Crustal Architecture of the Paleo-Pacific Rift Margin of East Antarctica: Evidence From U-Pb Ages and O-Hf Isotope Compositions of Ross Orogen Granitoids

Granitic batholiths of the similar to 500 Ma Ross Orogen in Antarctica are voluminous in scale, reflecting prolific magmatism along the active early Paleozoic convergent margin of Gondwana. New age and isotopic analysis of zircons from a large suite of Ross granitoids spanning >2,000 km along the orogen provide a wealth of geochronologic, tracer, and inheritance information, enabling us to investigate the pace of magmatism, along-strike temporal and geochemical trends, magmatic sources, and tectonic modes of convergence. Because granitoids penetrate the crust of the earlier Neoproterozoic rift margin, they also provide insight into the age and composition of the largely ice-covered East Antarctic craton. Zircon U-Pb ages from these and other samples indicate that active Ross magmatism spanned 475-590 Ma, much longer than generally regarded. Most samples have heavy zircon delta O-18 values between 6.5 and 11.5 parts per thousand and initial epsilon(Hf) compositions between 0 and -15; their isotopic co-variations are independent of age, as in other contemporary continental arcs, and reflect largely crustal melt sources. Samples near Shackleton Glacier have distinctly more mantle-like isotope composition (i.e., radiogenic epsilon(Hf) and low delta O-18) and separate two regions with distinctive isotopic properties and inheritance patterns-a more juvenile section of Mesoproterozoic crust underlying the southern TAM and an older, more evolved region of Paleoproterozoic and Archean crust in the central TAM. The isotopic discontinuity separating these regions indicates the presence of a cryptic crustal boundary of Grenvillian or younger age within the East Antarctic shield that may be traceable into the western Laurentian part of the Rodinia supercontinent.