Transcript Lecture 11 -Metamorphism and Metamorphic Rocks

Metamorphism and Metamorphic
Rocks
GLY 2010 - Summer 2012 - Lecture 11
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Metamorphism
• The mineralogical, chemical, and
structural adjustment of solid rocks to
physical and chemical conditions
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Metamorphic Rock
• Any rock derived from pre-existing rocks by
mineralogical, chemical, and/or structural
changes, essentially in the solid state
• Derivation is in response to marked changes in
temperature, pressure, shearing stress, and
chemical environment, generally at depth in
the Earth's crust
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Agents of Metamorphism
• Temperature
• Pressure
• Chemically Active Fluids
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Temperature
• Minerals in rocks which are heated
may become unstable
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Pressure
• Minerals in rocks which are under
increased pressure may become
unstable
• Pressure may be directed or
hydrostatic
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Directed Pressure
• Pressure may be directed (along a single
axis)
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Metamorphism from Compression
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Effect of Compression
• Compression has
deformed the
strata and caused
metamorphism
• This represents a
continentcontinent plate
collision
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Lithostatic Pressure
• Lithostatic pressure is equal in all
directions
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Confining Pressure
• At depth,
lithosphatic
pressure is due
to confinement
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Chemically Active Fluids
• Fluids from different sources
• Fluids are capable of dissolving ions and
ion-exchanging with exiting minerals, to
produce new minerals
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Regional Metamorphism
•
•
•
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Burial
Dynamothermal
Fault
Metasomatism
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Burial
• When sediments are deposited, they
begin to change
• Often associated with large sedimentary
basins
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Dynamothermal
• Rocks caught between two colliding plates
• Seen as the roots of ancient mountain
regions, such as in parts of Minnesota,
Wisconsin, and Michigan
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Fault
• Movement of rock along fault generates
shearing stress in rock (increased
pressure)
• Stress can lead to smearing of rocks
along the fault, and the creation of some
new minerals
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Metasomatism
• The presence of interstitial, chemically
active pore liquids or gases contained
within the rock body or introduced from
external sources are essential for the
replacement process
• Often, though not necessarily, this occurs at
constant volume with little disturbance of
textural or structural features
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Contact Metamorphism
aka Thermal
• Proximity to a heat
source may cause
new minerals to
form
• Zone of contact
metamorphism is
known as contact
aureole
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Contact Metamorphic Diagram
• Development
of contact
metamorphic
aureole
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Foliation
• A general term for a planar arrangement
of textural or structural features in any
type of rock
• Involves alignment of platy or elongated
minerals grains
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Foliation exposed in a mine
• Björnevatn Mine, Kirkenes, Norway
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Foliated Vs. Nonfoliated
• Metamorphic rocks produced by directed
pressure, such as tectonic or shearing
forces, will be foliated
• Rocks produced by load pressure, or not
involving pressure, will not be foliated
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Development of Foliation
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Metamorphic Grade
• Grade indicates the severity of conditions
at the time of metamorphism
• Grade is judged on the basis of:
 Foliation
 Grain size
 Mineral assemblage - index minerals
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Metamorphic Grade Indicators
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Metamorphic
Mapping
• The first
appearance of
grade minerals
can be used to
produce maps of
metamorphism
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Foliated Rocks
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Slate
•Red slate contains
oxidized iron
• A compact, fine-grained metamorphic rock
that possesses slaty cleavage and hence can be
split into slabs and thin plates
• Most slate was formed from shale
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Slaty Cleavage
Gray slate showing foliation (slaty cleavage) in quarry
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near Alta, Norway
Uses of Slate
Pool table at
Campobello, F.D.
Roosevelt's home
Slate Roof
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Phyllite
• A metamorphosed rock,
intermediate in grade
between slate and mica
schist
• (Upper) Cleavage
surfaces have a silky
sheen
• (Lower) Phyllites
commonly exhibit
corrugated cleavage
surfaces
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Schist
• A strongly
foliated
crystalline rock,
formed by
dynamic
metamorphism
•Garnet schist showing well-developed schistosity
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Mica Schist
• (Above) Mica
imparts a shiny luster
to rock
• (Left) Garnet mica
schist
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Gneiss
• High grade regional metamorphic rock
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Gneiss Continued
• Gneissic foliation often involves felsic
and mafic layers
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Contortion in Gneiss
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Migmatite
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Increasing Metamorphic Grade
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Non-foliated Rocks
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Marble
• Metamorphic
equivalent of
limestone
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Quartzite
• A metamorphic rock consisting mainly of
quartz
• Sioux Quartzite, South Dakota
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Hornfels
• A fine-grained rock
• Composed of a mosaic of
equidimensional grains
• No preferred orientation
• Typically formed by contact
metamorphism
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Table Mountain Capetown, South Africa
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Malmesbury
Hornfels
The Malmesbury Group was made famous by
Charles Darwin during his voyage of scientific
discovery on H.M.S. Beagle in 1844
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Anthracite
• Anthracite is metamorphosed coal
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Fault
Metamorphism
• Brittle fracture
under low
confining
pressure
• Ductile flow
under high
confining
pressure
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Skarn
• Lime-bearing silicates, of any geologic age,
derived from nearly pure limestone and
dolomite with the introduction of large
amounts of silicon, aluminum, iron and
magnesium
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Skarns As Ores
• Gold-bearing
vesuvianite and
grossular skarn,
near Benambra,
northeast
Victoria,
Australia
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