GEMOC ARC National Key Centre

Ming Zhang¹, Jon Stephenson², Suzanne Y. O'Reilly¹, M.T. McCulloch³, M. Norman¹

1. GEMOC, Macquarie,
2. Dept of Earth Sciences, James Cook University, Townsville,
3. Research School of Earth Sciences, ANU

Late Cenozoic basalts (6 Ma-13 Ka) of six volcanic provinces (Atherton, McBride, Chudleigh, Nulla, McLean, and Piebald) in North Queensland (NQld) consist of a chemical spectrum from nephelinite, through basanite, alkaline olivine basalt, and hawaiite, to olivine tholeiite. ⁸⁷Sr/⁸⁶Sr ratios (n=31) range from 0.70340 to 0.70472 and ¹⁴³Nd/¹⁴⁴Nd ratios (n=29) range from 0.51302 to 0.51279 (eNd =+7.5 ­ +3.0) (Zhang et al., 1998a).  Although these Sr and Nd isotopic ratios overlap those for oceanic island basalts and the NSW lava-field basalts, they clearly differ from the latter in their high 87Sr/86Sr at a given eNd, thus forming a high ⁸⁷Sr/⁸⁶Sr trend.  Pb isotopic ratios (n=12) range in ²⁰⁶Pb/²⁰⁴Pb of 17.93-18.62, in 207Pb/204Pb of 15.52­15.62, and in ²⁰⁸Pb/²⁰⁴Pb of 37.75­38.55, ubiquitously displaying the Dupal-type Pb isotopic signatures (D8/4Pb = 37-63, D7/4Pb = 3.3-10.9).  The Sr-Nd-Pb isotopic data fit with two-component mixing between an isotopically depleted Indian-MORB type component and an enriched one with EM2 signatures.  The basalts with the enriched isotopic signatures are generally high in K/Nb, K/U, Rb/Sr, and Zr/Nb, but low in U/Pb and Ce/Pb, consistent with derivation from a subduction-modified source and possible participation of phlogopite in the lithospheric mantle melting domain.  We propose that the enriched mantle source is a subcontinental lithospheric mantle modified by subduction-related processes during the late Paleozoic orogeny in the region.  On the other hand, the depleted mantle component represents an asthenospheric mantle chemically identical to the mantle source for the present-day Indian MORB.  The basalts with the depleted isotopic signatures show fractionated incompatible element patterns with high Ce/Pb, U/Pb, LREE/HREE, but low K/Nb, K/U, Rb/Sr, and Zr/Nb, similar to or even more fractionated than some HIMU oceanic island basalts.  We attribute these distinct incompatible element signatures to melting of amphibole- and apatite-bearing assemblages of metasomatic origin resided in the lithospheric mantle.  The high µ (>70) and low Sm/Nd (~0.15) of the nephelinites are obviously decoupled from their low 206Pb/204Pb (< 18.0) and high eNd (> 6.7).  This implies that the metasomatism must be a close precursor of the nephelinitic magmatism, probably connected with the early Tertiary subduction at the NE margin of the Australian Plate during the rapid northward motion of the Australian continent after 50 Ma.

Our data also show that some lava-field basalts (55-30 Ma) in NSW may require a Pacific-MORB upper mantle as one of their major sources.  This discovery is consistent with the proposed secular distribution of the Pacific and Indian MORB reservoirs surrounding the Australian continent in the eastern Gondwana, as deduced from studies of back-arc basin basalts in the SW Pacific and ocean floor basalts in the Southern Ocean (eg Hergt and Hawkesworth, 1994; Lanyon et al, 1995).  We further suggest that the Indian MORB mantle is a long-term mantle reservoir beneath most part of the East Gondwana and the westward migration of the P-MORB mantle may have partly associated with the Tasman Sea opening (ca 82-60 Ma) along a broad front southeast of the Australian continent (Zhang et al., 1998b).

An isotopic framework of mantle reservoirs, based on mixing relationships between various theoretical end-members, can now be established in eastern Australia.  The delineated source end-members include the Pacific and Indian Ocean asthenosphere, two mantle plumes (one presently located near the Bass Strait and another at Balleny Islands), and SCLM domains with both EM2 and EM1 signatures.  The secular distribution of these mantle reservoirs are consistent with the evolutionary history of the Tasman foldbelts in eastern Australia.

References

Hergt, J.M., and 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.

Lanyon, R., Crawford, A.J., and Eggins, S.M., 1995.  Western migration of Pacific Ocean upper mantle into the Southern Ocean region between Australia and Antarctica. Geology, 23, 511-514.

Zhang, M., Stephenson, J., O'Reilly, S.Y., McCulloch, M.T. and Norman, M., 1998a. Petrogenesis of late Cenozoic basalts in North Queensland and its geodynamic implications: trace element and Sr-Nd-Pb isotope evidence. Submitted to J. Pet.

Zhang, M., O'Reilly, S.Y. and Chen, D., 1998b.  Pacific- and Indian-MORB mantle as source reservoirs for the Cenozoic basalts in eastern Australia: Pb-Sr-Nd isotope evidence. Submitted to Geology.