PROTEROZOIC SUPERCONTINENTS, OROGENS, AND GIANT DYKE SWARMS: NEW INSIGHTS FROM ZIRCON AGE DISTRIBUTIONS

Both detrital zircon and subduction or collision-related granitoid zircon age spectra suggest that the supercontinent Nuna began to form at 2.2-2.0 Ga (overlapping breakup of Archean supercratons) with craton collisions principally in Africa and South America; most growth, however, occurred between 1.9 and 1.8 Ga. The final collisions were at 1.6 - 1.4 Ga between cratons in Australia, Africa (Kibaran) and SW Laurentia. Between the assemblies of Nuna (2.2-1.65 Ga) and Rodinia (1.2-1.0 Ga) there is a major gap in zircon ages from collisional orogens (1.7-1.3 Ga), which is filled with ages from accretionary orogens. This suggests that as subduction zones were lost during Nuna assembly, convergent plate margins shifted to the perimeter of the growing supercontinent. There is no evidence for global events recorded in the granitoid or detrital zircon ages from accretionary orogens between Nuna and Rodinia assemblies, but peak locations and heights are dictated by sample frequency from local geographic regions. There is also no evidence for global episodicity of events in either granitoid or detrital age spectra during this 400-My time interval.

Surprisingly, giant dyke swarms have an age distribution very much like that of orogenic granitoids and detrital zircons with major peak clusters at 2.2-2.0 Ga and 1.9-1.8 Ga corresponding to the assembly of Nuna, and clusters at 1.2-1.0 Ga corresponding to the assembly of Rodinia. These peak clusters are apparent in histograms of both age frequency and cumulative dyke swarm area. These results indicate that the age distribution of giant dyke swarms may not always be reliable indicators of supercontinent breakup. For the most part, dyke swarms occur in Archean cratons adjacent to collisional and accretionary orogens, and indicate that compressional tectonic regimes at craton margins were accompanied by extensional tectonic regimes in craton interiors. No maximum in LIP or giant dyke swarm ages is recorded at 1.3-1.2 Ga or at any other time between assemblies of Nuna and Rodinia. A minimum in dyke swarm ages is confirmed at 2.4-2.2 Ga, corresponding to the crustal age gap in granitoids, and implies a link in the mantle between subducted slabs and mantle plume generation.