Geochemistry and Petrogenesis of Basaltic Rocks from North Queensland: Has Subduction-Modified Mantle Played a Role?

Ming Zhang1, Jon Stephenson2, Suzanne Y O'Reilly1, Marc Norman1, Malcolm T McCulloch3

1, GEMOC, Macquarie University, Sydney, NSW 2109, Australia; 2, Dept of Earth Sciences, James Cook University, Townsville, Qld 4811, Australia; 3, Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

INTRODUCTION

Cenozoic basaltic volcanism is widespread in North Queensland (NQld) as a part of the eastern Australian volcanic zone. Basaltic rocks from 7 volcanic provinces, including Atherton, McBride, Chudleigh, Nulla and Mingela at Townsville-Cairns area and McLean and Piebald near Cooktown, were chosen for this study. The basalts are located cross the major tectonic boundaries between the Mesoproterozoic Georgetown Inlier and the Phanerozoic Hodgkinson and Thomson fold belts. The McBride Province is located mostly inside the Georgetown Inlier, whereas the Mingela, Atherton, and Cooktown Provinces are in the Phanerozoic fold belts. The Chudleigh and Nulla volcanoes straddle the tectonic boundary. 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 (44-31 Ma) plugs and dykes at Mingela (Stephenson, 1989). All these basaltic provinces are recognised as lava field provinces. Their age distribution does not show any relationship with the northward motion of the Indo-Australian plate over a mantle plume that resulted in the central volcano volcanism in eastern Australia during at least the last 35 Ma. In the following text, the term "Cairns" is used for Atherton, McBride, Chudleigh and Nulla Provinces, and "Cooktown" for McLean and Piebald.

PETROLOGY AND GEOCHEMISTRY

Sixty-one basalt samples from North Queensland were analysed for major (by XRF) and 40 trace elements (by XRF and ICPMS); up to 41 of them were also analysed for Sr-Nd-Pb isotopes (Pb isotope data not available for Chudleigh and Mingela Provinces). Based on a CIPW-norm classification (O'Reilly & Zhang, 1995), 57 samples are alkaline basalts, ranging from nephelinite (5%) and basanite (16%) to alkali olivine basalt (5%) and hawaiite (67%), and the rest are olivine tholeiite (7%). Mg numbers (Mg#=Mg/Mg+Fe2+) range 0.70-0.49, with an average of 0.64 (MgO=12.8-4.3 wt%), and average Ni content is 185 ppm (±60 ppm). The majority of these basaltic rocks (including some hawaiites) contain abundant mantle xenoliths and high-pressure megacrysts, indicating their primitive or near-primitive nature.

Major and trace elements: In general, basalts from Cairns area and Mingela are higher in SiO2, Al2O3, but lower in TiO2, Sc, Y, Nb and La at a given Mg# than the lava field basalts from New South Wales (NSW) and the Victorian Newer Basalts in eastern Australia (Table 1). They also have CaO/Al2O3 (0.56±0.08) lower than the NSW basaltic rocks (0.69±0.12), but similar to the Newer Basalts (0.57±0.11). In contrast, the Cooktown basalts are similar to many strongly alkaline basalts in NSW in their SiO2, Al2O3 and TiO2 contents, but have higher CaO/Al2O3 (0.82±0.20) than the later. Y abundances of the Cooktown samples are within the lower range for the NSW and Victorian basalts, but marginally higher than the other NQld basalts, whereas Nb, Th and LREE abundances are among the highest of all the basaltic rocks discussed here. No well-defined correlations between Mg# and minor and most incompatible trace elements are present for the NQld basalts.

Incompatible element patterns: The primitive NQld basalts exhibit three different types of incompatible element patterns (Fig. 1). The majority of basalts from Cairns area are moderately silica-undersaturated (eg alkali olivine basalts and hawaiites). They display identical incompatible element patterns with slight to moderate enrichments in K, Sr and P over Nb, Ta, La, Zr and Hf, reflecting a lesser degree of enrichments in both HFSE and LREE. On the other hand, strongly alkaline basalts (eg lc-normative nephelinites), the predominant rock type in Cooktown area and the subordinate in Cairns, show fractionated incompatible element patterns similar to many NSW nephelinites, basanites, and ne-hawaiites (eg Barrington, Kandos, and some from Southern Highlands, Monaro and Dubbo; O'Reilly & Zhang, 1995), with strong enrichments in Nb, Ta and LREE and relative depletions to variable degree in Rb, K, Zr, Hf, and Ti. One moderately evolved hawaiite and one ol-tholeiite from Cooktown are similar to the majority of the Cairns basalts. The relative K-depletion in the Cooktown basalts becomes more significant (ie decreasing K/Nb) with increasing SiO2-undersaturation. The incompatible element patterns of the early Tertiary Mingela basalts are similar to the typical OIB, the Newer Basalts, and some NSW lava field basalts (eg Blue Mts, Oberon, Grabben Gullen, and some Dubbo tholeiites). They exhibit 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 differences in the incompatible element patterns reflect the variations in some incompatible element ratios (eg K/Nb=340±80 for Cairns vs 180±80 for NSW; Table 1) and can be further illustrated using element-ratio plots such as Rb/Sr vs K/Nb (Fig. 2) and Sr/La vs K/Nb. For example, three distinct areas can be recognised on the Figure 2. The Cooktown strongly alkaline rocks fall in the area for the Barrington, Kandos and Dubbo alkaline basalts, forming a trend pointing to the region defined by amphibole- and apatite-bearing spinel peridotite xenoliths hosted by the Victorian Newer Basalts. The Mingela samples plot close to the Newer Basalts, Dubbo tholeiites and average OIB composition. The Cairns basalts mostly plot outside the region for the other Australian basalts and extend toward the N-MORB and the Kermadec-Tonga island arc basalts (Ewart & Hawkesworth, 1987) due to their high K/Nb and low Rb/Sr ratios.

$, chondrite-normalised ratios; ¶, basaltic rocks from Kermadec-Tonga arc (Ewart & Hawkesworth, 1987); *, estimated compositions of primitive mantle, ave. OIB, and ave. normal MORB (Sun & McDonough, 1989)

Figure 1 Incompatible element patterns for representative basaltic rocks in North Queensland. Atherton and Chudleigh from Cairns area and McLean from Cooktown area

Sr-Nd-Pb isotopic systematics: The NQld basaltic rocks have 87Sr/86Sr of 0.7034-0.7048 and 143Nd/144Nd of 0.51302-0.51279 (eNd=+7.5-+3.0) The Sr and Nd isotopic ratios correlate with neither their parent/daughter element ratios nor any fractionation or crustal contamination indicators such as Mg# and SiO2. Although the Sr and Nd isotopic ratios of the Cairns and Cooktown basalts are within the ranges of the NSW lava field basalts and the Victorian Newer Basalts, they differ in their high 87Sr/86Sr at a given 143Nd/144Nd, thus forming a high 87Sr/86Sr trend above the NSW and Victorian trend (Fig. 3). Coincident with this trend, spinel peridotite xenoliths found from the Cairns area are characterised by even higher 87Sr/86Sr at a given 143Nd/144Nd than their hosts (O'Reilly & Zhang, 1995). The most depleted NQld basalts are similar to relatively enriched Indian MORB, whereas the most enriched trend towards the EM2 component. In contrast to the high 87Sr/86Sr trend of the younger NQld basalts, the early Tertiary Mingela basalts plot with in the compositional range for the NSW and Victorian basalts.

Pb isotopic ratios of the NQld samples range in 206Pb/204Pb of 17.90-18.66, in 207Pb/204Pb of 15.56-15.63, and in 208Pb/204Pb of 37.72-39.24, displaying Dupal signatures with D7/4Pb = +3.3-+12.5 and D8/4Pb = +32-+106. They differ from the NSW lava field basalts in their generally low but variable 206Pb/204Pb and high D7/4Pb (18.60-19.14 and -4.4-+6.7, respectively, for NSW basalts). Therefore, on a 207Pb/204Pb vs 208Pb/204Pb plot (Fig. 4), the NQld basalts form a subparallel trend below the NSW basalts, indicating the presence of a source component with relatively low Th/U. The Dupal-type Pb isotopic signature and the correlations between 87Sr/86Sr and 206Pb/204Pb and between 143Nd/144Nd and 206Pb/204Pb also require contributions from mantle reservoirs of both an Indian-MORB type and an EM2 type.

Figure 2 Rb/Sr vs K/Nb for the lava field basalts in eastern Australia. amp- & ap-peridotites, amphibole- and apatite-bearing spinel peridotite xenoliths in the Victorian Newer Basalts (O'Reilly & griffin, 1988); Newer Basalts, after McDonough et al. (1985); Kermadec basalts, after Ewart & Hawkesworth (1987)

Figure 3 87Sr/86Sr vs 143Nd/144Nd for the lava field basalts in eastern Australia. Pacific and Indian MORB after Mahoney et al. (1995).

CHARACTERISTICS OF THE MANTLE SOURCES

Three distinct mantle sources can be inferred from the Sr-Nd-Pb isotope data. One is represented by the early Tertiary Mingela basalts. It has both isotope and element signatures likely derived from an OIB-type sublithospheric mantle source, possibly resulting from interaction of an enriched deep mantle with the overlying depleted asthenosphere of the Pacific-MORB type. A similar source has been considered as one of the dominant mantle reservoirs for some basalts in NSW and Victoria (O'Reilly & Zhang, 1995).

Figure 4 207Pb/204Pb vs 208Pb/204Pb for the lava field basalts in eastern Australia.

The isotopic trends for the other NQld basalts can be simply interpreted as mixing of two source components. One is likely to be an asthenospheric Indian-MORB source that has been widely recognised in the late-Tertiary to Present basalts from island arc and backarc basin settings in the SW Pacific (eg the Kermadec-Tonga arc and the Lau basin, Hawkins, 1995). This Indian-MORB-type asthenosphere is considered to have at least partially replaced the previous Pacific-MORB-type asthenosphere beneath the SW Pacific margins. The other is a component with EM2 geochemical signatures, likely hosted in the subcontinental lithospheric mantle (SCLM). However, this SCLM component must be heterogeneous and is likely to have multiple origins if the differences in element abundances and ratios between the Cairns and Cooktown basalts are taken into account. The Cairns basalts are characterised by relatively high K/Nb, Sr/La, Sr/Nd, but low La/Yb. This, in connection with their relatively high SiO2, Al2O3, but low TiO2, Y, HFSE and LREE abundances, indicates that the SCLM beneath the Cairns area may have been heterogeneously modified by subduction-related processes, probably during the Paleozoic - early Mesozoic times when the Tasman Fold Belts were formed by westward subduction of the proto-Pacific plate beneath the eastern margin of the Australian continent. Isotopic signatures of mantle xenoliths in the same region support this conclusion. On the other hand, the incompatible element patterns of the Cooktown alkaline basalts may require contributions from a metasomatised SCLM source containing amphibole (± apatite), similar to that for the Barrington nephelinites (O'Reilly and Zhang, 1995).

REFERENCES

Ewart, A, & Hawkesworth, C.J., 1987. The Pleistocene-Recent Tonga-Kermadec arc lavas: Interpretation of new isotopic and rare earth data in terms of a depleted mantle source model. J. Petrol., 28, 495-530.

Ewart, A, Chappell, B.W., & Menzies, M.A., 1988. An overview of the geochemical and isotopic characteristics of the eastern Australian Cainozoic volcanic provinces. J. Petrol., Spec. Vol., 225-274.

Hawkins, J.W., 1995. Evolution of the Lau Basin-Insight from ODP Leg 135, in Active Margins and Marginal basins of the Western pacific, Taylor, B., and Natland, J. (editors), AGU Geophysical Monograph 88, pp 125-173,

McDonough, W.F., McCulloch, M.T., Sun, S.-s., 1985. Isotopic and geochemical systematics in Tertiary - Recent basalts from southeastern Australia and implications for the evolution of the sub-continental lithosphere. Geochim. Cosmochim. Acta 49, 2051-2067

Mahoney, J.J., Lones, W.B., Frey, F.A., Salters, V.J.M., Pyle, D.G., & Davies, H.L., 1995. Geochemical characteristics of lavas from Broken Ridge, the Naturaliste Plateau and southernmost Kerguelen Plateau: Cretaceous plateau volcanism in the southeast Indian Ocean. Chem. Geol., 120, 315-345.

O'Reilly, S.Y., & Griffin, W.L., 1988. Mantle metasomatism beneath western Victoria, Australia, I: Metasomatic processes in Cr-diopside lherzolites. Geochim. Cosmochim. Acta 52, 433-447

O'Reilly, S.Y., & 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, P.J., 1989. Northern Queensland. In Intraplate volcanism in eastern Australia and New Zealand, Johnson, R.W. (editor), Cambridge Univ. Press, 89-97.

Sun, S.-S. & McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. in Magmatism in the ocean basins, Saunders, A.D. & Norry, M.J. (editors), Spec. Pub. Geol. Soc. London, 42: 313-346. Blackwell Scientific Publications.

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