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Metamorphic Grade
a. General term for describing the relative T and P
conditions under which metamorphic rocks form.
b. Metamorphic Rocks can be formed in labs but not
naturally occurring minerals or rocks
c. High P—Low T= Subduction
d. Low P—High T=Contact Metamorphism
(Intrusion)
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Role of Temperature
a. Rocks heat up as they are buried/pushed deeper
in the Earth’s crust (Burial, plate tectonics)
b. Rocks may heat up locally due to igneous
intrusions or lava
c. Plate tectonics is an important aspect in
metamorphism or temperature. Plate tectonic
(regional metamorphism) transport rocks and
sediments into hot depth of the crust
o Subduction
o Continent-continent
d. Proximity to igneous intrusions (localized
metamorphism)
e. Geothermal Gradient, is the increase in
temperature with increasing depth in Earth’s
interior
o Shallow/Smaller (20ºC / km) where the
lithosphere is thick
o Steep/Greater (50ºC / km) where the
lithosphere is thin or where magma is rising
o Instrument for measuring is Geothermometer
Gradient which is the use of a rock’s mineral
composition to gauge the temperature at
which it was formed Clay minerals
recrystallize into micas, which then
recrystallize into garnet.
The Role of Pressure (stress)
a. Two Types of Pressure
o Confining Pressure, the force applied
equally in all directions in proportion to the
weight of overlying rocks.
o Directed Pressure/Differential Stress, the
force exerted in a particular direction; usually
concentrated within particular zones or along
discrete planes (Ex. Continent-continent
collision)
b. Average pressure at depth of 15km is about
4000x the pressure at the surface
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Metamorphic Rocks are products of processes acting
on rocks at depths ranging the upper to the lower crust
• Changes in heat, pressure, and the chemical
environment of rocks can alter mineral compositions
and crystalline textures, making them metamorphic.
• Metamorphic changes occur in the solid state, so there
is no melting.
• Meta (change); Morph (Form) -- (Greek)
• Under metamorphism, a rock will change its minerals
and/or textures until it is in equilibrium with its new
Temperature and Pressure (or new fluids)
• Through metamorphism, limestone like Fossiliferous
when heated and pressurized will recrystallize
forming bigger crystals and losing the fossils (textural
change) and it will turn into marble.
• Clay minerals contain a lot of water, through
metamorphism and with increased pressure it will
remove the water and then form micas (Mineralogical
and textural change)
a. Shale—Slate—phyllite—schist—gneiss
(Increasing metamorphism)
Significance of Metamorphic Rocks
• To understand how the Earth’s crust evolved through
time—most metamorphism happens deep in the crust
• Commercial reasons (gemstone)
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Factors include:
a. Temperature
b. Pressure
c. Fluid Phases
d. Time
Driven by or causes of metamorphism:
a. Internal heat of Earth
b. Internal pressure of Earth
c. Fluid composition inside Earth
Temperature increases with depth
a. rate = 20º to 60ºC per km
b. at 15 km depth: 450ºC
Pressure and temperature increase with depth in all
regions
a. Pressure (P) increases with depth at about the
same rate everywhere
b. Temperature (T) increases at different rates in
different regions
c. Both P and T might increase locally due to
magmatic intrusions, plate tectonics, and etc.
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c. Rate of increase = 0.3 to 0.4 kbar / km
d. Minerals are measured by geobarometers
Role of Fluids
a. It can alter rock’s mineralogy by introducing or
removing chemicals components that are soluble
in heated water
b. Accelerated chemical reaction
o Hydrothermal fluid- carries dissolve CO2,
Na, K, silica, Co, and Zn—soluble in hot
water under pressure
c. Metasomatism
o Changing rock’s composition by fluid
transport of chemical substances into or out
of it.
d. Reactive fluids
o Are from minute pores (spaces between
grains) i.e. Chemical bound water in clays
(Mica and amphibole crystal structure); CO2
in hydrothermal fluids is largely derived from
sedimentary carbonates—limestone and
dolostones.
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Type of Metamorphism
a. Shock Metamorphism
o Results from the heat and shock waves of
a meteorite impact, transforms rock at
impact site.
o Shock metamorphism, also called impact
metamorphism, occurs when high heat
and pressures generated during an impact
deform the underlying rock layers.
b. Regional Metamorphism
o Mineralogy and texture of rocks are
changed over a wide area by deep burial
and heating associated with the largescale forces of plate tectonics.
o At convergent plate boundaries occurs at
moderate to deep levels under moderate
to ultra-high pressures and high
temperatures.
o Example: Himalayas, produces schist
and gneiss metamorphic rocks
c. High-pressure Metamorphism
o Usually found in along active trenches,
subduction zones, linear belts of
volcanic arcs and was produced by
continent-continent collision. Example.
Past of Surigao (Philippine or Pacific
plate underneath Mindanao).
o Ultra/high-pressure
metamorphism
refers to the metamorphism of crustal
rocks (both continental and oceanic)
brought to pressures high enough to
crystallize the index minerals like
coesite and/or diamond. (8-12 kbar or
greater than 28 kbar)
d. Contact Metamorphism
o Occurs adjacent to igneous intrusions
and results from high temperatures
associated with the igneous intrusion.
Affects
a
thin
zone
of
country/surrounding rock around the
igneous intrusion.
o This happens when the temperature of
the existing rocks rises and is also
infiltrated with the magma fluid.
e. Burial Metamorphism
o Transforms sedimentary rocks at
progressively increasing temperature and
pressure
o Occurs when sediments are buried
(deposition) deeply enough that the heat
and pressure cause minerals to begin to
recrystallize and new minerals to grow
f. Seafloor Metamorphism
o Occurs at mid-ocean ridges, where
intruding magma drives seawater
circulation (makes it hotter) and ethe
effect have a direct change to the
surrounding basalt.
3)
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Texture of metamorphic rocks is determined by
a. Size (Grains)
b. Shape
c. Arrangement of its constituent crystals
(Grains)
Some texture depends on the particular kinds of
minerals formed
Grain size increases as metamorphic grade
increases
Each textural variety tells us something about the
metamorphic process that created it
Two major textural classes:
a. Foliated rocks
b. Granoblastic rocks or non-foliated rocks
Two major textural classes: Foliation/Foliated
Rocks
a. A set of flat or wavy parallel cleavage planes
produced deformation of sedimentary or
igneous rocks under directed pressure
b. Foliation planes may cut through the bedding
of the original sedimentary rock at any angle
or be parallel to the bedding. (Ex. Cracks or
parallel lines in bedding planes caused by
directed pressure and other factors)
c. As the grade of regional metamorphism
increases,
formation
becomes
more
pronounced.
d. Creating Bedding planes is an example to
determine the foliation (definition of beds
depends on the person to make the sketch
either through grains, colors, structure and
etc.)
e. Foliation and Cleavage Illustration
(Bedding plane)
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1. Directed pressure causes sedimentary
rocks, such as shale, to form cleavage
planes that may differ from their bedding
plane.
2. The original bedding in a sample of shale
is marked by the thin sandstone layers.
3. Regional metamorphosed rocks show
foliation caused by compressive force
4. Rocks develop elation when they contain
platy minerals (sheets of minerals) that
align along with a preferred orientation
depending on their preferred orientation,
compressive forces cause the mineral
crystals in the rock to grow or to a line
perpendicular to the compressive forces.
Preferred orientation of the minerals
a. Planes of all the platy crystals are aligned to
the foliation
b. As platy minerals crystallize, the preferred
orientation is usually perpendicular to the
main direction of forces squeezing the rock
during metamorphism.
c. Crystals of resisting minerals may contribute
to the foliation by rotating until they also lie
parallel to the developing foliation plane.
Slaty Cleavage (different from perfect cleavage
of sheers silicates like mica)
a. Develops at small, regular intervals in the
rock
b. Rock splits into thin sheets along smooth,
parallel surfaces
Elongated, needlelike crystals habit
a. Also tend to assume a preferred orientation
b. Crystals normally line up parallel to the
foliation plane
c. Example are rocks with abundant amphiboles
(mafic rocks)
Foliated Rocks are classified on:
a. Metamorphic Grade
o Range of metamorphic change a rock
undergoes, progressing from low (little
metamorphic change) grade to high
(significant metamorphic change) grade
b. Crystal Size
c. Type of Foliation
o Type of foliation are: slate, phyllite,
schist, and gneiss
d. Banding
o Rock consists of alternating, thin layers
of two different mineral compositions.
Same mineral but different proportions.
Foliated Rock Names/Type of Foliation
(Increasing Metamorphism)
a. Slate
o Metamorphosed shale or volcanic ash
deposit. Dark gray to black from small
amount of organic matter in the parent
shale; very fine-grained.
b. Phyllite
o More or less glossy sheen from the
crystal of mica and chlorite that have
grown a little larger than those of slates.
Also tend to split into thin sheets, but less
perfectly than slates
c. Schist
o Platy crystals grow large enough to be
visible and minerals tend to segregate
into lighter and darker bands = coarse,
wavy foliation known as schistosity.
o Among the most abundant metamorphic
rocks types. It has >50% platy minerals
(mica, muscovite and biotite) and thin
layers of quartz, feldspar, or both
depending on the quartz content of parent
shale.
d. Gneiss
o Gneiss—gneissic foliation= segregation
of lighter colored quartz and feldspar
from darker and tables and other mafic
minerals
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Ratio of granular to plate in minerals is
higher than in slate or schist = poor
foliation and thus little tendency to split
e. Migmatite
o Surrounding parts of country-rock begin
to melt. Melt then migrates short
instances before they solidify again
o Rocks are badly formed and contorted,
penetrated by veins, small pods, and
lenses of melted rocks: resulting in a
mixture of igneous and metamorphic
rock
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Two major textural classes: Granoblastic rocks
or non-foliated rocks
a. Composed mainly of crystals that grow in
equant shapes (cubes or spheres) (ex. Garnet)
b. Contact metamorphism where directed
pressure is absent.
c. All except hornfels are defined by their
mineralogy rather than texture because all of
them have a homogeneous granular texture
(no identifiable feature; identifiable only by
mineralogy)
Non-foliated (granoblastic) rocks names:
a. Hornfels
o High-temperature
contact
metamorphism; uniform grain size; little
or no deformation; parent rock is
sedimentary or any rock with abundant
silicate minerals; granular overall
texture, not foliated, and it's platy or
elongated crystals are oriented randomly
(or all directions).
b. Quartzite
o From
quartz-rich
sandstone;
homogeneous, unbroken by preserved
bedding and foliation, other have thin
bands of slate or schist—relics of former
interbedded layers of clay or shale.
c. Marble
o Products of heat and pressure acting on
limestone and dolomites.
d. Greenstone
o Metamorphosed mafic volcanic rocks;
low-grade metamorphic rocks form by
seafloor metamorphism; large areas of
seafloor are covered with basalts that
have been slightly or extensively altered
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in this way at mid-ocean ridges; greenish
due to abundance of chlorite.
e. Amphibolite
o Amphibole and plagioclase feldspar;
medium to high grade metamorphism of
mafic
volcanic
rocks;
foliated
amphiboles are by directed by pressure
f. Granulite (Granofels)
o High
grade
metamorphism;
homogeneous granular texture; medium
to coarse grained rocks in which the
crystals are equant and has faint foliation
at most; form metamorphism of shale,
impure sandstone, and many kinds of
igneous rocks.
Classification of Metamorphic Rocks by
Texture
Grade of Metamorphism
a. Low
b. Intermediate
c. High
Mineral Isograds (zone of charge)
a. Index minerals reflect pressure and
temperature conditions
o Index minerals are abundant minerals
that each from under a limited range of
temperature and pressure
o Marks the point at which we move into a
new zone with a higher metamorphic
grade
o Make a map of the boundaries (isograds)
between metamorphic zones
o Reflect pressure and temperature
conditions
b. Groups of 2 to 3 index minerals form an
Isograds
Index Minerals Define Metamorphic Zones
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Increasing Metamorphism Grade
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Changes in the mineral composition of shale
during metamorphism; Changes in the mineral
composition
of
mafic
rock
during
metamorphism
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Contact Metamorphism around intruding
plutons
d. Transform plate margins
o Seafloor metamorphism
o Metamorphism by shearing forces along
transform faults
Metamorphic Pressure-Temperature Paths
a. Metamorphic grade in forms of the maximum
pressure or temperature to which the
metamorphic rock has been subjugated but
nothing about where the rocks encountered those
conditions
b. Does not tell us about how the rock was
exhumed, or transported back to earth’s surface
P-T Path
a. Sensitive recorder of many factors: and rates of
tectonic transport (burial and exhumation) that
changes pressures
b. Characteristics of particular plate tectonic
settings
Garnet (common porphyroblasts)
a. Grow steadily during metamorphism
b. As P and T of the environment changes, the
chemical composition of the garnet changes.
c. Core (oldest), edge (youngest): yields the history
of metamorphic conditions under which it is
formed
Tectonic Plates in response to the correlation
between pressure and temperature of Metamorphic
Facies
Metamorphism occurs in or near
a. Continental Plate interiors
o Contact Metamorphism
o Burial Metamorphism
o Regional Metamorphism
b. Divergent plate margins
o Seafloor Metamorphism
o Contact Metamorphism is likely to be best
preserved
c. Convergent plate margins
o Regional Metamorphism
o High-pressure
and
ultra-high-pressure
Metamorphism
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Metamorphic pressure-temperature paths
a. History of burial and exhumation
b. Prograde and retrograde paths
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The garnet crystal initially grows in a schist but ends
up growing in a gneiss.
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Rapid erosion (exhumation) rates of mountain
ranges show a relationship between
a. Tectonics (Orogeny)
b. Climate
Controls the flow of metamorphic rocks to the
surface.