A phacolith is a pluton of igneous rock 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.
The term was coined and initially defined by Alfred Harker in his The Natural History of Igneous Rocks in 1909.
- in the Franklin and Hamburg areas of Sussex County, New Jersey
- the Omey pluton in Ireland
- near Bayalan, Ajmer district, Rajasthan in India
- Baker, D.R.; Buddington, A.F. (1970). Geology and Magnetite Deposits of the Franklin Quadrangle and Part of the Hamburg Quadrangle, New Jersey (USGS Professional Paper 638) (PDF). Washington D.C.: USGS. p. 30.
- McCarthy, William; Reavy, R. John; Stevenson, Carl T.; Petronis, Michael S. (2015). "Late Caledonian transpression and the structural controls on pluton construction; new insights from the Omey Pluton, western Ireland". Earth and Environmental Science Transactions of the Royal Society of Edinburgh. 106 (1): 11–28. doi:10.1017/S1755691015000201.
- Dasgupta, N.; Paljoydeep, T.; Ghosh, S. (2011). "Characteristics of pegmatoidal granite exposed near Bayalan, Ajmer district, Rajasthan". Journal of Earth System Science. 120 (4): 617–626. doi:10.1007/s12040-011-0100-7.
- 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.
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