General geology of Archean cratons

Archean cratons expose two broad groups of rocks that are distinguished on the basis of their metamorphic grade: greenstone belts and high grade gneiss terrains (Windley, 1981). Both groups are intruded by large volumes of granitoids. Together these rocks form the Archean granite-greenstone belts. The structure and composition of these belts provide information on the origin of Archean crust and the evolution of the early Earth.

The greenstones consist of metavolcanic and metasedimentary rocks that exhibit a low pressure (200-500 MPa), low temperature (350-500°C) regional metamorphism of the greenschist facies. Their dark green color comes from the presence of minerals that typically occur in altered mafic (i.e. Mg- and Fe-rich) igneous rock, including chlorite, actinolite, and epidote. Three main stratigraphic groups are recognized within greenstone belts (Windley, 1981). The lower group is composed of tholeiitic and komatiitic lavas. Komatiites, named after the Komati Formation in the Barberton Greenstone belt of the Kaapvaal craton, South Africa (Viljoen & Viljoen, 1969), are varieties of Mg-rich basalt and ultramafic lava that occur almost exclusively in Archean crust. The high Mg content (>18 wt% MgO) of these rocks (Nisbet et al., 1993; Arndt et al., 1997) commonly is used to infer melting temperatures that are higher than those of modern basaltic magmas (Section 11.3.3). The central group contains intermediate and felsic volcanic rocks whose trace and rare earth elements are similar to those found in some island arc rocks. The upper group is composed of clastic sediments, such as graywackes, sandstones, conglomerates, and banded iron formations (BIFs). These latter rocks are chemical-sedimentary units consisting of iron oxide layers that alternate with chert, limestone, and silica-rich layers (see also Section 13.2.2).

High-grade gneiss terrains typically exhibit a low pressure, high temperature (>500°C) regional metamorphism of the amphibolite or granulite facies (Section 9.9). These belts form the majority of the area of Archean cratons. A variety of types commonly are displayed, including quartzofeldspathic gneiss of mostly granodiorite and tonalite composition, layered peridotite-gabbro-anorthosite or leucogabbro-anortho-site complexes, and metavolcanic amphibolites and metasediment (Windley, 1981). Peridotite (Sections 2.4.7, 2.5) is an ultramafic rock rich in olivine and pyroxene minerals. Leucogabbro refers to the unusually light color of the gabbroic rock due to the presence of pla-gioclase. Anorthosites are plutonic rocks consisting of >90% plagioclase and have no known volcanic equivalents. These latter rocks occur exclusively in Archean and Proterozoic crust. Most authors view Archean anortho-sites as having differentiated from a primitive magma, such as a basalt rich in Fe, Al and Ca elements or, possibly, a komatiite (Winter, 2001). High-grade gneiss terrains are highly deformed and may form either contemporaneously with, structurallybelow, or adjacent to the low-grade greenstone belts (Percival et al., 1997).

The granitoids that intrude the greenstones and high-grade gneisses form a compositionally distinctive group known as tonalite-trondhjemite-granodiorite, or TTG, suites (Barker & Arth, 1976). Tonalites (Section 9.8) and trondhjemites are varieties of quartz diorite that typically are deficient in potassium feldspar. These igneous suites form the most voluminous rock associations in Archean crust and represent an important step in the formation of felsic continental crust from the primordial mantle (Section 11.3.3).

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