5/1/2017. Foliated Metamorphic Rocks. Classification of Metamorphic Rocks. Classification & Textures. Metamorphic Classification. Metamorphic Textures

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Classification & Textures Metamorphic Classification Foliated Rocks Non-foliated Rocks Etc.

Sequence of deformations, Metamorphic events 1 Classified on the basis of texture and composition (either mineralogical or chemical) Much simpler classification than that for igneous rocks

Lineation: any linear fabric elements ----------------------------------------------------------------------------------------------------------------------------------- Cleavage Traditionally: the

1 Classification & Textures Metamorphic Classification Foliated Rocks Non-foliated Rocks Etc. Metamorphic Textures Relict textures Contact metamorphism Deformation & Recrystallization Dynamic/Regional metamorphism Foliations, Lineations, Sense of Shear Progressive development of textures Sequence of deformations, Metamorphic events 1 Classified on the basis of texture and composition (either mineralogical or chemical) Much simpler classification than that for igneous rocks Prefix-type modifiers used to indicate important or unusual textural or mineralogical aspects The key purpose is to convey information in a concise way 2 Foliation: and planar fabric element Lineation: any linear fabric elements Cleavage Traditionally: the property of a rock to split along a regular set of sub-parallel, closely-spaced planes A more general concept adopted by some geologists is to consider cleavage to be any type of foliation in which the aligned sheet silicates are too fine grained to see individually with the unaided eye 3 Schistosity A preferred orientation of inequaint mineral grains or grain aggregates produced by metamorphic processes Aligned minerals are coarse grained enough to see with the unaided eye The orientation is generally planar, but linear orientations are not excluded Gneissose structure Either a poorly-developed schistosity or segregated into layers by metamorphic processes Gneissose rocks are generally coarse grained 4 Slate: compact, very finegrained, metamorphic rock with a well-developed cleavage. Freshly cleaved surfaces are dull Phyllite: a rock with a schistosity in which very fine phyllosilicates (sericite/phengite and/or chlorite), although rarely coarse enough to see unaided, impart a silky sheen to the foliation surface. Phyllites with both a foliation and lineation are very common. b: Phyllite a: Slate Figure Examples of foliated metamorphic rocks. a. Slate. b. Phyllite. Note the difference in reflectance on the foliation surfaces between a and b: phyllite is characterized by a satiny sheen. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 5 Schist: a metamorphic rock exhibiting a schistosity. By this definition schist is a broad term, and slates and phyllites are also types of schists. In common usage, schists are restricted to those metamorphic rocks in which the foliated minerals are coarse enough to see easily in hand specimen. S. Kuehn 2005 Figure 22.1c. Garnet muscovite schist. Muscovite crystals are visible and silvery, garnets occur as large dark porphyroblasts. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 6 1

7 Non- 8 Gneiss: a metamorphic rock displaying gneissose structure. Gneisses are typically layered (also called banded), generally with alternating felsic and darker mineral layers.

Simpler than for foliated rocks Again, this discussion and classification applies only to rocks that are not produced by high-strain metamorphism Granofels: a comprehensive term for any isotropic

Hornfelses are tough, and tend to splinter when broken. Figure 22.1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An Introduction to Igneous and Metamorphic Petrology.

Quartzite: a metamorphic rock composed predominantly of quartz. The protolith is typically sandstone.

Drawings of quartz-mica schists. a. Closer spacing of micas in the lower half causes quartz grains to passively elongate in order for quartz-quartz boundaries to meet mica (001) faces at 90 o.

2 7 Non- 8 Gneiss: a metamorphic rock displaying gneissose structure. Gneisses are typically layered (also called banded), generally with alternating felsic and darker mineral layers. Gneisses may also be lineated, but must also show segregations of felsic-mineral-rich and dark-mineral-rich concentrations. Simpler than for foliated rocks Again, this discussion and classification applies only to rocks that are not produced by high-strain metamorphism Granofels: a comprehensive term for any isotropic rock (a rock with no preferred orientation) Hornfels is a type of granofels that is typically very finegrained and compact, and occurs in contact aureoles. Hornfelses are tough, and tend to splinter when broken. Figure 22.1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 9 Quartzite Fig 8.9 Understanding Earth 10 Marble: a metamorphic rock composed predominantly of calcite or dolomite. The protolith is typically limestone or dolostone. Quartzite: a metamorphic rock composed predominantly of quartz. The protolith is typically sandstone. Some confusion may result from the use of this term in sedimentary petrology for a pure quartz sandstone. Marble typically have little to no foliation Fig 8.9 Understanding Earth a Figure Drawings of quartz-mica schists. a. Closer spacing of micas in the lower half causes quartz grains to passively elongate in order for quartz-quartz boundaries to meet mica (001) faces at 90 o. From Shelley (1993). b. Layered rock in which the growth of quartz has been retarded by grain boundary pinning by finer micas in the upper layer. From Vernon, 1976) Metamorphic Processes: Reactions and Microstructure Development. Allen & Unwin, London. b 11 Greenschist/Greenstone: a low-grade metamorphic rock that typically contains chlorite, actinolite, epidote, and albite. Note that the first three minerals are green, which imparts the color to the rock. Such a rock is called greenschist if foliated, and greenstone if not. The protolith is either a mafic igneous rock or graywacke. 12 Amphibolite: a metamorphic rock dominated by hornblende + plagioclase. Amphibolites may be foliated or nonfoliated. The protolith is either a mafic igneous rock or graywacke. 2

13 14 Serpentinite: an ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine. Blueschist: a blue amphibole-bearing metamorphosed mafic igneous rock or mafic graywacke.

Granulite: a high grade rock of pelitic, mafic, or quartzofeldspathic parentage that is predominantly composed of OH-free minerals. Muscovite is absent and plagioclase and orthopyroxene are common.

Migmatites may appear layered, or the leucosomes may occur as pods or form a network of cross-cutting veins.

3 13 14 Serpentinite: an ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine. Blueschist: a blue amphibole-bearing metamorphosed mafic igneous rock or mafic graywacke. This term is so commonly applied to such rocks that it is even applied to non-schistose rocks. Eclogite: a green and red metamorphic rock that contains clinopyroxene and garnet (omphacite + pyrope). The protolith is typically basaltic. Skarn: a contact metamorphosed and silica metasomatized carbonate rock containing calc-silicate minerals, such as grossular, epidote, tremolite, vesuvianite, wollastonite, etc. Granulite: a high grade rock of pelitic, mafic, or quartzofeldspathic parentage that is predominantly composed of OH-free minerals. Muscovite is absent and plagioclase and orthopyroxene are common. Migmatite: a composite silicate rock that is heterogeneous on the 1-10 cm scale, commonly having a dark gneissic matrix (melanosome) and lighter felsic portions (leucosome). Migmatites may appear layered, or the leucosomes may occur as pods or form a network of cross-cutting veins. 15 Additional Modifying Terms: Porphyroblastic means that a metamorphic rock has one or more metamorphic minerals that grew much larger than the others. Each individual crystal is a porphyroblast Some porphyroblasts, particularly in low-grade contact metamorphism, occur as ovoid spots If such spots occur in a hornfels or a phyllite (typically as a contact metamorphic overprint over a regionally developed phyllite), the terms spotted hornfels, or spotted phyllite would be appropriate. 16 Geology - Chernicoff 17 Large garnet porphyroblasts in schist 18 Figure 23.14b. Spotted Phyllite. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. S. Kuehn 2005 Fig 8.10 Understanding Earth 3

Additional Modifying Terms: augen structure.

Individual grains of this sort are called auge (German for eye), and the (German) plural is augen. An augen gneiss is a gneiss with augen structure.

as grain-size, color, chemical aspects, (aluminous, calcareous, mafic, felsic, etc.). As a general rule we use these when the aspect is unusual.

Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

The terms are used only when they serve to dissipate doubt.

4 Additional Modifying Terms: augen structure Augens Some gneisses have large eyeshaped grains (commonly feldspar) that are derived from pre-existing large crystals by shear. Individual grains of this sort are called auge (German for eye), and the (German) plural is augen. An augen gneiss is a gneiss with augen structure. Additional Modifying Terms: Other modifying terms that we may want to add as a means of emphasizing some aspect of a rock may concern such features as grain-size, color, chemical aspects, (aluminous, calcareous, mafic, felsic, etc.). As a general rule we use these when the aspect is unusual. Obviously a calcareous marble or mafic greenschist is redundant, as is a fine grained slate. Figure Augen Gneiss. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 21 High Strain Rocks 22 Additional Modifying Terms: Ortho- a prefix indicating an igneous parent, and Para- a prefix indicating a sedimentary parent The terms are used only when they serve to dissipate doubt. For example, many quartzo-feldspathic gneisses could easily be derived from either an impure arkose or a granitoid rock. If some mineralogical, chemical, or field-derived clue permits the distinction, terms such as orthogneiss, paragneiss, or orthoamphibolite may be useful. High Strain Rocks Mylonite Ultramylonite Mylonite Produced by shearing in the deepest parts of fault zones. Arrows indicate the shear direction in this sample Understanding Earth 4

25 Scaleindependence 26 Figure 22.2. Schematic cross section through a shear zone, showing the vertical distribution of fault-related rock types, ranging from noncohesive gouge

Note that the width of the shear zone increases with depth as the shear is distributed over a larger area and becomes more ductile.

(texture) (structure) (structure) Diagram showing that structural and fabric elements are generally consistent in style and orientation at all scales. From Best (1982).

Metamorphic Textures Textures are small-scale penetrative features Relict Textures Inherited from original rock Blasto- = relict Any degree of preservation Pseudomorphs of

5 25 Scaleindependence 26 Figure Schematic cross section through a shear zone, showing the vertical distribution of fault-related rock types, ranging from noncohesive gouge and breccia near the surface through progressively more cohesive and foliated rocks. Note that the width of the shear zone increases with depth as the shear is distributed over a larger area and becomes more ductile. Circles on the right represent microscopic views or textures. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Berlin. (texture) (structure) (structure) Diagram showing that structural and fabric elements are generally consistent in style and orientation at all scales. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Metamorphic Textures Textures are small-scale penetrative features Relict Textures Inherited from original rock Blasto- = relict Any degree of preservation Pseudomorphs of minerals or pre-metamorphic textures/structures (bedding, pillows, etc.) 27 Textures of Contact Metamorphism Typically shallow pluton aureoles (low-p) Crystallization/recrystallization is near-static Monomineralic with low surface energy granoblastic polygonal Larger S.E. decussate Isotropic textures (hornfels, granofels) Relict textures are common Progressive thermal metamorphism of a diabase (coarse basalt). From Best (1982). Igneous and Sandstone texture Quartzite texture 5

Progressive thermal metamorphism of slate. From Best (1982).

on earlier regional events are common Thermal maximum later than

Spotted slates and phyllites Figure 23.14.

small porphyroblasts of cordierite develop in a preexisting phyllite.

Prentice Hall. Depletion haloes 35 Figure 23.13.

Fe and Mg were less plentiful, so that hornblende was consumed to a greater

zones. Sample courtesy of Peter Misch. Prentice Hall.

6 Progressive thermal metamorphism of slate. From Best (1982). Igneous and Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Textures Contact overprint on earlier regional events are common Thermal maximum later than deformational Separate post-orogenic (collapse) event Nodular overprints Spotted slates and phyllites Figure Overprint of contact metamorphism on regional: Spotted phyllite in which small porphyroblasts of cordierite develop in a preexisting phyllite. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Depletion haloes 35 Figure Light colored depletion haloes around cm-sized garnets in amphibolite. Fe and Mg were less plentiful, so that hornblende was consumed to a greater extent than was plagioclase as the garnets grew, leaving hornblende-depleted zones. Sample courtesy of Peter Misch. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Depletion haloes Depletion halo around garnet porphyroblast. Boehls Butte area, Idaho 36 6

Depletion haloes 37 Pressure Solution 38 Progressive development of a depletion halo about a

High-strain areas dissolve and material precipitates in adjacent low-strain areas (shaded).

Pressure solution of a quartz crystal in a deformed quartzite ( 1 is vertical).

and precipitation in low-strain areas (large arrow). ~ 0.5 mm across.

After Hibbard (1995) Petrography to Petrogenesis. Prentice Hall.

migration 40 and sub-grain rotation Figure 23.5.

Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure 23.

migration. Width 0.2 mm. From Borradaile et al. (1982). Fig 23.21 Types of foliations a.

Shape of deformed grains d. Grain size variation e.

7 Depletion haloes 37 Pressure Solution 38 Progressive development of a depletion halo about a growing porphyroblast. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco. Figure 23.2 a. Highest strain in areas near grain contacts (orange hatch pattern). b. High-strain areas dissolve and material precipitates in adjacent low-strain areas (shaded). The process is accompanied by vertical shortening. c. Pressure solution of a quartz crystal in a deformed quartzite ( 1 is vertical). Pressure solution results in a serrated solution surface in high-strain areas (small arrows) and precipitation in low-strain areas (large arrow). ~ 0.5 mm across. The faint line within the grain is a hematite stain along the original clast surface. After Hibbard (1995) Petrography to Petrogenesis. Prentice Hall. Dislocation migration forms two strain-free subgrains 39 Recrystallization by grain boundary migration 40 and sub-grain rotation Figure Illustration of a recovery process in which dislocations migrate to form a subgrain boundary. Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure 23.7a. Recrystallized quartz with irregular (sutured) boundaries, formed by grain boundary migration. Width 0.2 mm. From Borradaile et al. (1982). Fig Types of foliations a. Compositional layering b. Preferred orientation of platy minerals c. Shape of deformed grains d. Grain size variation e. Preferred orientation of platy minerals in a matrix without preferred orientation f. Preferred orientation of lenticular mineral aggregates g. Preferred orientation of fractures h. Combinations of the above 41 Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and 42 Figure Types of fabric elements that may define a foliation. From Turner and Weiss (1963) and Passchier and Trouw (1996). 7

43 44 Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982).

Igneous and 45 46 Progressive development (a c) of a crenulation cleavage for both asymmetric (top) and symmetric

Symmetrical crenulation cleavages in amphibole-quartz-rich schist. Note concentration of quartz in hinge areas.

24b. Asymmetric crenulation cleavages in mica-quartz-rich schist.

8 43 44 Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Progressive syntectonic metamorphism of a volcanic graywacke, New Zealand. From Best (1982). Igneous and Progressive development (a c) of a crenulation cleavage for both asymmetric (top) and symmetric (bottom) situations. From Spry (1969) Metamorphic Textures. Pergamon. Oxford. Figure 23.24a. Symmetrical crenulation cleavages in amphibole-quartz-rich schist. Note concentration of quartz in hinge areas. From Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag. 47 Figure 23.24b. Asymmetric crenulation cleavages in mica-quartz-rich schist. Note horizontal compositional layering (relict bedding) and preferential dissolution of quartz from one limb of the folds. From Borradaile et al. (1982) Atlas of Deformational and Metamorphic Rock Fabrics. Springer-Verlag. 8

Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured

Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured Chemical Systems Introduction to Metamorphism Reading: Winter Chapter 21 An assemblage of coexisting phases (thermodynamic equilibrium and the phase rule) A basaltic composition can be either: Melt Cpx