Paper No. 3
Presentation Time: 1:35 PM
ZIRCON AGE PEAKS: A BALANCE BETWEEN CONTINENTAL PRODUCTION, PRESERVATION, AND RECYCLING
Geologic and isotopic evidence from orogens suggests that the five global peaks in U/Pb zircon ages (2700, 1900, 1000, 600, 300 Ma) reflect selective preservation rather than production of juvenile continental crust. Supporting this interpretation are, 1) juvenile crust in orogens forms during ocean basin closings rather than actual collision, and 2) the ratio of juvenile to reworked crust (as inferred from Nd and Hf model ages) is about the same in zircon age peaks as in the valleys between age peaks. Nd isotopes in granitoids and metasediments suggest that a significant volume of juvenile continental crust is formed in external continental arcs during ocean basin closings and preserved during continent-continent collisions. Continental growth by crustal underplating (and overplating) is not included in this model because most underplates are mafic and do not produce significant numbers of zircons. Nd model ages from LIPs suggest that mafic underplates may contribute to continental growth and cannot be ignored in crustal production models. We now have evidence from the Andes and other sites in modern arcs that recycling of continental crust into the mantle has been important in the last 200 Myr as Pangea has fragmented. This observation suggests that supercontinent breakup with opening ocean basins drives external orogens into overriding modes, enhancing recycling by subduction erosion and delamination. Model ages from the continents indicate that, although only minor volumes are preserved, juvenile crust was produced in significant amounts in the time intervals between zircon age peaks. This observation requires recycling of continental crust into the mantle during supercontinent breakup. If this model is correct, any planet with plate tectonics should exhibit global episodic ages tied directly to the supercontinent cycle.