Title: Metamorphic Rocks, Part 2 HIGHER-GRADE REGIONAL METAMORPHICS
1Metamorphic Rocks, Part 2HIGHER-GRADE REGIONAL
METAMORPHICS
2High-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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4Regional 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
5Exposure of the Core
- Core is not always exposed
- Especially true of granulite facies rocks and, to
a lesser extent, of amphibolite facies rocks
6Mantling 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
7Granulite 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
8Eclogites
- 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
9Mineralogy 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
10Amphibolite 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
11Amphibolite 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
12Amphibolite 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
13Mineralogy 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
14Granulite Mineralogy Cont.
- Hornblende may be present
- Biotite is absent, unless formed by retrograde
metamorphism after the major metamorphic episode
ends
15Granulite 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)
16Granite-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
17Oldest 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
18Granulite 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
19Eclogite Mineralogy
- Composed of the high-pressure jaditic pyroxene
omphacite, and garnet - Origin is mafic volcanic rocks, under dry
conditions - High density is quite characteristic
20Eclogite 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)
21Retrograde 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)
22Economic 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
23Origin of Hydrothermal Fluids
- The hydrothermal solutions could be derived from
several sources including - Juvenile water
- Connate water
- Water released during progressive metamorphism
24Juvenile Water
- Juvenile means waters released by melting which
have never been at the surface before - Often from granitic intrusions
25Connate 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
26Economic 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
27Economic 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
28Gneiss
- 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
29Contortion 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
30Orthogneiss
- Common orthogneisses (gneisses formed from
igneous rocks) are similar in composition to
granite or granodiorite, and some may have
originally been lava flows
31Orthogneiss 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
32Quartzo-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
33LANDSAT 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
34Augen Gneiss
- Augen gneiss is a variety containing large eye
shaped grains (augen) of feldspar - Photo Close-up picture of Ponaganset augen
gneiss , Rhode Island
35Injection Gneiss
- Injection gneisses are formed by injection of
veinlets of granitic material into a schist or
some other foliated rock
36Migmatites
- 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
37Migmatite 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
38Typical Migmatite
- Skattøra gneiss
- Scale 13 cm.
39Migmatite 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
40Origin of Gneiss
- The origin of a gneiss can usually be determined
by its chemical composition and mineral content - Orthogneiss igneous origin
- Paragneiss sedimentary origin
41Differentiation 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
42Sillimanite
- 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
43Garnet
- Photo (CN) Note the zonal distribution of quartz
inclusions in this garnet porphyroblast - Garnet is isometric and remains in extinciton in
CN
44Perthite 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