Onset of the Ross orogeny, Antarctica: a northern Victoria Land perspective

Abstract

The Ross orogen of the Transantarctic Mountains and its eastern Australian equivalents such as the Delamerian orogen formed a 4,000-km-long Andean-style belt along the Pacific margin of Gondwana. Following a series of Neoproterozoic rifting events related to break-up of Rodinia, convergent tectonics is believed to have been active since as early as Ediacaran time. This late Neoproterozoic subduction initiation model is primarily based upon the large population of detrital zircon at the period, although the major pulse of arc granitoids was at the Cambro-Ordovician. In this study, we report the finding of rift-related mafic magmatism of Ediacaran age preserved in Cambrian eclogites of the Ross orogen. We propose that in northern Victoria Land, situated at the Pacific end of the Transantarctic Mountains, the onset of convergent tectonics should postdate the Ediacaran rifting event. Inherited zircon cores from two eclogite samples are characterized by the presence of subtle oscillatory zonation and acicular apatite inclusion. The zircon cores are relatively high in Th/U ratios (0.340.99), and yielded the 206Pb/238U ages of 590.8 ± 8.3 Ma and 603.2 ± 4.4 Ma (tσ), respectively. The Hf isotopic compositions of zircon from the same analytical pits of UPb analyses yielded initial εHf(t) values ranging from +9.2 to +18.8. The zircon U￢Pb and Hf isotopic data in combination with mildly alkalic, within-plate to continental basalt-like geochemistry of the eclogites suggest that their gabbroic protoliths should be a spatial-temporal equivalent to c. 600580 Ma rift to passive margin magmatic rocks of eastern Australia. Dominantly siliciclastic composition of associated metasedimentary rocks further supports an extensional setting of northern Victoria Land-eastern Australia pair at c. 600580 Ma. This is in marked contrast to an existing model for c. 590570 Ma arc initiation on the basis of age data from the Barrovian metamorphism in southern Victoria Land, and the detrital zircon and granitic tills in central Transantarctic Mountains. Therefore, we suggest diachronous tectonic inversion events from rift to arc in the Ross-Delamerian orogens: (1) from Ediacaran (c. 600580 Ma) rifting to early Cambrian (c. 540530 Ma) convergent orogeny in the eastern Australia-northern Victoria land pair; and (2) from Cryogenian (c. 670650 Ma) to Ediacaran (c. 590570 Ma) possibly in the rest of the Transantarctic Mountains.