IUGS Classification of Ultrabasic Rocks | UPSC Mains GEOLOGY-PAPER-II 2017

Discuss with the help of neat diagram the IUGS classification of phaneritic ultrabasic rocks.

How to Approach

This question requires a detailed understanding of the IUGS (International Union of Geological Sciences) classification scheme for phaneritic ultrabasic rocks. The answer should begin with defining phaneritic texture and ultrabasic rocks. Then, a clear, labelled diagram illustrating the classification based on mineralogy (olivine, pyroxene, and plagioclase content) is crucial. The explanation should cover the different rock types within the classification – peridotite, pyroxenite, and related varieties – and their characteristic mineral compositions. Focus on the key diagnostic features for identification.

Phaneritic texture, referring to rocks with visible crystals, is characteristic of slowly cooled intrusive igneous rocks. Ultrabasic rocks, also known as ultramafic rocks, are igneous rocks composed predominantly of olivine and pyroxene, with less than 45% silica content. These rocks are significant as they represent material from the Earth’s mantle and provide insights into the planet’s composition and formation. The International Union of Geological Sciences (IUGS) has established a standardized classification for phaneritic ultrabasic rocks based on their modal mineralogy, providing a framework for their systematic identification and understanding. This classification is essential for geologists studying igneous petrology and the Earth’s deep interior.

IUGS Classification of Phaneritic Ultrabasic Rocks

The IUGS classification of phaneritic ultrabasic rocks is primarily based on the relative proportions of three key minerals: olivine, pyroxene, and plagioclase. The classification utilizes a triangular diagram, with each apex representing 100% of one of these minerals. Rocks are plotted within the diagram based on their modal mineralogy, allowing for precise categorization.

Key Minerals and Their Significance

Olivine: (Mg,Fe)₂SiO₄ – A magnesium-iron silicate, typically olive-green in color. Its abundance is a primary indicator of the rock’s ultramafic nature.
Pyroxene: (Mg,Fe,Ca)₂Si₂O₆ – A group of silicate minerals, commonly dark green to black. Different pyroxene compositions (e.g., enstatite, diopside) are important for further classification.
Plagioclase: (Na,Ca)AlSi₃O₈ – A solid solution series of sodium and calcium aluminosilicates. Its presence, even in small amounts, can significantly alter the rock’s classification.

The Classification Diagram

(Note: Since I cannot directly display images, I have provided a link to a representative diagram. A hand-drawn, neatly labelled diagram would be expected in an exam setting.)

Major Rock Types and Their Characteristics

Peridotite: Dominantly composed of olivine (>40%), with varying amounts of pyroxene. Can be further subdivided based on olivine content and the type of pyroxene present. Two main types are:
- Alpine Peridotite: Rich in olivine and enstatite.
- Websterite: Contains significant amounts of olivine and diopside.
Pyroxenite: Predominantly composed of pyroxene (>90%), with minor olivine and/or plagioclase. Different pyroxene compositions lead to variations within this category.
Dunite: Almost entirely composed of olivine (>90%). Often associated with peridotite masses.
Lherzolite: A peridotite containing significant amounts of both olivine and pyroxene (typically enstatite and diopside). Considered a major constituent of the Earth’s upper mantle.
Harzburgite: A peridotite with a high olivine content and a low pyroxene content. Often found in ophiolite suites.

Influence of Accessory Minerals

While the primary classification relies on olivine, pyroxene, and plagioclase, accessory minerals like chromite, spinel, and amphibole can provide additional information about the rock’s origin and petrogenesis. For example, the presence of chromite is often associated with layered intrusions and can indicate the potential for economic mineralization.

Field Identification

Field identification of these rocks relies on careful observation of texture, color, and mineral composition. Hand lens examination is crucial for identifying the constituent minerals. The density of these rocks is generally high due to their iron and magnesium content.

Additional Resources

Key Definitions

Modal Mineralogy

Modal mineralogy refers to the actual proportions of different minerals present in a rock, determined through microscopic examination of thin sections or point counting techniques.

Ophiolite

An ophiolite is a section of oceanic crust and upper mantle that has been uplifted and exposed on land, typically during continental collision or subduction.

Key Statistics

The Earth’s mantle is estimated to be composed of approximately 84% peridotite (as of 2023).

Source: Understanding Earth's Mantle, National Geographic

Approximately 70% of the Earth’s surface is covered by oceanic crust, much of which is ultramafic in composition (as of 2022).

Source: Oceanic Crust Composition, Woods Hole Oceanographic Institution

Examples

Kalahari Manganese Field, South Africa

The Bushveld Igneous Complex in South Africa, including the Kalahari Manganese Field, contains significant deposits of ultramafic rocks like peridotite and pyroxenite, associated with large-scale magmatic intrusions and economic mineralization of platinum group elements (PGEs).

Frequently Asked Questions

▶What is the difference between peridotite and dunite?

Peridotite contains both olivine and pyroxene as major constituents, while dunite is almost entirely composed of olivine (>90%). Dunite is essentially an end-member composition within the peridotite family.