Porphyroclastic texture

Porphyroclasts are single crystals of a size exceeding the mean grain size in the surrounding matrix and typical for mylonites. They are relic structures of a more coarse-grained original fabric. Common minerals that form porphyroclasts are feldspar, garnet, muscovite, hornblende and pyroxenes. If they are equidimensional and have a sharp boundary with the matrix, they are known as naked clasts or, if the porphyroclasts have an elongate shape with monoclinic symmetry, mineral fish. In many cases, porphyroclasts have attached polycrystalline rims that differ in structure or composition from the matrix; such assemblages are known as porphyroclast systems. Porphyroclast systems can have a number of characteristic shapes that can be used to determine shear sense and other kinematic parameters. In many cases, the surrounding rim has a tapering shape on opposite sides of the porphyroclast. If the material in the rim is of the same composition as the porphyroclast, the rim is called a mantle and the structure is known as a mantled porphyroclast or mantled clast. If the rim has a composition different from the porphyroclast, the adjacent tapering domains are known as strain shadows and the entire structure as a porphyroclast with strain shadows. Such stain shadows are commonly composed of carbonate, quartz, mica or opaque minerals, which were apparently not formed by reaction with the porphyroclast, but by precipitation from solution. If it can be shown that the material in the rim is formed by transformation of the porphyroclast, the rim is known as a reaction rim (Fig.1).

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Fig.1: Schematic diagram of the principal types of objects encountered in the matrix of mylonites. This includes large single crystal shapes such as naked clasts and mineral fish, single crystal porphyroclasts with rims such as mantles, reaction rims or strain shadows, and polycrystalline aggregates such as sigmoids. A, B, etc. refer to mineral types. Modified from Passchier (2005).

Deformation quadrants

A stiff inclusion perturbs the flow in the soft matrix immediately adjacent to it (Fig.2). The perturbation area can be divided into four quadrants. Two quadrants are extensional; they alternate with the other two shortening quadrants. In bulk non-coaxial flow these quadrants of perturbed local flow are asymmetrically disposed about the stiff inclusion with respect to the main foliation, which is equated with the shear plane in strongly sheared rocks. This asymmetry directly reflects the shear-sense of the bulk flow in the matrix.

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Fig.2: Incremental deformation quadrants in the perturbed flow of a soft matrix around a hard object. From Jean-Pierre Burg.

Asymmetric tails

Porphyroblasts in mylonites have "tails" of very fine grain aggregates that are recrystallized from the edges of the blast itself (Fig.3). These tails are attached to the edges of the blast and are installed in the regions where the general flow is disturbed by the relatively rigid grain. In effect, rigid objects tend to rotate in their ductile matrix since the bulk deformation is non-coaxial. If the tails have the same mineral composition as the neighboring blast, the central grain + tails system is a mantled, winged porphyroblasts. The morphology of a pair of tails is defined with respect to a reference line, which is the imaginary line parallel to the bulk mylonitic foliation through the center of the porphyroblast in XZ planes of rock. The asymmetry is defined by the stair-stepping of an imaginary line running along the middle of the tails, called the median line, and joining the tails across the reference line through the clast. The sense of asymmetry of the tails defines the sense of shear in the deformed rock.

Four types of mantled porphyroclasts have been distinguished in the literature based on the shape of the wings:

Φ-type: these porphyroclasts are dominant in high grade rocks, likely due to rapid recrystallisation in non-coaxial flow. Due to their lack of stair-stepping in the tails, they cannot be used to determine shear sense.

σ-type: On σ-clasts, the tails extend parallel to the foliation and wedge out from each side of the grain in the "downstream" direction of the relative shear in the matrix (Fig.4). The tails are essentially on opposite sides of the reference line. The stepping up direction of the median line defines the sense of shear. This type of clast-tail system characterizes recrystallisation rates higher than clast rotation rates. σ-type objects can be subdivided into two groups:

- σ_a-type: Isolated in a mylonitic matrix; consist of weakly anisotropic minerals such as feldspar, hornblende or apatite; recrystallized wings are usually equigranular and structureless.
- σ_b-type: Part of developing C/S-fabrics: In quartz-feldspar mylonites the C-planes tend to enclose large porphyroclasts of feldspar, and dynamically recrystallized material from the feldspar grain mantle is deflected along them; tend to occur in clusters.

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Fig.4: σ-type porphyroclast. From Jean-Pierre Burg.

δ-type: The δ-type is derived from the σ-type by rotation of the clast (Fig.5). The clast entrains and coils the tails in a sense consistent with the bulk shear to produce embayed shapes. Consequently, the folded wings wrap around the clast and cross the reference line in the clast or blast. δ-type and complex mantled clasts mainly occur in high strain mylonites.

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Fig.5: δ-type porphyroclast. From Jean-Pierre Burg.

Complex objects: Extreme shear strain may lead to complex structures (Fig.6) combining σ- and δ-type tails. δ-and complex δ + σ clast systems are also described as rolling structures.

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Fig.6: Complex object porphyroclast. From Jean-Pierre Burg.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo/caption>
Delta (δ) porphyroclast in a Mylonitic Granite from khumbu (Nepal). PPL image, 10x (Field of view = 2mm)	Delta (δ) porphyroclast in a Mylonitic Granite from khumbu (Nepal). XPL image, 10x (Field of view = 2mm)	Sigma (σ) porphyroclast in a Mylonitic Granite from khumbu (Nepal). PPL image, 10x (Field of view = 2mm)
Delta (δ) porphyroclasts in a Mylonitic Granite from khumbu (Nepal). XPL image, 2x (Field of view = 7mm)	Sigma (σ) porphyroclast (Orthose) in a Mylonitic Granite from khumbu (Nepal). XPL image, 2x (Field of view = 7mm)	Delta (δ) porphyroclast in a Mylonitic Granite from khumbu (Nepal). PPL image, 10x (Field of view = 2mm)
Feldsapr porphyroclasts in a Mylonitic Granite from khumbu (Nepal). XPL image, 2x (Field of view = 7mm)	Feldsapr porphyroclast in a Mylonitic Granite from khumbu (Nepal). XPL image, 10x (Field of view = 2mm)	Feldsapr porphyroclast in a Mylonitic Granite from khumbu (Nepal). XPL image, 10x (Field of view = 2mm)
Sigma (σ) Garnet porphyroclast from Posada Asinara Shear zone, Sardinia (Italy). PPL image, 2x (Field of view = 7mm)	Sigma (σ) Garnet porphyroclast from Posada Asinara Shear zone, Sardinia (Italy). PPL image, 10x (Field of view = 2mm)	Sigma (σ) Oligoclase porphyroclast from Posada Asinara Shear zone, Sardinia (Italy). PPL image, 2x (Field of view = 7mm)
Sigma (σ) Oligoclase porphyroclast from Posada Asinara Shear zone, Sardinia (Italy). XPL image, 2x (Field of view = 7mm)	Staurolite porphyroclast from Posada Asinara Shear zone, Sardinia (Italy). PPL image, 2x (Field of view = 7mm)	Staurolite porphyroclast from Posada Asinara Shear zone, Sardinia (Italy). XPL image, 2x (Field of view = 7mm)
Sigma (σ) Garnet porphyroclast. PPL image, 10x (Field of view = 2mm)	Sigma (σ) Albite porphyroclast. PPL image, 10x (Field of view = 2mm)	Sigma (σ) Pyroxene porphyroclast. PPL image, 10x (Field of view = 2mm)
Sigma (σ) Albite porphyroclast. PPL image, 10x (Field of view = 2mm)	Sigma (σ) Albite porphyroclast from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Sigma (σ) Albite porphyroclast from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)
Albite and Tourmaline porphyroclast from Sardinia (Italy). PPL image, 10x (Field of view = 2mm)	Albite and Tourmaline porphyroclast from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)	Albite porphyroclasts from Sardinia (Italy). PPL image, 2x (Field of view = 7mm)
Albite porphyroclasts from Sardinia (Italy). XPL image, 2x (Field of view = 7mm)	Titanite porphyroclasts with chlorite tails from Sardinia (Italy). PPL image, 10x (Field of view = 2mm)	Titanite porphyroclasts with chlorite tails from Sardinia (Italy). XPL image, 10x (Field of view = 2mm)