ALBERT

Description: The Ross orogen in southern Victoria Land, Antarctica, is composed of Skelton Group metasedimentary rocks and varied plutons of the Granite Harbor intrusives. In the Baronick Glacier area, metasediments are dominated by thick sequences of coarse conglomerate, interbedded with pisolitic limestones. Clasts within metaconglomerates are predominantly volcanic, ranging in composition from basanite to rhyolite. Baronick basaltic clast compositions are mildly alkalic and ocean-island basalt-like, compatible with eruption in a continental-margin rift environment. Sr-Nd isotope ratios suggest rhyolites and quartz-trachytes were produced by assimilation and fractional crystallization mechanisms. Two rhyolites yielded U-Pb zircon ages determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) of 650.1 {+/-} 4.0 Ma and 648.5 {+/-} 3.2 Ma (both 2{sigma}). Studies of southern Victoria Land Skelton Group metasediments indicate that, although dominated by Grenville-aged detrital zircons, they contain an important component dated between 630 and 680 Ma, overlapping the age of volcanism in both the Baronick Glacier and central Transantarctic Mountain areas. The distribution of 650 Ma, rift-related volcanism in the southern Victoria Land segment of the Ross orogen, and the absence of significant older, but post-Grenville, extension-related zircon populations, is interpreted to reflect initiation of Rodinia fragmentation on the Antarctic margin at this time. It is proposed that rifting along the paleo-Pacific margin was diachronous, with the Antarctic segment of the rift opening substantially earlier than in Australia. Based on the dates of subduction-related calc-alkaline plutons formed during the Ross and Delamerian orogenies, the paleo-Pacific Ocean in Antarctica may also have closed earlier than in Australia, effectively defining the time of Gondwana assembly.