Archean granitoid-greenstone terranes are one of the most
distinctive components of Archean cratons. About 70–80 percent
of the Archean crust consists of granitoid materials,
most of which are compositionally tonalites and granodiorites.
Many of these are intrusive into metamorphosed and
deformed volcanic and sedimentary rocks in greenstone belts.
Greenstone belts are generally strongly deformed and metamorphosed,
linear to irregularly shaped assemblages of volcanic
and sedimentary rocks. They derive their name from
the green-colored metamorphic minerals chlorite and amphibole,
reflecting the typical greenschist to amphibolite facies
metamorphism of these belts. Early South African workers
preferred to use the name schist belt for this assemblage of
rocks, in reference to the generally highly deformed nature of
the rocks. Volcanic rocks in greenstone belts most typically
include basalt flows, many of which show pillow structures
where they are not too intensely deformed, and lesser
amounts of ultramafic, intermediate, and felsic rocks. Ultramafic
volcanic rocks with quench-textures and high MgO
contents, known as komatiite, are much more abundant in
Archean greenstone belts than in younger orogenic belts, but
they are generally only a minor component of greenstone
belts. Some literature leads readers to believe that Archean
greenstone belts are dominated by abundant komatiites; however,
this is not true. There have been a inordinate number of
studies of komatiites in greenstone belts since they are such
an unusual and important rock type, but the number of studies
does not relate to the abundance of the rock type. Sedimentary
rocks in greenstone belts are predominantly
graywacke-shale sequences (or their metamorphic equivalents),
although conglomerates, carbonates, sandstones, and
other sedimentary rocks are found in these belts as well.
Suites of granitoid rock that are now deformed and metamorphosed
to granitic gneisses typically intrude the volcanic
and sedimentary rocks of the greenstone belts. The deformation
of the belts has in many cases obscured the original relationships
between many greenstone belts and gneiss terrains.
Most of the granitoid rocks appear to intrude the greenstones,
but in some belts older groups of granitic gneisses have been
identified. In these cases it has been important to determine
the original contact relationships between granitic gneisses
and greenstone belts, as this relates to the very uncertain tec-
tonic setting of the Archean greenstones. If contact relationships
show that the greenstone belts were deposited unconformably
over the granitoid gneisses, then it can be supposed
that greenstone belts represent a kind of continental tectonic
environment that is unique to the Archean. In contrast, if contact
relationships show that the greenstone belts were faulted
against or thrust over the granitoid gneisses, then the greenstone
belts may be allochthonous (far-traveled) and represent
closed ocean basins, island arcs, and other exotic terrains similar
to orogenic belts of younger ages.
Prior to the mid-1980s and 1990s, many geologists
believed that many if not most greenstone belts were deposited
unconformably over the granitoid gneisses, based on a few
well-preserved examples at Belingwe, Zimbabwe; Point Lake,
Canada; Yellowknife and Cameron River, Canada; Steep
Rock Lake, Canada; and in the Yilgarn, western Australia.
However, more recent mapping and structural work on these
contact relationships has revealed that all of them have largescale
thrust fault contacts between the main greenstone belt
assemblages and the granitoid gneisses, and these belts have
since been reinterpreted as allochthonous oceanic and island
arc deposits similar to those of younger mountain belts.
There seems to be an age-dependent variation in the style
of Archean greenstone belts. Belts older than 3.5 billion years
have sediments, including chert, banded iron formation, evaporites,
and stromatolitic carbonates, indicating shallow water
deposition, and contain only very rare conglomerates. They
also have more abundant komatiites than younger greenstone
belts. Younger greenstone belts seem to contain more intermediate
volcanic rocks, such as andesites, and have more deepwater
sediments and conglomerates. They also contain
banded iron formations, stromatolitic carbonates, and chert.
However, since there are so few early Archean greenstone
belts preserved, it is difficult to tell if these apparent temporal
variations represent real time differences in the style of global
tectonics, or if they are a preservational artifact.
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