NUNA TO RODINIA: PRESERVATION AND RECYCLING OF CONTINENTAL CRUST

Proterozoic collisional orogens are of two types: those with ocean basin closing durations < 500 My (mean = 125 My) and those with durations ≥ 500 My (mean = 720 My), the latter with lengthy accretionary orogen stages. Zircon age peaks at 2100-1900 and 1200-1000 Ma correlate with craton collisions leading to the assembly of Nuna and Rodinia, respectively, with a time interval between collisions mostly < 50 My (range 0-250 My). Most rocks preserved in orogens date to the pre-collisional, ocean-basin closing stage and the most widely preserved tectonic setting is the continental arc (10-90 %, mean = 60%), with oceanic tectonic settings (oceanic crust, arcs, islands and plateaus, serpentinites, pelagic sediments) comprising < 20% and mostly < 10% (mean = 8%). Reworked crustal components are variable, comprising 20-80% (mean = 32%). Nd and Hf isotopic data indicate that Proterozoic orogens contain 10-60% of juvenile crust (mean = 37%) and 40-75% of reworked crust (mean = 63%). Neither the fraction nor the rate of preservation of juvenile crust, which reaches a maximum of about 50%, is related to collisional age nor to duration of ocean-basin closing.

Minima occur in zircon ages of rocks preserved in the Great Proterozoic Accretionary Orogen (GPAO) at 1600-1500 Ma (Laurentia), 1700-1600 Ma (Amazonia), and 1750-1700 Ma (Baltica). If these minima are due to subduction erosion (± delamination), as in the Andes in the last 200 My, as much as one third of the volume of the of the GPAO could have been recycled into the mantle before the Grenvillian collisions around 1100 Ma. Although a rapid decrease in Hf model ages between 1600 and 1400 Ma could reflect a decrease in production rate of continental crust, it could alternatively represent an increase in recycling rate of crust into the mantle caused by increasing rates of subduction erosion in Proterozoic accretionary orogens, perhaps in response to partial fragmentation of Nuna.