Phacolith

A phacolith is a pluton parallel to the bedding plane or foliation of folded country rock. More specifically, it is a typically lens-shaped pluton that occupies either the crest of an anticline or the trough of a syncline. In rare cases the body may extend as a sill from the crest of an anticline through the trough of an adjacent syncline, such that in cross section it has an S shape. In intensely folded terrain the hinge of folds would be areas of reduced pressure and thus potential sites for magma migration and emplacement.

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The term was coined and initially defined by Alfred Harker in his The Natural History of Igneous Rocks in 1909.

References

• Davis A. Young (2003) Mind Over Magma: The Story of Igneous Petrology, page 335, Princeton University Press. ISBN 0-691-10279-1
• American Geological Institute. Dictionary of Geological Terms. New York: Dolphin Books, 1962.