Structural Geology: Metamorphic Foliations
An important feature used to unravel a geological story
Background
What is Structural Geology?
Study of the architecture of features in the Earth’s crust at all scales (micro to regional) resulting from deformation, (Wilkerson Pers. Comm., 2019)
Geologic Structure
a definable shape and/or pattern in rock that represents a change in shape, position, and/or orientation of the rock, (Wilkerson Pers. Comm., 2019).
What is a Foliation?
A foliation is any sort of fabric-forming planar or curved planar geologic structure in a metamorphic rock, but could additionally include sedimentary bedding or magmatic layering (Wilkerson, 2019). A foliated rock holds a parallel alignment of certain minerals that are repetitively layered. Further, these rocks are cohesive (ductile), though rocks may break apart preferentially along the foliation. Foliated rocks are valuable assets, because they may provide clues about the history of the geology surrounding them.
Fabric
The fabric referred to in this definition is a geometrical order of component features in the rock (Wilkerson Pers. Comm., 2019). Fabrics are made up of minerals that align in a favored orientation that exist in the rock at the microscopic to centimeter-spacing scale.
Metamorphic Rock
Sedimentary and igneous rocks may undergo extreme levels of heat and pressure deep beneath the surface, which results in the transformation into metamorphic rocks. It is common for them to form with bands of crystals penetrating through them, (What Are Metamorphic Rocks?, https://www.usgs.gov/faqs/what-are-metamorphic-rocks-0?qt-news_science_products=0#qt-news_science_products.).
This image exhibits the stress applied to rock that results in the alignment of minerals that is reflected in metamorphic rocks through banding and foliation. https://www.colorado.edu/center/mortenson/sites/default/files/attached-files/metamorphic_rocks.pdf
This specimen is a Gneiss Metamorphic rock that contains banding (the layering of dark and light minerals) as well as foliation (the horizontal arrangement and layering present). The lighter minerals are primarily quartz and feldspar and the dark bands include some hornblende and biotite minerals. The sample is in the DePauw University Rock Room's collection.
Primary versus Secondary Foliation
Primary foliations are key sources of the creation of buckle folds and important for analyzing folds in rocks overall. They form during deposition of sediments and formation of magmatic rocks, while secondary foliations, such as axial plane cleavages, form in metamorphic rocks, (Fossen, 2016). Most foliations have a direct correlation with perpendicular shortening, which includes perpendicular to or at high angle to the shortening direction.
Axial Planar Cleavage
At the hinge, the stress is at the greatest level. The mineral grains order themselves in the direction of least strain, naturally moving into separate into layers of different minerals parallel to the axial plane. (https://www.geograph.org.uk/photo/1945635)
Image on the left: Axial Planar Cleavage
cc-by-sa/2.0 - © Anne Burgess - geograph.org.uk/p/1945635
Since cleavage falls under the umbrella of foliations, here's a general definition:
- It's a tectonic foliation that is ductile (maintained cohesion) in low-grade (small increase in temperatures during metamorphism) rocks which forms relatively flat, parallel surfaces, (Wilkerson Pers. Comm., 2019).
Classifications of Foliations
- Foliations are referred to as S-tectonites, unlike lineations that are referred to as L-tectonites. In the top figure on the right, the patterns formed for the S-tectonite compared to L-tectonite are depicted. Notice in the S-tectonite (foliation) figure, the top of the surface is a randomized grain pattern and not stretched, but the parallel layering (horizontal) arrangement is prevalent on the sides.
- The diagram depicting the L-tectonite and S-tectonite is sourced from a Scott Wilkerson lecture at DePauw University.
- In the bottom image, foliation is illustrated in figure (c) and lineation is shown in figure (d). The horizontal layering throughout the specimen is clear in (c), and the linear elongated minerals throughout the specimen in uni-directional way (d).
- Foliation and lineation are two separate entities, but are often found complementing one another in the same metamorphic rocks, as you will see later.
This Wilkerson diagram contains illustrations of rock behavior when (a) a foliation is present, (b) a lineation is present, and (c) when the rock contains a combination of the two, (Wilkerson, Pers. Comm., 2019).
Types of Foliations
Gneissic Foliation
- foliation in high-grade rocks defined by metamorphic compositional layering (Wilkerson, 2019).
- The image on the right is a gneiss rock that formed from high-pressure, high-temperature conditions (originally was a granite igneous rock). Source: Peter Davis.
Development of Gneissic Layering (found in the diagram below):
- Original Layering (a)
- Transposition (b)
- Metamorphic Differentiation(c)
- Lit-par-lit (sill) intrusion (d)
OR "Original compositional layering of a rock could also become transposed to a new orientation during metamorphism. ... In the beginning stages, a new foliation starts to develop in the rock as a result of compressional stress at some angle to the original bedding.,"(Wilkerson, Pers. Comm., 2019).
Source: W.W. Norton or Scott Wilkerson
Gneiss rock sample is shown above that was collected by Dr. Hazlett. Augen foliation is visible here: large, lens-like, eye-shaped mineral grains or mineral aggregates that are present in some foliated metamorphic rocks. This is a personal photograph taken of a Syenite Gneiss from the DePauw University rock room.
Schistosity
Foliation in medium-grade rocks defined by the alignment of metamorphic mica (platy minerals), (Wilkerson, Per. Comm., 2019).
- The image on the right is a personal photograph of foliated schist from the DePauw University Rock Room. This rock sample displays both lineations and foliations present together. The black Tourmaline crystals are clearly lined up parallel to one another, as they are lineated, (Mills, Pers. Comm., 2019).
- Lineation: a linear preferred fabric that is also known as L-tectonites, (Wilkerson, Pers. Comm., 2019)
This is a personal photograph of a schist sample from the DePauw University rock room collection. The schist rock illustrates folded foliations, as there is a distinguished bend of the layering structure, (Mills, Pers. Comm., 2019)
Green Schist from E. of Stockbridge, White River, Vt. sample is from the DePauw University rock room collection.
Slaty Foliation
Slate is a fine-grained, foliated metamorphic rock that is physically and chemically changed (mineral grains changed directions) through by the alteration of shale or mudstone by low-grade regional metamorphism. (https://www.radford.edu/jtso/GeologyofVirginia/Rocks/GeologyOfVARocks2-5a.html)
A slate specimen from the DePauw University rock room collection that illustrates the thin lamination foliations and common dull appearance of slate.
The image displays another slate sample from the DePauw University rock room collection. As noted, the micas are not visible to the naked eye.
Mylonitic Foliation
These Foliations are related to transposed layering and gneissic banding, but the distance between the foliation domains is smaller, typically on the millimeter or centimeter scale related to high strains involved: flatten objects and thin layers
Mylonitic foliations are linked with transposed layering and gneissic banding, but the distance between the foliation domains is smaller, oftentimes on the millimeter or centimeter scale. They are related to high strains involved, which flatten objects and thin out the layers . Mylonite is very high-grade, higher grade than Gneiss, (Fossen, 2016).
The mylonitic rock on the right is currently stored in the DePauw University rock room, but formed in Bancroft, Ontario. The specimen is sheared as it is a fault rock from a shear zone. There was a gigantic mountain range that slid downwards above the fault and a marble slab of rock that sheared up below the fault. The marble unit acted as a lubricant along the faulting. The sliding along the marble unit created the marble mylonite. Attachment fault crushed up in the fault zone. We see gneissic banding of dark and light minerals penetrating throughout the rock specimen, (Mills Pers. Comm., 2019)
Small boudins are present in this foliated rock (circled below) that formed during the stretching of layers causing them to break apart.
Zoomed in image on the boudins that are circled in the mylonite that is shown to the right of this image.
- A Protomylonite only experiences grain size reduction in less than 50% of the rock.
- Additionally, a protomylonite is a mylonitic rock produced from contact-metamorphosed rock, with granulation and flowage being due to overthrusts following the contact surfaces between intrusion and country rock, (Wilkerson Pers. Comm., 2019)
- Mylonites have grain size shrinking by plastic deformation forces acting upon them.
- Ultra mylonite is a type of mylonite defined by modal percentage of matrix grains more than 90%. Ultramylonite is often hard, dark, cherty to flinty in appearance and sometimes resemble pseudotachylite and obsidian, (Wilkerson Pers. Comm., 2019)
- Blastomylonites are coarse-grained, commonly sugary texture without defined tectonic banding; has a grain-size increase by recrystallization
Foliated Phyllite
Phyllite is a type of foliated metamorphic rock originated from slate that has been more metamorphosed so that very fine grained mica (not visible in slate) finds a preferred orientation. The layers in a phyllite are so fine, they are referred to as laminations just like slate rocks, (Mills Pers. Comm., 2019).
This is another foliated phyllite sample from DePauw University Rock Room collection that contains some oxidized iron secondary features and fine sand layering in it. The Mica is present, as the faces have a shean to them, but mica grains themselves are not yet visible.
This specimen is labeled as a Apanite Schist from Germany but has been metamorphosed into more of a Phyllite. The Phyllite contains relic bedding and foliation (DePauw University Rock Room).
References
- First figure as the Title page background is a Scott Wilkerson personal photograph.
- Fossen, Haakon. “Structural Geology by Haakon Fossen.” Cambridge Core, Cambridge University Press, https://www.cambridge.org/core/books/structural-geology.
- https://www.colorado.edu/center/mortenson/sites/default/files/attached-files/metamorphic_rocks.pdf
- Rocks 2.5, https://www.radford.edu/~jtso/GeologyofVirginia/Rocks/GeologyOfVARocks2-5a.html.
- What Are Metamorphic Rocks?, https://www.usgs.gov/faqs/what-are-metamorphic-rocks-0?qt-news_science_products=0#qt-news_science_products.
- Wilkerson, M. Scott (2019). Presentation on Foliations and Lineations. https://moodle.depauw.edu/pluginfile.php/357381/mod_resource/content/0/2019_10FoliationsLineations.pdf
