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Journal of Petrology Volume 42 Number 4 Pages 685-719 2001
© Oxford University Press 2001
Petrogenesis and Geodynamic Implications of Late Cenozoic Basalts in North Queensland, Australia: Trace-element and Sr–Nd–Pb Isotope Evidence
1GEMOC ARC NATIONAL KEY CENTRE, DEPARTMENT OF EARTH AND PLANETARY SCIENCES, MACQUARIE UNIVERSITY, SYDNEY, N.S.W. 2109, AUSTRALIA
2DEPARTMENT OF EARTH SCIENCES, JAMES COOK UNIVERSITY, TOWNSVILLE, QLD. 4811, AUSTRALIA
3RESEARCH SCHOOL OF EARTH SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
Received September 22, 1999; Revised typescript accepted June 26, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL BACKGROUNDCENOZOIC MAGMATISM IN NORTH...PETROGRAPHYANALYTICAL TECHNIQUESBASALT GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Radiogenic isotopic (Sr–Nd–Pb) and trace-element compositions of late Cenozoic basalts from two discrete geographical regions in North Queensland, Australia, can be used to identify contributions from geochemically distinctive mantle source components. The North Queensland basalts have positive
7/4Pb and 8/4Pb values (relative to the Northern Hemisphere Reference Line), and high 206Pb/204Pb and 87Sr/86Sr at given 
Nd relative to Tertiary basalts in New South Wales, eastern Australia. The northernmost Cooktown nephelinites in North Queensland are isotopically depleted with low 87Sr/86Sr and 206Pb/204Pb and high 143Nd/144Nd compared with the more southern enriched Atherton–Nulla basalts. Sr–Nd–Pb isotopic data fit with two-component mixing between an isotopically depleted Indian Ocean mid-ocean ridge basalt source component and an enriched mantle component with an EM2 signature. The geochemical characteristics of the isotopically enriched Atherton–Nulla basalts are consistent with contributions from a subcontinental lithospheric mantle modified by subduction-related metasomatism. The isotopically depleted Cooktown nephelinites show HIMU-like incompatible element signatures that can be attributed to contributions from amphibole- and apatite-bearing assemblages in the lithospheric mantle. The low 206Pb/204Pb and high Nd of these depleted basalts are not correlated with their respective high U/Pb and low Sm/Nd parent/daughter element ratios. This decoupling implies that the formation of the metasomatic amphibole and apatite assemblages must be a close precursor of the magmatism, possibly connected with the eastward migration of the Indian Ocean asthenosphere and/or subduction at the northeastern margin of the Australian plate during early Tertiary time. KEY WORDS: Cenozoic; basalt; Queensland; Australia; isotopes
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDCENOZOIC MAGMATISM IN NORTH...PETROGRAPHYANALYTICAL TECHNIQUESBASALT GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Basaltic magmas in continental settings are often inferred to form by decompression partial melting of an upwelling mantle plume or asthenospheric mantle close to the base of the subcontinental lithospheric mantle (SCLM) as discussed by White & McKenzie (1989)
. The primary magmas can be modified on their way to the surface by interaction with their lithospheric wall rocks. Small volumes of alkaline mafic magmas can also originate within the SCLM through direct partial melting of metasomatized layers as a consequence of thermal perturbation (McKenzie, 1989). Consequently, primitive continental basalts erupted within different crustal domains (or at different times) can record geochemical signatures of three major mantle reservoirs (mantle plumes, asthenosphere and SCLM), and can also trace the secular evolution of SCLM which, in general, ties in closely with the evolutionary history of the crustal domains mapped at the surface (e.g. Menzies, 1990; Wilson & Downes, 1991; O’Reilly & Zhang, 1995). Basaltic magmatism has been widespread in eastern Australia during the last 70 my (Fig. 1a). The basalt provinces stretch nearly 4000 km north–south in a belt from North Queensland to Tasmania, mainly within 200 km of the eastern and southern Australian coast (Fig. 1a). Most of the basalts have erupted through a variety of terranes in the Phanerozoic Tasman Foldbelt, but some provinces in North Queensland and Tasmania are located in Proterozoic terranes.
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Three types of volcanic provinces in eastern Australia have been previously recognized: (1) leucitite provinces; (2) central-volcano provinces; (3) lava-field provinces (Fig. 1a; Wellman & McDougall, 1974
; Duncan & McDougall, 1989). The leucitite provinces mainly consist of minor intrusions distributed 
500 km inland in the southern part of the Tasman Foldbelt. The central-volcano provinces consist of thick volcanic sequences dominated by fractionated basalts and with a high proportion of differentiated rocks (rhyolite, trachyte or phonolite). The central-volcano magmatism began 35 my ago in Central Queensland and progressively youngs southwards (Fig. 1a), consistent with the separation rate of the Australian and Antarctic plates (at a present rate of 65 ± 3 mm/yr). This time progression of the central-volcano magmatism is considered to reflect the northward drift of the Australian plate over a mantle plume (hotspot) now possibly situated beneath the Bass Strait between the Australian continent and Tasmania (Wellman & McDougall, 1974) and referred to as the Australian plume.
The lava-field provinces are composed predominantly of small-volume alkaline basalts (bearing abundant mantle xenoliths), occurring as dykes, volcanic necks, breccia pipes, diatremes and maars or isolated flows, and rarely as long lava flows (e.g. North Queensland, Stephenson et al., 1998). Their age distribution does not show any systematic relationship to the northward motion of the Australian plate as does the central-volcano magmatism (Johnson, 1989). In New South Wales and Victoria, most of the lava-field basalts are older than or approximately the same age as the central-volcano basalts located at the same latitude, whereas in North Queensland, the main lava-field basalts are much younger than the central-volcano basalts in the northern part of Central Queensland (Fig. 1a).
The chemical diversity and primary nature of many Cenozoic basaltic rocks in eastern Australia record the presence of and interaction between major mantle reservoirs (e.g. Ewart et al., 1988; Sun et al., 1989; O’Reilly & Zhang, 1995; Zhang et al., 1999). Two mantle plumes (hotspots) have been active beneath the Australian plate in the Cenozoic. The Australian plume is inferred to be the dominant mantle source for the central-volcano mafic magmatism during the last 35 my (Wellman & McDougall, 1974; Fig. 1a). Sun et al. (1989) showed that the primitive central-volcano basalts derived from the Australian plume have Dupal-type Pb–Sr–Nd isotopic signatures (Hart, 1984) and smooth incompatible trace element patterns, similar to average oceanic island basalts (Sun & McDonough, 1989). The other hotspot is at present located near the Balleny Islands in the Ross Sea region of Antarctica, forming a seamount chain from the Balleny Islands to the East Tasman Plateau (the Soela Seamount), 300 km SE of Tasmania (Duncan & McDougall, 1989). Lanyon et al. (1993) reported that some of the seamount basalts and some strongly alkaline basalts in Tasmania display Pb–Sr–Nd isotope signatures similar to but less extreme than those from the distinctive HIMU Cook–Austral plume in the South Pacific Ocean (Woodhead, 1996). They proposed a plume-related HIMU component as one of the mantle sources for the Tasmania basalts.
An isotopically depleted asthenospheric (MORB) source is proposed for many discrete lava-field basalts that are temporally and spatially distant from the plume-derived central-volcano basalts (Sun et al., 1989). On the basis of preliminary Sr–Nd–Pb isotopic data for some mantle-xenolith-bearing lava-field basalts from both New South Wales and North Queensland, Zhang et al. (1999) further identified the presence and secular distribution of asthenospheric mantle sources with Pacific MORB and Indian MORB chemical signatures in eastern Australia. Their data suggest that the Indian MORB source is a widespread long-term asthenospheric reservoir beneath the eastern Gondwana lithosphere and that the westward migration of the Pacific MORB source may have been associated with the Tasman Sea opening (80–65 Ma, Veevers et al., 1991).
The potential role of SCLM as a mantle source component for the Australian basaltic rocks has been considered by, for example, Nelson et al. (1986), Sun et al. (1989), O’Reilly & Zhang (1995) and Zhang & O’Reilly (1997). For example, an EM1-type SCLM component may have significantly contributed to the leucitites (Nelson et al., 1986) and Dubbo alkaline basalts (Zhang & O’Reilly, 1997) from the inland part of southeastern Australia (Fig. 1a).
In this paper, we present detailed geochemical data for late Cenozoic lava-field basalts erupted within both Proterozoic and Phanerozoic terranes in North Queensland (Fig. 1b), eastern Australia, to constrain the petrogenesis and nature of mantle reservoirs beneath this region. We focus particularly on the SCLM domains beneath different tectonic terranes. The geodynamic implications of the identified mantle components provide new information on the Phanerozoic tectonic evolution of the Tasman Foldbelt in eastern Australia and on regional plate motion since the late Mesozoic. The new data, combined with previous published data, allow us to identify isotopically distinct mantle sources that have contributed to the Cenozoic intraplate basalts in eastern Australia.
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GEOLOGICAL BACKGROUND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDCENOZOIC MAGMATISM IN NORTH...PETROGRAPHYANALYTICAL TECHNIQUESBASALT GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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The major tectonic units in North Queensland include the Mesoproterozoic Georgetown Inlier and several provinces of the Phanerozoic Tasman Foldbelt (Fig. 1b), separated by the Tasman Line, the major tectonic boundary in eastern Australia that divides Precambrian from Phanerozoic terranes (Plumb, 1979
). The Georgetown Inlier consists of major Mesoproterozoic granitoids and multiply deformed (1570–300 Ma) Carpentarian metamorphic rocks of granulite to greenschist facies, and minor Adelaidian metasediments (Blewett et al., 1998). The Phanerozoic Tasman Foldbelt in the region consists of the Hodgkinson Basin, the Broken River Province, the Lolworth–Ravenswood Province, and the Coastal Igneous Province (Stephenson et al., 1980). The Hodgkinson Basin formed during late Carboniferous–early Permian cauldron-subsidence, following the climactic late Devonian–early Carboniferous orogeny. The Broken River Province contains Silurian–Devonian greenschist facies flysch sediments and basaltic–andesitic volcanic rocks, overlain by Carboniferous terrestrial and marine sediments. Permo-Carboniferous post-orogenic felsic extrusive and intrusive rocks are widespread in the Coastal Igneous Province. The Lolworth–Ravenswood Province consists of Cambro-Ordovician sediments and felsic volcanic rocks, and possibly Precambrian metamorphic rocks of amphibolite to greenschist facies, intruded by middle Ordovician granitoid bodies.
The depth to the crust–mantle boundary is 30 km for both Precambrian and Phanerozoic terranes in the region, as inferred from mantle xenolith studies (O’Reilly et al., 1997). Shear-wave seismic tomography indicates that seismic velocity in the mantle is relatively slow to the depth of 170 km, but high below 210 km in North Queensland (Simons et al., 1999). Observed in situ stress data are not available in North Queensland, but a compressional stress regime with consistent NNE-oriented maximum horizontal stress is displayed in the Bowen Basin, south of the North Queensland region (Hillis et al., 1999). Numerical modelling of the intraplate stress field in the Indo-Australian plate produces contradictory results for the region, from an extensional stress regime induced by the New Hebrides and Tonga–Kermadec arc–trench systems (Cloetingh & Wortel, 1986) to a compressional stress regime with small stress magnitudes (Coblentz et al., 1998).
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CENOZOIC MAGMATISM IN NORTH QUEENSLAND |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDCENOZOIC MAGMATISM IN NORTH...PETROGRAPHYANALYTICAL TECHNIQUESBASALT GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Cenozoic basalts in North Queensland cover an area of
23,000 km2 with a total volume of >650 km3 and extending nearly 1200 km north–south as a part of the eastern Australian volcanic zone. The magmatism occurred in two episodes, at 44–25 Ma and 8·0–0·01 Ma (Stephenson, 1989). The older basaltic rocks include plugs and flow remains from Mingela (44–25 Ma) and Pentland (27–25 Ma; Fig. 1b), in addition to several isolated residual plugs and/or flow remains west of Atherton and near Mount Fox. These early-episode basalts are contemporaneous with plume-related central-volcano basalts from Nebo, Peak Range and Springsure (33–24 Ma) and lava-field basalts from Anakie (31–19 Ma) located 300–500 km to the south in the northern part of Central Queensland (‘Central Queensland plume-related basalts’ hereafter, Fig. 1a).
The younger episode is the major magmatic event in the region, with basaltic provinces composed of many shield, composite and scoria volcanoes (up to 164 eruptive centres in McBride; Stephenson, 1989) and long lava flows (e.g. Undara and Kinrara flows in McBride and Toomba flow in Nulla, Stephenson et al., 1998). Six of the young basaltic provinces, namely Atherton, McBride, Chudleigh, Nulla, McLean and Piebald (Table 1; Fig. 1b), were chosen for this study. The McBride and Chudleigh basalts erupted largely through the Mesoproterozoic Georgetown Inlier, whereas the McLean, Piebald, Atherton and Nulla Provinces are located in various blocks of the Phanerozoic Tasman Foldbelt. All the six provinces are lava-field provinces. In this paper, the term ‘Atherton–Nulla Provinces’ is used to represent Atherton, McBride, Chudleigh and Nulla Provinces, and ‘Cooktown Provinces’ to represent McLean and Piebald Provinces.
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PETROGRAPHY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDCENOZOIC MAGMATISM IN NORTH...PETROGRAPHYANALYTICAL TECHNIQUESBASALT GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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The North Queensland mafic volcanic rocks are dominantly (>90%) undersaturated alkaline basalts, with <10% olivine (ol) tholeiites (9–18% normative hy), based on the CIPW-normative classification of Johnson & Duggan (1989)
and O’Reilly & Zhang (1995). The alkaline basalts range from strongly undersaturated nephelinites (some being lc normative; Stephenson, 1989; Barron et al., 1996), through nepheline (ne) hawaiites and basanites to moderately undersaturated alkali olivine basalts and hawaiites. Nephelinites are found only in Cooktown Provinces, whereas hawaiites and/or ne hawaiites are the major rock types in Atherton–Nulla Provinces, associated with basanites. Minor ol tholeiites occur in Atherton, McBride and Piebald (Table 1).
All the mafic rocks are porphyritic with 5–30% microphenocrysts (e.g. Stephenson et al., 1998). Olivine is a ubiquitous phenocrystic phase (0.05–3 mm) and dominant in nephelinites and basanites. Clinopyroxene (0·1–5 mm) and plagioclase (<3 mm) are more abundant than olivine as microphenocrysts in ne hawaiites, hawaiites and ol tholeiites. Compositional zoning in clinopyroxene (including hourglass and sector zoning) is common, with Ti-augite rimming diopside–salite. Olivine, clinopyroxene, plagioclase, magnetite, ilmenite and apatite are the groundmass phases in all the mafic rocks. Alkali feldspar is an interstitial phase in the more fully crystallized alkaline basalts. Both nepheline and analcite are found as poikilitic grains in the groundmass of nephelinites and some ne hawaiites and basanites. The samples are generally fresh under the microscope. The majority (>30 samples) do not show any alteration of either microphenocrysts or groundmass. Fresh glass is still preserved in many young lava flow samples from Toomba and Kinrara (Table 2). However, some samples show slight alteration around olivine and, to a lesser extent, clinopyroxene microphenocrysts (e.g. MB281 from the Undara lava flow) with or without alteration in the aphanitic groundmass (e.g. MB355 from McBride). Secondary fillings are also present in several vesicular samples (e.g. BK114 from Atherton). Rare plagioclase and quartz xenocrysts are found in three Nulla lava flow samples.
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Mantle and granulite-facies lower-crustal xenoliths, such as peridotites, pyroxenites, and mafic and felsic granulites, occur in many studied samples. The host rocks include mainly strongly alkaline basalts (nephelinite, basanite and ne hawaiite) from all the provinces and, to a lesser extent, some alkali olivine basalts and hawaiites from Atherton and Nulla. Pyroxene, amphibole, phlogopite, anorthoclase, spinel, ilmenite and garnet (rare) megacrysts have also been found.
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ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDCENOZOIC MAGMATISM IN NORTH...PETROGRAPHYANALYTICAL TECHNIQUESBASALT GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Forty-four samples of North Queensland basaltic rocks (locations, ages and types listed in Table 2) were analysed for major element [by X-ray fluorescence (XRF)] and trace element [by XRF and inductively coupled plasma mass spectrometry (ICPMS)] compositions (Table 3). Ni, Cr, Zn, Cu, V, Rb, Sr, Ba, Zr, Ga and Nb contents listed in Table 3 were determined by XRF, whereas the other trace elements were determined by ICPMS. A subset of 22 basalt samples were analysed for Sr–Nd–Pb isotope ratios (Table 4). Some mantle peridotite xenoliths from North Queensland and New South Wales were also analysed for Rb, Sr, Nd and Sm abundances, and Sr–Nd isotope ratios.
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After weathered surfaces were removed from basalt sample blocks, thin (2–5 mm) slabs were cut and crushed to chips of <5 mm in size between polythene sheets. The chips then were carefully hand-picked to remove xenocrystic material and secondary fillings and then were ground, using an agate mortar, for elemental and Sr–Nd isotope analyses.
The XRF analyses for major and trace elements were carried out at Macquarie University, using methods described by O’Reilly & Zhang (1995). The solution ICPMS analyses were performed using a Perkin–Elmer ELAN 5100 instrument at Macquarie University. Sample preparation, instrument operating conditions, and calibration procedures of the analyses were described by Norman et al. (1998). Conservative estimation of the detection limits is much less than 50 ppb for all elements except for the transition metals and Zr (<100 ppb). The in-house standard Kilauea 93-1489 was analysed as an unknown to monitor drift and data quality during the period of the analyses. Precision [determined by repeated analysis of Kilauea 93-1489 and presented as relative standard deviation (% RSD) for the period of analysis] is better than 4% except for Li, Be, Zn, Cd, and Sb (5–12%). Analytical accuracy (presented as average measured value/recommended value) is better than 5%. Total analytical blank is <10 ppb for most elements except for Be, Cr, Ni, Cu, Zn, Sr, Ba and W (12–92 ppb), making blank corrections mostly <0·1% for the basalt samples. All the basalt samples were run in duplicate by both XRF and ICPMS; differences between the duplicate runs are <5%. Both XRF and ICPMS methods were used for 12 trace elements (Ni, Cr, Zn, Cu, V, Y, Rb, Sr, Ba, Zr, Ga and Nb) on all the samples. The difference between the two methods is generally <10% for all the elements except for Y (up to 20%).
Sr and Nd isotope ratios for the basalts (unleached powders) were analysed by thermal ionization mass spectrometry (TIMS) in the ARC University Consortium/CSIRO isotope laboratories at North Ryde, Australia, using a VG354 instrument. The analytical details have been given by O’Reilly & Zhang (1995). Average values of repeated standard analysis during the period of analysis at CSIRO (1995–1996) are 87Sr/86Sr = 0·710234 ± 3 (2 SE, n = 139) for SRM 987 and 143Nd/144Nd = 0·511107 ± 2 (2 SE, n = 5) for J&M Nd. Total procedural blanks for the isotopic analyses are <800 pg for Sr and <350 pg for Nd.
Sr and Nd isotope ratios and Sr, Rb, Nd and Sm contents for the peridotite xenoliths (unleached bulk-rock powders) were determined by TIMS at the Mineralogisk-Geologisk Museum in Oslo, Norway, using a VG354 instrument, with analytical methods described by Griffin et al. (1988). Average values of repeated standard analysis during the period of analysis (1983–1986) at Oslo are 87Sr/86Sr = 0·710239 ± 26 (2 SE, n = 104) for SRM 987, 143Nd/144Nd = 0·511129 ± 13 (2 SE, n = 62) for J&M Nd, and 143Nd/144Nd = 0·511864 ± 13 (2 SE, n = 12) for La Jolla Nd. Precision and accuracy for isotope dilution analyses of Sr, Rb, Nd and Sm contents are 
0·5%. Total procedural blanks are <1 ng for Sr, 100 pg for Nd, and <<100 pg for Sm and Rb. None of these blanks was significant at the levels of the samples analysed. The samples were carefully chosen from the interior of large xenoliths and were as free as possible from visible alteration or grain-boundary phases. Acid-leaching techniques were not adopted because it was recognized that acid-leaching could easily destroy metasomatic phases such as apatite and mica, which host a significant part of Sr and Nd. Although grain-boundary contamination could increase measured bulk-rock 87Sr/86Sr ratios, leaching experiments on an apatite-free xenolith (Griffin et al., 1988) demonstrated that the change is generally not significant. Therefore, the Sr isotope data obtained from unleached mantle xenoliths would not affect the conclusions of this paper.
Pb isotopic ratios of basalts were analysed by TIMS at the Research School of Earth Sciences, Australian National University (ANU), and the ARC Consortium/CSIRO isotope centre, using a Finnigan MAT-261 and a VG-ISOMASS instrument, respectively. All the samples are hand-picked rock chips leached by warm 6N HCl, and conventional HBr–HCl separation procedure was adopted in both laboratories. A 207Pb–204Pb double-spiking technique was applied for Pb isotope analysis preformed at ANU, with external precision (2 SD) on the corrected data for a rock sample 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of ±0·003, ±0·003 and ±0·010, respectively (Woodhead et al., 1995). The double-spike was calibrated by Woodhead et al. (1995) using SRM 981 standard values of 206Pb/204Pb = 16·937, 207Pb/204Pb = 15·492 and 208Pb/204Pb = 36·708. Six SRM 981 analyses during this work are 16·938 ± 0·007 for 206Pb/204Pb, 15·494 ± 0·007 for 207Pb/204Pb, and 36·713 ± 0·022 (2 SD) for 208Pb/204Pb. Total procedural blanks are 100 pg. Pb isotope analyses at CSIRO followed the procedures described by Gulson et al. (1984). Pb isotope ratios were normalized for mass fractionation to SRM 981 (Todt et al., 1984), by applying a correction factor of +0·08%/a.m.u.. External precision (2 SD) based on >2000 replicate analyses of SRM 981 is ±0·017, ±0·008 and ±0·018 for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, respectively.
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BASALT GEOCHEMISTRY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDCENOZOIC MAGMATISM IN NORTH...PETROGRAPHYANALYTICAL TECHNIQUESBASALT GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Major elements and transition metals
MgO contents of the North Queensland basalts range from 12·8 to 4·3 wt %, with mg-number [mg-number = Mg/(Mg + Fe2+), assuming Fe2O3/FeO = 0·2] ranging from 0·70 to 0·49 (Fig. 2). In general, the nephelinites have the highest mg-number, whereas the ol tholeiites have the lowest. Nepheline hawaiites display the largest variation in mg-number (0·68–0·49). Nine samples with mg-number <0·60 include ol tholeiites, hawaiites and ne hawaiites. SiO2 content for the North Queensland basalts ranges from 40·3 to 51·7 wt %, with nephelinites
42 wt % and ol tholeiites >50 wt %. SiO2 and Al2O3 contents display broad negative correlations with mg-number, whereas CaO/Al2O3 shows positive correlation with mg-number (Fig. 2).
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The majority of basalts in the Atherton–Nulla Provinces are hawaiites, alkali olivine basalts and olivine tholeiites (referred as Atherton–Nulla basalts). They differ significantly from the Cooktown nephelinites and several basanites and ne hawaiites from Cooktown, McBride and Chudleigh (referred as Cooktown nephelinites) in their high SiO2 and Al2O3, but low TiO2 and CaO/Al2O3 (Fig. 2). Within the Atherton–Nulla Provinces, however, basalts from the Georgetown Inlier (McBride and Chudleigh) overlap compositionally with those from the Phanerozoic foldbelts (Atherton and Nulla).
Compared with the lava-field basalts from New South Wales (O’Reilly & Zhang, 1995, and references therein), the Atherton–Nulla basalts are relatively high in SiO2 and Al2O3 (Fig. 2a and b), but low in TiO2 (Fig. 2c) and CaO at a given mg-number. Their CaO/Al2O3 ratios (0·58 ± 0·07) are lower than those of the New South Wales basalts (0·69 ± 0·12; Fig. 2d). In contrast, the Cooktown nephelinites are similar to strongly alkaline basalts in New South Wales. Total alkali (Na2O + K2O) contents of the North Queensland basalts are 4·2–8·8 wt %, generally higher than those for the New South Wales basalts, whereas the average K2O/Na2O ratios of the North Queensland basalts are similar to those for the New South Wales basalts (0·42 vs 0·44).
Ni and Cr contents are in the range of 306–35 ppm and 553–35 ppm, respectively, for the North Queensland basalts, and show well-defined positive correlations with mg-number. Co and V contents (51–27 and 249–127 ppm, respectively) do not correlate with mg-number. Scandium contents are 24–8 ppm for the North Queensland basalts. Most samples fall in a narrow range of 16 ± 3 ppm (Fig. 3a), significantly lower than that for the New South Wales basalts (23 ± 6 ppm).
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, weighted average of lower-crust mafic xenoliths from McBride (Rudnick & Taylor, 1987
, average lower-crust xenoliths from Chudleigh (Rudnick et al., 1986Incompatible trace elements
Most incompatible trace elements do not display coherent correlations with mg-number (Fig. 3) if all the North Queensland basalts are considered. There are systematic differences in incompatible trace element signatures between the Cooktown nephelinites and the Atherton–Nulla basalts. The Cooktown nephelinites are much higher in strongly incompatible elements such as light rare earth elements (LREE), Cs, Th, U and Nb (up to a factor of three), and marginally higher in Sr, Ba, Zr and Hf than the Atherton–Nulla basalts, but have abundances of some moderately incompatible elements [e.g. heavy rare earth elements (HREE) and Y] similar to those for the Atherton–Nulla basalts (Fig. 3).
Despite the lack of correlation between mg-number and incompatible elements for the North Queensland basalts, SiO2 contents display well-defined negative correlations with many incompatible trace elements such as Th, U, high field-strength elements (HFSE), LREE, Ba, Sr, P2O5, TiO2, Li and Be (Fig. 4). Only three samples plot above some of these trends; a hawaiite LKD9 (mg-number = 0·58), an ne hawaiite MB49 (mg-number = 0·58) and an ne mugearite 32-32 (mg-number = 0·49). Negative correlations of SiO2 vs Cs, Rb, K2O, Y and HREE are present for some individual provinces, but not for all the North Queensland samples. No positive correlation between SiO2 and any incompatible elements has been observed.
Pb contents fall in a narrow range of 1·2–2·9 ppm for >80% of the North Queensland samples, whereas Ce contents vary by a factor of six (145–25 ppm) and U by a factor of nine (0·4–3·6 ppm). The ranges for Ce/Pb and U/Pb are 19–57 and 0·25–1·5, respectively, for all but two (Fish199 and TT7) samples, with the Cooktown nephelinites having the highest Ce/Pb and U/Pb. Ewart et al. (1988) reported even higher Ce/Pb (117) and U/Pb (2·75) in a Cooktown nephelinite (LKD-5).
Compared with the New South Wales basalts, the Atherton–Nulla basalts are normally at the low end of the compositional ranges for Th, HFSE and REE contents at a given mg-number, whereas the Cooktown nephelinites are at the high end (Fig. 3b, d and e). The abundances for some large ion lithophile elements (LILE) such as Rb, Ba and Sr are broadly similar for the North Queensland and New South Wales basalts.
All the North Queensland basalts are LREE enriched with chondrite-normalized La/Yb of 6–41. The basalts from Atherton have the lowest La contents (18–38 ppm) and La/Yb ratios (mostly 9–22), whereas the Cooktown nephelinites have the highest (35–82 ppm and 16–41, respectively).
Incompatible element patterns of the North Queensland basalts vary with major element chemistry (i.e. rock types). The Cooktown nephelinites and basanites are characterized by high abundances of strongly incompatible elements and marked enrichment in Nb, Ta, Th, U and LREE, and depletion in Rb and K (Fig. 5e), similar to some strongly alkaline basalts elsewhere in eastern Australia (e.g. BR7 from Barrington, New South Wales, and BHT from Tasmania; Fig. 5f) and oceanic basalts with HIMU isotopic signatures (e.g. Weaver, 1991; Chauvel et al., 1992; Woodhead, 1996). HFSE do not behave coherently as a group, as variable degrees of depletion in Zr, Hf and Ti are shown in these samples. Some basanites and several ne hawaiites from the Atherton–Nulla Provinces have incompatible element patterns generally similar to the Cooktown nephelinites, but with lower elemental abundances (Fig. 5a–c). Others are depleted in Th and U relative to Ba and Nb. Enrichment in P and Sr relative to Ce and Nd is characteristic for basanites and, to a lesser extent, nephelinites.
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The North Queensland ol tholeiites and hawaiites have overall relatively low incompatible element abundances and different incompatible element patterns. Many of them are enriched in K relative to Nb, Ta and LREE, Ba relative to Rb and Th, and Sr relative to Ce, P and Nd (Fig. 5a–d). The hawaiites and ol tholeiites generally resemble primitive central-volcano basalts from Central Queensland, Mingela plug basalts and some isotopically enriched alkaline basalts from New South Wales (Fig. 5f) in terms of both incompatible element abundances and patterns. However, the latter rocks do not show any enrichment in K relative to Nb (Fig. 5f). Lead is strongly depleted relative to Ce and Sr in the North Queensland nephelinites and basanites, but only slightly depleted or even enriched in the hawaiites and tholeiites (Fig. 5).
Sr–Nd–Pb isotopic ratios
A subset of the North Queensland basalts chosen for Sr–Nd–Pb isotopic analyses covers the whole spectrum of major element variations, with a majority bearing mantle xenoliths or relatively primitive samples. The isotopic data for the North Queensland basalts are plotted in Figs 6 and 7. Five samples of the subset with mg-number <0·60 and/or SiO2 > 50 wt % are labelled in the isotopic diagrams for distinction (Figs 6 and 7). Published data for several North Queensland basalts (many containing mantle xenoliths; Rudnick et al., 1986; Ewart et al., 1988; Stolz & Davies, 1989), and our preliminary data for the early-episode Mingela plugs in North Queensland (Zhang et al., 1996) are also included in the dataset. Also plotted for the purpose of comparison are Sr–Nd–Pb isotopic data for the New South Wales lava-field basalts (Wilkinson & Hensel, 1991; O’Reilly & Zhang, 1995, and references therein) and those for Central Queensland plume-related basalts (Ewart et al., 1988; Sun et al., 1989; Sutherland, 1998).
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87Sr/86Sr ratios range from 0·70323 to 0·70472 and 143Nd/144Nd ratios range from 0·51304 to 0·51279 (Nd = +7·8 to +3·0; Fig. 6) for the North Queensland basalts. The most depleted sample is the McBride ne mugearite 32-32 (Stolz & Davies, 1989). Among the individual provinces, the Chudleigh basalts have high Nd (+7·5 to +6·5), whereas the McBride basalts have low 87Sr/86Sr (<0·7040 except for basanite MBR6). Atherton basalts have the highest 87Sr/86Sr (up to 0·70472) among the North Queensland samples. The Cooktown basalts vary widely in Sr–Nd isotopic ratios, with the nephelinites being low in 87Sr/86Sr and high in Nd. The five samples with low mg-number and/or high SiO2 have relatively high 87Sr/86Sr and low Nd.
Pb isotopic ratios range in 206Pb/204Pb from 17·86 to 18·63, in 207Pb/204Pb from 15·51 to 15·62, and in 208Pb/204Pb from 37·75 to 38·67. All the samples plot above the Northern Hemisphere Reference Line (NHRL) line, ubiquitously displaying Dupal Pb isotopic signatures (Fig. 7a and b; Hart, 1984) with 8/4Pb = +32 to +72 and 7/4Pb = +3·3 to +10·9 (Table 4). The Cooktown, Nulla and McBride samples cover almost the whole range of Pb isotope variation with the nephelinites at the low 206Pb/204Pb end and the ol tholeiites at the high 206Pb/204Pb end, whereas the Atherton basalts are mostly at the high 206Pb/204Pb (>18·30) end. Pb isotope data for Cooktown basalts reported by Ewart et al. (1988) are in agreement with our results. The Nulla lava flow samples reported by Ewart et al. (1988) are higher in 206Pb/204Pb than the Nulla lava flow samples analysed here (18·37–18·66 vs 17·86–18·30), but all the samples show elevated 7/4Pb (+8·7 to +12·5) relative to the other North Queensland basalts, thus plotting on the upper part of the 206Pb/204Pb –207Pb/204Pb trend (Fig. 7a). The low-mg-number and/or high-SiO2 samples cannot be distinguished from the more primitive ones in terms of lead isotopes (Fig. 7a and b).
In general, Sr–Nd–Pb isotopic compositions overlap each other at individual province levels although the Atherton basalts tend to have relatively high 87Sr/86Sr and 206Pb/204Pb, but low Nd. There are no systematic variations in terms of Sr–Nd–Pb isotope ratios between the McBride and Chudleigh basalts erupted from the Precambrian Georgetown Inlier on one hand and the Atherton, Nulla and Cooktown basalts from the Phanerozoic Tasman Foldbelt on the other.
Although the ranges of Sr and Nd isotopic ratios for the North Queensland basalts overlap those for the New South Wales basalts, they clearly differ from the latter in their high 87Sr/86Sr at a given Nd. In Fig. 6, all but three North Queensland samples define a well-defined negative 87Sr/86Sr vs Nd trend above the trend formed by the New South Wales basalts. Only one sample from McBride (MBR1) plots in the central part of the New South Wales sample population. In contrast, the Central Queensland plume-related basalts and the old Mingela hawaiites all lie below the North Queensland trend, within the field of the New South Wales basalts. The primitive (xenolith-bearing) Central Queensland plume-related basalts have Sr–Nd isotopic compositions similar to or more depleted than the Mingela hawaiites. Three North Queensland samples (CHD18, TOM and LKD2) have high 87Sr/86Sr at a given Nd, thus plotting to higher 87Sr/86Sr values at given Nd than the North Queensland trend, in the field of Tonga–Kermadec Island arc basalts (Fig. 6). The most depleted North Queensland samples plot within the Indian MORB field, whereas the most depleted New South Wales ones plot close to the enriched end of the Pacific MORB field.
Most of the North Queensland samples plot within the Indian MORB field on the 206Pb/204Pb vs 208Pb/204Pb diagram, but many plot above the Indian MORB field on the 206Pb/204Pb vs 207Pb/204Pb diagram. On the other hand, the New South Wales basalts have high 206Pb/204Pb (18·70–19·14), with some having negative 7/4Pb and 8/4Pb values (Fig. 7a and b; Zhang et al., 1999). In contrast to the nearly vertical trends shown by the New South Wales basalts on the 206Pb/204Pb vs 87Sr/86Sr and Nd diagrams (Fig. 7c and d), the North Queensland basalts show a positive correlation between 206Pb/204Pb and 87Sr/86Sr and a negative correlation between 206Pb/204Pb and Nd. The 208Pb/204Pb vs 87Sr/86Sr and Nd trends for the North Queensland basalts are also distinct from those for the New South Wales basalts. The Central Queensland plume-related basalts differ from their North Queensland counterparts in their high 206Pb/204Pb ratios and negative correlations between 206Pb/204Pb and 87Sr/86Sr (Fig. 7c), with the primitive samples having 7/4Pb and 8/4Pb values lower than the North Queensland basalts and close to the Pacific MORB field.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDCENOZOIC MAGMATISM IN NORTH...PETROGRAPHYANALYTICAL TECHNIQUESBASALT GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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Evaluation of crustal contamination
The observed isotopic trends of the North Queensland basalts reflect mantle source heterogeneity and/or crustal contamination. The enriched (high 87Sr/86Sr and 206Pb/204Pb, low
Nd) end-member could be a component from the upper or lower continental crust, a mantle plume, or enriched SCLM domains.
As shown in Fig. 6, the high-SiO2 and low mg-number North Queensland basalt samples have relatively high 87Sr/86Sr and low Nd values. These geochemical signatures could reflect crustal contamination (e.g. Ewart et al., 1988; Price et al., 1997). Alternatively, they could be derived from various degree of partial melting of heterogeneous mantle sources. Therefore, it is necessary to evaluate the extents of crustal contamination before using geochemical signatures of the North Queensland basalts to constrain their potential mantle sources.
If crustal contamination is the cause of the negative correlations between SiO2 and many incompatible elements (Fig. 4), the process must have resulted in dilution instead of enrichment for the incompatible element abundances, including those highly enriched in the felsic upper continental crust such as U, Th, LREE, Cs, Rb, K, Ba, Pb and Li. In Fig. 4, the compositional ranges of these elements for estimated average upper and lower continental crust (Taylor & McLennan, 1995; Wedepohl, 1995; and references therein) are plotted at the high-SiO2 end of the diagrams. Although bulk assimilation of upper continental crust could produce some of the observed trends (e.g. for Nb, Sr and Zr), the upper crust cannot serve as a ‘diluent’ for the other observed trends (e.g. for U, Th, LREE and Li; Fig. 4) because of its high abundances of these elements. The most SiO2-rich and low-mg-number basalts such as ol tholeiites CK25 and MB355 are among the North Queensland samples with the lowest Pb (Fig. 4f), K2O and Rb (Table 3). Thus, they are very unlikely to have been derived by upper-crustal contamination of any of the parent melts of the studied North Queensland basalts.
Silica contents of average lower continental crust estimated on the basis of exposed granulite terranes (Taylor & McLennan, 1995; Wedepohl, 1995; and references therein) are 55–61 wt %. This andesitic composition is suitable as an contaminant, but less effective (>30 wt % crustal material is required) in producing the elevated SiO2 in the evolved basalt samples. In addition, abundances of many incompatible elements such as U, Th, Sr and LREE in the estimated lower crust are similar to or even higher than those for the most evolved basalts CK25 and MB355 (Fig. 4a–c). Thus, contamination processes alone can hardly produce these negative trends. The negative correlation between SiO2 and Li (Fig. 4e) and low Pb contents of the evolved basalts (e.g. CK25 and MB355; Fig. 4f) do not fit a model dependent upon lower-crust contamination either.
The positive correlation between SiO2 and K/Nb both for all the North Queensland basalts and at the individual province level (Fig. 8a) could have resulted from crustal contamination, as both upper and lower continental crusts have elevated K/Nb ratios (500–2300). However, both estimated crusts have low Sr/La ratios that cannot account for the broad positive K/Nb–Sr/La trend of the basalts (Fig. 8b). In detail, the SiO2–K/Nb trend (Fig. 8a) consists of two parallel sub-trends for individual provinces; i.e. the Cooktown basalts and some McBride basanites and ne hawaiites form a sub-trend parallel to but below the sub-trend formed by the other North Queensland basalts, particularly the Nulla and Atherton basalts. This suggests that the primitive magmas are not homogeneous in terms of K/Nb among and within individual provinces as a result of variable degrees of partial melting of an inhomogeneous mantle source, and that crustal contamination, if it exists, may only superimpose insignificant contributions upon the trends. At the high-SiO2 end of some trends shown in Fig. 4, evolved samples such as LKD9, TT7 and MB217 plot above the general trend, and this may indicate some crustal assimilation and/or fractional crystallization (AFC) processes in these few samples.
Another element ratio sensitive to crustal contamination is Ce/Pb, as continental crust normally contains significant amounts of Pb. Although the negative correlation between Ce/Pb and 87Sr/86Sr and positive correlation between Ce/Pb and Nd for the North Queensland basalts may suggest potential crustal contamination, most North Queensland basalts have Ce/Pb ratios within or higher than the ratio range for most oceanic basalts (e.g. 25 ± 5, Hofmann et al., 1986) with only two samples (TT7 and Fish-199) being significantly low in Ce/Pb. As discussed above, the North Queensland basalts display limited variation in Pb, but significant variation in Ce correlated with SiO2 (Fig. 4c and f). Thus, the observed correlations involving Ce/Pb more probably reflect enrichment of Ce instead of Pb in the basalts, and this is likely to be achieved by decreasing melting degree or source enrichment, but not by crustal contamination.
The abundant granulite-facies mafic lower-crust xenoliths found from Atherton, McBride and Chudleigh are considered as products of basalt underplating and assimilation of the surrounding lower crust (e.g. Rudnick et al., 1986; O’Reilly et al., 1988). These xenoliths are characterized by (1) their mafic composition and low abundance of most incompatible elements (Fig. 4) and (2) highly heterogeneous Sr–Nd–Pb isotopic ratios (Fig. 9). Some xenoliths have Sr–Nd–Pb isotopic compositions indistinguishable from their host basalts, but many others have high 87Sr/86Sr, low Nd and high 7/4Pb values, lying on the continuation of the basalt trends (e.g. Fig. 9a). Therefore, in theory, they could have contributed to the observed chemical variations in the host basalts as potential contaminants. Two-component mixing curves between the primitive basalts from McBride, Chudleigh or Cooktown and the lower-crustal xenoliths from McBride and Chudleigh are calculated (Figs 8b and 9). The modelling suggests that assimilation of lower-crustal materials in moderate amounts can explain certain isotopic variations of the host North Queensland basalts. For instance, addition of up to 10 wt % Chudleigh xenoliths to Chudleigh basanite ChD11 may explain the three samples (CHD18, TOM and LKD2) plotting above the general 87Sr/86Sr–Nd and 87Sr/86Sr–206Pb/204Pb trends (Fig. 9).
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However, the mafic nature of the lower-crustal xenoliths makes them ineffective contaminants. Simple mass-balance estimation indicates that, for example,
40% of the xenoliths are needed to increase SiO2 contents of the host basalts from 46 wt % to 49 wt %. Although some of the modelling curves (e.g. ‘McB’ curves in Fig. 9 and ‘ChD’ curve in Fig. 8b) apparently well fit the basalt trends, at least 40–80 wt % of lower-crustal material is required if these trends had been formed by the contamination alone. This is extremely unlikely to be the case. Instead, it is possible that the North Queensland basalts contain a source component derived from the SCLM of the same age and similar geochemical characteristics as the least contaminated xenoliths (Wilson & Downes, 1991).
Mantle xenoliths and high-pressure pyroxene and spinel megacrysts occur in many North Queensland basalts, such as some of the Atherton basalts with enriched isotopic compositions (e.g. BK112; Fig. 6) and even some hawaiites with relatively low mg-number and/or high SiO2 (e.g. MB217 and 32-32 from McBride and LKD9 from Cooktown). The Chudleigh hawaiites and basanites contain mantle peridotite xenoliths up to 70 % by volume and up to 20 cm in diameter at three eruptive centres (Stephenson, 1989). Although high-pressure crystal fractionation in the upper mantle cannot be excluded, transportation of mantle fragments to the surface requires fast transmission of these mantle-derived melts to the Earth’s surface and precludes any significant interaction between the magma and the crustal wall rocks.
It is also evident in Fig. 10 that, although most evolved North Queensland basalts have high 87Sr/86Sr and low Nd, the most isotopically enriched samples include those with mg-number >0·67. The high mg-number (>0·67) samples cover almost the entire range of 87Sr/86Sr variation (0·70344–0·70472; Fig. 10a) and a large range of Nd variation (+6·2 to +3·0; Fig. 10b). Some basalts with low mg-number (e.g. 32-32 and some Chudleigh and Cooktown basalts) have even higher Nd (up to +7·8) than those with high mg-number. 206Pb/204Pb ratios of the primitive basalts also span almost the whole variation (from BK112 and C1 at the high end to CK40 at the low end; Table 3).
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In summary, the geochemical evidence indicates that crustal contamination is very unlikely to produce the main isotopic and trace element trends of the North Queensland basalts. These geochemical trends in the primitive and near-primitive basalts rather reflect heterogeneity in mantle sources and variations in partial melting processes and degrees. Therefore, these trends will be used to constrain their mantle sources.
Role of the Australian plume
The potential role of the Australian plume or other mantle plumes in the generation of the late Cenozoic North Queensland lava-field basalts needs to be evaluated because the region is only 300 km away from the plume-derived central-volcano provinces in Central Queensland (Fig. 1a) and because the early episode of basaltic magmatism in the region (Mingela and Pentland) was contemporaneous with or slightly later than the central-volcano magmatism (Fig. 1b). In this paper, the term ‘mantle plume’ (or ‘hotspot’) will be applied in a strict sense to an ‘active’ plume (Campbell & Griffiths, 1990) generated at the core–mantle boundary or the 660 km discontinuity (Hofmann, 1997) with recognizable geophysical features, but not to a ‘passive’ plume incubated at the base of the lithosphere (e.g. Kent et al., 1992). A plume ‘head’ could be responsible for generating voluminous flood basalts and causing continental break-up (e.g. Kerguelen Plateau) because of its large size and high temperature (White & McKenzie, 1989) whereas a plume ‘tail’ could account for long-lived seamount chains or continental intraplate volcanic trails, reflecting the locus of a moving plate over the narrow, stationary plume (e.g. Ninety-East Ridge of the Indian Ocean; Mahoney et al., 1995).
The evaluation should include two aspects. First, are the late Cenozoic North Queensland basalts directly (wholly or partially) sourced by a mantle plume from deep mantle sources in the region? If not, did the Australian plume leave any of its geochemical fingerprints in these basalts?
In North Queensland, the spatial distribution of the lava-field basalts does not provide any evidence for the migration of the locus of volcanism with time (Stephenson et al., 1980), as predicted by conventional hotspot models and manifested by the Australian central-volcano provinces (Duncan & McDougall, 1989). Nor do geophysical data support the presence of such a mantle plume. Seismic tomographic studies (Anderson et al., 1992; Zielhuis & van der Hilst, 1996; van der Hilst et al., 1997) suggest a pronounced mantle low-velocity zone at 80–140 km depths (approximately the depth of the asthenosphere–lithosphere boundary beneath the Tasman Foldbelt; Muirhead & Drummond, 1991) along the entire eastern Australian region. The North Queensland mantle becomes relatively cool with increasing depth, and is one of the currently coldest regions both in eastern Australia and globally at 300–600 km depth (Anderson et al., 1992). As the Australian plate has moved rapidly northward since 43 Ma, it may have overridden cold slab materials subducted from north of Papua New Guinea in the early Tertiary (Johnson et al., 1978; Müller et al., 1998). In contrast, the hottest mantle region in eastern Australia at such depths occurs in the Bass Strait and Tasmania, near the proposed present-day site of the Australian plume.
The thermal structure of the mantle beneath North Queensland therefore contrasts markedly with that beneath active plumes at oceanic settings that show a cylindrical zone of low P- and S-wave velocities extending from 100 km to at least 400 km beneath the surface [e.g. Hawaii (Anderson et al., 1992) and Iceland (Wolfe et al., 1997)]. A fossil plume (135 Ma) revealed in the Parana Basin, Brazil, by teleseismic travel-time study (VanDecar et al., 1995) also has a vertical-cylinder-shaped large-scale low-velocity structure extending to at least 500–600 km depth in the upper mantle. We believe that partial melting beneath North Queensland was mainly restricted to the vicinity of the lithosphere–asthenosphere boundary, consistent with the slow S-wave signal from 100–200 km depth. Generation of the North Queensland basalts could be induced by upwelling asthenospheric diapir(s) responding to regional extension and/or stress release (Cloetingh & Wortel, 1986). A mantle plume is unlikely to have played any direct role in the late Cenozoic magmatism in North Queensland.
The absence of an active mantle plume as a mantle source for the late Cenozoic North Queensland lava-field basalts does not necessarily mean that the Australian mantle plume had not played any indirect role in the late episode of magmatism in North Queensland. The asthenospheric mantle volume that interacted with the upwelling Australian plume 35 my ago is unlikely to have contributed to the late Cenozoic basalts, as it should have been located at least 2000 km to the south when the magmatism began 8 my ago. Nevertheless, the possibility for an SCLM source that had been metasomatized by the Australian plume warrants further discussion.
Original geochemical signatures of the Australian plume are not well constrained (Sun et al., 1989) because the majority of the central-volcano volcanic rocks are evolved and geochemical data for the primitive or near primitive central-volcano basalts that contain mantle xenoliths and/or have high mg-number (>0·67) are limited. Published Sr–Nd–Pb isotope data for the Central Queensland plume-related basalts (Figs 6 and 7) show trends clearly in contrast to the late Cenozoic North Queensland basalts. In the Sr–Nd isotope diagram (Fig. 6), they all plot below the North Queensland basalt trend as do the New South Wales lava-field basalts, with the primitive samples having low 87Sr/86Sr (<0·7036) and high Nd (>+4). On the Pb isotope plots (Fig. 7a and b), the data for the Central Queensland plume-related basalts plot as a continuation of the North Queensland basalt trends at the high 206Pb/204Pb end. However, the negative correlation between 87Sr/86Sr and 206Pb/204Pb (Fig. 7c) for the Central Queensland plume-related basalts indicates that the Australian mantle plume has higher 206Pb/204Pb and lower 87Sr/86Sr than required to be an enriched source component for the North Queensland basalts. It is interesting to notice from our preliminary data (O’Reilly & Zhang, 1995; Zhang et al., 1996) that the Mingela basalts have both incompatible element patterns and Sr–Nd isotope ratios similar to the primitive Central Queensland plume-related basalts of similar age (Figs 5f and 6). The similarities indicate that the Australian plume may have contributed to the early Mingela basalts, although it is difficult to further assess from present data if the SCLM beneath North Queensland (and particularly that beneath Mingela in the southern part) has been significantly modified by the plume.
In summary, geological and geophysical data do not support the presence of a plume as a mantle source for the late Cenozoic magmatism in North Queensland during the last 10 my. The possibility for mantle metasomatism in the SCLM beneath North Queensland by the Australian plume cannot be unambiguously precluded. However, such an SCLM terrane, if it exists, may have not provided recognizable geochemical fingerprints in the late North Queensland basalts.
Nature of mantle sources for the North Queensland basalts
As crustal contamination alone cannot produce the observed radiogenic isotope trends (Figs 6 and 7) of the late Cenozoic North Queensland basalts, and Sr–Nd–Pb isotopic ratios of the primitive and near-primitive North Queensland basalts span almost the entire ranges of observed isotopic variations, a feasible explanation for the coherent trends involves a mixing of partial melts from two mantle end-member components. The depleted component has low 87Sr/86Sr and 206Pb/204Pb, and high 143Nd/144Nd, similar to source of the Indian MORB. The complementary enriched component points to a source with high 87Sr/86Sr and 206Pb/204Pb, but moderately low 143Nd/144Nd, indicative of an EM2 component (Zindler & Hart, 1986). Both components, however, have the typical Dupal 7/4Pb and 8/4Pb values (Hart, 1984). The isotopically depleted North Queensland basalts are dominantly the Cooktown (northern) nephelinites, whereas the isotopically enriched samples are mostly from the Atherton–Nulla (southern) basalts. Therefore, in the following discussion, the terms ‘Cooktown nephelinites’ and ‘Atherton–Nulla basalts’, respectively, are used to refer to the basalts with depleted and enriched isotopic signatures.
Indian MORB asthenosphere component
As a mantle plume has been excluded as a potential mantle source for the North Queensland basalts, the depleted isotopic signature most probably simply reflects an Indian MORB-type asthenospheric mantle source. It has been widely recognized that many young basalts from Western Pacific arcs and back-arc basins are similar to Indian MORB in terms of Sr–Nd–Pb isotopic systematics, although there is no consensus as to whether they are derived from the same asthenospheric domain that supplies the modern Indian MORB (Hickey-Vargas et al., 1995). For example, in the Lau basin (Hergt & Hawkesworth, 1994) and the New Hebrides arc (Crawford et al., 1995), the Indian MORB-source mantle is considered to have at least partially replaced Pacific MORB-source mantle during the last 12 my. In the Southern Ocean, the present boundary between oceanic basalts with Pacific MORB and Indian MORB isotopic signatures is located near the Australian–Antarctic Discordance (AAD), whereas the early Tertiary basalts with Indian MORB signatures have been found at the South Tasman Rise, east of the AAD (Lanyon et al., 1995; Pyle et al., 1995).
The Sr–Nd–Pb isotopic data presented here demonstrate the presence of an Indian MORB-source mantle component for the Cooktown nephelinites and a more diluted Indian MORB-source component for the Atherton–Nulla basalts in North Queensland during the last 8 my. Using plate reconstruction positioning, Zhang et al. (1999) traced the locus of the eastern boundary of the Indian MORB-type basalts onto the Australian continent in two time slices (early Tertiary vs Present) and concluded that the Indian MORB-source mantle has been a long-term feature (
65 my) beneath the eastern Gondwana continent.
Subcontinental lithospheric mantle component with enriched (EM2) signatures
In addition to the asthenospheric mantle component with Indian MORB isotopic signatures, another source component isotopically similar to an enriched mantle component (EM2) can be recognized for the Atherton–Nulla basalts. The origin of the EM2 signatures in oceanic island basalts (OIBs) has been attributed to contributions from recycled terrigenous sediments via a mantle plume (Hart, 1988; Weaver, 1991; Chauvel et al., 1992). The resemblance between the EM2 OIBs and some basalts from island-arc settings where sediment subduction and contamination of upper-mantle wedge is strongly implicated (e.g. Banda and Antilles arcs, Hart, 1988), suggests that a subduction-modified SCLM may be another potential EM2-like mantle reservoir.
Isotope and incompatible element constraints. In the Sr–Nd–Pb isotope diagrams (Figs 6 and 7), many of the Atherton–Nulla basalts plot in or near the field for young Tonga–Kermadec arc basalts (Ewart et al., 1998, and references therein). The high 87Sr/86Sr trends of the Atherton–Nulla basalts are consistent with derivation from an SCLM mantle wedge modified by metasomatic fluids released from subducted oceanic crust with high 87Sr/86Sr (Hawkesworth et al., 1993). The positive 7/4Pb values (up to +11; Table 4) are also similar to the Tonga–Kermadec basalts. However, in the 206Pb/204Pb–208Pb/204Pb diagram (Fig. 7b), the North Queensland basalts overlap with the Indian MORB, plotting above the trend for the Tonga–Kermadec arc basalts (Fig. 7b); i.e. having higher 8/4Pb values than the arc basalts. This implies that the enriched mantle component involved in the petrogenesis of the Atherton–Nulla basalts may be different from the supra-subduction zone mantle wedge at the SW Pacific. Because subducted sediments from the SW Pacific arcs generally have low U/Pb but high Th/U [average 238U/204Pb (µ) = 0·8 and 232Th/238U (
) = 7·2 for bulk sediments from the Tonga arc; Plank & Langmuir, 1998], 300–400 my of ageing will suffice to increase the 8/4Pb value to the level of that for the Atherton–Nulla basalts.
The Atherton–Nulla basalts are generally enriched in K and Sr and depleted in Th, U and LREE in their incompatible element patterns (Fig. 5a–d). They have high K/Nb, Sr/La (Fig. 8b), K/U and K/Ba (Fig. 11), but low U/Pb and Ce/Pb relative to the Cooktown nephelinites. Although increase in partial melting degrees in a lherzolite source may increase SiO2 content in the resultant melts, it cannot significantly increase K/Nb, because of similar compatibility between K and Nb in mantle peridotites. The high K/Nb trend of the Atherton–Nulla basalts in the SiO2–K/Nb plot (Fig. 8a) must involve a high K/Nb source component in various proportions. The end-member of the positive Sr/La vs K/Nb and K/U trends (Figs 8b and 11a) represented by the Atherton–Nulla basalts is similar to the Tonga–Kermadec arc basalts (but not necessarily crustal materials or pelagic sediments). It is likely that the source component is subduction-modified mantle wedge located within the lithospheric mantle beneath North Queensland. The subducted slab (oceanic crust plus sediments) in the region is characterized as enriched in Sr and having high Sr/LREE ratios (McCulloch & Gamble, 1991).
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We propose therefore that the EM2 component recognized in the Atherton–Nulla basalts may have been derived from an SCLM modified by percolation of subduction-related metasomatic fluids during the late Palaeozoic when this part of the Tasman Foldbelt (Hodgkinson and Broken River provinces) was built up by orogenic magmatism and crustal accretion (Plumb, 1979). The partial melting probably occurred at the asthenosphere–lithosphere boundary, with inputs from both the Indian MORB-source asthenosphere and the SCLM in various proportions. This is consistent with Ewart’s (1989) suggestion that a subduction-modified SCLM is significant in the generation of some eastern Australian basalts.
Radiogenic isotope and incompatible element signatures of the basalts from McBride and Chudleigh and those from Atherton and Nulla show overlap. This implies that overprinted metasomatic geochemical signatures have largely obliterated the original geochemical signatures of both Precambrian and Phanerozoic terranes in North Queensland at the lower part of the lithosphere (i.e. within the garnet peridotite stability field and probably at the asthenosphere–lithosphere boundary). Geochemical signatures of spinel peridotite and pyroxenite xenoliths from the region will provide further constraints on the nature of the upper part of the contrasting SCLM terranes.
Mantle xenolith constraints. Whole-rock Sr–Nd isotope data for lithospheric mantle-derived peridotite and pyroxenite xenoliths from Atherton, McBride and Chudleigh in North Queensland and several provinces in New South Wales are listed in Table 5 and plotted together with lava-field basalts in North Queensland and New South Wales (Fig. 12). There are significant differences in Sr–Nd isotope signatures between the xenoliths from each side of the Tasman Line. The Atherton xenoliths have MORB-like or even more depleted Nd isotope ratios (Nd +8 to +19) and limited 87Sr/86Sr variation (0·7036–0·7042), whereas the McBride–Chudleigh xenoliths have relatively low Nd (<+10) and some have various enriched Sr–Nd isotopic ratios. It is likely therefore that mantle isotopic provinciality between the Precambrian and Phanerozoic terranes has been preserved in the upper part of the SCLM in North Queensland, with the mantle samples from the Precambrian terrane recording time-integrated enrichment, but not in the lower part of the SCLM as tapped by the Atherton–Nulla basalts.
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The seven Atherton peridotite xenoliths produce an Sm–Nd ‘isochron age’ of 264 Ma (R = 0·9754) with Nd(t) = +8. This age may be relevant to accretion of or melt extraction from a moderately refractory SCLM during the late stage of the regional orogeny beneath the Tasman Foldbelt in North Queensland.
Regional differences in Sr–Nd isotope ratios between the North Queensland and New South Wales basalts are mirrored by those between the xenoliths of the two regions but with more profound contrast. All but two of the North Queensland xenoliths form a high 87Sr/86Sr trend above the North Queensland basalt field, whereas the majority of the New South Wales xenoliths form a low 143Nd/144Nd trend largely below the New South Wales basalt field. Although these xenoliths from the shallower spinel peridotite field do not actually represent the direct source for the lava-field basalts, these data are compatible with the explanation that the uppermost SCLM in both regions was modified by such processes that mirror the geochemical characteristics in the deeper garnet peridotite stability field.
In addition, there are differences between the Atherton–Nulla basalts and most of the NSW lava-field basalt in some element signatures. The Atherton–Nulla basalts are lower in TiO2, CaO/Al2O3 (Fig. 2), Sc, Y and HREE (Fig. 3) than their New South Wales counterparts. These systematic variations may, among the other factors, also reflect the geochemical heterogeneity of the Phanerozoic SCLM sources beneath the Tasman Foldbelt. Combination of the basalt and xenolith data demonstrates the systematic variations in geochemical signatures in both the lower and upper parts of SCLM beneath North Queensland and New South Wales.
Origin of HIMU incompatible element signatures
Constraints from decoupled Nd and Pb isotope and parent/daughter element ratios. Although the two-component (Indian MORB-source asthenosphere–EM2 SCLM) mixing model fits the isotope trends, it cannot explain certain relationships between radiogenic isotope ratios and parent/daughter incompatible element ratios of the North Queensland basalts. The North Queensland basalts show broad negative correlations between 206Pb/204Pb and 238U/204Pb (µ) for the total sample population and for samples from several individual provinces (Fig. 13a), with the highest µ values (up to 172) for the Cooktown nephelinites. Negative correlations between Nd and 147Sm/144Nd also exist for the basalts from several individual provinces (Fig. 13b). These negative correlations require recent fractionation of parent/daughter element ratios in either or both of the two source components (or melts generated therefrom).
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Although the inferred SCLM source for the Atherton–Nulla basalts could have been enriched in Pb by subduction-related metasomatism and thus generated melts with low U/Pb and Ce/Pb (Miller et al., 1994
; Brenan et al., 1995), the positive correlations between U and U/Pb (Fig. 13c), Th and Th/Pb, and Ce and Ce/Pb are associated with large variations in U, Th and Ce, but very limited variation in Pb (Figs 3 and 4). This demonstrates that the high U/Pb, Th/Pb and Ce/Pb ratios are largely due to metasomatic enrichment of U, Th and LREE into the mantle source for the Cooktown nephelinites with the Indian MORB isotopic signatures. The low 147Sm/144Nd of the Cooktown nephelinites (0·11) may also be attributed to the same enrichment event. The negative correlation between 206Pb/204Pb and µ of the North Queensland basalts (Fig. 13a) allows constraints to be placed on the maximum age of metasomatism. The 206Pb/204Pb ratios would increase from 17·9 (the minimum value for the North Queensland basalts measured) to 18·6 (the maximum value) in about 40–60 my with µ = 65–90 (values for Cooktown nephelinites). Therefore, the time interval between the metasomatism and the magmatism must be <60 my. Otherwise, the observed negative correlation between 206Pb/204Pb and µ would be completely obliterated by time-integrated isotopic evolution.
The differentiated incompatible element patterns of the Cooktown nephelinites also require a third mantle component distinct from either the SCLM source for the Atherton–Nulla basalts or the Indian MORB source. It is possible for an enriched MORB source (e.g. Wood, 1979) to produce melts with incompatible-element levels similar to the Cooktown nephelinites if degrees of partial melting decrease from 10–20 wt % for MORB to 1–2 wt %. However, this process cannot change the smooth incompatible element patterns of enriched MORB (Fig. 5f) into the differentiated pattern of the Cooktown nephelinites (Fig. 5e) without the presence of mineral phase that can significantly modify certain key element ratios such as K/Nb, U/Pb and LREE/Pb in MORB-source melting region (Hofmann, 1997).
Repository of HIMU elemental signatures within SCLM. Geochemical signatures of HIMU components include high 206Pb/204Pb (>20), negative 8/4Pb (-40 to -90), and prominent differentiation of strongly incompatible elements; i.e. Th, U, Nb, Ta and LREE enrichment and Rb, K and Pb depletion (Zindler & Hart, 1986; Weaver, 1991; Hofmann, 1997). HIMU basalts occur in both oceanic (e.g. Mangaia Island, Woodhead, 1996) and continental (e.g. Tasmania, Lanyon et al., 1993) settings in connection with mantle plumes. Lead isotopic signatures of HIMU basalts require source evolution under conditions of elevated 238U/204Pb ratios over time periods of 1·5–2·0 by (e.g. Woodhead, 1996; Thirlwall, 1997). It is generally accepted that the HIMU sources have formed by incorporation of recycled oceanic crust into mantle plumes rising from the core–mantle boundary (e.g. Weaver, 1991; Hofmann, 1997).
Diagnostic incompatible element signatures of strongly alkaline basalts from Cooktown and New South Wales, eastern Australia (O’Reilly & Zhang, 1995) include (1) differentiated incompatible element patterns with prominent enrichment of Th, U, Nb, Ta, LREE, Sr and P relative to the other elements with similar compatibility in common four-phase mantle assemblages (Fig. 5), and therefore (2) high U/Pb (Fig. 13), Th/Pb and Ce/Pb, accompanied by low K/Nb, Sr/La (Fig. 8b), K/U, K/Ba and Rb/Sr (Fig. 11). Being similar to HIMU OIBs in the incompatible elemental signatures, these basalts do not show HIMU Pb isotope signatures and cannot be directly linked to any known plume activity. Therefore, generation of HIMU elemental signatures in the shallow mantle is possible. Lithospheric mantle is a preferred reservoir to asthenospheric mantle as mantle convection may homogenize the asthenosphere.
Present knowledge of partitioning behaviour between upper-mantle phases and mafic melts (e.g. Green, 1994; Halliday et al., 1995) indicates that various degrees of partial melting of upper-mantle assemblages consisting of only major upper-mantle phases (olivine, orthopyroxene, clinopyroxene, spinel and/or garnet) will not produce melts with HIMU incompatible element signatures unless the source had possessed such differentiated signatures before partial melting. On the basis of modal melting (both batch and fractional) modelling, Halliday et al. (1995) argued that fundamental HIMU elemental signatures can be generated by low-degree (1–2%) partial melting of a garnet peridotite source containing minor amphibole (2 wt %) and trace phlogopite and sulfide (0·2 wt %) in the upper mantle. Incompatible element ratios of MORB can be affected by contamination of such melts with the source or in transit to the surface to acquire HIMU elemental signatures.
O’Reilly & Zhang (1995) proposed a genetic model for nephelinites from Barrington, New South Wales, that have incompatible element signatures identical to the Cooktown nephelinites, but Sr–Nd–Pb isotopic ratios similar to Pacific MORB instead of Indian MORB. This model invokes interaction between low-degree melts derived from asthenospheric mantle and those from amphibole (amp)- and apatite (ap)-bearing mantle assemblages in SCLM to explain the HIMU elemental signatures. The model was mainly based on whole-rock geochemical data for metasomatized spinel peridotite xenoliths from the Victorian Newer Basalts Provinces (O’Reilly et al., 1988). In this paper, we will further evaluate the roles of apatite and amphibole in generating HIMU isotopic signatures with high-quality in situ trace element data for the accessory mantle phases for the Victorian xenoliths (O’Reilly et al., 1991; Yaxley & Kamenetsky, 1999; O’Reilly & Griffin, 2000;).
The differentiated incompatible element patterns of the amp- and ap-bearing spinel peridotite xenoliths in Victoria are similar to those of HIMU basalts [fig. 11 of O’Reilly & Zhang (1995)]. They have K/Nb, Sr/La, K/U, K/Ba and Rb/Sr ratios in the same ranges as or even lower than those for the Cooktown nephelinites and HIMU OIBs (Figs 8b and 11), suitable to be one of the mantle components to produce the incompatible element trends for the North Queensland basalts.
Mantle apatite dominates the budget for U, Th, Pb, LREE and Sr in the peridotite xenoliths, and contains significant amounts of Ba (100–360 ppm) but relatively low K (<1100 ppm), Rb (0·06–20 ppm) and Nb (<2·4 ppm) (O’Reilly et al., 1991; Yaxley & Kamenetsky, 1999; O’Reilly & Griffin, 2000). Therefore, addition of a small proportion of mantle apatite would produce melts with low Sr/La, Rb/Sr, K/Ba and K/U (Figs 8b and 11), but high U/Pb (Fig. 13c), characteristic of HIMU basalts. Although mantle apatite has K/Nb (>600) too high to serve as a mantle end-member for the positive K/Nb–Sr/La correlation of the North Queensland basalts, the very low Nb and K contents of apatite suggest that it contributes little to the K and Nb budgets of the alkaline basalts.
Mantle amphibole is the main repository for K, Ba and Nb if phlogopite is absent. The Victorian amphibole has low K/Nb but high Sr/La. Mass balance calculation using the median compositions of relevant elements suggests that a mixing of 3–5% apatite and 97–95% of amphibole will produce the low Sr/La and K/Nb ratios observed from the whole-rock xenoliths (Fig. 8b). This calculated ratio is similar to the modes of amp- and ap-bearing spinel xenoliths in Victoria (O’Reilly et al., 1991). If apatite is the only repository for phosphorus in SCLM, this model indicates that at least 3 wt % apatite in the melting mode of the source is required to increase phosphorus contents of an enriched MORB (0·14 wt %, Sun & McDonough, 1989) to the level for the Cooktown nephelinites (1·2–1·7 wt % P2O5).
The presence of mantle phlogopite as a residual phase during low-degree partial melting can generate melts depleted in K and Rb and with low K/Nb ratios, but it is not necessarily required by this model.
Amphibole- and apatite-bearing spinel peridotite xenoliths have been found at Sapphire Hills, Chudleigh in North Queensland (S. Y. O’Reilly, unpublished data, 1988). Amphibole-bearing spinel peridotite xenoliths have also been reported from a nephelinite vent at Hoskings Peaks, Cooktown (Barron et al., 1996) although apatite has not been reported from these xenoliths. However, as explained by O’Reilly & Zhang (1995) and O’Reilly & Griffin (2000), the presence of apatite as a mantle phase of metasomatic origin in peridotite xenoliths can often be petrographically and geochemically overlooked. Phosphorus-rich glasses that could have resulted from apatite resorption have been found in spinel peridotite xenoliths in subcontinental settings, e.g. Victoria (O’Reilly & Griffin, 1988) and the Massif Central (Rosenbaum et al., 1997). Therefore, partial melting of amphibole- and apatite-bearing mantle assemblages in the lithosphere is consistent with the HIMU incompatible element signatures observed from the Cooktown nephelinites.
Geodynamic implications
Our preferred model for the petrogeneses of North Queensland basalts and their relationships with regional tectonic evolution is illustrated in Fig. 14. This model involves three mantle source components that can be explained as an Indian MORB-source asthenosphere, an SCLM component with EM2 signatures modified by subduction-related processes, and an SCLM component with HIMU elemental signatures formed by amphibole–apatite type metasomatism. Magma was generated around the asthenosphere–lithosphere boundary as the upwelling asthenosphere interacted with the base of SCLM. Mixing of the asthenosphere-derived melts with partial melts from the subduction-modified SCLM would produce the Atherton–Nulla basalts. Low-degree asthenosphere-derived melts may interact with amphibole- and apatite-bearing assemblages of metasomatic origin in the SCLM to acquire the distinct HIMU incompatible element signatures of the Cooktown nephelinites. The whole compositional spectrum of the North Queensland basalts can be explained by varying contributions of the three components. The Atherton–Nulla basalts contain a more diluted Indian MORB-source asthenosphere component than do the Cooktown nephelinites.
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Formation of the EM2-type SCLM in North Queensland may have been related to an episode of regional pervasive metasomatism during late Palaeozoic to early Mesozoic, across different crustal domains in North Queensland (Fig. 14a). This event is consistent with the Sm–Nd ‘isochron’ age of 264 Ma for the Atherton spinel lherzolite xenoliths (Table 5) and the zircon U–Pb ages for the lower-crust mafic granulite xenoliths from McBride (320–220 Ma; Rudnick & Williams, 1987), inferred as a major tectonothermal event caused by basalt underplating. It is also contemporaneous with the extensive post-orogenic felsic magmatism in the region (320–270 Ma; Plumb, 1977).
The amphibole- and apatite-bearing mantle assemblages are inferred to have formed <60 my ago. This late episode of metasomatism produced a volumetrically insignificant but geochemically distinct component in the SCLM columns beneath North Queensland (Fig. 14b). The metasomatism may be connected with low-degree partial melting from the east-migrating Indian Ocean asthenosphere, which had a long residence beneath eastern Gondwana (Hickey-Vargas et al., 1995; 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 North Queensland at depths of 300–600 km (Anderson et al., 1992; van der Hilst et al., 1997) can be explained as the manifestation of the downgoing plate (Müller et al., 1998). In either case, this amphibole- and apatite-type metasomatism is likely to be a close precursor of the Cooktown magmatism.
The similarity in geochemical signatures between the early Tertiary Mingela basalts and the Central Queensland central-volcano basalts (Figs 5f and 6) suggests a possible genetic link between the two contemporaneous basalt provinces. Thus, the possibility that the melts generated at various part of the plume head had modified the SCLM in North Queensland cannot be entirely ruled out. However, geochemical signatures of the plume have not been recognized from the younger North Queensland basalts in this study.
Isotopic framework for the Australian basalts
On the basis of available data up to the late 1980s, Sun et al. (1989) proposed a four-component dynamic model to explain the isotopic and incompatible element variations of the Australian central-volcano and lava-field basalts. The four components include: (1) a mantle plume for the central-volcano basalts (Fig. 1a); (2) a shallow asthenosphere component for the lava-field basalts with most depleted Sr–Nd isotopic compositions; (3) an SCLM component for the leucitites in western New South Wales; (4) a crustal component as a contaminant.
Our previous and present studies (O’Reilly & Zhang, 1995; Zhang & O’Reilly, 1997; Zhang et al., 1999) are consistent with Sun et al.’s (1989) model as a baseline and allow us to refine their model into a more detailed isotopic framework of mantle components for the Australian basalts (Fig. 15). A first-order approximation to explain the isotopic data for the lava-field basalts in North Queensland and New South Wales comprises a series of two-component mixing relationships, each reflecting dynamic interactions between two mantle end-member components that contribute to magma generation. The two end-member components for the late Cenozoic North Queensland basalts (<6 Ma) are an Indian MORB-source asthenosphere and an EM2 SCLM terrane. On the other hand, the New South Wales lava-field basalts (55–12 Ma) represent mixing between a Pacific MORB-source asthenosphere and an SCLM component, isotopically similar to the inferred SCLM end-member in North Queensland.
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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 and therefore a plume-related origin is likely, although some tholeiites in the region may require contributions from SCLM and crustal contamination (Price et al., 1997). Both seismic tomography results (Zielhuis & van der Hilst, 1996) and plate motion reconstructions (Duncan & McDougall, 1989) locate the present Australian mantle plume beneath the Bass Strait, compatible with this model.
The early Tertiary Tasmanian alkaline basalts reflect mixing between the HIMU component and a Pacific MORB-type source (Lanyon et al., 1993). The former can be connected to a HIMU plume located at present near the Balleny Islands, Antarctica (Lanyon et al., 1993), whereas the latter is consistent with the recognized Pacific MORB-source component for the early Tertiary basalts from New South Wales (Zhang et al., 1999), the Tasman Seamounts and the Balleny Basin (Lanyon et al., 1993).
In addition, the New South Wales leucitites record time-integrated isotopic evolution of an EM1-type mantle source (Nelson et al., 1987), probably residing in the SCLM beneath the western part of the Lachlan Foldbelt with possible Precambrian basement. The Dubbo basanites may bear a similar EM1 signature (Zhang & O’Reilly, 1997).
Finally, the locus of central volcanoes shown in Fig. 16 (filled circles) tracks the northward movement of the Australian plate over the Australian plume during the past 35 my. The radiogenic isotopic composition of the plume has been constrained by mantle-xenolith-bearing primitive central-volcano basalts (e.g. Nebo, Peak Range, Springsure and Warrumbungles; Sun et al., 1989).
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Figure 16 summarizes the spatial and temporal distribution of mantle reservoirs in eastern Australia as indicated by geochemistry of basalts from 55 Ma to the present day.
This model represents a very simplified isotopic framework for the eastern Australian basalts. The mixing relationships could be considered as a manifestation of a continuum of changing chemical heterogeneity in the mantle sources (and melts derived therefrom) during their evolutionary history linked to regional tectonic evolution, rather than a literal mixing process of the mantle components. In this sense, the ‘plum-pudding’ model of a heterogeneous asthenospheric source directly beneath the SCLM, accompanied by a gradual increase in the degree of partial melting during magma generation (Sun et al., 1989), may represent a more realistic way to view the petrogenesis, although we would like to emphasize the significance of asthenosphere–lithosphere interaction at the base of SCLM.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDCENOZOIC MAGMATISM IN NORTH...PETROGRAPHYANALYTICAL TECHNIQUESBASALT GEOCHEMISTRYDISCUSSIONCONCLUSIONSREFERENCES |
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- Primitive basalts can be used to infer large-scale mantle geodynamics, as these geochemical compositions have been shown to reflect major mantle reservoirs in both the lithospheric mantle and asthenosphere.
- North Queensland basalts are generated from heterogeneous mantle sources with pronounced geochemical heterogeneities both between and within provinces. Although all the North Queensland basalts show the Indian MORB Pb isotopic signatures (positive 7/4Pb and 8/4Pb values), the Cooktown nephelinites are generally isotopically more depleted, with lower 87Sr/86Sr and 206Pb/204Pb and higher Nd, than most of the Atherton–Nulla basalts.
- Three mantle components are recognized as potential sources for the North Queensland basalts: an Indian MORB-type asthenosphere, an SCLM component with an EM2 signature and an SCLM component geochemically identical to lithospheric mantle modified by amphibole–apatite type metasomatism. The compositional spectrum of the North Queensland basalts can be considered in terms of mixing of melts derived from the three source components in different proportions. The Indian MORB asthenosphere component is more diluted in the Atherton–Nulla basalts than in the Cooktown nephelinites.
- The EM2 SCLM component may have resulted from subduction-related mantle metasomatism in the late Palaeozoic. This metasomatic event may have largely overprinted geochemical signatures of the lower part of SCLM in both the Precambrian and Phanerozoic terranes in North Queensland as inferred from similar geochemical signatures of the North Queensland basalts on each side of the Tasman Line. However, difference in Sr–Nd isotopic signatures between the Atherton and McBride spinel peridotite xenoliths indicates that the geochemical heterogeneity between the Precambrian and Phanerozoic terranes has been preserved in the upper part of SCLM.
- The metasomatism responsible for the inferred formation of amphibole- and apatite-bearing mantle assemblages must be a recent event (<60 Ma). It may be a consequence of the upwelling of the east-migrating Indian Ocean asthenosphere (which has had a long-term residence beneath eastern Gondwana continent) and/or the early Tertiary subduction of the Phoenix–Pacific plate at the northeastern margin of the Australian plate.
- Significant geochemical differences between the lava-field basalts in North Queensland and those in New South Wales reflect mantle source heterogeneity at a regional scale (in several hundred kilometres) in eastern Australia. The North Queensland basalts are characterized by their overall high 206Pb/204Pb ratios and high 87Sr/86Sr at a given Nd relative to the New South Wales basalts. In contrast to an Indian MORB-source asthenospheric component recognized from the late Cenozoic North Queensland basalts, the early Tertiary New South Wales basalts require a Pacific MORB-type asthenosphere as one of their source components.
- An isotopic framework of mantle source end-members can be refined for the intraplate volcanism in eastern Australia. The delineated components include the Pacific and Indian Ocean-type asthenosphere, two mantle plumes, and SCLM domains with EM2 and EM1 signatures. Secular distribution of these mantle sources is consistent with our understanding of the Phanerozoic tectonic evolution in eastern Australia.
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ACKNOWLEDGEMENTS |
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We would like to thank P. Whitehead for some Atherton samples used in this study. We thank N. Pearson, C. Lawson and A. Sharma for invaluable assistance with XRF and ICPMS analysis at Macquarie University, G. Denton for assistance with Sr–Nd isotopic analysis at CSIRO, and G. Mortimer for assistance with Pb isotopic analysis at ANU. Helpful discussions with W. L. Griffin and insightful and constructive reviews by M. Wilson, S.-s. Sun and an anonymous reviewer significantly improved the manuscript. This study was supported by Australian Research Council and Macquarie University grants to S.Y.O’R. This is Publication 219 of the ARC National Key Centre for the Geochemical Evolution and Metallogeny of Continents (GEMOC) at Macquarie University (www.es.mq.edu.au/GEMOC/).
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FOOTNOTES |
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*Corresponding author. Telephone: +61-2-9850-8414. Fax: +61-2-9850-8943. E-mail:ming.zhang{at}mq.edu.au
Present address: Centre for Ore Deposit Research, University of Tasmania, Hobart, Tas. 7001, Australia.
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