Crenulation cleavage

Crenulation cleavage is the most common type of cleavage in multiply deformed, intermediate to high-grade metapelitic rocks. Crenulation cleavage is characterized by phyllosilicate-rich (P) domains, in which phyllosilicates deï¬ne the overall cleavage, separated by quartz and feldspar-rich (QF)domains (Fig.1).

Fig.1: Photomicrograph of crenulation cleavage. Indicated are the P- (Phyllosilicate) and QF- (Quartz-Feldspar) domains.

Crenulation cleavage is created when an earlier foliation is folded (crenulated) on a meso- to microscale. An already established tectonic foliation can be affected by a later cleavage (S2 or higher) if the orientation of the ISA (Instantaneous Stretching Axes) changes locally or regionally at some point during the deformation, or if a later cleavage-forming deformation phase occurs. Because cleavages tend to form perpendicular to the maximum shortening direction (X), a new cleavage will form that overprints the preexisting one. In many cases this occurs by folding the previous foliation into a series of microfolds, in which case the cleavage is called a crenulation cleavage. Hence, a crenulation cleavage is a series of microfolds at the centimeter scale or less with parallel axial surfaces. Depending on the angle between the existing foliation and the secondary stress field, the crenulation cleavage will be symmetric or asymmetric (Fig.2). A symmetric crenulation cleavage has limbs of equal length, while an asymmetric crenulation cleavage is composed of small, asymmetric folds with S- or Z-geometry. Crenulation cleavage is restricted to lithologies with a preexisting well-developed foliation that at least partly is defined by phyllosilicate minerals. It is commonly seen in micaceous layers while absent in neighboring mica-poor layers.

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Fig.2: Symmetric and asymmetric crenulation cleavage.

As the microfolds become more closely compressed, the limbs become progressively thinned out and parallel while the fold hinges become relatively thicker. The new crenulation cleavage is parallel to the aligned limbs of stacked microfolds. Micas within the limbs of crenulations remain approximately parallel to the earlier fabric. They are still parallel to the earlier foliation but have been rotated toward parallelism with the new foliation (Fig.3). In this manner, the development of crenulation cleavage likely involves the mechanical rotation of existing grains accompanied by chemical processes such as modification of grain shapes and sizes by diffusive processes and growth of new grains with an orientation and shape compatible with the local strain history.

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Fig.3: Distribution of mica crystals in a crenulated rock. The grey rectangles represent muscovite grains and the grey lines outline the quartz grains. From Naus-Thijssen, F. M., Johnson, S. E., & Koons, P. O. (2010)

Several foliation types involve compositional layering. This layering (called banding in two-dimension observations) is attributed to some metamorphic differentiation (or segregation) during the foliation development. The solution, mass transfer and re-deposition of material (pressure-solution) cause segregation. Dissolution (removal) occurs on grain-to-grain or layer boundaries in porous rocks under nonhydrostatic stress at a rate controlled by the magnitude of normal stress across the boundary. Boundaries perpendicular to the direction of the greatest compression dissolve into the aqueous pore fluid most rapidly. The dissolved material reprecipitates, often as fibrous minerals on low-stress intergranular boundaries and opening veins. Quartz and feldspar may dissolve under pressure solution in the highly compressed limbs and be reprecipitated at the hinges where pressure is lower. As the process continues, the new foliation aligns itself perpendicular to maximum shortening and bands of micas or sheet silicates (limb sites) alternating with bands of quartz or feldspar (hinge sites) define a differentiation layering parallel to the new foliation (Fig.4).

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Fig.4: Development of quartz-rich hinges and mica-rich limbs. 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.
• Naus-Thijssen, F. M., Johnson, S. E., & Koons, P. O. (2010). Numerical modeling of crenulation cleavage development: a polymineralic approach. Journal of Structural Geology, 32(3), 330-341. • 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
Crenulation cleavage in a slate. PPL image, 2x (Field of view = 7mm)	Crenulation cleavage in a slate. PPL image, 2x (Field of view = 7mm)	Crenulation cleavage in a slate. PPL image, 2x (Field of view = 7mm)
Crenulation cleavage in a slate. PPL image, 10x (Field of view = 2mm)	Crenulation cleavage in a slate. XPL image, 10x (Field of view = 2mm)	Crenulation cleavage in a slate. PPL image, 2x (Field of view = 7mm)
Crenulation cleavage in a slate. PPL image, 2x (Field of view = 7mm)	Crenulation cleavage in a slate. XPL image, 2x (Field of view = 7mm)	Crenulation cleavage in a slate. PPL image, 2x (Field of view = 7mm)
Crenulation cleavage in a slate. PPL image, 2x (Field of view = 7mm)	Crenulation cleavage in a slate. PPL image, 2x (Field of view = 7mm)	Crenulation cleavage in a slate. XPL image, 2x (Field of view = 7mm)
Crenulation cleavage in a slate. PPL image, 2x (Field of view = 7mm)	Crenulation cleavage in a slate. XPL image, 2x (Field of view = 7mm)	Crenulation cleavage in a slate. XPL image, 2x (Field of view = 7mm)
Crenulation cleavage in a slate. XPL image, 2x (Field of view = 7mm)	Crenulation cleavage in a slate. XPL image, 2x (Field of view = 7mm)	Muscovite with crenulation clivage in schist. XPL image, 2x (Field of view = 7mm)
Muscovite with crenulation clivage in schist. XPL image, 2x (Field of view = 7mm)	Muscovite with crenulation clivage in schist. XPL image, 2x (Field of view = 7mm)	Muscovite with crenulation clivage in schist. XPL image, 2x (Field of view = 7mm)
Muscovite with crenulation clivage in schist. XPL image, 2x (Field of view = 7mm)