Metamorphic Rocks, Part 2 HIGHER-GRADE REGIONAL METAMORPHICS

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Metamorphic Rocks, Part 2HIGHER-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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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 (gt 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 by Sam Bowring of
  MIT has yielded ages up to 4010 million years and
  study of Neodymium isotopes indicates ages in
  excess of 4100 million years

The Acasta Gneisses now represent the oldest
intact terrestrial rocks yet discovered
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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 Higher-Grade 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 direction

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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 subsequent
  static hydrous retrogradation in amphibolite
  facies

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Quartzo-Feldspathic Orthogneiss

• Photograph of quartzo-feldspathic 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 meter scale

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LANDSAT Image

• 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)

True color Landsat image looking west from above
the city of Phoenix at the White Tank Mountains
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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 K-feldspar (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