Anukruti posted an Question
January 04, 2021 • 13:47 pm 30 points
  • IIT JAM
  • Geology (GG)

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    Rahul kumar

    Basalt can't be an answer because according to IUGS classification of volcanic igneous rocks, Basalt won't contain any quartz. Basalt will always contain feldspathoids. You can also note in the classification Diagram that basalt is in the lower half (towards the feldspathoid side) of the diagram so it shouldn't contain any quartz. However in the question data, quartz is present (2%), so it will never be a basalt according to IUGS classification.

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    Rahul kumar Best Answer

    Answer: Andesite Given, Qtz= 2%, Plg= 60%, Orthoclase= 1%, Olivine=6%, Opx=5% , Cpx= 18% , Others= 8% Here, Q+A+P> 10%, so according to the IUGS classification, we will use QAPF Diagram of volcanic rocks. For IUGS classification of volcanic rocks, we have to Normalise Q (Quartz), A (Alkali feldspar), P (plagioclase) to 100% [We will ignore other minerals such as olivine, Opx, Cpx etc. because Q+A+P>10%]. After normalisation, the modal percentage of Q,A and P are; Q= (2/63) × 100 = 3.17% A= (1/63) × 100 = 1.56% P= (60/63) × 100 = 95.24% On IUGS classification Diagram of volcanic igneous rocks (QAPF Diagram), this modal composition will lie in the zone of Andesite. So, according to the IUGS classification, this rock should be classified as Andesite.

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