Earth As an Evolving Planetary System by Condie Kent C.;

Author:Condie, Kent C.; [Condie, Kent C.]
Language: eng
Format: epub
Publisher: Elsevier Science & Technology
Published: 2021-09-20T00:00:00+00:00

Fig. 7.20 Distribution of U/Pb zircon ages in orogenic granitoids and detrital zircons for the last 4 Gyr with 1σ error envelope, where N is representative of the number of samples as a function of age that would be observed in a histogram with bins of width 30 Myr. Granitoids, red; ancient sediments, blue; modern sediments, green; all data, black. Major peak clusters labeled in Ma and supercontinent cycle with colored vertical bands. Modified after Condie and Aster (2010).

This brings up the question of how to define the duration of supercontinent assembly, and indeed whether the duration can be defined. For instance, at 1900 Ma numerous small cratons collided to make “megacratons,” but it was not until 1700–1500 Ma that the megacratons assembled into Nuna. In the case of Rodinia, numerous cratons collided at 1200–1000 Ma, but most or all of Rodinia assembly occurred between 1100 and 850 Ma (Pisarevsky et al., 2003, 2014b). Although most craton collisions related to the growth of Gondwana-Pangea occurred at 650–550 Ma, assembly of the Pangea phase continued until about 300 Ma (Cawood and Buchan, 2007). Thus, in all three of the supercontinents, just before or during the early stages of assembly, there is a peak in frequency of craton collisions. Should we count this peak as part of supercontinent assembly, or only count the final collisions of megacratons as the assembly? If we include paleomagnetic constraints (Pisarevsky et al., 2003, 2014a, b), it would appear that only the final megacraton collisions should be designated as supercontinent assembly.

Another question regarding the U/Pb zircon age patterns (Fig. 7.20) is the meaning of the age “valleys” at 2400–2200, 1400–1300, 900–650, and 185–120 Ma. These valleys, which correlate in part with supercontinent breakup, may represent times when crustal production and recycling rates were about the same. As discussed in Chapter 6, the prominent 2400–2200 Ma age gap may be an exception, if it represents a partial reversion back to stagnant lid tectonics. The Neoarchean supercratons appear to have broken up near the end of this time interval at 2200–2100 Ma (Bleeker, 2003), and the 1400–1300 Ma valley may correlate with the partial breakup of Nuna. The age valley at 900–650 Ma overlaps the assembly and breakup phases of Rodinia and perhaps the beginning of the assembly of Gondwana, and the 185–120 Ma valley seems to correlate with major phases in the breakup of Pangea.

If this interpretation of U/Pb ages of igneous and detrital zircons is correct, it implies that any planet with plate tectonics and a supercontinent cycle should exhibit episodic ages in continental crust.

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