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Hadean Archean 0 km Modern continental crust 0 km
10 km
hydrosphere hydrosphere sedimentary
clastic, volcanic dominated
volcanic & chem felsic crust 0 km & chem seds TTG succession
seds mafic crust Granite
mafic crust

reworked 10 km granite
Hadean schist
amphibolite 10 km amphibolite gneiss
granulite amphibolite
granulite
20 km 20 km

20 km felsic
granulite
mantle

mantle 30 km 30 km

mafic Moho
granulite 40 km

mantle

Figure 2. Schematic sections of Hadean, Archean, and modern continental crust. Hadean and Archean sections adapted from Kamber et al. (2005) and modern
crust from Hawkesworth and Kemp (2006). TTG—tonalite-trondhjemite-granodiorite.

GSA TODAY | SEPTEMBER 2016 species are marine (Mora et al., 2011), marine fossils dominate the and continental collision decrease, but the chances of the resultant
fossil record. Mineral deposits that form at or near Earth’s surface igneous rocks being preserved is high. Thus, ages from late stage
(e.g., epithermal silver-gold deposits) have a lower long-term pres- subduction- and collision-related magmatic rocks are likely to be
ervation potential than deeper deposits (orogenic gold; Wilkinson better preserved than those generated more generally above
and Kesler, 2007). subduction zones, and this results in peaks of ages coincident with
the ages of supercontinents. One implication is that the supercon-
It has similarly been argued that the peaks and troughs of crys- tinent cycle tends to bias the rock record (Cawood et al., 2013;
tallization ages that characterize the continental crust (Fig. 1) Hawkesworth et al., 2009).
reflect the better preservation of igneous rocks generated in some
tectonic settings compared to others. The peaks of ages are there- Crustal reworking is accentuated by continental collision, and
fore thought to reflect a biasing of the continental record, appar- the degree of crustal reworking has changed with time. The
ently linked to the development of supercontinents (Cawood et temporal distribution of crystallization ages of zircons with Hf
al., 2013; see also Condie et al., 2011; Hawkesworth et al., 2009, model ages greater than their crystallization ages can be used as a
2010). The implication is that they should not be taken as prima proxy for the degree of crustal reworking, and the periods of
facie evidence that in any global context the history of the conti- increased crustal reworking are those of supercontinent assembly
nental crust is marked by pulses of magmatic activity. Rather, (Dhuime et al., 2012). Similarly, there are peaks and troughs in
magmatic rocks generated in different tectonic settings have �18O values in zircons through time, and the periods of elevated
different likelihoods of being preserved over long periods. This is �18O are also those of supercontinent assembly (Fig. 1; Dhuime et
most marked in the contrast between the preservation of igneous al., 2012; Roberts and Spencer, 2014; Spencer et al., 2014). Elevated
rocks generated in continental and oceanic settings. However, �18O values indicate reworking of sedimentary material, and this
these differences in preservation are also a feature of rocks gener- is most readily achieved in sections of thickened crust in response
ated in subduction and collision-related tectonic environments in to continental collision. Thus, this is independent evidence that
the continents. the peaks of U-Pb crystallization ages are associated with periods
of crustal thickening, of continental collision, and the develop-
Along subduction zones, high volumes of magma are generated, ment of supercontinents.
but a number of studies have highlighted that the continental
crust is destroyed by erosion, subduction, and in some areas An alternative view is that peaks of ages reflect pulses of
delamination, at rates similar to, or greater than, those at which magmatic activity, and that as such, they might be associated with
new crust is generated (Clift et al., 2009; Scholl and von Huene, mantle plumes (Albarède, 1998; Arndt and Davaille, 2013;
2007, 2009; Stern, 2011). Island arcs have higher average rates of Condie, 1998; Parman, 2015; Rino et al., 2004). However, the
magma generation than Andean margins, yet island arcs are more composition of the continental crust appears to be dominated by
readily subducted and so they are even less likely to be preserved minor and trace element features that are characteristic of
than continental margins (Condie and Kröner, 2013). In contrast, subduction-related magmas (Rudnick and Gao, 2003), and even
the volumes of magma generated in the final stages of convergence for the relatively young age peaks, when the rock record is better

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