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.

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 (> 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
Upper Photo # b_103.gif
lower Photo #b_104.gif

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)
Photo # r_7.gif
Photo # granulite.gif

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
Photo #image19.gif

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)
Upper Photo #eclogite1_X.jpg
Lower Photo #eclogite1_UX.jpg

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)
Upper Photo # retroeclogite_X.jpg
Lower Photo # retroeclogite_UX.jpg

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
Cinema Expeditions CD-ROM 5 Rocks
Upper Photo \metamorp\gneiss\ jpg

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
Photo #gree-177.jpg

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
Photo #gneiss.jpg

33. LANDSAT Image
Two major types of rocks are found in the mountain range; 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
Photo #prettywtanks.jpg

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
Photo #1c22-3.gif

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
Photo #11.jpg

38. Typical Migmatite Skattøra gneiss Scale 13 cm.
Photo #12.jpg
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
Photo #15.jpg

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 Found in high-T metamorphic rocks that are rich in Al
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
Note the zonal distribution of quartz inclusions in this garnet
porphyroblast
Photo # garnet_X.jpg

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
Photo # a_4.gif