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Types of
metamorphism
Different kinds of metamorphism may
be defined on the basis of different criteria, including (1) the spatial extent
over which metamorphism occurred, (2) the geologic setting of metamorphism, (3)
the specific cause of metamorphism, (4) whether a rock equilibrated to a single
event or if one can discern superposed overprints, (5) whether or not
significant change in composition occurred, and (6) whether all or part of the
mineral assemblage developed in response to increasing or decreasing
temperature. Classifications of metamorphism therefore vary with the proclivity
of the author. The following classification is an attempt at compromise between
the various viewpoints.
Regional metamorphism occurs over an
area of great extent and affects a large volume of rock. It is thus associated
with large-scale processes, such as sea-floor spreading or mountain building
(orogeny). Local metamorphism is restricted to far more limited rock volumes and
can typically be related to a local cause/source such as a magmatic intrusion,
and fault zone. When it is possible to relate metamorphism to a particular
cause/source (as is generally the case) the following classification takes
precedence.
1. Orogenic metamorphism is a type
of regional metamorphism related to the development of mountain belts. Dynamic
and thermal effects are combined in varying proportions over a wide range of P-T
conditions, typically resulting in foliated metamorphic rocks such as slate,
schist, and gneiss (listed in order of increasing grade). Multiple deformational
and thermal phases may occur, resulting in several metamorphic overprints. Most
exposed metamorphic rocks belong to this category. See also: Gneiss; Schist; Slate
2. Burial metamorphism is a type of
regional metamorphism developed in rocks deeply buried under a sedimentary
and/or volcanic pile and is typically not associated with orogenic deformation
or extensive magmatic intrusion. It commonly involves only low to intermediate
metamorphic temperatures and low pressures, resulting in nonfoliated or poorly
foliated rocks.
3. Ocean-floor metamorphism is a
type of regional metamorphism developed near oceanic spreading centers in
response to the high local heat flow and extensive circulation of heated
seawater along pervasive fractures.
4. Contact metamorphism is a type of
local metamorphism that affects the host rocks adjacent to a magma body. It is
typically caused by heat transfer, perhaps accompanied by fluid emanations, from
a cooling magmatic intrusion. Metamorphic pressures are generally low, but
temperatures vary widely. Textures are predominantly nonfoliated, except in
cases of substantial dynamic effects due to the intrusion or when the host rocks
are already orogenically metamorphosed and the contact effects are superimposed.
5. Fault-zone metamorphism is a type
of local metamorphism associated with fault or shear zones. Dynamic effects
predominate, resulting in grain-size reduction and textures governed by the
interaction of deformation and recrystallization (typically strain
induced). See also: Earthquake;
Fault and fault structures
6. Impact metamorphism is a type of
local metamorphism caused by the impact of a meteorite or other projectile.
Extreme cases may result in melting and even vaporization.
7. Pyrometamorphism is an extreme
type of contact metamorphism characterized by very high temperatures at
relatively low pressures, generated by a very hot volcanic or subvolcanic body.
It is most typically developed in xenoliths enclosed in such bodies, but may
also occur at wall-rock contacts. It is typically accompanied by varying degrees
of partial melting. See also:
Xenolith
8. Hydrothermal metamorphism is a
type of metamorphism, typically of local extent, caused by hot water-rich
fluids. A common situation for this type of metamorphism is in geothermal fields
above cooling magmatic intrusions.
9. Polymetamorphism applies to any
situation in which one metamorphic event can be demonstrated to partially
overprint a previous metamorphism, while retaining relics of the original event.
10. Metasomatism is any metamorphism
accompanied by significant chemical alteration. This typically occurs in
situations involving fluid emanations from a crystallizing magmatic intrusion,
but may encompass a variety of situations involving contrasting rock
compositions and/or fluid mobility.
See also: Magma; Metasomatism
11. Prograde metamorphism refers to
the changes in a rock that accompany increasing metamorphic grade (the usual
case).
12. Retrograde metamorphism refers
to changes that accompany decreasing grade as a body of rock cools and recovers
from a metamorphic or igneous event. Equilibrium is better maintained during
prograde metamorphism when energy is being added, and retrograde effects are
generally minor and incomplete. Most metamorphic rocks thus retain the imprint
of the maximum metamorphic temperatures attained.
Metamorphic
gradients and zonation
Metamorphism is a response to
changes in external conditions. There are gradients in temperature, pressure,
and fluid composition in nature, so we can expect some sort of zonation in the
mineral assemblages constituting the rocks that equilibrate across an expanse of
these gradients. We should thus be able to traverse into an eroded metamorphic
area and cross from nonmetamorphosed rocks through zones of progressively higher
metamorphic grades. Figure 1 shows a cross section through a portion of a
contact aureole surrounding an intrusive igneous granite. The granite intrudes
into the shallow crust at temperatures in excess of 650°C (1200°F), and a
gradient in temperature develops as the granite heats the adjacent sediments.
Most granites also release water as they cool and crystallize, which may also
set up a gradient in fluid composition across the aureole. Because the different
sedimentary layers are of contrasting composition, they respond differently to
the thermal and fluid gradients. At the outer limits of rocks affected, shale
may develop small crystals of muscovite and chlorite. At higher temperatures
inward toward the granite, shale may progressively develop larger grains of
biotite + andalusite, cordierite + sillimanite. Very near the contact, the
original shale may be thoroughly recrystallized to a hornfels containing coarse
quartz, cordierite, and sillimanite. Because limestone and sandstone are
composed of virtually a single mineral each [limestone is calcite (CaCO3) and
sandstone is quartz (SiO2)], they are less reactive and the principal change
developed in these layers is recrystallization of the existing minerals to
produce progressively coarser marble and quartzite at higher temperatures. Only
very near the granite may some of the released silica-bearing aqueous fluids
interact with the limestone to form calcium-silicate minerals (skarn). Figure 2
shows a similar cross section through a mountain range that has been uplifted
and eroded to expose rocks that have experienced regional metamorphism. Here
gradients in both temperature and pressure over much larger distances (typically
hundreds of km) cause similar, but larger-scale transitions in the mineral
assemblages and textures of the affected rocks. See also: Andalusite; Contact aureole;
Cordierite; Granite; Limestone; Muscovite; Sandstone; Shale; Sillimanite; Skarn
Fig. 1 Diagrammatic cross section through a
contact-metamorphic aureole surrounding an intrusive igneous
granite.
Fig. 2 Cross section through an uplifted and
eroded mountain belt, exposing the orogenically metamorphosed
rocks.
The shales in Fig. 1 exhibit four
metamorphic zones of progressively higher-grade metamorphism, and the regionally
metamorphosed area in Fig. 2 exhibits four zones of much greater areal extent.
The transition from one zone to another is typically caused by changing
conditions; hence, a reaction between some or all of the lower-grade minerals to
produce the higher-grade assemblage. The transition in the field across which
this reaction occurs is called an isograd (line of constant metamorphic grade).
George Barrow pioneered the mapping of such zones in orogenically metamorphosed
shales in the Scottish Highlands. Barrow named the Scottish zones based on the
first appearance of a characteristic index mineral developed by a metamorphic
reaction marking the transition into that zone. The lower limit of any zone in
the field was thus marked by an isograd named for the index mineral that first
appeared at that isograd. Barrow's now-classical isograds and zones (in order of
increasing grade) are chlorite, biotite, garnet, staurolite, kyanite, and
sillimanite, but it is common practice to name zones on locally recognized
mineral sequences. See also:
Garnet; Kyanite; Staurolite
Metamorphic
facies
Pentii Eskola developed the concept
of metamorphic facies as an extension of the zonal method of characterizing
metamorphism and the grades developed. This approach is based on the typical
zones developed in metamorphosed basaltic igneous rocks (which span relatively
broad ranges of conditions) and attempts to assign pressure and temperature
limits to those zones. Figure 3 is a pressure-temperature (P-T) diagram
illustrating the most commonly accepted metamorphic facies and the P-T
conditions appropriate to each. See
also: Facies (geology)
Fig. 3 Pressure-temperature diagram showing the
generally accepted limits of the various facies. Boundaries are approximate and
gradational. Included are the typical, or average, continental geotherm and the
minimum melting curve for water-saturated granite. Broad gray swaths (1–3)
represent metamorphic field gradients from three contrasting terranes. 1,
average gradient for orogenic metamorphism in the Scottish Highlands; 2,
gradient developed in contact aureoles above large granitic plutons; 3, gradiant
from a subduction zone complex (Franciscan formation of
In Fig. 3, P-T trajectory 1 is representative of orogenic regional metamorphism and suggests that the typical facies series begins in the low-grade zeolite facies and extends progressively through the prehnite-pumpellyite facies, the greenschist facies, and the amphibolite facies. Large-scale melting is common in the upper amphibolite facies if sufficient water is available, and only dehydrated rocks make it into the granulite facies. Contact metamorphism, developed in aureoles around magmatic plutons, follows a P-T trajectory similar to 2 in Fig. 3, with a series of hornfels facies. Regional metamorphism accompanying the subduction of a cool plate is characterized by an unusually high P/T ratio (similar to trajectory 3 in Fig. 3), resulting in a series culminating in the blueschist facies. Very deep rocks, generally at mantle pressures, are typically metamorphosed in the eclogite facies (along the normal geotherm). It is common practice to combine the facies and zone approaches. Barrow's chlorite zone, for example, is in the greenschist facies and his staurolite zone is in the amphibolite facies.
- G. Barrow, On an intrusion of muscovite biotite gneiss in the S. E. Highlands of Scotland and its accompanying metamorphism, Quart. J. Geol. Soc., 49:350–358, 1893
- G. Barrow, On the geology of the lower Deeside and the southern highland border, Proc. Geol. Ass., 23:268–284, 1912
- K. Bucher and M. Frey, Petrogenesis of Metamorphic Rocks, 7th ed., Springer, 2002
- P. Eskola, On the relations between the chemical and mineralogical composition in the metamorphic rocks of the Orijärvi region, Bull. Commis. Geol. Finlande, vol. 44, 1915
- C. Lyell, Principles of Geology, James Murray, London, 1833
- J. Winter, An Introduction to Igneous and Metamorphic Petrology, Prentice Hall, 2001
- B. W. D. Yardley, An Introduction to Metamorphic Petrology, Longman, Essex, 1989
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