What is the Isua greenstone belt??

Question

What is the Isua greenstone belt??

Answer 1

The Isua greenstone belt is an Archean greenstone belt in southwestern Greenland dated at 3.8-3.7 Ga and contains the oldest known, well preserved, metavolcanic (metamorphosed mafic volcanic), metasedimentary and sedimentary rocks on Earth. It consists of five tectonic domains.

Almost all the rocks are deformed and substantially altered by metasomatism, however the transitional stages from the volcanic and sedimentary structures to schists can clearly be seen. The different episodes of metasomatic alterations can also be seen that produced a diversity of metamorphic mineral assemblages from similar protoliths. The metamorphic processes of garnet-growth events span both the early and late Archaean. New geological mapping studies are tracing the transitional gradations between the protoliths and their diverse deformed and metasomatised structures. These new mappings show that most of the Isua Greenstone Belt consists of fault bounded rock assemblies derived from basalt and high-magnesium basaltic pillow lava and pillow lava breccia, intruded by numerous sheets of tonalite and containing chert-banded iron formations, and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks.

It is thought that the recrystallized ultramafic bodies that occur in the belt are intrusions or komatiitic flows. Studies show that these komatiites are extremely similar to the 3.5 Ga Barberton basaltic komatiites of South Africa, and are both Archean equivalents of modern boninites produced by hydrous melting in subduction zones. The Barberton komatiites share some of the same geochemical characteristics with modern-day boninites, including petrologic evidence for high magmatic water content. The boninitic geochemical signatures provide evidence that plate tectonic processes are responsible for the creation of the belt, and that the pillow breccias and basaltic debris indicate that liquid water existed on the surface at the time of their formation. The most common sedimentary rocks are the chert banded iron formations. The 3.5 Ga Isua basalt-komatiite-chert are parental to the enclosing 2.8 Ga Amitsoq Tonalite-Trondhjemite-Granodiorite (TTG) gneiss suite. Current arguments favor a direct mantle melting to produce the diorites and high magnesian granodiorites found in these Archaean cratons. according to Rapp (1999): "A full continuum of processes can be envisioned from the generation of "pristine" TTG magmas by wet melting of garnet amphibolite/eclogite, to hybridization of these melts by assimilation of peridotite, and the consequent metasomatism of the sub-continental mantle. The Mg-number of Archean granitoids is perhaps the best indicator of TTG lineage, and the extent to which the mantle was involved in early continent formation. The evidence thus far suggests an increasingly important role in the late-Archean, with crustal, oceanic basaltic sources dominating in the early-Archean."

Quartz globules from undeformed pillow breccia are associated with a complex system of quartz veins and are interpreted as remnants of a sea-floor hydrothermal system, operating at the time of lava eruption and pillow basalt formation at 3.75 Ga. The quartz globules are thought to be interpreted as former gas vesicles filled with quartz and carbonate, and are embedded in an altered basaltic matrix now composed of biotite, muscovite and quartz. Silica filling in the vesicles is thought to have been contemporaneous with the formation of an intricate hydrothermal vein system. During deformation, the strain is thought to have been partitioned into the mica-rich rock matrix, when the vesicles behaved as competent objects, eventually cutting or deforming the thinner veins. Fluid inclusions from quartz in the vesicles resemble present-day sea-floor hydrothermal fluids and are thought to be responsible for the alteration of the pillowed breccia and coprecipitation of quartz and carbonate. Therefore, these carbonates may represent the products of an early sea-floor.

In the Southwestern Isua Greenstone Belt, kyanite has been found in the muscovite-rich schists and was formed by metasomatic reaction. This shows that this part of the belt was subjected to deformation in high strain zones and high grade metamorphism during the late Archaean. According to a study by Frei and Rosing (1999): The kyanites are Al-rich metasomatites formed by transformation of felsic gneisses by fluids derived from ultramafic schists. Peridotite and dunite bodies were transformed into talc-anthophyllite-chlorite-magnesite schists during prograde reaction with ambient fluids. During amphibolite facies metamorphism and deformation, the ultramafic schist released fluids, which interacted with neighboring lithologies. These fluids were buffered at silica activities...by the talc-anthophyllite-magnesite mineral assemblage. At this low silica activity, chlorite in the ultramafic schists buffered (aAl+)/(aH+)3 activity ratios close to that of corundum saturation. These fluids caused silica depletion, and high aluminium ion activities that intersected kyanite saturation in the quartz saturated felsic rocks. Progressive leaching in three steps defines a Pb-Pb isochron with an age of 2847 ± 26 Ma. This age is consistent with a Sm-Nd amphibole-plagioclase isochron of 2849 ± 116 Ma from mafic schists at Isua, and a lead (Pb) step leaching age of 2840 ± 49 Ma (MSWD=1.43) on magnetite from Isua banded iron formation, and with regional granulite facies metamorphism in other parts of the Greenland Archaean block.