GEMOC ARC National Key Centre

Ming Zhang1, S.Y. O'Reilly1 and J. Stephenson2
1. GEMOC, Macquarie
2. Dept of Earth Sciences, James Cook University

INTRODUCTION
Basaltic magmas in continental settings are commonly formed by decompression partial melting of an upwelling mantle plume or asthenosphere rising to the base of the subcontinental lithospheric mantle (SCLM). The magmas may contain components contributed by partial melting of small degrees of some enriched SCLM and primary magmas can also be modified on their way to the surface by interaction with the lithospheric wall rocks. Therefore, primitive continental basalts erupted through different crustal domains (or at different times) can record geochemical signatures of mantle reservoirs (mantle plume, asthenospheric mantle, and SCLM) and can trace the secular evolution of the SCLM which, in general, ties in closely with the evolutionary history of the crustal domains mapped at the surface.
Late Cenozoic basalts in North Queensland (8.0-0.01 Ma) constrain the nature and evolutionary history of their mantle sources.  The widespread basaltic volcanism in North Queensland (NQld) covers an area of ca 23,000 km2 with a total volume of more than 650 km3 (Stephenson, 1989).  Six of the NQld lava-field basaltic provinces (Atherton, McBride, Chudleigh and Nulla at Townsville?Cairns area and McLean and Piebald near Cooktown) were chosen for this study.  The McBride and Chudleigh basalts erupted largely through the Mesoproterozoic Georgetown Inlier. The McLean, Piebald, Atherton and Nulla Provinces are located in various blocks of the Phanerozoic Tasman foldbelts.  The term "Atherton-Nulla Provinces" refers to the Atherton, McBride, Chudleigh and Nulla Provinces, and the term "Cooktown Provinces" refers to the McLean and Piebald Provinces.
In this study, we use detailed geochemical data, including Sr-Nd-Pb isotopic ratios, of these basalts to constrain their petrogenesis and the nature of their mantle reservoirs and to provide new information on the Phanerozoic tectonic evolution of the Tasman foldbelt in eastern Australia. The new data, combined with previously published data for Cenozoic basalts from eastern Australia (eg McDonough et al., 1985; Ewart et al., 1988; O'Reilly and Zhang, 1995; Zhang et al., 1999), allow general isotopic characterisation of mantle sources for the widespread intraplate basalts in eastern Australia during Cenozoic times.  These data also document the locus of both Pacific-MORB and Indian-MORB mantle reservoirs beneath eastern Australia for two time slices (at ca. 35 Ma and present day) and thus provide additional constraints on the movement of the two global-scale mantle reservoirs with time and its relationship with regional plate-tectonic geodynamics.

PETROLOGY AND ELEMENTAL GEOCHEMISTRY
The NQld basaltic rocks are dominantly (>90%) undersaturated alkaline basalts, with less than 10% olivine tholeiites. The alkaline basalts range from strongly undersaturated nephelinite (found only in the Cooktown provinces), nepheline hawaiite and basanite to moderately undersaturated alkali olivine basalt and hawaiites.  More than 75% of the samples contain mantle xenoliths, reflecting their primitive nature.
SiO2 contents for the NQld basalts range from 40.3 to 51.7 wt%, with nephelinites ?42 wt% and ol-tholeiites >50 wt%.  MgO contents range from 12.8 to 4.3 wt% with Mg/(Mg+Fe2+) of 0.70-0.49 (mostly > 0.61).  The basalts from the Atherton-Nulla Provinces differ significantly from the Cooktown nephelinites in their high Al2O3 and low TiO2 (1.5-2.4 wt% vs >2.5 wt%) and CaO/Al2O3 (<0.7 vs 0.7-1.2).
Most incompatible trace elements do not correlate with Mg/(Mg+Fe2+) for the NQld basalts.  All the NQld basalts are LREE-enriched with chondrite-normalised La/Yb of 6-41. The Cooktown nephelinites have much higher contents (up to a factor of 3) of strongly incompatible elements such as Cs, Th, U, Nb, and light rare earth elements (LREE) and marginally higher Sr, Ba, Zr, and Hf contents than the Atherton-Nulla basalts.  The Pb contents of about 80% of the analysed samples fall in a narrow range of 1.4-2.6 ppm.  Some Cooktown nephelinites have Ce/Pb ratios much higher (up to 57) than the range for oceanic basalts (25±5).
The normalised incompatible element patterns of the Cooktown nephelinites are characterised by strong enrichment in Nb, Ta, Th, U and LREE and depletion in K and Rb.  In contrast, the Atherton-Nulla basanites generally show a gradual increase in strongly incompatible element abundances from Rb to Nb or Ta and a general decrease from Nb (or Ta) to Yb.  Enrichments in K relative to Nb and in Sr relative to Nd are common.

Sr-Nd-Pb ISOTOPIC SYSTEMATICS
87Sr/86Sr ratios for the NQld basalts range from 0.70340 to 0.70472 and 143Nd/144Nd ratios range from 0.51302 to 0.51279 (_Nd_=+7.5 ? +3.0).  Although the ranges of Sr and Nd isotopic ratios for the NQld basalts overlap those for the New South Wales (NSW) basalts, the NQld basalts clearly differ from the latter in their high 87Sr/86Sr at a given _Nd, thus forming a distinct high 87Sr/86Sr trend.  The most depleted NQld samples are similar to the most enriched Indian MORB, whereas the most depleted NSW samples plot close to the enriched end of the Pacific MORB field.
Pb isotopic ratios range in 206Pb/204Pb from 17.86 to 18.62, in 207Pb/204Pb from 15.51 to 15.62, and in 208Pb/204Pb from 37.74 to 38.55 for the NQld basalts, ubiquitously displaying a Dupal Pb isotopic signature with _8/4Pb = +32 ? +63 and _7/4Pb = +3.3 ? +10.9 (Fig. 1; Hart, 1984).  On the other hand, the NSW basalts are high in 206Pb/204Pb (18.70-19.14) and many of them have negative _7/4Pb and _8/4Pb values, similar to the Pacific MORB compositions (Zhang et al., 1999). In contrast to the nearly vertical trends shown by the NSW basalts on the 206Pb/204Pb vs 87Sr/86Sr and _Nd diagrams, the NQld basalts show a positive correlation between 206Pb/204Pb and 87Sr/86Sr (Fig. 2) and a negative correlation between 206Pb/204Pb and _Nd.

DISCUSSION
Mantle Sources of the North Queensland basalts
The simplest explanation for the coherent Sr, Nd and Pb isotope variations in the NQld basalts (Figs 1 and 2) involves a mixing of two mantle source components.  The depleted one has low 87Sr/86Sr and 206Pb/204Pb, and high 143Nd/144Nd, similar to the enriched Indian MORB.  The depleted isotopic signature most likely simply reflects an Indian-MORB asthenospheric mantle source that has been widely recognised from many young basalts from the Western Pacific arcs and back-arc basins (eg Hergt and Hawkesworth, 1994).
The complementary enriched mantle source points to a component with high 87Sr/86Sr, moderately low 143Nd/144Nd and high 206Pb/204Pb, characteristic of an enriched (EM2) mantle component.  Many of the Atherton-Nulla basalts have Sr-Nd-Pb isotopic ratios similar to the Tonga-Kermadec Arc basalts (eg Ewart et al., 1994 and refs therein).  In addition, they have generally high Rb/Sr, K/Nb, K/U, Sr/La and Zr/Nb ratios.  The high 87Sr/86Sr trend of the NQld basalts is consistent with derivation from a lithospheric mantle wedge modified by subduction-related processes. We propose that the EM2 mantle source recognised in the Atherton-Nulla basalts may have been derived from an SCLM modified by percolation of subduction-related metasomatic fluids during the late Paleozoic when this part of the Tasman foldbelt (Hodgkinson and Broken River Provinces) was built up by orogenic magmatism and crustal accretion.  This is consistent with Ewart et al.'s (1988) suggestion that subduction-modified SCLM has played a significant role in the generation of some eastern Australian basalts.
The Sr-Nd isotopic ratios of spinel peridotite xenoliths from the Atherton-Nulla provinces form an even higher 87Sr/86Sr trend, above the trend defined by the host basalts.  Although we do not think these xenoliths represent the direct mantle source for the basalts (which should be located in the garnet peridotite stability field), these data are compatible with the presence of a subduction-modified SCLM in North Queensland.  Seven spinel lherzolite xenoliths from Atherton produce a Sm-Nd "isochron" age of 264 Ma (R=0.9754) with _Nd(t) = +8.  This "age" may be relevant to the timing of metasomatism of a moderately refractory upper mantle wedge associated with or shortly after the regional Permo-Carboniferous post-orogenic magmatism.
However, the NQld basalts show a general negative correlation between 206Pb/204Pb and 238U/204Pb (µ).  Basalts from Cooktown and McBride provinces also display a negative correlation between__Nd and Sm/Nd.  The decoupling between the isotopic systems and relevant parent/daughter elements requires recent metasomatism to produce high U/Pb and LREE/HREE in the isotopically depleted mantle source or melts generated therefrom (such as the Cooktown nephelinites).  The age of this event should be younger than 40 m.y. as constrained by the high µ values (up to 170) of the low-206Pb/204Pb nephelinites.  The diagnostic incompatible element signatures of the Cooktown nephelinites include: (1) fractionated incompatible element patterns with strong depletion of Rb, and K and strong enrichment of Th, U, Nb and Ta and (2) high U/Pb, Th/Pb, Ce/Pb and La/Yb accompanied by low K/Nb, K/U, K/Ba, Rb/Sr and Zr/Nb.  These trace element signatures are similar to those of the plume-derived HIMU-type OIBs though their Pb isotope ratios bear little resemblance to one another.  A model to explain the incompatible element signatures (O'Reilly & Zhang, 1995) invokes interaction between ascending asthenosphere-derived melts and amphibole- and apatite-bearing mantle wall-rock in the upper part of SCLM.  Apatite not only has low Rb/Sr and Sr/La and high U/Pb, Th/Pb and Ce/Pb, but also dominates the budget for U, Th, LREE, and Sr in mantle xenoliths.  Nb contents are low in mantle apatite, but high in mantle amphibole.  Thus, addition of a small proportion of mantle apatite and amphibole would produce the observed incompatible element signatures in the Cooktown nephelinites.  Tectonically, metasomatism which produced apatite and amphibole in the upper mantle wall-rocks may be connected with low-degree partial melting from the east-migrating Indian Ocean asthenosphere which has a long residence beneath eastern Gondwana (Zhang et al., 1999).  Alternatively, this event can be linked to the early Tertiary SSW-directed subduction of the Phoenix-Pacific Plate north of Papua New Guinea (Johnson et al., 1978).  The presence of a high-velocity zone beneath the NQld at depths of 300-600 km (van der Hilst et al., 1997) may be explained as the manifestation of the down-going plate.  In either case, this amphibole- and apatite-type metasomatism is likely to be a precursor of the Cooktown magmatism.

Isotopic Characterisation of Mantle Sources for Eastern Australian Basalts
The present database for the Sr-Nd-Pb isotopes of the Australian basalts allow us to make a generalised characterisation of the radiogenic isotopic signatures of mantle sources in eastern Australia.  Sun et al .'s (1989) four-component dynamic model to explain the isotopic and incompatible element systematics of the Australian central-volcano and lava-field basalts provides baseline information.  A first-order approximation to explain the new data comprises several two-component mixing relationships, each reflecting dynamic interactions between two mantle components that contribute to magma generation (Figs 1 and 2).  The two source components for the late Cenozoic NQld basalts (< 6 Ma) are an Indian-MORB asthenosphere and an EM2-type SCLM.  On the other hand, the NSW lava-field basalts (55-14 Ma) can be accounted for by mixing between a Pacific-MORB asthenosphere and an SCLM component that is isotopically similar to the inferred SCLM component in NQld.
Alkaline basalts from the Victorian Newer Basalts Province have both incompatible element patterns and Sr-Nd-Pb isotopic compositions similar to the primitive central-volcano basalts. Therefore the Australian mantle plume presently beneath the Bass Strait may have contributed to these basalts.  The early Tertiary Tasmanian alkaline basalts reflect mixing between an Pacific-MORB source and a HIMU component connected to a HIMU plume presently located near the Balleny Islands, Antarctica (Lanyon et al., 1993).  In addition, the NSW leucitites and the Dubbo basanites may have tapped an EM1-type mantle source, likely residing in the SCLM beneath the western part of the Lachlan Foldbelt with possible Precambrian basement.

Pacific- and Indian-MORB Mantle in Eastern Australia
Sr-Nd-Pb isotopic data demonstrate the presence of an Indian-MORB source component for the Cooktown Provinces and a more diluted Indian-MORB source component for the Atherton-Nulla Provinces in North Queensland during the last 8 Ma.  In southeastern Australia, Pacific-MORB isotope signatures characterise some of the early (55-14 Ma) lava-field basalts. This discovery further constrains the secular distribution of major asthenospheric mantle reservoirs represented by the Pacific and Indian MORB sources during and following the breakup of eastern Gondwana.  These data, together with plate reconstruction positioning, track the locus of the boundary of the two reservoirs beneath the Australian continent (Fig. 3; Zhang et al., 1999) and fill the gap between previous boundary locations of the Indian-MORB and Pacific-MORB mantle sources in the region constrained from back-arc basin basalts in the southwestern Pacific Ocean (eg Hergt and Hawkesworth, 1994).and ocean floor basalts in the Southern Ocean (eg Pyle et al., 1995).  We conclude that the Indian MORB source is a long-term asthenospheric reservoir beneath most of eastern Gondwana continent and that the westward migration of the Pacific MORB source may have been associated with the opening of the Tasman Sea (at ca. 85-60 Ma) along a broad front southeast of the Australian continent.

REFERENCES
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.
Ewart, A., Bryan, W.B., Chappell, B.W. & Rudnick, R.L., 1994. Regional geochemistry of the Lau-Tonga arc and backarc systems. Proceedings of the Oceanic Drilling Program, Scientific Results 135, 385-425.
Hart, S.R., 1984. A large scale isotope anomaly in the Southern Hemisphere mantle. Nature, 309, 753-757.
Hergt, J.M. & Hawkesworth, C.J., 1994. The Pb, Sr, and Nd isotopic evolution of the Lau Basin: implications for mantle dynamics during the back-arc opening. Proceedings of the Oceanic Drilling Program, Scientific Results 135, 505-518.
Johnson, R.W., Mackenzie, D.E. & Smith, I.E.M., 1978. Delayed partial melting of subduction-modified mantle in Papua New Guinea. Tectonophysics 46, 197-216.
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
Lanyon, R., Varne, R. & Crawford, A.J., 1993. Tasmanian Tertiary basalts, the Balleny plume, and opening of the Tasman Sea (Southwest Pacific Ocean). Geology 21, 555-558.
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.
Pyle, D.G., Christie, D.M., Mahoney, J.J. & Duncan, R.A., 1995. Geochemistry and geochronology of ancient southeast Indian and southwest Pacific seafloor. Journal of Geophysical Research 100, 22261-22282.
Stephenson, P.J., 1989.  Northern Queensland. In: Johnson, R.W. (editor), Intraplate volcanism in eastern Australia and New Zealand. 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: Saunders, A.D. & Norry, M.J. (editors), Magmatism in the ocean basins. Spec. Pub. Geol. Soc. London, 42: 313-346. Blackwell Scientific Publications.
van der Hilst, R.D., Widiyantoro, S. & Engdahl, E.R., 1997. Evidence for deep mantle circulation from global tomography. Nature 386, 578-584.
Zhang, M., O'Reilly, S.Y. & Chen, D., 1999.  Pacific- and Indian-MORB mantle as source reservoirs for the Cenozoic basalts in eastern Australia: Pb-Sr-Nd isotope evidence. Geology 27, 39-43.

Fig. 1  206Pb/204Pb vs 208Pb/204Pb diagram for lava-field basalts from eastern Australia. NHRL, the Northern Hemisphere Reference Line from Hart (1984).
 

Fig. 2  206Pb/204Pb vs 87Sr/86Sr diagram for lava-field basalts from eastern Australia.

Fig. 3  Distribution of Australian basalts with Pacific MORB and Indian MORB geochemical signatures at two time slices, 35 Ma (fainter broad lines) and 5 Ma (heavy lines) on palinspastic reconstructions. Faint and heavy dashed lines represent inferred boundary between Indian MORB and Pacific MORB-type basalts at 35 Ma and 0 Ma, respectively.