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Xenoliths (foreign rocks in Ancient Greek) are fragments of various rocks entrained in (and, in some cases, picked up and transported by) magma. Reaction of xenoliths with their host magma causes changes on both sides of the magma/rock interface: recrystallization and metasomatism of primary mineral assemblage in the xenoliths, and chemical changes in the magma due to assimilation of xenolithic material. In a general case, xenoliths and their host rocks are not genetically linked (i.e. related by birth). If such a linkage is suspected, the term autolith should be used instead. Xenoliths in mantle-derived rocks (such as kimberlites) are an invaluable (and, in some cases, the only available) source of information about the makeup of the Earth's upper mantle and lower crust. The studies of xenoliths also help us understand how magmas are emplaced and crystallize. Shown here are:
[Upper right] Xenoliths of upper-mantle peridotite (yellowish green) in basanite from the San Carlos volcanic field, Arizona. These fragments of mantle rock were transported to the Earth's surface from a depth of 50-60 km! The San Carlos xenoliths are the most important source of gem-quality olivine (peridot) in North America. This piece was donated by Tracy Paul.
[Below] Xenolith of Proterozoic crustal rock (charnockite) in a dike of alkali-ultramafic rock (greenish black), Turiy Mys (Terskiy Coast, Kola, Russia).
[Lower right] Giant xenoliths of metamorphosed sedimentary rock (hornfels) and alkaline igneous rock (the one I am pointing at) in nepheline syenite, Mont Saint-Hilaire (Québec, Canada). Note reaction rims encircling some of the xenoliths.