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Axenolith ("foreign rock") is arock fragment (country rock) that becomes enveloped in a larger rock during the latter's development and solidification. Ingeology, the termxenolith is almost exclusively used to describeinclusions inigneous rock entrained duringmagma ascent, emplacement and eruption.[1] Xenoliths may be engulfed along the margins of amagma chamber, torn loose from the walls of an eruptinglava conduit or explosivediatreme or picked up along the base of a flowing body of lava on the Earth's surface. Axenocryst is an individual foreign crystal included within an igneous body. Examples of xenocrysts arequartz crystals in asilica-deficient lava anddiamonds withinkimberlite diatremes. Xenoliths can be non-uniform within individual locations, even in areas which are spatially limited, e.g.rhyolite-dominated lava ofNiijima volcano (Japan) contains two types ofgabbroic xenoliths which are of different origin - they were formed in different temperature and pressure conditions.[2]
Although the term xenolith is most commonly associated with inclusions in igneous rocks,[3] a broad definition could also include rock fragments which have become encased insedimentary rock.[4][5] Xenoliths have been found in somemeteorites.[6]
To be considered a true xenolith, the included rock must be identifiably different from the rock in which it is enveloped; an included rock of similar type is called anautolith or a cognate inclusion.
Xenoliths and xenocrysts provide important information about the composition of the otherwise inaccessiblemantle.Basalts,kimberlites,lamproites andlamprophyres, which have their source in theupper mantle, often contain fragments and crystals assumed to be a part of the originating mantle mineralogy. Xenoliths ofdunite,peridotite andspinellherzolite inbasaltic lava flows are one example. Kimberlites contain, in addition to diamond xenocrysts, fragments oflherzolites of varying composition. Thealuminium-bearing minerals of these fragments provide clues to the depth of origin. Calcicplagioclase is stable to a depth of 25 km (16 mi). Between 25 km (16 mi) and about 60 km (37 mi),spinel is the stable aluminium phase. At depths greater than about 60 km, densegarnet becomes the aluminium-bearing mineral. Some kimberlites contain xenoliths ofeclogite, which is considered to be the high-pressuremetamorphic product of basalticoceanic crust, as it descends into the mantle alongsubduction zones.[7]
The large-scale inclusion of foreign rock strata at the margins of an igneous intrusion is called aroof pendant.
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