Page 380 - Global Tectonics

PRECAMBRIAN TECTONICS AND THE SUPERCONTINENT CYCLE 363

dikes that fan out over a 100° arc and extend for more resistance of the cratons to large-scale lithospheric recy-
than 2300 km (Ernst et al., 2001). Some of these shield cling processes. The results of seismic and petrologic
regions also contain huge sills and layered intrusions of studies (Sections 11.3.1, 11.3.3) and numerical modeling

mafic and ultramafic rock that occupy hundreds to (Lenardic et al., 2000; King, 2005) all suggest that com-
thousands of square kilometers. These intrusions positional buoyancy and a highly viscous cratonic
provide information on the deep plumbing systems of mantle explain why the cratons have been preserved for
Precambrian magma chambers and on crust–mantle billions of years. These properties, and isolation from
interactions. Three of the best known examples are the the deeper convecting mantle, have allowed the mantle
∼1.27 Ga Muskox intrusion in northern Canada (Le lithosphere to maintain its mechanical integrity and to
Cheminant & Heaman, 1989; Stewart & DePaolo, resist large-scale subduction, delamination and/or
1996), the ∼2.0 Ga Bushveld complex in South Africa erosion from below. Phanerozoic tectonic processes
(Hall, 1932; Eales & Cawthorn, 1996), and the ∼2.7 Ga have resulted in some recycling of continental litho-
Stillwater complex in Montana, USA (Raedeke & sphere (e.g. Sections 10.2.4, 10.4.5, 10.6.2), however the
McCallum, 1984; McCallum, 1996). Unlike the layered scale of this process relative to the size of the cratons
igneous suites of the Archean high-grade gneiss terrains is small.

(Section 11.3.2), these intrusions are virtually The cores of the first continents appear to have

undeformed. reached a sufficient size and thickness to resist being
Anorthosite massifs (Section 11.3.2) emplaced during returned back into the mantle by subduction or delam-
Proterozoic times also differ from the Archean exam- ination some 3 billion years ago. Collerson & Kamber
ples. Proterozoic anorthosites are associated with gran- (1999) used measurements of Nb/Th and Nb/U ratios
ites and contain less plagioclase than the Archean to infer the net production rate of continental crust
anorthosites (Wiebe, 1992). These rocks form part of since 3.8 Ga. This method exploits differences in the
an association known as anorthosite-mangerite- behavior of these elements during the partial melting
charnockite-granite (AMCG) suites. Charnockites are and chemical depletion of the mantle. The different
high temperature, nearly anhydrous rocks that can be ratios potentially provide information on the extent of
of either igneous or high-grade metamorphic origin the chemical depletion and the amount of continental
(Winter, 2001). The source of magma and the setting crust that was present on Earth at different times. This
of the anorthosites are controversial. Most studies inter- work and the results of isotopic age determinations
pret them as having crystallized either from mantle- (Fig. 11.13) suggest that crust production was episodic
derived melts that were contaminated by continental with rapid net growth at 2.7, 1.9, and 1.2 Ga and slower
crust (Musacchio & Mooney, 2002) or as primary melts growth afterward (Condie, 2000; Rino et al., 2004). Each
derived from the lower continental crust (Schiellerup of these pulses may have been short, lasting ≤100 Ma
et al., 2000). Current evidence favors the former model.
Some authors also have suggested that these rocks were
emplaced in rifts or backarc environments following
periods of orogenesis, others have argued that they are
closely related to the orogenic process (Rivers, 1997). 15 Stage 3 Stage 2 2.7 Ga Stage 1
Their emplacement represents an important mecha-
nism of Proterozoic continental growth and crustal 10 1.9 Ga
Frequency ( % ) 1.2 Ga
recycling.

11.4.2 Continental growth 5
and craton stabilization 0
0.2 0.6 1.0 1.4 1.8 2.2 2.6 3.0 3.4 3.8
Age ( Ga )
Many of the geologic features that comprise Protero-
zoic belts (Section 11.4.1) indicate that the continental Fig. 11.13 Plot showing the distribution of U-Pb zircon
lithosphere achieved widespread tectonic stability ages in continental crust (after Condie, 1998, with
during this Eon. Tectonic stability refers to the general permission from Elsevier).