Introduction to Metamorphic Rocks

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Introduction to Metamorphic Rocks
Metamorphism is the solid-state transformation of
pre-existing rock into texturally or
mineralogically distinct new rock as the result
of high temperature, high pressure, or both.
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Differential Pressure
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The Concept of Geothermal Gradient
Temperature increases with depth at a rate of
20-30oC/km in the crust.
Temperature
B
Deeper in Crust
A
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3 Types of Metamorphism
Regional Metamorphism Large-scale deformation
associated with T and P
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Types of Metamorphism

• Regional Widespread changes in temperature and
  pressure bring about changes in rocks due to
  tectonic forces (convergent boundaries)
• Contact Intrusion of magma against colder rocks.
  Affected area is proportional to the size and
  temperature of the intrusion - always only a
  local phenomenon (continental interiors)
• Seafloor - Hydrothermal Changes in rocks at the
  mid-ocean ridge associated with chemical
  reactions promoted by the infiltration of heated
  seawater (mid-ocean ridges)

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Metamorphic Reactions

• Mineralogical changes (e.g., clay to mica)
  Series of complicated reactions that depends on
  pressure, temperature, and composition
• Depend on metamorphic grade, duration,
  composition.
• Common metamorphic minerals include
• Amphibole - Staurolite
• Garnet - Kyanite
• Mica - Sillimanite
• Textural changes recrystallization (grain
  boundaries become more compact) and foliation
  (preferred orientation of minerals)

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Textural Changes

• Grain size - commonly increases with time and
  temperature
• Grain shape - as mineralogy changes (e.g.,
  biotite -gt garnet), and as grains grow under
  directed pressure
• Grain preferred orientations
• Cleavage, Foliation - arrangements of platy
  grains
• Lineation - alignments of elongate grains

Fig. Story 9.4
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Textural Changes
Fig. Story 9.4
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Textural Changes Foliated Rocks
Fig. Story 9.4
Protolith Shale
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Granoblastic Rocks

• Equant Crystals - grains are generally
  equidimensional, rather than platy or elongate

Fig. 9.5
Protolith Sandstone
Protolith Limestone
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Stability of Minerals

• Most minerals are stable over a relatively narrow
  range of pressure and temperature (e.g., ice
  unstable above 0C).
• The stability range of different minerals
  sometimes overlap and provide insights into the
  metamorphic history of rocks
• Unique P T conditions

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3 Types of Metamorphism
Contact Metamorphism The main metamorphic agent
is heat.
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3 Types of Metamorphism
Mylonite from shear zone in Appalachians
Dynamic metamorphism Rock deformation associated
with fault zones
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Metamorphic Facies and Grade
B
A
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Spatial Distribution of Metamorphic Facies
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Metamorphism of Sedimentary Rocks
Protolith (Precursor)
Sandstone Quartzite, Metaquartzite Shale Ph
yllite Slate Schist Gneiss Limestone Ma
rble
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Progression of metamorphism
Start with a shale and then hit it with pressure
and heat.
Slate
Phyllite
Schist
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You end up with something that is really Gneiss!
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A foliation is any planar fabric in a metamorphic
rock. In this case, the foliation is defined by
aligned sheets of muscovite sandwiched
between quartz grains.
This slide is indicative of a phyllite.
The foliation in this rock is a crenulation
cleavage, and is developed after the
horizonal foliation.
This slide is indicative of a schist.
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Metamorphism of Igneous Rocks
For most of our purposes, just put meta in
front of the protolith name. Examples metabasalt
metarhyolite If a mafic or intermediate
metamorphic rock is dominated by amphibole and
feldspars -Amphibolite.
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High Grade Mafic Rocks Eclogites
Garnet and clinopyroxene are the two major
minerals in eclogite. Eclogite is basalt which
has been metamorphosed at very high pressures in
subduction zones.
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Index minerals for Regional Metamorphism
In regional metamorphic terranes, the temperature
and pressure regime is indicated by the
distribution of metamorphic minerals across a
large area. Low metamorphic grade (low T and P
lt200oC) Slate and phyllite chlorite,
muscovite, biotite Intermediate metamorphic
grade Schist garnet, staurolite, kyanite High
metamorphic grade - 800 degrees C (verging on
melting) Gneiss kyanite, sillimanite Highest
metamorphic grade Partial melting Migmatite
rock partially melts, but no mass loss. Hence
the mineralogy tells us the metamorphic grade of
the rock.
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How does Metamorphism Relate to Plate Tectonics?
T and P increase at collisional (convergent)
plate boundaries.