DM

Igneous Petrology Concepts

Overview of Igneous Petrology

  • Study of melts (magmas) and crystallization process leading to solid rocks.
  • Important processes include melting, evolution of magma, crystallization, and eruption or emplacement.

Classification of Igneous Rocks

  • Chemical Petrology: Focuses on the composition and variation of igneous rocks.
  • Igneous Rock Types:
    • Obsidian
    • Rhyolite
    • Andesite
    • Basalt
    • Granite
    • Diorite
    • Gabbro
    • Peridotite
  • These terms define the composition and texture of igneous rocks.

Classification Categories

  • Intrusive vs. Extrusive rocks:
    • Intrusive: Granite, Diorite, Gabbro, Peridotite.
    • Extrusive: Rhyolite, Dacite, Andesite, Basalt.
  • Classification often depends on:
    1. Mineral content
    2. Rock texture
    3. Chemical properties

IUGS Classification Sysytem

  • Established in 1967 by Albert Streckeisen; now widely accepted for classifying igneous rocks.
  • Based largely on mineral content and texture:
    • Aphanitic: Rocks with grain sizes not visible to the naked eye, indicative of volcanic rocks.
    • Phaneritic: Grains are visible, indicating a plutonic origin.

Textural Characteristics

  • PHANERITIC:
    • Grains are seen with the naked eye, suggesting slow cooling underground (plutonic).
  • APHANITIC:
    • Grains not visible; rapid cooling at the surface (volcanic).
  • Phenocrysts: Larger crystals found in an aphanitic groundmass.

Essential Minerals for Classification

  • Necessary for naming:
    • Quartz (Q)
    • Plagioclase Feldspar (P)
    • Alkali Feldspar (A)
    • Feldspathoids
  • Examples of Essential Minerals include:
    • Plagioclase, Olivine, Augite (Cpx), and Hornblende (Amphibole).

Varietal and Accessory Minerals

  • Varietal Minerals: Occur in abundances greater than 5% and modify rock names based on their proportion.
  • Accessory Minerals: Less than 5% and not included in the rock name.

QAPF Diagram for Classification

  • Provides a visual representation for determining rock names based on mineral percentages:
    • Q = Quartz, A = Alkali Feldspar, P = Plagioclase.
  • Total proportions must add up to 100% for classification.

Mafic and Ultramafic Rocks

  • Mafic Rocks: Dominated by darker minerals, generally containing:
    • >10% Plagioclase, Pyroxene, Olivine.
  • Ultramafic Rocks: Typically >90% olivine and pyroxenes.

Classification of Volcanic Rocks

  • Volcanic rocks classified similarly but focus on visible minerals due to smaller grain size.
  • Important examples include Rhyolite, Dacite, Andesite, and Basalt.

Chemical Petrology

  • Emphasis on modern techniques for chemical analysis of rocks (e.g., wt% oxides).
  • Three categories of elements classified by abundance:
    1. Major Elements: >1 wt.% affecting mineralogy.
    2. Minor Elements: 1-0.1 wt.% participating in solid solutions.
    3. Trace Elements: <0.1 wt.% for substitution in minerals.

Variation Diagrams

  • Bivariate plots (e.g., Harker diagrams) illustrate chemical data trends among related rocks.
  • Used to represent the evolution of magmas and associated rock suites.

Summary of Rock Types and Traits

  • Felsic Rocks: High in silica and lighter colored minerals.
  • Mafic Rocks: Higher mg and fe content leading to darker colors.
  • Chemical Classification: Notably helps distinguish between magmatic types like alkaline and subalkaline.

Alkaline versus Subalkaline Rocks

  • Alkaline Rocks may occur in oceanic or continental settings.
  • Tholeiitic and Calc-Alkaline Series derived from various tectonic settings including convergent and divergent plate margins.

Final Notes

  • Understanding the IUGS classification and chemical analysis is essential for accurate identification and naming of igneous rocks.
  • Remember that rock classification is crucial for geological studies and applications varying from research to natural resource exploration.