Remnants of ancient Australia in Vanuatu: Implications for crustal evolution in island arcs and tectonic development of the southwest Pacific Janrich Buys, Carl Spandler*, Robert J. Holm, and Simon W. Richards School of Earth and Environmental Sciences, James Cook University, Townsville 4811, Australia ABSTRACT We report new geochemical and geochronological data from igneous rocks of the little studied western belt of the Vanuatu intraoceanic arc. Ar-Ar dating of igneous hornblende from hornblende andesites and U-Pb dating of zircon from a tonalite place the time of formation of these rocks in the late Eocene to Miocene; therefore, they represent part of the earliest arc development at Vanuatu. The petrological and geochemical characteristics of these rocks are typical of island arc magmas, except they contain inherited zircon grains with significant age populations at ca. 2.8–2.5 Ga, 2.0–1.8 Ga, 1.75–1.5 Ga, 850–700 Ma, 530–430 Ma, and 330–220 Ma. This inheritance signature is unlike anything recognized from the oceanic realm of the southwest Pacific, but in general matches the age of major crustal blocks of the Australian continent. An exception is the significant proportion of zircons of Rodinia breakup age (ca. 800 Ma) that previously have not been found in such amounts in eastern Australia or the southwest Pacific. We propose that part of the Vanuatu arc basement comprises continental material that was rifted and transported thousands of kilometers from northeastern Australia prior to the Cenozoic. The presence of hitherto-unrecognized ancient continental material within an intraoceanic arc provides an alternative source for the crustal trace element and isotopic signature of island arc magmas, and may help reconcile the relatively large thickness and low density of the crust of Vanuatu and possible other intraoceanic arcs. INTRODUCTION Intraoceanic arcs that are far removed from large continental masses are considered ideal sites to study growth of new continental crust, as magmatic addition to the arc crust is thought to be sourced from the underlying subduction-modified mantle, with little to no contribution from the overlying plate. Under these conditions, rates of subduction zone element recycling and arc crust growth can be calculated by directly comparing subduction inputs and arc magmatic outputs over a time period (e.g., see Spandler and Pirard, 2013). However, global application of this approach is hampered by poor understanding of the temporal evolution of many individual arc systems, and recognition that some island arc volcanic rocks include a significant component of assimilated crust (e.g., Bezard et al., 2014). In this paper we examine igneous rocks from the western belt of the Vanuatu island arc (Fig. 1) with the aim of improving our understanding of the early stages of Vanuatu arc development. We not only document the petrology, geochemistry, and timing of formation of the rocks, but also report the unexpected finding of inherited zircon grains within the volcanic rocks ranging in age from Archean to Cretaceous. We discuss the origin of these inherited zircon populations, their significance for tectonic evolution of the southwest Pacific, and implications for models of island arc magmatism and arc crustal growth. GEOLOGICAL SETTING AND SAMPLE DESCRIPTION The island chain of Vanuatu, located 2000 km east of mainland Australia, is the subaerial exposure of one of many active island arcs that are *E-mail:
[email protected].
above the complex convergent boundary between the Australian and Pacific plates. The Oligocene to mid-Miocene western belt of the Vanuatu arc (Fig. 1) preserves the earliest known phase of arc growth that formed in response to west-dipping subduction of the Pacific plate at the Vitiaz Trench (Mitchell and Warden, 1971; Meffre and Crawford, 2001). Arrival of the Melanesian oceanic plateau at the Vitiaz Trench in the middle Miocene caused subduction reversal and development of new arc crust, first in the eastern belt, and then from 6 Ma in the central belt, where arc volcanism remains active. Subduction of the d’Entrecasteaux Ridge from ca. 3 Ma (Fig. 1) caused extensive uplift of the central portion of the Vanuatu forearc (Meffre and Crawford, 2001), yet subduction has continued unabated. Geochemical studies have focused on defining the mantle and slab components of modern Vanuatu arc lavas, with the aim of understanding subduction zone mass transfer, mantle dynamics, and growth of new crust in island arcs (Peate et al., 1997; Turner et al., 1999). Implicit in these studies is an assumption of negligible contribution from continental materials due to isolation of the arc from major continental landmasses. Geochemists have proposed that there is a significant slab component to these lavas, with major contributions from Pacific Ocean–type mantle in the north and south of the arc, and Indian Ocean–type mantle in the central section of the arc (Peate et al., 1997; Turner et al., 1999). There has previously been little research on the igneous rocks of the western belt exposed on Vanuatu’s largest islands of Espiritu Santo (herein called Santo) and Malekula (Fig. 1). This study focuses on a suite of dolerites, basaltic andesites, andesites, and a tonalite collected from southwestern Santo. Detailed petrographic descriptions of the samples are provided in the GSA Data Repository1. Hydrothermal alteration and weathering of these rocks is limited, so primary igneous textures and minerals are well preserved. The tonalite consists largely of zoned plagioclase and quartz, with minor altered amphibole and/or pyroxene and accessory titanite, apatite, and zircon. The volcanic rocks typically feature coarse phenocrysts of hornblende and calcic plagioclase, with or without augite and Fe-Ti oxides, set in a fine-grained to aphanitic groundmass. Most of the volcanic rocks contain gabbroic to pyroxenitic xenoliths, as well as numerous small quartz-carbonate (± apatite ± k-feldspar ± albite) xenoliths that show evidence for resorption (see the Data Repository). The xenoliths are interpreted to be crustal materials that were incorporated into the magma prior to, or during, eruption. GEOCHEMISTRY AND GEOCHRONOLOGY The chemical and isotopic compositions of the samples are typical of island arc volcanic rocks, with higher 87Sr/86Sr and lower 143Nd/144Nd than normal mid-oceanic ridge basalt (MORB), negative Nb anomalies, and relative enrichment in Cs, Rb, Ba, Th, U, K, Pb, and Sr (Fig. 2; for methods and geochemical data, see the Data Repository). They are chemi1 GSA Data Repository item 2014336, Item DR1 (petrological description of samples), Item DR2 (bulk rock geochemical and Sr-Nd isotope composition of samples, details of source mixing calculations), Item DR3 (analytical methods and results of Ar-Ar dating of igneous hornblende), Item DR4 (U-Pb dating and results), and Item DR5 (cathodoluminescence images of analysed zircon grains), is available online at www.geosociety.org/pubs/ft2014.htm, or on request from
[email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
GEOLOGY, November 2014; v. 42; no. 11; p. 939–942; Data Repository item 2014336 | doi:10.1130/G36155.1 | Published online 15 September 2014
© 2014 Geological Society2014 of America. For permission to copy, contact
[email protected]. GEOLOGY | November | www.gsapubs.org
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Figure 1. A: Tectonic configuration and continental geology of the southwest Pacific. Pie charts show the age distribution of detrital zircon records from New Caledonia, Papua New Guinea highlands, and eastern Australia (references in Item DR4 [see footnote 1]). Note that the eastern Australian samples are pre-Cretaceous and therefore do not record Cretaceous magmatism of Australia. d’E R—d’Entrecasteaux Ridge. Colorcoded age ranges designate the age of major crustal blocks or tectonic events in the region (outlined in C). B: Physiography and geology of central Vanuatu. ES—Espiritu Santo; M—Malekula. Red areas outline extent of the old arc terranes (western and eastern belts), and star denotes the sample location of this study. C: Significant age populations of detrital zircons from the Espiritu Santo volcanic samples. The number of analyzed zircon grains in each age range is shown in the respective segment of the chart. Note the significant fraction of grains of Neoarchean, Paleoproterozoic, and Rodinia breakup age. NEO—New England orogen; Ws—Whitsunday Province; NC—New Caledonia.
cally indistinguishable from modern lavas from central Vanuatu (Fig. 2) and, like most arc magmas, are interpreted to derive from melting of the mantle wedge with some components from either the subducting plate or the arc crust into which they were emplaced. To determine the magmatic age of the rocks from Espiritu Santo we combined Ar-Ar dating of pristine hornblende phenocrysts from three andesitic rock samples with U-Pb dating of zircons separated from the tonalite and selected volcanic rocks. (For details of analytical methods and results, see the Data Repository). We obtained 40Ar/39Ar isochron ages of 16.17 ± 0.34 Ma and 18.53 ± 0.41 Ma for andesites and a zircon age of 16.67 ± 0.22 Ma for the tonalite, which are consistent with the expected early Miocene magmatic development of Espiritu Santo (Mitchell and Warden, 1971). Ages of 35.5 ± 5.2 Ma (40Ar/39Ar) and 32.4 ± 0.57 Ma (U/ Pb) for two other volcanic rocks record an earlier phase of arc volcanism that has not previously been identified on Espiritu Santo, but has been recognized from the Torres Islands to the north (Mitchell and Warden, 1971). The magmas that formed most of these volcanic rocks are unlikely to have ever reached zircon saturation, given their relatively primitive, low Zr composition (Fig. 2; Watson and Harrison, 1983). Nevertheless, during electron microprobe analysis we observed small (10 m.y. The major zircon age populations in the Vanuatu samples are also recognized in detrital zircon signatures from sedimentary rocks of eastern Australia (Fig. 1) that record major crustal growth episodes of the Australian continent (Fig. 3). More specifically, key features of the Vanuatu zircon record, such as the high proportion of Archean and Paleoproterozoic zircons, a Mount Isa orogeny–age population, and a dearth of Devonian–early Carboniferous grains, are consistent with detrital and magmatic records of northeastern Queensland (Withnall and Henderson, 2012; Fig. 1), so we propose a northern Australian provenance for the continental material in Vanuatu. There are distinctive differences between the inherited signatures of Vanuatu and eastern Australia that are potentially informative for tectonic GEOLOGY | November 2014 | www.gsapubs.org
Figure 3. Relative probability and frequency histograms of inherited zircon ages from Espiritu Santo (Vanuatu) and igneous rocks from Australia (from Huston et al., 2012). Color-coded age ranges represent significant age populations used for the pie chart in Figure 1C.
reconstructions. The prominent record of Cretaceous magmatism along eastern Australia (Bryan et al., 1997) and New Caledonia (Cluzel et al., 2011) is poorly represented in the Vanuatu record, which indicates that the continental materials in Vanuatu were isolated from Cretaceous magmatism at the east Australian margin. In contrast, the significant middle Neoproterozoic (850–700 Ma) zircon population from Vanuatu does not feature in the detrital or magmatic record of eastern Australia or the southwest Pacific (Figs. 1 and 3). This time period corresponds to rifting and breakup of Rodinia, which caused basin formation and intrusion of the Gairdner Dolerite in South Australia, but there has previously been no geological record of Rodinia breakup in northern Australia (Li et al., 2008). If the Vanuatu continental basement derives from northern Australia, this record of Rodinia breakup may be crucial for testing Neoproterozoic plate reconstruction models. It is currently difficult to evaluate the transport mechanism (i.e., sedimentary or tectonic) or plate configurations that led to Australian crust being present in Vanuatu; however, it is possible that the proto– Vanuatu arc represents a continental ribbon separated from eastern Australia in response to Cretaceous backarc extension and rifting that occurred to the west of the eastward-retreating Pacific subduction zone (Schellart et al., 2006). Similar tectonic mechanisms have been invoked to explain separation of the Lord Howe Rise and Norfolk Ridge from eastern Australia (Schellart et al., 2006), and migration of a continental fragment from Western Australia to form part of the basement of the current East Java island arc (Smyth et al., 2007). Considering our finding and those of Smyth et al. (2007), we suggest that continental materials may reside in the basement of many other island arcs, including those far removed from continental masses. This global-scale dispersion of continental material and its cryptic manifestation in arc magmas of Vanuatu spell a warning for geochemical studies of oceanic magmatism that assume complete isolation from continental influences. In this paper we focus on an intraoceanic arc setting, but the case may also be relevant for
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ocean island basalts that are commonly assumed to be entirely mantle sourced (e.g., Hofmann, 2003). It has previously been assumed that the subducting slab provides the crustal component of Vanuatu arc lavas (e.g., Peate et al., 1997). However, the demonstrable assimilation of ancient continental materials undoubtedly influenced the trace element and isotopic composition of Vanuatu arc lavas, with the end-member case having the entire crustal signature derived from continental basement material within the arc. Testing this case, we were able to reproduce the Nd isotope composition of the lavas using a model of MORB parental magma with ~3% assimilation of representative continental material in the arc crust (see the Data Repository). While such assimilation rates would fit with petrographic observations and major element chemistry of the lavas, it cannot reproduce the trace element characteristics (Fig. 2) or other radiogenic isotope signatures (Peate et al., 1997; Turner et al., 1999), so other mantle and slab sources must be implicated. Nevertheless, assimilation of old continental crust provides an explanation for the variable helium isotope composition of Vanuatu lavas (Jean-Baptiste et al., 2009) and may in part account for the relatively radiogenic Sr and unradiogenic Nd of magmas from the central segment of the arc, previously attributed to an incursion of Indian Ocean mantle beneath the arc following collision of the d’Entrecasteaux Ridge at 3 Ma (Peate et al., 1997; Turner et al., 1999). This latter model may need revision, as the similar geochemical and isotopic characteristics of the Oligocene–Miocene volcanic rocks from Espiritu Santo and modern lavas from central Vanuatu (Fig. 2) require that the distinct magma source must have been present in this region well before d’Entrecasteaux Ridge collision. Considering that old continental basement may be a significant magma contaminant, it may be inaccurate to apply many of the commonly used chemical and isotopic criteria to fingerprint the slab component of arc magmas, as both slab and continental crust components can deliver similar source signatures (Plank and Langmuir, 1998). Therefore, models involving simple mixing of mantle and slab components may not suffice to explain arc magma geochemistry; rather, such models may provide maximum values for element recycling from the slab. Island arc crust is generally thought to develop primarily via addition and evolution of mantle-derived arc magmas, yet it is of significantly greater thickness and lower density than typical oceanic crust (Dimalanta et al., 2002). The presence of even minor amounts of low-density continental material, such as documented here for Vanuatu, would ease the requirements for complex magmatic differentiation and delamination processes (e.g., Jagoutz and Schmidt, 2012), and would imply that crustal growth rates calculated from estimates of temporal island arc development (e.g., Dimalanta et al., 2002) may be overestimated. ACKNOWLEDGMENTS We thank Camillia Garae, Christopher Ioan, James Ghislaine, and Grace Barber for access to and assistance in the field, and Richard Wormald for help with zircon separation procedures. We also thank Wouter Schellart and two anonymous reviewers for constructive comments on the manuscript. This work was supported by the Australian Research Council (DP1095280). REFERENCES CITED Bezard, R., Davidson, J.P., Turner, S., Macpherson, C.G., Lindsay, J.M., and Boyce, A.J., 2014, Assimilation of sediments embedded in the oceanic arc crust: Myth or reality?: Earth and Planetary Science Letters, v. 395, p. 51–60, doi:10.1016/j.epsl.2014.03.038. Bryan, S.E., Constantine, A.E., Stephens, C.J., Ewart, A., Schön, R.W., and Parianos, J., 1997, Early Cretaceous volcano-sedimentary successions along the
eastern Australian continental margin: Implications for the break-up of eastern Gondwana: Earth and Planetary Science Letters, v. 153, p. 85–102, doi: 10.1016/S0012-821X(97)00124-6. Cluzel, D., Adams, C.J., Maurizot, P., and Meffre, S., 2011, Detrital zircon records of Late Cretaceous syn-rift sedimentary sequences of New Caledonia: An Australian provenance questioned: Tectonophysics, v. 501, p. 17–27, doi: 10.1016/j.tecto.2011.01.007. Dimalanta, C., Taira, A., Yumul, G.P., Jr., Tokuyama, H., and Mochizuki, K., 2002, New rates of western Pacific island arc magmatism from seismic and gravity data: Earth and Planetary Science Letters, v. 202, p. 105–115, doi: 10.1016/S0012-821X(02)00761-6. Hofmann, A.W., 2003, Sampling mantle heterogeneity through oceanic basalts: Isotopes and trace elements, in Carlson, R.W., ed., Treatise on Geochemistry, Volume 2: The mantle and core: Amsterdam, Elsevier, p. 61–101, doi: 10.1016/B0-08-043751-6/02123-X. Huston, D.L., Blewett, R.S., and Champion, D.C., 2012, Australia through time: A summary of its tectonic and metallogenic evolution: Episodes, v. 35, p. 23–43. Jagoutz, O., and Schmidt, M.W., 2012, The formation and bulk composition of modern juvenile continental crust: The Kohistan arc: Chemical Geology, v. 298–299, p. 79–96, doi:10.1016/j.chemgeo.2011.10.022. Jean-Baptiste, P., Allard, P., Bani, P., Garaebiti, E., Pelletier, B., Fourré, E., and Metrich, N., 2009, Highly variable helium isotope ratios in the Vanuatu volcanic arc: American Geophysical Union Fall Meeting Abstracts, v. 1, p. 1774, abs. T21A-1774. Li, Z.X., et al., 2008, Assembly, configuration, and break-up history of Rodinia: A synthesis: Precambrian Research, v. 160, p. 179–210, doi:10.1016/j .precamres.2007.04.021. Meffre, S., and Crawford, A.J., 2001, Collisional tectonics in the New Hebrides arc (Vanuatu): Island Arc, v. 10, p. 33–50, doi:10.1046/j.1440-1738.2001 .00292.x. Mitchell, A.H.G., and Warden, A.J., 1971, Geological evolution of the New Hebrides island arc: Geological Society of London Journal, v. 127, p. 501–529, doi:10.1144/gsjgs.127.5.0501. Peate, D.W., Pearce, J.A., Hawkesworth, C.J., Colley, H., Edwards, C.M.H., and Hirose, K., 1997, Geochemical variation in Vanuatu arc lavas: The role of subducted material and a variable mantle wedge composition: Journal of Petrology, v. 38, p. 1331–1358, doi:10.1093/petroj/38.10.1331. Plank, T., and Langmuir, C.H., 1998, The chemical composition of subducting sediment and its consequences for the crust and mantle: Chemical Geology, v. 145, p. 325–394, doi:10.1016/S0009-2541(97)00150-2. Schellart, W.P., Lister, G.S., and Toy, V.G., 2006, A Late Cretaceous and Cenozoic reconstruction of the Southwest Pacific region: Tectonics controlled by subduction and slab rollback processes: Earth-Science Reviews, v. 76, p. 191–233, doi:10.1016/j.earscirev.2006.01.002. Smyth, H.R., Hamilton, P.J., Hall, R., and Kinny, P.D., 2007, The deep crust beneath island arcs: Inherited zircons reveal a Gondwana continental fragment beneath East Java, Indonesia: Earth and Planetary Science Letters, v. 258, p. 269–282, doi:10.1016/j.epsl.2007.03.044. Spandler, C., and Pirard, C., 2013, Element recycling from subducting slabs to arc crust: A review: Lithos, v. 170–171, p. 208–223, doi:10.1016/j.lithos .2013.02.016. Turner, S.P., Peate, D.W., Hawkesworth, C.J., Eggins, S.M., and Crawford, A.J., 1999, Two mantle domains and the time scales of fluid transfer beneath the Vanuatu arc: Geology, v. 27, p. 963–966, doi:10.1130/0091-7613(1999) 0272.3.CO;2. Watson, E.B., and Harrison, T.M., 1983, Zircon saturation revisited: Temperature and composition effects in a variety of crustal magma types: Earth and Planetary Science Letters, v. 64, p. 295–304, doi:10.1016/0012-821X(83) 90211-X. Withnall, I.W., and Henderson, R.A., 2012, Accretion on the long-lived continental margin of northeastern Australia: Episodes, v. 35, p. 166–176. Manuscript received 7 August 2014 Revised manuscript received 10 August 2014 Manuscript accepted 11 August 2014 Printed in USA
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