REWORKING OF CONTINENTAL CRUST DURING ULTRAHIGH-PRESSURE SUBDUCTION

The abundance of ultrahigh-pressure terranes testifies to frequent subduction of continental margins throughout the Phanerozoic. The extent to which such subduction led to recycling of continental material back into the mantle or to major reworking of the continental crust is, however, poorly constrained. One of the archetypal examples of subduction of continental crust, the Western Gneiss region of Norway, affords a prime opportunity for examining these issues. Through petrological–structural–geochronological study, we find that deformation and metamorphism of the Western Gneiss region during Caledonian subduction was locally intense, but regionally weak.

The Western Gneiss region extends 'down-plunge' ~200 km from the first appearance of eclogites in the east to the ultrahigh-pressure domains in the west; it contains almost exclusively orthogneiss. Although mafic igneous rocks are variably transformed to eclogite over this entire area, only a handful of intermediate-composition or felsic eclogite-facies outcrops are known. Large areas of Proterozoic granulites (chiefly charnockite and garnetiferous anorthosite–gabbro)—some exceeding 300 km2—show no Caledonian metamorphism or deformation. Between 50–75% of the Western Gneiss region preserves Proterozoic igneous textures and early (Proterozoic?) foreland-directed structures that underwent only minor lower amphibolite- to greenschist-facies brittle–ductile deformation during the Caledonian.

These observations imply minimal recycling of continental material into the mantle and only minor reworking of the continental crust during subduction.