Trace element and Sr-Nd-Pb isotopic systematics of North Queensland basalts: A preliminary study

Ming Zhang1, J. Stephenson2, S.Y. O'Reilly1, M. Norman1, M.T. McCulloch3

1, GEMOC, Macquarie University, NSW 2109, Australia

2, Dept of Earth Sciences, James Cook University, Townsville, Qld 4811, Australia

3, Research School of Earth Sciences, Australian National Univ., ACT 0200, Australia

Cenozoic basaltic rocks are widespread in North Queensland (NQld), with many bearing mantle and lower crust xenoliths and high-pressure megacrysts. Most of the basalts occurring as volcanic cones and long lava flows were erupted during a period of 8.0-0.01 Ma, with an exception of the Early Tertiary plugs and dykes at Mingela (44-31 Ma; Stephenson, 1989). 62 samples from 6 volcanic provinces (Atherton, McBride, Chudleigh, Nulla, Mingela, and Cooktown) were analysed for major and trace elements; many of them were also analysed for Sr-Nd-Pb isotopes (no Pb isotope data available for Chudleigh and Mingela). 58 out of the 62 samples are alkaline basalts, ranging from nephelinite and basanite to alkali olivine basalt and hawaiites, with the rest being olivine tholeiite.

The NQld samples have 87Sr/86Sr of 0.7034-0.7048, 143Nd/144Nd of 0.51302-0.51279, and 206Pb/204Pb of 17.90-18.66. They display the Dupal-type Pb isotopic signatures with D7/4Pb= +3.3 to +12.5 and D8/4Pb=+32 to +72. The Late Tertiary basalts differ from the alkaline basalts of the lava-field provinces in New South Wales (NSW) and Victoria in their high 87Sr/86Sr at given 143Nd/144Nd and generally low 206Pb/204Pb ratios (18.60-19.14 for NSW basalts). On a 207Pb/204Pb vs 208Pb/204Pb plot, they form a trend subparallel to but below the NSW basalts, indicating the presence of a source component with relatively low Th/U. However, Sr and Nd isotope ratios of the Early Tertiary Mingela plugs fall in the compositional range of the other Australian lava-field basalts (Fig. 1; Ewart et al., 1988; O'Reilly & Zhang, 1995).

The primitive basalts from both long lava flows and volcanic cones in the Atherton, McBride, Chudleigh, and Nulla provinces show identical incompatible element patterns. They display slight to moderate enrichments in K, Sr and P over LREE, with relatively low Nb and Ta contents (eg ave. K/Nb=330 (n=43) for NQld vs 190 (n=197) for NSW). The primitive strongly alkaline basalts (some with norm-lc) from the Cooktown area have fractionated incompatible element patterns similar to those shown by nephelinites and basanites from Barrington, Kandos, Monaro, and Dubbo in NSW (O'Reilly & Zhang, 1995), ie with strong enrichments in Nb, Ta and LREE and depletions of variable degree in Rb, K, Zr, Hf, and Ti. The moderately evolved hawaiites and ol-tholeiites are similar to the younger NQld basalts from the other provinces. The incompatible element patterns of the Mingela basalts are similar to the typical OIBs with a peak at Nb and Ta and a gradual decrease in mantle-normalised abundances from K to Yb, except for the presence of a slight Sr-enrichment, which is almost ubiquitous in the NQld basalts.

The evidence of Sr-Nd-Pb isotope ratios, incompatible element patterns, and variation trends in major and trace element abundances and ratios indicates that an upwelling asthenospheric source that was hot, chemically heterogeneous, and probably related to the opening of the Carol Sea, is one of the source components for all the NQld basalts. The geochemical characteristics of the Early Tertiary basalts probably are determined by the relatively enriched part of this source. On the other hand, the Late Tertiary basalts require substantial contributions from subcontinental lithospheric mantle (SCLM) sources. The observed high 87Sr/86Sr and the inferred high Th/U of the source may indicate that the SCLM component for the Late Tertiary basalts was a mantle wedge modified by subduction during Paleozoic. However, this component must be heterogeneous in trace element abundances and/or modal composition, as reflected by large variations of incompatible element ratios such as K/Nb and Sr/La. In particular, the incompatible element patterns of the Cooktown alkaline basalts may require contributions from a metasomatised SCLM containing amphibole (± apatite).

Reference:

Ewart A, Chappell BW, Menzies MA (1988) An overview of the geochemical and isotopic characteristics of the eastern Australian Cainozoic volcanic provinces. J Petrol Spec Vol 225-274.

O'Reilly, SY & Zhang, M. (1995) Geochemical characteristics of lava-field basalts from eastern Australia and inferred sources: connections with the subcontinental lithospheric mantle. Contrib. Mineral. Petrol., 121, 148-170.

Stephenson PJ (1989) Northern Queensland. In: Johnson RW (ed) Intraplate volcanism in eastern Australia and New Zealand. Cambridge University Press, pp 89-97.