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.