Deep-Sea Research II ] (]]]]) ]]]–]]]

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Deep-Sea Research II

0967-06

doi:10.1

n Corr

E-m

PleasRese

journal homepage: www.elsevier.com/locate/dsr2

Cenozoic volcanism of the Capel-Faust Basins, Lord Howe Rise,SW Pacific Ocean

Dadd K.A.1,n, Locmelis M.1, Higgins K.2, Hashimoto T.2

1 Department of Earth and Planetary Sciences, Macquarie University, Sydney NSW 2109, Australia2 Geoscience Australia, GPO Box 378, Canberra ACT 2601, Australia

a r t i c l e i n f o

Article history:

Received 27 October 2010

Accepted 27 October 2010

Keywords:

Capel basin

Faust basin

seamount chemistry

Lord Howe Rise

45/$ - see front matter & 2010 Elsevier Ltd. A

016/j.dsr2.2010.10.048

esponding author.

ail address: kelsie.dadd@mq.edu.au (Dadd).

e cite this article as: Dadd, K.A., et alarch II (2011), doi:10.1016/j.dsr2.20

a b s t r a c t

New bathymetry, geophysical data and samples were acquired in 2007 during a marine reconnaissance

survey using the R.V. Tangaroa in the frontier Capel and Faust basins, Lord Howe Rise (LHR) by Geoscience

Australia as part of the Australian Government’s Offshore Energy Security Program. This survey identified

a number of volcanic features including cones, flows and sill-related features on the seafloor. Based on

analysis of seismic data and swath bathymetry, there are at least two distinct ages of volcanism exposed

on the seafloor; Late Miocene–Pliocene cones with a largely unmodified conical shape and Eocene–

Oligocene volcanic features. The Middle Miocene Gifford Guyot, part of the Lord Howe seamount chain

was included in the survey area. Volcanic features are common on the seafloor of the LHR and in the

neighbouring Tasman Basin, with two identified north–south seamount hotspot chains to the west of the

Capel-Faust region that have been active from the Miocene to recent. The previously completed ZoNeCo

5 swath-mapping, seismic and sampling survey also identified a few small conical seamounts on the

northern LHR, along a roughly north-northwest trend, dated as Early Miocene. The younger Capel-Faust

seamounts are aligned with these and the age relationships suggest they could be part of a single

seamount chain. The older cones do not show any preferred alignment within the survey area and are too

old to be related to the current hotspot activity in the region. Samples are variably altered but trace

element abundances of all dredged mafic rocks indicate they are alkalic basalt. Samples from the younger

cones can be differentiated from each other and from the Gifford Guyot on a plot of Ti, Sr and Tb/Yb

reflecting different source depths or degrees of partial melting. A comparison of pyroxene from the older

more altered cones and the younger cones shows that these most likely represent two distinct magmatic

events and that the younger cones are similar to the ZoNeCo 5 seamounts.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The Lord Howe Rise (LHR) region (Fig. 1) forms one of Australia’slargest unexplored offshore frontiers, extending 1600 km fromsouthwest of New Caledonia to the Bellona Trough, west of MountCook in New Zealand. Pre-existing geologic data coverage of thearea is sparse, with many unknowns and data gaps throughout theregion. Programs by Geoscience Australia have focussed on acquir-ing new data and filling these gaps initially with the Big New OilProgram (2003–2007) and subsequently the Offshore EnergySecurity Program (2007–2011).

As a part of these programs, new geologic and geophysical datawere acquired over three surveys in the area. The joint Australian–French AUSFAIR MD153 (Australia–Fairway Basin bathymetry andsampling) Survey in 2006 focussed on acquiring swath bathymetry,sub-bottom profiler, gravity and magnetic data, as well as rock

ll rights reserved.

., Cenozoic volcanism of the10.10.048

dredge samples and giant CALYPSO piston cores (Colwell et al.,2006). The Capel-Faust GA-302 seismic survey by GeoscienceAustralia in 2006–2007 acquired approximately 6000 km of 106-fold 2D data (Kroh et al., 2007) over the Capel and Faust basins ofthe northern Lord Howe Rise (NLHR). The Tangaroa GA-2436survey in late 2007 aimed to investigate both petroleum prospec-tivity and seafloor environments in the Capel-Faust area, success-fully acquiring piston core and dredge samples, as well asapproximately 24,000 km2 of multibeam bathymetry and 11,000line km of shipboard gravity and magnetic data with a line spacingof 3–4 km (Heap et al., 2009).

A number of previously unknown volcanic features were mappedand sampled during the Tangaroa GA-2436 survey (Fig. 1). Volcanicfeatures previously identified in the LHR region include two oceanicnorth-south seamount chains, one to the west of the Lord Howe Rise,the Tasmantid seamount chain (McDougall and Duncan, 1988) and theNorfolk Ridge seamount chain to the east (Exon et al., 2004). TheZoNeCo 5 survey (1999, RV L’Atalante) also revealed a few small conicalseamounts on the NLHR, along a roughly north-northwest trend, andsampled two of them (Exon et al., 2004). During the GA-2436 survey,

Capel-Faust Basins, Lord Howe Rise, SW Pacific Ocean. Deep-Sea

Fig. 1. Regional setting and location of sample sites used in this study from the Tangaroa GA-2436 survey. The swath bathymetry data shown was also acquired during this

survey.

DaddK.A. et al. / Deep-Sea Research II ] (]]]]) ]]]–]]]2

sampling targets were identified on the basis of previously acquiredseismic data (GA-302) coupled with bathymetry data acquired duringthe survey.

In this paper we provide a description of the morphology of thevolcanic edifices in the Capel-Faust basins, a petrographic descrip-tion of the samples dredged during the survey and the results ofgeochemical analysis of whole rock samples and pyroxene grains.This work will allow correlation of these volcanoes with othersidentified on the LHR. The age range of volcanism will be outlinedand correlated with that of other volcanoes in the LHR region, thusproviding an insight into the tectonic setting of the Cenozoicvolcanic activity in the Capel-Faust basins.

2. Regional setting

The Capel and Faust basins are located in water depths between1500–3000 m on the NLHR (Fig. 1). This region was originally partof the eastern margin of Gondwana, characterised by convergenttectonics in the Palaeozoic and Mesozoic (Gaina et al., 1998;Sdrolias et al., 2001; Veevers, 2000; Crawford et al., 2003; Lafoyet al., 2005; Schellart et al., 2006; Collot et al., 2009). Subsequentoblique rifting starting in the Early Cretaceous, associated with thebreak-up of Gondwana and the formation of the Tasman Sea,separated the continental fragment from the rest of Australia.Rifting continued until the latter part of the Late Cretaceous,producing a number of basins along the length and width of LHRwith likely petroleum potential (Stagg et al. 1999; Blevin 2001;Willcox et al., 2001; Van de Beuque et al., 2003; Exon et al., 2007).The thickest sediments and largest structures were identified in theCapel-Faust basins and have been the focus of data acquisition inthe region.

The LHR is dominated by a thick cover of marine ooze, resultingin a paucity of rock exposures from which to obtain samples. Acontinental origin for the LHR has largely been inferred from crustalthicknesses of 29–34 km (Zhu and Symonds, 1994; Shor et al.,1971). A number of igneous rocks have been sampled across theLHR. The 2006 AUSFAIR Survey dredged along the Vening-Meinesz

Please cite this article as: Dadd, K.A., et al., Cenozoic volcanism of theResearch II (2011), doi:10.1016/j.dsr2.2010.10.048

fracture zone on the LHR and recovered volcanic and plutonicclasts within conglomerate (Colwell et al., 2006/05). These samplesrepresent at least two episodes, or a long-lived and continuousphase, of Late Cretaceous alkalic volcanism, dated at 97 and 74 Ma,likely to be associated with rifting. DSDP Hole 207 (1000 km to thesouth) also encountered Cretaceous volcanic activity with samplesof rhyolite dated at 93.7+1.1 Ma (McDougall and van der Lingen,1974). Seismic data analysis of the Capel-Faust basins has alsoidentified a number of probable rift-related volcanic units that havenot been validated by sampling (Cowell et al., 2010).

Hotspot volcanoes occur over areas of continental crust and inthe adjacent ocean basins. There are three main chains of suchvolcanoes, the Tasmantid, Lord Howe and Norfolk Ridge seamountchains that parallel other hotspot chains on the nearby Australiancontinent. Exon et al. (2004b) also suggested that seamountssurveyed during the ZoNeCo 5 survey over the NLHR were alsopart of a seamount chain, erupted through continental crust.

3. Morphology, distribution and ages of the volcanic features

The Tangaroa GA-2426 survey mapped and sampled threedistinct sets of volcanic edifices, grouped by age. The oldest setwas identified on previously acquired seismic data (GA-302) and istypically associated with the Paleocene–Eocene and Eocene–Oligocene unconformities interpreted from the DSDP 208 welldata to the north. The edifices can be seen on seismic profiles tobuild upward from these horizons or basement highs, through ablanket of marine ooze, to be exposed on the seafloor (Fig. 2). Thedetailed shape of the summits is evident on recently acquiredbathymetry data (Fig. 3A). Each cone is surrounded by a moatwhere the currents have eroded sediment. These volcanoes do nothave any preferred alignment within the surveyed area, similar tosome other groups of seamounts in the southwest Pacific (Finnet al., 2003).

The youngest set of cones can be seen clearly on the bathy-metry data and on the seismic sections and have their basesclose to the modern sea floor. These volcanic cones are likely Late

Capel-Faust Basins, Lord Howe Rise, SW Pacific Ocean. Deep-Sea

Fig. 2. Seismic profiles from GA-302 survey (A) Old volcanic cones buried by predominantly Neogene marine sediment. The base of the cones is associated with an older

unconformity, possibly Early Oligocene at the top and, in this case, overlies a basement high. (B) A young volcanic cone with its base within the Miocene ooze.

Fig. 3. Multi-beam bathymetric images. (A) An old volcanic cone with its summit exposed on the seafloor surrounded by a current-eroded moat. (B) Max, one of the younger

cones showing its largely unmodified conical shape. (C) the Gifford Guyot.

DaddK.A. et al. / Deep-Sea Research II ] (]]]]) ]]]–]]] 3

Miocene–Pliocene based on a combination of seismic interpreta-tion, their unmodified conical shape and the freshness of obtainedsamples (Fig. 3B). The cones are in approximately 1500 m waterdepth. The largest cone has a basal diameter of 4 km and extends

Please cite this article as: Dadd, K.A., et al., Cenozoic volcanism of theResearch II (2011), doi:10.1016/j.dsr2.2010.10.048

400 m above the seafloor. These volcanoes form a more linear arrayon the seafloor and may be part of the same north-northwesttrending chain recognised during the ZoNeCo 5 survey (Exon, et al.,2004).

Capel-Faust Basins, Lord Howe Rise, SW Pacific Ocean. Deep-Sea

DaddK.A. et al. / Deep-Sea Research II ] (]]]]) ]]]–]]]4

The third set of samples come from the seafloor approximately30 km east of the Gifford Guyot (Fig. 3C). The Gifford Guyot is partof the Lord Howe seamount chain that extends in a roughly north–south direction and includes the seamounts upon which Elizabethand Middleton Reefs occur. The Gifford Guyot has previously beendated at approximately 15.6 Ma (Quilty, 1993). It has a flat tomoderately sloping top approximately 27�20 km and approxi-mately 293 m below sea level (Slater and Goodwin, 1973) built on abase approximately 42�36 km (Fig. 3C). The steep slopes of theedifice are cut by numerous channels as well as slide and slumpfeatures. The top has rocky reefs in the northwest and southwestand dune fields in the east (Heap et al., 2009).

4. Petrography of the volcanic rocks

Each volcanic edifice identified during the Tangaroa GA-2426survey was described, and where possible, sampled. The youngerLate Miocene–Pliocene cones sampled and described in this paperhave unofficially been named Mount Doom, Max and Smeagol(Fig. 1). The older? Eocene–Oligocene edifices are here groupedtogether and referred to as the older cones.

4.1. Mount Doom TAN0713/17DR06

Four samples of scoriaceous basalt were collected from the flanksof Mount Doom. Irregular to spherical vesicles make up 20–70 volume% of the samples (Fig. 4A). The vesicles are commonly empty, althoughsome are filled with a brownish, possibly calcareous material. Somevesicles were also found to contain foraminifer tests.

The abundance and type of phenocrysts varies within thesample set. In several samples, pyroxene and olivine (o1–5%)

Fig. 4. (A) Photomicrograph of scoriaceous basalt from Mt Doom showing a large skeleta

the photo is approximately 2 mm long. (B) Photomicrograph of basalt from Smeagol show

large carbonate sediment-filled voids that contain planktic foraminifer. The base of the

supported basalt breccia from Max. Clasts are scoriaceous basalt and the matrix is carbona

2 cm. (D) Photograph of a thin section of vesicular basalt breccia from one of the older

Please cite this article as: Dadd, K.A., et al., Cenozoic volcanism of theResearch II (2011), doi:10.1016/j.dsr2.2010.10.048

comprise sub-rounded, strongly altered and/or broken grainstypically smaller than 50 mm but up to 1 mm in length. Plagioclase(5–10%) occurs as thin needles up to 20–100 mm long. All grains arehosted in a dark, red-brown, groundmass (20–25%). Pyroxene andolivine (�10%) in one sample are euhedral (rarely skeletal), up to600 mm in the longest direction, and often have long, sharp edges incontrast to the other Mount Doom samples. Some samples aredominated by an orange-golden carbonate-rich matrix (up to 70%)that contains round foraminifer shells up to 350 mm in diameter.Rare foraminifer are elongate and up to 2 mm in length.

4.2. Smeagol TAN0713/19DR07

Three rounded cobbles of scoriaceous basalt were collectedfrom Smeagol. All samples contain spherical, round or irregular-shaped vesicles that make up 20–40% of the sample and are oftenfilled with fine brown carbonate sediment with foraminifer shells.Samples are all similar in mineralogy comprising thin needles ofplagioclase up to 400 mm long (1–7%), and subhedral to euhedralolivine and pyroxene (3–7%) commonly less than 100 mm, butup to 700 mm in some samples in a dark and/or black groundmass(30–55%) (Fig. 4B).

4.3. Max TAN0713/05DR03

Samples from Max include clast-supported and matrix-sup-ported manganese-cemented basalt breccia and vesicular basalt.The matrix-supported breccia is interbedded with foraminifer-richand basalt-bearing micrite (Fig. 4C). The clast-supported brecciahas rounded to irregular-shaped, scoriaceous clasts up to 2 cm in amatrix of altered brown volcanic glass. The clasts make up 95% ofthe breccia and have delicate edges and blocky morphologies. The

l olivine grain in an altered glassy groundmass with plagioclase needles. The base of

ing a clinopyroxene grain in a glassy groundmass with plagioclase needles. There are

photo is approximately 2 mm. (C) Photograph of a thin section of bedded matrix

te sediment with abundant foraminifer tests. The base of the photo is approximately

cones with a carbonate mud matrix. The base of the photo is approximately 2 cm.

Capel-Faust Basins, Lord Howe Rise, SW Pacific Ocean. Deep-Sea

DaddK.A. et al. / Deep-Sea Research II ] (]]]]) ]]]–]]] 5

matrix-supported breccia is poorly-sorted and vaguely-bedded,with 2–20% rounded to irregular-shaped, scoriaceous mafic clastsin a foraminifer-rich carbonate matrix. Some clasts have a thinmanganese coating. The basalt is aphanitic and has approximately40–50% vesicles filled with zeolites.

The basalt contains small subhedral to anhedral olivine grains(5–7%) typically less than 100 mm but up to 2 mm in length. Noplagioclase was noted and only one sample contains 3–4% pyrox-ene needles up to 250 mm in length. Foraminifers are abundant(o80%) in the matrix and the same carbonate sediment fills somevesicles.

4.4. ?Eocene–Oligocene cones, TAN0713/06DR04

Samples from the ?Eocene–Oligocene cones comprise reddishbrown to grey vesicular basalt breccia with a carbonate mud matrix(Fig. 4D) and samples of scoriaceous basalt with 85% sphericalvesicles. Vesicles are occasionally filled with brown clayey material.The breccia samples have surface manganese encrustations. Pyroxeneand olivine phenocrysts are rare (1–5%) in the basalt samplescomprising anhedral to subhedral grains typicallyo50–100 mmbut with rare olivine grains up to 5 mm length.

Table 1Major and trace element data for selected samples of volcanic rock.

Survey

Code

TAN0713/

37BS09 B1.1

TAN0713/

37BS09_C1

TAN0713/

05DR03 C1.1

TAN0713/

05DR03 E1.1

T

1

Volcano

name

Gifford Gifford Max Max M

SiO2n 46.67 45.41 48.42 43.71

TiO2 2.56 2.88 2.09 2.99

Al2O3 15.01 14.50 13.60 7.73

FeO 3.42 1.95 3.89 5.92

Fe2O3 7.53 9.36 5.11 4.95

MnO 0.11 0.15 0.13 0.15

MgO 6.08 5.44 5.54 8.09

CaO 9.16 8.21 9.52 12.36

Na2O 3.02 2.92 2.86 1.09

K2O 0.80 1.25 0.54 0.34

P2O5 0.28 0.60 1.01 1.14

LOI 4.79 6.90 6.58 10.68

Total 99.43 99.56 99.31 99.14

Bann 195.40 215.91 127.91 83.11

Ce 32.70 42.46 20.30 65.29

Cr 330.88 393.63 242.94 309.44

Dy 3.70 4.23 2.67 5.07

Er 1.85 2.01 1.29 2.14

Eu 1.66 1.96 1.28 2.55

Gd 4.43 5.68 3.32 7.50

Hf 3.85 4.71 3.52 5.67

Ho 0.71 0.79 0.51 0.90

La 15.33 24.29 10.40 29.29

Lu 0.21 0.21 0.14 0.23

Nb 25.94 34.85 39.04 83.51

Nd 17.67 25.55 12.04 36.51

Ni 172.23 173.51 162.66 184.50

Pb 2.07 2.33 2.56 2.32

Pr 4.25 6.16 2.78 8.57

Rb 9.22 25.35 4.38 5.84

Sm 4.33 5.62 3.21 8.27

Sr 370.46 371.37 850.17 325.39 1

Tb 0.67 0.81 0.49 1.04

Th 1.96 2.29 1.41 3.68

V 202.40 254.06 194.50 251.07

Y 21.66 26.62 17.28 30.87

Yb 1.47 1.49 0.98 1.55

Zr 175.54 219.49 162.69 302.47

n Major element data as weight percent;nn trace element data as parts per million.

Please cite this article as: Dadd, K.A., et al., Cenozoic volcanism of theResearch II (2011), doi:10.1016/j.dsr2.2010.10.048

4.5. Gifford Guyot, TAN0713/37BS09

Three vesicular and fine-grained basalt samples were collectedfrom adjacent to the Gifford Seamount. One sample comprisesrounded basalt clasts in a matrix of finer clasts and another sampleappears to be a section of a pillow. Compared to the Mount Doom andSmeagol samples, the most notable difference in mineralogy is theabsence of olivine and the occurrence of small oxide grains. Samplescontain 10–30% plagioclase needles (o100 mm but up to 600 mm), 7%anhedral to subhedral pyroxene (up to 1.5 mm, but typically less than25 mm in length) and ilmenite needles, and rare rounded Al-spinelgrains less than 20 mm in length. All silicates are strongly altered. Thegroundmass is very fine-grained and makes up to 60–70% of samples.Foraminifer shells are frequent indicating that some of the fine-grained material in the samples is carbonate matrix.

5. Geochemistry

5.1. Analytical methods

We analysed nine volcanic rocks for whole rock major and traceelements (Table 1) and a further six samples by electron

AN0713/

7DR06 B1.1

TAN0713/

17DR06 B3.1

TAN0713/

19DR07 B2.1

TAN0713/

19DR07 C2.1

TAN0713/

19DR07 C3.1

t Doom Mt Doom Smeagol Smeagol Smeagol

46.35 46.08 49.99 46.08 47.70

2.42 2.43 2.20 2.24 2.40

13.79 13.81 12.49 12.70 15.20

4.99 3.86 3.16 6.25 3.85

5.46 5.22 5.40 4.06 6.56

0.13 0.11 0.17 0.13 0.44

6.97 6.40 4.35 7.79 4.07

8.82 8.96 7.27 8.81 8.58

3.10 2.87 2.36 2.71 3.48

0.71 0.60 0.56 0.60 0.81

0.75 1.05 0.40 0.72 0.48

5.59 7.78 11.08 6.98 5.77

99.07 99.16 99.44 99.06 99.33

181.43 184.69 127.87 154.41 205.01

30.33 24.56 21.42 36.73 29.49

322.72 354.95 283.10 299.32 250.22

2.80 2.53 2.06 3.33 2.64

1.21 1.11 0.92 1.43 1.15

1.58 1.44 1.20 1.75 1.58

3.77 3.43 2.76 4.53 3.57

3.73 3.66 3.68 3.44 3.87

0.51 0.46 0.38 0.59 0.48

17.72 14.81 9.21 17.70 12.04

0.12 0.11 0.10 0.14 0.12

41.87 44.06 44.98 40.34 44.84

15.35 13.08 11.32 19.38 14.48

200.22 189.63 121.81 220.07 115.64

2.45 2.10 3.21 1.45 4.38

3.80 3.14 2.66 4.66 3.39

3.97 4.52 9.92 5.10 8.83

3.79 3.37 2.85 4.73 3.66

682.17 1913.30 638.81 665.13 797.87

0.55 0.49 0.40 0.65 0.52

1.72 1.69 1.43 1.83 1.73

200.56 196.81 163.97 177.50 220.39

16.38 15.01 11.96 18.74 14.67

0.87 0.77 0.68 1.02 0.84

146.34 161.54 174.15 162.46 174.32

Capel-Faust Basins, Lord Howe Rise, SW Pacific Ocean. Deep-Sea

Table 2Electron microprobe analyses of clinopyroxene for selected samples of volcanic rock.

Survey

Code

TAN0713/

06DR04 F1

TAN0713/

06DR04 F1

TAN0713/

37BS09_C1

TAN0713/

37BS09_C1

TAN0713/

17DR06 B3.1

TAN0713/

17DR06 E1.1

TAN0713/

17DR06 E1.1

TAN0713/

19DR07 C2.1

TAN0713/

19DR07 C2.1

Volcano

name

Older cone Older cone Gifford Gifford Mt Doom Mt Doom Mt Doom Smeagol Smeagol

SiO2 43.29 43.63 49.11 50.73 51.38 51.80 48.50 51.22 51.38

TiO2 4.45 4.23 1.90 1.44 0.98 1.03 2.10 1.06 1.05

Al2O3 9.01 8.76 6.05 3.48 2.51 2.93 5.55 3.22 2.84

FeO 8.72 8.65 7.27 8.18 6.49 6.14 7.14 6.07 6.49

MnO 0.13 0.10 0.14 0.14 0.16 0.12 0.09 0.11 0.15

MgO 10.68 11.12 14.10 14.19 15.50 15.07 13.49 15.06 15.30

CaO 22.07 21.91 21.12 21.62 21.89 22.15 22.32 22.29 21.86

K2O 0.03 0.03 0.00 0.03 0.02 0.03 0.00 0.01 0.02

Na2O 0.64 0.57 0.44 0.44 0.35 0.39 0.49 0.47 0.48

Total 99.25 99.25 100.64 100.26 99.82 100.12 99.95 100.22 100.04

Fig. 5. Total alkalis versus silica diagram with fields after Le Maitre et al. (1989) and

the alkalic/sub-alkalic divide (dashed line) after MacDonald and Katsura (1964) for

the Late Miocene-Pliocene volcanic cones and the Gifford Guyot. All samples are

basalt or alkalic basalt.

Fig. 6. Zr/Ti versus Nb/Y diagram (Floyd and Winchester, 1977) showing that

samples from the Late Miocene-Pliocene volcanic cones and the Gifford Guyot are

alkalic based on immobile trace element abundances. The samples from the Gifford

Guyot have a lower Nb/Y than the younger cones.

DaddK.A. et al. / Deep-Sea Research II ] (]]]]) ]]]–]]]6

microprobe (EMP; Table 2). All whole rock analyses are from maficunits, with four from lava flow units, four from mafic pyroclasticunits, and one from a clast in a volcaniclastic rock. Seven of thesamples are from the Late Miocene–Pliocene cones and two arefrom the Gifford Guyot. Rocks were selected to characterise each ofthe volcanic peaks and from these, the least altered rocks wereanalysed. Mafic rocks analysed by EMP occur as scoriaceous clastsin sedimentary rocks and samples that were too small to be crushedfor whole rock analysis. This set contains all samples from the olderseries of volcanic peaks. For whole rock analyses, rock chips werepulverized in a tungsten carbide mill.

Whole rock major and some trace element analyses wereperformed at Geoscience Australia, using a Philips PW 2404 XRFspectrometer. Loss on ignition values (LOI) were determined byheating the samples to 1050 1C. Trace element analyses wereperformed in the Geochemical Analysis Unit (GAU) at MacquarieUniversity by inductively coupled plasma mass spectrometry(ICPMS) using an Agilent 7500 ICPMS. Electron microprobe ana-lyses were performed using a Cameca SX-100 electron microprobeequipped with five wavelength dispersive spectrometers (WDS)and a PGT energy dispersive system (EDS) in the GAU. Theanalytical conditions employed were: 15 kV acceleration voltage,20 nA sample current and 10 seconds measurement time (peak).Standards used were a combination of natural minerals and puremetals.

5.2. Alteration and metamorphism

Rocks of the study area vary from little to significantly alteredreflecting their age and eruptive style. Alteration is probably the resultof syn-depositional to Late Miocene changes resulting from interac-tion of the igneous rock with seawater. Small volcaniclastic and glassyparticles are most susceptible to alteration. These processes particu-larly affect concentrations of SiO2, Fe2O3, Na2O and K2O. Loss onignition values (LOI) are high and range from 4.79–11.08%.

Despite alteration, all analyses have been retained in the dataset due to its small size. Interpretations of the petrogenesis andtectonic setting are based largely on the abundance and behaviourof the immobile trace elements (Zr, Nb, Y and Ti) and rare earthelements. The systematic behaviour of these elements in thevolcanic rocks suggests that they were immobile during alteration.

5.3. Major and Trace Element Geochemistry

The Cenozoic volcanic rocks are all alkali basalt to basalt incomposition (Figs. 5 and 6). The Nb/Y ratio is less affected byalteration than the alkali major elements used on the traditional

Please cite this article as: Dadd, K.A., et al., Cenozoic volcanism of theResearch II (2011), doi:10.1016/j.dsr2.2010.10.048

TAS diagram and is considered a good indicator of chemicalaffinity (Pearce and Cann, 1973; Winchester and Floyd, 1977).The mafic rocks span a range in Mg# (MgO/(MgO+FeO), from 0.41to more evolved compositions as low as 0.27. Samples from eachvolcanic edifice show a range of Mg#, with those from Smeagolhaving the greatest range. Most of the edifices have samples that

Capel-Faust Basins, Lord Howe Rise, SW Pacific Ocean. Deep-Sea

DaddK.A. et al. / Deep-Sea Research II ] (]]]]) ]]]–]]] 7

cluster fairly tightly or show variation consistent with crystalfractionation. Samples from Max show more variation with onesample, having much lower SiO2, and plotting away from all otheranalyses. Although this sample has very high LOI, all elementalabundances show variation consistent with crystal fractionationand so the sample was not removed from the data set. However its’elevated Ca abundance may indicate contamination by carbonatesediment.

All units have a similar range of major and trace elementabundances (Figs. 7 and 8). SiO2 abundances vary from 43.7–50%and show an overall trend of increasing SiO2 with decreasing Mg#,

Fig. 7. Mg# (explain) vs major element plots. Mg# is used as an index of fractionation.

CaO increases and Al2O3 decreases slightly with increasing Mg# suggesting that

plagioclase is a fractionating phase. The range in Mg# for all units suggests the

involvement of at least one ferromagesium phase.

Please cite this article as: Dadd, K.A., et al., Cenozoic volcanism of theResearch II (2011), doi:10.1016/j.dsr2.2010.10.048

although this is not always the case when individual volcanicedifices are considered. A similar trend is shown by most majorelements and the incompatible trace elements. Ni shows a positivecorrelation with Mg# suggesting that olivine fractionation was animportant control on chemical variation. Cr decreases slightly forall units except those from the Gifford Guyot and may indicatepyroxene fractionation. Samples from Mount Doom have muchmore abundant Sr than the other volcanoes (Fig. 8) and most unitsshow a slight positive Eu anomaly (Fig. 9) suggesting someplagioclase accumulation.

All samples are light rare earth element (LREE)-enriched with Laabundances from 38 to 124 times chondrite (Fig. 9). Heavy rareearth element (HREE) abundances are from 3 to 9 times chondrite.REE profiles are parallel for all samples except those from theGifford Guyot that have flatter HREE. All samples have a profilesimilar to ocean island basalts on a primitive mantle-normalisedtrace element plot with slightly elevated Ta, Nb, Zr and Ti (Fig. 10)and plot in the ocean island basalt field on a Ti-Zr-Y diagram(Fig. 11).

6. Mineral chemistry

Pyroxene, olivine and plagioclase grains from all samples wereanalysed by EMP to characterise their chemistry and to allowcomparison between the three generations of volcanic edifices. Allpyroxene grains are clinopyroxene and probe analyses indicate thatthere is little zoning from core to rim except in more magnesiangrains from the Gifford Guyot. Compositions are aluminian andchromian diopside and augite with rare aluminian ferrian augite inthe Late Miocene–Pliocene basalts and samples from the GiffordGuyot, and aluminian ferrian subsilicic titanian wollastonite todiopside in one of the older cones (Fig. 12). The clinopyroxenegrains from the older cones are significantly more Ti-rich (Fig. 13).All grains from the Late Miocene–Pliocene cones have Mg#between 0.58 and 0.66. Grains from the Gifford Guyot are generallyless Mg-rich but range in Mg# from 0.54 to 0.73 while the singlegrain from the older cones is less magnesian with Mg# from 0.49(core) to 0.50 (rim).

Clinopyroxene analyses are typical of melts crystallising from abasaltic magma but are less magnesian than the grains fromseamounts discovered during the ZoNeCo 5 survey (Mg# 0.89–0.84; Exon et al., 2004b). However, the grains from the ZoNeCo5 survey are very similar to the Late Miocene–Pliocene grains on aplot of atomic Ti versus Ca+Na (Fig. 13) supporting the idea thatthey may be part of single seamount chain or magmatic event. Mostanalyses plot as alkalic based on the discrimination line of Leterrieret al. (1982) consistent with a hotspot origin.

Olivine grains from all series have similar forsterite contentswith compositions from Fo67 to Fo80 and another group with Fo81to Fo93 (Fig. 14). Some of the more magnesian grains are cores withmore iron-rich rims but others are unzoned. The composition ofolivine grains from all volcanic series overlap. Plagioclase grainsfrom the Late Miocene–Pliocene cones have anorthite contentsfrom An56 to An66 and those from the Gifford Guyot have An57 toAn76. No plagioclase grains were identified in the older cones.

7. Discussion

Samples collected from volcanic edifices on the Lord Howe Riseduring the Tangaroa survey can be divided into three groups basedon age, shape and stratigraphic position. Samples from the youngerMiocene cones are classified as alkalic basalt. Mafic rocks from eachof these sampled cones is similar in composition but differencesin Sr and Ti abundances allow two of the three cones to be

Capel-Faust Basins, Lord Howe Rise, SW Pacific Ocean. Deep-Sea

Fig. 8. Mg# vs trace element plots. The decrease in Ni and Cr for samples from the younger cones over their Mg# range indicates the fractionation of clinopyroxene and olivine.

The samples from the Gifford Guyot fall off the trends shown by the younger cones for several of the trace elements.

DaddK.A. et al. / Deep-Sea Research II ] (]]]]) ]]]–]]]8

distinguished. All samples from the young cones have traceelement abundance patterns similar to other ocean island basalts(Fig. 10). Samples from the Middle Miocene Gifford Guyot are alsoalkalic basalt of ocean island affinity and have similar major andtrace elements abundances to the smaller Miocene cones. Theydiffer however in their flatter HREE profile (Fig. 15). The olderedifices were identified in seismic surveys and the base of theseconical volcanoes occurs at the Paleocene–Eocene and Eocene–Oligocene boundaries. No samples were fresh enough for wholerock analysis but analyses of clinopyroxene phenocrysts using EMPindicate they are from alkalic magmas. Clinopyroxene grains can bedistinguished from those from the other cones based on theirhigher Ti abundances (Fig. 13).

The linear distribution of the younger volcanic edifices withinthis region suggests the presence of several hotspots. The Late

Please cite this article as: Dadd, K.A., et al., Cenozoic volcanism of theResearch II (2011), doi:10.1016/j.dsr2.2010.10.048

Miocene–Pliocene cones identified during the Tangaroa surveyalign with the seamounts identified and sampled during theZoNeCo 5 survey (Exon et al., 2004) and may be part of the samechain. The seamounts sampled on both surveys are alkalic basalttypical of seamounts in the area. Clinopyroxene grains for conesfrom both the ZoNeCo 5 and Tangaroa surveys form a single trendfrom low to high Ti, Ca+Na abundances and differ from others inthe sample set (Fig. 13).

The seamount chain identified by Exon et al. (2004b) and thosein this study are made up of small, low-volume volcanoes. Lowvolumes and a broad regional extent are typical of alkalic mag-matism in the SW Pacific (Finn et al., 2003). Finn et al. (2003)suggest that magmatism in this region only occurs where the low-velocity zone lies at less than about 80 km and that the magmas aregenerated from metasomatised upper mantle. Metasomatism may

Capel-Faust Basins, Lord Howe Rise, SW Pacific Ocean. Deep-Sea

DaddK.A. et al. / Deep-Sea Research II ] (]]]]) ]]]–]]] 9

be due to earlier periods of subduction-related magmatism andmodification of the mantle, however, there is no subductionsignature shown on the primitive-mantle normalised trace ele-ment diagram (Fig. 10). Zhang and O’Reilly (1997) looked atMiocene mafic rocks from the Dubbo area of NSW that coincidewith the locus of hotspot-related central-volcano magmatismin eastern Australia. These rocks have typical OIB chemistryand Zhang and O’Reilly (1997) suggested a plume source. Fig. 10shows a comparison of these rocks with the LHR mafic rocks.The overall patterns are very similar, but the Dubbo rocks haveslightly higher Rb, Ba, Th, La, Ce and HREE. Further sampling,geochemistry and age dating of all these sequences, includingfrom outside the current survey area, is required to test thehotspot model.

8. Conclusions

Three groups of volcanic edifices were sampled during the 2007Tangaroa GA-2436 survey: Late Miocene–Pliocene cones, olderprobably Eocene–Oligocene volcanic cones and the Miocene Gif-ford Guyot. The younger cones have a largely unmodified conicalshape and their geochemistry suggests they are alkalic basaltrelated to within-plate hotspot activity and may be part of a chainidentified to the north during the ZoNeCo 5 survey. Samples fromthe older cones were derived from the tops of cones that areexposed above the more recent blanket of marine ooze. Samples

Fig. 10. Primitive mantle-normalised trace element plot (after Sun and McDonough, 1989

The dashed field shows data for central volcanoes of the Dubbo region of NSW, Austral

Fig. 9. Rare earth element profiles with normalising values from Sun and

McDonough, 1989. All data has relatively elevated LREE and the younger volcanic

cones have parallel trends. Samples from the Gifford Guyot have slightly

flatter HREE.

Please cite this article as: Dadd, K.A., et al., Cenozoic volcanism of theResearch II (2011), doi:10.1016/j.dsr2.2010.10.048

were altered and consisted of scoriaceous basalt clasts in carbonatesediment. Clinopyroxene grains from these cones can be distin-guished from the younger cones based on their higher Ca and Ticontents. The Gifford Guyot is part of the Lord Howe seamountchain. Samples from this volcano can be distinguished from allother samples based on the lower Ca+Na of contained clinopyr-oxene grains and from the younger cones based on the flatter HREEprofile of the guyot samples. All samples resemble ocean-islandbasalts in their chemistry, displaying the high Ba, Th, Ta, Nb, Sr andTi typical of rocks from this setting.

). Patterns are similar to ocean island basalts with slightly elevated Ta, Nb, Zr and Ti.

ia (Zhang and O’Reilly, 1997) for comparison.

Fig. 11. Ti-Zr-Y tectonic discrimination diagram of Pearce and Cann (1973).

A¼ island arc tholeiites, B¼MORB, island arc tholeiites and calc-alkali basalts,

C¼calc-alkali basalts, and D¼within-plate basalts. All samples plot in the within-

plate basalt field.

Fig. 12. Classification diagram for Ca-Mg-Fe pyroxenes after Morimoto et al. (1988).

All analyses plot as diopside or augite. Clinopyroxene from the older cones is more

calcic.

Capel-Faust Basins, Lord Howe Rise, SW Pacific Ocean. Deep-Sea

Fig. 13. Atomic Ti versus atomic Ca+Na diagram showing the alkalic character of

most of the clinopyroxene grains; highlighted by the discriminant line of Leterrier

et al. (1982). The fields of the Gifford Guyot, Late Miocene-Pliocene cones and older

cones do not overlap suggesting different mantle sources for the respective magmas.

This is the only diagram clearly separating Gifford Seamount chemically from the

younger and smaller cones.

Fig. 14. Ni versus forsterite content for olivine grains in the younger and older series

of cones. Tie lines are shown for the cores and rims of the more magnesian grains

where possible.

Fig. 15. Plot of Sr-Ti-Yb/Lu for the Late Miocene-Pliocene cones and the Gifford

Guyot. This diagram is designed to separate each volcanic edifice based on the

differences in Sr and Ti between members of the younger cones and the flatter HREE

in the Gifford Guyot. Samples from the Late Miocene-Pliocene cone, Max, overlap

the other fields.

DaddK.A. et al. / Deep-Sea Research II ] (]]]]) ]]]–]]]10

Please cite this article as: Dadd, K.A., et al., Cenozoic volcanism of theResearch II (2011), doi:10.1016/j.dsr2.2010.10.048

Acknowledgments

Kelsie Dadd thanks Geoscience Australia and the University ofthe Sea program for the opportunity to take part in the 2007Tangaroa survey and access samples for analysis. The authors thankHeike Struckmeier and George Gibson of Geoscience Australia, andNeville Exon of the Australian National University, for their reviewof the manuscript. This is contribution 695 from the AustralianResearch Council National Key Centre for the Geochemical Evolu-tion and Metallogeny of Continents (http://www.gemoc.mq.edu.au) and is published with the permission of the Chief ExecutiveOfficer, Geoscience Australia. The analytical data were obtainedusing instrumentation funded by ARC LIEF, and DEST, SystemicInfrastructure Grants, industry partners and Macquarie University.

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