Endogenous and Exogenous Plutons: The Influence of Emplacement Style on Contamination of Granitic Magma

Whether granitic magmas can detach and digest fragments of wallrock or mingle with other magmas depends in large part on the style of magma emplacement. Many composite plutons grow by relatively small pulses of magma, some of which may reach their solidus before injection of the next pulse. Composite “nested ” plutons may grow as new pulses of magma invade and displace their predecessors, or “sheeted ” plutons may grow as new magma follows contacts between wallrock and older intrusions. Contrary to a common assumption, the innermost parts of a composite intrusion are not necessarily the last to be emplaced or the most fractionated. Emplacement kinematics of some plutons and lava domes are similar. Lava domes grow in two ways. Endogenous domes infl ate as new lava ascends and ponds within a cooler lava crust, and exogenous domes accrete as new lava ascends, breaches the lava crust, and spreads over the exterior. Exogenous growth of sheeted plutons offers less opportunity for magma mingling and mixing, but allows more wallrock assimilation (contamination) than endogenous growth of nested plutons.