Journal of Petrology Advance Access originally published online on May 4, 2006
Journal of Petrology 2006 47(9):1673-1704; doi:10.1093/petrology/egl024
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The Origin of HIMU in the SW Pacific: Evidence from Intraplate Volcanism in Southern New Zealand and Subantarctic Islands
1 DEPARTMENT OF GEOLOGY, BOWLING GREEN STATE UNIVERSITY BOWLING GREEN, OH 43403, USA
2 DEPARTMENT OF GEOLOGY & GEOPHYSICS, WOODS HOLE OCEANOGRAPHIC INSTITUTION WOODS HOLE, MA 02543, USA
3 DEPARTMENT OF EARTH & ENVIRONMENTAL SCIENCE, NEW MEXICO INSTITUTE OF MINING & TECHNOLOGY SOCORRO, NM 87801, USA
RECEIVED JULY 28, 2005; ACCEPTED MARCH 29, 2006
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONREGIONAL GEOLOGICAL HISTORYVOLCANIC GEOLOGYSAMPLE DESCRIPTION AND...RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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This paper presents field, geochemical and isotopic (Sr, Nd, Pb) results on basalts from the Antipodes, Campbell and Chatham Islands, New Zealand. New 40Ar/39Ar age determinations along with previous K–Ar dates reveal three major episodes of volcanic activity on Chatham Island (85–82, 41–35,
5 Ma). Chatham and Antipodes samples comprise basanite, alkali and transitional basalts that have HIMU-like isotopic (206Pb/204Pb >20·3–20·8, 87Sr/86Sr <0·7033, 143Nd/144Nd >0·5128) and trace element affinities (Ce/Pb 28–36, Nb/U 34–66, Ba/Nb 4–7). The geochemistry of transitional to Q-normative samples from Campbell Island is explained by interaction with continental crust. The volcanism is part of a long-lived (100 Myr), low-volume, diffuse alkaline magmatic province that includes deposits on the North and South Islands of New Zealand as well as portions of West Antarctica and SE Australia. All of these continental areas were juxtaposed on the eastern margin of Gondwanaland at >83 Ma. A ubiquitous feature of mafic alkaline rocks from this region is their depletion in K and Pb relative to other highly incompatible elements when normalized to primitive mantle values. The inversion of trace element data indicates enriched mantle sources that contain variable proportions of hydrous minerals. We propose that the mantle sources represent continental lithosphere that host amphibole/phlogopite-rich veins formed by plume- and/or subduction-related metasomatism between 500 and 100 Ma. The strong HIMU signature (206Pb/204Pb >20·5) is considered to be an in-grown feature generated by partial dehydration and loss of hydrophile elements (Pb, Rb, K) relative to more magmaphile elements (Th, U, Sr) during short-term storage at the base of the lithosphere. KEY WORDS: continental alkaline basalts; lithospheric mantle, mantle metasomatism; New Zealand; OIB, HIMU; Sr, Nd and Pb isotopes; West Antarctica
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGICAL HISTORYVOLCANIC GEOLOGYSAMPLE DESCRIPTION AND...RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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It is widely accepted that most mafic alkaline magmas erupted in continental areas that have HIMU (high time-integrated 238U/204Pb or high µ) isotopic signatures similar to oceanic island basalts (OIB: 206Pb/204Pb >20·5 and low 87Sr/86Sr <0·7030 and 3He/4He 5–7 R/Ra) are products of small-degree melts from sub-lithospheric mantle sources (Wilson et al., 1995
; Ballentine et al., 1997; Franz et al., 1999; Janney et al., 2002). The origin of the HIMU source component is often attributed to chemically modified oceanic lithosphere that was subducted and stored in the deep mantle, aged and eventually transported back to the surface via mantle plumes (Hofmann, 1997; Moreira & Kurz, 1999; Stracke et al., 2003, 2005). However, it is becoming increasingly recognized that HIMU sources may also exist within sub-continental lithosphere, generated by the infiltration of metasomatic fluids and melts (Stein et al., 1997; Blusztajn & Hegner, 2002; Pilet et al., 2005). A lithospheric versus sub-lithospheric origin of the HIMU component in continental basalts in the SW Pacific (New Zealand, West Antarctica and SE Australia) has been a matter of debate for several decades.
The intraplate igneous rocks of southern New Zealand are low in volume (<10 000 km3), widely scattered within a 1000 km x 800 km area and include several large shield volcanoes, mafic lava fields and small intrusive complexes (Weaver & Smith, 1989). The mafic rocks are predominantly alkaline and have OIB-like major and trace element compositions and isotopic signatures that resemble a HIMU mantle source (Gamble et al., 1986; Barreiro & Cooper, 1987; Weaver & Smith, 1989; Baker et al., 1994; Hoke et al., 2000). Coombs et al. (1986) and Sun et al. (1989) recognized the regional OIB–HIMU like character of continental volcanism in New Zealand, Tasmania and West Antarctica. Lanyon et al. (1993), Weaver et al. (1994) and Storey et al. (1999) related the HIMU volcanism to mantle plume sources that entered the upper mantle beneath Gondwanaland in the mid- to late Cretaceous and suggested that plume activity played a role in Antarctic–New Zealand break-up and the opening of the Tasman Sea. A younger and smaller plume has been invoked to explain Cenozoic uplift and volcanism in Marie Byrd Land (LeMasurier & Rex, 1989; Behrendt et al., 1992, 1996; Hole & LeMasurier, 1994; LeMasurier & Landis, 1996; Storey et al., 1999). An alternative plume model is one in which the source for all of the HIMU volcanism in the SW Pacific can be related to an ancient plume head that was fossilized in the uppermost mantle prior to Gondwanaland break-up (Rocholl et al., 1995; Hart et al., 1997; Panter et al., 2000a). Others have proposed magmatism without plume origins. Rocchi et al. (2002, 2004) suggested that the upper mantle source for the alkaline magmas was metasomatically enriched during late Cretaceous extension and subsequently melted in response to reactivation of trans-lithospheric fractures during Cenozoic times. Finn et al. (2005) proposed that the Cenozoic volcanism forming this Diffuse Alkaline Magmatic Province (DAMP) is a response to the sudden detachment and sinking of subducted slabs into the lower mantle causing vertical and lateral flow that triggered melting of a previously metasomatized mantle lithosphere.
Past studies are weighted heavily on data gathered from Cenozoic basalts but most of the models call upon sources that are at least Mesozoic in age. Prior to this study, Mesozoic igneous rocks that approach OIB–HIMU end-member compositions in terms of Pb isotopes (206Pb/204Pb > 20) have been identified only within the mid-Cretaceous (c. 90–100 Ma) Tapuaenuku Igneous Complex (Baker et al., 1994) on the South Island of New Zealand (Fig. 1). Other late Cretaceous continental basalts in New Zealand and Australia have similar geochemical characteristics but complete datasets, in particular Pb isotopes, are not yet published. We present here a detailed study of continental basalts from three islands off the east and southeastern coast of New Zealand (Fig. 1). The oldest basalts on Chatham Island are of late Cretaceous age (85 Ma) with initial 206Pb/204Pb values 
20 and trace element characteristics similar to OIB. The Cenozoic basalts from the same region, as well as basalts from Marie Byrd Land, West Antarctica (Hart et al., 1997; Panter et al., 2000a) have very similar isotopic and trace element characteristics. The purpose of this study is to: (1) constrain the processes involved in the genesis of mafic magmas in southern New Zealand; (2) determine mantle source compositions; (3) evaluate the origin of the HIMU-like signature; (4) use the new data to help constrain regional models proposed for the magmatism.
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REGIONAL GEOLOGICAL HISTORY |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGICAL HISTORYVOLCANIC GEOLOGYSAMPLE DESCRIPTION AND...RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The Campbell and Antipodes islands are situated along the southeastern margin of the submarine Campbell Plateau, whereas the Chatham Islands lie at the eastern end of the Chatham Rise (Fig. 1). The basement geology of the Chatham Rise and Campbell Plateau consists of Paleozoic–Mesozoic metasedimentary and plutonic rocks (Beggs et al., 1990
; Wood & Herzer, 1993) that correlate with major tectonostratigraphic terranes recognized on the mainland of New Zealand (Bishop et al., 1985; Bradshaw, 1989; Adams et al., 1998) and in Marie Byrd Land, Antarctica (Bradshaw et al., 1997; Pankhurst et al., 1998; Sutherland, 1999).
The late Cretaceous break-up of the proto-Pacific margin of Gondwanaland was foreshadowed by mid-Cretaceous extension and crustal thinning over a broad area (Bradshaw, 1989; Beggs, 1993; Davy, 1993; Laird, 1993; Luyendyk et al., 2001, 2003), rapid regional uplift (Tulloch & Kimbrough, 1989; Richard et al., 1994; Adams et al., 1995; Spell et al., 2000) and a sudden change from subduction-related to extension-related magmatism (Weaver et al., 1994; Tulloch & Kimbrough, 1995; Waight et al., 1998a). The switch in tectonic regime corresponded to the oblique arrival of the Pacific–Phoenix spreading center along the Gondwanaland margin at 100 Ma (Bradshaw, 1989; Luyendyk, 1995; Mukasa & Dalziel, 2000). Ultimately, the interaction between the Pacific–Phoenix spreading center and the subduction zone led to the break-up of the supercontinent and inception of the Pacific–Antarctic Ridge shortly before 83 Ma (Molnar et al., 1975; Larter et al., 2002).
Cretaceous extension-related magmatism along the New Zealand sector of the Gondwanaland margin includes the Mandamus (Weaver & Pankhurst, 1991), Tapuaenuku (Baker et al., 1994) and Blue Mountain (Grapes, 1975) layered igneous complexes, numerous other intrusions (Waight et al., 1998b) and dispersed volcanic fields, including a large basaltic shield volcano on the Chatham Islands (Grindley et al., 1977). These igneous rocks range in age from 100 to 60 Ma with a peak in activity between 100 and 90 Ma (Baker et al., 1994; Table 1). Following continental break-up, New Zealand experienced pronounced thermal relaxation as it moved northward on the Pacific Plate. Consequently, the Paleocene–Eocene epochs were a period of relative magmatic quiescence (Weaver & Smith, 1989) and volcanic rocks of late Eocene to early Oligocene age are minor in volume and widely dispersed.
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Over the past 45 Myr tectonic activity in New Zealand has been the result of the progressive development of the Australian–Pacific plate boundary (Sutherland, 1995
). Transtension and inception of the paleo-Alpine Fault system, whose age is constrained by dikes of the Westland swarm (Fig. 1) emplaced between 32 and 25 Ma (Adams & Cooper, 1996), was followed by transpression and rapid uplift of the Southern Alps beginning at 6 Ma (Walcott, 1998). Adams & Cooper (1996) proposed that the termination of a major volcanic episode, which produced several large igneous centers on the Banks and Dunedin Peninsulas in the middle to late Miocene, was a response to the change to a compressive tectonic regime.
Pliocene–Pleistocene volcanism in southern New Zealand was restricted to small areas on the South Island (Duggan & Reay, 1986) and on Antipodes and Chatham Islands (Cullen, 1969; Grindley et al., 1977). Although at present there is no active volcanism in southern New Zealand, geophysical evidence and helium emission studies indicate the existence of hot regions of decompressing upper mantle (Hoke et al., 2000; Godfrey et al., 2001).
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VOLCANIC GEOLOGY |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGICAL HISTORYVOLCANIC GEOLOGYSAMPLE DESCRIPTION AND...RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Campbell Island is New Zealand's southernmost subantarctic island (Fig. 1). Miocene volcanic rocks cover most of the island and consist of lava flows, pyroclastic rocks and volcanoclastic breccias (Morris, 1984
; Morris & Gamble, 1990). Apart from a shallow intrusion of gabbro that predates volcanism by 9 Myr, most of the deposits are late Miocene in age (7·0 Ma, Adams et al., 1979).
The Antipodes Islands are a small isolated island group that lies at the eastern edge of the Campbell Plateau (Fig. 1). The islands are composed of alkaline lavas and pyroclastic rocks, including tuff cone and ring deposits (Cullen, 1969; Gamble et al., 1986; Gamble & Adams, 1990). A Quaternary age for the islands is inferred based on the well-preserved volcanic morphology and two K–Ar dates (Cullen, 1969).
The Chatham Islands are a populated group of islands that lie over 700 km east of the mainland of New Zealand (Fig. 1). The group consists of two main islands, Chatham and Pitt, which are surrounded by several smaller islands and a number of islet clusters. Campbell et al. (1993) provided a comprehensive review of the geology, geochronology and biostratigraphy of the Chatham Islands. The petrology and geochemistry of the Eocene and Cretaceous volcanic rocks have been reported by Morris (1985a, 1985b).
The mid- to late Cretaceous volcanic and sedimentary rocks of the Chatham Islands rest unconformably on a pre-Cretaceous basement of metasediments. The Cenozoic sequence consists of thin biogenic and clastic sedimentary units interspersed with localized volcanic deposits. Based on mapping and 26 K–Ar dates, Grindley et al. (1977) recognized that volcanism on the Chatham Islands occurred in three distinct episodes; late Cretaceous (81–70 Ma) Southern Volcanics, late Eocene (41–36 Ma) Northern Volcanics andMiocene–Pliocene (5·0 Ma) Rangitihi Volcanics (Fig. 2).
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The late Cretaceous Southern Volcanics are volumetrically the most significant and widely distributed group. The Southern Volcanics consist of lava flows 2–5 m thick that often show reddened brecciated bases and scoriaceous tops indicative of subaerial emplacement. Exposed lava sequences, up to 300 m thick, occur on the southern coast of Chatham Island and thin progressively to the north, consistent with a southern source located in the Pitt Strait (Morris, 1985a
; Campbell et al., 1993). The late Eocene Northern Volcanics form small (<150 m in height) but prominent hills on the northern half of Chatham Island (Fig. 2). The hills represent variably eroded cones composed predominantly of interbedded basaltic lavas and pyroclastics. At Mairangi cone, reddened scoriaceous deposits containing bomb and block ejecta were produced by Strombolian activity. At Matakitaki cone, dark green palagonitized tuffs containing vesiculated lapilli-sized sideromelane fragments were produced by phreatomagmatic activity.
The Miocene–Pliocene Rangitihi Volcanics are concentrated in two areas on the northern coast of Chatham Island (Fig. 2). The deposits exposed at Cape Young and Maunganui consist of interbedded tuffs and tuff-breccias, lavas and fossiliferous marine sediments. The Rangitihi Volcanics are characterized by megacrysts of amphibole (up to 4 cm in length) and also contain abundant crustal and mantle xenoliths. The presence of tuffaceous foraminiferan packstone surrounding and interclated with lobes of pillow basalts at the base of the volcanic sequence at Maunganui suggests that volcanic activity began with shallow submarine eruptions (Campbell et al., 1993).
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SAMPLE DESCRIPTION AND ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGICAL HISTORYVOLCANIC GEOLOGYSAMPLE DESCRIPTION AND...RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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This study is based on 36 mafic igneous rocks from the Campbell, Antipodes and Chatham island groups. The majority of the samples were collected from lava flows and small shallow intrusions. Most of the samples are dense, holocrystalline and porphyritic with phenocrysts of olivine and clinopyroxene (Table 1). Plagioclase feldspar is abundant (10–20%) as phenocrysts in Campbell Island lavas and is also the dominant interstitial phase in gabbro. Magnetite along with plagioclase occurs in the groundmass of most samples (Table 1) and kaersutite is found in Pliocene basalts from Chatham Island and in Pleistocene lavas from Antipodes Island (Gamble et al., 1986
). Dense holocrystalline samples are fresh and unaltered but some glassy and more vesicular samples show secondary alteration. Devitrification of hypohyaline groundmasses is observed in some lavas and blocks of the Northern Volcanics, including a palagonitized sideromelane tuff from Matakitaki cone on Chatham Island.
Thirty-two samples were analyzed for major and trace elements (V, Cr, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba and Pb) by X-ray fluorescence (XRF) on a Philips PW2400 XRF spectrometer at New Mexico Tech. Analytical precisions on major elements with concentrations >1 wt % are <0·1% (1
) and for trace elements typically <4% (1). Lead concentrations for 12 samples were determined by inductively coupled plasma mass spectrometry (ICP-MS) with precisions of 1–2%, others by XRF with precisions of 10% (Table 2). Other trace elements were measured at New Mexico Tech by instrumental neutron activation analysis (INAA) using 26% and 18% efficient high-purity Ge detectors (Hallett & Kyle, 1993). Based on repeat analyses of standards, we estimate analytical precision to be <1% for Sc, La, and Sm; <3% for Ce, Eu, Hf, Ta, Th and Lu; <5% for Tb and Yb; and <10% for Nd, Cs, and U.
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Many of the Chatham Island samples have loss on ignition (LOI) values that exceed 1·0 wt % (Table 2). There is a broad correlation between higher LOI values and samples that contain glass and amygdules. However, only when considering the Northern Volcanics are correlations found between LOI and some major and trace elements. The geochemistry of the most highly altered samples (LOI >5 wt %), is not considered in subsequent petrogenetic discussions. For the remaining samples, major and trace elements were recalculated to 100% volatile-free (Table 2).
Sr, Nd and Pb isotopic analyses of 18 samples (Table 4) were performed at the Woods Hole Oceanographic Institution using conventional thermal ionization mass spectrometry (TIMS; Hauri & Hart, 1993). All sample chips were first leached in warm 6N HCl for 1 h before dissolution. Precisions of the Sr and Nd data are ± 0·003–0·005% (2). Reproducibility of Pb data is 0·05% per a.m.u. based on repeat runs of NBS981.
A total of 12 Chatham Island samples were dated by the 40Ar/39Ar method (Table 3). The whole-rock samples were crushed, sieved, leached with dilute HCl, washed in distilled water and hand-picked to remove phenocrysts and altered material to produce a groundmass concentrate. Amphibole separates were prepared by hand-picking crushed rock with the aid of a binocular microscope. The samples and Fish Canyon Tuff sanidine monitor (age 27·84 Ma) were irradiated for 14 h at the Nuclear Science Center reactor, College Station, Texas. Argon isotopic compositions of samples and monitors were determined at the New Mexico Geochronological Laboratory.
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RESULTS |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGICAL HISTORYVOLCANIC GEOLOGYSAMPLE DESCRIPTION AND...RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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40Ar/39Ar geochronology—Chatham Island
Twelve 40Ar/39Ar dates of basalts were obtained to better constrain the volcanic history of Chatham Island. Apparent ages were determined from age spectra and isotope correlation (36Ar/40Ar vs 39Ar/40Ar) diagrams (Table 3). The latter provide a quantitative measure of the initial argon composition and allow the identification of excess argon that has 40Ar/36Ar ratios greater than the present-day atmospheric value of 295·5. The 40Ar/36Ar intercept values in Table 3 indicate that excess argon is not significant in Chatham Island samples. The anomalous 40Ar/36Ar ratios for amphibole are a consequence of high uncertainties in the fit of the regression line. In all cases, the plateau age spectrum is interpreted to represent the eruption age of the basalts (Table 3 ‘preferred age’ and Fig. 3). Degassing of the three amphibole samples occurred over a very narrow range of temperatures, which resulted in a small number of steps comprising >90% of the total 39ArK released. Sample CHT-5 degassed primarily in one temperature step (1200°C, 39ArK = 91·7%) and, therefore, does not meet plateau criteria (Fig. 3b). Its age, however, is indistinguishable from the plateau age for sample CHT-8, which was collected from a stratigraphically and lithologically equivalent unit. Alteration is the likely reason for the discordant age spectrum of groundmass samples CHT-11 and CHT-20. The low radiogenic 40Ar yields caused by alteration translate into high uncertainties on the age of individual temperature steps. Radiogenic yields are significantly higher for amphibole separated from CHT-20, providing an apparent age with a higher precision (Table 3).
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The 40Ar/39Ar ages provide several important modifications to the volcanic history of Chatham Island based on the K–Ar dates of Grindley et al. (1977)
. Our results indicate that the late Cretaceous volcanism, which produced the Southern Volcanics, is significantly older than previously thought. The 40Ar/39Ar age determinations from this study range from 82·26 ± 0·79 Ma to 85·45 ± 0·59 Ma whereas prior K–Ar dates range from 70·4 ± 1·2 Ma to 81·4 ± 2·0 Ma [revised using the decay constants and isotopic abundances recommended by Steiger & Jäger (1977)]. Our samples were collected from units that encompass most of the volcanic stratgraphy of Chatham Island and from some of the same sites sampled for K–Ar dating. Two basalts collected 1 km apart on the same lava flow at Ohira Bay yield indistinguishable 40Ar/39Ar ages of 84·16 ± 0·73 Ma and 84·2 ± 1·2 Ma (samples CHT-1 and CHT-3, respectively; Table 3). These ages are significantly older than the revised K–Ar age of 79 ± 2·0 Ma [sample R3198 of Grindley et al. (1977)] from the same flow. Other locations where our results yield significantly older ages relative to previous K–Ar ages include samples collected from Waitaha Creek (CHT-12) and Cape Fournier (CHT-27). We, therefore, conclude that the age of the Southern Volcanics from the Chatham Islands are more tightly clustered around 84–85 Ma rather than the younger and longer (10 Myr) time-span proposed by Grindley et al. (1977). A relatively short period of eruption for the bulk of the Southern Volcanics is supported by the age of samples collected from the base and top of thick (80–100 m) lava sequences that are indistinguishable within the resolution of argon dating.
The Cenozoic volcanism on the Chatham Islands consists of at least 10 separate volcanic lithostratigraphic units, only half of which have been dated by radiometric methods (Campbell et al., 1993). Previous K–Ar ages for the Northern Volcanics are late Eocene (41–36 Ma). In this study, one sample of the Northern Volcanics collected from Mt. Chudleigh (Fig. 2) has a slightly younger age of 32·1 ± 4·8 Ma (CHT-11). Four samples of the Pliocene–Miocene Rangitihi Volcanics were dated by the 40Ar/39Ar method and generally agree with previous K–Ar ages. Two amphibole separates from basaltic lavas at Cape Young yield indistinguishable ages of 4·96 ± 0·25 Ma and 4·97 ± 0·10 Ma (CHT-5 and CHT-8, respectively; Table 3). Approximately 12 km to the west at Maunganui (Fig. 2), a lava flow on the coast is dated at 4·34 ± 0·45 Ma (CHT-17, Fig. 3d). Our 40Ar/39Ar age determinations indicate that Rangitihi Volcanics also exist at the Tawreikoko volcanic cone (Fig. 2). Tawreikoko cone is composed of breccias and lavas mapped as Northern Volcanics but is cut by a kaersutite-bearing basaltic dike of Rangitihi age (4·95 ± 0·27 Ma, CHT-20 amp, Table 3).
Major and trace elements
Mafic igneous rocks from the Chatham, Antipodes and Campbell islands range in composition from ultrabasic to basic (42–51 wt % SiO2), have Mg-numbers between 44 and 65, and are classified as basanite (ne-normative, >10% ol-normative), tephrite (ne-normative, <10% ol-normative), alkali- and transitional basalts (<10% hy-normative), subalkali basalt, and Q-gabbro (>10% hy-normative, Q-normative) according to the recommendations of Le Maitre (2002). One sample from Chatham Island (CHT-14) is intermediate in composition and classified as a basaltic trachyandesite (Table 2).
The broad correlation of MgO with major and trace elements displayed in Fig. 4 suggests that most of the samples have been modified by crystal fractionation. The fractionation of olivine is indicated by increasing Al2O3 and decreasing Ni (not shown in Fig. 4) (KDol/melt Ni 10–23, Hart & Davis, 1978), whereas fractionation of clinopyroxene is indicated by decreasing MgO with CaO/Na2O and Cr (KDCpx/melt Cr 3·8, Hart & Dunn, 1993). A negative correlation between Zr/Hf ratios and Sc content, particularly well defined in the Southern Volcanics, is also indicative of clinopyroxene control during crystallization (David et al., 2000).
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With decreasing MgO the Rangitihi Volcanics show a systematic increase in the concentration of most incompatible elements (e.g. Ba, Nb, La, Th) and depletion in elements compatible with olivine and clinopyroxene (Fig. 4). The trend of increasing FeOt with decreasing MgO (not shown) argues against significant fractionation of Fe–Ti oxides. The slight decrease in SiO2 with MgO (Fig. 4) and corresponding increase in Si-undersaturation (from
4% to >10% normative nepheline, Table 2) is in response to the removal of kaersutite; a Ti-rich, silica-poor amphibole that is found in high modal proportions in the Rangitihi Volcanics (Table 1). The fractionation of kaersutite has also buffered TiO2 concentrations. TiO2 will increase with crystallization of olivine and clinopyroxene with little or no fractionation of Fe–Ti oxides; however, the high TiO2 content of kaersutite (5 wt %) relative to the magma (<3 wt %) counteracts this affect. Removing kaersutite from the magma also depletes Ti relative to middle rare earth elements (MREE), Eu and Tb (Ti/Ti*, Fig. 4) and other large ion lithophile elements (LILE). The coherent fractionation trends of Rangitihi Volcanics can be modeled by the fractionation of 25% clinopyroxene, 19% kaersutite, 15% olivine, 10% plagioclase and 4% titanomagnetite from the least fractionated basanite (CHT-20, MgO >12 wt %) to produce a 27% residual that closely approximates the major element (
r2 = 0·04) and trace element concentrations (errors for U and Th <20%, Zr, Hf, Ba <15% and Sr, La, Ce, Ta, Nb <10%) of tephrite (CHT-17, MgO <6 wt %). This least-squares mass balance and Rayleigh fractionation model uses mineral analyses from the Rangitihi Volcanics and the mafic mineral–melt partition coefficients of Caroff et al. (1993). Mass balance models for other suites suggest that clinopyroxene and olivine (± plagioclase) controlled the evolution from alkali basalt to hawaiite for the late Cretaceous and Eocene sequences on Chatham Island (Morris, 1985a, 1985b). Morris (1984) also modeled the differentiation of transitional basalt to hawaiite on Campbell Island by the removal of plagioclase and lesser amounts of clinopyroxene, consistent with the observed petrography. To help evaluate the source of the basalts and constrain partial melting processes we have selected the most primitive samples from each island. Seven of the basalts are relatively unfractionated (Mg-number 62–66, Cr > 300 ppm and Ni > 250 ppm); these include one sample from Campbell Island (OU39796) and one from each of the three main age-delimited volcanic groups on Chatham Island. The least fractionated Antipodes Island sample (ANT-3) is comparable in concentration with respect to Al2O3, CaO, Cr and V but has a lower Mg-number (56) and lower Ni (148 ppm) content, indicating that greater amounts of olivine have been fractionated. To compensate for olivine fractionation the samples were normalized to Mg-number 73. This was accomplished by adding olivine, incrementally, to each composition while maintaining a constant Fe2+/Mg KD value equal to 0·30 until the Mg-number reached 73 and olivine reached Fo90. Trace element concentrations were reduced by the percentage of olivine added back to each composition (Table 2).
The olivine-normalized basanite, alkali- and transitional-basalts from the Antipodes, Campbell and Chatham Islands are compared on multi-element and REE plots in Figs 5 and 6, respectively. The samples display similar enriched trace element patterns with peaks at Na–Ta and prominent negative K- and Pb-anomalies (Fig. 5). The basalts also show parallel and moderately steep patterns on REE plots with LaN/YbN values between 12 and 28 and LaN/SmN values between 2·4 and 3·2 (Fig. 6), which implies that all of the basalts are the products of small melt fractions of a garnet-bearing source. A notable difference, however, is evident in their relative Ti concentrations (Fig. 5). The Ti/Ti* values for the basalts are variable (0·8–1·7) and span this range of values in samples from all localities (Fig. 4). There is also a difference in Zr/Nb and Hf/Nb values between the Late Cretaceous (Zr/Nb = 5·2–5·8, Hf/Nb = 0·12–0·13) and Cenozoic (Zr/Nb = 3·7–3·8, Hf/Nb = 0·09–0·10) basalts from Chatham Island. Setting aside these subtle variations it is apparent that the overall trace element characteristics (Fig. 5) and low LILE/Nb values (e.g. Fig. 7) of the New Zealand samples are comparable with those of other continental alkaline basalts from the SW Pacific (Johnson, 1989; Finn et al., 2005) and OIB, in particular, those that have the HIMU type isotopic signature (St. Helena and the Austral–Cook island chain).
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Sr, Nd and Pb isotopes
Results of isotope analysis for Chatham, Antipodes and Campbell island samples are presented in Table 4 and are plotted with reference to basalts from the south–central Pacific and South Atlantic oceans in Fig. 8. Also included in Fig. 8 are data for samples from alkaline magmatic suites on the South Island of New Zealand; the early Miocene Westland dike swarm (Barreiro & Cooper, 1987
), mafic clasts within the Port Chalmers Breccia (PCB) of the Dunedin Volcanic Group (Price et al., 2003), the mid-Cretaceous Tapuaenuku igneous complex (Baker et al., 1994; J. A. Baker, unpublished data, 1997) and Auckland Island (J. A. Gamble & R. J. Wysoczanski, unpublished data, 2006). All data plotted in Fig. 8 are measured values. Although it is customary to correct for the radiogenic in-growth of daughter nuclides to discuss source signatures we suggest that a better comparison between Mesozoic and Cenozoic samples can be made if the values are left uncorrected. We consider the measured values to be close approximations of their present-day source values. To evaluate this assertion, initial isotopic ratios for the Cretaceous Chatham Island basalts were projected to the present day using parent/daughter ratios calculated by means of a two-stage mantle evolution model. In Table 4, the model source ratios differ from measured ratios by 
0·00008 for 87Sr/86Sr, 0·00004 for 143Nd/144Nd and 0·19 for 206Pb/204Pb.
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Together, the basalts from the Chatham and subantarctic islands show a restricted range in measured 143Nd/144Nd (0·5128–0·5129) and 207Pb/204Pb (15·6–15·7) values, a moderate range in measured 87Sr/86Sr (0·7029–0·7040) and 208Pb/204Pb (39·1–40·5) values, and a wide range in measured 206Pb/204Pb values (19·2–20·8). With the exception of one sample (CHT-11) all Chatham Island and Antipodes Island basalts have high 206Pb/204Pb (20·3–20·8) and low 87Sr/86Sr (
0·703) values and define moderately tight clusters on isotope correlation plots (Fig. 8). In Fig. 8, the Chatham and Antipodes basalts mostly lie between the FOZO and HIMU mantle domains redefined by Stracke et al. (2005) based on oceanic basalts from the Austral–Cook island chain and St. Helena. The samples from Tapuaenuku and the Westland dike swarm show wider variations in 206Pb/204Pb and 208Pb/204Pb values, and along with the Dunedin samples (PCB), are intermediate between rocks from Campbell Island and those from the Chatham and Antipodes Islands. The gabbros from Tapuaenuku have the lowest 143Nd/144Nd values whereas the Q-gabbro and transitional to subalkaline basalts from Campbell Island have, on average, the highest 87Sr/86Sr values. |
DISCUSSION |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGICAL HISTORYVOLCANIC GEOLOGYSAMPLE DESCRIPTION AND...RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Crustal contamination
Continental alkaline basalts with high Sr isotope ratios and OIB-like trace element signatures are often attributed to small-degree melts of a metasomatically enriched mantle lithosphere (e.g. Baker et al., 1997
; Zhang & O'Reilly, 1997; Späth et al., 2001; Barry et al., 2003). High Sr isotope ratios in basalts may, however, reflect contamination from radiogenic crust. Chemical contributions from continental crust have previously been proposed for some of the mafic rocks from the Tapuaenuku igneous complex (Baker et al., 1994) and for the Westland dike swarm (Barreiro & Cooper, 1987). In each case, the samples that have slightly elevated Sr isotope ratios (>0·7035, Fig. 8c) also have low Sr concentrations and, therefore, are more susceptible to contamination from highly radiogenic crust.
The transitional to slightly silica-oversaturated basalts from Campbell Island have elevated 87Sr/86Sr values and low Sr contents (350–700 ppm) relative to Chatham and Antipodes samples (700–1300 ppm, Table 2). They also have lower average Nb/U values (34) relative to OIB (47, Hofmann et al., 1986) and Antipodes and Chatham island basalts (46), which may indicate fractionation and addition of low Nb/U crust [10–20 for total continental crust (Sims & DePaolo, 1997) and 5 for bulk metasediments from southern New Zealand (Mortimer & Roser, 1992)]. To evaluate the possibility that Campbell Island samples have been contaminated by crust we have modeled several evolutionary paths for combined assimilation and fractional crystallization (AFC) on isotope and trace element ratio plots (Fig. 9). A mildly fractionated basalt from Chatham Island (CHT-17) was selected for the starting composition and several possible assimilants, representing the pre-Cenozoic basement of New Zealand (Graham & Mortimer, 1992; Graham et al., 1992) and modern near-trench bulk sediments collected from the Kermadec–Hikurangi margin to the north of New Zealand (Gamble et al., 1996), were used. The AFC paths in Fig. 9 show that samples with higher 87Sr/86Sr and lower 206Pb/204Pb, 207Pb/204Pb, Nb/U and Nb/Rb values may be explained by assimilation of arc sediments concurrent with fractional crystallization of 30% (1 – F).
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Mantle metasomatism
It has long been proposed that metasomatic enrichment of the upper mantle by hydrous- or carbonate-rich fluids or low-volume partial melts may be a necessary precursor to alkaline magmatism (Lloyd & Bailey, 1975
; Sun & Hanson, 1975; Wass & Rodgers, 1980) and studies of alkaline rocks from New Zealand support this assertion (Gamble et al., 1986; Barreiro & Cooper, 1987; Weaver & Smith, 1989; Baker et al., 1994; Cook et al., 2005). Alkalis (e.g. K, Rb, Ba) and water are stored within metasomatized upper mantle in amphibole and phlogopite and are retained in these minerals relative to anhydrous minerals during melting (Adam et al., 1993; Dalpé & Baker, 1994; LaTourrette et al., 1995). This effect is displayed prominently by negative K-anomalies on primitive mantle-normalized multi-element plots (Fig. 5). The trace element patterns of other New Zealand mafic alkaline rocks also show negative K-anomalies (Gamble et al., 1986; Weaver & Smith, 1989), suggesting that this is a widespread characteristic of the mantle source (Gamble et al., 1986).
In an effort to constrain mantle source compositions and the role of hydrous phases during melt generation we use the inversion technique described by Hart et al. (1997) for modal batch partial melting. Source models are based on the regression of two dominant data arrays observed on reciprocal plots (Fig. 10). The first model consists of the inversion of the entire filtered dataset that includes basalts from each age-delimited suite on Chatham Island and basalts from Antipodes Island (Fig. 10a). The second model is based solely on the regression of the Cretaceous Southern Volcanics from Chatham Island (Fig. 10b). The results of these calculations are summarized in Table 5 and are shown in Fig. 11. The normalized trace element patterns for the two modeled sources are roughly parallel and are both enriched in highly incompatible elements and depleted in heavy REE (HREE) relative to primitive upper mantle (McDonough & Sun, 1995). Also, both patterns show pronounced negative Pb and positive Ti anomalies. The models differ most significantly with respect to absolute concentration of LILE (Rb, Ba, Th, U, and K) and HFSE (Zr and Hf). The composition derived for the source of the Southern Volcanics is considered to be homogeneous, consistent with their relatively restricted age (84–85 Ma; Table 3), limited geographical extent (Fig. 2) and similar petrological characteristics (Figs 4–6). The model composition derived from the entire dataset must, therefore, represent a combined signature that includes a less enriched component.
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We believe that some of the compositional disparity between the two models reflects differences in source mineralogy. As discussed previously, amphibole is an important host for K (KDamp/melt 1·36; Dalpé & Baker, 1994
) whereas Rb and Ba are compatible in phlogopite (KDphl/melt 5·8 and 2·9, respectively; Adam et al., 1993). Modal variations in the melt source of ±3% or less of amphibole and phlogopite, with their proportions set at 4:1, can adequately account for the differences in K and Ba concentrations between the two models, with the Southern Volcanics source containing a higher percentage of both minerals. The difference in Rb content between the two models is not well matched and would require a higher bulk proportion of phlogopite. The source estimates are based on simple addition using the compositions of vein minerals in peridotites reported by Ionov & Hofmann (1995) and Zanetti et al. (1996). If addition of amphibole and phlogopite to the melt source occurs at the expense of clinopyroxene, either by vein–wall-rock interaction or reactive porous flow, then the source may become depleted in Th and U (Schmidt et al., 1999; Tiepolo et al., 2000) relative to neighboring elements (Ba, Nb, Ta and K) on multi-element plots (Fig. 11). The addition of amphibole and phlogopite may also contribute to Ti enrichment, although other Ti-rich phases (e.g. ilmenite and rutile) are known to occur in significant modal abundance in some metasomatized peridotites (Haggerty, 1987; Ionov et al., 1999).
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PM) and the error bars, calculated from errors on regression line slopes and intercepts (e.g. The difference in Zr and Hf concentrations between the models cannot be explained by the simple addition of hydrous phases. However, van Achterbergh et al. (2001)
documented progressive modal metasomatism in natural peridotites by the reactive replacement of garnet and orthopyroxene by clinopyroxene and phlogopite (± Cr-spinel) accompanied by enrichment in Sr, Na, K, light REE (LREE) and high field strength elements (HFSE; Ti, Zr and Nb) and depletion in Y and HREE. The presence of garnet in the melt sources for New Zealand basalts can be inferred from high Zr/Hf (38–47) and Zr/Yb (120–220) values (see van Westrenen et al., 2001) and moderately steep mantle-normalized trace element patterns (LaN/YbN 12–28) (Fig. 6). The subtle negative Zr–Hf anomalies on mantle-normalized plots (Fig. 5) also imply garnet control during melting (Dgarnet/melt Zr > Hf 
Sm >> Sr; Hauri et al., 1994).
The origin of the negative Pb anomalies in Fig. 11 is less clear. Negative Pb anomalies are present in the trace element patterns of all of the basalts (Fig. 5) along with most OIB when normalized to a primitive mantle Ce/Pb value of 11. This may indicate that upper mantle sources are deficient in Pb relative to primitive mantle or that Pb is retained in the mantle by sulfide during melting (Salters et al., 2002; Hart et al., 2005). Alternatively, experimental results indicate that Pb, along with K and Rb, is partitioned into aqueous fluids relative to silicate melts and, therefore, may be fractionated from elements with lower Dfluid/melt (e.g. Sr, Nb, Th, U) during mantle metasomatism (Brenan et al., 1995; Keppler, 1996). Based on mineral–fluid partition coefficients it has also been suggested that dehydration of amphibole-rich peridotite could produce a residual source that is depleted in Pb and Rb relative to U, Th, Nb and Sr (Brenan et al., 1995; Stein et al., 1997). The characteristic negative Pb anomalies of the basalts (Fig. 5) may therefore reflect Pb loss during partial dehydration of metasomatized mantle.
Timing of metasomatism
The source metasomatism recorded by the alkaline rocks and mantle xenoliths extends to continental areas outside of New Zealand. During the Paleozoic and Mesozoic the continental blocks of New Zealand were part of the eastern margin of Gondwanaland, juxtaposed to southeastern Australia, Tasmania, and the Victoria Land and Marie Byrd Land provinces of present-day West Antarctica (Lawver et al., 1992; DiVenere et al., 1994; Sutherland, 1999). In each of these areas metasomatized mantle sources for Cenozoic alkaline magmas have been proposed (Hart et al., 1997; Zhang & O'Reilly, 1997; Orlando et al., 2000; Panter et al., 2000a; Rocchi et al., 2002; Handler et al., 2003). Estimates for the timing of metasomatism are highly variable, but most suggest that enrichment took place some 100–400 Myr before the mid-Cretaceous break-up of New Zealand from Antarctica.
Depth and location of melting
An estimate of the pressure (depth) of melting for the source of the Chatham and Antipodes basalts can be made using the algorithms derived by Herzberg & Zhang (1996). Our calculations using filtered data corrected for olivine fractionation yield average pressures of 4·1 GPa (FeO), 4·4 GPa (MgO), >3·4 GPa (CaO), 3·9 GPa (Al2O3) and >5·2 GPa (Na2O) for basalts from Chatham Island and 5·3, 5·5, >5·1, 4·3 and >3·1 GPa for basalts from Antipodes Island. The results suggest that melt-equilibration occurred at a greater depth beneath Antipodes Island than Chatham Island; 160 km vs 120–130 km based on the MgO and FeO contents. The pressures calculated for each of the three major episodes of volcanism on Chatham Island indicate that the depth of melting has not varied significantly (±0·15 GPa) over a period of 80 Myr. These results are broadly comparable with depths of melting estimated by Hart et al. (1997) for alkaline basalts from the Hobbs Coast, Marie Byrd Land (110–140 km) using the same Herzberg–Zhang equations, and those of Huang et al. (1997) for alkaline basalts from the North Island of New Zealand (80–140 km) using the experimental data of Hirose & Kushiro (1993).
Pressure estimates and evidence for residual hydrous potassic minerals place important limits on the thermal regime of the melt sources. Experimental studies show that the temperature and pressure stability limits of amphiboles vary with composition. Pargasitic amphibole is stable at temperatures <1100°C at pressures of 3 GPa (Mengel & Green, 1986; Niida & Green, 1999), whereas K-richterite is stable to higher temperatures (1200–1250°C) at higher pressures (3–5 GPa, Foley, 1991; Trønnes, 2002). At these pressures the delimited temperature requires a lithospheric source (see Class & Goldstein, 1997; LeRoex et al., 2001). Only synthetic F-rich amphiboles (Foley, 1991) are stable at temperatures that exist at the top of the asthenosphere (1300°C, McKenzie & Bickle, 1988) but they have not been identified in natural samples (Class & Goldstein, 1997).
Shear-wave velocity models consistently show that the regions of alkaline magmatism from eastern Australia to New Zealand and West Antarctica are characterized by slow velocity anomalies restricted to a zone between 60 and 200 km depth (see Finn et al., 2005, fig. 4). Lithospheric thickness estimates of 100–150 km based on elastic models and seismic anisotropy data from Australia (Debayle & Kennett, 2000; Simons et al., 2003) and New Zealand (Scherwath et al., 2002) and scattered broad-band seismometer data from West Antarctica (Winberry & Anandakrishnan, 2003), suggest that part of the observed low-velocity zones (<100–150 km depth) beneath the continental areas of the SW Pacific (see Finn et al., 2005, fig. 4) reflect melt or volatile sources within the lithosphere and not in the asthenosphere (Finn et al., 2005). A dominantly lithospheric source for volcanism is also consistent with the fact that over the last 85 Myr, the Chatham Islands have drifted nearly 3000 km north (65°S to 44°S) on the Chatham Rise–Campbell Plateau block of Zealandia (Fig. 12) without major changes in source composition or depth of melting (120–130 km).
Origin of metasomatism and implications for HIMU signatures
The strong HIMU isotopic signature (defined here as 206Pb/204Pb >20·5, 87Sr/86Sr 0·703) of basalts from the Antipodes and Chatham Islands, along with other mafic igneous rocks from the South Island, New Zealand (Figs 1 and 8), suggest the widespread occurrence of a pre-Cenozoic HIMU mantle component within the New Zealand lithosphere. Basalts with an equally strong HIMU character are also found in Marie Byrd Land (Hart et al., 1997; Panter et al., 2000a) and their melt sources, along with sources for other alkaline rocks with elevated 206Pb/204Pb ratios (19·5) found throughout the SW Pacific (Fig. 12), have been related to a common origin that pre-dates the Late Cretaceous break-up of the eastern margin of Gondwanaland. Previous studies called upon sources produced by mantle plume activity originating in Mesozoic times (Lanyon et al., 1993; Weaver et al., 1994; Rocholl et al., 1995; Hart et al., 1997) or pre-Cenozoic sources developed from metasomatized continental lithosphere (Rocchi et al., 2002; Cook et al., 2005; Finn et al., 2005) or a combination of both (Panter et al., 2000a). A strong case against active mantle plumes in the Cenozoic is based on geological and geophysical evidence that has been thoroughly reviewed and discussed by Finn et al. (2005). We will not reiterate these arguments and will limit our discussion to pre-Cenozoic scenarios for metasomatism and the origin of the HIMU component in the SW Pacific.
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Considering the geochemical and geophysical evidence presented above, we propose that the HIMU source (1) resides within the subcontinental lithosphere and (2) was produced by interaction of the lithosphere with metasomatic fluids and melts. The case for a lithospheric source for the HIMU magmatism is underscored by the fact that the highest 206Pb/204Pb ratios (>20–20·5) are measured only in continental basalts, whereas basalts from nearby ocean islands (Balleny, Scott, Peter I) and crust (Macquarie Island) all have consistently lower ratios (19·0–19·8; Fig. 12a). Indeed, a lithospheric mantle source for the HIMU magmatism has already been proposed, but as plume-derived proto-lithosphere that was introduced and ‘stagnated’ beneath Gondwanaland (Rocholl et al., 1995
; Hart et al., 1997; Panter et al., 2000a). With regard to our second assertion that the HIMU signature was generated by the evolution of metasomatic agents, we consider our findings, along with those of others (e.g. Gamble et al., 1986; Baker et al., 1994), to have firmly established the occurrence of residual K-rich hydrous minerals in the melt sources for HIMU-like alkaline magmas in southern New Zealand. However, the temporal relationship between the HIMU component and the metasomatism has not been discussed. It has been recognized that in Marie Byrd Land the strong HIMU component is preferentially sampled by low-degree (2%) partial melts (Hart et al., 1997; Panter et al., 2000a). This is indicated by the negative correlation between K/K* and Pb/Pb* and 206Pb/204Pb values; i.e. lower K/K* and Pb/Pb* values [values <1 appear as negative K- and Pb-anomalies on primitive mantle normalized multi-element plots (Fig. 5)] represent smaller degrees of melting when K- and Pb-compatible minerals are retained in the residuum. Similar correlations for the New Zealand data strengthen this interpretation (Fig. 13b and c), although, as discussed above, low relative Pb contents may be a relic of the source composition (Fig. 11) and not solely a product of partial melting. Nevertheless, it follows that melting localized around veins containing minerals with lower solidus temperatures (amphibole, mica, apatite, clinopyroxene) will consume a greater proportion of these minerals relative to more refractory assemblages (olivine and orthopyroxene) in the surrounding peridotites. This establishes a link between metasomatism and the strong HIMU signature. In situ storage of modified lithosphere over a sufficient time-span before reactivation and further fractionation by vein-plus-wall-rock melting mechanisms (see Foley, 1992) may explain some of the range in isotope compositions for SW Pacific basalts (see Finn et al., 2005, Fig. 7) and the variable enrichment of their melt sources [Fig. 11; also see Hart et al. (1997, fig. 8) and Rocchi et al. (2002, fig. 10)].
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(TaPM x LaPM) and PbPMAncient plumes and metasomatism
Amphibole and phlogopite may have been introduced into the Gondwanaland lithosphere by infiltrating melts or fluids derived from rising mantle plume material. A possible pre-Cenozoic plume has been associated with the fragmentation of the eastern margin of Gondwanaland in the mid–late Cretaceous (Lanyon et al., 1993
; Weaver et al., 1994; Storey et al., 1999). Weaver et al. (1994) placed the axis of their proposed 2000 km diameter plume head at the reconstructed boundary between New Zealand and Marie Byrd Land, which intersects the region delimited by the highest 206Pb/204Pb values (>20·5) in Fig. 12c. Arguments against a late Cretaceous plume are based on the lack of large-scale regional uplift (LeMasurier & Landis, 1996), the unrealistically short time interval (5–10 Myr) for the switch from subduction-driven magmatism to plume-driven magmatism (Dalziel et al., 2000; Mukasa & Dalziel, 2000) and the much lower than expected magma production rates relative to plume-related large flood basalt provinces (Finn et al., 2005). An earlier plume event may have been responsible for the magmatism that produced the Jurassic Karoo–Ferrar large igneous province (Storey, 1995; Storey & Kyle, 1997; Dalziel et al., 2000; Storey et al., 2001). However, the geochemical character of the mantle source of the Ferrar magmatism is unresolved (Kyle et al., 1983; Hergt et al., 1991; Molzahn et al., 1996; Demarchi et al., 2001; Hergt & Brauns, 2001). Hart et al. (1997) suggested that a large, two-component (HIMU + FOZO), mantle plume metasomatized and enriched the Gondwanaland lithosphere prior to break-up. In this model, the rising plume head, weakened by thermal entrainment of ambient mantle, was flattened and flowed over a vast region beneath the Gondwanaland lithosphere, with little, if any, associated volcanism. The plume and metasomatically enriched lithospheric mantle material was then preserved to form a young and geochemically stratified component of the sub-Gondwanaland lithosphere (compare with the fossil plume model of Stein & Hofmann, 1992).
Geochemical studies that favor ancient mantle plumes as sources for continental alkaline magmatism in the SW Pacific, however, do not adequately account for the seemingly unavoidable physical and chemical interaction between a rising plume head and subduction that was active along the proto-Pacific margin of Gondwanaland between 500 and 100 Ma (Bradshaw, 1989; Borg & DePaolo, 1991; Elliot, 1991; Lawver et al., 1992). Dalziel et al. (2000) presented a model in which the impingement of a plume head beneath subducted oceanic lithosphere in the late Paleozoic flattened the angle of slab descent and contributed to the early Mesozoic Gondwanide orogeny. The thermal and mechanical breakthrough of the slab by the plume is considered to have taken place some 30–60 Myr later and was followed by Karoo–Ferrar magmatism. Subduction was re-established along the proto-Pacific margin soon after the slab was penetrated. Although Dalziel et al. (2000) did not discuss the geochemical implications of their model, they do offer a plausible scenario for plume–lithosphere interaction beneath a region that would later produce alkaline magmatism.
Subduction and metasomatism
The most important mechanism responsible for the introduction of hydrous and CO2-rich melts and fluids back into the mantle is the subduction of oceanic lithosphere, including hydrothermally altered upper oceanic crust (± sediments). There are several lines of evidence to suggest that the metasomatized mantle sources for continental alkaline magmatism in the SW Pacific may be linked to subduction with, or without, plume influence. First, the calculated chronology of metasomatic enrichment estimated from basalts and mantle xenoliths coincides with subduction along the eastern margin of Gondwanaland (500–100 Ma). Second, if it is accepted that the products of metasomatism reside within the subcontinental mantle lithosphere and were in place before Gondwanaland break-up, then the sources of the alkaline magmas would be inboard of and roughly parallel to the subduction zone at 100 Ma (Fig. 12c). Additionally, it is important to note that the regions with the longest history of subduction correspond to sources having the strongest HIMU signatures (highest 206Pb/204Pb values, Fig. 12).
However, if the metasomatism is subduction-related, then why do the magmas lack the classic subduction fingerprint (high LILE/HFSE ratios, e.g. Ba/Nb, Rb/Nb, etc.) recorded in island arc and active continental margin magmas? We suggest that the alkaline magmas were generated, in part, within a region of the lithosphere that had been metasomatized (veined) by subduction-related melts or fluids but that did not contain what would be regarded as a typical arc signature. On the basis of the concept of chromatographic separation of trace elements in mantle environments (e.g. Navon & Stolper, 1987; Hawkesworth et al., 1993), Ionov & Hofmann (1995) and Stein et al. (1997) have developed subduction-related metasomatic models to explain the retention of Nb and Ta in hydrous minerals within the lowermost portion of the mantle wedge above a devolatilizing slab [Fig. 14a (1)]. In these models, slab fluids enriched in incompatible elements migrate upwards into the mantle wedge. Some of the fluids reach the melt-generation zone above the wet solidus for peridotite [the source for arc magmas; Fig. 14a (2)] while others react metasomatically with the surrounding peridotite near the cool base of the wedge to form Nb-rich amphiboles and micas in veins. This enriched region of the mantle is transported by buoyancy-induced convective flow (Davies & Stevenson, 1992) to areas above the P–T stability limit of amphibole where it becomes partially dehydrated [Fig. 14a (3 and 4)]. The post-metasomatic partial dehydration of the mantle causes it to become depleted in the hydrophile elements Rb and Pb relative to more magmaphile elements Th, U, Nb and Sr (Brenan et al., 1995; Keppler, 1996), producing high Th–U/Pb, low Rb/Sr and Rb/Nb, and relatively unmodified Sm/Nd ratios. If this mantle region is then isolated near the base of the lithosphere [Fig. 14a (5)] for a sufficient period of time it may develop isotopic signatures that are similar to mantle sources of HIMU-type ocean island basalts.
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Cause of Cretaceous–Cenozoic magmatism
The most likely trigger for the Cretaceous alkaline magmatism in New Zealand was extension and lithospheric thinning associated with the break-up of New Zealand from Antarctica, which was in turn a response to the collision (Bradshaw, 1989
) or near collision (Luyendyk, 1995) of the Pacific–Phoenix ridge with the eastern margin of Gondwanaland. According to Finn et al. (2005), most of the Cenozoic magmatism in the SW Pacific was a response to the sudden detachment and sinking of subducted slabs into the lower mantle during the late Cretaceous. This may have caused vertical and lateral flow of warm asthenosphere into ‘thin-spots’ at the base of the lithosphere (see King & Anderson, 1995), catalyzing melting of amphibole/phlogopite-rich veins (Fig. 14b). Also, mid-Cenozoic to active extension in the southern Ross Sea (Cape Roberts Science Team, 1998, 1999, 2000; Willis et al., 2004) and dextral transtension along faults in northern Victoria Land (Rocchi et al., 2002; Di Vincenzo et al., 2004) may have caused local decompression melting in these areas. |
CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGICAL HISTORYVOLCANIC GEOLOGYSAMPLE DESCRIPTION AND...RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Continental intraplate mafic magmatism in southern New Zealand consists of relatively small volume melts erupted from widely distributed centers located on the South Island and offshore on the submerged continental crust of the Chatham Rise and Campbell Plateau. We have presented geochemical data on mafic rocks from three islands; Campbell, Antipodes and Chatham, to elucidate the magmatic history, mantle sources and fundamental cause of alkaline volcanism in this region, as well as in adjacent continental areas of the SW Pacific. Our main conclusions are as follows.
(1) Volcanism on Chatham Island produced subaerial lavas and associated pyroclastic rocks in three distinct eruptive episodes: late Cretaceous (85–82 Ma), late Eocene (41–35 Ma) and Miocene–Pliocene (5 Ma). Dating by the 40Ar/39Ar method modifies earlier interpretations based on K–Ar dates (Grindley et al., 1977); most notably, that the Cretaceous basalts are significantly older than previously thought.
(2) Fractionation-corrected whole-rock compositions from all three islands display similar trace element characteristics that have OIB-like trace element ratios (e.g. Ce/Pb 28–36, Nb/U 34–36, Ba/Nb 4–7, LaN/YbN12–28) and prominent negative K- and Pb-anomalies on primitive mantle-normalized multi-element plots. The basalts show a restricted range in measured isotope ratios and the majority have high 206Pb/204Pb (20·3–20·8) and low 87Sr/86Sr (0·7030) values similar to HIMU OIB. The more radiogenic 87Sr/86Sr (>0·7036) rocks from Campbell Island can be explained by the local assimilation of continental crust.
(3) Inversion models for the Antipodes and Chatham islands suggest mantle sources that are enriched in highly incompatible elements relative to primitive mantle. Chemical heterogeneity, inferred from the models, is associated with modal variations in anhydrous and hydrous (amphibole and mica) minerals within a veined mantle.
(4) Calculated melt equilibration depths of 160 km (Antipodes) and 130 km (Chatham), together with estimates from geophysical data and the P–T stability limits of hydrous minerals, suggest that melting occurred primarily within the subcontinental lithospheric mantle. This is also supported by the fact that Chatham Island drifted 3000 km over 85 Myr without a significant change in source composition or depth of melting.
(5) Metasomatized lithosphere is considered the source for alkaline magmatism in other areas of New Zealand, as well as adjacent continental fragments of Gondwanaland (SE Australia, Tasmania, Victoria Land and Marie Byrd Land). The timing of metasomatism is constrained to between 500 and 100 Ma, coincident with subduction and the distribution of HIMU volcanism.
(6) Metasomatism and enrichment of the uppermost mantle by a mantle plume and subsequent preservation of this material at the base of the Gondwanaland lithosphere prior to break-up may provide the source for the late Cretaceous–Cenozoic alkaline magmatism in the SW Pacific. However, models that ascribe to an ancient plume source must also account for the likely physical and chemical interaction between a rising plume head and subducted oceanic lithosphere.
(7) Metasomatism, enrichment and the source of the strong HIMU signature may be the result of a complex multi-stage chromatographic separation process (see Stein et al., 1997) consisting of: (a) devolatilization of subducted oceanic crust, which enriched the overlying peridotite in incompatible elements (e.g. Th, U, K, Pb, Nb, Sr, Rb, LREE) and deposited K–Nb (Ta)-rich hydrous minerals in veins; (b) buoyant convection and partial dehydration of the veined peridotite, depleting hydrophile elements (e.g. Pb, Rb, ± K) relative to more magmaphile elements (e.g. Th, U, Sr); (c) preservation and storage of what has become a high Th–U/Pb and low Rb/Sr source at the base of the Gondwanaland lithosphere.
(8) The metasomatized lithospheric source was later sampled by low degrees of partial melting triggered by extension-related decompression and/or warm upwelling asthenosphere over the past 100 Myr.
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ACKNOWLEDGEMENTS |
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We would like to thank John Gamble, Steve Weaver, Tony Reay and Douglas Coombs for access to collection of subantarctic island samples in New Zealand; the Tuanui family and Kevin Bliss for their warm hospitality during our stay on Chatham Island; and Ann Beck for her support and assistance in the field. We would also like to thank Mark Kurz for access to the VG-354 facility at WHOI, Nelia Dunbar for microprobe analyses at NMT, Joel Baker for unpublished Tapuaenuku analyses, E. K. Esawi for instruction on the use of his AMPH-CLASS program (Esawi, 2004
), and Dietmar Müller for plate reconstructions. Special thanks go to Carol Finn (USGS) for informative discussions from a geophysical perspective. Official reviews by John Gamble, Joel Baker, Richard Wysoczanski and additional comments made by Craig Cook significantly improved the manuscript and are gratefully acknowledged. This study was supported by National Science Foundation Grants OPP-9419686 and OPP-0003702 awarded to K.S.P.
*Corresponding author. Telephone: (1) 419-372-7337. Fax: (1) 419-372-7205. E-mail: kpanter{at}bgsu.edu
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TOPABSTRACTINTRODUCTIONREGIONAL GEOLOGICAL HISTORYVOLCANIC GEOLOGYSAMPLE DESCRIPTION AND...RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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