Page 367 - Global Tectonics

350 CHAPTER 11

In addition to being cool and strong, studies of named after the Komati Formation in the Barberton
mantle xenoliths indicate that the Archean mantle roots Greenstone belt of the Kaapvaal craton, South Africa
are chemically buoyant and highly depleted in incom- (Viljoen & Viljoen, 1969), are varieties of Mg-rich basalt

patible elements (O’Reilly et al., 2001; Pearson et al., and ultramafic lava that occur almost exclusively in
2002). When mantle melting occurs elements such as Archean crust. The high Mg content (>18 wt% MgO) of
calcium, aluminum and certain radiogenic elements are these rocks (Nisbet et al., 1993; Arndt et al., 1997) com-
concentrated into and extracted by the melt whereas monly is used to infer melting temperatures that are
other elements, particularly magnesium, selectively higher than those of modern basaltic magmas (Section
remain behind in the solid residue. Those elements that 11.3.3). The central group contains intermediate and
concentrate into the melt are known as incompatible felsic volcanic rocks whose trace and rare earth ele-
(Section 2.4.1). Both buoyancy and chemical depletion ments are similar to those found in some island arc
are achieved simultaneously by partial melting and melt rocks. The upper group is composed of clastic sedi-
extraction, which, in the case of the mantle lithosphere, ments, such as graywackes, sandstones, conglomerates,
has left behind a residue composed of Mg-rich harzbur- and banded iron formations (BIFs). These latter rocks
gites, lherzolites, and peridotite (O’Reilly et al., 2001). are chemical-sedimentary units consisting of iron oxide
Eclogite also appears to be present in the cratonic litho- layers that alternate with chert, limestone, and silica-
sphere. However, high velocity bodies consistent with rich layers (see also Section 13.2.2).
large, dense masses of eclogite have not been observed High-grade gneiss terrains typically exhibit a low
in the continental mantle (James et al., 2001; Gao et al., pressure, high temperature (>500°C) regional metamor-
2002). An inventory of mantle xenoliths from the phism of the amphibolite or granulite facies (Section
Kaapvaal craton suggests that eclogite reaches abun- 9.9). These belts form the majority of the area of
dances of only 1% by volume in the continental mantle Archean cratons. A variety of types commonly are
(Schulze, 1989). These characteristics have resulted in displayed, including quartzofeldspathic gneiss of mostly
the mechanical and thermal stability of the cratons for granodiorite and tonalite composition, layered
up to three billion years (Section 11.4.2). peridotite-gabbro-anorthosite or leucogabbro-anortho-
site complexes, and metavolcanic amphibolites and
metasediment (Windley, 1981). Peridotite (Sections

11.3.2 General geology of 2.4.7, 2.5) is an ultramafic rock rich in olivine and pyrox-
ene minerals. Leucogabbro refers to the unusually light
Archean cratons color of the gabbroic rock due to the presence of pla-
gioclase. Anorthosites are plutonic rocks consisting of
Archean cratons expose two broad groups of rocks that >90% plagioclase and have no known volcanic equiva-
are distinguished on the basis of their metamorphic lents. These latter rocks occur exclusively in Archean and
grade: greenstone belts and high grade gneiss terrains Proterozoic crust. Most authors view Archean anortho-
(Windley, 1981). Both groups are intruded by large sites as having differentiated from a primitive magma,
volumes of granitoids. Together these rocks form the such as a basalt rich in Fe, Al and Ca elements or, pos-
Archean granite-greenstone belts. The structure and com- sibly, a komatiite (Winter, 2001). High-grade gneiss ter-
position of these belts provide information on the rains are highly deformed and may form either
origin of Archean crust and the evolution of the early contemporaneously with, structurally below, or adjacent
Earth. to the low-grade greenstone belts (Percival et al., 1997).
The greenstones consist of metavolcanic and The granitoids that intrude the greenstones and
metasedimentary rocks that exhibit a low pressure high-grade gneisses form a compositionally distinctive
(200–500 MPa), low temperature (350–500°C) regional group known as tonalite-trondhjemite-granodiorite, or
metamorphism of the greenschist facies. Their dark TTG, suites (Barker & Arth, 1976). Tonalites (Section
green color comes from the presence of minerals that 9.8) and trondhjemites are varieties of quartz diorite

typically occur in altered mafic (i.e. Mg- and Fe-rich) that typically are deficient in potassium feldspar. These

igneous rock, including chlorite, actinolite, and epidote. igneous suites form the most voluminous rock associa-
Three main stratigraphic groups are recognized within tions in Archean crust and represent an important step
greenstone belts (Windley, 1981). The lower group is in the formation of felsic continental crust from the
composed of tholeiitic and komatiitic lavas. Komatiites, primordial mantle (Section 11.3.3).