Page 371 - Global Tectonics

354 CHAPTER 11

(a) Modern
CFB OIB 2000
0 MORB Estimated
Cont. crust Mantle Melting Plume
Cont. crust
Cont. crust
40 Conditions origin
Depth (km) 150 Lithosphere Asthenosphere 1800
Plumes Temperature (°C) 1600
Depth of melting
CFB>OIB>MORB 1400 Boninite Subduction
origin
(b) Archean Archean tholeiites Mature
arc
0 MORB 1200
Mid-ocean Subduction Modern Archean
Cont. crust
Cont. crust
Cont. crust
40 ridge zone plume komatiite
Depth (km) 150 Lithosphere Fig. 11.6 The range of mantle melt generation
temperatures estimated for various modern tectonic
Asthenosphere
settings compared to temperatures inferred for komatiite
Plumes
melt generation by a plume model (black filled oval) and

a subduction model (gray filled oval) (after Grove &

Parman, 2004, with permission from Elsevier).
Fig. 11.5 Model of komatiitic and tholeiitic basalt
formation involving mantle plumes (after Arndt et al.,
1997, by permission of Oxford University Press). Model A variety of tectonic models also have been postu-

shows the influence of lithospheric thickness on depth of lated for the origin of Archean continental crust.
melting where CFB is continental flood basalt, OIB Windley (1981) noted the geologic and geochemical

oceanic island basalt, and MORB mid-ocean ridge similarities between Archean tonalite-trondhjemite-
basalt.
granodiorite (TTG) suites and exhumed granitoids
associated with Andean-type subduction zones (Section
9.8). He considered this to be an environment in which
mantle would have caused melting to begin at depths voluminous quantities of tonalite can be produced, and
that were much greater than occurs in subduction concluded that this represents a reasonable analogue for
zones, possibly in upwelling mantle plumes or at unusu- the formation of these rocks in Archean times. Subse-
ally deep levels (∼200 km) beneath mid-ocean ridges. quent work has led to a general consensus that these
The greater depths of melting would produce large subduction models are applicable to the Late Archean.
volumes of basalt and oceanic crust that was much However, their applicability to Early and Middle
thicker (20–40 km) than it is today (Bickle et al., 1994). Archean times when thick oceanic crust may have inhib-
Evidence of large volumes of mafic magma and high ited subduction is more controversial. As an alternative

eruption rates have suggested that oceanic plateaux and to subduction, Zegers & van Keken (2001) postulated

continental flood basalts are the best modern analogues that TTG suites formed by the removal and sinking of

for such thick mafic crust and invites comparisons with the dense, lower part of thick oceanic plateaux. The
Phanerozoic LIPs (Section 7.4.1) (Arndt et al., 1997, peeling away, or delamination, of a dense eclogite root
2001). In this latter context, the differences between the results in uplift, extension, and partial melting to
modern and ancient rocks are explained by variations produce TTG suite magmas. This process could have
in the depth of melting and in the effects of a thick returned some oceanic material into the mantle and
overriding lithosphere (Fig. 11.5). These and a variety may have accompanied collisions among oceanic ter-
of other models (Fig. 11.6) illustrate how information ranes in Early–Middle Archean times. However, the
on the depth and source of the melting that produced possible absence of subduction creates a problem
komatiites has important consequences for both the in that, assuming a nonexpanding Earth (Section 12.3),
tectonic setting and the thermal evolution of the early a high rate of formation of oceanic lithosphere
Earth. during these times must have been accompanied