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Tectonic evolution of the Barberton greenstone belt
The Barberton greenstone belt (BGB) is located in the Kapvaal craton of southeastern Africa. It characterizes one of the most well-preserved and oldest pieces of continental crust today by containing rocks in the Barberton Granite Greenstone Terrain (3.55–3.22 Ga). The BGB is a small, cusp-shaped succession of volcanic and sedimentary rocks, surrounded on all sides by granitoid plutons which range in age from >3547 to <3225 Ma. It is commonly known as the type locality of the ultramafic, extrusive volcanic rock, the komatiite. Greenstone belts are geologic regions generally composed of mafic to ultramafic volcanic sequences that have undergone metamorphism. These belts are associated with sedimentary rocks that occur within Archean and Proterozoic cratons between granitic bodies. Their name is derived from the green hue that comes from the metamorphic minerals associated with the mafic rocks. These regions are theorized to have formed at ancient oceanic spreading centers and island arcs.〔(【引用サイトリンク】 title=Geology:Greenstone Belts )〕 In simple terms, greenstone belts are described as metamorphosed volcanic belts. Being one of the few most well-preserved Archean portions of the crust, the BGB is well studied. It provides present geologic evidence of Earth during the Archean (pre-3.0 Ga). Despite the BGB being a well studied area, its tectonic evolution has been the cause of much debate.
==General geology of the BGB==
The BGB is contained within part of a larger system called the Barberton Granite Greenstone Terrain (BGGT) which includes two main components; the supracrustal succession, which defines the BGB portion, and the deeper-level intrusive units that surround the BGB. Major rock types found within the BGB are mafic to ultramafic volcanics, sedimentary, and shallow intrusive rocks covered by a thin sedimentary veneer. The deeper-level intrusive pluton units dome up under the greenstone belt and are divided into two major groups: the TTG group, (tonalite-trondhjemite-granodiorite) which consists of plagioclase dominant feldspar minerals and the GMS group (granite-monzonite-syenite), in which alkali feldspars are the dominant mineral composition. Pre-3.2 Ga, eruptions of mafic to ultramafic volcanics formed thick sequences. Following the formation of the thick volcanic layers was cyclic deposition of volcanic and sedimentary rocks. Then intrusions of plutonic TTG bodies began the formation of dome-and-keel structures. The volcanic layers deformed into synclines and the dome like TTG bodies created anticlines which is represented in the BGB today.
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