Assimilation of xenocrysts in granitic magmas: Principles, processes, proxies, and problems

Country-rock materials, either as intact xenoliths or as disaggregated xenocrysts, are inevitable contaminants in granitic magmas. Xenocrysts approach chemical equilibrium with the granitic magma in several ways. Two pre-assimilation reactions may change the nature of the material being assimilated: redox, involving ferromagnesian silicates. oxides, pyrobitumen, and sulfides in the country rocks, and thermal decomposition, such as the breakdown of calcite to lime and carbon dioxide if T exceeds 825 degrees C. Three assimilation reactions then eliminate the foreign material as a physical entity: 1) melting, either nominal or system-dependent, and either congruent or incongruent, depending on the individual phase, 2) dissolution, if the magma temperature is lower than the system-dependent solidus of the xenocryst and if the foreign phase is either temporarily or permanently under-saturated in the silicate melt, and this dissolution may also be congruent, if the mineral dissolves directly in the granitic melt, or incongruent. if the mineral dissolves indirectly in the granitic melt, leaving an insoluble new mineral, or 3) ion exchange, if the magmas temperature is lower than the system-dependent solidus of the xenocryst, and if the foreign phase is already present as a primary phase in the granitic magma, and also if the xenocryst is a more refractory member of the solid-solution series. Complete assimilation-type reactions leave chemical evidence of contamination, but no original physical evidence. Incomplete assimilation-type reactions leave partially reacted xenocrysts. with diagnostic sizes, shapes, reaction rims, inclusions, and compositions. As the partly reacted xenocrysts approach equilibrium with the granitic melt, their recognition as xenocrysts becomes difficult. Physical (modal abundances) and chemical (whole-rock major, trace, and isotopic composition) estimates of the amounts of contamination in granites are rarely in agreement. Glass-making and alloy-making are excellent industrial analogues for the contamination-assimilation process in granites.