Physical Constraints on Magma Contamination in the Continental Crust: An Example, the Adamello Complex

Igneous intrusions may move upwards through the crust by zone melting, by penetrative intrusion, by stoping or by some combination of these three mechanisms. Each mechanism offers different opportunities for contamination of the magma by country rock. Both zone melting and stoping offer the greatest possibility of assimilation, but in most natural situations the maximum amount of country rock assimilable by a magma is considerably less than its own original volume. Thermal and other constraints limit the amount of ascent that a magma body may accomplish by either zone melting or stoping: once, and twice to three times the original height of the magma body respectively. The assimilability of a xenolith sinking in a magma (i.e. the possibility of reaching the bottom without fully melting) depends on the fourth power of the radius r of the xenolith because the time required for assimilation increases as r² and the time available decreases as r² (because of increased sinking velocity). This can explain the observed size distribution of xenoliths in some intrusions. Applied to the Tertiary Adamello igneous complex of northern Italy, these considerations suggest that the intrusion may have been initiated by the emplacement of a mafic magma body in the lower crust. The body remained gravitationally stable until its composition had been modified and its density so lowered by zone melting of its roof that it began to ascend through the crust by either penetrative or stoping processes. The intrusion finally solidified at a depth of between six and ten kilometres and the last stages of emplacement occurred by stoping. The complex comprises a number of separate intrusions and this process was repeated seven or more times over a 10 Ma period (between 30 and 40 Ma). Each episode of intrusion, however, lasted less than 1 Ma.