Some aspects of contact metamorphism are more constrained than regional metamorphism. The heat source is evident and metamorphic temperatures increases toward the intrusion. Pressure is not likely to change much across a contact aurole, because of the limited distance scale. Deformation is less likely to have modified the simple distribution of temperature and pressure. Equlibrium relationships among minerals can be used and tested against thermal models of the setting. Complicating factors include complicated intrusion histories, reaction kinetics in the shorter time scales of contact metamorphism, and the H2O-CO2 fluid circulation that is driven by the heat of the intrusion and the fluid released from a cooling magma.
Intrusion of the Bregaglia (Bergell) tonalite into rocks of the Malenco serpentinite in the Oligocene produced a contact aureole in Switzerland. Prior to the intrusion, the ultramafic rocks underwent regional metamorphism as part of the Alpine orogeny and were still warm (~350°C, Trommsdorff and Connolly, 1996) at the time of the intrusion. Six of the samples in this activity were collected from the Bergell aureole. The other two samples (one from the Alps, one from New England) have assemblages like rocks that occur in the aureole. From this activity you should take away the following concepts:
- Contact aureoles provide a good opportunity to study metamorphic processes in a setting where there are constraints on the metamorphic conditions.
- Mineral assemblages, not minerals alone, are needed to constrain the possible equilibrium conditions represented by a metamorphic rock.
- The equilibrium mineral assemblage observed in a metamorphic rock depends not only on the temperature and pressure, but also on the bulk composition of the rock.
- Chemical reactions that may have occured during metamorphism can be deduced from the mineral assemblages of rock samples represented on composition diagrams.
The Bergell aureole has been studied by a number of petrologists including Trommsdorff and Evans, 1972, 1977; Ferry, 1995; Trommsdorff and Connolly, 1996. Examine one or more of these papers to learn more about the Bergell aureole and ways that petrologists have studied it. Acknowledgements
This activity follows closely a lab assignment designed by Jack Cheney for his Amherst College petrology classes. Jack assembled the samples and prepared thin sections from rock donations by Peter Crowley, Charlie Guidotti, and Peter Robinson. Clara Brill-Carlot photographed the thin sections and rocks loaned by Jack Cheney. Thanks, Jack, for recognizing the teaching value of these rocks, and for sharing them.
