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Xenoliths, meaning 'foreign stones', arise when fragments of metamorphically altered country rock fall into magma or lava and become enveloped within igneous rock. Xenoliths can be engulfed along margins of magma chambers, torn loose from the walls of a conduit for erupting lava or an explosive diatreme, or picked up along the base of flowing lava. Xenoliths can be quite large and xenoliths can occur in clusters within the plutonic rock or demonstrate dikelet intrusions by the magma.

Xenoliths are distinguished from paleosomes, which are older bodies comprising refractory minerals that failed to melt within migmatites (in which more fusible neosomal minerals melted during Barrovian regional metamorphism).

[images : xenolith of peridotite embedded in vesicular basalt?; olivine in peridotite xenolith in alkali basalt; xenolith of partly melted metamorphic rock embedded in solidied lava; xenolith embedded in granite, Garnet Canyon, Tetons, WY; xenolith in pink granite, 2; xenolith in granodiorite, and close-up of boundary; xenolith of biotite-rich schist enclosed in Petersburg Granite; xenolith; xenolith in a basaltic sill; spinel lherzolite xenoliths, San Carlos AZ; xenolith of spinifex-textured basalt within vesicular dacite glass; altered xenolith of spinifex rock in altered and veined dacite; xenoliths in boulder; Shiprock xenolith; formations: xenolith of a quartz-biotite gneiss into which granite was intruded; xenolith in granite quarry, Elberton, GA, 2; excellent, though large bandwidth, illustrations of a xenolith in the Halifax Pluton]

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