A Cenozoic Diffuse Alkaline Magmatic Province (DAMP) in the Southwest Pacific Without Rift Or Plume Origin

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Article Geochemistry 3 Volume 6, Number 1 Geophysics 16 February 2005 Q02005, doi:10.1029/2004GC000723 GeosystemsG G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society A Cenozoic diffuse alkaline magmatic province (DAMP) in the southwest Pacific without rift or plume origin

Carol A. Finn U.S. Geological Survey, Denver Federal Center, MS 945, Denver, Colorado 80226, USA R. Dietmar Mu¨ller School of Geosciences and University of Sydney Institute of Marine Science, University of Sydney, Edgeworth David Building F05, Sydney, New South Wales 2006, Australia Kurt S. Panter Department of Geology, Bowling Green State University, Bowling Green, Ohio 53503-0218, USA ([email protected])

[1] Common geological, geochemical, and geophysical characteristics of continental fragments of East Gondwana and adjacent oceanic lithosphere define a long-lived, low-volume, diffuse alkaline magmatic province (DAMP) encompassing the easternmost part of the Indo-Australian Plate, West Antarctica, and the southwest portion of the Pacific Plate. A key to generating the Cenozoic magmatism is the combination of metasomatized lithosphere underlain by mantle at only slightly elevated temperatures, in contrast to large igneous provinces where mantle temperatures are presumed to be high. The SW Pacific DAMP magmatism has been conjecturally linked to rifting, strike-slip faulting, mantle plumes, or hundreds of hot spots, but all of these associations have flaws. We suggest instead that sudden detachment and sinking of subducted slabs in the late Cretaceous induced Rayleigh-Taylor instabilities along the former Gondwana margin that in turn triggered lateral and vertical flow of warm Pacific mantle. The interaction of the warm mantle with metasomatized subcontinental lithosphere that characterizes much of the SW Pacific DAMP concentrates magmatism along zones of weakness. The model may also provide a mechanism for warming south Pacific mantle and resulting Cenozoic alkaline magmatism, where the oceanic areas are characterized primarily, but not exclusively, by short-lived hot spot tracks not readily explained by conventional mantle plume theory. This proposed south Pacific DAMP is much larger and longer-lived than previously considered.

Components: 16,174 words, 10 figures, 1 table. Keywords: geochemistry; geophysics; tectonics; alkaline magmatism; metasomatism. Index Terms: 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); 3099 Marine Geology and Geophysics: General or miscellaneous; 1099 Geochemistry: General or miscellaneous. Received 2 March 2004; Revised 5 November 2004; Accepted 22 November 2004; Published 16 February 2005.

Finn, C. A., R. D. Mu¨ller, and K. S. Panter (2005), A Cenozoic diffuse alkaline magmatic province (DAMP) in the southwest Pacific without rift or plume origin, Geochem. Geophys. Geosyst., 6, Q02005, doi:10.1029/2004GC000723.

1. Introduction Australia, the Tasman Sea, New Zealand, and the Antarctic plate extending east of the Australia- [2] Cenozoic alkaline igneous rocks cover conti- Antarctic discordance, south of the Pacific- nental fragments of East Gondwana and adjacent Antarctic Ridge and west of the Antarctic Peninsula oceanic lithosphere in parts of Antarctica, eastern (Figure 1). This region is distinguished by dis-

Figure 1. Topographic and bathymetric map of south Pacific [Sandwell and Smith, 1997]. The SW Pacific DAMP study area is indicated by the thick black line. AP, Antarctic Peninsula; AT, Adare Trough; BI, Balleny Islands; BFZ, Balleny Fracture Zone; CP, Campbell Plateau; CR, Chatham Rise; CS, Coral Sea; LHR, Lord Howe Rise; LR, Louisville Ridge; LTK, Lau-Tonga-Kermadec trench; NFB, North Fiji Basin; NQ, Northern Queensland; MBL, Marie Byrd Land; MI, Macquarie Island; P-DG, Peter I and De Gerlache Seamounts; RS, Ross Sea; TAM, Transantarctic Mountains; TS, Tasman Sea; TZ, Tasmania; WA, West Antarctica; WV, Western Victoria. tinctly low velocity upper mantle with variably developing a general model for magmatism is to enriched geochemical signatures (Figure 2). The synthesize a variety of the modern data sets, igneous activity has been related to adiabatic whichhasnotbeenpreviouslydone.Inthis decompression melting due to rifting [Johnson, paper, we describe the geological, geochemical, 1989; Tessensohn, 1994; Wo¨rner, 1999] or strike- and geophysical characteristics of the crust and slip faulting [Rocchi et al., 2002a, 2003], large mantle that we use to define a diffuse Cenozoic mantle plumes [Behrendt, 1999; LeMasurier and alkaline magmatic province (DAMP). On the Landis, 1996], or numerous separate, small hot basis of our synthesis and analysis of the limi- spots [Gaina et al., 2000; Lanyon et al., 1993; tations of existing models, we identify the key Sutherland, 1991]. As we will argue, all of these combination of elements required to bring about models are flawed, whether the magmatism is the regional alkaline magmatism in the largely considered as separate or related events. continental fragments of East Gondwana and suggest an alternate model linked to late Creta- [3] Accumulating geological, geophysical and ceous slab detachments to explain these charac- geochemical data collected over the last several teristics. Finally, this model will be used to show years provide a foundation to revisit the issue of that the largely continental magmatism in the SW the origin of the volcanism. Our approach toward Pacific DAMP may be part of a much broader

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Figure 2. Rayleigh wave 150s group velocity map (120 km depth) [Larson and Ekstro¨m, 2001]. Only long-lived hot spots with long traces [Clouard and Bonneville, 2001; Gaina et al., 2000; Ritsema and Allen, 2003] underlain by low-velocity perturbations in the upper mantle are shown. Abbreviations same as Figure 1 and AAD, Australian- Antarctic Discordance; LHR, Lord Howe Rise; RS, Ross Sea; TAM, Transantarctic Mountains, BH, Bellingshausen Sea. The SW Pacific DAMP study area is indicated by the thick black line. This region also includes the expected low velocities associated with the mid-ocean ridges. The white line locates the projection of the location of the postulated 130 Ma subducted plate in the mantle [Mu¨ller et al., 1993]. province encompassing much of the south igneous rocks extend 4400 km from offshore of Pacific. its northern coast south to Tasmania and Victoria (Figure 1). The 100–300 km wide belt contains 2. Characteristics of the Cenozoic scattered local volcanic centers whose estimated Alkaline Magmatism thickness derived from gravity modeling averages 70 m, yielding a total upper crustal (<2 km) volume of 0.02–0.07 Â 106 km3 (surface 2.1. Location of Alkaline Magmatism area times estimated thickness) [Wellman and [4] Cenozoic dominantly alkaline igneous rocks McDougall, 1974]. In New Zealand, Cenozoic cover large, but discontinuous portions of the intraplate volcanic rock is widely distributed along SW Pacific (Figure 1). The continental basement 1000 km of the coastlines in 3 distinct, 50– to the Cenozoic alkaline magmatism formed as a 100 km wide, mostly mafic provinces from the result of subduction processes and is composed of northeastern part of the North Island, to Auckland, Paleozoic-Mesozoic arc plutonic roots of magmatic and the southern portions of the South Island. arcs, and accreted sedimentary and oceanic crust, Compared to Australia, most of the predominantly covered in part by Jurassic igneous rocks [e.g., mafic volcanic centers are considerably smaller Dalziel, 1992]. In East Australia, the exposed, with estimated volumes an order of magnitude mainly mafic (alkaline and tholeiitic basalts), lower [Weaver et al., 1989]. The thinned continental

3of26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G crust of the Campbell Plateau region and seafloor [Weaver et al., 1989]. Intermittent Cenozoic between Australia and Antarctica host scattered volcanism in the Campbell Plateau region began alkaline volcanic islands [Johnson, 1989] and small 40 Ma (Chatham Islands) and extends to 1Ma seamounts (Figure 1). (Antipodes Islands) [Weaver et al., 1989]. In Ant- arctica, magmatism started at least 48 Ma in [5] Cenozoic alkaline igneous rocks crop out at the the Transantarctic Mountains–Ross Sea region edge and offshore of West Antarctica (Marie Byrd [Tonarini et al., 1997], 37 Ma in Marie Byrd Land seamounts and Peter I Island) and parts of Land [Rocchi et al., 2002b] and continues today East Antarctica (e.g., Balleny Islands), on islands [LeMasurier and Thomson, 1990] (Figure 1). in the Ross Sea and in the Transantarctic Moun- Because much of the region is covered by ice, tains (Figure 1). In contrast to much of the rest of age data are sparse, but indicate pulses of activity in the region, 20 volcanoes have summit elevations the Upper Oligocene-lower Miocene in the western greater than 2500 m, but thousands of small cinder Ross Sea and Marie Byrd Land regions and the cones and flows are also observed [LeMasurier last 10 Myr, with 20 volcanoes manifesting and Thomson, 1990]. In the mostly ice-covered Holocene activity [LeMasurier and Thomson, regions of West Antarctica, only a few volcanic 1990; Rocchi et al., 2002a]. edifices have been imaged in sub-ice bedrock topography, none of them large (<2 km diameter [7] Age dating of seamounts on the Pacific plate and 500 m elevation) and only one is inferred to be [Crawford et al., 1997; Duncan and McDougall, active [Blankenship et al., 1993]. High-amplitude 1989; Lanyon et al., 1993], inferences from stra- and frequency magnetic anomalies are interpreted tigraphy [Behrendt et al., 1987], bathymetry, and as evidence for sub-ice mafic igneous rocks that magnetic anomalies [Cande et al., 2000] suggest cover 50% of West Antarctica [Behrendt, 1999; that the seamounts are younger than the underlying Damaske et al., 1994; Ferraccioli et al., 2002; oceanic crust and therefore intraplate in nature. Luyendyk et al., 2003; Maslanyj and Storey, Volcanism in the Tasman Sea occurred during 1990; Pederson et al., 1981]. Joint modeling of and after its opening between 90 and 52 Ma [Gaina magnetic and gravity data over central west Ant- et al., 2000; Lanyon et al., 1993; McDougall and arctica shows that, with the exception of the large Duncan, 1988]. Notable Cenozoic alkaline features volcanoes, the exposed igneous rocks cannot be include the Balleny Islands (<10 Ma), Peter I more than 1000 m thick, and are probably less than Island (<12 Ma) [LeMasurier and Thomson, 600 m, leading to total volume estimates of 0.5– 1990] and the De Gerlache seamounts (20– 1 Â 106 km3 for much of the province [Behrendt et 21 Ma) (Figure 1) [Gohl et al., 1997]. al., 1994; Finn et al., 2001]. 2.3. Tectonic Setting 2.2. Timing of Magmatism [8] Measurements and models of modern and paleo-stress fields from east Australia show that [6] The earliest alkaline magmatism in Australia the continent has been under minor compression started about 70 Ma, but most has been emplaced since the Eocene, with little variation in intensity episodically between 55–15 Ma and 5 Ma to [Dyksterhuis, 2002; Hillis et al., 1999; Reynolds et Recent with young (< 13,000 years) volcanic al., 2002]. The change at 6 Ma from predomi- centers limited to NE Queensland and western nantly strike-slip to compressional tectonics led to Victoria (Figure 1) [Johnson, 1989, and references the cessation of volcanism in the South Island, NZ therein]. Intermittent activity over tens of millions [Walcott, 1998]. of years characterizes much of the region; for example, western Victoria has been active at var- [9] Little is known about the current stress field of ious times over the last 60 Myr [Johnson, 1989]. Antarctica. Plate motion studies suggest that the In New Zealand, Mid to Late Cretaceous alkaline continent is under compression [Lithgow- igneous rocks are exposed in several places on the Bertelloni and Guynn, 2004; Wuming et al., South Island and offshore (e.g., Chatham Islands). 1992] and that the last major regional extensional Magmatism increased around 30 Ma, but most event was the Late Cretaceous break-up of Gond- centers are younger than 15 Ma with Recent wana [Lawver and Gahagan, 1994]. GPS measure- activity on the North Island (Figure 1) [Hoke et ments collected in Marie Byrd Land, indicate al., 2000; Weaver et al., 1989]. As with Australia, no significant motion between East and West several centers exhibit pulses of activity over 10– Antarctica [Donnellan, 2003]. The lack of diffuse 30 Myr periods, notably on the South Island seismicity indicates that most of West Antarctica is

4of26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G no longer actively extending [Winberry and rather than real differences [Berg et al., 1989]. Anandakrishnan, 2003]. In addition, gravity lows Both geotherms have been explained by heat and low-velocity layers that typically reflect ex- transport by mafic magmatic intrusion at the tensional, young sedimentary basins are sparse base of the crust [Berg et al., 1989; Cull et al., and small (<40 km wide) in West Antarctica 1991; O’Reilly and Griffin, 1985; Sass and [Bell et al., 1998; Luyendyk et al., 2003; Lachenbruch, 1979]. Studinger et al., 2001; ten Brink et al., 1993]. The lower crust of much of eastern Australia and central West Antarctica, is electrically resistive, 2.5. Seismic Velocity Anomalies not conductive as observed in active rifts [12] Inversion of Rayleigh and Love waves [Tammemagi and Lilley, 1971; Wannamaker et referenced to models of the average velocity struc- al., 1996]. Lack of geologic evidence for ture for the crust and mantle yielding 3-D shear large Cenozoic faults does not support large- wave velocity perturbation models for the upper magnitude, regional extension in much of the [Bannister et al., 2000; Debayle and Kennett, region [Siddoway et al., 2003; Walcott, 1998]. 2000b; Larson and Ekstro¨m, 2001; Ritzwoller et However, marine magnetic anomalies in the al., 2001; Shapiro and Ritzwoller, 2002; Simons et Adare Trough [Cande et al., 2000] and stratigra- al., 1999] and whole mantle [Ritsema et al., 1999] phy and fault orientations of drill core in the provide information on mantle velocity structure western Ross Sea (Figure 1) identify contempo- and, indirectly, temperature and chemical varia- raneous Oligocene-mid-Miocene extension [Cape tions including volatile and melt content. Although Roberts Science Team, 1998, 1999, 2000]. Along resolution of models is difficult to determine, the Transantarctic Mountains front and offshore, estimates range from horizontal resolution of faults and fractures identify a stress regime com- 250 and vertical resolution of 50–100 km in patible with regional Late Cenozoic dextral trans- Australia and to 600 km horizontal and 50– tension [Rocchi et al., 2002a, 2003; Wilson, 1995] 200 km vertical resolution for much of the and GPS measurements indicate that the western south Pacific upper mantle including Antarctica Ross Sea is slowly extending today [Willis et al., [Ritzwoller et al., 2001]. Velocity perturbation 2004]. In addition, Cenozoic 40Ar/39Ar ages for values for the upper mantle generally range from pseudotachylyte suggest coseismic fault activity in ±6% and ±1.5% in the lower mantle. The North Victoria Land [Di Vincenzo et al., 2004]. resolution for the lower mantle is 1000 km laterally and 100–200 km vertically at 2.4. Heat Flow >1000 km depth with the poorest resolution (>250–300 km vertical resolution) in the transition [10] Averaged heat flow values in eastern Australia zone (500–1000 km depth) [Ritsema et al., 1999]. over the Recent volcanic fields of Queensland Areas with few earthquakes and seismic stations and Victoria, Tasmania, and New Zealand are like Antarctica will have lower resolution. 90 mW/m2, but near the 60 mW/m2 continental average elsewhere [Cook et al., 1999; Cull, 1991; [13] Consistently, the regions of alkaline magma- Godfrey et al., 2001; Pandey et al., 1981; Sass and tism are characterized by slow velocity anomalies, Lachenbruch, 1979]. Sparse measurements in the such as the velocity perturbations exceeding À2% western Ross Sea region [Blackman et al., 1987; from eastern Australia to New Zealand and West Decker and Bucher, 1982; Della Vedova et al., Antarctica (Figure 2). The low velocities are gen- 1991; Kyle, 1990] range from 60–100 mW/m2;a erally restricted to a zone between 60 and 200 km single measurement of 75 mW/m2 was obtained in depth (Figure 4) [Bannister et al., 2000; Debayle central West Antarctica [Gow et al., 1968]. and Kennett, 2000b; Larson and Ekstro¨m, 2001; Ritzwoller et al., 2001; Simons et al., 1999] except [11] Thermobarometric analyses of crustal xeno- beneath the Tasman Sea (Figure 5a) and South liths from the western Ross Sea [Berg et al., Pacific Ocean (Figure 5b) where low-velocity 1989] and eastern Australia [O’Reilly and Griffin, perturbations (>0.4%) extend to 670 km and 1985] reveal elevated temperature gradients in 800 km depths, respectively [Montelli et al., the crust (Figure 3). The SE Australia geotherm 2004; Ritsema et al., 1999]. However, the near crosses the McMurdo geotherm, giving some- constant velocity variations between 300 and what lower temperatures in the crust and higher 800 km depths might be due to limitations of the temperatures in the upper mantle, but this may model resolution at this depth range such that the be due to lack of data (dashed lines, Figure 3) actual depth is poorly determined [Ritsema and

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Figure 3. Pressure versus temperature diagram for McMurdo, Antarctica [Berg et al., 1989], SE Australia [O’Reilly and Griffin, 1986] to stability fields of amphibole and phlogopite [Class and Goldstein, 1997; Green and Falloon, 1998]. Also shown are water-saturated and water-undersaturated solidi and an adiabatic path for asthenospheric mantle. Orange areas outline incipient melt zone; blue regions denote major melt regimes [Green and Falloon, 1998].

Allen, 2003]. Although low seismic velocity trench) (Figures 2, 4b, and 4c). High-velocity anomalies are due to a largely undifferentiable Precambrian craton [Maslanyj and Storey, 1990] combination of temperature and chemical varia- terminates low-velocity mantle beneath Marie tions, elevated temperatures in the asthenosphere Byrd Land, as does the continuation of the are their most common explanation in the region 130-Myr slab east of the Antarctic Peninsula [Bannister et al., 2000; Debayle and Kennett, [Gurnis et al., 1998] that coincides with the bound- 2000b; Ritzwoller et al., 2001; Simons et al., ary of Pacific and Atlantic mantle determined by 1999]. geochemical tracing [Pearce et al., 2001] and seismic wave anisotropy studies [Helffrich et [14] The low-velocity zones terminate in the west al., 1999]. Beneath the region, lower mantle and south at the boundaries with thick, high- high seismic velocities (>0.6%) image detached velocity cratons in Australia and Antarctica, and Mesozoic subducted slabs (Figure 5) [Gurnis et the 130-Myr subducted Pacific slab [Gurnis et al., al., 2000]. 1998] that divides distinct Indian and Pacific geochemical reservoirs [Klein et al., 1988; Pyle [15] Magmatism is also generally absent from areas et al., 1992] at the Australia-Antarctic Discordance characterized by laterally extensive (>100 km), (Figures 2, 4, and 5). In the north and northeast, the high-velocities (perturbations >2% [Ritzwoller boundaries of the low-velocity zone coincide with et al., 2001]) in the upper 80 km (Figure 4) do high-velocity perturbations (>0.9%) of the sub- not typically host Cenozoic volcanic rocks. Exam- ducting Pacific plates beneath the Lau-Tonga- ples include areas of old (>100 Ma), thick (60– Kermadec trenches (Figure 5a), and old oceanic 80 km) lithosphere like the area offshore Marie lithosphere (e.g., east of the Tonga-Kermadec Byrd Land north of 70S and east of 230 and

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Figure 4

7of26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G east of the Campbell Plateau (compare Figure 1 to et al., 2002a]. This is also supported by melting Figures 4b and 4c.). experiments [Orlando et al., 2000] and the occur- rence of metasomatized, amphibole- and phlogo- 2.6. Geochemical Signature pite-bearing upper mantle xenoliths in alkaline rocks from West Antarctica, SE Australia and [16] For most of the region, alkaline magmatism is Southern New Zealand [Gamble et al., 1988; a result of low degrees of melting (1–3%) of a O’Reilly et al., 1989]. The origin of alkaline source enriched in incompatible elements relative magmas in New Zealand and Australia have also to primitive upper mantle. Some mantle sources been linked to sources with residual amphibole or have been metasomatically enriched. Indeed, it has phlogopite [Gamble et al., 1986; Panter et al., long been proposed that sources for primary silica- 2000; Zhang and O’Reilly, 1997] and province- undersaturated alkaline magmas contain volatiles wide models for a metasomatic source for alkaline (H O±CO)[Green and Falloon, 1998] and that 2 2 volcanism have been proposed [O’Reilly, 1987; metasomatic enrichment of the upper mantle may Panter et al., 2000; Sun and McDonough, 1989]. be a necessary precursor to alkaline magmatism [Best and Christiansen, 2001; Sun and Hanson, 1976; Wass and Rogers, 1980]. The volatiles are [18] Variations in Sr, Nd and Pb isotopes of SW stabilized in the hydrous phases of phlogopite and Pacific basalts have been explained by mixing of HIMU mantle (high 238U/204Pb sources that pro- pargasitic or kaersutitic amphibole, which can exist 206 204 at pressures >3GPa [Mengel and Green, 1986; duce high time-integrated Pb/ Pb signatures) Wallace and Green, 1991]. Their presence in the with depleted sources (e.g., sources for mid-ocean ridge basalts, MORB) and enriched sources (high residual source region of alkaline magmas can 87 86 143 144 be assessed using minor and trace element data Sr/ Sr and low Nd/ Nd) such as oceanic [Beswick, 1976; Class and Goldstein, 1997; mantle EM1 and EM2, and subcontinental litho- Greenough, 1988; Spa¨th et al., 2001] in particular spheric mantle (SCLM). The depleted mantle Rb, Ba and K, which are retained in hydrous source for volcanism in all but the northern-most relative to anhydrous minerals during melting region exhibits Pacific MORB mantle isotopic [Dalpe´andBaker, 1994; LaTourrette et al., fingerprints [Klein et al., 1988; Pearce et al., 1995]. If melting does not consume all of the 2001; Pyle et al.,1992;Zhang et al., 1999]. hydrous minerals then the liquid extracted from Beneath North Queensland [Zhang et al., 1999] the source will be low in Rb, Ba and K relative to and the Lau and North Fiji basins [Hickey-Vargas other elements (e.g., LREE, Th, U) that partition et al., 1995], Indian Ocean MORB-type mantle has more readily into the melt phase. partially displaced Pacific Ocean MORB-type mantle over the past 10 Myr; the present bound- [17] Negative K anomalies on mantle-normalized ary of the two distinct mantle domains is in the multielement plots (Figure 6) demonstrate the Vanuatu-Fiji-Tonga region (near NFB, Figure 2) retention of hydrous minerals in the source. While [Crawford et al., 1995]. In West Antarctica, Hart et low relative K contents of alkaline rocks may be an al. [1997] and Panter et al. [2000] suggest that the artifact of low source concentrations, modeling of depleted source for volcanism is not representative trace element data from West Antarctic basalts of MORB but is similar to sources for the Balleny predict sources with K concentrations 1–3Â prim- and Scott Islands, and possibly Macquarie Island itive upper mantle values and bulk partition coef- [Kamenetsky et al., 2000]. The Balleny source is ficients consistent with the presence of residual very close to the oceanic FOZO (‘‘focus zone’’) hydrous potassic phases [Hart et al., 1997; Rocchi mantle end-member as defined in 3-D isotope

Figure 4. Shear velocity anomalies from a global three-dimensional diffraction tomographic model created from inversion of surface wave fundamental model phases and group velocity measurements [Shapiro and Ritzwoller, 2002]. (a) Model slice at 150 km depth showing location of profiles. (b) Section at 34S. Gray line represents the thickness of the lithosphere derived from geoid and flexural models [Zhang et al., 1998]. Thick dashed black line represents the maximum lithospheric thickness based on seismic anisotropy [Debayle and Kennett, 2000a] and magnetotelluric [Simpson, 2002] data. White circles indicate earthquake locations. (c) Section at 50S. Dashed gray line represents the lithospheric thickness derived from flexural models [Godfrey et al., 2001]. (d) Section at 80S. Thick gray line represents the lithospheric thickness derived from seismic data [Winberry and Anandakrishnan, 2003]. EWM, Ellsworth-Whitmore Mountains. White boxes outline source regions of melt derived from geochemical modeling [Hart et al., 1997; Hoke et al., 2000; O’Reilly and Zhang, 1995; Panter et al., 2000]. Thin black outlines in Figures 4b–4d indicate features persistent in all models. 8of26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G

Figure 5. Two 180-wide cross sections of shear velocity anomalies from model S20RTS [Ritsema et al., 1999]. The thick dashed line indicates the 670-km discontinuity. (a) Section from the Southeast Indian Ridge, across the Tasman Sea, and Kermadec trench. (b) Section from the Indian Ridge, East Antarctic craton, South Pacific Ocean, Pacific-Antarctic Ridge and central Pacific. (c) Model slice at 2850 km depth [Ritsema et al., 1999]. Circles indicate locations of hot spots. space (87Sr/86Sr-143Nd/144Nd-206Pb/204Pb) by Hart [Ewart et al., 1988; Foden et al., 2002; O’Reilly, et al. [1992] (Figure 7). This FOZO-like compo- 1987; Rocholl et al., 1995; Wo¨rner, 1999]. nent is found in both continental and oceanic basalts and appears to be a large-scale geochemical [19] The HIMU signature is strongest in continen- feature in the uppermost mantle of the SW Pacific tal basalts from Marie Byrd Land and Southern DAMP. The enriched isotopic signatures (Figure 7) New Zealand, approaching 206Pb/204Pb values of are limited to continental basalts (including conti- 21 (Figures 7 and 8). Using ocean island basalts as nental fragments such as the Lord Howe Rise and a proxy, HIMU sources in the SW Pacific have Campbell Plateau [Weaver et al., 1994]) and have been linked to mantle plumes [Behrendt, 1999; been attributed to EM1+ EM2 [Hart et al., 1995, LeMasurier and Landis, 1996]. It has also been 1997] as well as ancient metasomatized SCLM suggested that an upwelling plume(s) played a role

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intermittence, broad regional extent, low magma production rates, metasomatized and enriched sources for volcanism that coincide with unusual upper mantle low-velocity zones and moderate to high-velocity lower mantle. Key issues for understanding the origin of magmatism include determining the timing of metasomatism, locating the metasomatized portion of the melt source (in the SCLM, the asthenosphere, or both), and defining the roles of lithospheric architecture (thickness and faults) and stress.

[22] The cause of the metasomatic enrichment is unclear. Several authors have proposed volatile Figure 6. Primitive mantle normalized [McDonough flux from plume derived melts [Panter et al., and Sun, 1995] multielement diagram comparing mean 2000; Hart et al., 1997]. But another potential values of continental alkaline basalts from West Antarctica [Hart et al., 1997; Panter et al., 1997b, source is metasomatic fluids (hydrous fluids and 2000, 2003; Rocchi et al., 2002a; Rocholl et al., 1995], volatile-rich silicate melts) derived from the pro- Australia and Tasmania [Ewart et al., 1988; McBride et longed subduction in the Paleozoic-Mesozoic al., 2001; McDonough et al., 1985], and southern New (500–100 Ma) along the Pacific margin of Zealand [Baker et al., 1994; Panter et al., 1997a]. Also Gondwana. It has been a long-held view that shown are oceanic samples that represent average the melting of subduction-related metasomatized HIMU end-member compositions [Hauri and Hart, sources will yield high La, Ba, Rb/Nb-Ta ratios as 1993, 1997; Woodhead, 1996], MORB from the Pacific- observed in island and continental arc magmas. Antarctic ocean ridge system [Ferguson and Klein, Alkaline rocks in the SW Pacific show OIB trace 1993], and three ‘‘near primitive’’ glasses from element signatures with high relative abundances Macquarie Island, an uplifted block of oceanic crust at of Nb and Ta, indicating the lack of the classic the Australia Pacific plate boundary south of New Zealand [Kamenetsky et al., 2000]. Macquarie samples subduction component (Figure 6). But if subduc- and all other basalts used to calculate mean values have tion-modified SCLM exists beneath much of the MgO concentrations greater than 7 wt.%. SW Pacific, then why is it not being tapped by the volcanism? One explanation would be that melting does not occur within the lithosphere; however, in the Mid to Late Cretaceous breakup of the proto- this is contrary to the geochemical and geophysical Pacific margin of Gondwana [Lanyon et al., 1993; evidence. A second explanation is that only SCLM Storey et al., 1999; Weaver et al., 1994]. Variants that has been altered by plume-derived melts and on the plume model call upon an ancient ‘‘fossil- fluids is being consumed, but this would require ized’’ source that was emplaced and frozen to the spatially discrete metasomatic domains. A third base of the SCLM prior to Gondwanaland breakup, possibility is that the alkaline melts may be gener- either in the Cretaceous or Jurassic [Panter et al., ated, in part, within a region of the lithosphere that 2000; Rocholl et al., 1995]. has been metasomatized by subduction but does not contain what would be regarded as a typical arc [20] The mixtures of distinctly different isotopic end-members by small volume alkaline magmas signature [e.g., Petrone et al., 2003]. imply mantle heterogeneity on a fine scale [23] On the basis of the concept of chromatographic [Meibom et al., 2002]. The broad region over separation of trace elements in mantle environ- which these magmas were erupted also implies that ments, Ionov and Hofmann [1995], Navon and this heterogeneity is a regional feature in the upper Stolper [1987], and Stein et al. [1997] have devel- mantle beneath the Pacific [Lassiter et al., 2003; oped subduction-related metasomatic models to Staudigel et al., 1991; Workman et al., 2004]. explain the retention of Nb (and Ta) in hydrous minerals (amphibole) within the lowermost portion 3. Key Characteristics of the Cenozoic of the mantle wedge above a dehydrating slab. SW Pacific Diffuse Alkaline Magmatism Further development of the model helped explain geochemical characteristics of the source regions [21] Striking features of SW Pacific Cenozoic for alkaline rocks from the northern Arabian- Nu- alkaline magmatism are its longevity (50 Myr), bian Shield [Stein et al., 1997]. In their model,

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Figure 7

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fluid-rock interactions retain Nb while Pb and Rb are lost relative to U, Th, Sr and REE during the preservation of the wedge. The process leads to the development of low Th-U/Nb and Rb/Sr ratios and high U/Pb ratios compatible with sources for alkaline magmas. Isolation of this type of geochemical domain over long periods of time may produce isotopic signatures that are similar to sources for HIMU-type ocean island basalts (high 206Pb/204Pb and low 87Sr/86Sr). Although a detailed evaluation of subduction-related versus plume-related metasomatism for the SW Pacific is beyond the scope of this paper, it is of interest to note that source regions in the SW Pacific with the longest history of subduction (Marie Byrd Land, Campbell Plateau, Chatham Rise, and southern New Zealand) correspond to the highest 206Pb/204Pb values (Figure 8). It has also been proposed that the impact of a Mid Creta- ceous plume head beneath this same region can explain the position of Antarctic-New Zealand rifting and the distribution of the HIMU source [Storey et al., 1999; Weaver et al., 1994]. How- ever, the model has been criticized because of the brief time interval (10–15 Myr) between the cessation of subduction-zone magmatism and magmatism related to the inception of a mantle plume [Dalziel et al., 2000; Mukasa and Dalziel, 2000], leaving subduction as the likely origin of metasomatism and enrichment.

[24] Evidence for metasomatized sources for 97 Ma mafic rocks in southern New Zealand [Baker et al., 1998] and 80 Ma volcanics on the Campbell Plateau [Weaver et al., 1994; Panter et al., 1997a], 440 Ma model ages from metasomatized ultramafic xenoliths from Figure 8. Plate reconstructions of Gondwana; colors the Ross Sea region [McGibbon, 1991] and indicate age of the ocean floor [Mu¨ller et al., 1993]. Neoproterozoic Nd model ages from basalts from Keys to abbreviations in Figure 1. (a) A 100 Ma Australia [Zhang and O’Reilly, 1997] and 500– reconstruction overlain by locations and contours for 206 204 300 Ma ages from metasomatized xenoliths alkaline samples with measured Pb/ Pb ratios (data [Griffin et al., 1988] from southeastern Australia from sources in Figure 7). (b) A 50 Ma reconstruction, approximate timing of onset of alkaline magmatism. suggest that the enrichment had occurred by the (c) A 15 Ma reconstruction. Mid-Cretaceous. No evidence exists for metasomatism concurrent with Cenozoic magmatism

Figure 7. Values of 206Pb/204Pb versus (a) 207Pb/204Pb, (b) 208Pb/204Pb, and (c) 87Sr/86Sr diagrams showing fields representative of basalts from the southwest Pacific Ocean and continental areas. Data sources are the same as in Figure 6 with additional samples from the Pacific-Antarctic Ridge [Vlastelic et al., 1999], Balleny and Scott Islands [Hart et al., 1992, 1995; Hart and Kyle, 1994], Antarctic Peninsula [Hole et al., 1993], and Peter I Island [Hart et al., 1995]. Also indicated are the approximate locations of possible source regions for magmatism: HIMU, FOZO (Focus Zone [Hart et. al., 1992]), EM1, and EM2 [Workman et al., 2004]. Torlesse metasediments, South Island, New Zealand [Graham et al., 1992] represent a potential mid-crustal contaminant. The Northern Hemisphere Reference Line [Hart, 1984] and 4.5 Ga geochron are also shown for reference. 12 of 26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G

[O’Reilly, 1987]. A rough estimate of the age plume [Crawford et al., 1997; Lanyon et al., for sources in the SW Pacific can be made on 1993] whose origin is disputed [Gaina et al., the basis of the correlation of data on 207Pb/ 2000] or a regional asthenospheric signature 204Pb–206Pb/204Pb plots (Figure 7a). If we assume [Kamenetsky et al., 2000]. Movement of the con- a two-stage mantle evolution from bulk-earth, the tinental fragments during Gondwana breakup slope of the data arrays (±1s) yield pseudo-iso- could have delaminated part of weak SCLM and chrons between 950 and 650 Ma. We believe the smeared the geochemical signature throughout model ages represent a mixture of sources, includ- much of the asthenosphere in the region. Also, ing metasomatized SCLM. A younger metasomatic perhaps relatively close (500 km) continental event may be responsible for higher U/Pb ratios fragments contaminated the ridge magmas as oce- (either addition of U or depletion of Pb). If so, how anic crust was formed (Figures 8b and 8c). There- much time would be required to produce a HIMU- fore older (>10 Ma) regions such as the Tasman like signature? Again we use a simple two-stage Sea (adjacent to Australia and the Lord Howe, New mantle evolution model. First, initial U/Pb and Pb Zealand, and Campbell Plateau continental frag- isotope ratios of a premetasomatized source were ments), Macquarie Island and Ridge (near the calculated for any time in the past using a present- Campbell Plateau) and Balleny Islands, Peter I day 206Pb/204Pb ratio of 19.5. This value approx- Island and De Gerlache seamounts (near West imates that of a ubiquitous FOZO-like lower U/Pb Antarctica) are contaminated, in contrast to oceanic component in the SW Pacific mantle that is recog- regions lacking enriched signatures which are far nized by the convergence of data arrays in 3-D from continental lithosphere (>500 km) such as isotope space [Hart et al., 1997; Panter et al., the Pacific-Antarctic Ridge (PAC, Figures 5 and 6; 2000]. In Figure 7, the lower U/Pb source lies in a crust <10 Ma, Figure 8c). A plate reconstruction region between the values for the Balleny and Scott from the approximate onset of alkaline magmatism Islands, Macquarie Island and the FOZO mantle at 50 Ma (Figure 8b) and at 15 Ma (Figure 8c) reservoir. Second, the U/Pb ratio was adjusted to illustrates the close proximity of enriched crustal evolve the lower U/Pb source to match what is fragments and adjacent oceanic crust during much considered to be the HIMU end-member of SW of the magmatism. We conclude that the sources of Pacific DAMP (206Pb/204Pb 21.0). Adjustments the magmas lie in SCLM below the continents, and to higher U/Pb ratios thus simulate fractionation in asthenosphere contaminated by adjacent or due to metasomatism. Our calculations suggest that smeared SCLM beneath oceanic crust. The pro- in-growth of HIMU-like values for Pb isotopes can posed melt and volatile sources, then, would dom- be obtained within 550 to 250 Myr, with U/Pb inate the low seismic velocity signature in the ratios between 0.5 and 0.8 (or 238U/204Pb (m)32– SCLM (white boxes, Figures 3b–3d), rather than 53). The model U/Pb ratios are comparable with temperature. values for metasomatized peridotites from SCLM [Hawkesworth et al., 1986; Ionov and Hofmann, [26] Lithospheric thickness has implications for 1995; Lee et al., 1996]. While speculative the locating sources of magmas. If the <80 km thick- calculations lend support, in conjunction with the ness of the high-velocity seismic lid for most of depth of low seismic velocities, evidence for re- SW Pacific approximates the lithosphere, the sidual hydrous phases and prior history of subduc- melts, inferred from geochemical evidence to orig- tion, for a relatively young and shallow inate at 100–140 km depth, would lie in the metasomatic origin of the HIMU signature in the asthenosphere. Temperatures at the top of the SW Pacific. asthenosphere are often defined to be 1300C [McKenzie and Bickle, 1988], which, for much of [25] If a SCLM source modified by metasomatism SW Pacific, would occur at shallow levels (50– can account for the geochemical signatures in the 80 km). In this case, the continental geotherms Gondwana fragments of SW Pacific, what is the would resemble the asthenosphere adiabat with explanation for similar signatures in the oceanic major melting and generation of tholeiites at islands such as Balleny Islands, seamounts to the 50–60 km (Figure 3). This is not observed north, Peter I Island and basalts that formed at and argues against shallow asthenospheric melt (Macquarie Island) [Kamenetsky et al., 2000] or sources. Lithospheric thickness estimates of 100– very near (e.g., seamounts east of Tasmania) 150 km based on elastic models and inferred spreading centers (Figure 8c) [Crawford et al., ‘‘frozen-in’’ directions of seismic anisotropy from 1997; Lanyon et al., 1993]? Previous interpreta- Australia [Debayle and Kennett, 2000a; Simons et tions include contamination from the Balleny al., 2003] and scattered broad-band seismometer

13 of 26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G data from West Antarctica [Winberry and Anan- the thinner (75 km) lithosphere of Marie Byrd dakrishnan, 2003], suggest also that part of the Land may help explain extensive magmatism observed low-velocity zones (<100–150 km there. depth) (Figure 4) reflects melt/volatile sources within the SCLM, not in the asthenosphere. 4. Possible Triggers for Cenozoic SW Pacific Diffuse Alkaline Magmatism [27] Lithospheric architecture and stress influences the localization of magmatism in a variety of [29] The triggers for magmatism include chang- ways. Uncertainties related to thickness estimates ing composition to lower melting temperature; aside, the magmatism is restricted to continental depressurization, usually by extension; and lithosphere no greater than 150 km thick and increasing temperature. The melting regime oceanic lithosphere <80 km thick. The discon- for mantle-derived basaltic magmas for the SW nected high-velocity anomalies of the continental Pacific alkaline rock types requires the presence fragments (e.g., Lord Howe Rise (Figure 4b), of volatiles such as carbon and hydrogen in the Tasman basin and Campbell Plateau (Figure 4c) melt phase, which reduces the solidus tempera- and West Antarctica (Figure 4d) contrast with the ture (compare dehydration to dry solidus, more coherent anomalies associated with Meso- Figure 3). Magmas are derived from an ‘‘incip- zoic oceanic lithosphere (Figures 4b and 4c), ient melting’’ regime which lies at temperatures suggesting that the continental fragments have below the dry solidus marking entry to the been broken up and are therefore more susceptible ‘‘major melting’’ regime [Green and Falloon, to melt incursion than more coherent oceanic 1998] (Figure 3). Assuming low melting temper- lithosphere of similar thickness. The coincidence ature SCLM underlies much of the region, we of volcanoes (Peter I Island, De Gerlache and evaluate various models for extension and heat- Marie Byrd Land seamounts, Figures 2 and 8c) ing triggers for volcanism. with major trans-lithospheric structures separating thick, old (>50 Ma) from younger (<20 Ma) oceanic lithosphere [Gohl et al., 1997] imply that 4.1. Extension Triggers these structures are necessary to promote volumi- [30] Can extension alone trigger incipient melting nous magmatism in thick lithosphere. In conti- in the region? A model for North Victoria Land nental areas, most volcanoes are not localized by suggests that reactivation of preexisting translith- large faults displaying significant Cenozoic mo- ospheric faults 43 Ma induced lithospheric tion [Siddoway et al., 2003; Wilson, 1995], with pull-apart and small-scale mantle convection at the exception of large trans-lithospheric bound- the edge of the East Antarctic craton and trig- aries in the western Ross Sea and Marie Byrd gered local decompression melting of mantle Land [Damaske et al., 1994; Kyle and Cole, enriched by veining associated with amagmatic 1974; LeMasurier and Rex, 1989; Luyendyk et late Cretaceous rifting [Rocchi et al., 2002a]. al., 2001]. However, as the lack of evidence for regional extension indicates, this model would only ac- [28] Although the magmatism has been linked count for magmatism in the western Ross Sea broadly to tensional stress fields, these are not region. On the basis of recent evidence from requirements as evidenced by the Australia vol- drilling in the western Ross Sea [Cape Roberts canism which has occurred in mildly compres- Science Team, 1998, 1999, 2000], rifting did not sional stress fields for most of its history occur amagmatically in the Cretaceous but contem- [Dyksterhuis, 2005; Zhao and Mu¨ller, 2001]. poraneously with magmatism from Oligocene- Stronger compressional forces such as those as- present. In addition, although mantle veining could sociated with subduction and translation along the produce enriched trace element signatures (high U, Alpine fault, as started in New Zealand in the Th, Rb, Ba, and LREE/HREE ratios; Figure 6), it Miocene inhibit volcanism [Hoke et al., 2000; would be difficult to explain the isotopic signatures Walcott, 1998]. Extension in the western Ross Sea in particular, high 206Pb/204Pb ratios (>20.5), if the coincides with heightened periods of magmatism enrichment was less than the 40 Ma initiation of in the Oligocene [McIntosh, 2000], a relation that rifting. If metasomatism did occur before Gond- continues today [Willis et al., 2004; Wilson, wana breakup as suggested above, the rifting in 2002]. Decompression melting due vertical flow most of the SW Pacific should have triggered of asthenospheric from 100–150 km thick litho- magmatism. This is generally not observed, sug- sphere in central West Antarctica and offshore to gesting that extension alone in the Cenozoic would 14 of 26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G also not trigger magmatism and a heating event is Although estimating magma production rates is required. difficult due to erosion, underplating of unknown amounts of material and paucity of age dates, 4.2. Hot Spots, Hot Lines, and Mantle comparison with flood basalt provinces (similarly Plumes computed) is revealing. In Australia and New Zealand, averaging the estimated volume of surfi- [31] Lack of evidence for significant regional ex- cial (upper 5 km) Cenozoic igneous rocks tension during the Cenozoic, as well as the over 30–50 Myr yields a production rate of enriched isotopic signatures of the alkaline rocks, .002 km3/Myr [Hoke et al., 2000; Wellman and motivated models of increased temperatures caused McDougall, 1974]. For the more voluminous by regional mantle plumes, local hot spots and hot magmatism reported for Antarctica [Behrendt et lines with origins in both the upper and lower al., 1994; Finn et al., 2001], over its 35–50 Myr mantle. Due to the recognition of their low mag- history, a crude rate of 0.026 km3/Myr obtains. In matic volumes (<100,000 km3), and long duration comparison, 600,000 km3 were erupted from of activity (>50 Ma), Australia and New Zealand the Deccan traps in 5Ma[Bhattacharji et al., have not been viewed as large volume flood basalt 1996] (magma production rate of 1km3/yr); provinces resulting from large, deep-seated mantle 1.3 Â 106 km3 from the Siberian traps in 1 Ma plumes [Johnson, 1989]. Instead numerous small [Reichow et al., 2002] (magma production rate of diameter hot spots with both lower and upper 1.3 km3/yr); and from the relatively small Columbia mantle origin, have been proposed to explain some Plateau, 40,000 km3 in 1 Ma (magma production [Duncan and McDougall, 1989; Gaina et al., rate of .2 km3/yr) [Swanson et al., 1975]. None of 2000; Johnson, 1989; McDougall and Duncan, these provinces are as areally extensive as SW 1988; Sutherland, 1998; Wellman, 1983] if not all Pacific DAMP (or even West Antarctica alone). [Sutherland, 1991, 1994] of the volcanism. [34] Assignment of other characteristics commonly [32] Several volcanic chains do indeed fit the hot spot reference frame and rates of movement of associated with mantle plumes to the SW Pacific is Australia relative to Antarctica. Alignments of also problematic (Table 1). In particular, with the tholeiitic, highly differentiated volcanic centers exception of local anomalies under the Tasman and contemporaneous with the alkalic volcanism Ross Seas, low seismic velocities are generally (<35 Ma) young progressively southward in eastern restricted to the upper 250 km of the mantle in Australia (colored triangles, Figure 1) and the the region, in contrast to low-velocity zones under Tasman Sea, consistent with the separation rate of East Africa [Nyblade et al., 2000], and the central the Australian and Antarctic plates, [Eggins et al., Pacific [Montelli et al., 2004; Ritsema and Allen, 1991; McDougall and Duncan, 1988; McDougall 2003] that extend to >400 km depth. The velocities et al., 1981; Sutherland, 1991; Wellman, 1983; in the lower mantle beneath the region are rela- Wellman and McDougall, 1974], and therefore tively high (e.g., Figures 4a–4c), in contrast to passage over the Bass hot spot [Gaina et al., regions under the central Pacific (Figures 4a and 2000] which has been imaged seismically (Figures 4c) and Africa [Montelli et al., 2004; Ritsema and 2 and 4a) [Montelli et al., 2004; Ritsema et al., Allen, 2003]. 2004]. However, other proposed hot spot tracks such as in the Balleny Islands [Lanyon et al., [35] The origin of HIMU signatures is hotly debat- 1993], New Zealand, and offshore Antarctica ed [e.g., Anderson, 1995; Hoffmann, 1997]. HIMU [Sutherland, 1991] do not fit hot spot models signatures have been attributed to recycling of (Table 1) [Gaina et al., 2000] and are not imaged ancient (>200 Ma) oceanic crust within plumes seismically (Figures 2, 4, and 5). rising from the deep mantle [Hart et al., 1992; Hoffmann, 1997] and, along with EM types, which [33] In contrast to the rest of the region, West are attributed to recycling of ancient sediments, Antarctica has been compared to flood basalt considered to be diagnostic of lower mantle plumes provinces resulting from large, deep-seated mantle because of the inferred long periods of isolation plumes [Behrendt, 1999; LeMasurier, 1990]. Com- required to generate the isotopic signatures [Hart, parison of various attributes of flood basalt prov- 1984]. Another model suggests that enriched layers inces often attributed to mantle plume activity formed by subduction recycling of sedimentary shows that West Antarctica and the SW Pacific rocks and oceanic lithosphere metasomatized by are not similar (Table 1). A key parameter hydrothermal activity at ridges resides in the upper for comparison is the magma production rate. 200 km of the mantle and provides the isotopic 15 of 26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G

Table 1. Conventional Characteristics of Deep-Seated Mantle Plume/Shallow Hot Spot Versus SW Pacific DAMP Flood Basalt/OIB Provinces SW Pacific DAMP

Dominantly tholeiitic volcanism Dominantly alkaline [Johnson, 1989] HIMU (lead isotope signatures >20.5) Only in Marie Byrd Land [Hart et al., 1997; [Hart, 1984] Panter et al., 2000] and S. New Zealand [Panter et al., 1997a]. Atmospheric normalized He3/He4 >12 Ra <8.5 Ra in NZ [Hoke et al., 2000] and [Lupton, 1983] 4–7 in western Ross Sea [Nardini et al., 2003]. Dome (stationary plate), uplift Marie Byrd Land dome only [LeMasurier and Landis, 1996] [Davies, 1988] and existence disputed [Luyendyk et al., 2001]. Timing of uplift of eastern Highlands, Australia not coincident with passage of hot spot [Johnson, 1989]. Linear age progression (moving plate) Only Bass, Louisville, and Tasman Sea hot [Morgan, 1971] spots produces tholeiitic magmatism in eastern Australia and offshore that fit in plate tectonic frame of reference [Gaina et al., 2000]. High heat flow (>75 mW/m2) 90–120 mW/m2 [Blackman et al., 1987; Cull, 1982; Della Vedova et al., 1991; Hoke et al., 2000; Pandey et al., 1981; Purss and Cull, 2001]. High magma production rates Low magma production rates (averaged over 50 Myr) (>1km3/yr, neglecting underplating) <0.002–0.03 km3/yr) [Finn et al., 2001; [Bhattacharji et al., 1996; Reichow et al., 2002] Hoke et al., 2000; Johnson, 1989; Wellman and McDougall, 1974]. Short-lived volcanism (1–5 Myr) Long-lived (50–70 Myr) [Richards et al., 1989] [Johnson, 1989; Tonarini et al., 1997] High (5–8%) degrees of melting Low (1–3%) degrees of melting [Hart et al., 1992] [Hart et al., 1995; O’Reilly and Griffin, 1985] >1450C upper mantle temperatures 1100–1300C upper mantle temperatures [McKenzie and Bickle, 1988] [Hart et al., 1997; O’Reilly, 1987; Panter et al., 2000] Vertically extensive (>400 km) Low velocity zone generally restricted to low-velocity zone upper 200 km of in the upper mantle [Debayle and Kennett, 2000b; Ritzwoller et al., 2001]. Geoid high [Davies, 1988] Geoid low for much of the region; high due to subducting slabs in north Dimensions 1000 Â 1000 km2 Minimum of 2000 Â 7,000 km2 [Davies, 1988] signatures [Anderson, 1995]. Alternate models in- Zhong and Gurnis, 1994] episodes of mixing and clude mantle heterogeneities at a variety of scales whole mantle flow in a primarily 2-layer sys- [Meibom and Anderson, 2004] or preservation tem. Detachment of subducted slabs also gen- of enrichment in SCLM following mantle erates vertical and lateral viscous upper mantle metasomatism [Hawkesworth et al., 1986; O’Reilly flow resulting in magmatism [Pysklywec et al., and Zhang, 1995]. The debate indicates that the 2003]. Lateral flow of mantle resulting from HIMU signatures are not sufficiently diagnostic to slab detachment [Kosarev et al., 1999] related uniquely define mantle plumes beneath a region. to continental collision in east Asia has been linked to widespread Cenozoic alkaline volca- 4.3. Slab Detachment Model nism [Flower et al., 1998] similar to that in the SW Pacific. [36] Unusual magmatism in modern subduction [Levin et al., 2002; Wortel and Spakman, 2000] [37] Mantle convection and subsidence models for and collision [Kosarev et al., 1999; Seber et al., Australia suggest that eastward migration of the 1996] zones has been linked to detachment of Gondwana continent over the subducting slab may subducting slabs. Numerical models of mantle have sheared it, causing detachment 130–90 Ma convection suggest that slabs deflected horizontally [Gurnis et al., 1998]. Subduction continued off the in the mantle transition zone are gravitationally Antarctic portion of Gondwana until 100 Ma, unstable [Christensen, 1997] and capable of trig- when the Phoenix plate may have been captured by gering dramatic [Solheim and Peltier, 1994; the north-moving Pacific plate, initiating separation Tackley et al., 1993] or moderate [Davies, 1995; of New Zealand, and other continental fragments

16 of 26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G

from Marie Byrd Land [Luyendyk, 1995]. Synthetic 3-dimensional density models of the mantle based on plate convergence and a simple simulation of whole mantle flow [Lithgow-Bertelloni and Richards, 1998] as well as recent mantle convection models [Steinberger et al., 2004] suggest that the detached Pacific Gondwana slab was present in the upper mantle for much of the region until 75– 65 Ma, when it sank into the lower mantle, where pieces remains today (e.g., Figures 5b and 5c). This detachment could have occurred episodically as in the Mediterranean region [Wortel and Spakman, 2000] with initial detachment off Aus- tralia, eventually migrating to West Antarctica 20–30 Myr later.

[38] A synopsis of the development of conditions that led to alkaline magmatism in the SW Pacific (Figure 9) suggests that subduction-related processes contribute to every phase. During the Paleozoic-Mesozoic, east Gondwana lithosphere formed largely by magmatism and accretion ac- companied by volatile flux from the subducting plate, and/or Jurassic plume activity that metasomatized the SCLM (Figure 9a). This same activity imprinted at least part of the enriched geochemical signature in the SCLM.

[39] Slab detachment in the late Cretaceous could have induced a change in mantle flow. The migration paths of the mantle (arrows, Figures 9b and 9c) mimic those from figures of geodynamic models of the North Fiji Basin [Pysklywec et al., 2003] which are used to guide discussion of the relation of mantle flow to magmatism, but here are only schematic and do not follow the geodynamic models exactly. Mantle flow during the initial tearing of the slab in Late Cretaceous [Lithgow- Figure 9. Cartoon depicting proposed model for Bertelloni and Richards, 1998] would be moderate Cenozoic alkaline magmatism in SW Pacific DAMP according to the geodynamic models for North Fiji based on subduction history models [Lithgow-Bertelloni (Figure 9b). The time gap between the proposed and Richards, 1998] with mantle flow velocity vectors slab detachment 90 Ma (Figure 9b) and initiation generalized (and meant to be schematic, not quantita- of magmatism (55–60 Ma in Australia) tive) from a model of the North Fiji Basin [Pysklywec et (Figure 9c) can be explained by the P-T diagram al., 2003]. (a) Paleozoic-Mesozoic subduction along the (Figure 3). The present-day SE Australia geotherm Gondwana margin (Figure 8a) formed the lithosphere would decay to the conductive Phanerozoic geo- and provided enrichment of the SCLM in the region. Additional fluids may have been introduced during therm in 40–50 My [O’Reilly et al., 1997; Sass postulated Jurassic mantle plume activity. (b) Late and Lachenbruch, 1979]. This also represents the Cretaceous detachment of the subducting slab along the maximum amount of time to conductively heat Gondwana margin. (c). Cenozoic alkaline magmatism from the Phanerozoic to present-day geotherm. If related to slab detachment and sinking into the lower convective processes occur, as evidenced by mag- mantle (see Figure 5 for high-velocity anomalies that matism and underplating in the region, the heating have been related to detached slabs) that resulted in flow time would be considerably shorter, in the extreme of warm Pacific mantle into the SCLM, catalyzing case of convective processes alone, nearly instan- melting and producing the observed seismic low- taneously [Lachenbruch and Morgan, 1990]. As velocity zone (pink box) and geochemical signatures.

17 of 26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G the onset of the magmatism was 55 Ma, the perhaps much of the rest of the SW Pacific. If processes to generate magmatism would have been the temperature of mantle material at 150– set in motion 55–95 Ma, coincident with the slab 200 km beneath >50 Ma oceanic lithosphere is detachments proposed by plate reconstruction 1300–1400C[Shapiro and Ritzwoller, 2002; models (Figure 9b). As indicated by the plate Ritzwoller et al., 2004], flow of this material history models [Lithgow-Bertelloni and Richards, beneath the 100–150 km thick continents 1998], the Pacific slab would have sunk completely (Figure 9c) could partially melt both the metasom- into the lower mantle by 65 Ma (Figure 9c). atized SCLM mantle (due to its low melting Mantle flow is supported by stage poles derived temperature) and rising asthenospheric mantle from plate motion rotations, which show west and (though decompression melting). Subsequent mag- southwesterly lateral flow of Pacific mantle beneath matism could produce the observed isotopic sig- the region between about 80 and 40 Ma [Gaina et natures interpreted to indicate mixing of different al., 2000]. reservoirs (SCLM and asthenosphere) that is a combination of the MORB and FOZO-like com- [40] As the plate detaches and sinks into the lower ponents, as discussed previously. Extension super- mantle (Figure 5c) (Late Cretaceous) [Lithgow- imposed on this system, such as in the western Bertelloni and Richards, 1998; Steinberger et al., Ross Sea during the Oligocene and currently, could 2004], relatively warm, Pacific mantle would flow account for the higher temperature gradients [Berg under the metasomatized SCLM (Figure 9c). The et al., 1989] (blue line, Figure 3) and more volu- Pacific mantle is considered to be warm enough to minous magmatism there than elsewhere in the SW generate melts, an observation supported by plate Pacific. Volcanism should persist until the low- history models, the low seismic velocity anomalies melting point metasomatized layer is depleted or in central Pacific mantle (Figure 5) and numerous subduction is renewed. seamounts [Anderson, 1994]. Lack of cooling by subduction for millions of years is one explanation for the warm temperatures [Anderson, 1995]. Sev- 5. A Regional Model for a Cenozoic eral lines of evidence suggest that the central South Pacific Diffuse Alkaline Pacific superswell, a broad region of uplift dynam- Magmatic Province ically supported by buoyant, low-viscosity low- velocity upper mantle material [McNutt, 1998], [42] Catastrophic slab detachments in the late Cre- was warmed in the late Cretaceous. Recent analysis taceous would mostly likely induce mantle flow in of bathymetry data suggest that warm mantle with a broad region and therefore may explain Cenozoic a temperature gradient of 0.014C/km flows from magmatism not only in the continental pieces the superswell to the East Pacific Rise and that the described here, but over much of the south Pacific, onset of the swell was 98 Ma [Hillier and Watts, including the superswell region, which contains 2004], roughly synchronous with the proposed slab many scattered, short-lived volcanic chains with detachments. In addition, thermal models con- linear age progression that cannot easily be strained by seismic surface wave disperson data explained by conventional plume theory (e.g., suggests that the central Pacific lithosphere Austral Islands; Figure 2) [McNutt and Bonneville, was reheated between ages of 70 and 100 Ma 2000; McNutt et al., 1997]. predominantly at depths between 70 and 150 km [Ritzwoller et al., 2004]. [43] In order to investigate the potential link between slab detachments and mantle flow leading to [41] On the basis of the above discussion, the effect warming in the south Pacific, we reconstruct man- of removing the slab and migration of Pacific tle density from 100 Ma to the present on the basis mantle beneath the Gondwana pieces could raise of a time-dependent global mantle flow model temperatures sufficiently to generate alkaline melts [Steinberger et al., 2004]. The models are gener- as indicated by the P-T diagrams (Figure 3). If the ated by integrating a current mantle density field late Cretaceous geotherm for Gondwana is similar backward in time with global plate motions model to that estimated for Phanerozoic Australia (green superimposed as boundary conditions. The integra- line, Figure 7), and pressure did not decrease (that tion is accomplished by reversing the sign of the is, little to no extension) in the Cenozoic, a density anomaly which effectively reverses the temperature increase of 100C (indicated by red convection back through time. The initial density arrow, Figure 3) is required to match the current SE structure in the flow model was derived from a Australia geotherm (orange line, Figure 3) and shear wave tomography model [Becker and Boschi,

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2002]. The velocity anomalies were converted to a) density anomalies with a conversion factor of 0.25 below 220 km. In our model, current mantle density anomalies are advected back to 100 Ma, not restricted to the Tertiary as in previous models [Steinberger et al., 2004]. At 100 Ma, high-density mantle reflecting subducting slabs partially under- lie the east Gondwana margin (Figure 10a). By 50 Ma (Figure 10b), the slabs have sunk into the lower mantle. Between 50 Ma (Figure 10b) and the present (Figure 10c), a superswell-size negative mantle density anomaly, presumably reflecting warm temperatures and perhaps volatile content, forms in the southwest Pacific, suggesting a link between slab detachment in the lower mantle and upwelling. Three-dimensional modeling of upper-mantle anelastic structure [Ekstrom and Dziewonski, 1998] of the prominent low-velocity anomaly beneath the Pacific superswell (Figures b) 4a, 4c, 5b, and 5c) also suggest that thermal upwelling from the lower mantle carry enough energy across the transition zone to create coherent upwelling flow in the upper mantle [Romanowicz and Gung, 2002] as observed in our mantle density models (Figure 10). This upwelling could then supply heat and horizontal flow to the low-viscosity asthenospheric channel, thereby feeding hot spots in the superswell [Romanowicz and Gung, 2002].

[44] Therefore we suggest that the continental alkaline magmatism described here may lie at the southwestern edge of a Cenozoic Pacific diffuse alkaline magmatic province (DAMP) largely defined by broad, discontinuous regions of relatively low volume alkaline basalts erupted inter- mittently since 55–30 Ma in 80–150 km thick lithosphere. The extent of the DAMP most likely c) covers the south Pacific region associated with unusual mantle low-velocity zones (e.g., most of the region of Figure 2 and the low-velocity region of Figure 5c). The thermal flow model for the superswell bathymetry suggests that warm temperature mantle extends at least to the East Pacific Rise [Hillier and Watts, 2004] as do our models

Figure 10. Mantle density anomaly reconstruction for 500 km depth (in units of 1/1000 kg/m3) at (a) 100 Ma, (b) 50 Ma, and (c) the present based on the time- dependent global mantle flow model from Steinberger et al. [2004]. High-density slabs lie beneath the upper mantle in parts of the SW Pacific DAMP at 100 Ma but have sunk into the lower mantle by 50 Ma. Lighter, presumably warmer mantle is developed during this time period. 19 of 26 Geochemistry 3 finn et al.: alkaline magmatic province Geophysics 10.1029/2004GC000723 Geosystems G

(Figure 10). However, definition of the entire Becker, T., and L. Boschi (2002), A comparison of tomo- province is beyond the scope of this paper. graphic and geodynamic mantle models, Geochem. Geophys. Geosyst., 3(1), 1003, doi:10.1029/2001GC000168. [45] Metasomatized SCLM (60–150 km depth, Behrendt, J. C. (1999), Crustal and lithospheric structure of the Figures 3b–3d) partially sources the melts in West Antarctic Rift system from geophysical investigations: A review, in Lithosphere Dynamics and Environmental continental regions and produces part of the ob- Change of the Cenozoic West Antarctic Rift System, Global served regional low-velocity signature as well as Planet. Change, 23(1–4), 25–44. enriched and HIMU-like geochemical signatures. Behrendt, J. C., A. K. Cooper, and A. Yuan (1987), Interpreta- The low-velocity signature outside of SCLM prob- tion of marine magnetic gradiometer and multichannel ably reflects warm temperatures in the astheno- seismic-reflection observations over the western Ross Sea shelf, Antarctica, in The Antarctic Continental Margin: sphere and, geochemically, is the source of Geology and Geophysics of the Western Ross Sea, Earth depleted (MORB) and FOZO-like components. Sci. Ser., vol. 5B, pp. 155–177, Circum-Pac. Counc. for On the whole, the isotopic heterogeneity is similar Energy and Miner. Resour., Houston, Tex. to that observed in basalts from the Central Pacific Behrendt, J. C., D. D. Blankenship, C. A. Finn, R. E. Bell, R. E. Sweeney, S. M. Hodge, and J. M. Brozena (1994), (e.g., Austral-Cook islands [Schiano et al., 2001]) CASERTZ aeromagnetic data reveal late Cenozoic flood and mixing of Gondwana SCLM with Pacific basalts in the West Antarctic rift system, Geology, 22(6), asthenosphere is consistent with the diversity of 527–530. sources that are required to explain the isotopic Bell, R. E., D. D. Blankenship, C. A. Finn, D. L. Morse, T. A. arrays in Figure 7. All of these lines of evidence Scambos, J. M. Brozena, and S. M. Hodge (1998), Influence of subglacial geology on the onset of a West Antarctic ice support the notion that mantle flow induced by slab stream from aerogeophysical observations, Nature, 394,58– detachments explains the SW Pacific DAMP and 62. perhaps short-lived volcanism along ephemeral hot Berg, J. H., R. J. Moscati, and D. L. Herz (1989), A petrologic spot tracks in the entire south Pacific as well. geotherm from a continental rift in Antarctica, Earth Planet. Sci. Lett., 93(1), 98–108. Best, M. G., and E. H. Christiansen (2001), Igneous Petrology, Acknowledgments Blackwell Sci, Malden, Mass. Beswick, A. E. (1976), K and Rb relations in basalts and other [46] Special thanks go to Mike Ritzwoller and Nikolai mantle derived materials: Is phlogopite the key?, Geochim. Shapiro for access to tomographic models and overall respon- Cosmochim. Acta, 40, 1167–1183. Bhattacharji, S., N. Chatterjee, J. M. Wampler, P. N. Nayak, siveness to questions. We thank Bernhard Steinberger for and S. S. Deshnukh (1996), Indian intraplate and continental providing the mantle density files for Figure 10. Carmen margin rifting, lithospheric extension, and mantle upwelling Gaina and Eric Anderson assisted with the figures. Carolina in Deccan flood basalt volcanism near the K/T boundary: Lithgow-Bertelloni provided a mantle density model for Evidence from mafic dike swarms, J. Petrol., 104, 379–398. 85 Ma. Very constructive and comprehensive reviews and Blackman, D. K., R. P. Von Herzen, and L. A. 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