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
The Concept of Lithostatic 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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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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Metamorphic Facies and Grade
Metamorphic facies a set of metamorphic rocks
characterized by particular mineral associations
indicative of environment.
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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.