PLUTONIC CATASTROPHES

Centuries after establishing the igneous origin of granite, interpretation of plutonic rocks still confounds geoscientists. Are plutons shaped like beer cans 10-20 km tall, or pancakes 1-2 km thick? Were they once huge volumes of interconnected melt, or composite bodies made up of a multitude of individual injections? Are their bases unseen or do they have well-defined floors? Do systematic chemical variations reflect in situdifferentiation or changes in the magma being supplied?

Many interpretations of pluton evolution involve processes that are catastrophist in nature. Early thermal models of plutons typically grew them instantaneously (e.g., a starting condition of a magma sphere 6 km in diameter, at 1200 °C, in contact with wall rocks at 200 °C). A more realistic recent model of incremental growth built an eruptible body of magma a few km thick in 600 ka. However, this model required intrusion of a 10 km-thick cylinder of magma with a sustained vertical inflation rate of 17 mm/a, a process that would cause many km of surface uplift, geologically overnight. The Skaergaard intrusion is interpreted from geochemistry as having been a mobile magma body 8 km thick, again requiring catastrophic rates of vertical inflation. Such things are not impossible but require rare events such as a large meteor strike (e.g., Sudbury).

We suggest that building plutons via slow incremental growth is consistent with uniformitarian processes as well as with geochronologic, geodetic, geomorphic, and seismic data. Vertical inflation rates of a few mm/a are consistent with geodetic measurements and solve the room problem via erosion and floor subsidence. Intrusion into releasing bends in faults is consistent with measured rates of deformation. Injecting myriad dikes to build a pluton is consistent with volcanic seismology, in which swarms of tens of thousands of earthquakes, unaccompanied by eruption, might be the signal of incremental emplacement. The problem of how to make a magma body capable of erupting ~1000 km³of magma, however, remains. Eruption of such large volumes is (fortunately) rare and not necessarily linked directly to processes that build the majority of plutons.