EVIDENCE FOR A LONG-LIVED STAGNANT LID DURING THE MESOPROTEROZOIC (Invited Presentation)

The Mesoproterozoic (1600-1000 Ma) is unusual in terms of both tectonic and magmatic regimes, and it partly overlaps the “Boring Billion” defined by carbon isotope anomalies in sediments and a slow pace of biological evolution. Assembly of the first supercontinent Nuna at 1.8-1.5 Ga led a large continental stagnant lid which survived for ≥ 300 myr and had profound effects on continental evolution as recorded by geological and geochemical signals in the geological record. During this time, four minor isotopic age peaks in zircons, LIPs and mantle xenoliths (1650, 1575, 1460, 1385 Ma) correspond to assembly and partial breakup of Nuna. In addition, assembly of Nuna destroyed many subduction zones. Between 1.7 and 1.3 Ga, orogen frequency dropped from 30 to 3 per 100-myr, and the remaining orogens are largely accretionary types found around the supercontinent margin. During this time, average plate speeds decreased from 40 to 25 deg/100 myr. Although growth rate of continental crust significantly decreased, Hf isotopes in both igneous and detrital zircons of this age indicate that juvenile continental crust was still produced in the marginal arcs. Only one passive margin is recognized, probably related to the breakup of an Archean supercraton at 2.1 Ga. Also tracking the decrease in orogen frequency is the small exposed surface area of rocks of this age and the lower abundances of metamorphic rocks, greenstones, granitoids, and orogen-related mineral deposits. A smaller input of radiogenic Sr into the oceans and a decrease in continental sediment production reflect low continental relief related to the decreasing numbers of Mesoproterozoic orogens. Increases in A-type granites and associated mineral deposits, massif anorthosites and granitoid heat production probably reflect melting of relatively juvenile deep continental crust or mantle lithosphere related to heating beneath the long-lived continental stagnant lid. Thelack of these signals in younger supercontinents may be due to large overlaps in assembly and breakup phases as the supercontinent cycle speeds up and the lifetime of supercontinent stagnant lids shortens.