NEW U-PB ZIRCON DATA BEARING ON THE DEVELOPMENT OF THE ROSS OROGEN AND BYRD GLACIER DISCONTINUITY, TRANSANTARCTIC MOUNTAINS, ANTARCTICA

Following the breakup of Rodinia, the Ross orogen developed along the Pacific margin of the Antarctic craton. Based on the roughly linear distribution of Cambro-Ordovician plutons and batholiths throughout the orogen, the continental margin must have approximately coincided with the present day Transantarctic Mtns. A major discontinuity in this linear trend occurs at Byrd Glacier where plutonic and high-grade metamorphic rocks to the north are juxtaposed to the south by a low-grade sedimentary sequence. Neoproterozoic sedimentation is suggested throughout the Transantarctic Mtns, while E to M Cambrian sedimentation and volcanism have been documented by fossils and isotopic dating only in that portion of the Transantarctic Mtns between Byrd Gl and the Pensacola Mtns. It remains an open question whether any of the metasedimentary rocks found north of Byrd Gl throughout Victoria Land are of Cambrian age. In Skelton Gl area, deformation of Skelton Gp was followed by intrusion at 551 Ma, but a similar constraint is lacking elsewhere in Victoria Land. In ongoing research focused on the tectonic implications of the Byrd Gl discontinuity, we have undertaken a U-Pb study of detrital zircon ages from metasedimentary rocks collected throughout Victoria Land, including Priestley Fm from the Priestley Gl area, Koettlitz Gp from the McMurdo Sound area, Skelton Gp from the Skelton Gl area, and Horney Fm from the Britannia Range north of Byrd Gl. In all these samples there are no discrete grain populations younger than 600 Ma, allowing that all the metasedimentary rocks in Victoria Land were deposited before the Cambrian. We also have obtained new U-Pb zircon dates on two plutons directly north of Byrd Gl: 545.7 +/- 6.8 Ma and 531.0 +/- 7.5 Ma. These ages slightly proceed to overlap the depositional age of E Cambrian Shackleton Limestone now found on the south bank of Byrd Gl only 22 km to the south. Combining these new data with known geology we develop a model in which a reentrant in the continental margin at Byrd Gl, that formed during the original breakup of Rodinia, was the site of subduction and subsequent collision of a terrane or ribbon continent upon which Shackleton Limestone was being deposited, and that during the E to M Cambrian the region of Victoria Land was a positive landmass with no deposition.