Page 381 - Global Tectonics

364 CHAPTER 11

(Condie, 2000). On the basis of available data, Condie PRIMITIVE MANTLE
4
(=ASTHENOSPHERE)
(2005b) concluded that 39% of the continental crust E. China
formed in the Archean, 31% in the Early Proterozoic, YOUNGER Vitim +
12% in the Middle–Late Proterozoic, and 18% in the Phanerozoic + + Zabargad
3 Spinel
Phanerozoic. Peridotite PHANEROZOIC
Two of the most important mechanisms of Late European +
Archean and Early Proterozoic continental growth and % CaO 2 Massifs + PROTEROZOIC
cratonic root evolution were magma addition (Section + S.Australia + Xenolith
9.8) and terrane accretion (Section 10.6). Several authors Kaapvaal averages
(e.g. Condie, 1998; Wyman & Kerrich, 2002) have sug- 1 + Gnt-SCLM
ARCHEAN Archean
gested that the ascent of buoyant mafic material in Proterozoic

mantle plumes may have initiated crust formation and Phanerozoic
0

may have either initiated or modified root formation 0 1 2 3 4 5
during periods of rapid net growth (Section 11.3.3). % Al 2 O 3
Schmitz et al. (2004) linked the formation and stabiliza-
Fig. 11.14 CaO-Al 2 O 3 plot showing the range of
tion of the Archean Kaapvaal craton in South Africa to
subcontinental lithospheric mantle (SCLM) compositions
subduction, arc magmatism, and terrane accretion at
for selected cratons that have been matched with ages
2.9 Ga. In this and most of the other cratons, isotopic
of the youngest tectonothermal events in the overlying
ages from mantle xenoliths and various crustal assem- crust (after O’Reilly et al., 2001, with permission from the
blages indicate that chemical depletion in the mantle Geological Society of America). Compositions have
lithosphere was coupled to accretionary processes in been calculated from garnet xenocrysts (Gnt). Xenolith
the overlying crust (Pearson et al., 2002). This broad averages shown for comparison. Plot shows that newly
correspondence is strong evidence that the crust and formed subcontinental lithospheric mantle has become
the underlying lithospheric mantle formed more or less progressively less depleted in Al and Ca contents from
contemporaneously and have remained mechanically Archean through Proterozoic and Phanerozoic time.
coupled since at least the Late Archean. A progressive Garnet peridotite xenoliths from young extensional
decrease in the degree of depletion in the lithospheric areas (e.g. eastern China, Vitim in the Baikal region of
mantle since the Archean (Fig. 11.14) indicates that the Russia, and Zabargad Island in the Red Sea) are
Archean–Proterozoic boundary represents a major shift geochemically similar to primitive mantle, indicating
in the nature of lithosphere-forming processes, with very low degrees of melt depletion.
more gradual changes occurring during the Phanero-
zoic (O’Reilly et al., 2001). The most obvious driving
mechanism of this change is the secular cooling of the derived from laboratory measurements (Lenardic et al.,
Earth (Section 11.2). In addition, processes related to 2003). These issues, and the extent to which the cratonic

subduction, collision, terrane accretion, and magma mantle interacts with and influences the pattern of
addition also helped to form and stabilize the con- mantle convection, presently are unsolved. Improved
tinental lithosphere. resolution of the structure, age and geochemical evolu-
Whereas these and many other investigations have tion of the continental crust and lithospheric mantle
identified some of the processes that contributed to the promise to help geoscientists resolve these problems in

formation and stabilization of Archean cratons, numer- the future.
ous questions still remain. Reconciling the composition
of craton roots determined from petrologic studies
with the results of seismic velocity studies is problem- 11.4.3 Proterozoic
atic (King, 2005). There are many uncertainties about
how stability can be achieved for billions of years plate tectonics
without suffering mechanical erosion and recycling in
the presence of subduction and mantle convection. Early tectonic models of Proterozoic lithosphere envis-
Another problem is that the strength of mantle materi- aged that the Archean cratons were subdivided by
als required to stabilize craton roots in numerical exper- mobile belts in which deformation was wholly ensialic,
iments exceeds the strength estimates of these materials with no rock associations that could be equated with