Page 372 - Global Tectonics

PRECAMBRIAN TECTONICS AND THE SUPERCONTINENT CYCLE 355

by a mechanism by which oceanic lithosphere also was melts but produced basaltic melts instead. As the oceanic
destroyed at high rates. crust cooled and became thinner, a point was reached
An important aspect of the origin of TTG suites is in the Late Archean when the entire crust could subduct
the type of source material that melted to produce the (Fig. 11.7b). At this time hydrothermally altered crust,
magma. Early petrologic studies suggested that these such as amphibolite, was introduced into subduction
magmas could result from the partial melting of sub- zones and led to the widespread formation of TTG
ducted oceanic crust in the presence of water (Martin, suites. This model supports the view that the formation
1986). However, more recent work has emphasized of the earliest continental crust requires subduction and
other sources, including the lower crust of arcs and the the melting of a hydrous mafi c source.
base of thick oceanic plateaux (Smithies, 2000; Condie,
2005a). The importance of the source material is illus-
trated by a two-stage model proposed by Foley et al.
(2003). This model envisages that during the Early 11.3.4 Crustal structure
Archean, oceanic crust was too thick to be subducted
as a unit, and so its lowermost parts delaminated and Granite-greenstone belts display a variety of structural
melted (Fig. 11.7a). These lower roots are inferred to styles and outcrop patterns, many of which also occur
have been pyroxenites that were produced by the meta- in Phanerozoic orogens (Kusky & Vearncombe, 1997).

morphism of ultramafic cumulate layers. The melting Those common to both Archean and Phanerozoic belts
of the pyroxenite did not favor the generation of TTG include large tracts of metamorphosed igneous and

(a) Oceanic crust section
Basalt 3
Hydrothermally altered 2
4 km Granulite/
eclogite
Gabbro
1
20 km Delamination:
Ultramafic Melts of pyroxenites
cumulates are basalt/nephelinite
28 km

TTG TTG
(b)
Oceanic crust section
Basalt
Hydrothermally altered
4 km
3
Gabbro
13 km
Subduction:
Ultramafic cumulates
Melts of garnet
18 km amphibolite are TTG
Fig. 11.7 Two-stage model of Archean crustal evolution (after Foley et al., 2003, with permission from Nature 421,
249–52. Copyright © 2003 Macmillan Publishers Ltd). (a) Early Archean delamination of thick oceanic crust and the
melting of pyroxenites (1). Local melting of lower crust (2) and garnet amphibolite (3) may also occur to produce small
volumes of felsic magma. (b) Late Archean whole-crust subduction and the large-scale melting of garnet amphibolite
to produce TTG suites (3).