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Metamorphic Rocks
•  Rocks that recrystallize without melting
(solid state) at high temps and pressures
•  Caused by changes in T, P or pore
fluids
•  New environment often = new minerals
•  Growing minerals create a new texture*
Fig. 6.4. Sources of metamorphic change
Fig. 6.1b. Metamorphic rocks exposed at Mt. Everest.
Deformation occurs at various scales
Origin of Metamorphic Rocks
•  Metamorphism begins when
– Temperature exceeds 200OC
– Pressure exceeds 3 kb (approx. 3 km or
10,000 ft deep)
•  Metamorphism ends when melting
begins (~700°C)
Two types of Metamorphic
Environments
– Contact metamorphism
•  Close proximity to igneous intrusions; high temps
and chemical activity of fluids (hydrothermal
alteration)
•  Few meters to a few hundred meters from intrusion
•  Small volumes of rock; localized
– Regional metamorphism
•  Roots of mt. belts; plates collide; high T&P and
directed stresses (foliated rocks)
•  Tens to hundreds of kilometers
•  Involves large volumes of rock
Fig. 6.6. Sources of heat
for metamorphism
Contact Metamorphism
Regional Metamorphism
Origin of Metamorphic Rocks
•  T & P combined determine degree of
metamorphism & mineral assemblage
–  Low-grade metamorphism- 200 to 350 OC and
relatively low pressures
–  Intermediate-grade metamorphism - 350 to
550 OC and moderate to high pressures
–  High-grade metamorphism - very high
temperatures, usually above 550OC and/or very
high pressures
Deformation
•  Mineral growth responds to stress
–  Differential stress causes metamorphic rocks to
have distinct textures - Foliation
–  Uniform stress does not create a mineral fabric
– Nonfoliated
•  Generally, grain size in foliated rocks increase
with intensity of metamorphism
•  In foliated rocks, mineral alignment is
perpendicular to regional horizontal stress
Metamorphic Rocks
•  Classified by texture and composition
– Texture is divided into foliated &
nonfoliated rocks
– Mineral composition qualifies textural
name ex. Chlorite schist
•  Index minerals provide information of
specific T & P conditions
Foliated vs. Nonfoliated textures
Foliated Rocks
•  Slate
– Fine grained rock showing slaty cleavage
– Slaty cleavage produced by
recrytallization of platy minerals
– Growth is perpendicular to stress
– Slaty cleavage does not always coincide
with bedding planes
– Originate from shales
Fig. 6.10a. Slate
Foliated Rocks
•  Schist
– Strongly foliated rock
– Medium to coarse grained, dominated by
platy minerals
– Further classified by dominant platy
mineral & accessory minerals, ex.
Chlorite schist
– Numerous parent rock types
Fig. 6.10b. Schist
Foliated Rocks
•  Gneiss
– Coarse grained granular rock
– Foliation occurs as alternating bands of
light & dark minerals
•  Light: Quartz, feldspar
•  Dark: Amphibole & biotite
– Numerous parent rock types
Fig. 6.10c. Gneiss
Nonfoliated Rocks
•  Marble
– Interlocking, coarse grained calcite
– Recrystallization of limestone or
dolostone
– Sedimentary features are destroyed
Fig. 6.10f. Marble
Nonfoliated Rocks
•  Quartzite
– Metamorphism of quartz sandstone
– Pore space in sandstone is filled with
silica cement
– Entire rock is recrystallized
Nonfoliated Rocks
•  Metaconglomerate
–  A conglomerate altered by heat & pressure
–  Individual pebbles are stretched, deformed, and
fused
–  Very hard and dense (indurated); if fractured or
broken, could break across pebbles as easily as
around them
Regional Metamorphic Zones
•  Large scale changes in thick masses of
rock
– Systematic change in degree of
metamorphism
•  Changes in P & T
•  Metamorphic zones are defined by index
minerals
Regional Metamorphic Zones
•  Precursor rock controls final mineral
assemblage
– Different mineral assemblages form at a
given P & T for various parent rocks
– Metamorphic facies describe a group of
minerals formed under similar conditions
Metamorphic Rocks & Tectonics
•  Most metamorphic rocks develop due
to plate collisions at convergent
boundaries – high grade
metamorphism (hi T &P) but can also
get low grade (low T&P) in shallow
crust
•  Divergent boundaries – usually just
low grade metamorphism
Fig. 6.19. The origin of metamorphic rocks
End of Chapter 6
Extras
Metamorphism
•  Textural changes:
–  New platy minerals grow in direction of least
stress producing planar texture called foliation.
•  Three types of foliation:
–  Slaty cleavage – tendency of rock to split into
thin layers
–  Schistosity – parallel arrangement of large platy
minerals like mica, chlorite, talc
–  Gneissic layering – alternating light & dark
layers
Regional Metamorphic Zones
•  Index minerals
– A mineral that forms within a
specific,often narrow range of conditions
– Identifies a specific grade of
metamorphism, ex. Low grade: chlorite,
High grade: sillimanite
Metamorphic Index Minerals