A lithospheric transect in southern Australia

Oliver F. Gaul (GEMOC, Earth Sciences, Macquarie Univ., Sydney 2109, Australia)

Lithospheric sections, constructed using geochemical information from pyrope garnets entrained by alkaline volcanism, have been used to produce a lithospheric transect across the Tasman Line, which divides the Phanerozoic, accreted terranes of eastern Australia from the Proterozoic and Archean cratonic areas of western Australia. The localities used in constructing the transect span a distance of over 1000 km, from Jugiong in central NSW to El Alamein at the northern end of the Spencer Gulf in South Australia.

A geotherm for each section is obtained from temperature and pressure estimates for each garnet grain, derived using the Ni thermometer and Cr barometer of Ryan et al. (1996). Plots of Y content against temperature give an estimate of the temperature at the base of the lithosphere, using the criterion that Y-depleted garnets are restricted to the lithosphere. Lithospheric thickness then is estimated by referring this temperature to the geotherm. Likely source-rock types of individual garnet grains are inferred from calcium and chromium contents and trace element patterns are used to define metasomatic processes that a garnet's source area has undergone.

The results show a decrease in lithospheric thickness and a rise in geotherm from west to east. El Alamein, at the western end, shows a lithospheric thickness of 160 km and a geotherm of 42 mWm-2. The White Cliffs locality, in the central part of the transect, has a 130 km thick lithosphere and a conductive geotherm of 48 mWm-2 Preliminary results from Jugiong indicate that the geotherm is hotter and the lithosphere thinner at the eastern end. Although there is a systematic change in lithospheric thickness from west to east across the transect there appears to be little variation in the stratigraphy of the sections. All of the sections show a predominantly lherzolitic lithosphere; minor amounts of calcic harzburgite occur in the South Australian mantle; wehrlite is abundant beneath White Cliffs. Depleted rocks make up the bulk of all of the sections but the amount of melt- and fluid-related metasomatism varies from section to section. Y/Ga and Zr/Y ratios of garnets indicate that the South Australian kimberlites have sampled typical Proterozoic mantle, while the White Cliffs mantle is more like that of Phanerozoic areas. Limited data from Jugiong indicate a lithosphere typical of Phanerozoic areas.