TECTONIC HISTORY OF RODINIA: CONSTRAINTS FROM STRONTIUM AND CARBON ISOTOPES AND CONTINENTAL CRUST PRODUCTION RATES

Possible fragmentation of a Paleoproterozoic supercontinent between 1.6 and 1.4 Ga correlates with only a weak decrease in the 87Sr/86Sr ratio of seawater (0.706 to 0.705), little or no change in d 13C in marine carbonates (d 13C~0-1), and possibly a minor increase in the production rate of continental crust from 1 to 1.5 km3/y, as monitored by U-Pb zircon ages and Nd isotopic data. Formation of Rodinia between 1.3 and 1.0 Ga does not affect seawater Sr isotope ratios until about 1 Ga during terminal Grenvillian collisions, when 87Sr/86Sr rises rapidly to about 0.707. d 13C shows a small increase during this time interval (+1 to +3.5), and continental crust production increases slightly but remains very low (0.5-1 km3/y). The breakup of Rodinia (800-600 Ma) is recorded by a minimum in seawater 87Sr/86Sr ratio at 850-750 Ma (0.7055) followed by a puzzling increase at 700 Ma. During this breakup, d 13C rises (+2 to +8) and crustal production rate increases (from 1 to 2 km3/y). Formation of Gondwana at 650-550 Ma is accompanied by a rapid rise in seawater 87Sr/86Sr ratio (0.707 to 0.710), a drop in d 13C (+8 to +4), but little change in crustal production rate.

Long-term variation in Sr and C isotopes in seawater, and to a smaller extent, continental crust production rate, may be controlled in part by the supercontinent cycle, at least between 1.6 and 0.5 Ga. Unlike 1.9 and 2.7-Ga supercontinents, the absence of a major increase in continental crust production rate during formation of Rodinia at 1.2-1.0 Ga may be due to the absence of a mantle superplume event at this time. The fact that Sr and C isotopes and crustal production rate show only a weak imprint of supercontinent fragmentation at 1.6-1.4 suggests that the Paleoproterozoic supercontinent underwent only minor breakup at this time. This observation, which is consistent with the ages of 2.0-1.0-Ga sutures, suggests that Rodinia formed from only a few relatively large cratons.