Australian Proterozoic Intraplate Igneous Activity - Abstract 4

The Metallogenic Potential of Australian Proterozoic Granites, Record 2001/12

Lesley A.I. Wyborn, Mart Idnurm, Anthony R. Budd, Irina Bastrakova, Murray S. Hazell, Suzanne M. Edgecombe

• Download PDF
• What does the analysis tell us?
• References
• Related links

Download PDF

• Download this file as a PDF [14k]

[back to top]

What do ~10 000 whole rock geochemical analyses tell us about Australian proterozoic intraplate igneous activity?

A major compilation of Australian Proterozoic Igneous rocks reveals broad patterns which provide major constraints on geodynamic reconstructions in the Proterozoic. The key results show that the Australian Proterozoic can be subdivided into major magmatic provinces each of which is coincident with one or several major recognised fold belts/orogenic domains and contains magmatism that is clearly episodic. The majority of Australian Proterozoic granites are I-(granodioritic) type derived by melting of pre-existing crust. Proterozoic granites are also dominantly Sr-depleted and Y-undepleted, signifying high geothermal gradients and sources dominated by plagioclase. Genuine S-type granites with visible cordierite and garnet in the more mafic compositions are rare, as are intermediate or felsic types that are distinctly Sr-undepleted, Y-depleted: a signature which is most commonly found in granites associated with subduction in island arc or continental margin settings, and infers the presence of garnet in the source and lower geothermal gradients.

[back to top]

Wyborn et al. (1997) have shown that in most major Proterozoic magmatic provinces the dominant Sr-depleted, Y-undepleted I-(granodioritic) types can be divided into three groups which show a time progression in geochemistry. The oldest group (Group 1) at 1870-1850 Ma comprises restite-rich suites. Group 2, emplaced at 1840-1800 Ma is a low-Ca type that shows evidence of magmatic fractionation. The youngest group (Group 3) is enriched in incompatible elements and comprises three subgroups: Subgroup 3₁, dated at around 1800-1780 Ma, has very high values of Zr, Nb and Y; Subgroup 3₂, usually emplaced between 1760 and 1650 Ma, is enriched in F and has variable amounts of Y, Zr, and Nb; and Subgroup 3₃, emplaced from 1640 to 1500 Ma, is more oxidised with a wide range in SiO₂ values. A simple explanation for the geochemical evolution from Groups 1 to 3 is that as the temperature in the source region increases, the magma is dominated first by minimum melt, then by biotite breakdown and finally by amphibole breakdown, with evidence of source temperatures of up to 1000 °C during the latter phase. The temperature increase of granite melts contrasts with a general decrease in the temperature of the mafic melts with time, with high Mg-tholeiites dominating before ~1850 Ma and continental tholeiites after ~1850 Ma (with the exception of high Fe-tholeiites at Broken Hill and Mt. Isa).

[back to top]

It is difficult from the geochemical viewpoint to relate these consistent, continent-wide changes in composition to magmatism at the plate boundaries. Further, given the abundance of continent-wide magmatic activity, this would require an unlikely large number of small plates. Likewise. an explanation for melt emplacement in terms of tectonism, related to coeval mantle heating events is difficult to reconcile with time constants for heat transfer from the mantle to the crust. For crustal thicknesses of 25, 30, 40 and 50 kms the time lags have been calculated as 19, 27, 48, and 76 Ma respectively (Upton et al. 1997). Given the likely Proterozoic crustal thicknesses of at least 30 kms, the time lag between mantle magmatic activity and crustal melting is at least 25 Ma and may exceed 70 Ma, suggesting that melting of the lower crust and the emplacement of granitic magmas were due to different tectonic events. The clue to solving these problems may be in the shape of the apparent polar wander path (APWP) which, for those parts of the Australian Proterozoic where it is defined, confirms plate mobility and also shows that magma emplacement was coincident in time with inflection points on the APWP. The latter are recognised as significant interplate tectonic events with associated intraplate effects that cause major episodic migration of basinal fluids. Similar intraplate tectonic responses to later plate boundary tectonic events may have also allowed granitic melts to migrate into the upper crust, with the composition of the melt being simply dependant on the temperatures in the lower crust at the time of 'escape'. This hypothesis is consistent with:

1. The increase in temperature of the felsic melts with decreasing age
2. The progressive change in chemical compositions with time in each province, and
3. The related decrease in the temperature of the mantle melts. Finally, the hypothesis implies that major magma emplacement in the Proterozoic of Australia may have occurred in the interior of the plate rather than along numerous plate boundaries.

[back to top]

References

Upton, P., Hobbs, B., Ord, A., Zhang, Y., Zhao, C., Drummond, B. and Archibald, N. 1997. Thermal and deformation modelling of the Yilgarn Deep Seismic transect. Abstracts, Geodynamics and Ore Deposits Conference, Australian Geodynamics Cooperative Research Centre, Ballarat, 22-25.

Wyborn, L., Ord, A., Hobbs, B., and Idnurm, M. 1997. Episodic crustal magmatism in the Proterozoic of Northern Australia - a continuum crustal heating model for magma generation. AGSO Record, 1997/44, 131-134.

[back to top]

Related links

• The Metallogenic Potential of Australian Proterozoic Granites
• Original Proposal for Project
• Project Methodology