The Itsaq Gneiss Complex in southwest Greenland comprises the most extensive and best-preserved fragment of early Archean continental crust.
Southwestern Greenland's Isua Greenstone Belt is an Archean greenstone belt that dates from 3.8-3.7 Ga. The belt's five tectonic domains contain the best preserved, metavolcanic, metasedimentary and sedimentary rocks on Earth.
Most of the Isua Greenstone Belt comprises fault-bounded rock assemblies derived from basaltic, high-Mg basaltic pillow lavas, and pillow lava breccias, intruded by numerous sheets of tonalite, chert-banded iron formations, and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks.
The recrystallized ultramafic bodies within the belt are thought to be intrusions or komatiitic flows, and these komatiites are very similar to the 3.5 Ga Barberton basaltic komatiites of South Africa. Both are Archean equivalents of modern boninites that are produced by hydrous melting in subduction zones. The boninitic geochemical signatures provide evidence that plate tectonic processes were responsible for the creation of the belt, while the pillow breccias and basaltic debris indicate that liquid water existed on the surface at the time of their formation.
Chert banded iron formations comprise the commonest sedimentary rocks, and the 3.5 Ga Isua basalt-komatiite-chert was parental to the enclosing 2.8 Ga Amitsoq Tonalite-Trondhjemite-Granodiorite (TTG) gneisses. It is considered that direct mantle melting produced the diorites and high magnesian granodiorites found in these Archaean cratons.
Quartz globules in undeformed pillow breccias are associated with a complex system of quartz veins that probably represent remnants of a sea-floor hydrothermal system that was contemporaneous with lava eruption and pillow basalt formation (3.75 Ga).
In the Southwestern Isua Greenstone Belt, kyanite in muscovite-rich schists developed when the region was subjected to deformation in high strain zones during the late Archaean. In the 2.7 Ga Manjeri Formation in the Belingwe Greenstone Belt, oxide and sulphide facies ironstones indicate a complex bacteria/archaea eclogical community.
On the opposite coast of Greenland from the Isua Greenstone Belt, the Eocene Skaergaard intrusion lies in contact with Archean gneisses. Geological features of the 55 Ma layered intrusion are the subject of excellent photographs on Kurt Hollocher's webpage and a Vanishing Ice, and view Greenland's Thinning Icecap.)
Maps : Greenland: 1:2 500 000 geological map; 1: 500 000 geological maps : geological maps of Greenland; main periods of crust formation and orogeny;
links: images: formations: Isua: aerial of Isua, satellite image of Isua (large), spectacular 20 000 nT anomaly of the Isua banded iron formation (magnetic total field); thrust-nappes in tonalite mylonite in the western footwall to the Isua Greenstone Belt; cross-cutting felsic dikes in the "Central gneisses", Isua Greenstone Belt, contact between a mafic Tarssartoq dike and tonalitic gneiss, Isua, Isua, oldest conglomerate, ancient sedimentary, oldest sedimentary, oldest pillow basalt, flattened pillow basalt, pillow lava, oldest ophiolite, oldest BIF, den båndende jernmalm ved Isua (BIF), BIF contact with ultramafic, felsic gneisses, rock types of the Isua, finely banded and intensely folded calc-silicate rocks (thought to have formed by carbonate metasomatism of predominantly basaltic protoliths), magnetite-tremolite, hand-specimen BIF, thin-section of graphite-bearing garnet in the ~3.8 Ga metasediment from the Isua Greenstone Belt; non-Isua: strongly metamorphosed Precambrian rocks, near Kangerlussuaq (West Greenland); deformed granites, gneisses and migmatites, Nordvestfjord; granitic vein, Liverpool Land Grundvigskirken - a needle-like mountain peak of hard, Caledonian cristalline rocks (granite, gneiss) in the Øfjord, shaped by ice-age glaciers; close-up: strongly deformed and metamorphosed rocks, Øfjord, inner Scoresbysund; gneiss, Nanortalik; webpages: The Precambrian shield; The Caledonian fold belt; Old Red and New Red; Basalts of the Blosseville Küste; World's 'oldest' volcanic rocks (Canada); Oldest evidence of photosynthesis
Labels: Amitsoq TTG, Archean Eon, Belingwe, boninites, hydrous melting, Isua Greenstone Belt, Itsaq Gneiss Complex, komatiites, Manjeri Formation, metasedimentary, metavolcanic, pillow lavas, subduction zones