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Journal of Petrology Volume 41 Number 6 Pages 759-788 2000
© Oxford University Press 2000
The Isotope and Trace Element Budget of the Cambrian Devil River Arc System, New Zealand: Identification of Four Source Components
1INSTITUT FÜR GEOLOGIE UND DYNAMIK DER LITHOSPHÄRE AND GEOCHEMISCHES INSTITUT, GOLDSCHMIDTSTR. 3, 37077 GÖTTINGEN, GERMANY
2MAX-PLANCK-INSTITUT FÜR CHEMIE, ABTEILUNG GEOCHEMIE, POSTFACH 3060, 55020 MAINZ, GERMANY
Received December 18, 1998; Revised typescript accepted November 11, 1999
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGEOLOGICAL SETTING AND PETROLOGYRESULTSDISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: TRACE ELEMENT...REFERENCES |
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The Takaka Terrane in the South Island of New Zealand contains a well-preserved Cambrian arc system (Devil River Volcanics) that displays a complete assemblage of interbedded low- to high-K arc rocks, back-arc rocks and boninites. Most volcanic rocks are mafic. A coherent dataset was obtained including major elements, trace elements and Sr–Nd–Pb isotope compositions from clinopyroxene and amphibole separates. With time, 207Pb/204Pb in the arc rocks become more unradiogenic and 143Nd/144Nd more radiogenic, and Th/Yb and La/Yb increase. La/Yb values range from one in the boninites and back-arc rocks to 30 in the high-K arc rocks. Corresponding
Nd values range from -5 to +6. High field strength element systematics rules out an OIB source component in the mantle wedge. The isotope variations in the volcanic rocks indicate addition of subducted sediments derived from Archaean sources to the mantle source of the magmas. Simple two-component mixing between depleted mantle wedge and melt derived from subducted sediment, however, cannot accommodate all observed trace element and isotope features. Pb–Nd isotope and La/Yb mixing models provide evidence for mixing of at least four components in the mantle sources of the Devil River Volcanics. The components include the mantle wedge, melts from subducted sediment, mid-ocean ridge basalt (MORB)-derived slab fluid, and MORB-derived slab melt. The concomitant increase in Nd, Th/Yb and La/Yb in the arc rocks through time reflects an increasing contribution from MORB-derived slab melts. Addition of up to 0·5% melt from subducted sediment, 3% MORB-derived slab fluid and 6% MORB-derived slab melt (all in wt %) to the mantle wedge is required to explain all magmatic compositions. Such mixing proportions are consistent with those obtained for recent arc systems. KEY WORDS: arc; Cambrian; isotopes; trace elements; mixing
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PETROLOGYRESULTSDISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: TRACE ELEMENT...REFERENCES |
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Chemical variations in subduction-related magmas reflect the interplay of multiple sources in plate collision systems. These sources include the mantle wedge beneath the arc as well as fluids and melts derived from the subducting plate. Most recent arc magmas derive from the mantle wedge above the subducting plate (e.g. Ringwood, 1974
; Gill, 1981; Wyllie, 1982; Arculus & Powell, 1986). The composition of the ‘primary’ mantle wedge is generally thought to be that of the mid-ocean ridge basalt (MORB) source or even more depleted mantle, although in some cases ocean island basalt (OIB) source components in the mantle wedge have been proposed (e.g. Morris & Hart, 1983; Woodhead, 1989; Crawford et al., 1995; Danyushevski et al., 1995; Peate et al., 1997). Fluids and melts derived from both the MORB and sediment portions of the downgoing slab have been known to modify the composition of the subarc mantle significantly (Tera et al., 1986; Ellam & Hawkesworth, 1988; von Huene & Scholl, 1993; Miller et al., 1994; Kamenetsky et al., 1997). Element transfer from the slab to the mantle wedge in most recent arc systems is the consequence of two major processes: (1) melting of subducted sediment; (2) dehydration of the MORB portion of the slab (Nichols et al., 1994; Elliot et al., 1997; Hawkesworth et al., 1997). Melting of the MORB portion requires unusually steep thermal gradients, and is thought to be limited at present-day geothermal gradients to young and relatively hot oceanic crust (Drummond & Defant, 1990; Peacock et al., 1994; Yogodzinski et al., 1995; Stern & Kilian, 1996). The composition of subduction-related melts can be further modified during their ascent to the surface. Reaction of the ascending melt with wall rock peridotite in the subarc mantle can change major and trace element compositions substantially and can even account for selective depletion of the high field strength elements (HFSE) (Navon & Stolper, 1987; Kelemen et al., 1993). Assimilation of continental crust and crystal fractionation can also modify the composition of subduction-related magmas at active continental margins through assimilation–fractional crystallization (AFC) or melting, assimilation, storage and homogenization (MASH) processes (DePaolo, 1981; Hildreth & Moorbath, 1988). This study presents a coherent isotope, major and trace element dataset for a well-preserved Cambrian arc–back-arc system in the Takaka Terrane, New Zealand, to assess the relevance of the different arc-related processes for magma petrogenesis. The Devil River Volcanics of the Takaka Terrane are ideally suited to this task because they display several outstanding features:
- An association of different magma types, such as back-arc tholeiites, low- to high-K arc rocks and boninites are exposed in a single stratigraphic succession.
- Mafic compositions are dominant, which suggests a minor role of crustal contamination and thus allows insights into mantle source processes.
- Mafic phenocrysts are abundant and well preserved because the metamorphic overprint of most of the Devil River Volcanics is limited to prehnite–pumpellyite facies. Analyses of these phenocrysts yield primary magmatic isotopic compositions, especially for the Rb–Sr and U–Pb systems.
- Incompatible trace element abundances show significant variations and a large range of isotope compositions.
- Incompatible trace element abundances display a systematic change of isotopic signatures as the arc evolved, thus providing a rare opportunity to study temporal variations in subarc mantle processes.
In modern arc settings, estimates of the extent of fluid and sediment transfer from the subducting plate to the mantle wedge have relied particularly on large ion lithophile element (LILE) systematics of the volcanic rocks (e.g. Ellam & Hawkesworth, 1988; Plank & Langmuir, 1988, 1993; Hawkesworth et al., 1997). Because low-grade metamorphism may have mobilized most LILE elements (K, Rb, Sr, Ba, Cs, U) in the Devil River Volcanics, these elements cannot be used reliably for petrogenetic modelling. Abundances of rare earth elements (REE), HFSE, Th and most major elements, however, vary systematically between the rock types and behave consistently within individual volcanic suites (Table 1). Any postmagmatic changes in these element abundances are therefore likely to be minor, thus allowing their use for petrogenetic evaluation together with Sr–Nd–Pb isotope data from pristine phenocrysts.
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Particular attention is paid here to the large compositional spread observed in the arc rocks, which include interbedded low- to high-K types. Previous studies have suggested that such variations are caused by amounts and compositions of the sediment subducted beneath arcs (e.g. Plank & Langmuir, 1988, 1993), subarc slab melt enrichment (e.g. Stolz et al., 1996) or differences in the degree of melting (e.g. Pearce et al., 1995). Other studies suggest the presence of enriched ‘OIB source’ type mantle domains in the sources of high-K suites (e.g. Edwards et al., 1994). Compositional systematics of the Devil River Volcanics, as evaluated below, suggests mixing of mantle- and slab-derived components in their sources as the relevant process involved. On this basis, isotope and trace element systematics of the Devil River Volcanics are therefore explained here using multi-component mixing models. Each component is identified using specific trace element or isotope features of the volcanic rocks.
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GEOLOGICAL SETTING AND PETROLOGY |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PETROLOGYRESULTSDISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: TRACE ELEMENT...REFERENCES |
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The Takaka Terrane in Northwest Nelson on the South Island of New Zealand (Fig. 1) is part of a voluminous, mainly intra-oceanic Early Palaeozoic arc–back-arc assemblage that is now dispersed as fragments in Australia, Antarctica and New Zealand (Crawford & Keays, 1978
; Weaver et al., 1984; Cooper & Tulloch, 1992; Crawford & Berry, 1992; Münker & Cooper, 1995). These subduction-related rocks all originated from the collision of the palaeo-Pacific plate with the Gondwana continent, which lasted from the late Early to the Late Cambrian. Most of the Cambrian volcanic rocks in the Takaka Terrane have been grouped together as Devil River Volcanics and are interbedded with partly continent-derived turbidites (Haupiri Group, Münker & Cooper, 1999). Both Devil River Volcanics and Haupiri Group were deposited in submarine facies. The Devil River Volcanics were formed within 
20 My as constrained by both palaeontological and isotopic age constraints (Münker & Cooper, 1999). U–Pb SHRIMP zircon and amphibole 40Ar/39Ar ages for the Devil River Volcanics vary from 490 to 515 Ma (C. Münker, T. Ireland, J. Wijbrans & S. D. Weaver, unpublished data, 1997).
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The Devil River Volcanics constitute a coherent arc–back-arc succession and are grouped into three major units: (1) calc-alkaline arc rocks of the Benson Volcanics; (2) back-arc tholeiites of the Mataki Volcanics; (3) boninitic intrusives of the Cobb Igneous Complex (Münker & Cooper, 1999). The Benson Volcanics are by far the most voluminous unit and are subdivided into nine suites, of which eight are predominantly of basaltic to basaltic–andesitic composition. One suite, the Heath volcanic suite, is andesitic, and is treated here separately, together with other rare felsic Benson Volcanics.
A subdution-related setting of the Benson Volcanics is indicated by the presence of the Balloon mélange in the Takaka Terrane. This soft sediment mélange of known Late Cambrian age (Münker & Cooper, 1999) has incorporated parts of all volcanic sequences, which indicates that a compressional regime was present until the Late Cambrian–Early Ordovician. In the Late Palaeozoic, the Cambrian sequence of the Takaka Terrane was faulted into 12 fault-bounded slices, following final amalgamation to the Australian Antarctic Gondwana margin. Stratigraphic overlap between the Cambrian fault slices [documented in detail by Münker & Cooper (1999)], however, demonstrates that all of the above volcanic units are part of one sequence and, consequently, have been temporally and spatially related.
Most Devil River Volcanics are overprinted by prehnite–pumpellyite facies metamorphism. They contain abundant relict phenocrysts of clinopyroxene, amphibole, plagioclase and magnetite. The Benson Volcanics can be classified into low- to high-K groups based on their distinctive ranges in Th/Yb (Fig. 2). A systematic enrichment of the rocks in Th/Yb through time is also evident (Fig. 2). The boninitic samples come from gabbros, cumulates and melt layers within shallow layered magma chambers, which were emplaced into Haupiri Group sediments and back-arc sequences of the Mataki Volcanics (Münker & Cooper, 1999). Mg-rich clinopyroxenes and refractory compositions of chromite [Cr/(Cr + Al) > 0·6, Hunter, 1977) indicate the boninitic compositions of the parental melts.
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The oldest (early Mid to middle Mid Cambrian) section of the Devil River Volcanics comprises the Mataki Volcanics (back-arc), interbedded with rare low-K arc rocks of the Benson Volcanics. The lower part of the Mataki Volcanics and interbedded Haupiri Group sediments are intruded by boninitic intrusives of the Cobb Ignous Complex (515 ± 7 Ma, U–Pb SHRIMP). This observed co-occurrence of boninites and back-arc rocks is known from present-day nascent back-arc basins, where such magmatism is caused by the ascent of MORB source mantle diapirs into refractory arc source mantle (e.g. Crawford et al., 1989). The youngest part of the Devil River Volcanics (late Middle to early Late Cambrian) consists of medium- to high-K rocks of the Benson Volcanics (490–500 Ma, U–Pb SHRIMP, Ar–Ar). High-K rocks are predominant in the uppermost part of this section, but also occur interbedded with medium-K rocks. Because all these volcanic units occur together in a single sequence and were both erupted over a longer time interval (10 My), a genetic link between them seems plausible.
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RESULTS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PETROLOGYRESULTSDISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: TRACE ELEMENT...REFERENCES |
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Major and trace element analyses and Sr–Nd–Pb isotope data for volcanic rocks and sediments of the Takaka Terrane are listed in Tables 1 and 2. Major and trace elements were analysed by X-ray fluorescence (XRF) (Phillips PW 1480) and inductively coupled plasma mass spectrometry (ICPMS) (VG Plasma Quad II+) on a total of
120 samples (both at the Geochemisches Institut, Universität Göttingen). Particular attention was paid to an accurate and precise determination of HFSE. Details of the analytical procedure have been given by Münker (1998). H2O contents were analysed by Karl-Fischer titration, and CO2 analyses were performed using an Eltra CS 1000RF IR absorption spectrometer. Major element contents of all samples, as reported below, were recalculated on an H2O- and CO2-free basis.
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87Sr/86Sr and 143Nd/144Nd data were obtained for 20 clinopyroxene and amphibole separates and six whole-rock samples covering all the above volcanic units. The whole-rock samples comprise three high-K rhyolites and three samples from the boninitic Cobb Igneous Complex, of which one (NZ-376/1) is a late-stage plagiogranite. Additional Sr–Nd data for 15 Haupiri Group sediments have been given by Wombacher & Münker (1998). Twelve representative mineral separates and seven Haupiri Group sediments were also analysed for their Pb isotope composition. All isotope values were calculated back to an age of 500 Ma using the measured U–Pb, Rb–Sr and Sm–Nd abundances in the mineral separates and rocks, respectively. The uncertainties in the ages (500 ± 15 Ma) have a negligible effect on the recalculated initial isotopic ratios and are within the analytical uncertainty.
SiO2 contents are consistent with modal compositions of the examined samples (Table 1, and Münker & Cooper, 1999) and show the predominance of basalts and basaltic andesites. Andesitic to rhyolitic compositions are rare and are mainly confined to the Heath volcanic suite of the Benson Volcanics. The arc rocks (Benson Volcanics) are generally rich in Al2O3 (14–21 wt %) and poor in TiO2 (0·4–1·5 wt %). By comparison, back-arc tholeiites of the Mataki Volcanics contain less Al2O3 (13–16 wt %) and much more TiO2 (1·4–3·5 wt %). MgO, Ni and Cr contents of both units cover wide ranges (0·5–11 wt %, 40–400 ppm and 5–100 ppm, respectively), reflecting olivine and clinopyroxene fractionation.
The Benson Volcanics display an extremely wide range in their incompatible trace element abundances [e.g. Th/Yb 0·3–11 (Fig. 2) and La/Yb 2–30]. Th/Yb, Ta/Yb and La/Yb are strongly correlated (Fig. 2 and Table 1) and increase through time. Zr and Y abundances are in the range 20–150 ppm and 8–23 ppm, respectively. Back-arc samples are confined to lower Th/Yb (0·3–0·8) and La/Yb (2–6) but show larger ranges in Zr (80–230 ppm) and Y (20–50 ppm) than the arc rocks. Nb abundances in both units also vary considerably and range from 0·7 to 30 ppm.
The boninitic samples of the Cobb Igneous Complex show extreme variations in their MgO contents from 0·6 wt % (plagiogranite) to >40 wt % (dunitic cumulate). Ni and Cr vary over similarly large ranges (7–470 and 6–3450 ppm). Two samples from melt layers (NZ-330, NZ-333, Table 1) that are intercalated with the gabbros and cumulates show major element compositions that are similar to those of present-day low-Ca boninites (Crawford et al., 1989). Even the most differentiated samples of the Cobb Igneous Complex are extremely depleted in incompatible trace elements. Compositions are in the range 0·3–4 ppm for Nb, 4–60 ppm for Zr and 0·5–12 ppm for Y. Th/Yb and La/Yb also show large variations (0·4–0·9 and 1–12, respectively).
87Sr/86Sr and 143Nd/144Nd compositions at 500 Ma of the Devil River Volcanics (Fig. 3) cover an extremely large range from -5 (boninites) to +6 (back-arc) in Nd and from 0·703 to 0·712 in 87Sr/86Sr with no systematic difference between the groups. Arc rocks of the Benson Volcanics cover a large range in Nd500 from -1·5 to +4, and display a systematic increase in Nd500 with increasing enrichment in their incompatible trace element contents (Fig. 3, and Fig. 10b below). Nd500 values for the low-K group range from -1·5 to +2 and are much higher for the medium-K (+2 to +3·5) and high-K groups (+3 to +4). Sr isotope compositions do not show this trend and overlap between the different groups, except for sample NZ-113b, which has an anomalously high (87Sr/86Sr)500 of 0·71158 (Fig. 3). The three high-K dacite–rhyolite samples have Nd500 from -2 to +2 and (87Sr/86Sr)500 from 0·70442 to 0·70587, lying between the compositions of the mafic high-K rocks and those of the Haupiri Group sediments of the Takaka Terrane (-11 to +2, Wombacher & Münker, 1998).
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Pb isotope compositions of the Devil River Volcanics differ significantly from those of the interbedded Haupiri Group sediments (Fig. 4). Initial clinopyroxene/amphibole compositions range from 18·4 to 19·3 in (206Pb/204Pb)500 and from 15·60 to 15·70 in (207Pb/204Pb)500. Whole-rock samples from the interbedded Haupiri Group sediments show significantly less radiogenic compositions from 17·4 to 18·1 in (206Pb/204Pb)500 and from 15·58 to 15·62 in (207Pb/204Pb)500.
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Two observations that need to be explained by any successful petrogenetic model are (1) the unusually radiogenic Pb isotope compositions in the Devil River Volcanics, and (2) their spread in Nd. The latter is unusual, because in recent intra-oceanic arc systems, Nd values are generally within the range +2 to +9 (White & Patchett, 1984; Arculus & Powell, 1986). A further exceptional feature is the observed increase in incompatible trace element ratios and abundances (e.g. Th/Yb, La/Yb, Fig. 2, and Fig. 10b below) with increasing Nd. Moreover, the medium- and high-K arc rocks of the Devil River Volcanics show a pronounced enrichment in HFSE (see Figs 7 and 8 below; Table 1) compared with average arc tholeiitic compositions (e.g. Ewart, 1982). Such a feature is often explained by an OIB component (high Nb/La and Nb/Th) in the source of arc magmas (e.g. Morris & Hart, 1983; Woodhead, 1989; Edwards et al., 1994; Danyushevski et al., 1995).
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The trace element and isotope geochemistry of the Devil River Volcanics is first evaluated below by assessing the role of combined fractional crystallization and crustal assimilation processes (AFC). On this basis, the trace element and isotope characteristics of primary magmas are reconstructed by back-correction. In a second step, the influences of partial melting processes and mantle wedge depletion on incompatible element abundances and ratios are examined. Using HFSE systematics in the Devil River Volcanics, the possible presence of an OIB source mantle component in the magma sources is then discussed. In a final step, the nature of element transfer from the subducting slab to the mantle wedge (melt or fluid) is discriminated and the amount of slab component is quantified by multi-component modelling.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PETROLOGYRESULTSDISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: TRACE ELEMENT...REFERENCES |
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Fractional crystallization–crustal assimilation processes
Fractionation of olivine, clinopyroxene and magnetite is evident from large variations in mg-number, Ni, Cr, Ti and Sc (Table 1, Fig. 5), and probably controls the compositions of major and compatible trace elements. Amphibole becomes important for the more differentiated rocks (e.g. Heath volcanic suite of the Benson Volcanics), in which it occurs as a phenocryst phase. The back-arc rocks show strong enrichment in TiO2 with decreasing MgO, which suggests little or no magnetite fractionation and supports their classification as tholeiites. Conversely, the arc rocks show constant to slightly increasing TiO2, suggesting a calc-alkaline trend, where magnetite fractionation controls the TiO2 contents. High Al2O3 contents in the arc rocks (Benson Volcanics) indicate plagioclase accumulation as suggested for recent high-alumina rocks (Crawford et al., 1987
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Correlations between isotope ratios and compatible elements (Fig. 5) help to delimit crustal assimilation processes. Constant values of Nd500 and (87Sr/86Sr)500 at different mg-numbers in most suites (Fig. 5) preclude significant shallow-level assimilation of continental crust during magma crystallization (AFC). Shallow-level assimilation of continental material, such as Haupiri Group sediment, would cause a decrease in Nd and an increase of 87Sr/86Sr in the magma suites as indicated by schematic arrows in Fig. 5. Some samples of the low- and high-K calc-alkaline groups, in fact, do show an increase in 87Sr/86Sr at relatively low mg-number (high-K group and NZ-113b from the low-K group). However, crustal contamination effects observed for Sr isotopes are not detectable for Nd (Fig. 5). Pb isotope compositions in the Devil River Volcanics are different from those of the Haupiri Group sediments and do not lie on a mixing array between the sediments and recalculated Cambrian MORB source mantle (Fig. 4a). The Pb isotope compositions in the analysed samples therefore appear not to be affected by shallow-level assimilation of Haupiri Group sediments. Only sample NZ 113b has anomalously low 206Pb/204Pb (Fig. 4a) and overlaps in its Pb composition with compositions obtained for Haupiri Group sediments. This observation is consistent with the elevated 87Sr/86Sr in this sample, thus confirming assimilation of Haupiri Group sediment in this case. In summary, variations in Pb and Nd isotopes are therefore much more suitable tracers for source processes than Sr isotopes because both appear not to be significantly modified by shallow-level crustal contamination. Models for the mantle sources of the Devil River Volcanics below are therefore limited to the Pb–Nd isotope systems.
Dacitic–rhyolitic samples from the high-K calc-alkaline group have Nd500 and (206Pb/204Pb)500 that are significantly lower than those of the mafic high-K rocks (Table 2 and Fig. 4a). Pb isotope compositions of rhyolites (e.g. NZ-380) plot in the sediment field (Fig. 4a), suggesting a major Pb contribution from Takaka Terrane sediment. Dacite–rhyolite generation therefore involved assimilation of large amounts of Takaka Terrane sediment by mafic melts rather than closed system fractional crystallization.
As shown in Fig. 6, primary melt compositions for the different groups of the Devil River Volcanics were obtained from the measured abundances by back-correction. Following Plank & Langmuir (1988), the fractionation-corrected trace element abundances are obtained by extrapolation of observed fractionation trends to 8 wt % MgO, indicated as continuous lines. The boninites were extrapolated to 10 wt % MgO, which is the highest MgO content found in samples with mg-numbers between 70 and 75 (i.e. in equilibrium with olivine). These higher initial Mg contents in the boninites reflect larger (>30%) degree of melting (Fig. 7). Except for highly incompatible elements (e.g. Nb) in the high-K rocks, all mafic arc rocks of the Benson Volcanics lack any significant incompatible element enrichment by fractionation. In contrast, all abundances and ratios in the boninites, back-arc tholeiites and felsic high-K rocks increase steadily with increasing degree of fractionation. Corrected trace element abundances (e.g. Nb8·0, Zr8·0) are used for these suites below.
Partial melting, wedge depletion and the HFSE budget
Partial melting processes involved in generating subduction-related magmas are mainly controlled by (1) adiabatic mantle decompression and (2) the hydration of the mantle through slab-derived fluids, which lower the mantle solidus (Plank & Langmuir, 1988; Pearce & Parkinson, 1993). In a case study on the Mariana arc, Stolper & Newman (1994) showed a strong correlation between the degree of partial melting and the fluid flux from slab to mantle wedge. In their study, the large-scale influx of highly incompatible elements from slab-derived fluids or melts is reflected in positive correlations for all LILE (soluble in fluids) with H2O contents in fresh volcanic glasses. On the other hand, Ti, Y and Zr (relatively insoluble in fluids) show negative correlations with H2O. This trend indicates an increase in the degree of partial melting with increasing H2O flux in the mantle wedge because Ti, Zr and Y are moderately incompatible. The budget of highly incompatible elements (LILE) in the magmas is therefore controlled by fluid influx, whereas the budget of the moderately incompatible elements [heavy REE (HREE), Zr, Y, Ti] is controlled by partial melting processes and mantle depletion. Empirically and experimentally determined slab–fluid partition coefficients (McCulloch & Gamble, 1991; Brenan et al., 1995b; Keppler, 1996; Stalder et al., 1998) confirm this element behaviour. On the basis of this systematics, Pearce & Peate (1995) proposed the classification of the above element groups as ‘conservative’ (HFSE and HREE) and ‘nonconservative’ (LILE) with respect to their mobilization by slab-derived fluids or melts in subduction systems. Conservative elements are thus preferred here to constrain variations in partial melting, whereas variations in nonconservative elements and isotope ratios (Sr and Pb isotopes, Th) are used to quantify slab-derived inputs.
Some approaches for estimating the degree of melting and wedge depletion in subduction-related rocks have been presented by Pearce & Parkinson (1993) and Woodhead et al. (1993). These studies demonstrate that compatible elements such as Sc or Cr (Dmantle/melt > 1) are buffered by the residual mantle and remain virtually constant over large variations in degree of melting. Moderately incompatible elements, such as Y, Yb or Ti (Dmantle/melt = 0·1–1), are more sensitive to the degree of melting, but are still buffered sufficiently by the mantle residuum to be relatively insensitive to the degree of mantle depletion. These elements are therefore well suited to monitor variations in degree of partial melting; furthermore, the abundances of these elements differ little between the different mantle sources of arc and back-arc magmas. More incompatible elements (Dmantle/melt < 0·1), such as Nb, are extremely sensitive to depletion events because they enter the melt very efficiently. Fractionation-corrected Nb, Zr, Ti and Y abundances (all conservative elements) have therefore been used for recent arc rocks to estimate degrees of melting and wedge depletion (Pearce & Parkinson, 1993; Woodhead et al., 1993). A strictly conservative behaviour of HFSE in the Devil River Volcanics, however, is questioned by Nb/Ta systematics, which suggests that Nb and Ta were slightly enriched in the mantle source by slab components (Münker, 1998). To clarify the isssue, variations in Nb (highly incompatible) vs Yb (moderately incompatible) and of Zr/Y vs Y are therefore modelled in a two-stage melting model (Appendix A; Fig. 7). Zr has been chosen, because its mobility in slab components and its compatibility during melting are transitional between Nb and Y–Yb.
In Fig. 7, only the boninites, some low-K arc rocks and some back-arc rocks fall within the field for the calculated mantle melts. In most arc rocks, Nb is enriched by up to a factor of 10 compared with primitive mantle melts at similar degrees of melting (and Yb contents). Similarly, most samples of the Devil River Volcanics plot at higher Zr/Y than the primitive mantle melting curve. Moreover, Zr/Y ratios are strongly correlated with Zr contents, but not with Y contents, which overlap in the arc magmas. This observation is important for two reasons:
- the influence of residual garnet in the magma sources on Zr/Y can be largely excluded because of its high partition coefficient for Y (Dgarnet/melt = 4–11, Jenner et al., 1994) relative to Zr (Dgarnet/melt = 0·4–0·7, same reference). Varying amounts of residual garnet in the sources would change the Y contents in the magmas and Zr/Y would correlate with Y.
- The Nb and Zr contents of most Devil River Volcanics can therefore not be explained by melting of primitive mantle or its depleted residues. Contribution of the ‘excess’ Nb and Zr to the mantle source is required, either from an ‘OIB’ source component or from the subducted slab.
The Zr–Nb systematics of the Devil River Volcanics therefore rules out any quantification of mantle wedge depletion for this arc system. However, higher mg-numbers and Cr abundances and low CaO/Al2O3 in the boninitic melts (Fig. 5, Table 3) suggest that the boninitic mantle source was significantly more depleted than those of the arc and back-arc rocks. This is consistent with recent arc systems where subarc mantle wedge compositions are known to be variably depleted, often more than typical MORB source mantle (e.g. Woodhead et al., 1993). Trace element modelling for the boninites below therefore needs to take into account a more depleted mantle source than for the arc and back-arc rocks.
Fractionation-corrected Y–Yb abundances suggest extraction of the back-arc rocks after 5–15% melting and of the arc rocks after 15–35% melting. Y–Yb abundances of the boninites suggest >30% partial melting. Assuming a pooled fractional melting process as above, such estimates of course only represent averages, but are strikingly similar to estimates obtained for recent arc and back-arc settings (Plank & Langmuir, 1988; Pearce & Parkinson, 1993; Woodhead et al., 1993; Stolper & Newman, 1994).
OIB mantle source vs slab component
In particular, high Nb abundances have been used as major evidence for an OIB component in the mantle source of subduction-related rocks [e.g. for high-K rocks in Indonesia (Edwards et al., 1994)]. Such an approach, however, has been challenged by, amongst others, Stolz et al. (1996) and Hoogewerff et al. (1997), who have attributed the high Nb abundances in the Indonesian high-K rocks to addition of melts derived from subducted sediment to the mantle wedge. Stolz et al. (1996) have reported extremely high Nb/Ta in Indonesian high-K rocks (>20). Because such high Nb/Ta are not known for OIB (Jochum et al., 1996), it has been suggested that Nb and Ta in the arc rocks originate from melts derived from the subducting slab. Apart from Nb/Ta ratios, Nb anomalies are another important discriminant between subduction components (negative anomaly) and ‘OIB’ components (no or positive anomaly). In contrast to Nb/Th and Nb/Ta, Sr–Nd–Pb isotope ratios of these two potential components partially overlap (White & Patchett, 1984; Hofmann, 1997) and are therefore a less suitable discriminant.
Figure 8a shows that Nb/Th in the arc rocks of the Devil River Volcanics ranges between one and six and is not correlated with Nb/Yb. Such systematics rules out any significant role of an OIB component in the source because OIB generally has much higher Nb/Th (>10, Jochum et al., 1991). A correlation of Nb/Th with Nb/Yb would be expected in the case of mixing between MORB source and OIB source mantle. Nb–Ta enrichment by a subduction component in the Devil River Volcanics is also suggested by Nb/Ta ratios ranging up to 25 ± 2 (Münker, 1998). Nb/Ta ratios in OIB are limited to chondritic (17·5 ± 1) or lower values (Jochum et al., 1996). Correlations of Nb/Ta with Th/Yb and La/Yb (both ‘nonconservative’) in the Devil River Volcanics (Münker, 1998) furthermore indicate that significant amounts of Nb and Ta were added by the slab component. Because rutile in the subducted slab has DNb–Ta > 100 with fluids and melts (Green & Pearson, 1987; Brenan et al., 1994), even small amounts of this mineral can control the Nb–Ta budget in the slab component. Subchondritic ratios in the boninites and low-K arc rocks (<17·6) can reflect fluid enrichment because the partitioning of Nb between rutile and fluid is higher than for Ta (Brenan et al., 1994). Superchrondritic ratios in most high-K rocks (>17·6) suggest melt enrichment as a result of lower partitioning of Nb compared with Ta between rutile and melt (Green & Pearson, 1987).
In Fig. 8b, samples with high La/Yb and high Nb enrichment relative to Zr (low Zr/Nb) always display Nb/Ta higher than chondrite. Zr/Nb ratios of the Devil River Volcanics lie below the values for average N-MORB [Zr/Nb =30, after Hofmann (1988)]. Addition of slab components to the mantle wedge can explain this feature: Nb is much more incompatible than Zr and, hence, much more enriched than Zr in small degree melts such as slab melts (see also Appendix B). Addition of slab melts to the depleted mantle wedge therefore causes extreme enrichment in Nb relative to Zr and a decrease in Zr/Nb (Fig. 8c). Alternatively, the Nb–Ta budget of samples having low Zr/Nb and low La/Yb at subchondritic Nb/Ta (Fig. 8b) must be controlled by slab fluids. Recent experimental work (Brenan et al., 1995b; Stalder et al., 1998) reports a higher solubility of Nb in subduction fluids relative to Zr.
Evidence for source contamination by subducted sediment
Constant Nd at different mg-numbers within single suites of the Devil River Volcanics together with predominantly mafic compositions (Fig. 5) rule out AFC (e.g. Hildreth & Moorbath, 1988). Figure 4a also shows that AFC contamination of magmas derived from ancient MORB source mantle with Haupiri Group sediment alone [or a Lachlan Fold Belt basement (Culloch & Woodhead, 1993)] cannot account for the Pb compositions observed in the Devil River Volcanics. Such contamination would result in significantly less radiogenic compositions, as indicated by the schematic mixing trends shown in Fig. 4a. Mixing between MORB and different OIB reservoirs (Hofmann, 1997), calculated to 500 Ma (Fig. 4b), also cannot explain the high 207Pb/204Pb observed in some samples of the Devil River Volcanics.
Hence, source contamination by subducted sediment is a preferred hypothesis. This subducted sediment must have extremely radiogenic Pb and unradiogenic Nd, not known from Palaeozoic foldbelts of southeastern Australia (McCulloch & Woodhead, 1993; Turner et al., 1993). Subduction of sediment beneath the Devil River arc that was at least in part derived from Archaean sources is strongly supported by the Cambrian palaeogeography (Dalziel, 1997; Münker, 2000): large areas of Archaean crust were exposed along the rims of the palaeo-Pacific ocean. These areas include Archaean provinces in Antarctica (DePaolo et al., 1982) and North America (Wyoming and Slave Provinces, Bowring et al., 1989; Mueller et al., 1993). Extremely radiogenic (207Pb/204Pb)500 (15·5–17), highly variable (206Pb/204Pb)500 and unradiogenic Nd500 (-15 to -40) (Fig. 4b) are typical features of these Archaean domains.
207Pb/204Pb vs 206Pb/204Pb mixing lines between depleted MORB source mantle and subducted sediment are shown as continuous lines in Fig. 4b. Bulk mixing of subducted sediment derived from Archaean sources with mantle wedge material was assumed. Compositions of the endmembers used are listed in Table 3. Three representative compositions of Archaean sources, named ‘low-’, ‘medium-’ and ‘high-Pb sediment’, referring to their enrichment in radiogenic Pb, were used. Admixture of only 0·1% of these sediment endmembers to the mantle wedge is sufficient to account for the compositions of the Devil River Volcanics, as calculated for recent arcs (e.g. White & Patchett, 1984; White & Dupré, 1986). This amount, however, is much lower than up to 2% added sediment obtained by Nd mixing calculations (not shown); therefore refinement of the above mixing model is required.
Mixing of sediment derived from Archaean sources on the subducting plate with Takaka Terrane sediment in the subduction trench, a geologically most realistic case, would significantly lower the 207Pb/204Pb composition of the contaminant. It is furthermore likely that the sediment on the subducting plate is not purely ‘Archaean’ in its provenance but also contains components derived from younger sources with unradiogenic Pb. Hence, a mixture of pure Takaka Terrane sediment and sediment derived from Archaean sources of ‘high-Pb’ composition is calculated (dotted line in Fig. 4b). A mixture of 70% average Takaka Terrane sediment (Table 3) and 30% ‘high-Pb’ Archaean source-derived sediment can best explain the volcanic compositions. Only such a mixture has the appropriate 206Pb/204Pb value (18·9, Fig. 4b) to account for the volcanic compositions by mixing with MORB source mantle. Pb–Nd isotope compositions of this sediment mixture are listed in Table 3.
Entrainment of subducted sediment into the mantle wedge occurs as melt rather than by bulk mixing (Nichols et al., 1994; Elliot et al., 1997; Hawkesworth et al., 1997). Melts of sediments can have significantly more fractionated trace element ratios than their protoliths, thus resulting in different mixing proportions with the mantle wedge. The composition of melts derived from average pelagic sediment were therefore calculated (Appendix B; Table 3). Plots of La/Yb, Pb/Nd, and Nd vs degree of melting for the calculated melts are shown in Fig. 9. Pb/Nd of the melts are close to those of the bulk sediment (Table 3), but La/Yb and Nd contents might increase by a factor of 10 at low degrees of melting. In what follows, a 15% sediment melt was used as representative of an (upper) average composition.
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Mixing of up to 0·4% sediment-derived melt with the mantle wedge is required to explain the Nd isotope compositions in the Devil River Volcanics (Fig. 10a), a value that is much lower than for bulk sediment mixing. The calculated Pb isotope compositions, however, are still too radiogenic to fit Nd compositions (Fig. 10a), as Pb/Nd is not changed much during sediment melting (Fig. 9). Neither variation in isotope composition of the sediment nor variation in the way in which the sediment is transferred to the mantle (melt or bulk mixing) can therefore explain the discrepancy between Pb and Nd results. Independent addition of a third component to the wedge, i.e. slab-derived fluid, is thus required to explain the observed isotope systematics in the Devil River Volcanics.
Three components—fluids derived from subducted MORB
Fluids derived from the MORB portion of the subducted slab have been invoked in many recent arc systems as additional mixing endmembers in addition to subducted sediment (Ellam & Hawkesworth, 1988; McCulloch & Gamble, 1991; Miller et al., 1994; Hawkesworth et al., 1997). Fluids generally do selectively enrich Pb in the mantle wedge compared with Nd (Table 3 and references therein). Addition of MORB-derived fluids to the mantle wedge can thus explain why the Pb isotope ratios in the Devil River Volcanics are too high at fitting Nd, if mixing of pure sediment with the wedge is modelled. Selective admixture of MORB (= mantle)-derived Pb by the fluid lowers the Pb isotope ratio of the mantle wedge although Nd is virtually unaffected.
Nd–Pb mixing lines between mantle wedge and sediment melt for different amounts of fluid addition are shown in Fig. 10a. The isotopic compositions of the Devil River Volcanics can be explained by admixture of up to 3% MORB-derived fluid and up to 0·4% melt of subducted sediment to the mantle. Both results are within the range observed for recent arc systems (e.g. White & Patchett, 1984; White & Dupré, 1986; Ellam & Hawkesworth, 1988). Calculated fluid fluxes for individual samples increase with calculated sediment contributions. These results therefore suggest a scenario where slab dehydration and sediment melting occur simultaneously.
Three-component mixing of wedge, MORB fluid and sediment melt can therefore explain the isotopic compositions of the Devil River Volcanics. In contrast, the enriched incompatible trace element compositions in most arc rocks of the Devil River Volcanics cannot be explained (Fig. 10b). The Nd would be too low at the La/Yb8/10 (and Th/Yb) observed in the medium and high-K rocks. This is because the sediment melt and not the slab fluid controls the light REE (LREE) and Th budget of the slab component (Table 3).
A fourth component: melts derived from subducted MORB
High Nd at high La/Yb and Th/Yb in the high-K rocks strongly suggest addition of slab melt that is at least in part derived from subducted MORB (high Nd). Incorporation of such MORB-derived slab melts into the sources of recent subduction-related magmas has been reported by, amongst others, Kepezhinskas et al. (1995), Yogodzinski et al. (1995) and Stern & Kilian (1996).
A calculation of slab-derived MORB melts is difficult because the source compositions involved could range from purely eclogitic to amphibolitic, depending on how far the phase transformation in the subducting slab has progressed (Drummond & Defant, 1990). Therefore, melts from both eclogitic and garnet amphibolitic sources were calculated (Table 3, Fig. 9). Variations in La/Yb, Pb/Nd and Nd vs degree of melting are shown for the calculated melts in Fig. 9. The Pb/Nd ratios of both eclogite and garnet amphibolite melts remain more or less constant over a large range of melting and are similar to MORB. The La/Yb and Nd abundances in the melts are significantly higher than in MORB. Because of the strong effect of garnet on REE fractionation, La/Yb ratios and Nd abundances in the eclogitic melts are higher than those in the garnet amphibolite melts. As average MORB melt composition, a 5% melt from an eclogitic MORB residue (together with the 15% sediment melt) was chosen for modelling (Table 3). With respect to sediments, MORB eclogite or MORB garnet amphibolite has a much higher solidus (Nichols et al., 1994), so that such low degrees of MORB melting are realistic.
Figure 11a shows Nd vs La/Yb8/10 for melts derived from different mixtures of MORB source mantle wedge, MORB-derived slab melts and sediment-derived slab melts. A comparison with compositions of the Devil River Volcanics shows that addition of an 80:20 MORB:sediment melt mixture to the mantle wedge can explain the Nd–La/Yb8/10 ranges of the medium- to high-K groups. Only small changes in sediment contribution to the slab melt cause large shifts in Nd of the mantle source (Fig. 11b). The medium- and high-K rocks plot tightly along an array parallel to the slab melt mixing lines, suggesting that the slab melts mixed into the sources of all samples were homogeneous. Homogeneous slab melt compositions strongly suggest that the MORB and sediment components were already mixed shortly after slab melting and were not added to the wedge as separate components. The La/Yb8/10 ratios of the volcanic rocks suggest addition of up to 6% slab melt (Fig. 11b). This estimate might vary between 3 and 20%, depending on the slab petrology and degree of slab melting chosen (Fig. 9).
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Using trace element abundances, it has been shown above that the mantle wedge component in the sources of the Devil River Volcanics must be (1) a MORB source mantle (arc rocks) or (2) an even more depleted source (boninites). The above mixing model is therefore expanded in Fig. 11b, where a very depleted (boninitic) mantle source instead of a MORB source is used. The isotope composition of this boninitic mantle source (Table 3) is assumed to be that of the boninites with the most unradiogenic Nd. Mixing of such a mantle source with slab melts having sediment/MORB ratios between 5:95 and 20:80 can explain most compositions of the Devil River Volcanics (Fig. 11b). Calculated amounts of slab melt addition increase from <1% (low-K) to 3% (high-K), slightly lower than average results obtained using a MORB source mantle (Fig. 11a). Variations in mantle wedge composition therefore do not affect the mass balance calculations for the Devil River Volcanics significantly. Hence, the Devil River Volcanics were the mixing product of four components: mantle wedge, sediment melt, MORB-derived slab fluid and MORB-derived slab melt.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PETROLOGYRESULTSDISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: TRACE ELEMENT...REFERENCES |
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The Devil River Volcanics are derived from heterogeneous mantle sources that originated from mixing of a depleted mantle wedge with components derived from subducted sediment and MORB. Transfer of sediment and MORB components from the subducting slab occurred via separate melt and fluid phases. A quantification of slab contributions to the magma compositions suggests addition of up to 0·4% sediment melt, 3% MORB-derived slab fluid and
6% (3–20%) MORB-derived slab melt to the mantle wedge. The amount of melt derived from the MORB portion of the slab increases through time. Melting of subducted MORB is suggested to require a relatively young and hot subducted oceanic crust (Peacock et al., 1994). Current palaeogeographic reconstructions (e.g. Dalziel et al., 1997) suggest that the opening of the Pacific Ocean pre-dates eruption of the Devil River Volcanics by 100 Ma. Models for equivalents of the Devil River Volcanics in the Lachlan Fold Belt of Australia (e.g. Glen et al., 1997) suggest that, similar to the present western Pacific, several arc chains and spreading ridges might have been present. Subducted oceanic crust might therefore have been substantially younger than 100 Ma, thus providing an ideal scenario for subduction of ‘hot’ oceanic crust.
Systematics of the HFSE in the Devil River Volcanics suggests that this element group did not behave ‘conservatively’ and was in part contributed by slab components. Explanation of HFSE enrichment by an ‘OIB source’ rather than ‘MORB source’ mantle wedge composition can be ruled out by Nb/Th, Nb/Ta and Nb/Yb systematics in the magmas. Combined Nb/Th, Nb/Ta and Nb/Yb ratios are therefore a powerful discriminant between OIB source components and slab components in the sources of present-day arc rocks, in particular for high-K suites. The observed contribution of HFSE by the slab components prevents a precise mass balance of the trace element budget in the Devil River Volcanics. Such mass balances frequently rely on HFSE contents in the volcanic rocks as indicator for mantle wedge depletion and composition (e.g. McCulloch & Gamble, 1991). If the HFSE budget is at least in part controlled by the slab component, it is virtually impossible to constrain the depletion of the mantle wedge, in particular with respect to the highly incompatible trace elements. Mass balance calculations will, under these circumstances, slightly underestimate the amount of added slab component.
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APPENDIX A: ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PETROLOGYRESULTSDISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: TRACE ELEMENT...REFERENCES |
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Handpicked mineral separates (20–100 mg) were leached in cold 2 N HCl for 15 min and washed in purified H2O before dissolution. A stronger leaching procedure (e.g. using dilute HF) was specifically avoided. Stronger leaching might cause differential leaching of parent and daughter elements and could result in the wrong calculation of initial isotopic compositions. A duplicate analysis for sample NZ-311, run on leached and unleached splits of one clinopyroxene separate (Table 2), yielded a similar 87Rb/86Sr ratio. More importantly, recalculated (500 Ma) 87Sr/86Sr values for both splits lie only slightly outside analytical error. The age-corrected 143Nd/144Nd results (Table 2) also agree well. This shows that the HCl leaching procedure applied here apparently does not cause significant Rb/Sr fractionation in the residue.
Two additional mineral separates (NZ-198, NZ-113b) were handpicked twice and analysed for Rb–Sr in duplicate. Duplicate analyses for sample NZ-198 gave 87Sr/86Sr at 500 Ma of 0·70501 and 0·70497, which are within analytical error. The duplicate separates of sample NZ-113b differed in their purity. One handpicked separate included 50% milky clinopyroxene, and the other consisted entirely of glassy clinopyroxene. The glassy separate gave (87Sr/86Sr)500 of 0·71158 and is preferred over the milky one, which gave (87Sr/86Sr)500 of 0·71223. This difference is 22 times the external reproducibility, but is none the less negligible in the petrogenetic context discussed below, where variations in (87Sr/86Sr)500 are much larger. Nevertheless, only glassy grains were handpicked from all other mineral separates.
Pb and Sr–Nd isotope analyses were generally performed on different splits of a once homogenized mineral separate or whole-rock powder. All samples were dissolved in HF–HNO3. From the Pb splits, aliquots of 5 wt % were taken after sample dissolution and separately spiked with 208Pb and 235U spikes. In contrast, Sr–Nd splits were spiked with mixed 150Nd–149Sm and 84Sr–85Rb spikes without further sample aliquoting (total spiking).
Blanks for Sm and Nd were less than 25 pg and 75 pg, respectively, and are negligible. Rb and Sr blanks were less than 200 pg and 60 pg, respectively. Pb blanks, run in parallel with each batch of samples, were <150 pg (n = 7), excluding two outliers at 218 and 333 pg, and are negligible for the isotope composition (IC) splits. Results of isotope dilution measurements of Pb (5% aliquots of the IC solutions) needed to be corrected for Pb blanks. Blank correction is always <15% of the total amount in the sample and <5% in most cases.
Isotope measurements were performed on a Finnigan MAT 261 mass spectrometer at the Max-Planck-Institut für Chemie in Mainz (White & Patchett, 1984). Sr–Nd results have all been normalized to 86Sr/88Sr of 0·1194 and 146Nd/144Nd of 0·7219. Nd values have been calculated at 500 Ma relative to a present-day CHUR of 0·512638 for 143Nd/144Nd and of 0·1966 for 147Sm/144Nd (Jacobsen & Wasserburg, 1980). 87Sr/86Sr obtained for the NBS 987 standard was 0·710247 ± 29 (2
, n = 15). Measurements of the La Jolla standard yielded 0·511850 ± 23 (2, n = 16) for 143Nd/144Nd. Four whole-rock powders of Devil River Volcanics were dissolved and analysed in duplicate; each yielded reproducibilites for 143Nd/144Nd better than 0·000013 (n = 3) and for 87Sr/86Sr better than 0·000031 (n = 4, Table 2). Measured Pb isotope ratios were corrected for mass fractionation by 1·3
per a.m.u. relative to the values of Todt et al. (1996), based on 18 analyses of the NBS 982 standard (external reproducibility 0·36 per a.m.u., 2). Pb isotope compositions for each mineral separate were analysed in duplicate, with reproducibilities better than 0·36 per a.m.u. and most values below 0·2 per a.m.u. (2 errors).
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APPENDIX B: TRACE ELEMENT AND ISOTOPE MODELLING CALCULATIONS |
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TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTING AND PETROLOGYRESULTSDISCUSSIONCONCLUSIONSAPPENDIX A: ANALYTICAL METHODSAPPENDIX B: TRACE ELEMENT...REFERENCES |
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Mantle melting models
Previous melting–depletion models of Pearce & Parkinson (1993)
and Woodhead et al. (1993) are modified here, with the following improvements:
- residues of first-order depletion events are calculated from a primitive mantle composition (Hofmann, 1988) to avoid uncertainties in estimating a depleted MORB source mantle;
- recently determined partition coefficients were used, of which many were obtained by microbeam techniques and are of higher accuracy.
Partition coefficients used throughout the modelling calculations are those of Hart & Dunn (1993) for clinopyroxene and Kennedy et al. (1993) for olivine and orthopyroxene. For Nb, a maximum Dmineral/melt of 0·01 was used for olivine and orthopyroxene. Spinel partition coefficients are from Horn et al. (1994) for Nb and Zr, whereas the partition coefficient for Yb is from Nagasawa et al. (1980). The Y partition coefficient for spinel was chosen to be equal to that of Yb. A spinel lherzolitic mantle composition of 60% olivine, 10% orthopyroxene, 25% clinopyroxene and 5% spinel was assumed as starting material. A non-modal melt extraction model was assumed using a composition of 10% olivine, 20% orthopyroxene and 70% clinopyroxene. These compositions were selected according to petrological observations that suggest clinopyroxene is present during extraction of up to 30% melt from peridotite (e.g. Hirose & Kushiro, 1993). In a first step, residues after 5, 10, 15, 20 and 25% melt extraction were calculated. Non-modal batch melting was assumed during the first depletion event using the equation
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Melts derived from subducted sediment
Sediment-derived melts were calculated from an average pelagic sediment composition taken from White et al. (1985). Available petrological data (Irifune et al., 1994; Nichols et al., 1994; Plank & Johnson, 1997) suggest an assemblage of clinopyroxene + garnet + amphibole + muscovite + biotite + coesite at the solidus. Partitioning of incompatible elements into coesite is negligible. For clinopyroxene and amphibole, partitioning of REE and Pb can be assumed to be approximately identical (i.e. Hart & Dunn, 1993; Brenan et al., 1995a; LaTourette et al., 1995). A generalized modal sediment composition of 40% clinopyroxene, 40% garnet and 20% biotite [following Stern & Kilian (1996)] is therefore used. Partition coefficients used are those from Hart & Dunn (1993) for clinopyroxene, Johnson (1994) for garnet [Pb from Hauri et al. (1994)] and LaTourette et al. (1995) for phlogopite. Melting is assumed to be accumulated fractional melting in modal mode using the equation
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Fluids derived from subducted altered MORB
A realistic calculation of element abundances in slab fluids is not straightforward because fluid–mineral partitioning data are scarce. Furthermore, the possible influences of complexing agents or fluid matrices (e.g. Cl, F, Si) and the physical fluid properties on partitioning are poorly understood at present. Available fluid–mineral or fluid–slab partitioning data (McCulloch & Gamble, 1991; Brenan et al., 1995a; Keppler, 1996; Stalder et al., 1998), however, give surprisingly consistent results for Sr, Pb and REE. Slab–fluid partition coefficients used here (Table 3) are average values derived from the above references. A fluid in equilibrium with average N-MORB (Hofmann, 1988) was calculated (Table 3) using the simplified equation
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Melts derived from subducted altered MORB
Modal compositions of subducted MORB were assumed to be either (1) 50% clinopyroxene and 50% garnet (eclogite), or (2) a 10% garnet amphibolite. Melting of the garnet amphibolite was assumed to be plagioclase free [10% garnet, 90% amphibole; after Drummond & Defant (1990)]. Partition coefficients used were the same as those used for the sediment melts. Amphibole partition coefficients used were taken from LaTourette et al. (1995). Average N-MORB (Hofmann, 1988) was used as starting composition and accumulated fractional melting was assumed [equation (3)].
Mixing endmembers
The depleted MORB source mantle wedge composition used (DMM) is an N-MORB source average from Wood (1979), Sun & McDonough (1989) and depleted lherzolite compositions of Jagoutz et al. (1979). Nd isotope compositions of the MORB source were recalculated at 500 Ma using the depleted mantle values of Goldstein et al. (1984). The Pb isotope composition for the MORB source is taken from the radiogenic end of the 500 Ma MORB field in Fig. 4b, and the composition for the sediment component is calculated by mixture of 30% average Takaka Terrane sediment and 70% of ‘high-Pb’ Archaean source-derived sediment. An average sediment composition is estimated from values given by DePaolo et al. (1982), White et al. (1985), McCulloch (1987), Ellam & Hawkesworth (1988) and Turner et al. (1993). Calculation of an Nd for the subducted sediment is based on Pb mixing proportions in Fig. 4 (70% Takaka Terrane and 30% Archaean source derived sediment). As slab melts, a 15% sediment melt and a 5% MORB melt are used. A ‘boninite source’ mantle wedge composition is estimated using the boninite source with the lowest Nd ( -5). Trace element concentrations of this mantle source were calculated as the source for NZ-330, assuming 15% non-modal batch melting and D and P values of Fig. 7.
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ACKNOWLEDGEMENTS |
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This study was sponsored by the DFG project II C6—Schn 392/2-1 ‘Zur Geodynamik des kambrischen Nordostrandes Gondwanas am Beispiel des neuseländischen Takaka Terrans’. Constructive comments and support by G. Wörner, S. Galer, K. Mezger, A. Schneider and D. Kamenetsky are appreciated. A. W. Hofmann is thanked for the hospitality at MPI Mainz. K. Simon, B. Dietrich and E. Schiffczyk supported laboratory work in Göttingen. T. Funk, N. v. Wolfersdorff, C. Siebert, F. Wombacher, I. Hertel and U. Hinrichs are thanked for laboratory assistance work. The Department of Conservation, New Zealand, provided invaluable support during sampling. Journal reviews by I. Nicholls, G. Yogodzinski and three anonymous reviewers are appreciated. G. Bergantz and J. Davidson are thanked for editorial handling.
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FOOTNOTES |
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*Present address: Zentrallaboratorium für Geochronologie, Institut für Mineralogie, Corrensstr. 24, 48149 Münster, Germany. e-mail: muenker{at}nwz.uni-muenster.de
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