GEOCHRONOLOGY OF DEFORMATION, INTRUSION, AND COOLING DURING THE ROSS OROGENY, BYRD GLACIER AREA, ANTARCTICA

The Ross orogen was active at the Pacific margin of Antarctica during the transition from Rodinia to Gondwanaland. New U-Pb and Ar-Ar data on rocks from the Byrd Glacier area of the central Transantractic Mountains constrain events during the orogenic phase of the cycle. The Byrd Group encompasses an Early Cambrian carbonate to clastic transition wherein Atdabanian-Botomian Shackleton Limestone is conformably overlain by sandstones of the Dick Formation. The Dick Formation interfingers upwardly with Douglas Conglomerate, which at some localities unconformably overlies folded Shackleton Limestone, and is itself brittlely deformed. We correlate Byrd Group with Selborne Group, a sequence of lower-amphibolite grade marble and schist immediately south of Byrd Glacier. Likewise, we interpret the main phase of deformation (pre-Douglas Conglomerate) of both Byrd and Selborne Groups to be the same due to similar structural trends. A biotite granite pluton intruded Selborne Group post-tectonically. Nearly concordant zircons from this pluton give a U-Pb age of 492±2, interpreted to record granite crystallization of the granite, which constrains the main deformation in the area. This thermal pulse produced the static growth of biotite and/or muscovite, and scapolite and/or vesuvianite. Incremental heating ⁴⁰Ar/³⁹Ar analyses of selected minerals constrains the (>~300°) cooling history to the Early Ordovician. An amphibole gives a well-defined plateau age of 485±3 Ma. Muscovite and biotite separates give ages within a very restricted range at 460±3 Ma. Folded Shackleton Limestone is crosscut by a set of dikes that are fractured and faulted. Preliminary results on zircon from one of these dikes suggest an age in the range of 490 Ma, although uncertainties due to discordance and possible inheritance need to be resolved. This preliminary age is similar to the age of plutonism in Selborne Group, but importantly it precedes a phase of brittle deformation in the area. Two possibilities for this brittle deformation are that 1) it is related to the deformation that affected Douglas Conglomerate (late stage Ross orogeny), or 2) it is related to a younger event, such as uplift of the Transantarctic Mountains, or postulated strike-slip movement along the Byrd Glacier discontinuity.