Textures and Structures of Metamorphic Rocks | Geology Optional Notes for UPSC PDF Download

Textures in Metamorphic Rocks

• Metamorphic Textures Overview:
  • Definition: Metamorphic textures refer to the varied size, shape, orientation, and spatial arrangement of crystals in rocks, influenced by different pressure and temperature conditions during metamorphism.
  • Diversity: These textures exhibit a wide range of characteristics due to the changing P-T conditions.
• Categories of Metamorphic Textures:
  1. Palimpsest Texture: This texture shows evidence of earlier stages of metamorphism that have been partially or wholly overprinted by subsequent changes.
  2. Typomorphic Texture: These are textures that are characteristic of a particular mineral or group of minerals, aiding in their identification.
  3. Reaction Textures: These textures form as a result of reactions between minerals due to changes in pressure and temperature during metamorphism.
  4. Corona Texture: A texture where a mineral is surrounded by concentric layers of other minerals due to reactions at the mineral's rim.
  5. Intergrowth Texture: This texture involves the interlocking growth of different minerals, creating intricate patterns within the rock.

Textures and Structures of Metamorphic Rocks

Palimpsest Texture

• The primary texture of the original rock, known as palimpsest texture, sometimes persists through metamorphism, showcasing the inheritance of protolith rock textures.
• Palimpsest texture is also referred to as relict texture because it survives metamorphism, especially in low-grade metamorphic rocks where deformation is limited, allowing for preservation.
• Examples of palimpsest (relict) textures include:
  • Blasto-ophitic texture: Plagioclase laths suspended in a pyroxene matrix, where the plagioclase laths can be fully or partially surrounded by the matrix.
  • Blasto-intergranular texture: Igneous relict textures in metamorphic rocks characterized by ferromagnesium minerals occupying interstices between plagioclase crystals.
  • Blasto-porphyritic texture: Larger grains or crystals surrounded by a fine-grained matrix or glassy groundmass, reminiscent of textures found in igneous rocks.

Palimpsest textures, such as blasto-ophitic, blasto-intergranular, and blasto-porphyritic textures, are remnants from the original rock structure that persist through metamorphism, providing valuable insights into the rock's history.

Metamorphic Petrology

Typical Textures in Metamorphic Rocks

• Blasto-intergranular texture:
  • Features coarse-grained appearance with angular interstices filled with pyroxene minerals.
  • Commonly seen in rocks derived from basalt.
• Blasto-porphyritic texture:
  • Characterized by larger crystals of plagioclase, biotite, and clinopyroxene surrounded by a fine-grained groundmass.
  • Illustrates a clustered crystal arrangement.
• Blasto-cumulate texture:
  • Results from the settling of early-formed, high-density crystals, leading to a cumulate texture.
  • Survives metamorphism and is preserved as a relict texture.

Typomorphic Texture

Typomorphic textures are distinct features found in metamorphic rocks, influenced by various factors such as dynamic forces, thermal effects, or crystallization.

Textures Based on Dynamic Forces

• Porphyroclastic texture:
  • Occurs due to deformation within the rock, resulting in the crushing of softer minerals to form a groundmass, while more resistant minerals appear as larger fragments known as porphyroclasts.
  • Shows two distinct grain sizes of the same mineral.
• Mylonitic texture:
  • Characterized by a fine-grained, streaky appearance due to intense shearing forces.
  • Commonly formed in rocks subjected to significant tectonic stress.

Unit 14: Metamorphic Rock Textures

Porphyroclastic Texture

• A porphyroclast grain in metamorphic rocks consists of a rounded quartz grain surrounded by softer minerals in the groundmass, displaying a distinct porphyroclastic texture.
• Porphyroclastic texture is observable under cross-polarized light as shown in Fig. 14.4.
• Example: Imagine a rock sample where a large quartz grain is enveloped by smaller, softer minerals, creating a unique texture.

Mylonitic Texture

• In mylonitic texture, foliation planes or oriented minerals in metamorphic rocks lead to the alignment of platy minerals or quartz in a specific direction.
• Ductile deformation, attributed to cataclastic metamorphism, is the primary cause of this textural development.
• Types of mylonitic textures:
  • Protomylonitic: Shows an initial stage of foliation mylonitic texture or the beginning of mylonitisation.
  • Orthomylonitic: Rocks exhibit a well-defined foliation with quartz grains oriented in a ribbon-like manner.
  • Ultramylonitic: Represents the most advanced stage of cataclastic metamorphism, resulting in highly strained crystals recrystallizing into smaller ones, forming a granoblastic polygonal texture.
• Illustration: Visualize a mylonite outcrop displaying aligned minerals or a photomicrograph showcasing ultramylonite bands under cross-polarized light, as depicted in Fig. 14.5.

Textures Based on Thermal Effect

• Distinct textures result from the thermal effect on metamorphic rocks.
• In the absence of deformation during thermal metamorphism, mineral grains exhibit a random orientation, leading to granoblastic or hornfelsic textures.
• For example, when minerals undergo thermal metamorphism without any tectonic stress, they may show a disorganized, scrambled arrangement in the rock.

Metamorphic Rock Textures

• Development of Granoblastic Texture:
  • Occurrence in Regionally Metamorphosed Rocks: Granoblastic texture can develop in regionally metamorphosed rocks as well.
  • Types of Textures based on Thermal Effects:
    • Nodular Texture: Oval-shaped porphyroblasts of minerals like cordierite or scapolite grow alongside randomly distributed quartz and other minerals, forming nodular texture.
    • Granoblastic Texture: Found in non-foliated metamorphic rocks like marble or quartzite, where grains are equigranular to nearly equigranular, creating a mosaic of recrystallized mineral grains without porphyroblastic grains.
• Characteristics of Granoblastic Texture:
  • Variability: Granoblastic textures can range from equidimensional grains with straight grain boundaries and well-developed crystal faces to irregular grain boundaries.
  • Types of Granoblastic Textures:
    • Polygonal Granoblastic Texture: Seen in monomineralic quartz in quartzite and polymineralic quartz-mica and quartz-pyroxene rocks.
    • Interlobate Granoblastic Texture: Characterized by irregular grain boundaries.
    • Amoeboid Granoblastic Texture: Features irregularly shaped mineral grains.
    • Decussate Granoblastic Texture: Mineral grains are interlocked, randomly oriented, prismatic, or elongated with common triple junctions.

Unit 14: Metamorphic Rocks

(a) Decussate

• Decussate granoblastic texture is observed in non-foliated metamorphic rocks.
• Granoblastic texture refers to a texture composed of interlocking equidimensional grains.

(b) Textures and Structures of Metamorphic Rocks

Fig. 14.8: Diagrammatic representations of decussate granoblastic texture in non-foliated metamorphic rock.

• Metamorphic rocks exhibit various crystallisation textures.

(c) Crystallisation Textures

• Crystallisation textures are part of typomorphic metamorphic textures.
• These textures involve the development of coarse-grained structures.

Porphyroblastic Texture

• Porphyroblastic texture features large mineral grains within a fine-grained ground mass.
• Minerals like garnet and staurolite tend to recrystallize into large, individual crystals.
• Example: Large garnets embedded in a matrix of fine-grained muscovite or biotite.
• Resembles porphyritic texture found in igneous rocks.

Poikiloblastic Texture

• Poikiloblastic texture includes fine-grained inclusions within porphyroblast grains.
• Inclusions can be randomly oriented or exhibit helictic, spiral, or rotated patterns.

Block 4: Reaction Textures in Metamorphic Petrology

Introduction to Reaction Textures

• When new minerals form through the replacement of older minerals or by reactions between phases, they develop in concentric rings around pre-existing grains, known as a reaction texture.

Types of Reaction Textures

1. Reaction-Rim Texture

• Occurs when a new mineral forms by replacing an older mineral along the rim, creating an irregular rim that signifies a reaction between the two phases.
• Example: Formation of a new mineral along the rim of an existing mineral like garnet between plagioclase and hornblende.

2. Kelyphitic Texture

• Involves the replacement of a mineral through the intergrowth of two or more minerals, resulting in the new minerals completely surrounding the replaced mineral.
• This texture is commonly found in retrogressive metamorphic rocks and is also known as a replacement texture.
• Example: Complete encircling of a replaced mineral by new minerals in a retrogressive metamorphic rock.

Corona Texture

• Develops in both prograde and retrograde phases of metamorphism.
• One or more minerals form a complete rim around the older phase in the center, resembling a corona.
• The number of rim layers can vary based on the number of reactions that have occurred.

Intergrowth Texture

• Symplectite texture exhibits a worm-like appearance of minerals along the boundaries of older reacting minerals.
• For example, symplectite intergrowth of orthopyroxene-garnet-cordierite.
• Myrmekitic texture, like vermicular or wormy quartz intergrowing with plagioclase, is common in high-temperature metamorphic rocks.

Metamorphic Petrology

• Recrystallization:
  • Process where crystals in a rock change their size and shape without melting
  • Occurs due to heat and pressure during metamorphism
  • Example: Limestone turning into marble
• Denser Polymorphs of Quartz:
  • Forms of quartz that have higher density than normal quartz
  • Include coesite and stishovite
• Pressure Solution:
  • Process where minerals dissolve at high pressure at grain boundaries and recrystallize in areas of lower pressure
  • Leads to the development of foliation in metamorphic rocks
  • Example: Formation of stylolites in limestone
• Palimpsest Texture:
  • Also known as relict texture
  • Occurs when the original texture of a rock is partially or wholly preserved during metamorphism
  • Example: Fossil shells preserved in a metamorphic rock
• Porphyroblastic Texture:
  • Texture characterized by large crystals (porphyroblasts) surrounded by a finer-grained matrix
  • Forms during metamorphism under specific pressure and temperature conditions
  • Example: Garnet porphyroblasts in a schist matrix
• Kelyphitic Texture:
  • Texture where minerals grow over pre-existing grains, forming a rim around them
  • Commonly observed in serpentinites and other rocks undergoing metasomatism
  • Example: Chlorite forming around pre-existing pyroxene grains

Unit 14: Structures in Metamorphic Rocks

• Introduction to Structures in Metamorphic Rocks

  In the previous section, we explored the various textures present in metamorphic rocks. Now, let's delve into the common structures often observed in metamorphic rocks.

• Foliation and Lineation

  Foliation is a characteristic texture seen in metamorphic rocks where mineral grains exhibit a distinct directional orientation. This texture is characterized by the alignment of platy minerals or alternating layers of light (felsic) and dark (mafic) minerals.

  • Definition of Foliation

    Foliation refers to the layering observed within metamorphic rocks, derived from the Latin word folia, meaning "leaves." It is represented by penetrative surfaces that are nearly or fully parallel, defining planar fabric elements.

  • Definition of Lineation

    Lineation, on the other hand, consists of penetrative sets of parallel or nearly parallel lines, forming a linear fabric element within metamorphic rocks.

  • Characteristics of Foliation and Lineation

    Foliation is primarily a planar feature, visible on all sides of rocks as lines, while lineations are observed as circular to irregular specks or dots on at least one surface of the rock.

  • Development of Features

    The development of planar and linear features occurs in a plane perpendicular to the maximum principal stress applied on parallelly arranged flaky minerals like mica and chlorite.

• Relationship Between Foliation, Lineation, and Cleavage

  Cleavage is the tendency of a rock to break along surfaces of a specific orientation. All cleavages are essentially foliations, with both terms often used interchangeably to describe the same structural feature.

  • Differentiation between Foliation and Cleavage

    Foliation is considered a broader term than cleavage as it encompasses planar geometric features that may not necessarily result in a cleavage.

• Morphological Classification of Foliations and Lineations

  Foliations and lineations can be morphologically classified as per the system proposed by Twiss and Moore in 2007.

  • Classification of Foliations

    Foliations are categorized based on certain criteria outlined in Table 14.1.

  • Classification of Lineations

    Similarly, lineations are classified according to specific characteristics as detailed in Table 14.2.

Metamorphic Petrology

Spaced and Continuous Foliation

• Differences exist in the preferred orientation of mineral grains in structure or composition.
• Foliations are categorized based on spacing: spaced foliations (10 µm or more) and continuous foliations (less than 10 µm).
• Spaced foliations are further classified as compositional, disjunctive, and crenulation foliations.
• Compositional foliations feature layers of different mineral compositions.
• Disjunctive foliations are characterized by thin cleavage domains or seams with concentrations of oxides and aligned platy minerals.
• Crenulation foliations are formed by harmonic wrinkles or chevron folds in pre-existing foliations.
• Common rock types showcasing foliation include phyllites, schists, and gneisses.
• Feldspars in granites commonly exhibit lineation and foliation.

Morphological Classification Scheme for Foliation

• Compositional Foliation
• Disjunctive Foliation and Cleavage
• Spaced Lineations
• Continuous Coarse or Mineralogical

Structural and Mineral Lineation

• Lineation may be structural or mineralogical.
• Structural lineation involves the selective orientation of linear structures in rocks.
• Mineral lineation results from the alignment of minerals in parallel.
• Mineral orientation can be associated with elongated or acicular mineral grains.
• Structural lineation can be discrete or constructed.
• Discrete lineations arise from the orientation of discrete objects like ooids and pebbles.
• Constructed lineations form from planar features such as intersection of foliations, crenulation hinge lines, and boudin lines.

Morphological Classification Scheme for Lineation

• Structural Lineation
• Crenulation
• Mineral Fine
• Discrete Constructed

Unit 14: Crenulation Lineation and Textures of Metamorphic Rocks

Crenulation Lineation

• Crenulation lineation is a structural feature found in metamorphic rocks.

Textures and Structures of Metamorphic Rocks

Slaty Cleavage

• Slaty cleavage is characterized by strong parallelism in the foliation of fine-grained clay minerals.
• Platy minerals like micas contribute to the development of a strong slaty cleavage.
• When fine-grained platy minerals align parallel to each other in a specific direction, perpendicular to the stress maximum, a plane parallel to bedding known as slaty cleavage is formed.
• Slaty cleavage is commonly observed in rocks such as slate and phyllite.
• During metamorphism, clay minerals transform into chlorite, while other platy minerals, like micas, may be present.
• Slaty cleavage typically forms under low-grade metamorphic conditions and diminishes as metamorphic grade increases.
• Compression during metamorphism causes clay flakes to reorient and grow perpendicular to the direction of compression, forming a foliation plane.
• The resulting cleavage allows the rock to split easily into thin sheets or layers, ideal for roofing shingles.

Development of Slaty Cleavage

• In sedimentary protoliths, the alignment of platy minerals is crucial for the development of slaty cleavage.

Metamorphic Petrology

• Schistose Structure/Texture
• 
  
  • Schistose structure, also known as schistosity, refers to a strongly foliated texture resulting from mineral growth.
  • Schist is characterized by a foliation or alignment of medium to coarse-grained minerals, particularly large mica flakes.
  • This texture is commonly found in mica schist, chlorite schist, and hornblende schist.
• Formation of Schistosity
• 
  
  • Schistosity develops through the parallel arrangement of micaceous or platy minerals under directed pressure during metamorphism in low to medium grade coarse-grained rocks.
  • Grain size increases, making the grains easily identifiable even without magnification.
  • The resulting structure forms a planar feature known as a foliation plane.
• Mineral Alignment
• 
  
  • In schistose structure, minerals like chlorite transform into minerals such as feldspar, mica, and quartz.
  • The newly formed minerals align themselves so that their longer axes are parallel to the directed maximum stress.
  • Minerals like kyanite, sillimanite, and staurolite are also part of the foliation plane.
• Characteristics of Schistosity
• 
  
  • Rocks exhibiting schistosity are termed schists and are relatively weak along the foliation plane, requiring some effort to cleave.
  • Schistose rocks can be observed in polished rock cuttings, demonstrating the schistosity or schistose structure.

Unit 14: Schistosity and Gneissose Structure

Schistosity

• Schistosity refers to the tendency of minerals in metamorphic rocks to align in layers or foliation planes due to intense pressure and heat.

Quartz

• Quartz is a common mineral found in metamorphic rocks, known for its resistance to chemical weathering and its hexagonal crystal structure.

Maximum Stress Direction

• Maximum stress direction influences the alignment of minerals in metamorphic rocks, creating distinct textures and structures.

Textures and Structures of Metamorphic Rocks

• Metamorphic rocks exhibit a variety of textures and structures based on the conditions of pressure and temperature during their formation.

Feldspar

• Feldspar is a common mineral group in metamorphic rocks, known for its hardness and variety of colors.

Preferred Orientation of Sheet Silicates

• Sheet silicates in metamorphic rocks tend to align parallel to the direction of pressure, leading to a preferred orientation within the rock.

Gneissose Structure/Texture

• Gneissose structure features alternating bands of light-colored felsic minerals like quartz and feldspar, and dark-colored mafic minerals like pyroxene and hornblende.
• This banding is a result of mineral segregation during high-grade metamorphism, creating distinctive striped patterns in gneiss rocks.
• The lighter bands consist of felsic minerals, while the darker bands contain mafic minerals, with differences not only in color but also in texture.
• Gneiss fabric is relatively weaker than schistosity and may undergo further changes, such as migmatisation, where melted parts separate from unmelted parts, forming distinct layers.

Metamorphic Rocks and Structures

Regional Metamorphism

• Occurs due to increasing intensity of metamorphism
• Characterized by changes in rock texture
• Metamorphic grades:
  • Low grade: Gneissic banding, increasing crystal size, coarseness of foliation
  • Intermediate grade: Schistosity, presence of schist and gneiss
  • High grade: Migmatite, slaty cleavage
• Metamorphic rocks:
  • Phyllite: Low grade, quartz-rich
  • Slate: Intermediate grade, dark-colored minerals
  • Schist: Intermediate grade, abundant micaceous minerals
  • Gneiss: High grade, fewer micaceous minerals

Cataclastic Structure

• Tectonically developed structure
• Occurs under intense shearing without high temperature
• Results from stress along fault zones
• Features:
  • Fragmentation of rocks due to intense shearing
  • Formation of crushed breccias
  • No crystallization of new minerals
  • Porphyroclastic or pseudoporphyritic structure

It's essential to understand these concepts to grasp the complexities of metamorphic rocks and structures.

Unit 14: Metamorphic Rocks

Development of Mylonitic Structure

• Reasonably raised temperatures lead to the development of mylonitic structure.

Processes in Cataclastic Metamorphism

• Micro-fracturing and granular flow contribute to cataclastic metamorphism.
• Frictional sliding plays a significant role in the process.

Textures and Structures of Metamorphic Rocks

• Granular flow-rock fabric is evident in pre-deformation stages.
• Cataclastic structure is observed in post-deformed rocks.
• Examples: Granular flow-rock fabric and Cataclastic structure.

Mylonite Texture

• A microstructure example of mylonite texture is shown in Fig. 14.26.

Pseudotachylite

• Pseudotachylite is a dark, fine-grained rock resulting from the devitrification of glass.
• Formation occurs due to frictional melting caused by seismic faulting, impacting, or landslides.
• It resembles basaltic glass (tachylite) in appearance.
• Examples: Seismic faulting, Impacting, Landslides.

Pseudotachylites in Metamorphic Petrology

• Definition: Pseudotachylites are rock formations created during seismic faulting events. They can manifest as injected vein types (found within walls) or fault vein types (along fault surfaces) and even as pseudotachylite breccia within tectonic breccias.
• Significance: Pseudotachylites serve as indicators of past seismic activity, particularly earthquakes in geological history.
• Characteristics:
  • Vein Injection Type: Refers to pseudotachylites that appear as injected veins within rock walls.
  • Fault Vein Type: Describes pseudotachylites occurring along fault surfaces.
  • Pseudotachylite Breccia: These are pseudotachylites found within the matrix of tectonic breccias.
• Palaeoseismic Indicators: These formations provide valuable insights into seismic events that have impacted the Earth in the past.

Now that you have grasped the structures present in metamorphic rocks, consider engaging in an exercise to evaluate your understanding before moving on to the next section.