A hafnium isotopic record of magmatic arcs and continental growth in the Iapetus Ocean: The contrasting evolution of Ganderia and the peri-Laurentian margin
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We test the sensitivity of combined U-Pb-Hf isotopic analyses of detrital and magmatic zircon to document complex processes in accretionary orogens, using Ganderia from the Canadian Appalachian orogen as an example. Ganderia hosted a long-lived magmatic arc that began at least by 650 Ma and continued until 450 Ma, during which time it drifted northward from the Amazonian margin. Its isotopic record suggests arc activity began earlier, possibly at ~750 Ma (proto-Ganderia) within an oceanic domain. The U-Pb-Hf zircon isotopic array suggests that Ganderia formed on a sliver of Paleoproterozoic-Mesoproterozoic basement (TDMc = 2.2–1.1 Ga). A transition towards more evolved Hf isotopic compositions between 650 and 600 Ma is interpreted to coincide with its accretion to the Amazonian margin of Gondwana, consistent with other geological evidence. Accretion was followed by increasing amounts of juvenile crustal inputs between ~550 and 500 Ma, which is interpreted to reflect subduction roll-back and the onset of drift from the Gondwanan margin. Trench retreat culminated in the separation of the Ganderia arc and formation of the exclusively oceanic ~510–450 Ma Penobscot-Popelogan-Victoria arcs on Ganderia. Accretion of Ganderia to the active Notre Dame Subzone along the Laurentian margin occurred in the late Ordovician and is marked by the abrupt termination of Ganderian arc magmatism. The Notre Dame arc evolved along the Laurentian margin between ~515 and 425 Ma and was built on Paleoproterozoic-Archean (T(DM)c = 2.9–2.5 Ga) crust. Supra-subduction zone magmatism occurred at ~490–460 Ma during the closure of the Taconic Seaway and was dominated by the recycling of Laurentian crust, with progressive mixing of evolved and juvenile arc magmas over time. The preservation of very evolved Notre Dame arc zircon in Ganderian overstep sequences confirms the arrival of the leading edge of Ganderia to Laurentia by ~450 Ma. The hafnium isotopic method is proven here to be a powerful tool when used in conjunction with geological constraints, and is able to identify the changing nature of magmatism during the evolution of complex accretionary systems.
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