The Crustal Origin of Eclogite - Static or Dynamic

Abstract

Griffin and colleagues have presented an impressive wealth of new information on the crustal origin of eclogites. With reference to Backlund’s 1936-paper, in which eclogites are considered to be the products of extreme dynamical pressure or stress concentrated in layers of amphibolite, it is recommended to combine an evaluation of the dynamic factors of metamorphism with mineral-chemical data when studying massive rocks as eclogite and some types of alpine peridotites.

The eclogites of Western Norway, which were made world-famous by Eskola’s 1921-paper, have attracted renewed interest in recent years since Bryhni et al. (1970) proposed a crustal origin for these rocks.

In an impressive series of publications Griffin and collaborators have presented a wealth of new information not only about the Norwegian occurrences of eclogites but also about high pressure metamorphism and its products. Two main types of occurrence of eclogite have been described in these papers:

• 1. In the Western Gneiss Region eclogites are demonstrated to have been formed by prograde metamorphism of gabbroic or amphibolite rocks at high temperatures and pressures (750o C, 20 kbar) in a thickened crust. The enclosing Pre-cambrian gneisses are thought to have been exposed to similar high temperatures and pressures, but retrograde metamorphism has obliterated all traces of the eclogite facies mineralogy of these rocks. The eclogites are also extensively retrograded (Griffin et al., 1985).

• 2. In the granulite facies assemblage of anorthositic-noritic rocks in the Bergen Arcs (Austrheim & Griffin, 1985; Austrheim, 1987) eclogites are demonstrated to have formed in the vicinity of Caledonian shear zones in the anorthosite complex at 700–750o C, 16–19 kbar. Granulite-facies mineral assemblages are preserved a few centimetres from the shear zones which is interpreted as an indication of the importance of deformation in forming the eclogites. Tectonic “overpressure” is not considered important (Austrheim & Griffin, 1985).