Metamorphic Rocks Petrography and Indian Occurrences

Khondalite: Petrographic Notes and Indian Occurrence

Khondalite is a high-grade metamorphic rock belonging to the gneiss-schist group, composed mainly of a quartzo-feldspathic-garnet-sillimanite assemblage with graphite. The term was coined by T. L. Walker (1902) after the Khond tribe of Odisha.

Megascopic Characters

• Colour: Brown to greyish brown
• Grain size: Medium to coarse-grained
• Texture: Porphyroblastic (garnet as porphyroblasts)
• Structure: Schistose to gneissose, well-developed foliation

Microscopic (Petrographic) Characters

• Texture: Granoblastic to lepidoblastic
• Quartz: Colourless, anhedral, shows undulose extinction
• Feldspar (K-feldspar ± plagioclase): Subhedral, shows twinning
• Garnet: Euhedral to subhedral porphyroblasts, isotropic
• Sillimanite: Fibrolite or prismatic, high relief
• Biotite: Brown, pleochroic, aligned along foliation
• Accessory minerals: Graphite, ilmenite, rutile, zircon

Protolith and Metamorphism

• Protolith: Argillaceous sediments (shale/mudstone)
• Metamorphic grade: Upper amphibolite to granulite facies
• Type of metamorphism: Regional metamorphism
• Represents high-temperature deep crustal conditions

Indian Occurrence

• Occurs prominently in the Eastern Ghats Mobile Belt (EGMB):
  • Odisha
  • Andhra Pradesh
• Forms part of the Khondalite Group, associated with charnockite, leptynite, quartzite, and calc-granulite.
• Also occurs in the Southern Granulite Terrain (Kerala).

Schist: Petrographic Notes and Indian Occurrence

Schist is a medium to high-grade metamorphic rock characterised by a well-developed schistosity due to the parallel alignment of platy and elongated minerals. It is derived mainly from argillaceous sedimentary rocks such as shale.

Megascopic Characters

• Colour: Grey, green, brown, or black depending on mineral content
• Grain size: Medium to coarse-grained
• Texture: Schistose
• Structure: Strong foliation with visible flaky minerals; rock easily splits along schistosity planes

Microscopic (Petrographic) Characters

• Texture: Lepidoblastic
• Quartz: Colourless, anhedral, shows undulose extinction
• Mica (biotite/muscovite): Platy, strongly oriented, shows pleochroism
• Feldspar: Subhedral to anhedral, less abundant
• Garnet (in garnet schist): Euhedral porphyroblasts, isotropic
• Accessory minerals: Chlorite, tourmaline, zircon, iron oxides

Mineralogical Types of Schist

• Mica schist
• Garnet schist
• Chlorite schist
• Talc schist
• Hornblende schist

Protolith and Metamorphism

• Protolith: Argillaceous sediments (shale, mudstone)
• Metamorphic grade: Medium to high grade
• Type of metamorphism: Regional metamorphism
• Pressure-temperature conditions favour the development of schistosity

Indian Occurrence

• Himalayan region
• Aravalli Range (Rajasthan)
• Eastern Ghats Belt
• Dharwar Craton (Karnataka)
• Southern Granulite Terrain

Gneiss: Petrographic Notes and Indian Occurrence

Gneiss is a coarse-grained, high-grade metamorphic rock characterised by a distinct gneissose structure, produced by the segregation of light and dark minerals into bands. It is formed under high temperature and pressure conditions, generally by regional metamorphism.

Megascopic Characters

• Colour: Light grey to dark grey, sometimes pinkish
• Grain size: Coarse-grained
• Texture: Gneissose
• Structure: Alternating light (felsic) and dark (mafic) bands; massive and hard; does not split easily like schist

Microscopic (Petrographic) Characters

• Texture: Granoblastic to gneissose
• Quartz: Colourless, anhedral, shows undulose extinction
• Feldspar (orthoclase and plagioclase): Subhedral, shows twinning
• Biotite/Hornblende: Elongated, pleochroic, aligned in bands
• Garnet (in garnet gneiss): Euhedral porphyroblasts, isotropic
• Accessory minerals: Apatite, zircon, magnetite

Types of Gneiss

• Orthogneiss: Derived from igneous rocks (granite)
• Paragneiss: Derived from sedimentary rocks (shale, sandstone)
• Augen gneiss: Large feldspar “eye-shaped” crystals

Protolith and Metamorphism

• Protolith: Igneous or sedimentary rocks
• Metamorphic grade: High grade
• Type of metamorphism: Regional metamorphism
• Forms under intense deformation and recrystallisation

Indian Occurrence

• Aravalli Range (Rajasthan)
• Dharwar Craton (Karnataka)
• Bundelkhand Craton
• Eastern Ghats Belt
• Himalayan region

Charnockite: Petrographic Notes and Indian Occurrence

Charnockite is a coarse-grained, high-grade metamorphic rock belonging to the granulite facies, characterised by the presence of orthopyroxene (hypersthene) along with quartz and feldspar. The term was introduced by T. H. Holland (1900) from St. Thomas Mount, Chennai.

Megascopic Characters

• Colour: Dark greenish-grey to brown
• Grain size: Medium to coarse-grained
• Texture: Granoblastic
• Structure: Massive or weakly gneissose; hard and compact; fresh surfaces appear greenish due to pyroxene

Microscopic (Petrographic) Characters

• Texture: Granoblastic
• Quartz: Colourless, anhedral, shows undulose extinction
• Feldspar (orthoclase and plagioclase): Subhedral, shows twinning
• Orthopyroxene (hypersthene): Pale green to brown, weak pleochroism, high relief
• Biotite/Hornblende: Minor or retrogressive
• Accessory minerals: Garnet, magnetite, apatite, zircon

Protolith and Metamorphism

• Protolith: Igneous or sedimentary rocks
• Metamorphic grade: Granulite facies (high grade)
• Type of metamorphism: Regional metamorphism
• Formed under high temperature and moderate pressure, often associated with dehydration

Indian Occurrence

• Southern Granulite Terrain (Tamil Nadu, Kerala)
• Eastern Ghats Mobile Belt (Odisha, Andhra Pradesh)
• Nilgiri Hills
• Shevaroy and Palni Hills
• St. Thomas Mount, Chennai (type locality)

Geological Significance

• Marker of deep crustal, high-temperature metamorphism
• Important in understanding the Precambrian tectono-thermal evolution of India

Slate: Petrographic Notes and Indian Occurrence

Slate is a fine-grained, low-grade metamorphic rock formed by the metamorphism of argillaceous sedimentary rocks such as shale and mudstone. It is characterised by a well-developed slaty cleavage, which allows it to split into thin sheets.

Megascopic Characters

• Colour: Grey, black, green, purple
• Grain size: Very fine-grained
• Texture: Compact and dense
• Structure: Slaty cleavage, splits into thin plates; dull appearance; original bedding may be faintly preserved

Microscopic (Petrographic) Characters

• Texture: Lepidoblastic to cryptocrystalline
• Quartz: Very fine, anhedral
• Micas (sericite/muscovite): Fine flakes, oriented parallel to cleavage
• Chlorite: Green, fine-grained
• Feldspar: Minor, altered
• Accessory minerals: Pyrite, hematite, carbonaceous matter

Protolith and Metamorphism

• Protolith: Shale or mudstone
• Metamorphic grade: Low grade
• Type of metamorphism: Regional metamorphism
• Directed pressure produces slaty cleavage perpendicular to maximum stress

Indian Occurrence

• Himalayan region
• Aravalli Range (Rajasthan)
• Vindhyan Belt
• Chhattisgarh Basin
• Kumaon and Garhwal regions

Uses (Scoring Add-on)

• Roofing tiles and flooring stones
• Writing slates and blackboards
• Acid-resistant laboratory slabs

Marble: Petrographic Notes and Indian Occurrence

Marble is a crystalline, non-foliated metamorphic rock formed by the recrystallisation of limestone or dolomite under regional or contact metamorphism. It is composed predominantly of calcite or dolomite.

Megascopic Characters

• Colour: White, grey, pink, green, or variegated
• Grain size: Fine to coarse crystalline
• Texture: Granoblastic
• Structure: Massive, non-foliated; effervesces with dilute HCl

Microscopic (Petrographic) Characters

• Texture: Granoblastic mosaic
• Calcite: Colourless, high birefringence, shows polysynthetic twinning
• Dolomite (in dolomitic marble): Rhombohedral cleavage, lower birefringence
• Quartz: Minor, anhedral grains
• Accessory minerals: Graphite, mica, tremolite, diopside, wollastonite
• Interlocking grains with straight to curved grain boundaries

Protolith and Metamorphism

• Protolith: Limestone or dolostone
• Metamorphic grade: Low to high grade
• Type of metamorphism: Contact and regional
• Recrystallisation increases grain size and purity

Types of Marble

• Calcitic marble
• Dolomitic marble
• Impure marble (with silicates)

Indian Occurrence

• Makrana (Rajasthan): Famous white marble
• Rajsamand, Udaipur, Alwar (Rajasthan)
• Salem (Tamil Nadu)
• Katni (Madhya Pradesh)
• Himachal Pradesh

Uses

• Building and decorative stone
• Sculptures and monuments (e.g., Taj Mahal marble from Makrana)
• Raw material for lime and cement

Eclogite: Petrographic Notes and Indian Occurrence

Eclogite is a high-pressure, high-grade metamorphic rock characterised by the absence of plagioclase and the presence of garnet and omphacitic clinopyroxene. It represents metamorphism under deep crustal to upper mantle conditions.

Megascopic Characters

• Colour: Dark green to reddish-brown
• Grain size: Medium to coarse-grained
• Texture: Granoblastic
• Structure: Generally massive, non-foliated; garnet visible as red porphyroblasts in a green matrix

Microscopic (Petrographic) Characters

• Texture: Granoblastic to porphyroblastic
• Garnet: Euhedral to subhedral, isotropic, often fractured
• Omphacite (Na-Ca clinopyroxene): High relief, inclined extinction
• Absence of plagioclase: Diagnostic feature
• Quartz/coesite (rare): Indicates ultra-high pressure conditions
• Accessory minerals: Rutile, kyanite, amphibole

Protolith and Metamorphism

• Protolith: Basic igneous rocks (basalt, gabbro)
• Metamorphic grade: Eclogite facies
• Pressure-temperature conditions: High pressure (≥12 kbar) and moderate to high temperature
• Type of metamorphism: Regional / subduction-related metamorphism

Geological Significance

• Indicates deep subduction of oceanic or continental crust
• Important marker of plate tectonic processes
• Retrogression may form amphibolite during exhumation

Indian Occurrence

• Dharwar Craton (Karnataka)
• Eastern Ghats Belt
• Himalayan region
• Bastar Craton

Granulite: Petrographic Notes and Indian Occurrence

Granulite is a high-grade metamorphic rock formed under granulite facies conditions, characterised by a granoblastic texture and an anhydrous mineral assemblage. It represents metamorphism at high temperature and moderate to high pressure.

Megascopic Characters

• Colour: Grey, brown, pink, or dark green
• Grain size: Medium to coarse-grained
• Texture: Granoblastic
• Structure: Massive or weakly foliated; rock is hard, dense, and compact

Microscopic (Petrographic) Characters

• Texture: Granoblastic with polygonal grains
• Quartz: Colourless, anhedral, shows undulose extinction
• Feldspar (orthoclase & plagioclase): Subhedral, common twinning
• Orthopyroxene / clinopyroxene: High relief, characteristic of granulites
• Garnet: Euhedral to subhedral, isotropic
• Biotite: Scarce or absent (due to high-temperature dehydration)
• Accessory minerals: Ilmenite, magnetite, zircon, rutile

Mineral Assemblage

• Quartz + feldspar + pyroxene ± garnet
• Absence of hydrous minerals indicates high-temperature conditions

Protolith and Metamorphism

• Protolith: Igneous and sedimentary rocks
• Metamorphic grade: Very high grade
• Metamorphic facies: Granulite facies
• Type of metamorphism: Regional metamorphism
• Temperature conditions: ~700–900°C

Indian Occurrence

• Southern Granulite Terrain (Tamil Nadu, Kerala)
• Eastern Ghats Mobile Belt
• Aravalli Belt
• Himalayan crystalline complex
• Associated with charnockite and khondalite

Phyllite: Petrographic Notes and Indian Occurrence

Phyllite is a low to medium-grade metamorphic rock, representing an intermediate stage between slate and schist. It is characterised by a well-developed phyllitic foliation and a silky sheen due to fine-grained mica minerals.

Megascopic Characters

• Colour: Grey, greenish grey, black, or purplish
• Grain size: Fine-grained
• Texture: Phyllitic
• Structure: Foliated, with wavy cleavage surfaces; shows satiny or silky lustre

Microscopic (Petrographic) Characters

• Texture: Lepidoblastic
• Quartz: Fine-grained, anhedral
• Mica (sericite / muscovite ± biotite): Fine flaky minerals, strongly oriented
• Chlorite: Common, green pleochroic
• Feldspar: Minor and altered
• Accessory minerals: Graphite, hematite, zircon

Mineral Assemblage

• Quartz + mica (sericite/muscovite) + chlorite
• Fine grain size due to relatively lower metamorphic grade

Protolith and Metamorphism

• Protolith: Argillaceous sedimentary rocks (shale, mudstone)
• Metamorphic grade: Low to lower-medium grade
• Type of metamorphism: Regional metamorphism
• Transitional rock sequence: slate → phyllite → schist

Indian Occurrence

• Himalayan region
• Aravalli Range (Rajasthan)
• Dharwar Craton
• Eastern Ghats Belt
• Satpura Range

Quartzite: Petrographic Notes and Indian Occurrence

Quartzite is a hard, compact, non-foliated metamorphic rock formed by the metamorphism of quartz-rich sandstone. It is composed almost entirely of recrystallised quartz and commonly formed under regional or contact metamorphic conditions.

Megascopic Characters

• Colour: White, grey, pink, or reddish
• Grain size: Medium to coarse-grained
• Texture: Granoblastic
• Structure: Massive, non-foliated; very hard and compact; breaks across grains

Microscopic (Petrographic) Characters

• Texture: Granoblastic with interlocking polygonal grains
• Quartz: Colourless, anhedral, shows undulose extinction
• Sutured grain boundaries common due to recrystallisation
• Original clastic texture completely obliterated
• Accessory minerals: Feldspar, mica, zircon, iron oxides (minor)

Mineral Assemblage

• Almost entirely quartz
• Minor impurities reflect original sandstone composition

Protolith and Metamorphism

• Protolith: Quartz sandstone
• Metamorphic grade: Low to high grade
• Type of metamorphism: Regional or contact metamorphism
• Recrystallisation produces interlocking quartz mosaic

Indian Occurrence

• Aravalli Range (Rajasthan)
• Himalayan region
• Dharwar Craton
• Eastern Ghats Belt
• Vindhyan Supergroup

Relation between Metamorphism and Deformation

Definitions

• Metamorphism: The process by which pre-existing rocks undergo mineralogical, chemical, and textural changes in the solid state due to changes in temperature, pressure, and chemically active fluids, without melting.
• Deformation: The change in shape, size, volume, or internal arrangement of minerals in rocks caused by the application of stress during tectonic processes.

Key Relationships

• Common Geological Environment: Metamorphism and deformation commonly occur together during tectonic activities such as plate convergence, crustal thickening, and mountain building.
• Role of Stress: Stress responsible for deformation also acts as an important metamorphic agent, especially directed stress, influencing mineral growth and rock fabric.
• Formation of Foliation and Lineation: During deformation under metamorphic conditions, platy and elongate minerals align perpendicular to maximum stress, producing foliation, schistosity, and mineral lineation.
• Regional Metamorphism: Characterised by the simultaneous action of elevated temperature, pressure, and deformation, leading to large-scale metamorphic belts.
• Dynamic Metamorphism: Intense deformation along faults and shear zones causes dynamic metamorphism, forming rocks such as mylonite, cataclasite, and fault breccia.
• Syn-metamorphic Deformation: When deformation occurs at the same time as metamorphism, syn-tectonic mineral growth occurs, producing porphyroblasts with inclusion trails that record deformation history.
• Recrystallization: Deformation enhances recrystallization by increasing lattice defects, allowing minerals to grow and stabilize under new pressure-temperature conditions.
• Metamorphic Grade and Deformation Style: Metamorphic grade controls rock behavior; low-grade rocks deform in a brittle manner, while high-grade rocks deform ductilely, producing folds and flow structures.

Metamorphic Mineral Reactions

Metamorphic mineral reactions are chemical reactions that occur between minerals in a rock during metamorphism, leading to the breakdown of unstable minerals and the formation of new, stable mineral assemblages under changing temperature (T), pressure (P), and fluid conditions, without melting.

Prograde Metamorphic Reactions

Prograde metamorphism refers to metamorphism that occurs during increasing temperature and pressure as rocks are buried deeper into the Earth’s crust. Prograde reactions are mainly dehydration and decarbonation reactions, involving the consumption of hydrous and carbonate minerals and the release of fluids.

• Examples:
  • Chlorite + Quartz → Garnet + H₂O
  • Muscovite + Quartz → K-feldspar + Sillimanite + H₂O
  • Calcite + Quartz → Wollastonite + CO₂
• Characteristics: Occur at increasing T–P conditions, produce higher-grade minerals, release H₂O or CO₂, and usually proceed nearly to completion.

Retrograde Metamorphic Reactions

Retrograde metamorphism occurs during decreasing temperature and pressure as rocks are uplifted and exhumed toward the Earth’s surface. Retrograde reactions involve hydration and carbonation, where high-grade minerals react with infiltrating fluids to form low-grade mineral assemblages.

• Examples:
  • Garnet + H₂O → Chlorite
  • Pyroxene + H₂O → Amphibole
  • Feldspar + H₂O → Sericite
• Characteristics: Occur at falling T–P conditions, require the presence of fluids, are often incomplete due to limited fluid access, and are commonly seen as replacement textures.

Significance

• Control the mineral assemblage and metamorphic grade of rocks.
• Help define metamorphic zones and isograds.
• Record the pressure–temperature history (P–T path) of rocks.
• Influence rock strength and deformation behavior.

Migmatites and Their Origin

Migmatite is a composite rock exhibiting characteristics of both metamorphic and igneous rocks, formed due to partial melting (anatexis) of high-grade metamorphic rocks under deep crustal conditions. The term was introduced by Sederholm (1907) and means “mixed rock”.

Megascopic Characters

• Distinct banded appearance with alternating light and dark layers
• Mixed igneous–metamorphic texture
• Structures may be streaky, banded, folded, or veined
• Intimate association of felsic and mafic layers

Components of Migmatite

• Leucosome: Light-coloured, quartz–feldspar rich; represents the felsic melt fraction.
• Melanosome: Dark-coloured, rich in biotite/hornblende; represents the residual solid (restite).
• Paleosome: Original host metamorphic rock (gneiss/schist).

Microscopic (Petrographic) Characters

• Texture: Granoblastic to gneissic
• Quartz and feldspar show recrystallised, interlocking grains
• Biotite / hornblende define foliation
• Garnet may occur as porphyroblasts
• Locally preserved igneous textures in leucosome

Types of Migmatites

• Stromatic migmatite: Regular alternation of leucosome and melanosome bands.
• Agmatitic migmatite: Angular fragments of country rock in a granitic matrix.
• Schollen migmatite: Large blocks of unmelted rock enclosed in melt.
• Nebulitic migmatite: Diffuse, irregular banding due to intense melting.

Origin and Conditions of Formation

• Origin: Migmatites originate mainly by partial melting (anatexis) of high-grade metamorphic rocks. Hydrous minerals undergo dehydration melting, felsic melt segregates to form leucosome, and residual solid forms melanosome.
• Temperature: ~650–850 °C
• Pressure: Moderate to high
• Facies: Upper amphibolite to granulite facies
• Type of metamorphism: Regional metamorphism

Indian Occurrence

• Eastern Ghats Mobile Belt
• Southern Granulite Terrain (Tamil Nadu, Kerala)
• Dharwar Craton
• Aravalli Range
• Himalayan crystalline complex

Metasomatism and the Role of Fluids

Metasomatism is the process of chemical alteration of a rock involving the addition and/or removal of chemical components through the action of chemically active fluids during metamorphism, resulting in a change in the bulk chemical composition of the rock. It represents open-system metamorphism.

Metamorphic Fluids

Metamorphic fluids are mobile phases composed mainly of H₂O, CO₂, and dissolved ions. They originate from dehydration/decarbonation reactions, magmatic fluids, or connate/meteoric waters.

Role of Fluids in Metamorphism

1. Facilitation of Reactions: Fluids act as reaction media, increasing ion mobility and reaction rates.
2. Control of Mineral Assemblages: Fluid composition (H₂O-rich vs CO₂-rich) controls mineral stability.
3. Mass Transfer: Fluids transport ions (K⁺, Na⁺, Ca²⁺, etc.), leading to metasomatism.
4. Lowering of Melting Temperature: Presence of fluids lowers the solidus, promoting anatexis.
5. Development of Textures: Enhance grain growth, pressure solution, and foliation.

Types and Products of Metasomatism

• Contact Metasomatism: Near igneous intrusions.
• Hydrothermal Metasomatism: Alteration by hot aqueous fluids.
• Regional Metasomatism: Large-scale fluid flow during orogeny.
• Products: Skarn (Ca-Mg), Greisen (K-F rich), Serpentinite (hydration), and Fenite (alkali).

Classification of Metamorphic Rocks

Metamorphic rocks are classified based on texture, metamorphic grade, metamorphic environment, and parent rock (protolith).

I. Classification Based on Texture

• Foliated: Show planar structures. Sequence: Slate → Phyllite → Schist → Gneiss.
• Non-Foliated: Lack planar structures. Examples: Marble, Quartzite, Hornfels.

II. Classification Based on Metamorphic Grade

• Low-Grade: Slate, Phyllite.
• Medium-Grade: Schist.
• High-Grade: Gneiss, Migmatite, Granulite.

III. Classification Based on Metamorphic Environment

• Contact: Near intrusions (e.g., Hornfels).
• Regional: Large areas (e.g., Schist, Gneiss).
• Dynamic: Fault zones (e.g., Mylonite, Cataclasite).

IV. Classification Based on Parent Rock (Protolith)

• Orthometamorphic: Derived from igneous rocks (e.g., Orthogneiss).
• Parametamorphic: Derived from sedimentary rocks (e.g., Paragneiss).

Metamorphic Differentiation

Metamorphic differentiation is the process by which a chemically and mineralogically uniform parent rock becomes segregated into compositionally distinct layers or bands during metamorphism without complete melting. It is essentially a solid-state process.

Mechanisms of Differentiation

• Diffusion and Ionic Migration: Migration through crystal lattices and grain boundaries.
• Pressure Solution: Dissolution at high-stress sites and re-precipitation in low-stress zones.
• Recrystallization: Stable minerals grow at the expense of unstable ones.
• Fluid-Assisted Mass Transfer: Fluids enhance ion mobility for large-scale differentiation.

Types and Examples

• Chemical Differentiation: Redistribution of components like Si, Al, Fe, Mg.
• Mineralogical Differentiation: Segregation into felsic (leucocratic) and mafic (melanocratic) bands.
• Examples: Gneissic banding, schistosity, and stromatic banding in migmatites.