Transcript Metamorphic Rocks

Metamorphic
Rocks
Metamorphic Rocks
• Recrystallization in the solid state
• Caused by changes in T, P or
fluids
• New environment = new minerals
• Growing minerals create a new
texture
Sources of metamorphic change
Metamorphism
• Recrystallization
– T, P or change in pore fluids initiate
change in the mineral assemblage
– Reaction occur entirely in the solid state
Shale
Schist
• Textural changes occur during
metamorphism
– New minerals grow during
metamorphism
– Precursor rock textures are modified
Protolith
The parent rock subjected to Metamorphism
-can be any rock type: igneous,
sedimentary, or metamorphic
Shale
Schist
Protolith
Origin of Metamorphic Rocks
• Metamorphism
Temperature exceeds 200OC
Pressure exceeds 300 M Pa (approx.
10,000 ft deep)
• Metamorphism ends when melting
begins
Melting begins at ~700oC
Stability ranges for Al2 SiO5 minerals
• Source of heat
– Proximity to igneous intrusions
• Contact metamorphism
• Different metamorphic grade ring the
intrusion
– Depth of burial
• 15-30oC increase per km
• Geothermal Gradient
Zone of Contact
Metamorphism
Magma Body
-source of heat
Pressure and Stress
Lithostatic Pressure- pressure increases
with depth due to the weight of the
overlying rocks.
-equal in all directions: uniform stress
-a cubic foot of granite weighs 167.9
pounds.
Lithostatic Pressure - equal from all directions
Tectonic Pressure/Stress
-directional/non-uniform
-referred to as deviatoric stress
Folded Metamorphic Rocks
-subjected to non-uniform deviatoric stress
• T & P determine degree of
metamorphism
Low-grade metamorphism- 200 to 350 OC
Intermediate-grade metamorphism-350 to
550 OC
High-grade metamorphism - very high
temperatures, above 550OC
Progressive metamorphism of shale
Fig. 6.5. Stability ranges for Al2O5 minerals
Index Minerals
Metamorphic Rocks
• Classified by texture and
composition
– Texture is divided into foliated &
nonfoliated rocks
– Mineral composition qualifies
textural name
Foliated
Nonfoliated textures
Foliation:
-preferred orientation
or location of minerals
Slate
• Phyllite
– Similar to slate
– More Recrystallization
• Crystals are larger
– Common Protolith:
– shales
• Schist
– Strongly foliated rock
– Medium to coarse grained
-dominated by platy minerals
– Protoliths: multiple??
[Garnet] Schist w/porphyroblasts
• Gneiss
– Coarse grained granular rock
– Foliation: alternating bands of light
& dark minerals
• Quartz, feldspar, amphibole,
• biotite
– Protoliths: ??
Nonfoliated Rocks
• Marble
– Interlocking, coarse grained calcite
– Recrystallization of limestone or
dolostone
– Sedimentary features
destroyed
• Quartzite
– Metamorphism of quartz sandstone
– Entire rock is recrystallized
• Amphibolites & Eclogites
– Metamorphism of mafic rocks
– Mineral assemblage
Amphibolite [pillow basalt]
gives green color
Eclogite from Norway
Regional Metamorphic Zones
• Index minerals
– A mineral that forms within a
specific,often narrow range of
conditions
– Identifies a specific grade of
metamorphism
– Allows further subdivision of rock
types
Metamorphic Index Minerals
• Protolith 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
• May be found in different rocks
Regional
Metamorphic
Gradients
Metamorphic facies
Metamorphic Rocks & Tectonics
• Most metamorphic rocks develop due
to plate collisions
– Processes are not observable in the field
– Conditions at various plate boundaries
define possible P & T range
– Parent rocks present control mineral
assemblages