high-grade gneiss terrain

high-grade gneiss terrain ‘High-grade gneiss terrain’ is the name traditionally given to one of the two main types of Archaean crust, the other being the ‘granite–greenstone terrain’. High-grade gneiss terrains are characterized by metamorphic rocks of amphibolite to granulite facies and dominated by gneissose igneous material with a wide range of composition, but containing a large proportion of granodiorites and tonalites. Gneisses of sedimentary and volcanic origin have also been recognized in these terrains but these generally form only a minor component. These rocks are typically highly deformed, which accounts for their gneissose appearance, and ductile shear zones ranging from centimetres to many tens of kilometres across are characteristic.

Although it was once thought that high-grade gneiss terrains represented a fundamentally different kind of Archaean crust from the granite–greenstone type, it is now realized that the two kinds of terrain represent only different levels of exposure. In the Superior province of Canada, for example, which contains perhaps the best-known examples, it has been shown that high-grade gneiss-type crust underlies the granite–greenstone terrain at depths of around 10 km, and is in several places upthrust to the surface along what are interpreted as major collisional suture zones. These high-grade rocks are indistinguishable from the high-grade gneiss terrains exposed, for example, in the nearby Godthaab craton of south Greenland, long considered to be a type example of the high-grade gneiss terrain.

R. G. Park