Early Ordovician terrane accretion along the Gondwanian margin of the EastAntarctic Craton: new Pb/Pb titanite ages from the Tonalite Belt, North Victoria Land, Antarctica

Early Palaeozoic subduction of the palaeo-Pacific: plate and terrane accret ion along the palaeomargin of the East Antarctic Craton is well-documented in North Victoria Land, where the Tonalite Belt is a complex of synkinemati c intrusions emplaced within the Lanterman-Murchison Shear Zone at the boun dary between the Wilson Terrane and the allochthonous Bowers Terrane. Stepw ise leaching Ph/Pb and U-Pb studies of titanite separates carried out on tw o well-foliated samples of tonalites yielded ages of deformation bracketed between 490 and 480 Ma with an isochron age of 480 +/- 13 Myr. Ar/Ar and K- Ar ages of 477 Myr in the metamorphic: rocks of accreted terranes point to fast cooling and uplift after accretion. The new titanite ages, compared wi th a regional distribution of magmatic and metamorphic ages, indicate an ea rly Ordovician age for terrane collision and amalgamation. As a consequence of collision. subduction shifted to an outward position along the palaeoma rgin of the East Antarctic Craton.