Detail
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| DIORITE
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Typical Minerals Na/Ca PLAGIOCLASE & MAFICS (amphibole)about 50/50 Quartz - absent to trace | Description
An intermediate igneous rock from the middle of Bowen's Reaction Series. This rock indicates that superficial appearance is not always a good guide to identifying a rock since this specimen does not look much like the previous two specimens. It does not have the blank white plagioclase of the others; instead this plagioclase is grayish and translucent. There is also some pinkish orthoclase (see detail) and the mafics (amphibole) are not as abundant. The color is also non-descript, not distinctly light or dark and without the "salt and pepper" appearance, which adds confusion. Thus this specimen brings a note of caution to rock identificaiton. Ultimately, identification must be based on mineral abundances, such as indicated in the key to phaneritic rock identification. Another thing we learn from this specien is that igneous rocks do not fall into nice neat categories. Their variation ranges widely and diffusly, reflecting the complex history behind igneous rock evolution. |
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Tectonic Association
Diorite is the result of fractional melting of a mafic parent rock above a subduction zone. It is commonly produced in volcanic arcs, and in cordilleran mountain building (subduction along the edge of a continent, such as with the Andes Mountains). It emplaces in large batholiths (many thousands of square miles) and sends magma to the surface to produce composite volcanoes with andesite lavas.
 Tectonic Cross Section - pdf version
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| Last Update: 9/29/00 | e-mail: (Fichtels@jmu.edu) |