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Geol 4110
Spring 2007
Primer on Metamorphic Rocks
Metamorphism - Changes in the mineral assemblage and textures of igneous and sedimentary
rocks due to prolonged exposure to elevated temperatures and pressures (see S&P, Chapter
5). These changes happen in the solid state. Changes to rock below 200° C and 300 bars
pressure are not considered metamorphic, but rather are termed diagenetic.
Types of Metamorphism - Five types of metamorphism are generally recognized on the basis of
1) temperature range (T), 2) the type and intensity of pressure (P - confining or differential),
3) the volume of rock affected, and 4) the chemical openness of the process:
Contact metamorphism - High T, low-mod. confining P; forms adjacent to igneous
intrusions.
High-pressure metamorphism - High P (strongly differential), low-mod T; forms in
convergent boundary zones
Burial metamorphism - Low T, low-mod confining to differential P; original
sedimentary/igneous features generally preserved; forms in deeper parts of thick sedimentary
sequences in non-tectonic area (e.g., on passive continental margins)
Regional metamorphism - Variable T, mod-high differential P; characteristic of overthickened plates (i.e., mountain belts) above convergent boundaries; affects large areas
Cataclastic metamorphism - Variable T, very high directed P; typically localized to narrow
zones of intense mechanical deformation (shear zones).
Metasomatism - Variable T&P; distinguished from other forms of metamorphism by the loss
and/or gain of material (usually transported by a fluid). Other types of metamorphism are
thought to occur as nearly closed processes (except for water loss).
Metamorphic grade - At temperatures and pressures above diagenetic conditions, three general
grades of metamorphism are recognized - Low, Medium, and High. The upper limit of
metamorphism corresponds to the onset of partial melting which will vary depending on the
rock type and the amount of water in the rock.
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Rates of metamorphic change: The rate at which a rock will modify its mineral assemblage to be
in equilibrium with prevailing conditions of pressure and temperature depends on three
general factors:
1. Fluid content (particularly water) of the rock. Water helps to catalyze the mineral
transformations.
2. Temperature - chemical reactions occur faster at higher temperatures.
3. Time - For a rock to develop a new metamorphic mineral assemblage corresponding to a
particular P & T, it must exist under those condition for a sufficiently long period of time
(generally tens of thousands to millions of years).
Changes in grade - Change in mineral assemblage reflecting an increase in the grade of
metamorphism (e.g. due to sedimentary burial or tectonic thickening of the crust) is termed
prograde metamorphism. Change due to a decrease in metamorphic grade (e.g., due to
tectonic uplift and erosion) is termed retrograde. Note that retrograde metamorphism is
more difficult to accomplish because of lowering temperature and because much of the
water in the rock may have been expelled at the peak metamorphic grade. Depending on the
factors controlling the rate of change, noted above, equilibrium may not be maintained
during prograde or retrograde metamorphism such that a mix of mineral assemblages
representing different P-T regimes are preserved in the rock. Such a rock is said to have a
disequilibrium mineral assemblage.
Mineralogic Responses to Metamorphism - Changes in the mineral assemblage of a
metamorphic rock are driven by chemical components striving to achieve the lowest energy
configuration at the prevailing pressure and temperature. The types of minerals which form
are dependent not only on T & P but also on the bulk composition of the rock (note different
mineral assemblages resulting from metamorphism of a shale vs. a basalt in Figure 7.8).
These changes are well documented by laboratory experiments, which investigate the
temperature, pressure, and water content at which a particular chemical reaction will occur.
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Textural Responses to Metamorphism - Several types of textural changes occur during
metamorphism which reflect the intensity and directionality of pressure (or stress).
1) Increased grain size - During prograde metamorphism or at a particular grade that is
maintained for a long period of time, minerals will tend to increase in size.
2) Foliation - As new platy minerals grow, they will align themselves perpendicular to the
maximum stress direction. For clay mineral and fine-grained micas, the planar fabric that
results is referred to as a slaty cleavage. In higher grade rocks, coarser grained mica
minerals are said to impart a schistosity to the rock.
3) Gneissic Banding - In very high grade rocks, the dark Mg-Fe minerals (biotite, amphibole,
pyroxene, sillimanite) tend to segregate from the lighter colored minerals (feldspar and
quartz) resulting in banded rock. Why this occurs is not entirely clear.
4) Porphyroblastic texture - When some new metamorphic minerals begin to form, they grow
large and well-formed crystals that stand out in the matrix. The crystals are called
porphyroblasts and are common to minerals such as garnet (see Fig. B5.1), sillimanite,
alkali feldspar.
5) Granoblastic texture - Under contact metamorphic conditions of sustained high
temperature, rocks will develop a very granular texture. The rock is then termed a
hornfels.
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Tectonic Regimes and Metamorphism - As with volcanism, variable types and grades of
metamorphism are commonly associated with crustal rocks in a convergent tectonic
environment as shown in the figure below.
Common Types of Regional Metamorphic Rocks
Protolith
Low Grade
Shale, Mudstone,
Slate
Siltstone
Graywacke
Metagraywacke
High Grade
Schist, Gneiss
Conglomerate
Metaconglomerate
Gneiss
Sandstone
Quartzite
Quartzite
Limestone
Marble
Marble
Basalt
Greenstone
Amphibolite, Mafic gneiss
Rhyolite
Felsic shist
Granitic gneiss
Gabbro
Metagabbro
Amphibolite, Mafic Gneiss
Granite
Granite
Granitic gneiss
Schist, Gneiss