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Metamorphic Rocks, Part 2 HIGHER-GRADE REGIONAL METAMORPHICS

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HIGHER-GRADE REGIONAL METAMORPHICS ... Eclogite is basalt which has been ... Minerals from Hydrothermal Alteration Economic Deposits in ... – PowerPoint PPT presentation

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Title: Metamorphic Rocks, Part 2 HIGHER-GRADE REGIONAL METAMORPHICS


1
Metamorphic Rocks, Part 2 HIGHER-GRADE REGIONAL
METAMORPHICS
  • Gneiss and Eclogite

2
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.

3
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4
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

5
Exposure of the Core
  • Core is not always exposed
  • Especially true of granulite facies rocks and, to
    a lesser extent, of amphibolite facies rocks

6
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

7
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

8
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

9
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

10
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

11
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

12
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

13
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

14
Granulite Mineralogy Cont.
  • Hornblende may be present
  • Biotite is absent, unless formed by retrograde
    metamorphism after the major metamorphic episode
    ends

15
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)

16
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

17
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
18
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

19
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

20
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)

21
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)

22
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

23
Origin of Hydrothermal Fluids
  • The hydrothermal solutions could be derived from
    several sources including
  • Juvenile water
  • Connate water
  • Water released during progressive metamorphism

24
Juvenile Water
  • Juvenile means waters released by melting which
    have never been at the surface before
  • Often from granitic intrusions

25
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

26
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

27
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

28
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

29
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

30
Orthogneiss
  • Common orthogneisses (gneisses formed from
    igneous rocks) are similar in composition to
    granite or granodiorite, and some may have
    originally been lava flows

31
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

32
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

33
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
34
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

35
Injection Gneiss
  • Injection gneisses are formed by injection of
    veinlets of granitic material into a schist or
    some other foliated rock

36
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

37
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

38
Typical Migmatite
  • Skattøra gneiss
  • Scale 13 cm.

39
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

40
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

41
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

42
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

43
Garnet
  • Photo (CN) Note the zonal distribution of quartz
    inclusions in this garnet porphyroblast
  • Garnet is isometric and remains in extinciton in
    CN

44
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
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