GL 272 Igneous and Metamorphic Petrology Study Notes

GL 272 IGNEOUS AND METAMORPHIC PETROLOGY: GENERAL OVERVIEW

• Institution: University of Mines and Technology (UMaT), Tarkwa.
• Faculty: Faculty of Geosciences and Environmental Studies.
• Department: Department of Geological Engineering.
• Course Title: GL 272 Igneous and Metamorphic Petrology.
• Compiled By: Dr. G. M. Tetteh.
• Date: May 2026.

COURSE OUTLINE: IGNEOUS PETROLOGY

• Theoretical Topics:

  1. Physical Properties of Magma.
  2. Sources of Magma.
  3. Mechanisms of Magma Generation and Extraction.
  4. Magma Diversity and Crystallisation.
  5. Forms, Structures and Textures.
  6. Classification of Igneous Rocks.
  7. Geological Setting of some Igneous Rocks.
  8. Granitoid Formation.

• Practical Topics:

  1. Identification of Mineralogy of common Igneous Rocks.
  2. Modal Percentage and Colour Index.
  3. Structures and Textures.
  4. Naming of Igneous Rocks using the QAP Diagram.
  5. Description of common Igneous Rocks and Interpretation.

COURSE OUTLINE: METAMORPHIC PETROLOGY

• Instructional Focus:
  1. Introduction of Metamorphism and Historical Perspective.
  2. Metamorphic Processes and factors of Metamorphism.
  3. Naming of Metamorphic Rocks.
  4. Criteria of Metamorphic Grade and Concepts.
  5. Metamorphic Differentiation.
  6. Migmatites.
  7. Determination of Protoliths of Metamorphic Rocks.
  8. Tectonics and Mineral Deposits.

PRINCIPLES OF IGNEOUS ROCKS

• Naming and Origin: Igneous rocks are named from the belief that they crystallised from a molten mass (magma) produced by internal "fires" of the earth.
• Magma Definition: A molten rock material formed at different levels in the earth, mobile enough to be erupted as lava at the surface. It is a complex system of molten silicates containing water and dissolved gases.
• Representation: Igneous rocks rarely represent the exact magma composition from which they formed because volatile constituents (water, gases) are lost during crystallisation.
• Classifications by Location:
  • Intrusive/Plutonic Rocks: Solidified inside the earth.
  • Extrusive/Volcanic Rocks: Flowed onto the surface, either subaerially (air) or subaqueously (water).

PHYSICAL PROPERTIES OF MAGMA

• Temperature Measurements:

  • Measured using thermocouples or optical pyrometers.
  • Basalt (Hawaii): $1150 - 1225\,^{\circ}\text{C}$.
  • Andesite (Pericutin, Mexico): $1020 - 1110\,^{\circ}\text{C}$.
  • Alkali basalt (Zaire, C. Africa): $980 - 1095\,^{\circ}\text{C}$.
  • Rhyolite (Mono Craters, California): $790 - 820\,^{\circ}\text{C}$.
  • Dacite (Mt. St. Helens, Washington): Specific data omitted but listed as a reference.

• Density:

  • Determined from melt experiments.
  • Obsidian: $2.4\,g\,cm^{-3}$.
  • Basalt: $2.9\,g\,cm^{-3}$.

• Viscosity:

  • Heavily dependent on water content.
  • Water Effect: Water lowers viscosity because it depolarises $\text{SiO}_{4}$ structures, replacing binding atoms with non-binding $\text{OH}$ groups.
  • Comparative Viscosity (Poises):
  • Rhyolite (Obsidian): $10^{6.5}$ at $700\,^{\circ}\text{C}$; $10^{5.6}$ at $800\,^{\circ}\text{C}$.
  • Basalt Liquid Glass: $10^{4}$ at $1000\,^{\circ}\text{C}$; $10^{2.6}$ at $1200\,^{\circ}\text{C}$; $10^{1.9}$ at $1300\,^{\circ}\text{C}$.
  • $SiO_2$ Glass: $10^{12}$ at $1300\,^{\circ}\text{C}$.
  • Water: $10^{-2}$ at $20\,^{\circ}\text{C}$.

GEOTHERMAL GRADIENT AND MAGMA SOURCE

• Geothermal Gradient: The rate of temperature increase with depth, averaging $30\,^{\circ}\text{C/km}$ near the surface.
• Depth vs. Heat: At the standard gradient, magma temperatures ($800 - 1200\,^{\circ}\text{C}$) should be reached at $30 - 40\,km$. However, evidence suggests the gradient slows at greater depths because radioactive heat sources are concentrated in the crust.
• Radioactive Heat Sources: Uranium, Thorium, and Potassium isotopes. These have affinities for granitic rocks.
• Heat Production Rates ($J/gy$):
  • Granite: $3.4 \times 10^{-5}$.
  • Basalt: $5.0 \times 10^{-6}$.
  • Peridotite: $3.8 \times 10^{-8}$.
• Revised Depth Estimates: Magma usually originates at depths of $60 - 100\,km$. Generation is a specialized event requiring geotherm perturbation.

SEISMIC AND COMPOSITIONAL MODELS OF THE MANTLE

• Seismic Evidence:

  • P-waves: Velocity is a function of incompressibility, elastic rigidity, and density.
  • S-waves: Velocity is a function of elastic rigidity and density. S-waves do not pass through liquids (zero rigidity).
  • Mantle Nature: Seismic discontinuities occur at $100\,km$ and $670\,km$. The mantle is likely partially molten (a few percent liquid) within a solid rock matrix.

• Mantle Xenoliths: Enclosed pieces of foreign rock in basalt/peridotite include gabbro, eclogite, amphibolite, and granulite. They represent:

  1. Early-crystallized magma fragments.
  2. Residual (depleted) mantle rock from anatexis.
  3. Unrelated wall rocks captured during magma ascent.

• Chemical Data: REE (Rare Earth Elements) and radioactive isotopes are used to model source regions. Sources must have identical compositions to the resulting rock (e.g., granite from crustal rocks).

• Cosmic Models: Chondrite meteorites help establish primordial solar system chemistry, suggesting an ultramafic mantle serves as the source for basaltic magmas.

• Pyrolite Model: A hypothetical artificial rock (1 part basalt + 3-4 parts peridotite/dunite). It is designed to melt into basalt and leave depleted peridotite. Criticisms include its artificial nature and lack of accounting for post-separation modifications.

MECHANISMS OF MAGMA GENERATION

• Melting Factors: Induced by changes in Pressure ($P$), Temperature ($T$), or Composition ($X$).

• Decompression Melting:

  • Most effective means involves an adiabatic rise of hot rock (no heat loss).
  • Common at oceanic or continental rifts and mid-ocean ridges (upwelling mantle).

• Flux Melting (Compositional Changes):

  • Induced by hydrating the mantle, typically in subduction zones.
  • Subducting oceanic plates carry hydrous minerals (epidote, mica, serpentine) which dehydrate, releasing water into the overlying mantle wedge to decrease the solidus and cause isothermal melting.

• Tectonic Site Comparisons:

  • Mid-Ocean Ridge: Decompression melting shifts the geotherm across the solidus.
  • Hotspot: Decompression plus heat addition.
  • Subduction Zone: Addition of volatiles lowers the melting point (shifts solidus left).

MAGMA TRANSPORT AND EXTRACTION

• Buoyancy: Magma moves because the solid matrix is denser than the liquid. Magma rises until it reaches its level of "neutral buoyancy" (where surrounding rocks are less dense).
• Transport Characteristics:
  • Lava flows often form a "river" flowing at $5 - 50\,km/h$.
  • Flow Dimensions (Hawaii Example): $10\,km$ long, $200\,m$ wide, $3\,m$ thick.
• Movement Controls: Composition, volatiles, phenocrysts, temperature contrast, heat content, viscosity, volume increase (approx. $15\%$ upon melting), and fracture processes.
• Transport Processes:
  • Fracturing/Crack Propagation: Rapid movement; buoyancy and thermal expansion generate cracks (associated with earthquakes).
  • Diapirism: Passive, slower transport of spherical/elliptical magma masses. Common in island arcs without deep earthquakes. Diapirs increase wall rock viscosity to reduce drag. Granitic diapirs often struggle to reach the surface due to cooling and high water content.

MAGMA DIVERSITY PROCESSES

• Magma Mixing: Blending of two different magmas. Intermediate rocks like andesite/dacite can result from basalt/rhyolite mixing.
• Assimilation: Chemical reaction with or melting of wall rocks. Basic (hot) magmas are more effective at assimilating cool siliceous wall rocks. Requires superheating or freezing of large magma portions to provide latent heat.
• Differentiation:
  • Crystal-Liquid Fractionation: Separation of crystals from liquid.
  • Liquid Immiscibility: Separation into two unmixable liquids (e.g., carbonate magmas).
  • Vapour Transport: Vapour enriched in elements like $Na$ and $K$ boils out early.
  • Diffusion: Chemical migration due to $P$, $T$, or chemical gradients.

CRYSTALLISATION PHASES

• Crystallisation Principles: First minerals are pyrogenetic (anhydrous, high T, e.g., olivine, calcic-plagioclase). Later minerals are hydatogenetic (volatile-dependent).
• Evolutionary Stages:
  1. Orthomagmatic: Separation of pyrogenetic silicates.
  2. Pegmatitic: Lower viscosity, high volatiles, temperature $400 - 600\,^{\circ}\text{C}$; generates exceptional crystal sizes and distinctive veins/dykes.
  3. Hydrothermal: Final residual aqueous fluid at low temperatures; forms deposits.

BOWEN’S REACTION SERIES

• Continuous Series: Plagioclase feldspar changes composition from anorthite (core) to albite (rim) without structural overhaul. Rapid growth leads to compositional zoning.
• Discontinuous Series: Crystals react with liquid to form new phases. Example: Olivine reacting with $SiO_2$ to form Pyroxene.
  • Reaction Example: $(Mg,Fe)_2(SiO_4) + SiO_2 \rightarrow 2(Mg,Fe)(SiO_3)$.
• Factors: The series is not a fixed sequence; order depends on melt saturation. In calc-alkaline magmas, amphibole may replace pyroxene as water builds up.

FORMS AND STRUCTURES OF IGNEOUS ROCKS

• Extrusive (Volcanic):

  • Gases: $CO_2$, $SO_2$, $H_2S$, $Cl$, $F$, $B$, and steam. Nuees ardentes are destructive glowing clouds of gas and dust.
  • Lavas: Surface textures include aa (jagged/viscous), pahoehoe (ropey/low viscosity), and pillow lavas (sack-like blocks formed subaqueously).
  • Pyroclastics: Fragments classified by size:
  • Dust: Finest.
  • Ash: Under $2\,mm$.
  • Bombs: Over $6\,cm$.
  • Tephra: Loose accumulation; Tuff (cemented); Ignimbrite (hot glassy fragments).

• Intrusive (Plutons):

  • Minor: Veins (irregular tongues), Dykes (discordant sheets), Sills (concordant tabular bodies), Ring Dykes (circular/elliptical).
  • Major: Laccolith (domed upper surface), Lopolith (saucer-shaped), Phacolith (lenticular in fold crests/bases), Batholith (> $100\,km^2$ granitic masses), Stocks (smaller batholiths).

CLASSIFICATION OF IGNEOUS ROCKS

• Chemical Classification ($SiO_2$ weight percent):

  • Ultramafic: < $45\%$.
  • Mafic: $45 - 52\%$.
  • Intermediate: $52 - 66\%$.
  • Felsic: > $66\%$.

• Colour Index (IUGS):

  • Holo-leucocratic: $0 - 5\%$.
  • Leucocratic: $5 - 35\%$.
  • Mesocratic: $35 - 65\%$.
  • Melanocratic: $65 - 90\%$.
  • Ultramafic: $90 - 100\%$.

• Felsic Mineral Content (QAPF Diagram):

  • Q: Quartz.
  • A: Alkali feldspar.
  • P: Plagioclase.
  • F: Feldspathoids (Note: Q and F are never together).

• Variation Diagrams:

  • Harker Diagram: Oxides vs. $SiO_2$.
  • Peacock Diagram (Alkali-Lime Index): Intersection of (Na2O + K2O) and CaO curves.
  • AFM Diagram: Triangle plot of $A$ (Total alkali), $F$ (Total iron), and $M$ (MgO). Tholeiitic trends show iron enrichment.

TEXTURES OF IGNEOUS ROCKS

• Categories of Grain Size:

  • Aphanitic: Grains too small for the unaided eye.
  • Fine-grained: < $1\,mm$.
  • Medium-grained: $1 - 5\,mm$.
  • Coarse-grained: > $5\,mm$.

• Inter-relationship Names:

  • Equigranular: Comparable diameters.
  • Porphyritic: Large phenocrysts in a fine groundmass.
  • Vitrophyric: Phenocrysts in glass.
  • Glomeroporphyritic: Phenocrysts aggregated in groups.
  • Poikilitic: Large crystal (megacryst) containing small inclusions.

• Volcanic-Specific Textures:

  • Vesicular: Gas holes.
  • Amygdaloidal: Vesicles filled with secondary minerals (calcite, quartz).
  • Miarolitic: Angular gas holes in granitic stocks.

• Gabbroic/Peridotitic Textures:

  • Cumulus: Settled grains isolated by "intercumulus" cement.
  • Corona: Reaction rims on individual grains.
  • Spinifex: Bladed/skeletal olivine in Archaean ultramafic rocks.

TECTONIC SETTINGS AND ROCK TYPES

• MORB (Mid-Ocean Ridge Basalt): Tholeiitic basalt, extension stress.
• Subduction (Volcanic Arcs): Calc-alkaline suite (andesite, dacite, rhyolite); compressive/strike-slip stress.
• Hotspots (Oceanic): Tholeiitic basalt dominant; seamount chains.
• Continental Rifts: Bimodal suite (tholeiitic basalt + rhyolite).
• Cratons: Typically little magma reaches the surface; stress is compressive/tectonically stable.

GRANITOID FORMATION THEORIES

• Granitisation (S-type): Metamorphic transformation of pre-existing rocks without significant melting. Shows gradational changes from gneiss to structureless rock.
• Fractional Crystallisation: Derivative of basaltic parents. Supported by sharp discordant contacts, chilled zones, and stoping evidence.
• Hybridisation: Assimilation/mixing of lower crustal rocks and mantle magmas.
• Anatexis: Partial melting of crust/mantle. Supported by evidence that granites are restricted to continental areas and experimental data.

INTRODUCTION TO METAMORPHISM

• Historical Perspective: Abraham Gottlieb Werner (Neptunian theory). Charles Lyell (1833) proposed "metamorphic" from Greek trans and forma.
• Definition: Mineralogical and structural changes in solid-state rocks due to physical/chemical conditions varying from original ones. Excludes weathering, diagenesis, and full melting.
• Processes:
  • Cataclasis: Crushing/breaking of grains.
  • Recrystallisation: Crystal lattice re-organisation without breakage.
  • Neocrystallisation: Formation of entirely new minerals.

TYPES OF METAMORPHISM

• Contact Metamorphism: Localized; affects heated rocks bordering intrusions (aureoles). Depth-based sub-divisions: LP-contact (low pressure), MP-contact, and HP-contact (rare, found in subduction accretionary complexes).
• Cataclastic (Dynamic): High pressure, low temp; associated with faults and meteorite impacts. Produces breccias, mylonites, and pseudotachylites (glassy appearance).
• Static Metamorphism: High lithostatic pressure from burial (e.g., passive continental margins).
• Regional Dynamic: In mountain belts; deviatoric stress yields tectonic melanges.
• Dynamothermal (Regional): Induced by both pressure and temperature. Characteristic of developing orogens.
• Metasomatism: Allochemical process dominated by chemical change via fluids. "Alteration" in ore deposits.
• Prograde vs. Retrograde: Prograde (increasing T); Retrograde (decreasing T, often incomplete/pseudomorphic).

FACTORS OF METAMORPHISM

• Temperature: Ranges from $100\,^{\circ}\text{C}$ (shallow burial) to $800\,^{\circ}\text{C}$ (anatexis).
• Pressure:
  • Lithostatic/Load Stress: Uniform in all directions.
  • Deviatoric Stress: Compression, tension, and shear. Produces distinctive textures.
  • Magnitude: $1\,kbar$ ($0.1\,GPa$) to $15\,kbar$ ($1.5\,GPa$) at the base of the crust.
• Fluids: $H_2O$ and $CO_2$ facilitate reactions and impact pressure ($P_{fluid}$).

TEXTURES OF METAMORPHIC ROCKS

• Planar Elements:
  • Foliation: Parallel arrangement/distribution.
  • Schistosity: Parallel platy minerals (mica); gives fissility.
  • Slaty Cleavage: Parallel fine micas in slate.
  • Gneissosity: Alternating light/dark layers.
• Linear Elements:
  • Lineation: Parallelism of linear features (hornblende prisms, microfold axes).
• Other Textures:
  • Crenulation Cleavage: Deformed schistosity forming new cleavage.
  • Hornfelsic: Nondirectional, fine-grained; typical of contact metamorphism.
  • Granoblastic: Even-grained mosaic.
  • Porphyroblast: Large grain grown in solid rock. Idioblast (euhedral); Xenoblast (anhedral).
  • Augen: Eye-shaped porphyroclasts (usually feldspar).
  • Poikiloblastic: Sieve texture with groundmass inclusions.
  • Helicitic: Curved inclusion lines preserving earlier foliation.
  • Corona/Reaction Rim: New mineral rimming an unstable core.
  • Cataclastic/Mylonitic: Sheared, crushed, or streaked fabric.

NAMING CONVENTIONS FOR METAMORPHIC ROCKS

• Descriptive Criteria:

  1. Mineralogical: Compounds of minerals in increasing abundance (e.g., biotite-garnet schist).
  2. Chemical: Reflects chemistry (e.g., carbonaceous phyllite).
  3. Protolithic: Name uses the parent rock (e.g., orthogneiss from igneous, paragneiss from sedimentary).

• Common Rock Series (Increasing Grade):

  • Slate: Very fine, platy cleavage.
  • Phyllite: Lustrous sheen, fine schistosity.
  • Schist: Coarse enough for hand-specimen ID.
  • Gneiss: Banded rock (quartzo-feldspathic vs. mica-rich layers).
  • Granulite: Very high grade, poor in mica, contains hypersthene + quartz.
  • Skarn (Tactite): Calc-silicate from metasomatism of carbonate.
  • Amphibolite: Hornblende + plagioclase.
  • Eclogite: Green omphacite + red garnet (extreme pressure).

METAMORPHIC GRADE CONCEPTS

• Depth Zones (Grubenmann & Niggli, 1924):
  • Epizone: Upper zone, moderate T, missing/strong stress.
  • Mesozone: Middle, higher T/P.
  • Katazone: Lower, high T, very high P.
• Index Mineral Concept (George Barrow, 1893):
  • Zones mapped by the first appearance of minerals in pelitic beds: Chlorite $\rightarrow$ Biotite $\rightarrow$ Garnet (Almandine) $\rightarrow$ Staurolite $\rightarrow$ Kyanite $\rightarrow$ Sillimanite.
  • Isograd: A line on a map marking the first appearance of an index mineral.
• Facies Concept (Eskola, 1920):
  • A set of mineral assemblages repeatedly associated, where mineralogy depends on chemical composition. Refined by Fyfe and Turner (1966).

METAMORPHIC FACIES SERIES

• Initial Types (Miyashiro, 1961):

  1. Contact Facies Series: Very low pressure ($> 80\,^{\circ}\text{C/km}$).
  2. Buchan Facies Series: Low pressure ($40 - 80\,^{\circ}\text{C/km}$).
  3. Barrovian Facies Series: Medium pressure ($20 - 40\,^{\circ}\text{C/km}$).
  4. Sanbagawa Facies Series: High pressure, moderate T ($10 - 20\,^{\circ}\text{C/km}$).
  5. Franciscan Facies Series: High pressure, very low T ($< 10\,^{\circ}\text{C/km}$).

• Key Facies Descriptions:

  • Zeolite: Lowest grade.
  • Greenschist: Abundant green minerals (chlorite, epidote, actinolite).
  • Blueschist (Glaucophane Lawsonite): High pressure, low temperature; characteristic mineral is glaucophane.
  • Amphibolite: Medium-high grade; hornblende + plagioclase.
  • Granulite: Maximum regional T; plagioclase + hypersthene.
  • Eclogite: Most deep-seated; pyrope garnet + omphacite.

METAMORPHIC DIFFERENTIATION AND MIGMATITES

• Differentiation: Development of compositional layering/segregations from a homogeneous parent via diffusion. Mobile constituents migrate from high to low-pressure sites.
• Migmatites: Composite rocks mixture of granitoid and metamorphic parts.
  • Neosome: The whole newly formed rock.
  • Leucosome: Light, quartz/feldspar-rich part (non-foliated/igneous-like).
  • Melanosome: Dark, ferromagnesian-rich (foliated).
  • Paleosome: Unaltered parent rock.

PROTOLITH DETERMINATION CRITERIA

• Sedimentary Relicts: Bedding, graded bedding (often reversed grain size in metamorphism), cross bedding, ripple marks.
• Igneous Relicts: Cross-cutting relationships, xenoliths, cumulus textures, phenocryst zoning, amygdules, pillows.
• Chemical Criteria: Sedimentary parents often have high ratios of $\text{Al}_2\text{O}_3$ to ($\text{Na}_2\text{O}$ + $\text{K}_2\text{O}$ + $\text{CaO}$). Trace elements and REE help resolve ambiguity.

QUESTIONS & DISCUSSION (EXAM ARCHIVE)

• Viscosity: Factors increasing viscosity include high silica content and decrease in water content (de-polymerisation of $\text{SiO}_4$).
• Diversity: A single parent magma creates variety through differentiation, assimilation, and fractionation.
• Granitoids via Anatexis: Evidence includes field occurrences of lenses in meta-terranes, studies of REE, and experimental melting of sediments.
• Metamorphic Zones: Barrow’s zones (Chlorite to Sillimanite) and their tectonic implications.
• Tectonic Settings: Volcanism reconstruction via MORB, Hot Spot, and Subduction models.
• Specific Textures: Explain concepts like ophitic texture (plagioclase laths in ferromagnesian crystal), poikilitic texture, and cumulus texture.
• Barrovian Facies Sequence: For shaly rocks: Slate $\rightarrow$ Phyllite $\rightarrow$ Schist $\rightarrow$ Gneiss.