metamorphosed 3400–3100 Ma Ameralik–Tarssartôq
swarm in the area around Godthåb (Figure 19.2) that
intrudes the 3800 Ma Amîtsoq granitic gneisses (Figure
14.38).
Thick sequences of flood basalts and local komatiite
were emplaced at about 2715 Ma on the Pilbara,
Superior, and Kaapvaal cratons and 2760–2680 Ma on
South American and Indian cratons. Because these cra-
tons don’t share earlier geologic histories and therefore
evolved as separate entities, Nelson et al. (1999) specu-
late that this episode of volcanism reflects a global-scale
convective overturn of the mantle and massive decom-
pression melting. Overlying thick accumulations of late
Archean–early Proterozoic banded iron formations in
these cratons may have resulted from related fluctua-
tions in global sea level and by enrichment of seawater
in Fe from the partly submarine lavas.
As many as nine distinct swarms of Proterozoic
basaltic dikes are evident in the southern Superior
Province (Figure 19.12). The largest of these is the
north- to northwest-striking, 2470–2450 Ma Hearst–
Matachewan swarm which intrudes an area 250 000 km
2
and represents a minimum volume of 50 000 km
3
of
basaltic magma. This swarm may well be the oldest
preserved large igneous province that was formed
above a major mantle plume (Heaman, 1997). Several
large-volume basaltic intrusions of about the same age
are known around the world (Table 12.5), including
the 2460 Ma Great Dyke of Zimbabwe, 2440–2450 Ma
layered mafic intrusions, flood basalts and dike swarms
in the Karelian craton of Finland and Russia, and
2420 Ma dike swarms in the Lewisian craton, Scotland,
in the Dharwar craton, India, in the Vestfold craton,
Antarctica, and in the Yilgarn block, Western Aus-
tralia, which also includes the Jimberlana layered
intrusion. One or more mantle plume systems, depend-
ing on how the cratons were then amalgamated, may
be represented in these widespread basaltic intrusions.
They possibly reflect a major shift in global mantle dy-
namics at 2460–2420 Ma near the transition between
the greenstone-belt dominated Archean and more
modern mafic petrotectonic associations of the Pro-
terozoic and Phanerozoic.
Another facet of the shift in global dynamics near
the Archean–Proterozoic time boundary resulted in
contrasts in the mantle lithosphere underpinning con-
tinental crusts. Using geophysical and xenolith data,
O’Reilly et al. (2001) show that Archean lithosphere
consists mostly of depleted peridotite, including lher-
zolite and large proportions of low Ca–Al, clinopyroxene-
free harzburgite, whereas Proterozoic lithosphere is
significantly less depleted and Phanerozoic still less.
The refractory, relatively buoyant, thick keel beneath
Archean cratons must have contributed to their stabil-
ity and longevity for the past 2.5 Gy, limiting their elim-
ination by gravitational forces and by partial melting.
19.8 MODELS FOR THE EVOLUTION OF
THE PRECAMBRIAN CRUST
In this last section, we return to the basic questions
posed at the beginning of the chapter regarding the
nature of the Precambrian mafic and sialic crusts of the
Earth and how they have evolved throughout geologic
time. We attempt to draw together ideas concerning
the magmatic and tectonic processes involved in this
evolution (Hoffman, 1989; Windley, 1995). Some ten-
tative conclusions can be drawn but they should only
be considered as steps toward final answers. Compar-
isons are made with modern processes of continental
growth, including arc magmatism drawing juvenile
partial melts from the mantle and accretion of oceanic
terranes.
For some geologists, the bimodal association of
granitoid and greenstone represents vestiges of ancient
felsic crust and mafic ocean basins. But other geologists
are unwilling to accept such a simple explanation or are
unwilling to believe in their formation by plate tectonic
processes as we know them today. This unwillingness
arises in part from occurrence of unique rocks and
associations during the Archean and the absence of
others found in the Phanerozoic (Figure 19.39b).
Komatiite, megacrystic anorthosite, and widespread
TTG and greenstone sequences occur in the Archean
and anorthosite suite massifs and rapakivi granite in
the Proterozoic. Alkalic rocks, ophiolite, eclogite, and
blueschist-facies rocks are absent through most of the
Archean and into the Proterozoic.
Despite the naysayers, an experienced Precambrian
geologist, Hoffman (1989), concludes that Archean
rocks are compositionally similar to those formed at
modern subduction zones, with differences attribut-
able to higher mean mantle T and possibly greater
melting of subducting oceanic crust at mantle depths.
At the risk of being dogmatic, Windley (1995, p. 409)
that “the conclusions of hundreds of international
earth scientists of every possible discipline working on
Archean rocks and theoretical models...all point in
their independent ways to broadly the same conclu-
sion, namely, that some form of modern-style plate
tectonic activity was responsible for the growth of the
continents in the Archean.” We have to find explana-
tions for the differences!
If plate tectonics had prevailed in the young Earth,
its character would likely have been different from today
because it was significantly hotter (Figure 19.39a).
Because rocks preserved in remnants of Archean
continents do not show elevated geothermal gradients
relative to young rocks, the excess heat must have been
dissipated through the oceanic crust where, today,
most internal heat is efficiently dissipated by advecting
seawater and mantle convection; the latter would have
been more vigorous than today because of the reduced
Precambrian Rock Associations
649