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| gabbro
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igneous rocks
metamorphic rocks
sedimentary rocks
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Description
A mafic igneous rock from the top of Bowen's Reaction Series. Typically dark colored with a mixture of dark gray calcium plagioclase and dark colored (dark green to black; dull; not showing good cleavage) pyroxene. Although the plagioclase may be dark gray it can also be light gray (as in this specimen) or even white. What is critical here is the prominence of pyroxene from the top of Bowen's Reaction series, although it may be mixed with amphibole.
olivine is sometime present but is not essential.
Typical Minerals
Tectonic Association
gabbro is typically found on continents in dikes, sills, and laccoliths from slow cooling of mafic magma, mostly derived from hot spots. gabbro is also part of the oceanic lithosphere (ophiolite suite) forming as a thick layer below the top-most pillow lavas across all the oceanic floors.
Another Sample
Description
A mafic igneous rock from the top of Bowen's Reaction Series. This specimen appears darker than the last one and is closer to what we expect in a color/texture classification, a coarse-grained dark-colored rock. And
although there is more dark gray plagioclase here, there is still a lot of light colored plagioclase too. What is crucial, of course, is the prominence of pyroxene as the mafic mineral. The next specimen is darker yet and more what we expect a gabbro to look like.
Description
A mafic igneous rock from the top of Bowen's Reaction Series. In a color/texture classification this is more what we expect a gabbro to look like, dark in color with both dark pyroxene and mostly dark gray calcium plagioclase (although some white plagiolclase is also present).
Detail
Description
In this detail we see the mi xed pyroxene and light
gray calcium-rich plagioclase.
In a color/texture classification we expect gabbro to be a dark rock, but what is crucial is the presence of pyroxene, see n here as black, irregular fractured masses.
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Thursday, November 11, 2021