contact metamorphism

contact metamorphism (thermal metamorphism) Probably the earliest understanding of the causes of metamorphism came from the study of contact-metamorphic aureoles, that is, metamorphic rocks that occur around an igneous intrusion, and have been produced by the transformation of pre-existing sedimentary or metamorphic rocks which still persist at a distance from the igneous contact. Most of the classic metamorphic aureoles studied in the nineteenth century are associated with granitic plutons emplaced late in the history of an orogenic belt; hence the metamorphic assemblages produced by magmatic heating are superimposed on earlier, regional metamorphic assemblages. One of the distinctive features of most contact-metamorphic rocks is that mineral growth is not accompanied by deformation. As a result, contact metamorphic minerals grow in random interlocking patterns giving rise to hornfels, a tough rock with no direction along which it will split preferentially. This texture is often found in rocks that appear to retain a pre-existing schistosity, because this can remain picked out as a fine compositional segregation, irrespective of subsequent recrystallization. One of the best-known rock types of contact metamorphism is spotted slate, a rock found in the outer parts of an aureole hosted by low-grade slaty regionally metamorphosed rocks. It has conspicuous spots, up to a few millimetres in diameter, which overprint the slaty fabric and are, or were, poikiloblasts (large inclusion-riddled crystals) of cordierite. In many instances the cordierite proves under the microscope to have been extensively altered to a fine-grained hydrous products known as pinite.

Most typical contact-metamorphic aureoles formed in the upper half of the crust, at pressures less than about 4 kbar. The effects of magma emplacement at greater depth are likely to be associated with more widespread heating of a regional character. For example, many granulite-facies terrains reflect additional magmatic heat contributions to areas of regional metamorphism. Because aureoles form at shallow depths, they are characterized by minerals stable at low pressures, typical of the hornfels facies (see metamorphism, metamorphic facies, and metamorphic rocks). Most typical are the assemblages of pelitic rocks in which cordierite is widespread but garnet generally rare, while andalusite normally occurs in place of kyanite. So strong is this association that when kyanite was first reported from the aureole of the Main Donegal Granite by W. S. Pitcher, an eminent reviewer condescended to declare that the mineral identification must have been incorrect, since it was well known that kyanite could not form during contact metamorphism.

Contact-metamorphic aureoles show marked zoning in their assemblages, and the rocks nearest the contact may have experienced sufficiently high temperatures for the onset of partial melting. This is seen in one of the best-studied modern examples, the Ballachulish aureole in Scotland. The strong variation in metamorphic grade across the zones in an aureole is indicative of conductive heating, and a number of workers have used simple models of conductive heating to estimate the duration of contact metamorphism and the pattern of heating. Simple calculations suggest that the maximum temperature in the aureole should not exceed the midpoint between the temperature of the magma and the initial temperature of the country rocks. Higher temperatures are, however, readily achieved if the pluton represents a conduit through which magma passed to an overlying volcano.

The simplicity of the classic contact-metamorphic aureole is somewhat misleading; there are a number of important aspects of contact metamorphism that remain obscure or at least controversial. One of the most elementary difficulties, alluded to above, is where to draw the distinction between regional and contact metamorphism. In areas of voluminous magmatism, low-pressure, high-temperature metamorphism commonly occurs beyond the immediate proximity of the igneous bodies, has zones that are not concentric about the pluton margins, and is typically accompanied by deformation. (north-eastern Maine is an excellent example.) This is much more a problem of classification that of understanding. It is clear that where magmatism results in extensive volcanism, as well as forming plutonic rocks, the distribution of metamorphism will reflect the total magma flux rather than the volume of pluton remaining. Furthermore, prograde metamorphism can result in weakening and superplastic behaviour of the rocks affected, as fluid pressure rises, minerals are weakened, and the grain-boundary structure is transformed by breakdown and growth. Modern work on stresses in the crust has demonstrated that large areas of apparently stable continent are in fact stressed close to the point of failure, and so deformation can be triggered as readily by weakening of the rocks as by increased stress. It is not therefore surprising that, if a sufficiently large volume of rock is weakened by thermal metamorphism, it will begin to deform in response to the ambient stress regime.

One of the most curious features of contact metamorphism is the places where it does not occur. Despite the fact that basic magmas are emplaced at higher temperatures than granitic ones, basaltic intrusions emplaced into sedimentary rocks at shallow levels commonly lack contact aureoles, or display extreme melting effects only close to the contact. It appears that at shallow levels the properties of water and the nature of the host rock to the intrusion play an important role in determining whether contact metamorphism, in the normally accepted sense, will take place. It is much easier to understand this by studying what is happening today around volcanoes where contact metamorphism might reasonably be expected to be occurring at depth. Many volcanoes have areas of geothermal activity on their flanks, and exploitation for geothermal energy means that their thermal structure has often been investigated by drilling. Frequently, temperature increases with depth in a way that is close to the boiling point-depth curve of water. A steep increase in temperature over a distance of a few hundred metres below the water-table is followed by rather uniform temperatures over a long vertical distance, typical of fluid convection. R. O. Fournier has pointed out in the context of the Yellowstone geothermal area in the USA that, when the temperature in the geothermal field is taken into account, there must be a very steep increase in temperature beneath it towards the magma body that remains at depth. These two thermal regimes correspond to an upper zone of relatively cool permeable rocks (at up to 400 °C), with extensive circulation of hot water, and a deeper zone in which the steep temperature gradient indicates conductive heat loss and is typical of a thermal aureole. This is illustrated in Fig. 1; indeed direct evidence for such a structure has been reported from deep drilling at the Larderello geothermal field in Italy.

Bruce W. D. Yardley

Bibliography

Lux, D. R.,, De Yoreo, J. J.,, Guidotti, C. V.,, and and Decker, E. R. (1986) Role of plutonism in low-pressure metamorphic belt formation. Nature, 323, 794–7.
Voll, G., Töpel, J., Pattison, D. R. M., and Seifert, F. (eds) (1991) Equilibrium and kinetics in contact metamorphism. Springer-Verlag, Berlin.