Classification of ultramafic rocks
Ultramafic rocks are a vast family of rocks consisting predominantly of mafic minerals (olivine, pyroxene, and amphibole) with very limited contents of felsic minerals (generally calcium plagioclase) and other accessories (garnet, spinel, phlogopite, etc.). These rocks comprise peridotites, constituting the Earth’s mantle, and various ultramafic cumulates that form by segregation of mafic minerals within peridotites or by settling at the base of magma chambers, generally of gabbroid rocks. The QAPF classification diagram considers ultramafic any rock with a modal content of mafic minerals (color index M) > 90%. Therefore, their classification requires specific ternary diagrams based on the proportion of olivine – orthopyroxene – clinopyroxene (Ol – Opx – Cpx), and olivine – pyroxene – hornblende (Ol – Px – Hbl) [see also: how to use a ternary plot]. Let’s consider just the Ol – Opx – Cpx diagram for the moment.
Most ultramafic rocks consist of the three minerals in this diagram: olivine, ferromagnesian pyroxene (orthopyroxene, enstatite), and calcium pyroxene (clinopyroxene, diopside). For ultramafic rocks containing hornblende (extremely rare) the diagram below should be used instead. Two broad fields are separated based on the modal content of olivine: peridotites (olivine > 40%) and pyroxenites (olivine < 40%). The peridotite field is characterized by four major rock types:
Dunite: consisting mostly of olivine (> 90%).
Harzburgite: consisting predominantly of olivine and orthopyroxene.
Lherzolite: a peridotite with olivine, orthopyroxene, and clinopyroxene.
Wehrlite: consisting largely of olivine and clinopyroxene.
Pyroxenites are further divided in:
Orthopyroxenite: with orthopyroxene > 90%.
Websterite: consisting of clinopyroxene and orthopyroxene.
Clinopyroxenite: with clinopyroxene > 90%.
‘Olivine’ is specified in the name in pyroxenite varieties with an olivine content between 5 and 40% (e.g. olivine websterite).
This diagram defines very well the first-order chemical variability of ultramafic rocks. SiO2-poor compositions lie close to the top vertex (dunite), whereas SiO2-rich compositions occur at the base of the diagram. This is because pyroxenes stoichiometrically contain more silica than olivine. Similarly, Ca-rich compositions lie towards the right of the diagram, since clinopyroxene is the only mineral in the diagram that contains Ca.
Amphibole-bearing ultramafic rocks
Ultramafic rocks containing hornblende, a type of calcium amphibole, are very rare on Earth, but they are found in some mafic and ultramafic complexes as a result of igneous processes that are currently poorly understood. These rocks can be classified using the olivine – pyroxene – hornblende (Ol – Px – Hbl) ternary diagram, where Px comprises both ortho- and clinopyroxene.
This classification diagram defines three broad fields: peridotites (olivine > 40%), pyroxenites (olivine < 40%, pyroxene > 50% of the pyroxene/hornblende ratio), and hornblendites (olivine < 40%, hornblende > 50% of the hornblende/pyroxene ratio). Hornblendite, strictly speaking, is an igneous rock containing more than 90% of hornblende. When hornblende < 90%, the presence of other minerals must be specified in the rock name (e.g. olivine pyroxene hornblendite). Similarly, the presence of hornblende must be specified in any other ultramafic rock name according to the diagram (e.g. hornblende peridotite).
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Streckeisen, A. (1976). To each plutonic rock its proper name. Earth-science reviews, 12(1), 1-33.