Zircon U-Pb age constraints for a Cambrian age for metasedimentary rocks at O'Brien Peak, Antarctica

Zircon U-Pb isotopic data provide new depositional age constraints for metasedimentary rocks in the O'Brien Peak area of the Queen Maud Mountains, Antarctica. Zircon grains from a sample of the metasedimentary succession display unabraded, euhedral-subhedral forms, oscillatory zoning and U/Th ratios 2.9, pointing to an igneous origin. A coherent group of 34 analyses yield a 520 +/- 6.6Ma (2 sigma) weighted mean age that is likely close to the depositional age, a result consistent with a c. 489Ma crystallisation age for granites that discordantly cross-cut the sequence. The 520-489Ma depositional age constraints overlap with depositional ages indicated by biostratigraphic and U-Pb zircon ages for interbedded sedimentary and volcanic rocks found elsewhere in the Ross orogen, supporting a growing body of evidence that sedimentary-volcanic successions in the Queen Maud Mountains, and elsewhere, previously thought to be Neoproterozoic and possibly exotic, are Cambrian and part of the autochthonous peri-Gondwana realm.