Xenocrystals

Xenocrystals are mineral grains that originate in a source rock different from the host rock in which they are found. In igneous petrology, a xenocrystal is a crystal incorporated into a magma or magma-derived rock from surrounding crustal or mantle material rather than crystallizing from the host melt itself. The term distinguishes these grains from crystals that grew in situ in the host rock (for example, phenocrysts) and from xenoliths, which are larger fragments of foreign rock rather than individual mineral grains.

Xenocrysts form when magma assimilates surrounding rocks as it moves through the crust or when existing rocks are dragged into an intrusion. They can survive partial dissolution or resorption and may show textures such as embayed margins or discordant zoning, indicating a history prior to the host rock's solidification. Common xenocryst minerals include quartz, feldspar (plagioclase and K-feldspar), zircon, and sometimes olivine or pyroxene, depending on the crustal or mantle source. Zircon xenocrysts are especially valuable for dating crustal components because zircons resist melting and preserve older ages.

Interpreting xenocrysts aids reconstruction of magmatic processes, crustal composition, and the timing of geological events. They provide direct evidence of crustal or mantle material interacting with magmas and can reveal the presence of ancient crustal provinces within the region associated with the host rock.

Xenocrystals should not be confused with xenoliths, which are larger rock fragments, or with antecrysts and phenocrysts, which have different crystallization histories.