Lithos 125 (2011) 659–674

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Geochemistry and age of the Nouméa Basin lavas, New Caledonia: Evidence forCretaceous subduction beneath the eastern Gondwana margin

K.N. Nicholson a,⁎, P. Maurizot b, P.M. Black c, C. Picard d, A. Simonetti e, A. Stewart a, A. Alexander a

a Department of Geological Sciences, Ball State University, Muncie, Indiana, USA, 47306b BRGM, 1ter, rue E.-Unger, Vallée du Tir, BP 56, 98845 Nouméa cedex, New Caledoniac School of Environment, The University of Auckland, Private Bag 92019, Auckland, New Zealandd Université de Franche-Comté, UMR Chronoenvironnement n° 2642, IUFM de Besançon, 57 av de Montjoux, 25000 Besançon, Francee Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick Hall, University of Notre Dame, Notre Dame, IN USA, 46556

⁎ Corresponding author. Tel.: +1 765 285 8268; fax:E-mail addresses: knichols@bsu.edu (K.N. Nicholson

(P. Maurizot), pm.black@auckland.ac.nz (P.M. Black), Ch(C. Picard), Antonio.Simonetti.3@nd.edu (A. Simonetti).

0024-4937/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.lithos.2011.03.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 November 2010Accepted 24 March 2011Available online 1 April 2011

Keywords:New CaledoniaNew ZealandVolcanismLate CretaceousTectonicsGondwana

The Nouméa Basin in New Caledonia is perhaps the best preserved sequence of in-situ Late Cretaceous marinesediments and volcanic rocks in the western Pacific region. Previous tectonic interpretations suggest that thebasin formed during a period of large-scale extension between New Caledonia and Antarctica during thebreak-up of the eastern Gondwana margin. However, new geochemical analyses have identified continentalarc signatures in the lavas, suggesting a well-developed Late Cretaceous volcanic arc system active in the NewCaledonia sector of the eastern Gondwana margin, possibly extending as far south as New Zealand. There aretwo distinct suites of lavas in the Nouméa Basin. The older lavas are predominately mafic, low to high-K, andhave a calc-alkaline fractionation trend. Chondrite normalised trace element plots show patterns that are lightrare earth element (LREE) enriched, andmid-ocean ridge basalt (MORB) normalised trace element plots showenrichment of most incompatible trace elements with discernable negative Nb, Ta and Ti anomalies. Traceelement ratios identify a continental arc signature in these lavas which were generated from an N-MORB-likesource. Overlying the mafic lavas is a sequence of younger voluminous siliceous, generally subalkaline lavas(+/−88 Ma). These lavas are LREE enriched with slight positive Nb–Ta anomalies and negative Eu and Tianomalies. The geochemical data indicates these lavas have within plate characteristics with minorcontinental affinities and an enriched source. We propose that the older mafic lavas were generated duringlarge scale subduction under the eastern Gondwana margin during the Late Cretaceous. Whereas the youngerlavas may have been generated during extension; caused by slab roll-back of the subduction system along theSouthwest Pacific plate boundary. The presence of fragments of a detached slab in this process would result inlavas chemically similar to those found in the Nouméa Basin, with minor continental characteristics, andgenerated from an enriched mantle source. What is of fundamental importance is the evidence that the arcsystem extended from New Caledonia southwards to New Zealand and was likely contemporaneous.

+1 765 285 8265.), maurizot@canl.ncristian.Picard@fcomte.iufm.fr

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Over the past ten years there have been a number of modelsproposed for the break-up of the eastern Gondwana margin(Crawford et al., 2003; Finn et al., 2005; Korsch et al., 2011; Schellartet al., 2006; Sdrolias et al., 2003; Tulloch et al., 2009), based onobserved structural features, seismic analyses and computer simula-tions. Although there is some consensus in the models, the basic

complication is a lack of on-land information (i.e. rocks) formedduring the 120 Ma to 50 Ma interval. Throughout its history,especially during the Cretaceous, the eastern Gondwana margin hasbeen a complex mixture of convergence and divergence resulting inthe formation of a series of marginal basins, volcanic arcs, accretedterranes, and isolated crustal wedges (Cayley, 2011; Crawford et al.,2003; Finn et al., 2005; Korsch et al., 2011; Schellart et al., 2006;Sdrolias et al., 2003). Extension in the region associated with theopening of the South Fiji Basin in the early – mid Tertiary resulted incrustal thinning, which combined with Miocene subduction, resultedin the consumption and destruction of most on land exposures. Hencelittle of the Cretaceous to early Tertiary sedimentary basins and arcsystems are preserved, and our understanding of the Cretaceous–Miocene plate margin dynamics in the Southwest Pacific is limited.

On land remnants of Cretaceous–Miocene volcanism are limitedand most are allochthonous: e.g. Tangihua Complex of New Zealand,

660 K.N. Nicholson et al. / Lithos 125 (2011) 659–674

Poya Terrane of New Caledonia and the Papuan Ophiolite of PapuaNew Guinea (Cluzel et al., 1994; Davies, 1968; Nicholson et al., 2000a,b; respectively; Fig. 1A). However, there appear to be two importantexceptions: the Mt Camel terrane in New Zealand represents asequence of in-situ continental arc lavas (Nicholson et al., 2008), andthe Nouméa Basin in New Caledonia, identified by Tissot andNoesmoen (1958) as a sequence of in-situ Cretaceous sedimentsand associated volcanic units (Fig. 1). Both New Caledonia and NewZealand represent fragments (or wedges) of preserved Gondwanacrust which may have allowed preservation of the Cretaceous lavas inthe region (Black, 1993; Paris, 1981; Sutherland, 1999).

The Nouméa Basin represents the remnants of a transgressivesedimentary sequence that contains cogenetic mafic-to-siliceousvolcanic rocks (Fig. 2). The basin itself is ~30 km long and a maximumof 15 km wide. Since the initial description (Tissot and Noesmoen,1958), the basin has been the subject of only two small-scalegeochemical studies (Black, 1995; Debeuf, 1999). The Nouméa Basinoccurs in an ideal geographical and geospatial location for study withparticular emphasis on the break-up of the eastern Gondwanamargin.In this study we describe the lavas in the Nouméa Basin and provide

Fig. 1. (A) Regional map of the SW Pacific showing the location of New Caledonia, New Zealaagain in B) and the Mt Camel terrane in New Zealand. (B) Generalised geology map of New(red star). The Nouméa Basin study area is outlined in black. Note that the Boghen and Kohconsidered to be two discrete terranes.

critical evidence to further our understanding of both the tectonicprocess active during the late Cretaceous–Tertiary break-up of easternGondwana and the associated volcanism.

2. Location and geology

2.1. General geology

The islands of New Caledonia cover 18,580 km2 and are located inthe SW.

Pacific between latitudes 18° and 23° south and longitudes 158°and 172° east (Fig. 1A). The territory is made up of an archipelagocomprising the main island, Grande Terre (16,350 km2), Île des Pins(152 km2), the Loyalty Islands (1980 km2), and the much smaller ÎlesBélep.

Geologically, New Caledonia, specifically the Grande Terre, isunusual in the SW Pacific region as it is a preserved slice of Gondwanamargin composed predominantly of terranes accreted to the easternGondwana margin prior to the break-up of Gondwana (Cluzel et al.,1994; Fig. 1B). New Caledonia is composed of eight major terranes in

nd and Papua New Guinea. Open boxes represent the location of New Caledonia (shownCaledonia showing the locations of all eight terranes and the capital city of Nouméaterranes are now considered one single terrane whereas the Teremba terrane is now

Fig. 2. Generalised geology of the Nouméa Basin. The volcanic units which are the focus of this proposal are shown in red (rhyolite) and purple (basalt).

661K.N. Nicholson et al. / Lithos 125 (2011) 659–674

total: three that accreted to the eastern Gondwana margin during thePermian, and five that were tectonically emplaced during and afterthe break-up of Gondwana.

The Central Chain, Koh-Boghen and Teremba terranes (Avias andGonord, 1973; Cameron, 1989; Campbell, 1984; Grant-Mackie et al.,1976; Guérangé et al., 1975; Meffre and Aitchinson, 1991), wereaccreted to the eastern Gondwanamargin probably in the late Jurassic(Blake et al., 1977). An Upper Cretaceous overlap sequence is formedof locally derived conglomerate, mature deltaic sandstone and coalshale. The Upper Cretaceous rocks are in turn overlain by pelagicPaleocene to lowermost Eocene chert and micrite, followed bydisconformable or unconformable upper Eocene conglomerates,shallow water limestones, and marls. Finally, upwards coarseningupper Eocene detrital sediments (Gonord, 1970) characterized byclasts derived from the Paleocene pelagic sediments and ophioliticdetritus, most likely accumulated in a foreland basin.

Terranes emplaced during and after the break-up of Gondwanaare: the Koumac (formerly part of the Teremba terrane), Diahot,Pouébo, Poya and the ultramafic (“nappe ophiolitique”) terranes. Theexact nature and the relationships between the terranes are oftenunclear. The Koumac terrane is formed of a strongly tectonized upperCretaceous to lowermost Eocene sequence, similar to the Nouméa-Bourail unit but lacking volcanic units. The Diahot terrane is formed ofstrongly foliated and metamorphosed Cretaceous to Paleocene(Maurizot et al., 1989) sediments and volcanic rocks. The Pouéboterrane is an undated (possibly Jurassic; Maurizot et al., 1989, orEocene; Arnould and Routhier, 1956) mélange and forms metre to

kilometre-size mafic boulders enclosed in an argillaceous, calcareousor serpentinitic matrix (Maurizot et al., 1989). The Poya terrane formsa mafic sheet overlying all pre-Neogene rocks (Nicholson et al.,2000b). The ultramafic terrane (“nappe ophiolitique”; Avias, 1967) isthe uppermost unit forming an extensive lithospheric mantle pile; asa result of erosion or detachment, the crustal sequence is almostabsent. According to geophysical data, the ultramafic terranerepresents an obducted part of the Loyalty Basin (Collot et al., 1988).

The terranes of New Caledonia were formed during two periods: aPermian to Jurassic accretion, and Cretaceous to Oligocene breakup,drift and final collision with an island arc (Cluzel et al., 1994; Collotet al., 1988; Maurizot et al., 1989). The first period is characterized byintraoceanic arc volcanism and back-arc basin formation. The secondperiod corresponds to the late Cretaceous break-up of the easternGondwana margin which generated several marginal plateaus duringthe opening of the Tasman Sea, New Caledonia Basin and LoyaltyBasin.

2.2. Geology of the Nouméa Basin

The Nouméa Basin, as discussed in this study, begins within thecity limits of Nouméa and extends 30 km to the northwest (Figs. 1and 2). As described by Tissot and Noesmoen (1958) the basinrepresents a transgressive sedimentary sequence that containscogenetic rhyolitic lavas and less voluminous mafic and intermediatevolcanic rocks (Fig. 2). The Late Cretaceous sedimentary units consistof basal conglomerates, derived largely from the pre-Cretaceous

662 K.N. Nicholson et al. / Lithos 125 (2011) 659–674

sediment: fine grained clastic carbonaceous sediment with local verythin coal seams; and extensive horizons of mainly rhyolitic flows,lenses of course volcanoclastic debris and tuffs. The Late Cretaceoussedimentary units unconformably overlie Lower Jurassic volcanoclas-tic rocks, and are in turn overlain by olisthostromes of ultramaficrocks, Paleocene–Eocene basic volcanics and Eocene flysch and chert-limestone sequences.

Historically there are three distinct volcanic horizons recognized inthe lower part of the basin: Pic Jacob, Catiramona and NogoutaHorizons (Tissot and Noesmoen, 1958). The Pic Jacob Horizon iscomposed of extensive rhyolitic lavas and associated silica tuffs. TheCatiramona Horizon is dominated by voluminous rhyolitic flows andwelded tuffs, tuffs and conglomerates, autobrecciated basaltic lavas,and re-deposited basic volcanic debris. The uppermost unit, theNogouta Horizon consists predominantly of altered rhyolitic lavaswith thick lenses and beds of basic and intermediate volcanic debris,associated bedded tuffs and rare mafic intrusions. The igneouslithologies include olivine-augite basalts, augite andesites, horn-blende-augite andesites, hornblende andesites, rhyodacites andhigh-Si rhyolites. Paleontological evidence places the Catiramonaand Nogouta Horizons Santonian–Campanian in age (85.8 Ma–70.6 Ma), and the Pic Jacob Horizon as Coniacian (89.3 Ma–85.8 Ma)Tissot and Noesmoen, 1958).

Better road access, deforestation, and development in Nouméahave resulted in improved field access to the units and henceinterpretation of field relations which indicate that the threedescribed formations aremore closely related to geographical location

Fig. 3. (A) Back scattered electron images zircons analyzed from NC45. The small pits indicbetween core and rim analyses. (B,C,D) Concordia plot showing the individual U–Pb laser a

as opposed to geochemical characteristics. The more mafic units areexposed in valley floors in the southern portion of the basin, and in thehills in the northeast portion of the basin. Initially described aspredominately autobrecciated, many flow units have been found inthe study area. The stratigraphically higher siliceous units (includingthe Noguota Horizon) overlie the mafic units throughout the area.However it is important to note that there are only two or threelocations where contact relationships are exposed and these are oftencompromised by intrusions and deformation.

Black (1995) also grouped the historical ‘three’ suites of lavas intotwo, possibly bi-modal, geochemical groups. Our new geochemicalanalyses (as discussed below) confirm that there are only twoformations, roughly corresponding to the Pic Jacob Horizon and thecombined Catiramona/Nogouta Horizon: herein referred to as olderand younger respectively. The shoshonitic units of Black (1995)correspond to the Nogouta Horizon, and are likely the result ofpotassium enrichment during alteration.

3. Petrography and alteration

3.1. Petrography

The lavas are grouped for description as follows: rhyoliteflow, andesite-basalt flow, tuffs/ignimbrite, and andesite-basaltvolcanoclastic.

The rhyolite flow samples are generally fine grained to microcrys-talline in thin section with preservation of flow direction as indicated

ate locations of laser ablation spots. As shown the zircons are too small to differentiateblation analyses for NC45, 49 and 72 zircons analyzed (Table 1).

663K.N. Nicholson et al. / Lithos 125 (2011) 659–674

by orientated microlites and phenocrysts. The samples containbetween 1 and 10% phenocrysts consisting of: small b2 mm euhedralplagioclase laths (occasionally glomerphyric), small (bb0.1 mm)subehdral clinopyroxene, b0.1 mm potassium feldspar, rare euhedralzircons, and small disseminated anhedral–subhedral opaquemineralsgrains. Most samples contain almost no clay alteration, and no zeolitemetamorphism except where veins and fractures are present.

The andesite-basalt flow samples are micro to cryptocrystallinewith a groundmass consisting mostly of plagioclase and lesseramounts of clinopyroxene, orthopyroxene and opaque minerals.Primary phenocrysts are: 5–15% euhedral clinopyroxene crystals upto 1.5 mm long with poikiocrysts of plagioclase and opaques, b5%small (b0.3 mm) euhedral orthopyroxene. Some samples includephenocrysts of subhedral hornblende (5–10%) which is generallyunstable, pitted, sieved and altered to clay and opaqueminerals. Small(b0.1 mm) olivine phenocrysts, generally b1% but sometimes up to 5%of the mineral assemblage, are present in a few of the samples.Alteration is non-pervasive and includes zeolites, chlorite, clay,quartz, and pumpellyite.

In thin section the ignimbritie samples are generally fine grainedwelded ash flows with preservation of glass shards, including cuspateshards and preserved bubble walls. The samples contain between 5and 10% lithic fragments which include rhyolite flow material,opaques and fine grained mudstones. Between 1 and 5% phenocrystsare present and include b1 mm subhedral–euhedral plagioclase laths(occasionally glomerphyric), b0.1 mm opaque minerals, and raresmall (b0.1 mm) anhedral–subhedral potassium feldspar grains andeuhedral zircons. Plagioclase is unstable, pitted, sieved and partiallyaltered to clay minerals. Where present small discontinuous fracturesare filled with quartz and iron oxides.

The intermediate to mafic volcanoclastic samples are predomi-nately andesitic through to basaltic andesite in composition. Thesesamples are clast supported with 70–90% porphyritic mafic clasts,euhedral–subhedral plagioclase (5–10%) and 5–10% pyroxene (oftenfound as remnant crystal shapes). The mafic clasts are predominatelyfine grained euhedral plagioclase in a cryto-microcrystalline ground-

Table 1Representative data table for samples from the Nouméa Basin. Analyses were analyzed by

Spot size 206Pb 238U 232Th Th/U 2

Sample # Analysis # um cps cps cps

NC45 P1_1 15 470 28138 10422 0.37 0P1_2 a 15 398 23521 7696 0.33 0P1_3 a 15 352 20634 5834 0.28 0P2_1 15 433 25574 7485 0.29 0P2_2 a 15 345 21188 6043 0.29 0P2_3 15 379 22079 6357 0.29 0P2_4 a 15 506 26627 9340 0.35 0P2_5 15 548 33023 12071 0.37 0P4_1 15 233 12625 3341 0.26 0P4_2 a 15 220 11050 2810 0.25 0P4_3 15 385 22658 15651 0.69 0P4_4 a 15 241 13329 3105 0.23 0P4_5 a 15 247 11484 2457 0.21 0P5A_1 15 426 26974 7450 0.28 0P5A_2 a 15 310 17669 4985 0.28 0P5B_1 15 521 33124 11778 0.36 0P5B_2 15 564 33003 11628 0.35 0P5B_3 15 544 31888 11086 0.35 0P5B_4 15 288 16349 4386 0.27 0

NC72 1-1 15 990 29998 8692 0.29 01-2 15 1749 56550 27861 0.49 03 15 864 29595 8543 0.29 05 15 1489 54769 22488 0.41 010 15 2145 75940 39553 0.52 0

NC49 N1 15 270 8214 2804 0.34 0W1a 15 876 35361 13474 0.38 0W2 15 690 26481 9270 0.35 0

a Not included in concordia age calculation – high error assoc. with 207/235 ratio.

mass. Between 0 and10% secondary quartz is present, in addition tochlorite and smectite. Rare prehnite and actinolite are occasionallypresent, and late stage calcite in-fills veins and open spaces.

3.2. Alteration

Every effort has been made to minimize the effect of alteration byscreening samples for primary volcanic features, such as preservedglass shards, and for absence of secondary mineralization. Within thebasin itself alteration andmetamorphism appear to increase upwards,such that the stratigraphically highest rhyolitic and tuffaceoussamples are the most pervasively altered. This is probably the resultof ophiolite emplacement. In addition, alteration and metamorphismis more pervasive in the volcanoclastic samples. Within flow unitsalteration and metamorphism are generally non-pervasive andconfined to veins and open spaces. Within the uppermost units it ispossible to find minimally altered samples, and below the upper unitsthe alteration and metamorphism are non-pervasive and rarelyexceed prehnite–pumpellytite facies. The highest metamorphicgrade found is lower greenschist facies.

We consider it likely that all of our data is reliable, however, ourpresentation of geochemical data and interpretation relies veryheavily on immobile elements, such as the high field strengthelements (HFSE), rare earth elements (REE), Th and Zr in order toreduce the dependence on potentially mobile elements.

3.3. Zircon descriptions

Within the rhyolite flow and the ignimbrite/tuff samples zirconswere common (although always much less than 1% of the phenocrystassemblage). Only zircons from rhyolite flow units were selected foranalyses to avoid possible zircon inheritance. Within the flow unitszircons were euhedral to subhedral blades and polygons (predomi-nantly squares and diamond shapes), ranging in length between 20and 150 μm, with widths of between 15 and 30 μm and all were clearand light coloured. Representative zircons are shown in Fig. 3a. Some

sodium peroxide fusion ICP-AES and ICP-MS methods. b.d. below detection limits.

07Pb/235U 2 sigma 206Pb/238U 2 sigma 206Pb/238U 2 sigma

age (Ma)

.1361 0.0996 0.0158 0.0011 101 7.3

.2138 0.2260 0.0164 0.0012 105 7.7 a

.2049 0.1919 0.0152 0.0015 97 9.5 a

.1389 0.1048 0.0157 0.0010 101 6.4

.2590 0.1870 0.0147 0.0016 94 10.5 a

.1833 0.1412 0.0157 0.0012 101 7.9

.4118 0.1895 0.0166 0.0017 106 11.0 a

.1796 0.1080 0.0161 0.0012 103 7.9

.1461 0.1800 0.0176 0.0017 112 10.9

.3339 0.1940 0.0185 0.0015 118 9.4 a

.1257 0.1036 0.0159 0.0012 102 7.9

.2508 0.2532 0.0175 0.0024 112 15.3 a

.2019 0.2249 0.0183 0.0023 117 14.9 a

.0309 0.0675 0.0152 0.0011 97 7.1

.3321 0.3083 0.0187 0.0017 119 10.7 a

.1099 0.0985 0.0151 0.0011 97 7.2

.1555 0.0799 0.0166 0.0010 106 6.7

.1674 0.1079 0.0166 0.0011 106 7.3

.0977 0.1809 0.0172 0.0011 110 7.2

.106 0.097 0.0149 0.0011 96 7.3

.116 0.096 0.0144 0.0015 92 9.6

.105 0.127 0.0137 0.0012 88 7.5

.101 0.061 0.0125 0.0012 80 7.7

.200 0.066 0.0134 0.0013 86 8.1

.246 0.457 0.0159 0.0023 102 15

.101 0.081 0.0136 0.0011 87 7

.119 0.090 0.0140 0.0013 90 8

Table 2Representative data table for samples from the Nouméa Basin. Analyses were analyzed by sodium peroxide fusion ICP-AES and ICP-MS methods.

Sample # NC36 NC25 NC26 NC22 NC65 NC64 NC60 NC56 NC54 NC53 NC49

Location Col Pirogue Petroglyphs Petroglyphs Petroglyphs Col Pirogue Col Pirogue Pic Malawi Conception Conception Conception Tina

Rock type andesite andesite andesite andesite basalt andesite andesite basalt basalt basalt rhyolite

Latitude 22°05′03.20″S

22°08′01.41″S

22°08′01.41″S

22°08′01.41″S

22°04′53.10″S

22°04′45.69″S

22°11′58.26″S

22°13′56.83″S

22°13′44.03″S

22°13′44.03″S

22°14′09.99″S

Longitude 166°19′09.32″E

166°26′12.47″E

166°26′12.47″E

166°26′12.47″E

166°19′06.23″E

166°18′45.23″E

166°29′54.14″E

166°29′16.02″E

166°29′34.21″E

166°29′34.21″E

166°29′47.24″E

wt.% LimitSiO2 0.01 54.78 56.86 59.47 60.69 44.31 62.75 60.40 55.22 55.61 55.00 80.43TiO2 0.01 0.66 1.29 1.03 0.70 0.85 0.85 1.23 0.82 0.82 0.83 0.21Al2O3 0.01 16.46 16.13 16.28 19.12 12.12 13.79 17.25 13.98 13.94 14.25 11.34Fe2O3 0.01 9.30 10.66 9.67 6.49 12.90 8.55 8.12 8.15 8.20 7.87 1.11MnO 0.001 0.20 0.18 0.18 0.09 0.24 0.20 0.11 0.18 0.15 0.14 0.01MgO 0.01 6.79 3.80 3.60 2.14 8.03 3.24 3.08 6.75 6.47 6.32 0.12CaO 0.01 4.43 4.84 3.25 7.48 20.74 5.41 2.27 7.33 7.47 7.91 0.09Na2O 0.01 6.23 4.29 4.06 2.20 0.11 3.38 5.39 3.00 2.48 2.45 5.94K2O 0.01 0.27 1.00 1.93 0.12 0.20 0.86 0.87 3.38 3.60 3.72 0.19P2O5 0.01 0.25 0.17 0.11 0.15 0.27 0.35 0.13 0.52 0.51 0.52 0.03Total 99.37 99.22 99.58 99.18 99.77 99.39 98.85 99.33 99.25 99.01 99.47LOI 3.66 3.67 3.34 1.15 3.71 3.86 2.90 3.10 2.74 3.62 1.32

ppm LimitBa 0.50 105 256 697 89 61 143 214 730 600 696 108Rb 0.20 4.95 16.57 30.72 4.00 6 26 23 85 104 105 2Sr 0.10 550 313 306 521 317 414 601 612 1237 1179 101Ta 0.10 0.24 0.13 0.15 0.37 0.24 0.62 0.40 0.66 0.71 0.67 7.46Th 0.10 2.26 1.69 2.17 4.10 1.83 5.17 3.51 5.10 5.11 5.12 13.86Zr 0.50 58 80 87 119 70 118 135 169 176 179 513Nb 1.00 4 2 2 5 4 10 6 10 10 10 113Y 0.50 15 25 22 20 17 19 18 21 20 20 55Hf 1.00 2 2 3 3 2 3 4 4 4 4 14V 5.00 330 321 279 215 285 224 194 234 227 224 4Cr 10.00 145 39 29 48 124 57 79 193 168 170 5Co 0.50 28 28 25 17 36 23 29 28 34 31 31U 0.05 0.91 0.47 0.62 1.10 0.48 1.54 0.93 1.30 1.33 1.32 3.56Sc 5.00 35 37 31 22 28 17 17 24 22 22 5Cu 5.00 69 44 54 75 29 21 11 31 33 24 11Zn 5.00 68 86 76 58 44 42 45 36 40 39 43Pb 5.00 4 5 5 8 3 7 6 6 6 5 8La 0.10 11.50 7.80 8.00 15.30 13.60 27.68 16.99 30.41 30.63 30.88 64.54Ce 0.10 26.85 16.70 16.99 37.76 24.69 53.50 34.05 58.90 59.50 59.39 137.36Pr 0.05 3.58 2.40 2.39 4.74 3.48 6.71 4.62 7.76 7.67 7.68 16.43Nd 0.10 16.03 11.58 11.58 20.20 16.48 30.41 20.51 35.07 34.67 34.40 67.74Sm 0.10 3.57 3.33 3.11 4.24 4.00 6.38 4.66 7.31 7.28 7.22 14.23Eu 0.05 1.13 1.15 1.14 1.19 1.32 1.91 1.52 2.01 2.03 2.00 2.74Gd 0.05 3.36 3.95 3.71 3.75 3.97 5.44 4.15 6.44 6.42 6.08 12.90Tb 0.05 0.50 0.67 0.63 0.59 0.65 0.73 0.60 0.82 0.79 0.80 1.84Dy 0.05 2.85 4.48 4.08 3.67 3.57 4.28 3.71 4.70 4.68 4.43 12.50Ho 0.05 0.56 0.93 0.87 0.76 0.73 0.76 0.71 0.84 0.83 0.79 2.19Er 0.05 1.57 2.92 2.56 2.22 2.07 2.06 2.06 2.22 2.10 2.10 6.21Tm 0.00 0.23 0.41 0.38 0.32 0.28 0.30 0.30 0.31 0.30 0.31 0.78Yb 0.10 1.66 2.83 2.75 2.23 2.03 1.88 1.97 2.02 2.12 2.01 5.14Lu 0.05 0.23 0.44 0.40 0.32 0.29 0.27 0.30 0.30 0.29 0.29 0.74

664 K.N. Nicholson et al. / Lithos 125 (2011) 659–674

of the zircons had inclusions, both light and dark, and some weresignificantly fractured. These were not selected for U–Pb analyses. Inaddition, Th/U ratios for the zircons analyzed (Table 1) rangepredominantly between ~0.3 and ~0.4, which are values characteristicof igneous magmatic zircons (e.g., Simonetti and Neal, 2010; Zartmanand Richardson, 2005).

4. Age dating

4.1. Analytical methods

U–Pb analyses were performed on zircon grains at the Universityof Notre Dame's LA-ICP-MS facility using a ThermoFinnigan Element2ICP-MS instrument coupled to a UP213 laser ablation system fromNewWave. The operating conditions and data acquisition parameters

employed for the U–Pb age determinations are similar to thosereported by Simonetti and Neal (2010) and Frei and Gerdes (2009).The aperture beam delivery system of the UP213 systemwas used andall data were acquired with single spot analyses employing a 15 μmspot size. In order to minimize laser induced elemental fractionation(LIEF) a repetition rate of 2 Hz was employed with a nominal energyoutput of ~55%, corresponding to laser energy of ~0.025 mJ/pulse andfluence of ~2 J/cm2. Individual analyses took ~75 s to complete withthe first ~30 s for measurement of the background ion signals,followed by 30 s of ablation, and 15 s of washout. Prior to the start ofthe data acquisition, the laser was fired for 20 s with the shutterclosed in order to stabilize laser output power.

All measurements were conducted using electrostatic scanning(E-scan)with themagneticfield resting atmass 202Hg. The following ionsignals were acquired: 202Hg, 204(Pb+Hg), 206Pb, 207Pb, 208Pb, 232Th,

Table 2Representative data table for samples from the Nouméa Basin. Analyses were analyzed by sodium peroxide fusion ICP-AES and ICP-MS methods.

NC72 NC47 NC45 NC66 NC41 NC67a NC62 NC63 NC58 NC37 NC38 NC28 NC29 NC06-21

Piata Tina Tina Sanatarium Callioux Sanatarium les Koghis les Koghis Yahoue Dumbea Dumbea Petroglyph Petroglyph LesKoghis

rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite rhyolite

22°10′21.58″S

22°14′10.06″S

22°14′16.09″S

22°04′52.62″S

22°05′08.05″S

22°04′54.36″S

22°10′58.46″S

22°11′17.85″S

22°12′09.89″S

22°04′49.53″S

22°09′42.68″S

22°08′21.91″S

22°08′21.91″S

22°12′58.77″S

166°27′55.43″E

166°29′53.72″E

166°29′51.05″E

166°19′09.45″E

166°21′02.87″E

166°19′22.06″E

166°29′07.63″E

166°28′52.12″E

166°29′50.43″E

166°18′47.33″E

166°26′41.34″E

166°23′37.91″E

166°23′37.91″E

166°29′18.67″E

72.42 78.30 74.11 76.72 74.21 71.61 76.13 76.09 73.59 75.39 73.51 72.68 75.07 76.530.21 0.59 0.32 0.29 0.30 0.30 0.26 0.35 0.37 0.36 0.37 0.24 0.23 0.29

13.25 11.13 14.41 10.11 14.57 10.66 13.11 13.15 11.27 11.02 11.46 12.34 12.57 15.160.43 0.24 2.80 4.41 2.65 12.25 0.23 1.91 5.15 5.60 5.68 3.55 3.67 0.750.00 0.00 0.00 0.03 0.02 0.10 0.00 0.04 0.13 0.05 0.10 0.06 0.05 0.010.01 0.04 0.00 0.32 0.31 1.15 0.00 0.11 0.11 0.45 0.09 0.00 0.10 0.200.02 0.07 0.11 0.09 0.01 0.01 0.07 0.02 2.83 0.01 1.65 1.52 0.06 0.033.92 3.50 6.79 1.83 4.98 0.52 4.62 5.70 3.93 3.44 4.15 3.86 3.87 3.243.93 2.74 0.21 3.89 2.48 3.04 2.97 1.67 1.64 2.62 2.43 3.77 3.61 2.920.02 0.07 0.03 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

94.21 96.68 98.79 97.70 99.55 99.66 97.41 99.06 99.04 98.96 99.47 98.04 99.25 99.151.55 1.89 1.75 1.04 1.22 2.36 1.22 1.25 1.74 0.58 2.53 1.51 1.04 2.36

885 1159 167 289 245 263 1018 380 233 268 162 788 921 31890 42 4 80 59 69 48 25 27 41 37 65.18 60.66 74.0385 118 177 146 21 12 186 25 48 7 58 79 122 487.71 7.42 9.81 10.70 9.63 12.05 9.31 10.81 10.59 11.15 11.44 7.62 8.01 10.07

14.30 14.31 18.05 17.89 13.60 20.93 15.34 15.49 15.54 15.03 16.30 13.33 13.13 15.37635 639 759 791 909 923 944 953 967 1051 1070 637 646 962119 111 145 153 151 168 146 156 153 164 156 118 120 16748 61 73 60 72 80 62 96 82 82 90 77 77 6817 17 21 21 22 25 21 24 25 26 27 17 17 21b.d. 18 6 2 b.d. 4 4 1 1 3 6 84 84 916 5 5 4 4 4 4 4 4 4 4 8 7 9

115 52 112 25 27 11 83 32 18 20 19 41 38 264.51 3.67 4.71 2.57 2.68 2.74 4.52 3.81 4.34 3.31 5.40 3.65 3.73 4.453 6 5 2 4 2 2 3 4 3 3 5 5 4

18 19 2 6 2 7 14 4 3 3 2 30 6 7167 31 20 93 107 138 77 80 93 96 121 145 163 2932 14 12 9 15 8 24 12 9 29 10 11 21 240.93 104.78 90.51 162.11 175.21 73.54 66.08 383.31 121.30 74.52 121.33 66.70 68.60 56.0093.40 217.66 183.34 296.81 341.18 157.05 164.85 639.46 237.71 163.35 240.09 181.35 186.19 115.8911.30 26.00 22.97 33.65 38.34 18.16 20.45 82.29 28.85 19.65 28.47 22.29 22.42 14.7747.90 108.02 98.82 128.54 151.18 74.39 81.32 338.32 118.62 86.62 121.78 95.29 95.21 59.109.91 22.30 20.21 21.03 26.35 17.64 14.96 60.56 23.14 18.67 23.98 20.39 20.28 12.582.20 4.86 4.21 2.90 5.31 2.65 2.80 12.89 5.18 4.64 5.33 4.55 4.51 2.839.01 18.78 17.61 15.99 20.03 17.36 12.94 43.40 19.81 17.90 21.23 18.76 19.00 12.581.52 2.51 2.54 2.21 2.66 2.72 1.98 4.61 2.89 2.81 3.11 2.78 2.75 2.07

10.89 15.63 16.57 13.72 16.23 17.99 13.42 23.65 18.54 18.76 20.55 16.36 16.42 13.392.07 2.74 3.02 2.59 2.93 3.45 2.55 3.88 3.43 3.38 3.75 3.02 3.07 2.686.24 7.06 8.25 7.22 8.01 9.85 7.35 10.06 9.24 9.24 10.14 8.29 8.34 7.610.91 0.95 1.22 1.08 1.18 1.38 1.04 1.35 1.33 1.29 1.42 1.12 1.15 1.096.02 6.14 7.83 7.29 7.59 8.92 7.40 8.72 8.92 8.34 9.33 7.29 7.31 7.390.91 0.85 1.10 1.06 1.09 1.32 1.03 1.23 1.22 1.25 1.30 1.01 1.03 1.06

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235U, and 238U. All data were acquired on four samples per peak with asampling and a settling time of 0.001 s for every isotope. 202Hg wasmeasured to monitor the 204Hg interference on 204Pb (using a 204Hg/202Hg value of 0.229883; Rosman and Taylor, 1999). A common Pbcorrection was not applied; however, individual measurement scansthat recorded 204Pb cps subsequent the 204Hg correction based on the202Hg ion signal were simply rejected. In addition, the ages reported inthis study (Table 1) are based on the weighted mean (WM) 206Pb/238Uratio due to the larger 206Pb ion signals measured (compared to 207Pb)and thus consideredmore robust. LIEFwasmonitoredwith the repeatedlaser ablation analysis of the Mudtank zircon (732±5 Ma; Black andGulson, 1978) as the external standard using a ‘sample standardbracketing’ technique (e.g. Simonetti et al., 2005, 2006). The Mudtankzircon was chosen as the optimal standard due to its relatively youngage and low Pb content of ~2 ppm (Black and Gulson, 1978). A typical

sequence of analyses consisted of 5 measurements of the Mudtankzircon both prior and subsequent themeasurement of 10 unknowns. Alluncertainties associatedwith themeasurement of the 206Pb/238U ratios,including the external reproducibility associated with the repeatedmeasurements of the Mudtank standard zircon (2σ standard deviation~4%) are propagated byquadratic addition using themethod outlined inHorstwood et al. (2003).

4.2. U–Pb dating results

Geochronological data obtained in this study are listed in Table 1and shown in Fig. 3. Fig. 3a shows a photomicrograph of two zirconsfrom the younger lavas before and after analyses and Fig. 3b illustratesthe U–Pb analyses of the zircons from the area of younger lavas on aconcordia plot. A total of 19 analyses were conducted with 8 being

666 K.N. Nicholson et al. / Lithos 125 (2011) 659–674

discarded because of high associated errors due primarily to low ionsignals measured. The concordia age for the younger lavas is 102.7±2.2 Ma, based on 11 analyses from 5 different grains. Two additionalsamples from the younger lava suite yielded zircons large enough foranalyses. The results are shown in Fig. 3c and d. A total of 5 analyseswere carried out on sample #NC72, with one discarded due to highassociated errors, and 3 analyses were made of sample #NC49.Samples #NC72 and #NC49 yielded concordia ages of 88.4±3.5 Maand 88.5±5.5 Ma respectively. These ages are slightly older than theaverage paleontological ages for the units given above, however, theyconcur with the upper age limited of the different Catiramona andNogouta Horizons described by Tissot and Noesmoen (1958).Alternatively, it may be argued that the younger ~88 Ma dates reflectPb loss incurred subsequent zircon crystallization at ~103 Ma.However, the calculated ages do not correlate inversely with U andTh ion signals (hence absolute U and Th contents; Table 1), and Th/Uratios are relatively uniform (most range between ~0.3 and ~0.4).Both features do not support the idea that the younger U/Pb ages arethe result of recent Pb loss. It is possible that the oldest age (103 Ma)

Fig. 4. Plots showing (A) wt.% SiO2 versus total alkalis (Na2O+K2O) after Le Maitre (1989), ashowing the calc-alkaline fractionation trend of the older lavas. The younger lavas are shoconstant in all figures. Shaded gray field represent theMt. Camel terrane lavas: dark shaded, hNicholson et al., 2008 and text for details).

represents inherited zircons, however every effort was made to selectzircons from flow units to minimize the chance of zircon inheritance.In addition the underlying sediments are significantly older (pre-Cretaceous) and finally this age falls within the paleontological agerange for the Nogouta Horizon (Tissot and Noesmoen, 1958). Thus farzircons in the older lavas have proven too small for dating by LA-ICP-MS. Given that the younger lavas range in age between 103 Ma and88 Ma, and using paleontological constraints, the older suite of lavasmust be Late Cretaceous in age.

5. Geochemistry

5.1. Analytical methods

Samples were prepared at Ball State University by removingexternal surfaces, including all visible signs of alteration or veining,and any cut surfaces were ground using a pure aluminum oxidepowder to remove any reside from the saw blade. The samples werethen broken into chips and ground to a powder using a tungsten

nd (B) ternary diagram of Na2O+K2O−FeOtotal−MgO after Irvine and Baragar (1971)w as open squares and the older lavas are shown as filled diamonds. Symbols remainigh-Si lavas are fromGroup 1, and the light shaded, lower-Si lavas are fromGroup 2 (see

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carbide ring mill. Samples were analyzed using facilities at SGSMinerals laboratories. Crushed rock samples were fused by sodiumperoxide in graphite crucibles and dissolved using dilute HNO3. Thesample was split and analysed by ICP-OES, ICP-MS (Perkin ElmerOptima 5300 DV and Perkin Elmer Elan 6100, respectively) and aBruker S8 Tiger XRF. XRF methodology uses the fusion techniquedescribed by Claisse and Blanchette (2004), with theoretical matrixcorrections after Rousseau (1987), in the PANalytical SuperQ/BrukerSpectraPlus software environment.

ICPmethod validation includes use of certified referencematerials,limit of quantification, specificity and measurement uncertainty. Aninstrument blank and calibration check was analysed with each run.Method blanks, reference materials and replicates are randomlyplaced within each batch. The Standards Council of Canada hasaccredited this test in conformance with the requirements of ISO/IEC17025. See http://www.scc.ca for scope of accreditation. All data isavailable in an online table.

5.2. Major element chemistry

Geochemical interpretations are based on 60 new analyses.Average loss on ignition (LOI) for all samples is 2.9 wt.%. Sampleswith LOI values larger than 4 wt.% were discarded. Iron was analysedas Fe2O3 with data recalculated for both FeO and Fe2O3 (afterMiddlemost, 1989). Major and trace element data is shown in Table 2.

There are two distinct geochemical groups present within thesample set. The younger lavas are indicated by open squares in allfigures. The younger lavas have greater than 65 wt.% SiO2, with thevast majority of the samples being ~76 wt.% SiO2. These lavas havelow wt.% TiO2 (b1%), and are generally subalkaline (Le Maitre, 1989;Fig. 4). All major elements have a negative correlation with SiO2,although both K2O and Na2O show considerable scatter due toalteration (Fig. 5). These patterns indicate the fractionation ofplagioclase with minor amounts of magnetite and pyroxene, followedby K-rich feldspar (as seen in thin section and discussed above).Magnesium numbers (Mg#) for the younger lavas have an averagevalue of 11, as is expected given their low SiO2 contents.

The older lavas (shown as solid diamonds in all figures) rangebetween 45 and 65 wt.% SiO2, and are predominately basaltic-

Fig. 5. Bivariate plots of major elements against SiO2 wt.%. Solid diamonds

andesites (Figs. 4 and 5). The older lavas range from low to high-Kand have a distinctive calc-alkaline fractionation trend. All the majorelements except Al2O3 show a negative correlation with SiO2,although both K2O and Na2O3 show considerable scatter (Fig. 5).Magnesium numbers (Mg#) for the older lavas range between 24 and48. Decreasing concentrations of Fe2O3, MgO and CaO are explainedby the formation of olivine, pyroxene and plagioclase feldspar as seenin thin section. Decreasing values of TiO2, MgO and Fe2O3 also reflectthe crystallisation of magnetite and titanomagnetite, and decreasingP2O5 is the result of apatite formation.

5.3. Trace element chemistry

In the older lavas most trace element contents increase withincreasing Zr (Fig. 6). The exceptions are: V, Ni, and Sc which correlatenegatively with Zr, and Ba, Rb, Sr, Cu, Cr, Zn and Pb with scatteredpatterns. Similarly, in the younger lavas most trace element contentsincrease with increasing Zr, the exceptions are: Ba, V, Cr, Ni, and Cuwhich all correlate negatively with Zr.

Chondrite normalised rare earth element (REE) plots for both theyounger and older lavas (Fig. 7a) show patterns that are light rareearth element (LREE) enriched, although the younger lavas are moreenriched in the heavy rare earth elements (HREE) and show a distinctEu anomaly characteristic of plagioclase fractionation. On MORBnormalised trace element plots (Fig. 7b) all the lavas have similarpatterns and are more enriched in large ion lithophile elements (LILE)relative to high field strength elements (HFSE). The younger lavashave a slight positive Nb–Ta anomaly and a negative Ti anomaly,whereas the older lavas have distinct negative Nb–Ta and Tianomalies.

6. Geochemical interpretations

From Figs. 4 and 5, it is clear that there are two distinct groups oflavas. The older lavas are predominantly basaltic-andesite lavas andthe younger lavas are rhyolitic. MORB and chondrite normalised traceelement plots (Fig. 7) illustrate further differences between theyounger and older lavas. The younger lavas have characteristicssimilar to within plate granites with a positive Nb–Ta anomaly

: older, open squared: younger. Symbols are the same in all diagrams.

Fig. 7. Chondrite (A) and MORB (B) normalised diagrams for both younger (dashedlines) and older (solid lines) lavas; normalisation after Evensen et al. (1978) and Sunand MacDonough (1989) respectively. Differences such as the negative Ta and Nbanomaly exhibited by the older lavas cannot be explained by fractionation and arerelated to different tectonic environments of formation.

Fig. 6. Bivariate plots of trace elements against Zr (ppm).

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whereas the older lavas have negative Nb–Ta anomalies and are moretypical of lavas formed in a volcanic arc environment. Furtherinvestigation into the chemistry of the older lavas begins to clarifythe effects of varying slab input and the spatial relationship betweenvolcanism and subduction as illustrated in trace element tectonicdiscrimination diagrams.

On Fig. 8a–b the older lavas plot within the island arc and volcanicarc fields, where as the younger lavas (Fig. 8c–d) show characteristicssimilar to within plate granites. For comparison, the more evolvedsamples from the older lavas are also shown on this diagram (Fig. 8c–d) and plot within the field of volcanic arc granites. On a Ta/Th versusTh/Yb diagram (Fig. 9a), the younger lavas have a more enrichedsource whereas the older lavas have a more depleted source (as istypical in SW Pacific island arc systems, e.g. Dupuis et al., 2005; Pearceand Parkinson, 1993). This diagram also reinforces the distinctly calc-alkaline affinities of the older lavas.

Nb and Ce are more incompatible in mantle phases than Zr and Y,respectively, relative to the melt, hence the smallest degree melts willshow the lowest values of Zr/Nb and the highest values of Ce/Y. InFig. 9b the Nouméa Basin lavas are shown in comparison to modernarc systems (after Clift et al., 2002). Ratios of Zr/Nb and Ce/Y in theolder lavas are similar to those observed in the active Izu and Tongansystems (Clift and Dixon, 1994; Clift et al., 2002; Ewart andHawkesworth, 1987), and overlap into the fields for the Marianasand Honshu arcs (Clift and Lee, 1998; Clift et al., 2002). This supports amodel whereby the older lavas have been formed in a continentalenvironment from a depleted source; over time, the lavas experienceddecreasing continental contamination and an increasing oceanicsignature. With increasing Ce/Y ratios and decreasing Zr/Nb ratios,the younger lavas trend towards continental crust compositions(Rudnick and Fountain, 1995) and overlap with both the Neogenecontinental Honshu tephras and Aegean glasses (Clift and Blusztajn,1999; Clift et al., 2002, respectively): suggesting both sourceenrichment and increased influence of continental crust. Zr/Nb ratiosin the younger lavas trend towards upper continental crust compo-sitions. Therefore these lavas were likely generated by small degreesof partial melting of an enriched mantle source, and experiencedassimilation of continental material.

Using a plot of (La/Sm)n versus (Ta/Th)n it is possible to differ-entiate between the effects of fractionation and assimilation (Fig. 10).Lavas with continental arc signatures are more likely to have under-gone some combination of assimilation and fractionation (AFC) intheir differentiation history. The younger lavas are clearly derivedfrom an enriched source (possibly similar to Ocean Island Basalts:OIB) which may have assimilated upper continental crust material.

Fig. 8. (A) A ternary plot of La–Y–Nb after Cabanis and Lecolle (1989) and (B) a ternary plot of 2Nb-Zr/4-Y after Meschede (1986) illustrating the arc-like signatures found in theolder lavas. (C) and (D) Y ppm versus Nb ppm and Yb ppm versus Ta ppm respectively after Pearce et al. (1984) showing the within-plate signatures of the younger lavas and thevolcanic arc signatures of the older lavas.

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However it is possible to generate the compositions of the youngerlavas through fractionation alone. Generation of the older lavasrequires partial melting of a more depleted source compositionfollowed by both fractionation and assimilation of continentalmaterial.

Basic AFC modeling of the composition of the older basaltic lavasusing a slightly enriched MORB source (cf. Fig. 9a) for the lavas andassuming a representative crystallisation history of plagioclase,

Fig. 9. (A) Bivariate plot illustrating source characteristics and possible fractionation and/or aC: crustal contamination, W: within plate enrichment, F: fractionation after Pearce (1982). (Bsystems such as the Honshu arc and the Marianas. This plot illustrates the changing signatursignatures in the younger lavas. Modified after Clift et al. (2002). Upper (U), middle (M) an

augite, and spinel, (up to the onset of apatite crystallisation), suggeststhat these lavas can be modeled by the addition of GLOSS (averageglobal subducting sediment) and possibly minor UCC (uppercontinental crust). By varying the amount of fractionation between60 and 70%, and the assimilation factor between 0 and 0.2, it ispossible to account for most of the older mafic compositions.

Similar AFC modeling on the younger lavas yielded very differentresults. The younger lavas were modeled using an E-MORB source

ssimilation histories for the younger and older lavas. (A) S: subduction zone enrichment,) Zr/Nb versus Ce/Y binary plot comparing the Nouméa Basin lavas with globally similares in the lavas from oceanic signatures in the older lavas, to well developed continentald lower (L) crustal sources (open squares) from Rudnick and Fountain (1995).

Fig. 10. (La/Sm)n versus (Ta/Th)n plot showing possible differentiation trends in the older and younger lavas. Dashed arrows represent AFC modeling from an N-MORB sourcethrough to CAB (continental arc basalt) and from an OIB-type source through to UCC (upper continental crust). DM: depleted mantle, PM: primitive mantle, GLOSS: globalsubducting sediment, IAT: island arc tholeiite, LCC: lower continental crust, N-MORB: normal MORB, E-MORB: enriched MORB. Modified after Dupuis et al. (2005).

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(cf. Fig. 9a) and upper continental crust as the assimilated material.Using UCC assimilation and minor fractionation it is possible togenerate the compositions of the younger lavas. However, it was alsopossible to generate the younger lavas through 90 to 95% fractionalcrystallization, starting fromthe least evolved chemistry in the sequenceto the most evolved.

7. Comparison to the Mt Camel terrane, New Zealand

Both New Caledonia and New Zealand contain fragments ofGondwana crust (Black, 1993; Cluzel et al., 1994; Paris, 1981). Henceit seems possible that lavas of similar age erupted onto these twolandmasses might be broadly similar, and is therefore worthy ofdiscussion. TheMt Camel terrane in Northland, New Zealand, containstwo suites of igneous rocks that are broadly similar to those found inthe Nouméa Basin. A geochemical analysis of primarily mafic lavas hasrevealed two different geochemical signatures within the Mt Camelterrane. On Fig. 4 the predominantly medium-K series lavas withMg#b40 appear similar to the Nouméa Basin suite. Nicholson et al. (2008)has divided the Mt Camel lavas into two suites: the older Group 1lavas contain distinct continental arc signatures, and the youngerGroup 2 lavas are intraplate lavas with minor continental affinitiesandwere generated from amore enriched source. Further comparisonin Fig. 11 shows that the Group 1 lavas formed in a similar tectonicsetting and have similar source characteristics to the older lavas in theNouméa Basin, and the Group 2 lavas formed in a similar tectonicenvironment and have a similar source to the younger lavas.

In terms of age, the lavas found in the Nouméa Basin are slightlyyounger than those in the Mt Camel terrane. Paleontological evidenceindicates the Mt Camel terrane sediments are Late Cretaceous (Isaacet al., 1994). U–Pb analyses from zircons within ignimbite units gaveages of 101–102 Ma (Tulloch et al., 2009) and associated sandstoneunits yield ages of between 108 and 119 Ma (Nicholson et al., 2008).The age of the younger lavas is, however, speculative and may beanywhere between Oligocene and Late Cretaceous. In the NouméaBasin the older lavas are constrained by paleontology to be LateCretaceous, and they are stratigraphically older than the younger lavasuite which has yielded U–Pb zircon ages of between 103 Ma and88 Ma. We suggest that the older Nouméa Basin lavas are eithercontemporaneous with, or slightly older than the older (Group 1) MtCamel terrane units. The younger lava suites in both New Zealand and

New Caledonia may also be contemporaneous, with both suiteserupting around 100–88 Ma.

Geochemically the Nouméa Basin lavas and the Mt Camel lavas arevery similar (Fig. 11) and their ages overlap. This data indicates thatthere was active continental arc volcanism in New Zealand and NewCaledonia, at approximately 101–103 Ma, possibly migrating north-ward. These results also suggest that the system changed to anextensional environment, resulting in within plate chemistries by88 Ma. In New Zealand the Late Cretaceous (?) extension generatedbasaltic and basaltic-andesite lavas, whereas in New Caledoniaextension involved continental crust and generated predominatelysiliceous lavas.

8. Tectonic implications

The Nouméa Basin contains an in-situ (autochthonous) LateCretaceous sedimentary basin sequence with continental arc magmas.The presence of the in-situ sedimentary basin is not as extraordinaryas the evidence for subduction-related magmatism under the easternmargin of Gondwana. The combination of the basin sediments withthe volcanic units erupted onto or near remnants of Gondwana crust(both in New Caledonia and New Zealand) gives an unprecedentedopportunity to study, and date, the processes active along the easternGondwana margin during the Late Cretaceous when little geologicalevidence is preserved.

Throughout the history of the eastern Gondwana margin, therehave been periods of both extension associated with the formation ofmarginal basins separated by slivers of continental crust, andcompression associatedwith accretion and arc volcanism (e.g., Cayley,2011Cluzel et al., 2011; Korsch et al., 2011). Evidence for Early to mid-Cretaceous marginal rifting is widespread in Southeast Australia(Bryan et al., 1997) and in the South Island of New Zealand (Deckertet al., 2002; Tulloch and Kimbrough, 1989). The end of the southernGondwana active margin activity occurred around 110 Ma (Mortimeret al., 1999) and was likely related to the subduction of a spreadingridge (Bradshaw, 1989), and/or the collision of the Hikurangi OceanicPlateau with the southeast Gondwana active margin (Mortimer andParkinson, 1996).

The opening of the SW Pacific marginal basins has been interpretedin terms of a pure extensional setting, possibly triggered by a mantleplume (Bryan et al., 1997), whereby slab break-off followed the end of

Fig. 11. Chondrite andMORB normalised REE diagrams for both the Nouméa Basin lavas and the Mt Camel terrane lavas (A and B) Group 2 lavas (dark shaded fields) (C and D) Group1 lavas (light shaded fields); chondrite normalisation after Evensen et al. (1978) and MORB normalisation Sun and MacDonough (1989).

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subduction (beneath the Gondwana margin), generated adakiticmagmas and triggered uplift of asthenospheric mantle that in turnwas responsible for the extensional break-up of the southeastGondwanamargin (Cluzel et al., 2010a,b). In Australia andNewZealandthe lack of evidence for post-Early Cretaceous activemarginmagmatismhas been taken as evidence for this purely extensional tectonic model.

An alternative model for the Late Cretaceous break-up of theeastern Gondwana margin has been presented by Finn et al. (2005).They argue that the small volume of volcanism, combined with thechemistry of observed lavas and a lack of characteristic seismicanomalies, contradicts the mantle plume model. Instead, Finn et al.(2005) argue that the evidence supports a slab detachment model.Using mantle convection and subsidence models, they suggest thatCretaceous eastward migration of the Gondwana continent over thesubducting slab may have sheared the slab, causing detachment,possibly as early as 130 Ma (Gurnis et al., 1998). Shearing began in theAustralian sector of the margin and migrated southward into theAntarctic portion of Gondwana until ~100 Ma, when the Phoenixplate may have been captured by the north-moving Pacific plate,initiating separation of New Zealand, and other continental fragmentsfrom Marie Byrd Land (Luyenduk, 1995). This model is broadlysupported by Sdrolias et al. (2003) who suggest that between 120 Maand 100 Ma convergence between the Australian and Pacific platesoccurred along the Norfolk Ridge, followed by fragmentation of theeastern Gondwana margin (Aitchison et al., 1995; Ali and Aitchison,2000; Bernardel et al., 2002; Cluzel et al., 2001; Eissen et al., 1998;Nicholson et al., 2000a,b).

Combining the models of Crawford et al. (2003), Finn et al. (2005)and Schellart et al. (2006) we suggest a model involving continentalfragmentation, episodic slab detachment and slab roll-back along theeastern Gondwana margin during this time frame (Fig. 12). BothCrawford et al. (2003) and Schellart et al. (2006) argue that continuedextension along the eastern Gondwana margin is only possible ifaccommodated by slab roll-back and subduction.

We propose that the older lavas in the Nouméa Basin and in theMtCamel terrane (New Zealand) represent remnants of continental arclavas associated with the Cretaceous–Paleocene slab roll-back, asindicated by their continental arc signatures and MORB source

(Fig. 12). It is most likely that subduction involved continentalfragments, or slice(s) of continental material separated from, butrunning parallel to, the eastern Gondwana margin. Therefore, thepresence of these Cretaceous continental arc lavas supports the modelproposed by Crawford et al. (2003), Finn et al. (2005) and Schellartet al. (2006) that the break-up of the eastern Gondwana margin wasaccompanied by westward subduction and significant slab roll-back.

The younger lavas in both New Caledonia and New Zealandpresent equally intriguing evidence to support the model of Finn et al.(2005). If we assume slab detachment occurred between 100 and90 Ma, then as the detached plate fragment sinks into the lowermantle, relatively warm Pacific mantle would flow under themetasomatized subcontinental lithospheric mantle. Magmas gener-ated in this manner are characteristically OIB-like magmas (oftenattributed to mantle plumes; Finn et al., 2005).

Geochemical evidence suggests the younger Nouméa Basinrhyolitic lavas were generated in within plate environment withslight continental influences and E-MORB source characteristics.Similar lavas in New Zealand (the younger Mt Camel terrane lavas)have been attributed to the Oligocenemagmatism associated with theLoyalty–Three Kings Ridge (Crawford et al., 2003; Schellart et al.,2006; Sutherland et al., 2004). Between 82 and 52 Ma west tosouthwest subduction occurred along the Southwest Pacific plateboundary during northeast to east directed roll-back of the Pacific slab(Crawford et al., 2003; Schellart et al., 2006; Sdrolias et al., 2003). Thepresence of a fragment of a detached slab in this process would resultin lavas chemically similar to those found in the Nouméa Basin, withOIB-like signatures, with minor continental characteristics, generatedfrom an enriched mantle source. This magmatism may have occurredimmediately after the initial continental arc-like volcanism or as lateas 30 Ma later. The younger Mt Camel terrane lavas, which are similarto those of the Nouméa Basin were likely generated by roll-back of thesouthernmost extension of the subduction zone.

9. Conclusions

The lack of on-land exposures of Late Cretaceous igneous rocks hassignificantly hampered the development of tectonic models for the

Fig. 12. Tectonic cartoon depicting the relative location of subduction between the Pacific and Australian plates between 100 Ma and 90 Ma. Geochemical data, paleontologicalevidence and U–Pb dating suggests that subduction occurred along this plate margin, generating lavas such as those found in New Zealand and New Caledonia for much of the LateCretaceous. Note for simplicity the outlines for Australia, New Zealand and New Caledonia are based on their current form.

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New Zealand and New Caledonia sectors of the Gondwana marginbetween the Late Cretaceous and the early Miocene. Critical to ourunderstanding of tectonic processes active during this time is the LateCretaceous medium- to high-K calc-alkaline volcanic activity reportedat the eastern boundary of the system, mainly in New Caledonia (i.e.the Nouméa Basin). Based on our geochemical results we suggest thatslab detachment and subduction roll-back along the southeastGondwana margin generated incipient west-dipping subductionalong the eastern end of the system and generated continental arcmagmas. Continued subduction rollback resulted in the generation ofyounger within plate rhyolites.

Supplementary material related to this article can be found onlineat doi:10.1016/j.lithos.2011.03.018.

Acknowledgments

This work was supported through a National Science FoundationGrant to KNN: NSF OISE IRES Grant #0727323. Special thanks toDrs. N. Mortimer (GNS, New Zealand) and M. Santosh (KochiUniversity, Japan) for their very helpful reviews of this manuscript.

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