In the IUGS classification diagram for mafic and ultramafic rocks shown below, choose the correct option for the rocks labelled X and Y.
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In the IUGS classification diagram, the rocks are classified based on their relative content of olivine, orthopyroxene, and clinopyroxene. Lherzolite contains a combination of all three, and Olivine Webstite is dominated by olivine with some clinopyroxene.
Step 1: Interpreting the IUGS classification diagram.
In the IUGS classification diagram, the relative positions of the rock compositions are determined by the proportion of Olivine, Orthopyroxene, and Clinopyroxene.
Lherzolite: Lherzolite is a rock made up of olivine, orthopyroxene, and clinopyroxene, typically with a higher proportion of orthopyroxene and clinopyroxene. The rock "X" in the diagram is positioned toward the olivine-orthopyroxene side, fitting the composition of Lherzolite.
Olivine Webstite: This rock is primarily composed of olivine, with some clinopyroxene. The rock "Y" is positioned near the olivine axis but has a notable presence of clinopyroxene, consistent with Olivine Webstite.
Step 2: Evaluating the options.
Option (A) is incorrect because Harzburgite typically has a high concentration of orthopyroxene, but "X" is closer to the olivine-orthopyroxene boundary.
Option (B) is correct because "X" is positioned in the region for Lherzolite and "Y" is in the region for Olivine Webstite.
Option (C) is incorrect because Dunite is predominantly olivine, and "X" contains both olivine and orthopyroxene, so it is not consistent with Dunite.
Option (D) is incorrect because Anorthosite is a plagioclase-rich rock and does not fit the diagram, and Wehrlite is not correctly positioned either.
