Crystalline rocks older than 1600 Ma
About half of the ice-free area of Greenland consists of Archaean and early Proterozoic crystalline basement rocks, mainly orthogneisses with some enclaves of supracrustal rocks. They belong to three distinct types of basement province: (1) Archaean rocks (3100–2600 Ma old, with local older units), almost unaffected by Proterozoic or later orogenic activity; (2) Archaean terranes reworked during the early Proterozoic around 1850 Ma ago; (3) terranes mainly composed of juvenile early Proterozoic rocks (2000–1750 Ma old). Terranes of categories (2) and (3) often contain high grade early Proterozoic.
Nearly all unreworked Archaean gneisses occur within the Archaean craton of southern Greenland . They are cut by swarms of basic dykes, most of which belong to a c. 2000–2200 Ma suite; the dykes are undeformed and non-metamorphosed and the gneisses of the Archaean craton cut by the dykes cannot have been significantly affected by early Proterozoic orogenic activity around 1850 Ma ago.
Reworked Archaean gneisses are prominent in the Nagssugtoqidian and Rinkian mobile belts that occur north of the Archaean craton in West Greenland, and in the Ammassalik mobile belt of East Greenland. Juvenile early Proterozoic gneisses and granitoid rocks (1900–1750 Ma) make up most of the Ketilidian mobile belt of South Greenland, and they a lso form a large proportion of the crystalline basement within the Caledonian fold belt of North-East Greenland; they are also present in the Inglefield mobile belt in North-West Greenland.
Simplified map showing the distribution of Archaean and early Proterozoic basment province in Greenland.
preserved Archaean (southern Greenland).
Archaean rocks reworked during the early Proterozoic (areas dominated by early Proterozoic metasediments are dotted).
Dominantly juvenile early Proterozoic rocks.
Younger formationsand ice.
indcate schematically localities where the presence of respectively, Archaean and early Proterozoic rocks have been documented in poorly known areas, and in cases where these ages in contrast to the age of the surrounding rocks.