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contact metamorphism As a body of magma invades the upper portion of Earth's crust, its heat will cause recrystallization and possibly neomorphism of the surrounding, cooler rocks. Metasomatism may also occur as hydrothermal fluids circulate through the surrounding rock. The zone of contact-metamorphosed rock surrounding the body of magma is called an aureole. Many profitable mines are situated in metal-rich aureoles formed by contact metamorphism.
A view of
Vulcan Mine which was active in the 1940's. The aureole of the mine consists
of an iron zone formed by metasomatism (shown in the lower portion of the image), and a
light-colored marble zone formed by recrystallization of calcite (shown in the middle of
the image). The original, parent rock, the Bonanza King Limestone is visible
in the distance.
Another view
of Vulcan Mine showing more clearly the iron and marble zones of the aureole.
The iron removed from this open-pit mine in the early 1940's was required for industrial
and military needs during World War II. The mine was abandoned at the end of
the war due to its remote location in the Mojave Desert of southern California.
Small igneous intrusions, like these igneous dikes within the rocks of Canyon Sin Nombre,
Anza Borrego State Park, will cause only a little contact metamorphism in the rocks
directly adjacent to the intrusion.
This
closeup of one of the dikes from the image above shows that little change occurred to the
original rock that was invaded by this small amount of magma.
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