Xenolith

Gabbroicxenolith in granite in Rock Creek Canyon, easternSierra Nevada,California

Olivineweatheringtoiddingsitewithin amantlexenolith

Axenolith( "foreign rock" ) is arockfragment (country rock) that becomes enveloped in a larger rock during the latter's development and solidification. Ingeology,the termxenolithis almost exclusively used to describeinclusionsinigneous rockentrained duringmagmaascent, emplacement and eruption.Xenoliths may be engulfed along the margins of amagma chamber,torn loose from the walls of an eruptinglavaconduit or explosivediatremeor picked up along the base of a flowing body of lava on the Earth's surface. Axenocrystis an individual foreign crystal included within an igneous body. Examples of xenocrysts arequartzcrystals in asilica-deficient lava anddiamondswithinkimberlitediatremes. Xenoliths can be non-uniform within individual locations, even in areas which are spatially limited, e.g.rhyolite-dominated lava ofNiijimavolcano (Japan) contains two types ofgabbroicxenoliths which are of different origin - they were formed in different temperature and pressure conditions.

Although the term xenolith is most commonly associated with inclusions in igneous rocks,a broad definition could also include rock fragments which have become encased insedimentary rock.Xenoliths have been found in somemeteorites.

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 anautolithor a cognate inclusion.

Xenoliths and xenocrysts provide important information about the composition of the otherwise inaccessiblemantle.Basalts,kimberlites,lamproitesandlamprophyres,which have their source in theupper mantle,often contain fragments and crystals assumed to be a part of the originating mantle mineralogy. Xenoliths ofdunite,peridotiteandspinellherzoliteinbasalticlava flows are one example. Kimberlites contain, in addition to diamond xenocrysts, fragments oflherzolitesof varying composition. Thealuminium-bearing minerals of these fragments provide clues to the depth of origin. Calcicplagioclaseis stable to a depth of 25 km (16 mi). Between 25 km (16 mi) and about 60 km (37 mi),spinelis the stable aluminium phase. At depths greater than about 60 km, densegarnetbecomes the aluminium-bearing mineral. Some kimberlites contain xenoliths ofeclogite,which is considered to be the high-pressuremetamorphicproduct of basalticoceanic crust,as it descends into the mantle alongsubduction zones.

The large-scale inclusion of foreign rock strata at the margins of an igneous intrusion is called aroof pendant.

Examples

• 
  Xenoliths ingranodioriteof the AltaStock,Little Cottonwood Canyon,Utah
• 
  Large xenolith ofsandstone(probably from the Albee Formation) within the FairleePlutoninVermont
• 
  Peridotite (green) mantle xenolith within a (dark)volcanic bombfromVulkaneifel,Germany (coin of one euro for scale)
• 
  Rounded, yellow, weatheredperidotitexenolith in anephelinitelava flow atKaiserstuhl,SW Germany
• 
  Large xenolith of BaltimoreGneissin theGuilford Quartz Monzonitein a wall of the old Waltersville Quarry,Granite,Maryland (about 1895)
• 
  Lamprophyrewith xenolith at Ontario, Canada.
• 
  Xenolith in granite nearDonner Pass,California (foot for scale).

Sources

• Nixon, Peter H. (1987).Mantle Xenoliths.J. Wiley & Sons.ISBN0-471-91209-3.