Xenocrysts

Xenocrysts are crystals embedded in an igneous rock that did not crystallize from the melt of that rock. They originate as foreign crystals carried into the magma from surrounding pre-existing rocks (country rocks) or from earlier magmas that interacted with the molten material during ascent or emplacement. In the course of crystallization, xenocrysts may be preserved as isolated grains or as part of the crystallizing assemblage, and they can retain textural or chemical signatures of their origin. Xenocrysts are distinct from xenoliths, which are larger fragments of foreign rock enclosed in an igneous body, whereas xenocrysts are individual crystals within the host.

During magma ascent, fragments or crystals from wall rocks or earlier magmas can be assimilated; some survive, while others dissolve and are overgrown by the host mineralogy. The set of xenocrysts in a rock depends on the crustal composition and magmatic history. Common xenocrysts include zircon, garnet, quartz, and certain feldspars, though their presence depends on the source rocks involved.

The study of xenocrysts yields information about crustal contamination, magma mixing, and the thermal history of magmatic systems. Dating xenocrystic zircons by U-Pb provides minimum ages for the crustal components and constrains magma emplacement times. Petrographic and geochemical analyses, including electron microprobe and isotopic methods, help distinguish xenocrysts from autocrystic crystals formed in situ.

Xenocrysts are a key tool in petrology for reconstructing magmatic processes and crustal architecture, especially in granitoids and volcanic rocks where assimilation during ascent is likely.