
(contains dark-colored schistose layers or lenses that alternate with non-schistose granular layers of quartz and feldspar.)

Gneiss
(Examples: biotite gneiss, hornblende gneiss, mica gneiss, etc.)

 

What metamorphic rocks tell usThe mineral assemblages in a metamorphic rock reflect the pressure and temperature conditions that created them.   because pressure can be equated with depth we can determine how much uplift and erosion has occurred since the rocks were metamorphosed.  The composition of a metamorphic rock reflect the rock (protolith) from which is was created. Some examples are given in Table 2. Table 1. Common metamorphic rocks and their protoliths.
Metamorphic Rock
Composition: dominant mineral(s)
Protolith
marble
calcite
limestone
quartzite
quartz
quartz sandstone
serpentinite
serpentine
ultramafic rock

amphibolite
(hornblende schist)
amphibole (e.g. hornblende)
basalt or gabbro

greenstone/greenschist

chlorite, grn. amphibole
mafic rock
muscovite (garnet) schist
muscovite, garnet
mudstone or shale
orthogneiss1
quartz, feldspar, mica, etc.
granite or other igneous rock
paragneiss2
quartz, feldspar, mica, etc.
layered detrital rock (e.g. shale with interbedded sandstone
1 Orthogneiss contains thin wavy discontinuous schistose lenses resulting from the alignment of micaceous minerals when an igneous rock is regionally metamorphosed.
2 Paragneiss contains parallel schistose layers.