PERSPECTIVES ON CONTINENTAL EVOLUTION FROM XENOLITHS AND GEOPHYSICAL DATA

For some time my thinking about the evolution of the continental cratons has been based on the idea that the chemistry and temperature of the deep (sublithospheric) tectosphere are mutually regulated by the dynamic requirement of isopycnic balance. The applicability of isopicnicity to the current state of the continents can be tested through combinations of petrologic data from mantle xenoliths, seismological imaging, and gravimetric measurements. The available information supports the hypothesis, at least as a first approximation, but its extrapolation backward in time places severe constraints on continental formation and evolution that have not yet been reconciled with salient aspects of geologic history. The transition sometime in the early to mid Proterozoic from thick to relatively thin tectosphere can plausibly be explained by the sequestration of mantle peridotites with Mg# > 92 that were depleted prior to 4 Ga, but the long history of intracratonic volcanism, as evidenced by pervasive dyke swarms and episodes of basaltic flooding, remains a puzzle. I will therefore explore the possibility that the magmatic stabilization of the cratons proceeded during an extended interval following their tectonic stabilization by supercontinent aggregation and advective thickening. According to this line of thinking, remnants of fertile, formerly convecting upper mantle trapped within the accreting cratons conductively cooled, subsided to form sedimentary basins, and eventually became unstable in basin inversion events that produced large volumes of basaltic magma by decompression melting. The resulting depletion led to the stabilization of these tectospheric “flaws”, although their existence remains evident as regions of lower Mg numbers and higher temperatures (and thus lower seismic velocities) in near-isopycnic balance with the surrounding tectosphere. Data from southern Africa supporting this speculative model will be discussed.