METAMORPHISM
Fig. Ta 7.1
Definition of Metamorphism
The alteration of a preexisting igneous,
sedimentary, or
metamorphic rock (called
the “parent rock” or
“protolith”) by the
application of heat and/or
pressure. Complete melting
does not occur.
Metamorphism depends on:
Parent Rock
Geothermal Gradient
Equilibrium Exchange
Change in P-T-X Confining/lithostatic pressure
Differential Stress: Shearing & Compression
TIME
Confining Pressure
Compressive
Stress
Shearing
Metamorphosed Conglomerate
Effects of Metamorphism
Mineralogical: - change minerals to reflect
new P-T (equilibrium) conditions.
Recrystallization: change of existing crystal size and
shape to form layers (e.g., shale to schist),
interlocking crystals (e.g., sandstone to quartzite).
Recombination: Recombination of elements in preexisting minerals to form stable new ones.
Addition/subtraction of ions common when fluid is
present.
Effects of Metamorphism
Textural: Re-orientation of mineral grains,
especially platy (micaceous) grains in
response to differential stress.
Non-foliated: random orientation – either no platy
minerals or no directed stress (isotropic);
Granoblastic (equigranular or isotropic) or
Porphyroblastic (metamorphic porphyritic texture).
Foliated: fabric developed through directed pressure
and/or shearing.
Development of Foliation
Non-Foliated
Foliated
Foliation
Oriented Minerals: platy or needle-like minerals
growing in a plane either re-orientation or existing or
growing of new.
Gives cleavage or planes of weakness.
Slate: oriented clay minerals allows the rock to be
easily cleaved.
Phyllite: growth of microscopic micas to give strong
foliation – rock is “shiny”.
Schist: strongly foliated with visible micas and feldspars.
Gneiss: strongly banded rock with dark bands of micas
& pyroxenes, light bands of feldspars & quartz.
Migmatite: strongly veined rock as it has partially melted.
Table 07.01
Typical Metamorphic Minerals
Chlorite:
Talc:
Serpentine:
Graphite:
Garnet:
Staurolite:
Hydrous Fe-Mg-Al silicate
Hydrous Mg-silicate
Hydrous Mg silicate
Carbon
Ca-Mg-Fe-Al silicate
Fe-Al silicate
Also:
Quartz, muscovite, biotite, feldspar, amphibole, calcite.
Metamorphic Grade
Degree of parent rock alteration, mostly
dependent on increasing temperature for
increasing grade.
Prograde: slate-phyllite-schist-gneiss-migmatite (melting).
Prograde metamorphic Reaction:
CaCO3
Calcite
+
SiO2 
Quartz
CaSiO3
Wollastonite
+
CO2
Fluid
Classification
Mineralogical and textural if foliated:
“garnet-mica schist”
“quartz-feldspar gneiss”
If non-foliated, named on compositional
basis (and often color):
“(white) quartzite” or “(pink) marble”
Types of Metamorphism: Contact
Thermal, local, around intrusions. Size of aureole depends
on:
Size of intrusion
Heat (composition)
Fluid content of
magma
Fluid content of
country rock
Country rock type
Types of Metamorphism: Contact
Produces non-foliated, granoblastic rocks:
Hornfels (if clastic - shale)
Quartzite (if sandstone)
Marble (if carbonate)
Types of Metamorphism: Regional
Also known as dynamothermal - produces both foliated
and non-foliated metamorphic rocks.
Associated with mountain belts - affects very large areas.
Heat & directed pressure on rocks buried deep within
the Earth - Prograde or Retrograde.
Prograde metamorphic reactions liberate a fluid.
Retrograde is difficult – pore spaces decrease during
prograde so getting fluid back in is not easy.
Regional
Metamorphism
Types of Metamorphism: Shock
Meteorite impact – sudden and intense deformation.
See www.meteorcrater.com
Types of Metamorphism: Shock
High-pressure polymorph of quartz – coesite – can form.
Impact melt can form.
Lots of rock fragmentation & mineral deformation.
Moon – no
atmosphere so lots
of meteorite
impacts (micro and
macro!). Produces
regolith, rock flour,
impact melt,
breccias.
Water in Metamorphism
Provides transport mechanism and can promote reactions.
Hydrothermal metamorphism: hot water streams
add/remove ions. May promote ore formation.
Water in Metamorphism
Metasomatism: addition of ions from external source.
Water in Metamorphism
Metamorphic aureole is greater around granitic plutons than around
gabbroic plutons, even though the magma temperature is lower.
Ore-bearing
veins in a mine
Metamorphic Grade:
Degree of parent rock
alteration, mostly
dependent on
increasing temperature
for increasing grade
Prograde: SLATE phyllite-schist-gneissmigmatite (melting).
SLATE: oriented clay
minerals allows the
rock to be easily
cleaved.
Foliation can be deformed
Prograde: slatePHYLLITE-schistgneiss-migmatite
(melting).
PHYLLITE: growth of
microscopic micas to give
strong foliation – rock is
“shiny”.
SCHIST: strongly foliated with visible micas & feldspars.
Prograde: slate-phyllite-SCHIST-gneiss-migmatite.
Garnet Schist
Fig. 07.14
GNEISS: strongly banded
rock with dark bands of micas
& pyroxenes and light bands
of feldspars and quartz.
Prograde: slatephyllite-schistGNEISS-migmatite.
Gneissose Banding
Prograde: slatephyllite-schistgneissMIGMATITE.
MIGMATITE:
strongly veined
rock as it has
partially melted.
0065
Metamorphic Facies:
Rocks having broadly similar mineral assemblages =
same metamorphic facies – analogous to climatic zones.
The metamorphic facies is determined by the specific
mineral assemblage present.
The facies is defined as a mineral assemblage that may
only occur over a relatively narrow range of P-T
conditions.
Formed under broadly similar P-T conditions.
Mineralogy dependent upon protolith composition.
Metamorphic Facies:
Plate Tectonics and Metamorphism
Plate Tectonics and Metamorphism
Gravitational collapse & spreading – central part of mountain
becomes too high & collapses. Rock forced downward.
Foliation developed parallel to collapse & spreading.
Index Minerals
Form over a restricted range of pressure & temperatures
(determined in the laboratory).
When found in rocks, the P-T of formation can be
reasonably estimated.
Index Minerals
Kyanite, Andalusite, Sillimanite: all Al2SiO5, different
crystal structures - polymorphs.
Andalusite - low P & T; Kyanite - high P;
Sillimanite - high T.
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