Page 370 - Global Tectonics

PRECAMBRIAN TECTONICS AND THE SUPERCONTINENT CYCLE 353

100 km
S W-E offset N
Abitibi greenstone domain Opatica gneiss-plutonic domain
0
4 Sh
Sh
Time (s) 12 8 Sh Moho
Sh
Moho
Moho
16 Moho 0 50
S S km
20

Fig. 11.3 Seismic reflection profile of the Opatica–Abitibi belt in the Superior Province of northern Canada (modified

from van der Velden et al., 2006, by permission of the American Geophysical Union. Copyright © 2006 American
Geophysical Union). Interpretation is modified from Calvert et al. (1995), Lacroix & Sawyer (1995), and Calvert & Ludden

(1999). S, fossil subduction zone; Sh, shingle reflections suggesting imbricated material in the middle crust.
with these comparisons is that no chemically unaltered,
complete example of Archean ocean crust is preserved.
(a) Subduction initiation
In addition, the Archean mantle was hotter by some Komatiitic crust
amount than the modern mantle (Section 11.2), which
undoubtedly influenced the compositions, source continent

depths, and patterns of the volcanism (Nisbet et al.,
1993). These problems have complicated interpreta-
tions of the processes that produced and recycled (b) Mature subduction zone
Archean crust and how they may differ from those in Calc-alkaline arc magmas
modern environments.
Most authors have concluded that the high magne- continent
sium contents and high degrees of melting associated
with the formation of komatiites reflect melting tem-

peratures (1400–1600°C) that are higher than those of
modern basaltic magmas (Nisbet et al., 1993). Exactly
how much hotter, however, is problematic. Parman
et al. (2004) proposed a subduction-related origin for (c) Subduction termination – continent collision
these rocks similar to that which produced boninites in
the Izu-Bonin-Mariana island arc (Fig. 11.4). Boninites continent
are high-Mg andesites that are thought to result from
continent
the melting of hydrous mantle in anomalously hot
forearc regions above young subduction zones (Craw-
ford et al., 1989; Falloon & Danyushevsky, 2000). If the
komatiites were produced by the melting of hydrous
mantle, then the depth of melting could have been Highly depleted mantle lithosphere
relatively shallow, as in subduction zones, and the
Archean mantle need only be slightly hotter (∼100°C) Fig. 11.4 Conceptual model for the generation of
than at present (Grove & Parman, 2004). In this inter- komatiites and cratonic mantle by partial melting in a
pretation, shallow melting and subduction result in the subduction zone (after Parman et al., 2004. Copyright ©
formation and thickening of highly depleted mantle 2004 Geological Society of South Africa). (a) Partial
lithosphere that some time later is incorporated into the melting produces komatiitic magma in a forearc setting.
cratonic mantle below a continent. (b) Mature subduction cools and hydrates residual
Alternatively, if the source rocks of komatiites were mantle. (c) Obduction of komatiitic crust occurs during
dry then high ambient temperatures in the Archean collision.