Role of Assimilation and Fractional Crystallisation in Crustal Magma Transfer

Assimilation and fractional crystallization (AFC) is a ubiquitous process in mantle-derived magma emplaced into the Earth's crust. AFC is driven by the latent and specific heats of the magma, and therefore mainly takes place in large, staging chambers. This is demonstrated by a negative correlation between the An contents of plagioclase (decreases with fractional crystallization) and initial whole-rock ratios of isotopes 87 and 86 of strontium (increases with crustal assimilation) in layered mafic intrusions. AFC equations fitted to Sr and Nd isotopic data from well-exposed sections of mafic cumulates demonstrate that the ratio of the mass assimilated to the mass crystallized (the r-value) is nearly constant within each intrusion and ranges from 0.12 to 0.27. The highest degree of assimilation occurred in a magma chamber emplaced into hot and fusible metasedimentary country rocks in the middle crustal (Hasvik Layered Intrusion, Norway). Lesser degrees of assimilation took place in magma chambers emplaced into colder, upper crustal rocks or emplaced into refractory country rocks such as basaltic lavas (Fongen-Hyllingen Intrusion, Norway) and anorthosite (Bjerkreim-Sokndal Intrusion, Norway). The way in which the crust physically interacts with magma is also important. Innumerable country rock xenoliths occur in the studied intrusions; they spalled off the roofs of the chambers during filling or recharge. Calculations based on the energy balance show that up to 75 percent of the heat available was absorbed by xenoliths, promoting higher rates of cooling and crystallisation than would have resulted from loss of heat to the country rock envelope alone. The high r values reflect the large amount of heat used in heating and melting country rock within the magma chambers themselves, and their constancy reflects the ready availability of fusible xenoliths.