THE PERSISTENCE OF OFF-CRATONIC LITHOSPHERIC MANTLE - OS ISOTOPIC SYSTEMATICS OF VARIABLY METASOMATISED SOUTHEAST AUSTRALIAN XENOLITHS

The Re-Os systematics of a suite of spinel peridotite xenoliths from s outheast Australia provide insight into the effects of melt extraction and metasomatism on Re and Os and place strong constraints on the evo lution and long-term stability of post-Archean lithospheric mantle in a tectonically complex region. Data from variably melt-depleted and no n-modally metasomatised xenoliths demonstrate that Re abundances are l argely controlled by melt extraction, with Re similarly distributed to Os. Ratios of Re/Os correlate strongly with indices of melt extractio n (e.g. Al2O3, Ni and Yb), and with the calculated bulk partition coef ficient of Re, comparable to that of Yb over a large range of melt ext raction (similar to 4-20%). Hence, if Re is controlled by sulfide phas es, sulfur:clinopyroxene ratios should remain essentially constant ove r large degrees of melt extraction. Eight of the 24 samples analysed w ere wehrlites or apatite-bearing peridotites, representing residual pe ridotite which has interacted with a carbonatitic melt. In comparison with the non-modally metasomatised xenoliths, these samples show no ev idence for disturbance of Os isotopic composition, or addition of Re o r Os during metasomatism. The entire suite provides a 220 km long, WNW -ESE lithospheric mantle transect, east of, and perpendicular to, the presumed Australian Precambrian shield margin. The Os model ages indic ate at least three episodes of mantle depletion: ca. 1960 Ma, 800-1000 Ma and < 500 Ma. The older age is found only in the two westernmost l ocalities where a subset of four samples define a Re-Os age of 1959 +/ - 100 Ma, with an initial gamma Os = + 0.2. Although the oldest expose d rocks in the region are of Cambrian age, and the presence of early P roterozoic basement is highly contentious, the Os isotopic data requir e that early Proterozoic basement extends some 400 km further east tha n the easternmost exposed early Proterozoic crust. Model ages of 800-1 000 Ma are common to all but one locality, indicating at least two mel t extraction events in the western localities. Paleozoic ages are only identified in the eastern localities, suggesting the lithospheric man tle becomes younger to the east. Importantly, this and other Re-Os iso topic studies provide increasing evidence for the long-term stability and persistence of lithospheric mantle of Proterozoic as well as of Ar chean age. (C) 1997 Elsevier Science B.V.