Precambrian Earth and Life History - The Changing Earth: Exploring Geology and Evolution - page 496

◗ Figure 19.4 Origin of Granitic Continental Crust

Sea level

Island arc Island arc

Oceanic
lithosphere

Asthenosphere

a An andesitic island arc forms by subduction of oceanic lithosphere and partial melting of basaltic
oceanic crust. Partial melting of andesite yields granitic magma.

(a)

(b)

b The island arc in a collides with a previously formed island arc, thereby forming a continental core.

c The process occurs again when the island arc in b collides with the evolving continent, thereby
forming a craton, the nucleus of a continent.

(c)

A greenstone belt has three main components; its lower
and middle parts are mostly volcanic rocks, whereas the
upper rocks are mostly sedimentary (Figure 19.6a). The ori-
gin of the mineral chlorite during low-grade metamorphism
gives the rocks their greenish color, hence the name green-
stone . Greenstone belts have a syncline-like structure and
measure 40 to 250 km wide and 120 to 800 km long. In ad-
dition, they have been intruded by granitic magma and com-
plexly folded and cut by thrust faults.
Much of the volcanism responsible for the lower and
middle units in greenstone belts must have been subaqueous
because pillow lavas are common (Figure 19.6b), but some
large volcanic centers built above sea level. Some of the most
interesting volcanic rocks are ultramafi c lava fl ows. We noted
previously that eruptions producing these fl ows are rare in
rocks younger than Archean because Earth's radiogenic heat
production has decreased and near-surface temperatures are

not high enough for ultramafi c magma to reach the surface
(see Chapter 4).
Sedimentary rocks are a minor component in the lower
parts of greenstone belts, but they become increasingly
abundant toward the top (Figure 19.6a). Associations of
graywacke, a sandstone with abundant clay, quartz, feldspars,
and rock fragments, and argillite (slightly metamorphosed
mudrock) are particularly common. Small-scale graded
bedding and cross-bedding indicate that deposition by
turbidity currents accounts for the graywacke-argillite succes-
sions. Some sedimentary rocks were deposited in delta, tidal
fl at, barrier island, and shallow marine shelf environments.
A variety of other sedimentary rocks are also present
in Archean greenstone belts, including conglomerate, car-
bonates, and chert. Banded iron formations (BIFs) are also
found in Archean greenstone belts, but BIFs are much more
common in areas of Proterozoic rocks.