AN ICONOCLASTIC VIEW OF PLUTONS: WHY BIG FIERCE MAGMA CHAMBERS ARE RARE

Plutons are generally assumed to represent frozen magma chambers that were once largely molten and behaved as viscous liquids. However, several lines of evidence indicate that some plutons accumulated over time scales an order of magnitude longer than required for a large magma chamber to cool. We suggest that many plutons were never large tanks of magma but instead accumulated incrementally over millions of years by addition of small batches.

Several lines of evidence suggest that the “big-tank” concept of magma chambers may not apply generally. First, field examination of many plutons indicates that they are composite products of numerous injections, accumulated in a vertical stack, as steep dikes, or irregularly. Second, precise U-Pb geochronology demonstrates that some plutons, such as the Half Dome pluton of Yosemite National Park, accumulated over time spans an order of magnitude longer (1-3 m.y.) than “big tank” cooling models allow (ca. 200 ka). Such plutons must have accumulated as separate pulses that solidified between injections. Third, geophysical studies have consistently failed to locate significant accumulations of largely molten magma. Even in magmatically active areas, geophysical data are typically consistent with only 5-20 vol% liquid in “magma chambers”. Although pluton-sized volumes of magma must exist at least ephemerally in the Earth’s crust to produce sporadic ignimbrite eruptions with volumes of hundreds or thousands of cubic km, such large bodies of highly molten magma are probably uncommon and short-lived.

We therefore suggest that plutons that formed by solidification of large bodies of highly molten magma (>50%) may be uncommon, and that many or most plutons represent piecemeal accumulation of magma over time scales of 1 to 10 m.y. If this hypothesis is correct, then it has several significant implications for how magmatic systems evolve. In particular, (1) pluton-scale geochemical variation cannot reflect in situ crystal fractionation or magma mixing; (2) commonly cited emplacement mechanisms such as diapirism, ballooning, and stoping cannot work as envisaged; (3) strain rates during growth of a pluton may be an order of magnitude slower than rapid-emplacement scenarios require.