Greenstone belt

Greenstone belts are generally elongate, Archean to Proterozoic terrains comprising intrusive and extrusive mafic to ultramafic igneous rocks , felsic volcanics, and inter-flow or cover sedimentary rocks . Greenstone belts occur sandwiched between regions dominated by granitoids and gneiss . Greenstones are generally of low to moderate metamorphic grade. The term greenstone comes from the green color of many mafic to ultramafic constituents due to an abundance of chlorite. A common igneous rock in greenstones is komatiite. Komatiites are rocks with greater than 18 weight percent magnesium oxide and a well-developed spinifex texture of inter-locking bladed or acicular (pointed) crystals of olivine or pyroxene. Spinifex texture (named after similarities in crystal shape and pattern to the pointed spinifex grass that grows in South Africa and Western Australia ) implies rapid cooling or decompression of the magma . Komatiites formed as volcanic flows and less commonly as intrusive sills. Sedimentary sequences within greenstone belts comprise both clastic (e.g., conglomerate, quartz arenite, shale and graywacke) and chemically precipitated (e.g., banded iron formation and chert ) components. Greenstones may also be intruded by syn-to post-tectonic granitoids. Greenstone belts check host many major mineral deposits, such as gold and nickel. Greenstone belts were previously often thought to continue to large depths in the crust . Reflection seismic profiles over the Norseman Wiluna Belt of the Yilgarn Craton , Western Australia, however, indicate that this greenstone belt has a relatively shallow (3.7–5.6 mi [6–9 km]) flat-base and overlies a uniformly thick crust.

Contrasting models have been proposed for the origins of greenstone belts. Some geologists believe magmatic and tectonic processes during formation of greenstone belts in Archaean times were different to present-day plate tectonics . Earth's mantle would have then been far hotter. They cite differences between greenstone belts and Phanerozoic orogens (such as the abundance of komatiitic lavas) and point out that there are no modern analogues to greenstone belts. Opponents to Archean plate tectonics contend that greenstone belts commonly represent a laterally continuous volcano sedimentary sequence (sometimes on a granite-gneiss basement) essentially undeformed prior to late tectonism and may not therefore represent relics of volcanic chains. They consider that Archaean tectonics was dominated by mantle plumes and was possibly analogous to the tectonics of Venus. Greenstone belts are interpreted as oceanic plateaus generated by mantle plumes, similar to plume-generated oceanic plateaus in the southern Caribbean. A mantle plume origin is also proposed for neighboring tonalite-trondhjemite-granodiorite sequences.

The alternate view is that tectonic processes comparable to present-day plate tectonics were operative during the Late Archaean, and possibly were similar to plate tectonics since the Hadean-Archean transition (between 4.0 and 4.2 billion years ago). In a plate tectonic context, greenstones may have formed in volcanic arcs or inter-arc or back-arc basins. Greenstone belts are interpreted to represent collages of oceanic crust, island arcs , accretionary prisms, and possible plateaus. Recent experimental work on the origin of komatiitic magmas indicates that they were hydrous and that temperatures for their formation do not indicate that the Archean upper mantle was significantly hotter than today. Komatiites and similar rocks have also been found in younger orogens. Komatiites may not therefore require different tectonic processes or conditions for their formation, as previously thought.

In many granitoid-greenstone terrains, greenstone belts constitute synformal keels between circular to elliptical granitoid bodies. This outcrop pattern is generally thought to be due to deformation resulting solely from the greater density of greenstones compared to underlying granitoid and gneiss. Due to gravitational instability, the underlying, less dense granitoid-gneiss basement domed upwards and rose to form mushroom-shaped bodies called diapirs whilst the denser greenstones sank into the basement. Shear zones were formed along some granite-greenstone contacts due to differential vertical displacement and upright folds developed in the greenstones. This process of either solid-state and/or magmatic diapirism was independent to any tectonic processes that may have acted on margins to granite-greenstone terrains. The formation of granitoid domes in granite-greenstone terrains has also been attributed to crustal extension (producing metamorphic core complexes) or polyphase folding during regional shortening. The more linear form of some greenstone belts is due to subsequent deformation, especially the superposition of regional-scale transcurrent shear zones on early-formed structures.

See also Geologic time

greenstone belt Large geologic formation, up to 250 km across, that is largely of Archaean age. Greenstone belts are considered to represent ancient volcano—sedimentary basins bordered and intruded by granitic plutons. These formations represent an important phase of crustal evolution and currently it is commonly considered that they are remnants of back-arc basins.