Transcript Metamorphic Rocks, Part 1 LOWER

Metamorphic Rocks, Part 1
LOWER-GRADE REGIONAL
METAMORPHICS
Slate, Phyllite,
“Greenstone” and Schist
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Metamorphic Rock Definition
• A sedimentary, igneous, or previously
existing metamorphic rock which has
undergone textural, structural, and/or
mineralogical changes due to the
action of one or more agents of
metamorphism
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Agents of Metamorphism
• Changes in pressure or stress
• Changes in temperature
• Chemically active fluids
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Recrystallization
• Metamorphism involves recrystallization of original
minerals in the rock that is undergoing metamorphism
• In some minerals, notably quartz, feldspar, and calcite,
this may lead to a simple increase in grain size - the
orientation of the grain may be modified in the process.
• In other minerals, especially clays, chlorites, dolomite,
and other carbonates, recrystallization is accompanied
by neomineralization, the formation of new minerals not
present in the rock prior to metamorphism
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Stress
• Stress (differential pressure) is one of the
most powerful factors influencing
metamorphic rock textures.
• Effects
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Development of mechanical fractures
Complex minor folding
Development of foliation
Rapidly applied stress may result in “crush”
rocks
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Stress Continued
• Stress plays a chemical as well as physical
role - reduction in grain size increases the
surface area available for reaction
• Developments of shear planes and fractures
provides routes for the movement of
chemically active fluids
• Stress may also be a source of localized
heat - friction of rocks moving past each
other may heat up the rocks along the
contact
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Foliation Example
Quartz-mica schist
• A foliation is any
planar fabric in a
metamorphic rock
• Here, foliation is
defined by aligned
sheets of muscovite
sandwiched between
quartz grains
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Strain
• Strain is deformation due to applied stress
• Strain in crystals partially breaks bonds,
thus facilitating the conversion to new
mineral species
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Temperature
• Change in temperature is a potent
metamorphic agent
• In otherwise unmetamorphosed rocks
exposed to the heat from an igneous
intrusion, changes in the type of mineral
present are often observed
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Effects of Temperature
• The solubility of minerals in water generally
increases as temperature rises, although there are
many exceptions to this rule (gypsum, calcite, etc.)
• Phase boundaries are crossed
 Some minerals become unstable, while others become
stable
• Chemical reaction rates increase rapidly with
increasing temperature
 Endothermic reactions are favored
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Effects of Temperature
• Generally minerals with more open crystal
structures are favored at higher
temperatures
• Increasing temperature tends to drive off
water and carbon dioxide (devolitilization),
which serve to increase the activity of the
fluid phase which they enter
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Effects of Pressure
• Pressure tends to counter the effects of
temperature
 Water and carbon dioxide are retained to higher
temperatures as pressure increases
• Pressure tends to favor minerals with
closed, compact (denser) mineral structures
 Metamorphic rocks produced at greater depths
are denser than those produced near the surface
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Effects of Pressure
• Pressure is determined largely by burial
depth
 At great depths, fluids are usually unable to
escape and any fluid pressure present will be
added to the load pressure
 Toward the surface, fluids usually escape
 Load pressure is hydrostatic (equal in all
directions)
 An increase in load pressure tends to prevent
stress fractures from forming, and to close those
that exist
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Chemically Active Fluids
• Many progressive reactions (from lower to
higher grade across a metamorphic terrain)
liberate water, carbon dioxide, and other
mineralizing agents
• Mineral assemblage present will vary
tremendously in environments high or low
in these mineralizing agents, particularly
water
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Fluid Release
• Presence of these fluids often depends on
the original compositions of the rock
 Wet sediments will release large quantities of
water during metamorphism
 Basalts will not
 Limestones will release carbon dioxide during
metamorphism
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Combination of Agents
• Although it is possible for any agent acting alone
to produce metamorphism, in most cases two or
more agents will work synergistically.
• Many combinations are possible
• Several types of metamorphism, involving one or
more agents, are commonly recognized
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Types of Metamorphism
•
•
•
•
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Regional metamorphism
Contact (or thermal) metamorphism
Dynamic metamorphism
Impact metamorphism
Metasomatism
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Regional Metamorphism
• By far the most volumetrically important as the name suggests, that these rocks occur
over extensive areas
• Results from the combined effects of heat,
pressure, and stress, with chemically active
fluids often playing a role, at least with
parts of a regional metamorphic complex
• Often regional metamorphism is associated
with orogenesis
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Regional Metamorphism, Cont.
• Granitic intrusions are often associated with
regional metamorphic complexes
 This association raises many questions,
especially as to whether the granite is the cause
of or the effect of metamorphism
• It is often possible to define several zones
of progressive metamorphism in regional
complexes
 Either the “grade” or the “facies” system may
be used to classify these zones
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“Grade” Classification
• The grade system is the product of the
British petrologists, principally Barrow,
Harker, and Tilley
• Classification based on the first appearance
of certain characteristic minerals
• Minerals are chlorite, biotite, garnet,
staurolite, kyanite, and sillimanite
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Facies Classification
• The facies system of Eskola is more commonly used
in the United States
• Classification based on the characteristic mineral
associations
• The facies system allows a parallel classification
with mafic igneous rocks
 This means that a rock formed from a magma, by
recrystallization from a solid, or from hydrothermal
solution, will have similar mineralogical associations
• The facies system puts somewhat more emphasis on
pressure than the grade system
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Eskola
Facies
System
• The diagram shows
the principal
regional
metamorphic facies
• Note that pressure
increases
downward
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Contact Metamorphism
• Contact metamorphism is the result of heat
from an intrusion of magma altering the
country rock around it
• This type of metamorphism was formerly
called “thermal” metamorphism because it
was believed that heat was the only agent of
metamorphism involved - this may
sometimes be true
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Contact Metamorphism, Cont.
• However, the heat often releases water and
carbon dioxide, and these fluids play an
important role in many cases
• Thus, the name contact metamorphism is
more appropriate
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Dynamic Metamorphism
• Dynamic metamorphism occurs when rocks
move along fault zones
• Stresses involved are large
• Frictional heat and fluids also may play a
role
• Fluid metamorphism is often temporally
later than the stress metamorphism
 Volumes involved are small
 No samples are available, and we will not
examine this type of rock
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Impact Metamorphism
• Result of large scale impacts, usually of
meteoritic origin
• Temporary pressures of megapascals are
possible
• Temperature spikes of short-duration may
also play a role
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Impact Metamorphism, Cont.
• This type of metamorphism was important
during the early history of the earth, but has
become less frequent with time
• The volume of rock metamorphosed is not
large
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Impact Metamorphism, Cont.
• The major importance of impact
metamorphic studies are in recognizing
and/or confirming the occurrence of events
such as the K-T impact that caused mass
extinctions
• The establishment of the frequency of these
events is far more significant than the
petrographic study of the rocks
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Metasomatism
• Metamorphosis through the action of
chemically active fluids
• Frequently occurs during other types of
metamorphosis, but has begun to be recognized
as a type of metamorphism in its own right
• We will examine rocks, often carbonate
containing, that may be either regional or
contact
 These rocks likely involve metasomatic changes,
sometimes with the aid of other agents
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Parent Rock Compositions
• Original composition of the country rock
plays a tremendous role in the type of
metamorphic rock formed
• While a complete discussion of this topic is
nearly a course in itself, but some of the
principles can be outlined here
• We can recognize five basic categories of
country rock
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Pelite
• A sediment or sedimentary rock composed
of the finest detritus, clays or mud-size
particles, or a calcareous sediment
composed of clays and minute quartz
particles
• Often these sediments are aluminous
• Pelite is sometimes used to mean the
metamorphic equivalent of an argillaceous
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rock
Psammite
• A clastic sediment or sedimentary rock
composed of sand-sized particles
• Synonymous term is arenite
• Sometimes called the metamorphic
equivalent of arenite
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Carbonate
• Limestone or dolomite
• Argillaceous and arenaceous types
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Felsic to Intermediate Igneous
Rocks
• “Granite”, Diorite or equivalent among the
intrusive rocks
• Extrusive igneous rocks are less commonly
metamorphosed, but may be if they are
buried - Rhyolite to Dacite
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Mafic to Ultramafic Igneous Rocks
• Gabbros, Peridotites, Pyroxenites may be
metamorphosed
• Extrusive rocks such as basalt are
frequently metamorphosed, because they
are dragged down a subduction zone, or cut
in an accretionary prism melange
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Slates and Phyllites
• Slates and phyllites characteristically form
from pelitic rocks.
• Little difference in mineralogy from the
original rock
• Typical principal minerals are quartz,
feldspar, sericite, and chlorite
• At slightly more advanced metamorphic
levels, biotite will be present, usually with
either chlorite or sericite
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Slates and Phyllites
• A close relative of biotite, stilpnomelane, is
another new mineral that may form in these
rocks
• If manganese is present, garnets may form
in the biotite zone
• Some loss of water occurs during the
formation of these rocks
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Foliation in Slates and Phyllites
• Both slates and phyllites show welldeveloped rock cleavage
• The cleavage may be parallel to the original
bedding in some phyllites
• In other phyllites and in most slates, the
cleavage cuts across the bedding
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Elongation Perpendicular to
Applied Stress
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Slaty Cleavage
• Gray slate showing well-developed slaty cleavage
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Slate Photomicrograph
• Note the fine grain
size and the
unimpressive foliation
in this weaklymetamorphosed rock
• Locality: Vermont
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Andalusite
• Upper (CN) and Lower (PP): The
distinct chiastolite cross that is
characteristic of andalusite is easily
seen in this section
• First order white/gray interference
colors
• Moderately high positive relief
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Phyllite
• Phyllite showing the sheen typically associated with it
• Larger grain size of phyllite produces the sheen
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Phyllite Crenulations
• Crenulations are often seen in phyllite
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Phyllite Photomicrographs
• Sample: Ira Phyllite
• Note the wavy
foliation and the
overall fine-grain size
of this rock
• Location: Vermont
• Upper photo CN
• Lower photo PP
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Slates and Phyllites of Psammitic
Origin
• Psammitic rocks show little changes in hand
specimen under low-grade metamorphism
• The grains of quartzitic sandstones may
become elongated and interlocking, but this
can only be seen by microscopic
examination of thin sections
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“Greenstones”
• Greenstones are usually mafic to ultramafic
rocks that formed under conditions of hightemperature and often high-pressure
• If these rocks undergo low-grade
metamorphism, the formation condition is
greatly different than the conditions of
metamorphism
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Greenstones Continued
• As a result, they undergo almost a complete
mineralogical change.
• Chief changes involve addition of water and
carbon dioxide
• Many of these rocks are undeformed, and may
preserve igneous textures
• The most important new mineral is usually
chlorite
• The feldspar is often albitized
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Schists
• Schists exhibit much larger grain sizes than
slates or phyllites
• They correspond to phaneritic igneous
rocks.
• Hand specimen examination of schists
reveals much more about composition than
from the examination of slates and phyllites
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Mica Schist
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•
•
Most common type of schist
Not all schists are micaceous
Mica schists are typically of pelitic origin
The micaceous minerals include sericite,
muscovite, chlorite, biotite or
stilpnomelane, and sometimes talc
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Mica Schist Continued
• Quartz is almost invariably present
• Unless formed from a graywacke or similar
rock, the quartz is recrystallized and larger
than in the original rock
• Feldspars include microcline, perthite, and/or
sodic plagioclase (albite to oligoclase)
• Generally speaking, the higher the grade of
metamorphism, the higher the calcium content
in plagioclase, although the availability of
calcium affects this generalization
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Mica Schist Continued
• As metamorphism intensifies, typical
metamorphic minerals include chloritoid, garnet,
staurolite, and kyanite
• Cordierite and andalusite are typical of contact
metamorphosed pelites, but may be present in
regionally metamorphosed terrains in which
metasomatism is involved
• Sillimanite is indicative of the highest grades of
metamorphism in schist, although it is more
commonly found in gneisses
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Mica Schist
• Mica makes this schist quite shiny
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Garniferous Mica Schist
• Garnets often develop in higher-grade schists
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Crenulation Cleavage
Photomicrograph, CN
• The vertical foliation
in this rock is a
crenulation cleavage,
and developed after
the horizonal foliation
• Muscovite-biotite
-garnet schist
• Location: New
Mexico
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Whiteschist
• This is a photomicrograph of a
high-pressure schist from the
famous Dora Maira massif in
Parigi, Italy
• The region of coarser-grained
quartz in the upper center
portion of this photomicrograph
was probably originally
Metamorphic rocks from the
occupied by coesite, the highDora Maira Massif show other
pressure polymorph of quartz
evidence of being exhumed
• Quartz-kyanite-garnetfrom EXTREMELY deep
levels in thickened crust (>28
muscovite schist
kbar)
Photo: K. Stewart
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Glaucophane Schist
• Upper (CN): Glaucophane schist
with a large, isotropic garnet in
the center, surrounded by highly
birefringent glaucophane and
white mica
• Lower (PP): A mixture of lightly
colored glaucophane, colorless
white mica, and dark, high relief
epidote in a low grade schist
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Glaucophane
• Occurrence: In low grade metamorphic
rocks, associated with white mica,
albite, quartz, chlorite, epidote, and
occasionally lawsonite and jadeite
• Strong pleochroism, blue to violet to
colorless to pale brown; amphibole
cleavage
• Photos at left are in plane polarized
light, illustrating the unique
pleochroism of glaucophane
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Stilpnomelane
• Golden stilpnomelane in plane
polarized light (below) and
under crossed nicols (above)
• Stilpnomelane occurs in some
low grade, burial metamorphic
rocks, with quartz, white mica,
garnet, etc.
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Stilpnomelane
• Stilpnomelane in a
characteristic sheaflike arrangement
• Plane polarized
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Quartz Elongation in Schist
• Quartz grains have been stretched perpendicular to
the direction of stress
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Tourmaline Schist
• Hexagonal crystals of tourmaline clearly visible
• Tourmaline commonly occurs in metamorphosed
sedimentary rock
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Psammitic Schist
• Psammitic schists may be similar to pelitic
schists if the rock is exposed to the activity
of alkaline solutions, which convert quartz
to mica, or if the rock is arkosic or a
feldspathic sandstone
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Carbonate and Schist
• Limestones and dolomites associated with
schists are often deformed by plastic flow
• Silication reactions (reaction of carbonate
with silica) may occur if the rock is an
impure carbonate, with a clay or silica
component, or if siliceous fluids from
hydrothermal or pegmatitic fluids occurs
• Typical minerals include tremolite,
actinolite, diopside, epidote, phlogopite,
scapolite, and serpentine
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Greenstone Schists
• Commonly form from mafic igneous rocks
• Hornblende is a major mineral
• May be accompanied by epidote and albite
in the epidote part of the amphibolite facies,
or by plagioclase more calcic than albite in
the amphibolite facies
• Hornblende becomes darker as grade
increases, the grains are coarser, and the
habit more pronounced
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Greenstone Schist Continued
• Large structures, such as pillows, may be
preserved
• Fine-grained mafic lavas generally lose
their structure as the result of
recrystallization
• Primary diabasic texture from dikes or sills
may be preserved
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Hornblende Schist
• Hornblende schist is a typical greenstone schist,
probably derived from a mafic igneous rock
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Actinolite
• Upper (CN): Actinolite in a
groundmass of Mg-rich chlorite photo shows the upper first-order
to mid second-order interference
colors of actinolite
• Lower (PP) Actinolite in a
groundmass of Mg-rich
chlorite
• Crystal form: columnar, bladed,
or acicular crystals, elongate
parallel to the c-axis, basal
sections (cleavage visible) are
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diamond shaped