Unit 3: Igneous Petrology

Concepts of Igneous Petrology

Igneous Petrology is the study of igneous rocks—rocks that are formed from the cooling and solidification (crystallization) of molten rock material.

• Magma: Molten rock beneath the Earth's surface.
• Lava: Molten rock that has erupted onto the Earth's surface.
• Intrusive (Plutonic) Rocks: Formed from magma that cools slowly deep underground. (e.g., Granite, Gabbro).
• Extrusive (Volcanic) Rocks: Formed from lava that cools quickly on the surface. (e.g., Rhyolite, Basalt).

Heat Flow and Origin of Magma

Heat Flow and Geothermal Gradient

The Earth is hot on the inside. This heat flows from the hot core and mantle towards the cool surface.

• Geothermal Gradient: The rate at which temperature increases with depth. In the crust, this is typically 25-30 °C per kilometer.
• Heat Flow through Time: The Earth was significantly hotter in its early history (e.g., Archean Eon). This means geothermal gradients were steeper, and melting occurred more easily.

Origin and Ascent of Magma

Magma forms by melting pre-existing rock in the crust or (more commonly) the upper mantle. The mantle is mostly solid, so melting requires special conditions:

1. Decompression Melting: At mid-ocean ridges, hot mantle rock rises. The pressure decreases, which lowers the melting point, causing it to melt (even though temperature stays the same).
2. Addition of Volatiles (Flux Melting): At subduction zones, water is forced out of the subducting oceanic plate. This water rises into the overlying hot mantle, lowers its melting point, and causes it to melt.
3. Heat Transfer (Conduction): Hot magma from the mantle can get "stuck" at the base of the crust. It transfers its heat to the crust, causing the crustal rock (which has a lower melting point) to melt.

Ascent of Magma: Once formed, magma is less dense than the surrounding solid rock, so it rises buoyantly, like a hot air balloon, often fracturing the rock above it (stoping) or moving through cracks (dikes).

Bowen’s Reaction Series (BRS)

This is the most important concept in igneous petrology. It describes the sequence in which minerals crystallize from a cooling mafic magma. It is divided into two branches that merge.

• Discontinuous Series (Mafic Minerals): A series of reactions.
  1. Olivine crystallizes first (High Temp: ~1200 °C).
  2. Olivine reacts with the melt to form Pyroxene.
  3. Pyroxene reacts with the melt to form Amphibole.
  4. Amphibole reacts with the melt to form Biotite Mica.
• Continuous Series (Felsic Minerals): A simple substitution.
  • Ca-rich Plagioclase (Anorthite) crystallizes first (High Temp).
  • It continuously reacts with the melt, becoming progressively more Na-rich (ending with Albite).
• Lower-Temperature Crystallization: After the two branches merge, the remaining melt (now rich in Si, K, Al) crystallizes in this order:
  1. Potassium Feldspar (K-Feldspar)
  2. Muscovite Mica
  3. Quartz (Last to crystallize, at the lowest temp: ~600-700 °C)

Magmatic Differentiation and Assimilation

Magmatic Differentiation

The process by which a single parent magma can produce a variety of different igneous rocks. The main mechanism is:

• Fractional Crystallization: As magma cools, the first-formed crystals (like Olivine, per BRS) are denser and sink to the bottom of the magma chamber.
• This removes Fe and Mg from the melt, leaving the remaining (differentiated) magma richer in Si, Al, K, and Na.
• The settled crystals form one rock type (e.g., Peridotite), while the leftover magma will cool to form another (e.g., Basalt or even Andesite).

Assimilation

As magma rises, it can melt and incorporate the surrounding "country rock." This "contaminates" the magma, changing its chemical composition. For example, a mafic (basaltic) magma assimilating silica-rich (granitic) crust will become more felsic.

Magmatic Associations

This topic refers to the "primary and reaction series," which is another name for Bowen's Reaction Series.

• Primary Minerals: Minerals that crystallize directly from the melt (e.g., Olivine, Ca-rich Plagioclase).
• Reaction Minerals: Minerals that form by reacting with the melt (e.g., Pyroxene, Amphibole, Biotite, and the changing Plagioclase).

These associations also refer to the fact that minerals from the same temperature regime (from BRS) are "associated" and found together.

• Mafic Association (High T): Olivine, Pyroxene, Ca-rich Plagioclase (found in Gabbro/Basalt).
• Felsic Association (Low T): K-Feldspar, Muscovite, Quartz (found in Granite/Rhyolite).

IUGS Classification of Igneous Rocks

The syllabus mentions "RUGS," which is a common typo for IUGS (International Union of Geological Sciences). This is the standard classification system for plutonic rocks, using the QAPF Diagram.

The diagram is a double-triangle (diamond shape) based on the relative modal percentages of four mineral groups:

• Q = Quartz
• A = Alkali Feldspar (K-Feldspar, Albite)
• P = Plagioclase Feldspar
• F = Feldspathoids (Rare, form when melt is Si-poor)

How it works:

1. Estimate the percentages of Q, A, P, and F in the rock.
2. Q and F cannot exist together (they would react). So, a rock will be in either the QAP or FAP triangle.
3. Recalculate the percentages of Q+A+P or F+A+P to equal 100%.
4. Plot the point on the triangle to find the rock name.

Textures and Structures

Textures (Grain Size and Relationships)

Texture describes the size, shape, and arrangement of mineral grains in the rock.

• Grain Size (Phaneritic vs. Aphanitic):
  • Phaneritic: Coarse-grained (crystals visible to naked eye). Indicates slow cooling (intrusive).
  • Aphanitic: Fine-grained (crystals too small to see). Indicates fast cooling (extrusive).
• Porphyritic Texture: A mixed texture with large crystals (phenocrysts) in a fine-grained matrix (groundmass). Indicates two-stage cooling (slow cooling at depth, then fast cooling on the surface).
• Glassy Texture: No crystals (e.g., Obsidian). Indicates instantaneous cooling.
• Vesicular Texture: Contains holes (vesicles) left by escaping gas bubbles (e.g., Pumice, Scoria).
• Amygdaloidal Texture: Vesicles that have been filled in later by secondary minerals (e.g., Calcite, Quartz).

Structures (Large-scale Features)

Structures are large-scale features seen in the field.

• Columnar Jointing: Polygonal columns (often 6-sided) that form as a thick lava flow cools and contracts (e.g., Giant's Causeway).
• Pillow Lavas: Bulbous, pillow-shaped structures that form when lava erupts underwater.
• Xenoliths: A piece of "foreign" country rock that was ripped off and trapped inside the magma as it rose.

Mode of Occurrence (Igneous Bodies)

This describes the 3D shape and orientation of an igneous rock body.

Intrusive (Plutonic) Bodies

• Concordant: Bodies that are parallel to the existing rock layers (bedding).
  • Sill: A tabular (sheet-like) intrusion.
  • Laccolith: A mushroom-shaped intrusion that pushes the overlying layers up.
• Discordant: Bodies that cut across the existing rock layers.
  • Dike: A tabular (sheet-like) intrusion.
  • Batholith/Stock: A massive, irregular, deep-seated intrusion. Batholiths are >100 km²; Stocks are smaller. These are the "roots" of volcanoes.

Extrusive (Volcanic) Bodies

• Lava Flows: Sheets of lava that flow over the surface.
• Pyroclastic Deposits: Layers of ash, pumice, and rock fragments ejected explosively from a volcano (e.g., Tuff).