Download 07 Chapter 7_Metamorphism and Metamorphic Rocks

Engineering Geology
CVL 3315
Chapter 7
Metamorphism and Metamorphic
Rocks
Dr. Sari Abusharar
University of Palestine
Faculty of Applied Engineering and Urban Planning
Civil Engineering Department
2nd Semester 2014-2015
1
Outline of Presentation
What Is Metamorphism?
What Drives Metamorphism?
Metamorphic Textures
Common Metamorphic Rocks
Metamorphic Environments
Metamorphic Zones
2
3
Metamorphism
The transition of one rock into another by
temperatures and/or pressures unlike those in
which it formed
Every metamorphic rock has a parent rock—
the rock from which it was formed.
Metamorphic rocks are produced from
• Igneous rocks
• Sedimentary rocks
• Other metamorphic rocks
The agents of metamorphism include:
– heat
– pressure (stress)
– chemically active fluids
During metamorphism rocks are usually subjected
to all three agents and the contribution of each
agent vary greatly from one environment to
another.
Metamorphism progresses incrementally from lowgrade to high-grade
During metamorphism the rock must remain
essentially solid
Metamorphism
Metamorphic settings
• Contact or thermal metamorphism – (heat
from magma) driven by a rise in temperature
within the host rock
• Hydrothermal metamorphism – chemical
alterations from hot, ion-rich water
• Regional metamorphism
– Occurs during mountain building
– Produces the greatest volume of metamorphic rock
Agents of metamorphism
Agents of metamorphism
Heat as a metamorphic agent
• Most important agent – it provides the
energy needed to drive the chemical
reactions that results in recrystallization.
• Recrystallization results in new, stable
minerals
• Two sources of heat
1. Heat from magma (contact metamorphism)
2. An increase in temperature with depth due
to the geothermal gradient
Agents of metamorphism
Pressure and differential stress
• Increases with depth
• Confining pressure applies forces equally
in all directions
• Rocks may also be subjected to
differential stress which is unequal in
different directions
Confining pressure in metamorphism
When confining
pressure increases,
rocks deform by
decreasing in volume.
Differential stress in metamorphism
During mountain building, rocks subjected to
differential stress are shortened in the direction
that pressure is applied and lengthened in the
direction perpendicular to that force.
Differential stress in metamorphism
Metaconglomerate,
also called stretched
pebble (4-64mm)
conglomerate. These
once nearly spherical
pebbles have been
heated and flattened
into elongated
structures.
Agents of metamorphism
Chemically active fluids
•
•
•
•
Mainly water with other volatile components
Enhances migration of ions
Aids in recrystallization of existing minerals
Sources of fluids
– Pore spaces of sedimentary rocks
– Fractures in igneous rocks
– Hydrated minerals such as clays and micas
Agents of metamorphism
The importance of parent rock
• Most metamorphic rocks have the same overall
chemical composition as the parent rock from
which they formed
• Mineral makeup (‫ )بنية المعدن‬determines, to a large
extent, the degree to which each metamorphic
agent will cause change
• For example, when magma forces its way into a clean
quartz sandstone, very little alteration may take place
• But if host rock is a “dirty” limestone (some clay), calcite
(CaCO3) may react with silica in clay to form
wollastonite (CaSiO3) plus carbon dioxide (CO2)
Metamorphic textures
Texture refers to the size, shape, and
arrangement of grains within a rock
Foliation ‫التورق‬/‫ –التصفح‬any planar
arrangement of mineral grains or
structural features within a rock
Examples of foliation
– Parallel alignment ‫ ارتصاص متوازي‬of flattened
mineral grains and pebbles
– Slaty cleavage ‫انفصام اردوازي‬where rocks can be
easily split into thin, tabular sheets
Metamorphic textures
Foliation
• Foliation can form in various ways
including
– Rotation of platy and/or elongated minerals
– Recrystallization of minerals in the direction
of preferred orientation
– Changing the shape of equidimensional
grains into elongated shapes that are aligned
Mechanical rotation of platy or elongated mineral
grains.
A. Existing mineral
grains keep their
random orientation
if force is uniformly
applied.
B. As differential
stress causes rocks
to flatten
Foliation resulting from
directed stress
This mechanism for changing
the shape of mineral grains
occurs when units of the
mineral’s crystalline structure
slide relative to one another.
‫‪Metamorphic textures‬‬
‫النسيج المتصفح ‪/‬المتورق‬
‫‪Foliated textures‬‬
‫النسيج المتصفح اإلردوازي ‪• Slaty‬‬
‫النسيج الشيستوزي ‪• Schistosity‬‬
‫النسيج المتطبق النيسوزي‪• Gnissosticy‬‬
Metamorphic textures
Foliated textures
‫المتورق‬/ ‫النسيج المتصفح‬
• Slaty ‫النسيج المتصفح اإلردوازي‬
– Low grade metamorphism
– Closely spaced planar surfaces along which
rocks split
– Can develop in a number of ways depending
on metamorphic conditions and parent rock
Development of rock cleavage
Slaty Cleavage
Excellent
slaty
cleavage
is
exhibited by the
rock in this slate
quarry.
Because
slate breaks into
flat slabs, it has
many uses. The
inset photo shows
the use of slate for
the roof of this
house
in
Switzerland.
Metamorphic textures
Foliated textures
• Schistosity ‫النسيج الشيستوزي‬
– Medium grade metamorphysim
– Platy minerals are discernible
with the unaided eye and exhibit
a planar or layered structure
– Rocks having this texture are
referred to as schist
schist
Metamorphic textures
Foliated textures
• Gnissosticy‫النسيج المتطبق النيسوزي‬
– During higher grades of
metamorphism, ion
migration results in the
segregation of minerals
– Gneissic rocks exhibit a
distinctive banded
appearance
Gneiss
Metamorphic textures
Other metamorphic textures
• Nonfoliated texture ‫النسيج غير المتصفح‬
Those metamorphic rocks that
lack foliation are referred to as
nonfoliated
– Develop in environments where
deformation is minimal
– Typically composed of minerals that
exhibit equidimensional crystals
Metamorphic textures
Other metamorphic textures
• Porphyroblastic textures
‫النسيج البورفير التحولي‬
– Large grains, called
porphyroblasts, surrounded
by a fine-grained matrix of
other minerals
– Porphyroblasts are typically
garnet ‫جارنت‬, staurolite ‫شتوروليت‬
, and/or andalusite ‫أندالوسيت‬
Common metamorphic rocks
Common metamorphic rocks
Foliated rocks
• Slate ‫صخر اإلردواز‬
– Very fine-grained
– Excellent rock cleavage
– Most often generated
from low-grade
metamorphism of
shale, mudstone, or
siltstone
Common metamorphic rocks
Foliated rocks
• Phyllite ‫صخر الفيلليت‬
– Gradation in the degree of
metamorphism between slate
and schist
– Platy minerals not large enough
to be identified with the unaided
eye
– Glossy sheen and wavy surfaces
– Exhibits rock cleavage
– Composed mainly of fine
crystals of muscovite and/or
chlorite
Common metamorphic rocks
Foliated rocks
• Schist
‫شست‬
– Medium- to coarse-grained
– Platy minerals predominate
– Commonly include the micas
– The term schist describes the
texture
– To indicate composition,
mineral names are used (such
as mica schist)
Common metamorphic rocks
Foliated rocks
• Gneiss ‫نيس‬
– Medium- to coarse-grained
– Banded appearance
– High-grade metamorphism
– Often composed of white or
light-colored feldspar-rich
layers with bands of dark
ferromagnesian minerals
Common metamorphic rocks
Nonfoliated rocks
• Marble
– Coarse, crystalline
– Parent rock was limestone
or dolostone
– Composed essentially of
calcite or dolomite crystals
– Used as a decorative and
monument stone
– Exhibits a variety of colors
Common metamorphic rocks
Nonfoliated rocks
• Quartzite ‫كوراتزيت‬
– Formed from a parent
rock of quartz-rich
sandstone
– Quartz grains are fused
together
Metamorphic environments
contact or thermal metamorphism
‫التحول التماسي أو الحراري‬
hydrothermal metamorphism
‫التحول الحرمائي‬
Types of
Metamorphic
Environments
burial and subduction zone
metamorphism
‫التحول بالدفن ونطاقات االندساس‬
regional metamorphism
‫التحول االقليمي‬
metamorphism along faults
‫تحول خالل الصدوع‬
impact metamorphism
‫تحول االرتطام‬
Metamorphic environments
Contact or thermal metamorphism
• Occurs due to a rise in temperature when
magma invades a host rock
• A zone of alteration called an aureole ‫ھالة‬forms
in the rock surrounding the magma
• Most easily recognized when it occurs at the
surface, or in a near-surface environment
Metamorphism
environment
The dark layer consists
of metamorphosed
host rock adjacent to
the upper part of the
light-colored igneous
pluton.
Contact Metamorphism
Contact
metamorphism
of shale yields
hornfels, while
contact
metamorphism
of quartz
sandstone and
limestone
produces
quartzite and
marble,
respectively.
Metamorphic environments
Hydrothermal metamorphism
• As large magma bodies cool and solidify,
silica-rich fluids (mainly water) are driven
into the host rocks.
• When the host rock is highly fractured,
mineral matter contained in these
hydrothermal solutions may precipitate to
form a variety of minerals, some of which
are economically important.
Metamorphic environments
Hydrothermal metamorphism
• Chemical alteration caused when hot, ionrich fluids, called hydrothermal solutions,
circulate through fissures and cracks that
develop in rock
• Most widespread along the axis of the
mid-ocean ridge ‫حيد وسط المحيط‬
Hydrothermal Metamorphism
Hydrothermal
metamorphism
along a midocean ridge.
Hydrothermal Metamorphism
As seawater percolates through the new, hot
oceanic crust, it is heated and chemically reacts
with the newly formed basaltic rocks.
The result is the conversion of ferromagnesium
minerals, such as olivine and pyroxene, into
hydrate silicates, such as serpentinite, chlorite,
and talc.
Hydrothermal Metamorphism
Hydrothermal solutions circulating through
the seafloor also remove large amounts of
metals, such as iron, cobalt, nickel, silver,
cooper and gold from the crust.
These hot, metal rich waters eventually rise
along fractures to the seafloor at temperatures
of 350° C generating black smokers.
Eventually the metals precipitates to form
deposits of economic value.
Metamorphic environments
Regional metamorphism
• Produces the greatest quantity of
metamorphic rock
• Associated with mountain building
Metamorphic environments
Regional
metamorphism
occurs where
rocks are
squeezed
between two
converging
lithospheric
plates during
mountain
building.
Metamorphic environments
Other metamorphic environments
• Burial metamorphism
– Associated with very thick sedimentary strata
– Required depth varies from one location to
another depending on the prevailing
geothermal gradient (‫)تدرج حرارة األرض السائدة‬
Metamorphic environments
Other metamorphic environments
• Metamorphism along fault zones
– Occurs at depth and high temperatures
– Pre-existing minerals deform by ductile flow
As large slabs of
rock move in
opposite
directions, the
minerals in the
fault zone
between them
tend to form
elongated grains
that give the rock
a foliated
appearance
(Figure B).
Metamorphic environments
Other metamorphic environments
• Impact metamorphism
– Occurs when high speed projectiles called
meteorites ‫ نيازك‬strike Earth’s surface
– Upon impact the energy of the once rapidly
moving meteorite is transformed into heat
energy and shock waves that pass through the
surrounding rocks, the result is
pulverized(‫)مطحون‬, shattered(‫)مھشم‬, and
sometimes melted rock.
– Products are called impactites ‫إمباكتيت‬
Metamorphic zones
Systematic variations in the mineral content
and often the textures of metamorphic rocks
are related to the variations in the degree of
metamorphism
Index minerals (‫ )معادن دالة‬and metamorphic
grade
• Changes in mineralogy occur from regions of
low-grade metamorphism to regions of highgrade metamorphism
Idealized illustration of progressive
regional metamorphism
From left to right, we progress
from low-grade metamorphism
(slate)
to
high-grade
metamorphism (gneiss).
The typical transition in mineral content
that results from the progressive
metamorphism of shale
Metamorphic zones
Index minerals and metamorphic grade
• Certain minerals, called index minerals, are
good indicators of the metamorphic
conditions in which they form
• Migmatites ‫صخر متحول مختلط عموما ً من مصدرين ( صخرالميجماتيت‬
‫)مختلفين‬
– Highest grades of metamorphism that is
transitional to igneous rocks
– Contain light bands of igneous components
along with areas of unmelted metamorphic rock
• Facies
-- Metamorphic rocks that contain the same
assemblage of minerals
Zones of metamorphic
Metamorphism and plate tectonics
Most metamorphism occurs along
convergent plate boundaries
• Compressional stresses deform the edges
of the plate
• Formation of the Earth’s major mountain
belts including the Alps, Himalayas, and
Appalachians
Metamorphism and plate tectonics
Large-scale metamorphism also occurs
along subduction zones at convergent
boundaries
• Several metamorphic environments exist
here
• Important site of magma generation
Metamorphism and plate tectonics
Metamorphism at subduction zones
• Mountainous terrains along subduction
zones exhibit distinct linear belts of
metamorphic rocks
– High-pressure, low-temperature zones
nearest the trench
– High-temperature, low-pressure zones
further inland in the region of igneous
activity
Metamorphic Facies and Plate
Tectonics
End of Chapter 7