GEMOC ARC National Key Centre

SOUTHERN AUSTRALIA AND SIBERIA: A COMPARISON OF TWO LITHOSPHERIC TRANSECTS

Oliver F. Gaul¹, William L. Griffin^1,2 and Suzanne Y. O'Reilly¹

1. GEMOC, Macquarie,
2. CSIRO Exploration and Mining

Two lithospheric transects have been constructed, in southern Australia and Siberia, using data from mantle xenocrysts brought to the Earth's surface by kimberlitic and basaltic activity.  The southern Australian transect extends from the Eyre Peninsula of central South Australia to Jugiong in southeastern New South Wales.  This transect encompasses both cratonic and fold belt environments and crosses the Tasman Line, which separates Archean/Proterozoic western Australia from the Phanerozoic accreted terrains of eastern Australia.  The Siberian transect also crosses a number of terrains from the Archean Eastern Siberian craton in the south to Proterozoic mobile belts in the north.  These two transects provide an opportunity to examine the changes in lithospheric structure and chemistry from cratonic to off-craton environments.  Comparison of the two transects can be used to determine what patterns are common to both cratons and what factors are independent of tectonic setting.

Samples studied consist of garnet and chromite heavy mineral concentrates and were analysed by electron microprobe (major elements) and proton microprobe or laser ablation ICPMS (minor and trace elements).  Temperature and pressure estimates have been made for each garnet grain through the use of the single mineral Ni-thermometer and Cr-barometer (Ryan and Griffin, 1996).  These data have been used to estimate model conductive paleogeotherms for each locality.  Results show a wide range of geotherms from 35 mWm-2 in the Archean of Siberia, through 40-43 mWm-2 in Proterozoic regions of both transects, to in excess of 50 mWm-2 in Phanerozoic eastern Australia.  Estimates of lithospheric thickness, based on trace element concentrations in garnet, vary from 240 km in the core of the Siberian craton, through 160-170 km in Proterozoic sections, to ~100 km in eastern Australia.

Temperature and pressure estimates for garnet also allow the construction of "chemical sections".  These are plots of garnet chemistry against depth which provide a useful tool for interpreting changes in a number of variables with depth within the lithosphere.

Results from these chemical sections reveal some interesting patterns:
 

• minor rock types are more commonly harzburgitic in Archean sections and wehrlitic in Phanerozoic    areas
• Cr2O3 content of garnet is lower in younger sections
• a steady trend to higher Y/Ga and lower Zr/Y ratios in garnet exists between Archean and Phanerozoic    lithosphere
• the pattern of metasomatic activity appears to vary little between Archean and Phanerozoic times
• TiO2 in garnet increases with depth in most sections regardless of age

A method for estimating olivine Mg content from garnet chemistry has been developed which involves the inversion of the O'Neill and Wood (1979) garnet-olivine Fe-Mg exchange thermometer.  The Ni-temperature of a given garnet grain can be used, in conjunction with the Mg, Fe and Ca content of the garnet, to solve for olivine Mg and Fe content.  The Mg content of olivine is an important indicator of the degree to which lithospheric mantle has been depleted through the extraction of basaltic melts.  Also, because olivine is the most common mineral in the lithospheric mantle, the Mg content of olivine is one of the main factors in determining the density of mantle rocks.  Results of this procedure show an overall decrease in olivine Mg content with depth in most sections.  There is also a trend to more Fe-rich compositions in younger areas.  Values in the shallow portions of sections range from Mg# >94 in Archean areas to Mg# of 90-91 in Phanerozoic sections.  This indicates a lower degree of depletion in younger lithospheric sections and could also account for a density increase of ~0.03 g/cc due to olivine chemistry alone.

References
O'Neill, H. StC. and Wood, B. J. 1979. An experimental study of Fe-Mg partitioning between garnet and olivine and its calibration as a geothermometer. Contributions to Mineralogy and Petrology, 70, 59-70.

Ryan, C. G. & Griffin, W. L. 1996. Garnet geotherms: Pressure-temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks. Journal of Geophysical Research, 101, 5611-5625.