Download Lab 10 - Gneiss and Eclogite

Metamorphic Rocks, Part 2
HIGHER-GRADE REGIONAL
METAMORPHICS
Gneiss and Eclogite
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High-Grade Regional
Metamorphic Facies
• Rocks in this laboratory represent high to very high
grade regional metamorphic rock
• Facies represented are the amphibiolite, granulite,
and eclogite facies
• Figure 1 shows the facies
• The rock types are usually gneiss or eclogite
• Gneisses come in many forms, and in the granulite
facies, the gneisses gradually become massive,
losing all trace of foliation.
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Figure 1
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Regional Metamorphic
Complexes
• Regional metamorphic rocks often occur in
layered complexes associated with an
orogenic event
• The core of the complex is the highest grade
of metamorphism that was achieved
• Core may be granulite, or some lower facies
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Exposure of the Core
• Core is not always exposed
• Especially true of granulite facies rocks and,
to a lesser extent, of amphibolite facies
rocks
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Mantling of Core Rocks
• If the core is granulite it will be mantled by
amphibolite facies, which in turn will be mantled by
greenschist facies
• If erosion is extensive, the granulite may be exposed
• If erosion is slight, only the greenschist may be
visible
• Granulite facies rocks are seen on the surface in only
a few places, but may be present in many more at
depth
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Granulite and Amphibolite Facies
• Granulite facies rocks represent very high
temperatures, which are normally only achieved at
great depth within the lower-most crust
• Granulites are found primarily in exposed Archean
terrains
• Exposures of amphibolite facies are far more
common
• Bryson Dome and Maggie-Dellwood areas of North
Carolina represent amphibolite facies rocks
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Eclogites
• Eclogites are restricted to moderate to very high
pressure and moderate to high temperatures
• The higher the temperature, the higher must be
the pressure to generate eclogite facies rocks
• They are typical subduction zone metamorphics,
usually associated with the much more
voluminous blueschist facies rocks
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Mineralogy of Amphibolite
Facies Rocks
• The amphibolite facies rocks are characterized
by plagioclase (> An20)
• Hornblende, or epidote with diopside and quartz
• Pelitic origin: staurolite or sillimanite with
muscovite is a diagnostic assemblage
• Calcareous origin: diagnostic assemblages are
diopside with tremolite and calcite or grossular
with clinozoisite or zoisite
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Amphibolite Mineralogy Cont.
• Granitic rock origin: Changes in mineralogy
will be most notable in the mafic to ultramafic
rocks
• Minerals present in high-grade metamorphosed
mafic rocks include talc, tremolite, and
anthophyllite
• Forsterite or enstatite may replace serpentine
formed at lower temperatures or pressures
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Amphibolite Mineralogy Cont.
• Changes in metamorphosed intermediate to
felsic rocks (andesite, diorite, granodiorite,
dacite, rhyolite, or granite) are much less
pronounced
• Pyroxene may alter to amphibole
• Garnet is common, but the source is unclear
• Garnet may form by reaction among the original
igneous minerals, or by contamination with
sedimentary or volcanic wall rock
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Amphibolite Photomicrographs
• The photographs (crossed
polarized above, plane polarized
below) show plagioclase (white,
light gray), hornblende (strongly
colored in lower photo), and
moderately birefringent
clinopyroxene
• Note that in metamorphic rocks,
plagioclase is typically
xenoblastic (anhedral) and
unzoned
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Mineralogy of Granulite Facies
Rocks
• Granulite facies rocks have mineralogy, and
sometimes appearance, very similar to
granite
• Common assemblages include hypersthene
with quartz or sillimanite with perthite and
quartz
• Muscovite, tremolite, actinolite, and
anthophyllite are absent
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Granulite Mineralogy Cont.
• Hornblende may be present
• Biotite is absent, unless formed by
retrograde metamorphism after the major
metamorphic episode ends
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Granulite Photomicrographs
• The photographs at left (both
CN) show clinopyroxene
(brightly colored grains),
orthopyroxene (high relief, gray
to first order yellow), perthite
(gray, left side of upper picture),
plagioclase (straight twins), and
quartz (light gray, top of
picture)
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Granite-Gneiss Association
• Granites and gneisses are often intimately
associated
• Some granites are thought to form by partial
melting, which could be associated with
high-grade regional metamorphism, or by
metasomatism, which makes the granite
itself a metamorphic rocks
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Oldest Rock - Acasta Gneiss
• The Acasta Gneisses are an
assemblage of massive to
foliated granite and tonalitic to
granitic gneiss exposed in the
western part of the Slave
Province
• Precise U-Pb dating of zircons
The Acasta Gneisses now
by Sam Bowring of MIT has
represent the oldest intact
yielded ages up to 4010 million
terrestrial rocks yet
years and study of Neodymium
discovered
isotopes indicates ages in excess
of 4100 million years
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Granulite Mineralogy Cont.
• Intermediate to felsic igneous rocks altered by
granulite facies metamorphism do show marked
changes
• Pyroxene will form from pre-existing amphibole
or biotite
• Hypersthene is commonly formed, often
accompanied by diopside
• Garnet is quite common
• Scapolite replaces plagioclase in many cases
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Eclogite Mineralogy
• Composed of the high-pressure jaditic
pyroxene omphacite, and garnet
• Origin is mafic volcanic rocks, under dry
conditions
• High density is quite characteristic
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Eclogite Photomicrograph
• (Upper CN) Garnet and
clinopyroxene are the two
major minerals in eclogite
• Eclogite is basalt which has
been metamorphosed at
very high pressures in
subduction zones
• The clinopyroxene is
omphacite
• Lower (PP)
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Retrograde Eclogite
• Upper (CN) The presence of
amphibole (probably
hornblende) is a tip-off that
this eclogite has been
subjected to retrograde
metamorphism
• Note the reaction
relationship between
clinopyroxene and
amphibole in the upper right
• Lower (PP)
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Economic Deposits in Regional
Metamorphic Rocks
• Economic deposits are most common in
rocks of the greenschist facies
• Chlorite schists and greenstones are known
to be associated with hydrothermal
alteration under the conditions of
greenschist facies metamorphism
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Origin of Hydrothermal Fluids
• The hydrothermal solutions could be
derived from several sources including:
• Juvenile water
• Connate water
• Water released during progressive
metamorphism
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Juvenile Water
• Juvenile means waters released by melting
which have never been at the surface before
• Often from granitic intrusions
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Connate Water
• Connate water is water trapped in the
interstices of sediments at the time of
deposition, and which has been out of
contact with the surface for a substantial
time, often a large part of a geologic period
or longer
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Economic Minerals from
Hydrothermal Alteration
• Rocks formed in this manner are hosts for
gold, copper, and copper-zinc
• In sheared or metamorphosed mafic rocks,
nickel-copper and asbestos deposits are
found
• Lead-zinc-silver veins are found in slates,
quartzites, and phyllites, and sometimes as
replacement minerals in limestones
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Economic Deposits in HigherGrade Rock
• Base metals ore bodies are also found with
some amphibolite facies rocks, especially
those with the cordierite-anthophyllite
association
• Ore bodies are scarce in granulite facies
rocks
• Ore bodies are non-existent in eclogites
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Gneiss
• Gneiss is a coarse to medium grained banded
metamorphic rock formed from igneous or
sedimentary rocks during regional metamorphism
• Rich in feldspars and quartz, gneisses also contain
mica minerals and aluminous or ferromagnesian
silicates
• In some gneisses thin bands of quartz feldspar
minerals are separated by bands of micas
• In others the mica is evenly distributed throughout
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Contortion in Gneiss
• Gneiss rocks form
under great pressure
and at high
temperatures
• They may show
contorted folding
• Folding is a response
to directed pressure the rock has shortened
along the horizontal
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direction
Orthogneiss
• Common orthogneisses (gneisses formed
from igneous rocks) are similar in
composition to granite or granodiorite, and
some may have originally been lava flows
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Orthogneiss from Greenland
•
•
Outer coast north of
Fiskefjord, point west
of Pâtôq
Angular dark fragments
are homogeneous
amphibolite
• Coastal exposure of purplish grey orthogneiss retrograded
from granulite facies
• The purplish grey orthogneiss displays indistinct foliation and
migmatization fabrics, which have been blurred during
recrystallisation under granulite facies P-T conditions and
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subsequent static hydrous retrogradation in amphibolite facies
Quartzo-Feldspathic Orthogneiss
• Photograph of quartzofeldspathic gneiss in the
southern White Tank Mtns.
• The gneiss is banded on the cm
scale with amphibolitic gneiss.
• Proterozoic pegmatite vein
(right center of view) has locally
disrupted and complexly folded
the gneiss
• These metamorphics are
strongly banded on both cm and
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meter scale
LANDSAT Image
True color Landsat image
looking west from above
the city of Phoenix at the
White Tank Mountains
• Two major types of rocks
are found in the mountain
range; 1.7-1.6 billion
years old proterozoic
metamorphic rocks (which
appear dark on the top and
in the southern part of the
range) and a Tertiary or
Cretaceous age granitic
intrusion (which is lighter
colored on the image)
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Augen Gneiss
• Augen gneiss is a
variety containing
large eye shaped
grains (augen) of
feldspar
• Photo: Close-up
picture of Ponaganset
augen gneiss , Rhode
Island
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Injection Gneiss
• Injection gneisses are formed by injection
of veinlets of granitic material into a schist
or some other foliated rock
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Migmatites
• Banded gneisses called migmatites are
composed of alternating light colored layers
of granite or quartz feldspar and dark layers
rich in biotite
• Some migmatites were formed by injection
• Others were formed by segregation of
quartz and feldspars
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Migmatite and Amphibolite
• Migmatite with amphibolitic
restite
• The Skattøra gneiss of the
north Norwegian Caledonides,
consists of partly migmatitic
gabbroic to amfibolitic gneiss
which are net-veined by
numerous (up to 50%)
anorthositic and luecodioritic
dikes
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Typical Migmatite
• Skattøra gneiss
• Scale 13 cm.
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Migmatite with Melt Pocket
• Migmatite with an anorthosite
melt pocket to the left side
• The migmatites show different
degrees of melting
• Melted regions often form
concordent bands
• More intense anatexis forms melt
pockets cutting the foliation
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Origin of Gneiss
• The origin of a gneiss can usually be
determined by its chemical composition and
mineral content
• Orthogneiss = igneous origin
• Paragneiss = sedimentary origin
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Differentiation of
Gneiss and Schist
• A distinction between gneiss and schist is
difficult to draw, for many gneisses look far
richer in mica than they are, when mica rich
parting plane is seen
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Sillimanite
• Upper (CN): Note the parallel
extinction of one of the crystals
and the end on view of another.
Birefringence is usually first
order, however, lower second
order colors may be seen
• Lower (PP): The slender
prismatic crystals show high
relief and are colorless in ppl.
• Found in high-T metamorphic
rocks that are rich in Al
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Garnet
• Photo (CN): Note the
zonal distribution of
quartz inclusions in this
garnet porphyroblast
• Garnet is isometric and
remains in extinciton in
CN
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Perthite photomicrograph
• Perthite is actually two
minerals: an intergrowth of
sodic plagioclase in Kfeldspar (orthoclase or
microcline)
• Intergrowths are commonly
stringy (as in the photo
above), but they may be
globular, lensoid, or other
shapes
• First order gray interference
colors
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