Inclusions in magmatic rocks fall into two broad categories: cognate xenoliths are genetically related to the magma in which they occur and accidental xenoliths have no direct relation to their host. The latter may be derived from all levels down to the source region of the magma, and some volcanic vents that erupt explosively contain fragments derived from the entire section through which the magma has risen. It may be difficult to decide whether some coarse-grained mafic rocks are xenoliths torn from the mantle or accumulations of crystals from the magmatic liquid, because, with increasing depth, the composition of the lower crust and mantle become more like that of basic igneous rocks.
Cognate xenoliths
Cognate xenoliths have a variety of origins. Some are simply coarse-grained rocks formed by slow crystallization at depth, and their chemical composition may be very close to that of the lavas that bring them to the surface. Others are fragments of the lower crust or mantle and...
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Bibliography
Bowen, N. L., 1928, The Evolution of the Igneous Rocks. New York: Dover Publications.
McBirney, A. R., 1979, Effects of assimilation, in H. S. Yoder ed., The Evolution of the Igneous Rocks: Fiftieth Anniversary Perspectives. Princeton University Press, 307–338.
Wyllie, P. J., ed. 1967, Ultramafic and Related Rocks. New York: Wiley.
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© 1989 Van Nostrand Reinhold
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McBirney, A.R. (1989). Xenolith . In: Petrology. Encyclopedia of Earth Science. Springer, Boston, MA. https://doi.org/10.1007/0-387-30845-8_260
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DOI: https://doi.org/10.1007/0-387-30845-8_260
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