Question

If a coarse-grained igneous rock is composed of >90% of plagioclase and <10% of olivine and pyroxene, then the name of this rock according to the IUGS classification is

• A. anorthosite
• B. olivine gabbro
• C. tonalite
• D. olivine websterite

Hint:

Remember: Rocks with >90% plagioclase → Anorthosite; with >90% pyroxene → Pyroxenite.

Answer 1

The Correct Option is A

Step 1: Understanding IUGS classification.
According to the IUGS (International Union of Geological Sciences) classification, igneous rocks are classified based on the relative proportions of essential minerals — mainly plagioclase, pyroxene, olivine, and amphibole.
Step 2: Identification.
When plagioclase constitutes more than 90% of the total minerals, and mafic minerals (olivine and pyroxene) are less than 10%, the rock is termed as anorthosite.
Step 3: Comparison.
(A) Anorthosite: >90% plagioclase — correct.
(B) Olivine gabbro: Contains significant pyroxene and olivine.
(C) Tonalite: Dominated by plagioclase and quartz — not applicable.
(D) Olivine websterite: Consists mainly of pyroxene and olivine.
Step 4: Conclusion.
Hence, the correct answer is (A) Anorthosite.