CONTRASTING CHARACTERISTICS OF PROTEROZOIC AND PHANEROZOIC MANTLE TYPES: PRELIMINARY RESULTS.

Eloise E. Beyer, GEMOC Macquarie

Fundamental differences in the composition of Archean and Phanerozoic lithospheric mantle have been well documented over the last decade, but little attention has been paid to the nature of Proterozoic mantle and how it compares with mantle of different ages. To establish a basis for comparison of Phanerozoic and Proterozoic mantle in Australia sampling was carried out at several localities across Tasmania. Sampling was carried out on either side of the Tasman Line, which delineates the surface expression of the younger Palaeozoic Tasman Fold Belt in the east from the older Proterozoic Central Australia Mobile Belt in the west, to determine any differences in mantle composition beneath the two tectonic regimes.

Lithosphere mapping represents a methodology which allows the construction of geological sections of the subcontinental lithospheric mantle (SCLM) and follows the evolution of the lithosphere through time (O'Reilly and Griffin, 1996). The basis of lithosphere mapping in Tasmania is the direct evidence for the petrology and geochemistry of the upper mantle provided by xenoliths entrained in Tertiary basalts. Results from in-situ microanalysis for major element abundances in spinel-bearing xenoliths and their constituent minerals make it possible to determine the distribution of temperature with depth; the depth to the crust-mantle boundary; and the stratigraphic distribution of rock types

Electron microprobe analyses have been carried out on xenoliths from two localities; Blessington (east of the Tasman Line) and South Riana (west of the Tasman Line). The mantle wall-rock xenoliths from both localities are exclusively lherzolites ± spinel. EMP data has delineated two distinct geochemical populations (low-Al2O3 vs high-Al2O3 in clinopyroxene) in xenoliths from Blessington. Al2O3 content in the low-Al2O3 group ranges from 2.5-4.0 wt% while the high-Al2O3 group shows a range of 4.0-6.0 wt%. Modal clinopyroxene in the low-Al2O3 group is 2.1-2.7 compared with 4.8-13.6 in its high-Al2O3 counterpart. This dichotomy is also reflected in olivine composition. The magnesium numbers (mg#) for olivines from the low-Al2O3 group are uniformly high (91.5-91.7) and all lie in the Proterozoic field of a Boyd diagram; modal olivine ranges from 54.9-82.4%. The high-Al2O3 group display much lower mg# (89.6-90), have a more restricted range in modal olivine (44.5-55.0%), and plot in Boyd's Phanerozoic field.

Preliminary work on South Riana has revealed two main xenolith types. The majority of South Riana samples are fine-grained and cpx-poor (<3.0%) with rarer coarse-grained cpx-rich (>15%) xenoliths. Al2O3 content of clinopyroxene is significantly higher in the coarse-grained samples though both types overlap the high-Al2O3 group of Blessington. Mg# for South Riana samples ranges from 89.2-90.3 placing them in the Phanerozoic field of a Boyd diagram. Modal olivine is higher in the fine-grained group (72.6-78.9% compared to 60.8-65.0%).

Temperature calculations from EMP data indicate that the two Blessington groups have originated from different thermal regimes indicating sampling from different depths. The low-Al2O3 group shows a range in temperature from 850-900°C compared to the high-Al2O3 group which has a temperature range of 925-975°C. This difference in temperatures is also seen in the two South Riana groups again indicating that sampling had occurred at different levels. The fine-grained samples range in temperature from 880-900°C whereas the coarse-grained group are considerably hotter at 990-1020°C. The absence of garnet-bearing xenoliths at the two localities prevents direct pressure calculations. An approximate depth of origin can be found by plotting known temperatures on the southeastern Australian geotherm. The low-Al2O3 group of Blessington and fine-grained xenoliths of South Riana give an estimated depth of 30-35km. This depth is inferred to be the distance to the crust-mantle boundary at these two localities. In contrast the high-Al2O3 group of Blessington appears to have originated from depths between 40-45km and the coarse-grained South Riana group from depths of 45-52km.

References

O'Reilly, S.Y. and Griffin, W.L. 1996. 4-D lithosphere mapping: methodology and examples. Tectonophysics, 262, 3-18.

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