THERMAL EFFECT PRODUCED BY MAFIC-ULTRAMAFIC MAGMA INTRUSION: AN EXAMPLE FROM SIERRA DE SAN LUIS, CENTRAL ARGENTINA

Traditionally, the intrusion of hot mafic-ultramafic magma in the crust was thought to be the cause of the increase of metamorphic grade from amphibolite to granulite facies conditions. However, it was demonstrated in some areas that this linkage does not necessarily exist. A case study is found in the Sierra de San Luis, Central Argentina, where a mafic-ultramafic layered set of rocks (Las Aguilas Group) occur as a belt of small (maximum width and length: 1 and 3 km), discontinuous lenses. They intrude a polydeformed crystalline basement, and are thought to be the cause of the local increase of metamorphic grade from amphibolite to granulite facies conditions.

The thermal effect of the emplacement of mafic sills and dikes on the host rocks was evaluated by applying a simple analytical solution (error function) for heating of a semi-infinite half space (the country rocks) in contact with a hotter sheet of finite thickness (the dike). This solution was specified by Carslaw and Jaeger (1959), and subsequently simplified by Philpotts (1990).

Results indicate that beyond a few hundred meters from the contact zone, temperatures never exceed 600 ºC, and a few km from the intrusion they barely increase by about 50 ºC relative to the initial temperature. These results agree with previous studies of the thermal effects of intrusion of hot mafic magmas, according to which the effect of the intrusion is local. Even large bodies of mafic rocks such as the Mafic Complex of the Ivrea zone (Southern Alps, Northern Italy) produce only a local metamorphic aureole of ~2km. This is in part because they are relatively dry intrusions, and fluid circulation is restricted to a small area around the mafic bodies.

These results, together with the preservation of primary igneous characteristics (such as rhythmic layering) being overprinted by metamorphic textural changes, indicate that the intrusion occurred before regional deformation (surely in extensional conditions). It could have caused a contact aureole in the host rock, but it could not be responsible for the local metamorphic increase from amphibolite to granulite facies conditions.

Carslaw H.S. and J.C.Jaeger, 1959. Conduction of heat in solids. Oxford University Press, New York, 510pp.

Philpotts, A.R., 1990. Principles of igneous and metamorphic petrology. Prentice Hall, New Jersey, 498pp.