Page 369 - Global Tectonics

352 CHAPTER 11

(a) Model 1: Segregation of residue from (b) Model 2: Segregation of recycled refractory residue
an upwelling mantle plume
Oceanic plateau
Komatiite lavas

Extraction and
Primitive crust or lithosphere accumulation of
Primitive crust or lithosphere
recycled refractory
residue

Residue of
high-degree melting
(c) Model 3: Preservation of remnants of the crust of
a magma ocean
Residue of
low-degree melting
Crust
Flotation and/or
in-situ crystallization
of ol opx
Liquid interior
of magma ocean

Fig. 11.2 (a–c) Three possible mechanisms that could allow the segregation and accumulation of high-Mg olivine and
orthopyroxene near the surface of the Earth (after Arndt et al., 2002, with permission from the Geological Society of
London).

evolution of Archean lithosphere (Tomlinson & Condie, that Archean mantle roots probably resulted from more
2001; Ernst et al., 2005). Data from seismic profi les, than one tectonic environment and that no single
geochronologic studies, and isotopic analyses indicate setting or event is applicable to all cases.
that many roots were affected by large pulses of mafi c The distinctive rock associations that comprise
magmatism during the Late Archean (Wyman & granite-greenstone belts (Section 11.3.2) provide
Kerrich, 2002; James & Fouch, 2002). Other studies, another important means of evaluating the mecha-
however, have emphasized a subduction zone setting to nisms that contributed to the formation and evolution
explain the evolution of Archean mantle lithosphere. of Archean lithosphere. One of the key questions to
Most of the cratons display evidence for the signifi cant answer is whether the komatiitic and tholeiitic lavas that
modifi cation of cratonic roots by terrane collisions and form the majority of the greenstones formed in envi-
thickening during at least some stage in their history ronments that were broadly similar to modern tectonic
(James & Fouch, 2002; Schmitz et al., 2004). In support environments. For example, if these lavas loosely rep-
of a subduction zone mechanism, a Late Archean (2.8– resent the Archean equivalent of modern mid-ocean
2.6 Ga) fossil subduction zone (Fig. 11.3) has been found ridge basalts, as is commonly believed, then they might
within the Abitibi craton in northern Canada using be used to infer that much of the volcanism in Archean
seismic data (Calvert & Ludden, 1999; van der Velden times involved the creation and destruction of ocean
et al., 2006). Nevertheless, it is important to recognize crust (Arndt et al., 1997). However, one of the problems