Igneous Rocks: Formation, Textures, Magmatism and Occurrences
Mode of Occurrence of Igneous Rocks
Igneous rocks are formed by the cooling and solidification of molten magma either beneath the Earth’s surface or on it. Their mode of occurrence is closely related to the depth of crystallization, the nature of magma intrusion, and the environment in which the magma solidifies. Broadly, igneous rocks occur in two main modes: intrusive (plutonic) and extrusive (volcanic).
Intrusive (Plutonic) Igneous Rocks
These rocks form when magma cools slowly beneath the Earth’s surface, allowing large crystals to grow. They are coarse-grained (phaneritic) in texture due to slow cooling.
Examples include granite, gabbro, and diorite.
They occur as plutons, which are large irregular bodies, or in smaller forms such as dikes, sills, laccoliths, and batholiths.
- Batholiths: Massive irregular intrusions extending over hundreds of square kilometers.
- Laccoliths: Lens-shaped intrusions that push overlying rocks upward.
- Dikes: Tabular intrusions cutting across bedding planes.
- Sills: Tabular intrusions parallel to bedding planes.
Extrusive (Volcanic) Igneous Rocks
These rocks form when magma erupts onto the Earth’s surface as lava or volcanic ash and solidifies rapidly. They are fine-grained (aphanitic) or glassy in texture due to quick cooling.
Examples include basalt, rhyolite, and andesite. Extrusive rocks are commonly associated with volcanic cones, lava flows, pyroclastic sheets, and volcanic plugs.
Other Modes
- Hypabyssal rocks: Intermediate rocks that crystallize at shallow depths, showing characteristics of both intrusive and extrusive rocks. Examples include dolerite and microgranite.
- Pegmatites: Exceptionally coarse-grained intrusive rocks forming in late-stage crystallization in magma chambers.
- Pyroclastic rocks: Formed from explosive volcanic activity; consist of fragmented material like tuff and ignimbrite.
Texture of Igneous Rocks
The texture of an igneous rock refers to the size, shape, arrangement, and mutual relationship of the mineral grains or crystals within it. It is a key feature that helps in understanding the cooling history, crystallization process, and environment of formation of the rock. Texture can also provide clues about magma composition and the rate of solidification.
Igneous textures are mainly controlled by cooling rate, composition of magma, and crystallization sequence. Based on these factors, igneous textures are broadly classified into the following types.
Aphanitic (Fine-grained) Texture
Crystals are too small to be seen with the naked eye. Forms due to rapid cooling of lava at or near the Earth’s surface. Example: basalt, andesite.
Phaneritic (Coarse-grained) Texture
Crystals are large enough to be identified with the naked eye. Forms due to slow cooling of magma deep within the crust. Example: granite, gabbro.
Porphyritic Texture
Consists of large crystals (phenocrysts) embedded in a fine-grained groundmass. Indicates two-stage cooling: first slow (phenocrysts) then rapid (groundmass). Example: porphyritic andesite, granite porphyry.
Glassy Texture
No visible crystals; the rock appears glassy due to extremely rapid cooling. Example: obsidian.
Vesicular Texture
Contains small cavities (vesicles) formed by trapped gas bubbles. Common in volcanic rocks. Example: scoria, pumice.
Pyroclastic or Fragmental Texture
Composed of broken fragments of minerals, glass, and rock debris. Formed from volcanic eruptions. Example: tuff.
Pegmatitic Texture
Very coarse-grained, often with crystals several centimeters to meters in size. Forms from residual magmas rich in volatiles. Example: pegmatite.
Magmatism Along Plate Margins
Magmatism along plate margins is directly related to the movements and interactions of tectonic plates. Based on plate tectonics, magmatism is most prominent at divergent (constructive) and convergent (destructive) margins, with minor activity at transform boundaries.
Divergent Plate Margins
Mechanism: Plates move apart, causing decompression melting in the underlying asthenosphere.
Magma Type: Typically mafic (basaltic) in composition due to partial melting of peridotite.
Characteristics: Forms mid-ocean ridges and rift valleys; produces oceanic crust at spreading centers. Magmas are generally low in viscosity, leading to gentle eruptions and pillow lava formation.
Example: Mid-Atlantic Ridge; East African Rift.
Convergent Plate Margins (Subduction Zones)
Mechanism: One plate subducts beneath another, leading to flux melting due to the addition of water and volatiles into the overlying mantle wedge.
Magma Type: Mainly andesitic to rhyolitic, often forming calc-alkaline volcanic rocks.
Characteristics: Associated with volcanic arcs, trenches, and deep earthquakes. Magmas are more viscous, producing explosive volcanism. May form composite volcanoes and generate plutonic rocks at depth.
Example: Oceanic-continental convergence: Andes (South America); oceanic-oceanic convergence: Mariana Islands.
Transform Plate Margins
Magmatism is rare because there is little vertical movement of mantle material. Any minor magmatism is usually related to localized extension or nearby hot spots.
Importance of Plate Margin Magmatism
Formation of new crust and continental growth. Generates a wide variety of igneous rocks depending on tectonic setting. Controls volcanic hazards and geothermal activity.
Petrography of Granite
Granite is a coarse-grained, felsic, intrusive igneous rock composed mainly of quartz, feldspar, and mica, with occasional accessory minerals like amphiboles, magnetite, and zircon.
Texture: Granite shows a phaneritic texture, meaning all its minerals are visible to the naked eye. It often exhibits a granular to inequigranular texture, sometimes porphyritic if larger crystals of feldspar or quartz are present. The crystals are usually interlocked, giving it a compact and hard nature.
Mineralogical Composition
- Quartz: Colorless or smoky, 20–60%, contributes to hardness and transparency.
- Feldspars: Potash feldspar (orthoclase, microcline): pink, salmon, or reddish, 20–50%. Plagioclase: white to grey, sometimes shows twinning.
- Mica: Biotite (black) or muscovite (silver-white), 5–15%.
- Accessory minerals: Hornblende, magnetite, zircon, sphene, apatite.
Classification and Structure
Based on feldspar composition, granite is divided into syenogranite, monzogranite, and alkali granite. Granites may also be petrographically classified as megacrystic, porphyritic, or equigranular types. Granite may show jointing due to cooling and contraction and sometimes contains pegmatitic patches where very large crystals occur.
Indian Occurrence of Granite
Granites are widespread in India and are mainly associated with Archaean and Proterozoic shields.
Peninsular India: Archaean granites are found in the Peninsular gneissic complex (PGC), Karnataka, Tamil Nadu, and Andhra Pradesh. Examples include charnockites of Karnataka, Nilgiri, and Salem.
Youngers granites (Proterozoic) occur in eastern and central India: Rajasthan (Ajmer, Udaipur), Madhya Pradesh, Odisha. Specific Indian localities: Rajasthan (Kota, Bundi – pink and red granites); Madhya Pradesh (Mandla, Jabalpur – grey granites); Andhra Pradesh & Telangana (Hyderabad, Anantapur – coarse-grained granites); Karnataka (Chitradurga, Tumkur – large exposures).
Economic Importance: Used as dimension stone, flooring, decorative purposes, and monuments.
Magma Generation in the Mantle and Its Evolution
Magmas are generated in the Earth’s mantle due to partial melting of rocks under specific physical and chemical conditions. Mantle-derived magmas are the primary source of igneous rocks and play a vital role in plate tectonics and crustal evolution.
Mechanisms of Magma Generation
- Decompression melting: Occurs when hot mantle material rises (e.g., at mid-ocean ridges or mantle plumes). As pressure decreases during upward movement, the solidus temperature is reached and partial melting occurs. Produces basaltic magmas typical of oceanic crust formation.
- Flux melting (addition of volatiles): Happens at subduction zones where water and CO2 are released from the subducting slab. Volatiles lower the melting point of mantle rocks, causing partial melting and leading to andesitic to dacitic magmas.
- Heat transfer melting: Mantle rocks melt when intruded by hotter magmas or due to mantle upwelling. Less common than decompression or flux melting.
- Partial melting: Only a fraction of the mantle rock melts, usually producing silica-poor (mafic) magma first. Residual solid becomes more refractory, rich in olivine and pyroxene.
Evolution of Mantle-Derived Magmas
- Fractional crystallization: Early-formed crystals settle out, changing magma composition and potentially transforming basaltic magma into more evolved types like andesite or rhyolite.
- Assimilation: Magma may melt surrounding crustal rocks and incorporate them, altering its composition.
- Magma mixing: Interaction of different magmas can modify chemistry and produce hybrid compositions.
- Differentiation: Combined processes of crystallization, assimilation, and mixing lead to chemical evolution of magma, responsible for the diversity of igneous rocks.
Structures Found in Igneous Rocks
Igneous rocks exhibit various structures that provide insights into their mode of formation, cooling history, and environment of crystallization. These structures can be broadly divided into primary (formed during solidification) and secondary (formed after solidification due to later processes).
Flow Structures
Planar, wavy, or curvy alignment of minerals or vesicles due to movement of magma before complete solidification. Example: observed in rhyolite, basalt, or granite. Minerals like feldspar and biotite align along flow direction; vesicles may elongate along the flow.
Vesicular and Amygdaloidal Structures
Cavities or holes (vesicles) formed due to trapped gas bubbles in magma. Amygdaloidal: vesicles later filled with secondary minerals like quartz, calcite, or zeolites. Example: basalt.
Porphyritic Structure
Coarse crystals (phenocrysts) embedded in a fine-grained groundmass. Formed by two-stage cooling: slow cooling at depth (phenocrysts) followed by rapid cooling at surface (groundmass). Example: andesite, rhyolite.
Graphic or Intergrowth Structure
Intergrowth of quartz and feldspar resembling written letters or cuneiform script. Example: granites. Indicates simultaneous crystallization of quartz and feldspar from magma.
Pegmatitic Structure
Very coarse-grained igneous texture where crystals are often several centimeters in size. Forms by slow cooling in water-rich residual magma pockets. Example: pegmatites.
Ophitic and Subophitic Structures
Ophitic: Plagioclase laths enclosed by pyroxene crystals (typical in diabase). Subophitic: Pyroxene partially encloses plagioclase. These indicate crystallization sequence of minerals.
Glassy or Vitreous Structure
No crystalline structure; rock is amorphous due to rapid quenching of lava. Example: obsidian, pumice.
Layering
Amygdaloidal, tuffaceous, and vesicular layering: repetitive bands in volcanic rocks caused by flow or sedimentation of fragments during eruptions. Example: basaltic lava flows.
Different Forms of Igneous Rocks
Igneous rocks are formed from the solidification of molten magma or lava and are classified based on their mode of occurrence, texture, and mineral composition. One key classification is based on their forms, which reflect how magma reaches the surface or crystallizes within the Earth’s crust. The main forms are:
1. Plutonic (Intrusive) Rocks
Definition: Rocks that solidify slowly beneath the Earth’s surface.
Characteristics: Coarse-grained (phaneritic) texture due to slow cooling; large, visible crystals. Examples: granite, diorite, gabbro.
Occurrence: Found in large bodies such as batholiths, stocks, laccoliths, sills, and dikes.
2. Volcanic (Extrusive) Rocks
Definition: Rocks formed from lava that solidifies on the Earth’s surface.
Characteristics: Fine-grained (aphanitic) or glassy texture due to rapid cooling; may contain vesicles from trapped gases. Examples: basalt, andesite, rhyolite, obsidian.
Occurrence: Found in lava flows, volcanic cones, ash sheets, and pyroclastic deposits.
3. Hypabyssal (Subvolcanic) Rocks
Definition: Crystallize at shallow depths between plutonic and volcanic conditions. Characteristics: medium to fine-grained texture; often porphyritic. Examples: dolerite (diabase), microgranite. Occurrence: dikes, sills, and laccolithic intrusions.
4. Pyroclastic Rocks (Fragmental)
Definition: Formed from explosive volcanic eruptions that eject fragments of lava, ash, and other materials. Characteristics: fragmental or clastic texture; includes tuffs, volcanic breccias, and agglomerates. Examples: tuff, ignimbrite, volcanic breccia. Occurrence: around volcanic cones or as deposits from explosive eruptions.
Assimilation in Magma
Assimilation is a process in igneous petrology where magma incorporates and melts the surrounding country rock (wall rock or host rock) as it rises through the crust. This process changes the magma’s composition and may result in hybrid rocks with characteristics of both the magma and the assimilated rock. Assimilation plays a crucial role in the evolution of magmas, especially in the formation of differentiated plutons and hybrid igneous rocks.
Types of Assimilation
- Partial Assimilation (Incomplete): Only a part of the country rock melts and mixes with the magma. The degree of assimilation depends on temperature, magma composition, and the melting point of the wall rock. Example: granitic magma assimilating small amounts of surrounding metamorphic rocks like schist.
- Complete Assimilation: The entire portion of the country rock in contact with magma melts and is incorporated into the magma, producing a composition very different from the original. Example: basaltic magma completely melting limestone or sandstone, forming hybrid magmas enriched in silica or calcium.
- Chill Zone or Marginal Assimilation: Occurs at the edges of magma bodies where magma contacts cooler country rock, forming a fine-grained or porphyritic texture near the contact. Example: a granite pluton showing a fine-grained border due to assimilation of surrounding metamorphic rocks.
- Mingling and Mixing: Sometimes assimilation is accompanied by magma mingling, where two magmas of different composition interact without completely mixing, producing partial assimilation. Example: andesitic magma mingling with rhyolitic magma in volcanic arcs, producing hybrid lavas with mixed textures.
Significance of Assimilation
Assimilation changes the chemical composition of magma (increase in SiO2, Al2O3, or CaO depending on assimilated rock), produces hybrid igneous rocks, and influences the crystallization sequence, mineral composition, and texture of the resulting rock.
Kimberlite: Petrography and Indian Occurrence
Kimberlite is a mafic-ultramafic, volatile-rich igneous rock that originates deep within the mantle, often at depths greater than 150 km. It is the primary source of diamonds and is typically found as pipes, dykes, or sills. Kimberlites are important both economically and petrologically due to their mantle-derived minerals and diamond content.
Petrography of Kimberlite
Texture: Kimberlites generally exhibit a phanerocrystalline to porphyritic texture. They may contain macrocrysts (large crystals) embedded in a fine-grained or aphanitic matrix. Vesicular or brecciated textures are common due to volatile-rich magma.
Mineralogy: Primary minerals include olivine (usually forsteritic, sometimes altered to serpentine), pyrope garnet (indicative of deep mantle origin), chromian diopside, phlogopite mica (often large and sheared), spinel, and ilmenite. The groundmass commonly contains serpentinized olivine, phlogopite, carbonates, and clay minerals. Accessory minerals include diamond (rare), chromite, ilmenite, perovskite, and apatite.
Chemical characteristics: Rich in magnesium, potassium, and volatile components (CO2, H2O). Alkaline in nature, often potassic or ultrapotassic. Kimberlites are broadly divided into Group I (common) and Group II (orangeite) based on petrography and geochemistry.
Indian Occurrence of Kimberlite
Kimberlites are found mainly in the Precambrian cratonic regions of India. Major kimberlite provinces include:
- Jhabua–Dhar District (Madhya Pradesh): kimberlite pipes near Jhabua and Dhar.
- Singhbhum Craton (Jharkhand): minor kimberlite occurrences.
- Bundelkhand Craton (Madhya Pradesh & Uttar Pradesh border): several kimberlite dykes reported.
- Rajasthan: kimberlite dykes near Sawai Madhopur.
Economic significance: India has several kimberlite pipes, but diamondiferous kimberlites are rare. The Panna diamond field (Madhya Pradesh) is associated with kimberlite and alluvial deposits.
Pegmatite: Petrography and Indian Occurrence
Definition: Pegmatites are extremely coarse-grained igneous rocks, usually granitic in composition, with crystals typically larger than 2.5 cm and often several meters in size. They are the last fractions of magma to crystallize and are enriched in volatile components like water, boron, lithium, fluorine, and rare elements.
Petrography of Pegmatite
Texture: Pegmatites exhibit pegmatitic texture, characterized by very large interlocking crystals, sometimes exceeding several meters. The texture is holocrystalline, meaning entirely crystalline with no glassy material.
Mineralogy: Typically granitoid in composition, containing large quartz crystals, feldspar (orthoclase or microcline, sometimes albite), mica (muscovite and biotite), and accessory minerals such as tourmaline, beryl, topaz, fluorite, and rare-metal minerals (e.g., lithium, tantalum, niobium).
Zoning: Many pegmatites show mineralogical zoning: border zone (fine-grained quartz, feldspar, mica), intermediate zone (coarser feldspar and quartz), and core zone (large crystals of quartz, feldspar, and rare minerals).
Origin: Pegmatites represent the late-stage fractionation of granitic magma. High volatile content promotes rapid growth of large crystals.
Classification
Simple pegmatites: mostly quartz, feldspar, mica. Complex pegmatites: contain rare-element minerals like beryl, spodumene, lepidolite.
Indian Occurrences of Pegmatite
- Aravalli region (Rajasthan): rich in mica and feldspar pegmatites.
- Bihar (Gaya, Jamui): lithium, spodumene-bearing pegmatites.
- Andhra Pradesh (Nellore, Chittoor): feldspar pegmatites.
- Madhya Pradesh (Chhatarpur, Katni): quartz and feldspar pegmatites.
- Kerala and Tamil Nadu: rare-metal pegmatites with beryl, garnet.
- Mica belt of Jharkhand and Bihar: large muscovite and biotite pegmatites.
Economic importance: Source of feldspar, quartz, mica, beryl, spodumene, and other rare metals.
Magmatism Along Mid-Ocean Ridges (MORB)
Mid-Ocean Ridges (MOR) are linear underwater mountain ranges where two tectonic plates diverge. Magmatism along MORBs (mid-ocean ridge basalts) results from partial melting of the upper mantle caused by decompression as mantle material rises to fill the gap created by diverging plates. MORB magmatism is tectonically controlled, highly extensive, and produces basaltic lavas that form new oceanic crust.
Nature and Source
Tectonic setting: divergent plate boundaries where asthenospheric mantle upwells, drops in pressure, and partially melts to produce basaltic magma.
Source and composition: upper mantle peridotite. Partial melting produces tholeiitic basaltic magma, which is low in potassium and volatile elements. MORB magmas are generally homogeneous, though some chemical variations exist along the ridge (e.g., N-MORB, E-MORB, T-MORB).
Magma Generation and Eruption
Magma generation: decompression melting is the main process. Melt fraction is usually low (~10–20%), but continuous supply along the ridge sustains magmatism.
Mode of eruption: magma erupts effusively at the ridge axis to form pillow lavas; some magma crystallizes below the surface, forming gabbroic layers in the lower oceanic crust.
Crustal Structure and Geochemistry
The oceanic crust formed at MORB has a typical three-layer structure: Layer 1 — sediment cover; Layer 2 — basaltic lavas (pillow basalts + sheeted dykes); Layer 3 — gabbroic intrusions. MORB is mafic, low in SiO2 (~50%), and relatively low in incompatible elements due to a depleted mantle source. MORB geochemistry helps in understanding mantle composition and melting processes.
Significance
MORB magmatism is responsible for creating new oceanic lithosphere, drives sea-floor spreading, and plays a key role in plate tectonics and mantle convection.
Classification of Igneous Rocks
Igneous rocks are classified based on origin, texture, mineral composition, and chemical characteristics. Several methods are used to systematically classify them:
1. Based on Mode of Occurrence
Intrusive (Plutonic): slow cooling beneath the surface. Example: granite, gabbro. Extrusive (Volcanic): rapid cooling at the surface. Example: basalt, rhyolite. Hypabyssal (Subvolcanic): emplaced at shallow depths, intermediate between intrusive and extrusive (dolerite, microgranite).
2. Based on Texture
Phaneritic, aphanitic, porphyritic, glassy, vesicular, pyroclastic — texture reflects cooling rate and environment of crystallization.
3. Based on Mineral Composition
Felsic (high silica, light minerals), intermediate (mixed), mafic (iron and magnesium rich, dark minerals), ultramafic (very low silica, very high Mg and Fe). Examples: felsic — granite/rhyolite; intermediate — diorite/andesite; mafic — gabbro/basalt; ultramafic — peridotite/komatiite.
4. Based on Chemical Composition
Uses total silica content (SiO2) and other oxides. Normative classification: QAPF diagram for plutonic rocks; TAS diagram (total alkali vs. silica) for volcanic rocks.
5. Based on Geological Setting or Origin
Primary igneous rocks (direct crystallization), pegmatitic rocks (late-stage fluids), carbonatites, kimberlites, and lamproites (specific mantle-derived magmas).
Textural Elements of Igneous Rocks
Texture refers to the size, shape, arrangement, and interrelationship of mineral grains or crystals within the rock. It provides important information about cooling history, crystallization environment, and magma evolution.
1. Grain Size
Phaneritic (>1 mm), aphanitic (<1 mm), porphyritic (large phenocrysts in fine matrix), glassy (no crystals).
2. Grain Shape
Equigranular (similar sizes), inequigranular (varying sizes), euhedral (well-formed faces), subhedral (partly formed faces), anhedral (no faces).
3. Grain Arrangement / Fabric
Interlocking (common in intrusive rocks), alignment/foliation (flow-banded rhyolites), random orientation (most plutonic rocks).
4. Special Textural Features
- Porphyritic texture: large crystals in fine groundmass.
- Vesicular texture: gas cavities; e.g., pumice.
- Amygdaloidal texture: vesicles filled with secondary minerals.
- Spinifex texture: skeletal olivine or pyroxene in ultramafic rocks.
- Pillow structures: rounded lobes in submarine extrusive rocks.
5. Microtextures
Graphic texture (quartz–feldspar intergrowth), sieve texture (porous feldspar or pyroxene), and exsolution textures (e.g., perthitic feldspar).
Fractional Crystallisation
Definition: Fractional crystallisation is the process by which different minerals crystallize from a cooling magma at different temperatures and are removed from the liquid, causing the composition of the remaining magma to change progressively.
Process: Magma cools slowly in a magma chamber. Minerals crystallize in a specific order according to Bowen’s reaction series. Early-formed minerals (e.g., olivine, pyroxene, Ca-rich plagioclase) settle due to gravity or are removed by filtration. The removal of these crystals changes the chemical composition of the residual melt, usually making it more silica-rich.
Significance: Explains the differentiation of magma into various rock types and the formation of layered intrusions and ore deposits (e.g., chromite in ultramafic rocks). Example: formation of granitic magma from a basaltic parent by fractional crystallisation.
Magmatism Along Island Arcs
Definition: Island arc magmatism occurs along oceanic-oceanic convergent plate boundaries, where one oceanic plate subducts beneath another.
Process: Subduction causes the descending slab to heat and dehydrate, releasing water into the overlying mantle wedge. Partial melting produces mafic magma, which ascends and may interact with crustal rocks.
Characteristics: Magma is typically calc-alkaline, rich in SiO2 and volatile elements. Rocks formed include basalt, andesite, dacite, and minor rhyolite. Island arcs are associated with volcanic hazards, stratovolcanoes, and geothermal activity. Examples include the Andaman Islands, Aleutian Islands, and the Japanese island arcs.
Petrographic Note on Basalt
Basalt is a mafic, fine-grained extrusive igneous rock formed by the rapid cooling of lava at or near the Earth’s surface. It is the most abundant volcanic rock, constituting the oceanic crust and large continental lava plateaus such as the Deccan Traps of India.
Megascopic Characters
Colour: Dark grey to black. Grain size: Fine-grained (aphanitic), sometimes porphyritic. Texture: Compact, dense, commonly vesicular or amygdaloidal. Structure: Massive or vesicular.
Microscopic (Petrographic) Characters
Essential mineral constituents: Plagioclase feldspar (labradorite to bytownite), typically lath-shaped with albite or polysynthetic twinning; pyroxene (augite) with two cleavages at nearly right angles; olivine in olivine basalts often altered to iddingsite or serpentine.
Accessory minerals: Iron oxides (magnetite, ilmenite), apatite, spinel.
Groundmass and Textures
Groundmass may be crystalline, glassy (tachylitic basalt), or show volcanic glass flow structures. Basalt textures include intergranular, intersertal, ophitic/sub-ophitic, porphyritic, vesicular/amygdaloidal.
Secondary alteration: olivine → iddingsite/serpentine; plagioclase → saussuritization; development of secondary minerals like chlorite and calcite.
Occurrence: lava flows, pillow lavas (submarine), dykes and sills, flood basalts (e.g., Deccan Traps, India).
Petrographic Note on Anorthosite
Anorthosite is a coarse-grained, plutonic igneous rock composed predominantly of plagioclase feldspar. It is important in igneous petrology because of its distinct composition, texture, and origin, and its occurrence in large layered intrusions.
Megascopic Characters
Colour: Light coloured (white, grey to bluish-grey). Texture: Coarse-grained, massive. Structure: Generally non-foliated. Composition dominated by plagioclase feldspar (>90%), often showing iridescence (labradorescence) when labradorite is present.
Microscopic / Petrographic Characters
Essential mineral: Plagioclase feldspar (usually labradorite to bytownite) with polysynthetic (albite) twinning, zoning, and alteration to sericite. Accessory minerals: pyroxenes (augite, hypersthene), olivine, ilmenite, magnetite, apatite. Secondary minerals: sericite, chlorite, epidote from alteration.
Texture and Genesis
Phaneritic, hypidiomorphic to idiomorphic granular texture, cumulate textures commonly seen in layered anorthosite complexes. Formed by fractional crystallization of basaltic magma with early crystallization and accumulation of plagioclase by flotation or settling in large magma chambers. Often associated with layered igneous complexes.
Occurrence
Occurs as large batholithic masses, common in Precambrian shield areas. Indian occurrences: Sittampundi complex (Tamil Nadu), Chotanagpur Plateau, Eastern Ghats. Economic importance: source of aluminium and calcium, associated with ilmenite, magnetite, and apatite, used as dimension stone.
Peridotite and Its Indian Distribution
Peridotite is an ultramafic igneous rock composed mainly of olivine (>=40%), with pyroxenes and minor spinel or garnet. It is coarse-grained, dense, and dark green to black. Peridotite represents upper mantle composition and is rarely exposed at the surface except in tectonically uplifted areas.
Petrography and Types
Major minerals: olivine (Fo70–90), orthopyroxene, clinopyroxene. Accessory minerals: spinel, garnet, amphibole, or serpentine (if altered). Texture: coarse-grained, granular, sometimes porphyroclastic. Types include harzburgite (olivine + orthopyroxene), lherzolite (olivine + orthopyroxene + clinopyroxene), and dunite (almost entirely olivine).
Indian Distribution
Peridotites in India occur mostly as ophiolitic complexes and ultramafic bodies: Kinnaur (Himachal Pradesh), Ladakh and Sikkim (ophiolite belts), Andaman Islands (small ultramafic lenses), Rajasthan (Dungarpur, Bundi) associated with Archaean greenstone belts. Significance: source rock for basaltic magmas and hosts chromite deposits.
Carbonatite and Its Indian Distribution
Carbonatite is a rare, carbonate-rich igneous rock composed of >50% carbonate minerals such as calcite, dolomite, or ankerite, with minor silicate or phosphate minerals. Carbonatites are alkaline and often associated with mantle-derived magmatism. They are important economically for REEs (rare earth elements), phosphate, and niobium.
Petrography and Types
Major minerals: calcite, dolomite, ankerite. Accessory minerals: apatite, magnetite, pyrochlore, bastnsite. Texture: medium to coarse-grained; can be porphyritic or holocrystalline. Types include calciocarbonatite (dominant calcite), dolomitic carbonatite, svite (coarse-grained), and alvikite (fine-grained).
Indian Occurrences
Carbonatites in India occur mainly in Proterozoic and Cenozoic alkaline complexes: Mundwara (Rajasthan), Sukinda (Odisha; minor occurrences associated with REE mineralization), Cuddapah Basin (Andhra Pradesh; rare carbonatite dykes), Amba Dongar (Gujarat; well-known complex containing apatite and rare earth minerals), Khadakwasla (Maharashtra; small occurrences).
Significance: Source of rare earth elements, niobium, and phosphate; associated with alkaline magmatism and mantle metasomatism.
Heat Flow and Geothermal Gradient Through Time
Heat flow and geothermal gradient describe the thermal state of the Earth and its evolution through geological time. Heat flow represents the amount of heat escaping from the Earth’s interior to the surface, whereas the geothermal gradient expresses the rate of increase of temperature with depth. Both parameters have played a crucial role in magma generation, crustal evolution, metamorphism, and plate tectonics.
Heat Flow Through Geological Time
Heat flow is measured as the amount of heat transferred per unit area per unit time (mW/m²). The main sources of Earth’s internal heat are primordial heat retained from accretion and core formation, and radiogenic heat produced by decay of radioactive elements (U, Th, K).
Early Earth (Hadean–Archaean): Heat flow was extremely high (2–3 times present value), caused by higher abundance of radioactive elements and residual primordial heat. Resulted in extensive mantle melting, widespread volcanism, thin lithosphere, and formation of komatiitic magmas.
Proterozoic: Gradual decline in heat flow due to radioactive decay and mantle cooling; reduced mantle melting and stabilization of continental crust (craton formation).
Phanerozoic to present: Heat flow is comparatively low and stable. Average present-day continental heat flow is ~60 mW/m², higher at mid-ocean ridges and lower in old cratons. Igneous activity is mainly controlled by plate tectonics rather than global thermal conditions.
Geothermal Gradient Through Time
Geothermal gradient is the rate of temperature increase with depth (°C/km). Early Earth had very steep gradients (40–60 °C/km or more), promoting shallow melting and high-grade metamorphism. Proterozoic saw a gradual decrease in gradient due to cooling and thickening of the lithosphere. Present-day average continental gradient is about 25–30 °C/km, with higher gradients in volcanic regions, rift zones, and mid-ocean ridges, and lower gradients in stable shields.
Significance: Higher heat flow and geothermal gradients in early Earth favored extensive magma generation. Decline in thermal regime through time led to compositional diversification of magmas and controls depth of partial melting, magma types, and tectonic style of igneous activity.
Bowen’s Reaction Series
Bowen’s Reaction Series, proposed by N. L. Bowen (1928), explains the systematic sequence in which minerals crystallize from a cooling magma. It demonstrates the relationship between temperature, mineral stability, and magma composition and is fundamental to understanding the origin and evolution of igneous rocks.
Discontinuous Reaction Series
Characterized by the crystallization of ferromagnesian minerals where each mineral reacts with the melt to form a new mineral structure as temperature decreases. The sequence is: olivine → pyroxene → amphibole → biotite. Each mineral is stable over a limited temperature range and is replaced by another mineral with decreasing temperature. These minerals are rich in Fe and Mg and form at high temperatures.
Continuous Reaction Series
Involves plagioclase feldspar, which changes composition continuously from calcium-rich (anorthite) at high temperature to sodium-rich (albite) at lower temperature. The crystal structure remains the same, but chemical composition varies gradually.
Late-Stage Minerals
At lower temperatures, felsic minerals crystallize from the remaining silica-rich melt: orthoclase (K-feldspar), muscovite, and quartz (last to crystallize at the lowest temperature).
Implications
- Explains mineral stability and why certain minerals occur together.
- Accounts for the formation of ultramafic, mafic, intermediate, and felsic rocks.
- Demonstrates how fractional crystallization leads to magma evolution.
- Helps interpret texture and composition of igneous rocks and predict ore mineral associations.
Crystallization Behaviour of Bicomponent Magma
A bicomponent magma consists of two components (A and B). When these components are completely soluble in each other in the solid state, they form a solid-solution series. The crystallization behaviour of such magma is best explained by a binary phase diagram with complete solid solution (e.g., albite–anorthite or forsterite–fayalite systems).
At high temperature, the magma exists entirely as a homogeneous liquid. On cooling, crystallization begins when the liquid reaches the liquidus curve. The first crystals to form are richer in the high-melting component (A) compared to the original melt.
As cooling continues, both liquid and solid coexist between the liquidus and solidus curves. The composition of crystals continuously changes, becoming progressively richer in the low-melting component (B). Simultaneously, the residual liquid also changes composition along the liquidus curve. At the solidus temperature, the last remaining liquid crystallizes, and the system becomes completely solid.
Important Features and Geological Significance
- Crystallization occurs over a range of temperatures, not at a single point.
- There is continuous reaction between early-formed crystals and the melt, allowing crystals to adjust composition; slow cooling yields equilibrium solid-solution crystals, while rapid cooling can cause zoning (normal zoning).
- Explains mineral zoning in plagioclase feldspar and olivine and is important for understanding magmatic differentiation and mineral compositional variation in igneous rocks.
Examples
Forsterite (Mg2SiO4) – fayalite (Fe2SiO4) series in olivine; albite – anorthite series in plagioclase feldspar.