Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Cataclasite

Cataclasite: Cataclasite: Fault-related rock that maintained primary cohesion during deformation, with matrix comprising greater than 10 % of rock mass; matrix is fine-grained material formed through grain size reduction. Cataclasites are subdivided into those that have 10-50% matrix (protocataclasite), 50-90% matrix (cataclasite) and > 90% matrix (ultracataclasite).

Brittle fault rocks

Brittle fault rocks form by fault propagation through intact rock, commonly along some older plane of weakness, and formation of a volume of brittle fault rock in a fault zone along the active fault. There are several types of fault rocks, depending on lithology, confining pressure (depth), temperature, fluid pressure, kinematics etc. at the time of faulting. It is useful to distinguish between different types of fault rocks, and to separate them from mylonitic rocks formed in the plastic regime (Fig.1). Sibson (1977) suggested a classification based on his observation that brittle fault rocks are generally non-foliated, while mylonites are well foliated. He further made a distinction between cohesive and non-cohesive fault rocks (Fig.2). Further subclassification was done based on the relative amounts of large clasts and fine-grained matrix. Its relationship to microscopic deformation mechanism is also clear, since mylonites, which result from plastic deformation mechanisms, are clearly separated from cataclastic rocks in the lower part of the diagram.

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Fig.1: Conceptual model of a fault zone showing fault rock location at depth. Modified from Mike Norton.

Fig.2: Sibson's fault rocks classification. From Fossen, H. (2016).

Brittle fault rocks classification

Incohesive brittle fault rocks are usually found in faults, which have been active at shallow crustal levels. They occur in fault zones of variable thickness and can be subdivided into:

Fault breccia is an unconsolidated fault rock consisting of less than 30% matrix. If the matrix fragment ratio is higher, the rock is called a fault gouge. A fault gouge is thus a strongly ground-down version of the original rock. These unconsolidated fault rocks form in the upper part of the brittle crust. They are conduits of fluid flow in non-porous rocks but contribute to fault sealing in faulted porous rocks.

Cohesive fault rocks can be subdivided into cohesive breccia, cohesive cataclasite and pseudotachylyte. Cohesive cataclasite and breccia commonly show evidence for abundant pressure solution and precipitation effects. Rock fragments are transected by healed cracks aligned with fluid inclusions. Veins of quartz, calcite, epidote or chlorite, and in ultramafic rocks, serpentine, are common. These veins form during and after brittle deformation, since they have commonly been fractured.

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Fault with fault breccia in interbedded metagraywacke-slate. Peyla, Minnesota, USA. From James St. John.

Cataclasite. From Busan National Geopark.

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.

Deformed plagioclase crystal in a cataclastic gabbro from Isola di Vocca, Ivrea Verbano, Italy. XPL image, 10x (Field of view = 2mm)	Faulted plagioclase crystal in a cataclastic gabbro from Isola di Vocca, Ivrea Verbano, Italy. XPL image, 10x (Field of view = 2mm)	Faulted plagioclase crystal in a cataclastic gabbro from Isola di Vocca, Ivrea Verbano, Italy. XPL image, 10x (Field of view = 2mm)
Faulted plagioclase crystal. Val Passiria (Alpi Venoste), Merano, Italy. XPL image, 1x (Field of view = 9mm)	Faulted plagioclase crystal. Val Passiria (Alpi Venoste), Merano, Italy. XPL image, 2x (Field of view = 7mm)	Faulted plagioclase crystal. Val Passiria (Alpi Venoste), Merano, Italy. XPL image, 2x (Field of view = 7mm)
Faulted plagioclase crystal. Val Passiria (Alpi Venoste), Merano, Italy. XPL image, 2x (Field of view = 7mm)	Faulted plagioclase crystal. Val Passiria (Alpi Venoste), Merano, Italy. XPL image, 2x (Field of view = 7mm)	Faulted plagioclase crystal. Val Passiria (Alpi Venoste), Merano, Italy. XPL image, 2x (Field of view = 7mm)
Faulted plagioclase crystal in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 10x (Field of view = 2mm)	Faulted plagioclase crystal in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 10x (Field of view = 2mm)	Faulted plagioclase crystal in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 10x (Field of view = 2mm)
Faulted plagioclase crystal in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 10x (Field of view = 2mm)	Faulted plagioclase crystal in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 2x (Field of view = 7mm)	Faulted plagioclase crystal in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 20x (Field of view = 1mm)
Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 2x (Field of view = 7mm)	Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 2x (Field of view = 7mm)	Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. PPL image, 2x (Field of view = 7mm)
Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 2x (Field of view = 7mm)	Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. PPL image, 2x (Field of view = 7mm)	Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 2x (Field of view = 7mm)
Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 2x (Field of view = 7mm)	Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 2x (Field of view = 7mm)	Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 2x (Field of view = 7mm)
Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 10x (Field of view = 2mm)	Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. PPL image, 2x (Field of view = 7mm)	Fragmented plagioclase and hornblende crystals in a cataclastic gabbro from Campegli mine, Val Petronio, Liguria, Italy. XPL image, 2x (Field of view = 7mm)
Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. XPL image, 2x (Field of view = 7mm)	Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. XPL image, 2x (Field of view = 7mm)	Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. Chlorite in green. PPL image, 2x (Field of view = 7mm)
Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. XPL image, 2x (Field of view = 7mm)	Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. XPL image, 2x (Field of view = 7mm)	Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. XPL image, 2x (Field of view = 7mm)
Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. XPL image, 2x (Field of view = 7mm)	Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. XPL image, 2x (Field of view = 7mm)	Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. Chlorite in green. PPL image, 2x (Field of view = 7mm)
Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. Chlorite in green. PPL image, 2x (Field of view = 7mm)	Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. XPL image, 2x (Field of view = 7mm)	Fragmented Quartz and plagioclase crystals in a granitic Cataclasite. XPL image, 2x (Field of view = 7mm)
Carbonate cataclasite. Immagine a N//, 2x (lato lungo = 7 mm)	Carbonate cataclasite. Immagine a N//, 2x (lato lungo = 7 mm)	Carbonate cataclasite. Immagine a N//, 2x (lato lungo = 7 mm)
Carbonate cataclasite. Immagine a N//, 2x (lato lungo = 7 mm)	Carbonate cataclasite. Immagine a N//, 2x (lato lungo = 7 mm)	Carbonate cataclasite. Immagine a NX, 2x (lato lungo = 7 mm)
Cataclastic surface in a phyllite. PPL image, 2x (Field of view = 7mm)	Cataclastic surface in a phyllite. PPL image, 2x (Field of view = 7mm)	Cataclastic surface in a phyllite. PPL image, 2x (Field of view = 7mm)
Cataclastic surface in a phyllite. PPL image, 2x (Field of view = 7mm)	Cataclastic surface in a phyllite. XPL image, 2x (Field of view = 7mm)	Cataclastic surface in a phyllite. XPL image, 2x (Field of view = 7mm)
Cataclastic surface in a phyllite. XPL image, 2x (Field of view = 7mm)