High-grade granitoid gneiss terranes form the second main
type of Archean terrane. Examples include the Limpopo belt
of southern Africa, the Lewisian of the North Atlantic
Province, the Hengshan of North China, and some less-welldocumented
belts in Siberia and Antarctica. The high-grade
gneiss assemblage seems similar in many ways to the lower-
grade greenstone belts, but more strongly deformed and
metamorphosed, reflecting burial to 12.5–25 miles (20–40
km) depth. Strongly deformed mylonitic gneisses and partially
melted rocks known as migmatites are common, reflecting
the high degrees of deformation and metamorphism. Most of
the rocks in the high-grade gneiss terranes are metamorphosed
sedimentary rocks including sandstones, graywackes,
and carbonates, as well as layers of volcanic rocks. Many are
thought to be strongly deformed continental margin
sequences with greenstone-type assemblages thrust over
them, deformed during continent-continent collisions. Most
high-grade gneiss terranes have been intruded by several generations
of mafic dikes, reflecting crustal extension. These are
typically deformed into boudins (thin layers in the gneiss),
making them difficult to recognize. Some high-grade gneiss
terranes also have large layered mafic/ultramafic intrusions,
some of which are related to the mafic dike swarms.
The strong deformation and metamorphism in the
Archean high-grade gneiss terranes indicates that they have
been in continental crust that has been thickened to double
crustal thicknesses of about 50 miles (80 km), and some even
more. This scale of crustal thickening is typically associated
with continental collisions and/or thickened plateaus related
to Andean style magmatism. High-grade gneiss terrains are
therefore typically thought to represent continent-continent
collision zones.
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