Introduction to Igneous Rocks and Their Textures lec 2

Why Classify Igneous Rocks?

Classification of igneous rocks provides a systematic framework to:

  • Identify shared magmatic origins: Both basalts and gabbros are fundamental parts of the oceanic crust.

  • Infer magma’s source region:

    • Basaltic: Typically mantle-derived.

    • Granitic: Typically crustal-derived.

  • Link rocks to their tectonic environments: Such as convergent margins, divergent margins, or intraplate settings.

  • Reconstruct magma evolution pathways and identify ore deposits:

    • Layered mafic intrusions: Formed by magma differentiation; often host valuable ores like chromitite and platinum-group elements.

    • Porphyritic intrusions: Found in subduction zones; often concentrate copper and gold.

Basis of Igneous Rock Classification

All classifications are based on three primary criteria:

  1. Mineralogy: The constituent minerals and their abundances. This reflects a rock’s chemical composition and depends on magma chemistry.

  2. Texture: The size, shapes, and arrangement of crystals. This reflects the cooling rate, environment of crystallization, and presence of volatiles.

  3. Chemical Composition: Often expressed via oxides like SiO{2}, MgO, and K{2}O + Na_{2}O. This reflects the rock's source region and tectonic environment.

Compositions and Components of Magmas

  • Silica Content: Most magma is largely silica, ranging from (SiO_{2} = 40 - 80 \text{ wt.%}).

  • Major Elements: Magmas contain oxides of Al, Ca, Mg, Fe, Na, \text{ and } K.

  • Elemental Solution: Magma usually consists of a solution of the most abundant elements in the Earth's crust.

Common Minerals in Igneous Rocks

The Silica Tetrahedron

  • Building Block: All silicate minerals are built from the [SiO_{4}]^{4-} unit.

  • Charge Balance: Silicon has a +4 charge and each oxygen has a -2 charge, resulting in a net charge of -4.

  • Cations: This charge is balanced by cations such as Mg^{2+}, Fe^{2+}, Ca^{2+}, Na^{+}, \text{ or } K^{+}.

Common Igneous Minerals

  • Feldspar: Most abundant group (\sim 51\% of the crust). From German Feld (field) + Spat (cleavage).

    • Plagioclase: From Greek plagios (oblique) + klasis (cleavage).

  • Quartz: Known for its hardness (7 on Mohs scale), derived from German Quarz.

  • Olivine: Abundant in mantle peridotites; named for its olive-green color.

  • Pyroxene/Augite: Pyroxene (Greek pyro "fire" + xenos "stranger") and Augite (Greek augē "brightness").

  • Amphibole/Hornblende: Amphibole (Greek for "ambiguous") and Hornblende (German for "horn-deceiver").

  • Mica: Named for its glittery look. Includes Biotite (dark) and Muscovite (light; "Muscovy glass").

Mafic vs. Felsic Minerals

  • Mafic Minerals (Magnesium + Ferric):

    • Rich in Mg and Fe; dark-colored and dense.

    • Crystallize at high temperatures.

  • Felsic Minerals (Feldspar + Silica):

    • Rich in Si and Al; light-colored and less dense.

    • Crystallize at lower temperatures.

Bowen’s Reaction Series

  • Function: Explains mineral crystallization/melting temperature relationships and why certain minerals (like Quartz and Olivine) are rarely found together.

  • Temperature Gradient: Ranges from approximately 1250^{\circ}C (2300^{\circ}F) at the high end to 700^{\circ}C (1300^{\circ}F) at the low end.

  • Crystallization Sequence:

    • Discontinuous Series: Involves a magnesium-iron change in minerals like Olivine to Pyroxene.

    • Continuous Series: Involves a transition from Ca-rich to Na-rich plagioclase.

The Crystallization Process

  • Undercooling (\Delta T): Occurs when magma temperature drops below the equilibrium crystallization temperature (\Delta T = T{equilibrium} - T{actual}).

  • Nucleation and Growth:

    • Low Undercooling: Low nucleation rate (N); growth dominates. Results in coarse-grained textures.

    • High Undercooling: Nucleation rate exceeds growth rate. Results in fine-grained textures.

    • Extreme Undercooling: Atoms cannot rearrange; results in glass (e.g., obsidian).

  • Growth Mechanics: Crystals grow by adding ions from the melt to the crystal lattices of seed nuclei.

Texture Classifications

Cooling Environments

  1. Intrusive (Plutonic): Magma cools slowly underground; insulated by surrounding rocks. Results in phaneritic (coarse-grained) textures.

  2. Extrusive (Volcanic): Lava cools quickly at the surface. Results in aphanitic, glassy, or vesicular textures.

Specific Texture Types

  • Phaneritic: Individual crystals are visible to the naked eye.

  • Aphanitic: Fine-grained crystals requiring a microscope.

  • Porphyritic: Two distinct sizes: large phenocrysts surrounded by a smaller groundmass.

  • Vesicular: Characterized by holes (vesicles) left by gas bubbles.

  • Glassy: No visible crystals even under a microscope.

  • Pyroclastic: Formed by fragments (ash and rock) during explosive eruptions.

  • Pegmatitic: Very coarse-grained (crystals > 1 \text{ cm}).

    • Cause: Forms from the last, volatile-rich portions of magma. Volatiles (H_{2}O, F, B) enhance ion mobility.

    • Importance: Source of gemstones (tourmaline, emerald) and rare elements (Li, Be, Ta).

  • Ophitic: Slender, euhedral plagioclase laths are enclosed by larger, late-forming pyroxene crystals. Common in diabase and gabbro.

  • Fast Crystallization Textures:

    • Spinifex: Dendritic olivine and pyroxene crystals forming bladed shapes; common in komatiite.

    • Skeletal/Dendritic: Crystals grow faster than atoms can be supplied, resulting in non-equilibrium shapes (blades or needles).

Crystal Shape

Determined by how freely a crystal grew (from the root hedra meaning "face"):

  • Euhedral: Crystals with well-developed planar faces reflecting their internal structure.

  • Subhedral: Partially bounded by crystal faces due to competition for space or reaction with melt.

  • Anhedral: Highly irregular grains with no ideal crystal shapes.

Crystallinity and Grain Size

  • Crystallinity: Ratio of crystals to glass (Holohyaline = 100\% glass; Hypocrystalline = mixture; Holocrystalline = 100\% crystals).

  • Relative Grain Size:

    • Equigranular: All crystals are approximately the same size.

    • Inequigranular: Crystals are of different sizes (e.g., Porphyritic).

IUGS Classification System

  • Mechanism: Use of ternary diagrams based on QAPF parameters (Quartz, Alkali feldspar, Plagioclase, Feldspathoid).

  • Plotting on Ternary Diagrams:

    1. Ignore Mafic Minerals: Base the name on felsic components only.

    2. Normalize Abundances: Mineral volumes are normalized to 100\%.

    3. Calculate Verticals: Determine Q / (Q + A + P) \times 100 (plotting up from the A-P base) or F / (F + A + P) \times 100 (plotting down).

    4. Calculate Diagonals: Determine the ratio of plagioclase to alkali feldspar: P / (A + P) \times 100.

Classification of Mafic/Ultramafic Rocks

  • Gabbroic Rocks: Classified as Anorthosite (>90\% calcic plagioclase), Gabbro (augite + calcic plagioclase), or Diorite.

  • Ultramafic Rocks: Defined by having mafic minerals >90\%.

  • Peridotite: Rocks containing 40-100 \text{ vol.%} olivine. QAPF diagrams do not apply to these compositions.