Age and significance of ‘outboard’ high-grade metamorphics and intrusives of the Ross orogen, Antarctica

We present new U–Pb zircon and 40Ar–39Ar hornblende isotopic data for gneisses and undeformed intrusions from three important and widely separated localities from the northern–eastern ‘outboard’ edge of the Ross orogen in the Queen Maud and Horlick mountains, Antarctica. U–Pb zircon analyses of gneisses that have long represented possible candidates for Precambrian cratonic crust have yielded latest Late Proterozoic to early Paleozoic ages. One paragneiss from the Fallone Nunataks, near Scott Glacier yields a 546 + 10.6/− 18.9 Ma age for its youngest zircon population, indicating a maximum latest Neoproterozoic–early Paleozoic (Ross orogeny) age for deformation and fabric development. Two orthogneisses from the Bravo Hills near Shackleton Glacier yield igneous protolith crystallization ages of 535 + 8.5/− 8.9 Ma and 517 + 6.9/− 6.5 Ma, and one orthogneiss from Woodall Peak near Ramsey Glacier yields a protolith crystallization age of 503.8 + 6.9/− 6.5 Ma. Intrusion of the orthogneiss protoliths therefore occurred concomitant with other pretectonic (deformed) subduction-related magmatic rocks associated with the early Paleozoic Queen Maud batholith. The younger limit for deformation of the paragneiss and orthogneiss protoliths is constrained by U–Pb crystallization ages of post-tectonic (undeformed) intrusions of the batholith that intrude the gneisses. These intrusions yield U–Pb crystallization ages of 503.5 + 6.2/− 6.5 Ma (post-tectonic, Woodall Peak), 498.4 + 8.8/− 7.6 Ma (post-tectonic, Bravo Hills), and 484.7 ± 8.4 Ma (syntectonic to post-tectonic, Fallone Nunataks). The younger limit for deformation is further constrained in the Bravo Hills by 40Ar–39Ar hornblende analyses that yield a metamorphic cooling age of 502 ± 7 Ma for an orthogneiss. On a regional scale, magmatism commenced by ~ 535 Ma within the Queen Maud batholith. Deformation locally extended into the Late Cambrian to Ordovician, with several areas transitioning to post tectonic magmatism and orogenic cooling by the late Middle Cambrian to Early Ordovician possibly concomitant with phases of slab roll back along the Gondwana margin.