Constraining continental structure by integrating Os isotopic ages of lithospheric mantle with geophysical and crustal data: An example from southeastern Australia

Rocks exposed at the Earth's surface provide an incomplete record of the va ried and complex processes that have acted to form and modify the continent s. The continental lithospheric mantle (CLM), sampled as xenoliths carried in explosive volcanics, is an alternative reservoir of age information that has only recently begun to be tapped. Data from southeastern Australia, a tectonically complex region, illustrate that the CLM can record important a spects of continental evolution that are not indicated by the exposed crust al rocks. Re-Os isotopic data and the resultant model ages for 15 spinel pe ridotite xenoliths from five localities in southeastern Australia show that parts of the CLM are similar to 1960 Myr old, some 1400 Myrs older than th e oldest crust exposed immediately in the sampled region. As the five sampl ed localities are east of the presumed eastern edge of the Precambrian inte rior of Australia, the preservation of Proterozoic ages in the lithosphere indicates that the supposed Precambrian edge of the Australian continent (t he Tasman Line) is a surficial feature rather than a lithospheric scale bou ndary. This conclusion is consistent with recent Skippy seismic tomography models for Australia. The mantle xenolith age distribution also suggests a significant age change from Proterozoic to dominantly Phanerozoic, which ap pears coincident with the surface expression of the boundary between two ma jor Fold belts (Lachlan and Delamerian) and with a deep lithospheric bounda ry previously proposed on the basis of differences in Sr and Pb isotopic sy stematics of southeast Australian basalts.