Gneiss and Eclogite
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• 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 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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• Core is not always exposed
• Especially true of granulite facies rocks and, to a lesser extent, of amphibolite facies rocks
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• 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 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 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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• The amphibolite facies rocks are characterized by plagioclase (> An
20
)
• 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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• 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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• 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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• 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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• 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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• Hornblende may be present
• Biotite is absent, unless formed by retrograde metamorphism after the major metamorphic episode ends
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• 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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• 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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The Acasta Gneisses now represent the oldest intact terrestrial rocks yet discovered
• 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
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• 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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• 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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• (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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• 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 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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• The hydrothermal solutions could be derived from several sources including:
• Juvenile water
• Connate water
• Water released during progressive metamorphism
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• Juvenile means waters released by melting which have never been at the surface before
• Often from granitic intrusions
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• 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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• 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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• 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 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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• 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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• 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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• 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

• 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 meter scale
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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 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 gneisses are formed by injection of veinlets of granitic material into a schist or some other foliated rock
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• 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 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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• Skattøra gneiss
• Scale 13 cm.
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• 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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• 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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• 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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• 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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• 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 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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