New ideas on the formation of granite plutons in the crust

New ideas on granite formation

Large granitic plutonic bodies are common in the upper part of the continental crust and it is generally believed that the magma from which these bodies originate is formed much lower down, probably at the base of the crust. Most geology textbooks describe the mechanism of granite pluton emplacement in terms of the very slow (millions of years) ascent of a large bodies (diapirs) of magma. In the lower – relatively ductile - part of the crust this is envisaged as a process of deformation of the surrounding rock. The upper – more brittle – crust is not expected to behave in this way, and processes such as stoping (breaking off of chunks of country rock) and melting have been considered to be applicable.

Ideas of granite emplacement have changed in the past few years, and some of the new theories are summarized in a recent Nature review article by geologists from Britain, Canada and France (Petford et al., 2000).

The suggested scenario for formation of upper-crustal granitic bodies can be summarized as follows:

Magma is formed by partial melting in the lowermost crust, resulting from upwelling of hot mantle material. In many cases this would occur above a subduction zone. It is estimated that a granitic melt layer two-thirds as thick as a 950º C mafic intrusion could be produced in as little as 200 years.

The magma is then segregated from the source rock and ascends along a fracture or other conduit, over time periods as short as years to hundreds of years.

The magma is emplaced in the upper crust (within kilometres of surface) by lateral spread along planes of weakness, and then by vertical thickening. The time scale is hundreds to thousands of years.

The Dinkey Creek pluton of the Sierra Nevada of California provides an example of how the mechanism might work.

As shown on the cross-section A-B, Dinkey Creek is a tabular shaped pluton which has been fed from below by a narrow dyke-like structure. The pluton has expanded outward from its source dyke along the interface between a previous plutonic body (in blue) and a series of overlying metamorphosed sedimentary rocks (in green).

The magnetic foliation trajectories (black lines) show the process of magma emplacement. It is implied, therefore, that the magma is ascending in pulses, and that each new pulse pushes pre-existing still molten magma farther away from the source dyke. The continued intrusion of magma also leads to a thickening of the pluton, either as a result of uplifting of the roof rock, or sagging of the floor.

If these ideas are correct, then emplacement of large granitic plutons need not takes tens of millions of years. Instead it may be a rapid and dynamic process which takes place on time scales of less than 100,000 years.

Petford, N., Cruden, A., McCaffrey, K and Vigneresse, J-L., Granite magma formation, transport and emplacement in the Earth's crust, Nature, V. 408, p. 669-673, December 2000.