Journal of the Geological Society, London, Vol. 163, 2006, pp. 997–1010. Printed in Great Britain.
Provenance of Cambrian conglomerates from New Zealand: implications for the
tectonomagmatic evolution of the SE Gondwana margin
M A R C U S G U T JA H R 1,2 , J O H N D. B R A D S H AW 1, S T E V E W E AV E R 1 , C A R S T E N M Ü N K E R 3;4 &
T R E VO R I R E L A N D 5
1
Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand
2
Present address: Institute for Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETH-Zentrum,
NW81.2, Clausiusstrasse 25, 8092 Zürich, Switzerland (e-mail: [email protected])
3
Institut für Mineralogie, Westfälische Wilhelms Universität Münster, Corrensstrasse 24, 48149 Münster, Germany
4
Present address, Universität Bonn, Mineralogisch-Petrologisches Institut, 53115 Bonn, Germany
5
Australian National University, Canberra, ACT0200, Australia
Abstract: The oldest rocks in New Zealand are the Mid- to Late Cambrian intra-oceanic island arc rocks of
the Takaka terrane (Devil River arc). The provenance of Cambrian conglomerates stratigraphically above the
exposed arc succession was studied to constrain the late stages of arc evolution and its accretion to continental
crust. The Dead Goat Conglomerate contains two distinct groups of igneous clasts: (1) intermediate to felsic
volcanic clasts with moderately enriched light rare earth element (LREE) and high field strength element
(HFSE) contents and positive ENd500 (+2.1) that were derived from a medium-K calc-alkaline source, probably
the main sequence of the Devil River arc; (2) dioritic to metagranitic plutonic clasts strongly enriched in
LREE and HFSE and with ENd500 of +3.5 to +5.9 that were derived from a high-K arc source, probably the
uppermost units of the Devil River arc. This is consistent with a new U–Pb sensitive high-resolution ion
microprobe age of 496 6 Ma. The Lockett Conglomerate also contains two distinct groups of igneous clasts:
(1) ultramafic to intermediate igneous clasts identified as boninitic to transitional low-K calc-alkaline arcrelated rocks based on depleted REE and HFSE abundances; (2) ‘I’-type metagranitoid clasts derived from a
distinct Andean type continental margin, as indicated by ENd500 as low as 7.1. Both conglomerates contain
sandstone clasts derived from a common old, multi-cycle continental source with ENd500 of 14.2 to 15.7,
and no suitable source has been found in present-day New Zealand. The new provenance data from these
conglomerates constrain the time of accretion of the Devil River arc to the palaeo-Pacific Gondwana margin
and provide new information on the structural evolution of the accretionary event.
coherent internal stratigraphy that differs somewhat from that of
adjacent slices, but that in some instances can be correlated
(Cooper & Tulloch 1992; Rattenbury et al. 1998; Münker &
Cooper 1999). Each slice contains igneous rocks of the Devil
River Volcanics Group and associated Haupiri Group sediments.
The Devil River Volcanics Group is divided into the older,
back-arc tholeiitic Mataki Volcanics, the younger back-arc
boninitic Cobb Igneous Complex, and the calc-alkaline arcrelated rocks of the Benson Volcanics, which evolved through
time from low-K to high-K type magmas (Münker & Cooper
1995, 1999). Basaltic to andesitic compositions are typical in the
Devil River arc (Münker & Cooper 1995, 1999) and felsic rocks
are rare, largely being restricted to the latest stages of volcanism
(McLean 1994; Funk 1996). Both the Benson and the Mataki
Volcanics formations are interbedded with Haupiri Group sediments. Furthermore, the Mataki Volcanics are intruded by dykes
of the Benson Volcanics type (Münker & Cooper 1999), supporting a close spatial relationship during the activity of the Devil
River arc. The back-arc boninitic Cobb Igneous Complex was
emplaced at shallow depths into Haupiri Group sediments and
the Mataki Volcanics.
Münker & Cooper (1999) reported the occurrence of boninitic
volcanic clasts in early Mid-Cambrian (Floran) conglomerates of
the Haupiri Group. Although similar in composition to rocks of
the Cobb Igneous Complex, these boninitic clasts stratigraphically underlie the Cobb Igneous Complex and Mataki Volcanics.
The oldest rocks of New Zealand are exposed in the Early to
Mid-Palaeozoic Takaka terrane in NW Nelson and SW Fiordland
(Fig. 1) (Cooper & Tulloch 1992), which are part of the Western
Province (Landis & Coombs 1967). These oldest units comprise
remnants of the late Mid- to Late Cambrian intra-oceanic Devil
River arc and the associated Haupiri Group sediments, forming
the western part of the Takaka terrane (Fig. 1). In the past two
decades many of the individual sedimentary and igneous suites
have been described in detail (e.g. Grindley 1980; Cooper 1989;
Pound 1993; Münker & Cooper 1999; Jongens et al. 2003). A
first reconstruction of the sequence was largely limited to the
igneous units (Münker & Cooper 1999). However, a fully
comprehensive reconstruction including the various sedimentary
units in the Haupiri Group has yet to be established. The aim of
the present study is to determine the provenance of igneous and
sedimentary clasts in two major conglomerate units within the
youngest sediments of the Devil River arc assemblage, to
constrain the later stages of arc evolution and, if possible, the
time of accretion.
Regional geology
The relics of the Cambrian Devil River arc are restricted to a
north–south-trending belt less than 15 km wide near the western
margin of the Takaka terrane (Fig. 1; Cooper 1989). The belt is
made up of 10 tectonic fault-bounded slices, each with a
997
Fig. 1. (a) Generalized geology of West Nelson–North Westland, South Island, New Zealand, as given by Cooper & Tulloch (1992). The Anatoki Thrust separates the Western Buller terrane from the
Eastern Takaka terrane. Cambrian arc-related rocks are restricted to the Central Belt. Grey box indicates position of the field area shown in (b). Inset: inferred offshore extent of Early Palaeozoic rocks of the
Buller and Takaka terranes. Box in inset indicates location of map (a). MTZ, Median Tectonic Zone. (b) Schematic outcrop of the Lockett and Dead Goat conglomerates North of Cobb Valley. Map is
compiled from Grindley (1980) and Pound (1993), and modified from new observations. Numbers in circles refer to component count locations where the data presented in Table 1 were collected.
Topographic contour interval is 40 m. The simplified stratigraphic column shows the main sedimentary formations of the Devil River arc with the corresponding New Zealand Cambrian stages. Ages for
New Zealand stages are taken from Cooper (2004).
998
M . G U T JA H R E T A L .
L O C K E T T A N D D E A D G OAT C O N G L O M E R AT E S T U DY, N Z
999
ever, clear erosional discordant contacts between the Tasman
Formation and the Lockett Conglomerate are not exposed. In
most cases, later faulting disrupted the contacts between the two
formations. The matrix of the Lockett Conglomerate contains
detrital garnet and amphibole, which Coleman (1978) and Pound
(1993) interpreted to be derived from metamorphic continental
sources.
The uppermost part of the Tasman Formation contains trilobites, including Dorypyge and Nepea cf. avara, that suggest a
Boomerangian age (Fig. 1b) (Münker & Cooper 1999). The
Lockett Conglomerate is regarded as Mindyallan. The conglomerate is over 1000 m thick (Grindley 1980) and the stratigraphic
top is unknown. The Lockett Conglomerate is cut by the Balloon
Mélange, interpreted as a tectonic diapir by Jongens et al.
(2003). Both the Lockett Conglomerate and the Balloon Mélange
are cut by dykes of Gendarme Dolerite with an 40 Ar/39 Ar age of
486 25 Ma (Münker & Cooper 1999).
These clasts will be referred to hereafter as ‘pre-Devil River’
rocks.
The Haupiri Group sediments are interbedded with and overlie
the volcanic rocks and range in grain size from conglomerate to
mudstone, reflecting the wide variety of sedimentary environments in the Devil River arc. The provenance of the Haupiri
Group sediments is variable. Some units comprise predominantly
arc-derived material. The Junction Formation, on the other hand,
predominantly consists of continent-derived quartzo-feldspathic
sandstone, siltstone and channel-fill conglomerate (Pound 1993;
Roser et al. 1996; Wombacher & Münker 2000).
The Haupiri Group comprises mainly intra-arc sediments,
although the interlayering of back-arc tholeiitic Mataki Volcanics
at a number of localities (Siebert 1998; Münker & Cooper 1999)
suggests proximity to or gradation into a back-arc basin. The
distinctly continental character of the Junction Formation has led
to suggestions that the back-arc basin was bounded by the
Gondwana continent (Cooper & Tulloch 1992; Pound 1993;
Wombacher & Münker 2000).
Sampling and analytical methods
In our study, the relative abundance of the various components in the
conglomerates was assessed at one location in the Dead Goat Conglomerate and eight well-separated sites in the Lockett Conglomerate (Fig. 1b,
Table 1). Samples from the Dead Goat Conglomerate were taken from
the section SW of Mount Lockett and samples from the Lockett
Conglomerate were collected along strike between Cobb Valley and
Mount Lockett (Fig. 1b).
Major and trace element concentrations were determined by XRF using
a Philips PW2400 system at the University of Canterbury and following
the method of Norrish & Hutton (1969). All major element contents
discussed below are given loss on ignition (LOI) free. REE and high field
strength element (HFSE) analyses were made by inductively coupled
plasma-mass spectrometry (ICP-MS) using a VG Plasmaquad PQ1
system and following the procedure of Garbe-Schönberg (1993) at the
Universität Kiel, Germany. Cross-calibration of the two methods was
ensured through a comparison of XRF data with ICP-MS data. Results
agree within better than 30%. ICP-MS results are presented in Table 2.
Detailed XRF results are available online at http://www.geolsoc.org.uk/
SUP18248. A hard copy can be obtained from the Society Library.
Felsic igneous clasts and sandstone clasts were analysed for whole-rock
Nd isotope composition and Sm–Nd concentrations by thermal ionization
mass spectrometry (TIMS) at the Institut für Mineralogie, Universität
Münster, Germany, using a VGSector54 multiple-collector system (Table
3). Samarium–neodymium concentrations were determined using a
Geological setting of the Dead Goat and the Lockett
conglomerates
The Dead Goat Conglomerate (new name) is the thinner of the
two conglomerate units and lies stratigraphically near the top of
the Tasman Formation (see simplified stratigraphic column in
Fig. 1b). The Dead Goat Conglomerate is a granule- to cobblesized conglomerate deposited in a broad channel in a marine
environment. Pound (1993) included associated siltstones together with the conglomerate in the Dead Goat Member, the
youngest part of the Tasman Formation. Other sections of the
Dead Goat Conglomerate are likely to be exposed further to the
north of the study area shown in Figure 1b.
The stratigraphically overlying Lockett Conglomerate is more
diverse than the Dead Goat Conglomerate, ranging from granuleto boulder-sized conglomerates, with single boulders reaching
1 m in diameter. At exposed contacts sandstone–siltstone lithologies of the Tasman Formation that are deposited above the Dead
Goat Conglomerate coarsen upward into pebble–cobble conglomerate of the Lockett Conglomerate. The basal Lockett
Conglomerate is marine, but the unit as a whole is regressive and
grades upward into braided fluvial deposits (Pound 1993). How-
Table 1. Summary of eight component counts for the Lockett Conglomerate (LC) and one count in the Dead Goat (DGC) Conglomerate
Count number
Volcanic clasts
Qz-diorites, granitoids
Gabbros, diorites
Ultramafic clasts
Pink granitoids
Purple volcaniclastic breccia
White, grey, black chert
Red chert
Sandstone clasts
Laminated sediment
Unidentifiable
Total
1 (LC)
2 (LC)
3 (LC)
4 (LC)
5 (LC)
6 (LC)
7 (LC)
8 (LC)
LC average (%)
9 (DGC) (%)
32
22
14
3
0
0
19
0
2
0
8
39
23
6
7
0
0
7
5
7
1
5
33
22
12
0
0
0
22
0
5
2
4
40
28
7
2
0
0
17
0
3
0
3
40
20
7
5
0
0
23
0
3
0
2
38
24
10
3
0
0
18
0
3
0
3
31
24
15
4
0
0
20
0
1
0
5
35
17
14
4
0
0
21
0
2
0
7
36
23
11
3
0
0
18
1
3
,1
4
100
15
0
0
6
5
10
17
5
33
5
4
100
Locations of the individual counts are shown in Figure 1b. Fresh surfaces of at least 1 m2 were chosen. Count 6 was counted twice for the calculation of the average
composition of the Lockett Conglomerate (observed surface for count 6 was 2.7 m2 ). The gabbroic and dioritic clasts occur only in the Lockett Conglomerate, whereas the pink
granitoid and volcaniclastic clasts occur only in the Dead Goat Conglomerate. Sandstone clasts are much more abundant in the Dead Goat Conglomerate. The igneous and
sandstone clasts were analysed petrographically and geochemically.
1000
M . G U T JA H R E T A L .
Table 2. Individual major and trace element contents of the mafic to intermediate samples
Unit:
Rock type:
Sample:
Volatile-free
(wt%)
SiO2
TiO2
Al2 O3
Fe2 O3T
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
Total
LOI
Original total
ppm
Li
Sc
Rb
Sr
Y
Zr
Nb
Mo
Cd
Sb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
Tl
Pb
Th
U
Mg-no.
CaO/Al2 O3
Th/Y
Ti/Zr
Zr/Y
Th/Yb
Ta/Yb
LaN /SmN
DyN /YbN
Eu/Eu*
LaN /YbN
LC
boninite
LC94
LC
boninitic
andesite
LC52
LC
boninitic
andesite
LC49
LC
hbmetapyroxenite
LC85
LC
hb–epidiorite
LC130
LC
hbmetadiorite
LC71
LC
hbmetadiorite
LC123
LC
hbmetadiorite
LC63
DGC
diorite
DG102
DGC
volcanic
breccia
DG150
55.15
0.17
12.43
10.65
0.15
10.60
6.69
0.94
3.19
0.02
100.00
2.39
100.21
56.46
0.30
14.32
10.16
0.17
6.56
7.12
3.21
1.67
0.03
100.00
2.06
100.10
58.05
0.27
14.44
7.72
0.09
7.22
5.65
3.43
3.09
0.04
100.00
2.09
100.45
50.40
0.35
18.50
11.46
0.11
6.70
7.19
0.35
4.92
0.02
100.00
2.63
99.94
52.86
0.23
20.01
7.14
0.13
6.63
6.90
1.39
4.64
0.06
100.00
3.16
100.30
53.83
0.19
19.07
7.99
0.14
8.62
3.80
0.54
5.78
0.03
100.00
3.36
100.20
55.97
0.70
18.01
10.12
0.15
5.48
3.34
1.14
5.02
0.07
100.00
2.07
99.32
56.22
0.32
15.35
8.95
0.15
7.97
6.15
1.23
3.61
0.04
100.00
2.41
100.25
56.49
0.83
13.38
14.67
0.11
4.81
4.41
3.81
1.25
0.26
100.00
7.41
100.40
53.30
1.16
14.90
11.95
0.14
2.99
8.38
6.24
0.59
0.36
100.00
3.98
99.97
11.1
41
108
59
6.0
9
0.45
0.10
0.01
0.18
1.45
693
1.03
2.53
0.34
1.61
0.51
0.16
0.69
0.13
0.97
0.23
0.71
0.11
0.82
0.13
0.33
0.05
0.36
0.67
0.32
0.09
58.4
0.54
0.05
110
1.5
0.39
0.06
1.27
0.77
0.80
0.85
9.7
40
49
269
9.6
15
0.56
0.12
0.02
0.18
1.13
751
2.59
4.70
0.61
2.68
0.82
0.31
1.14
0.21
1.56
0.36
1.09
0.17
1.18
0.18
0.54
0.06
0.26
1.55
0.56
0.17
47.7
0.50
0.06
121
1.6
0.48
0.05
2.00
0.86
1.00
1.49
6.4
28
99
148
7.0
27
1.58
0.15
0.07
0.15
2.28
537
3.92
9.14
1.00
4.09
1.02
0.22
1.13
0.19
1.25
0.26
0.77
0.11
0.78
0.12
0.81
0.16
0.48
0.93
1.43
0.41
56.9
0.39
0.20
60
3.8
1.84
0.21
2.43
1.05
0.64
3.40
7.5
45
197
96
5.0
8
0.46
0.10
0.26
0.16
1.52
444
1.99
3.4
0.49
2.12
0.54
0.30
0.65
0.11
0.80
0.18
0.56
0.09
0.60
0.10
0.27
0.05
0.81
1.69
0.25
0.11
45.2
0.39
0.05
269
1.6
0.42
0.08
2.32
0.86
1.54
2.23
9.2
32
119
158
5.6
20
1.50
0.08
0.06
0.12
3.37
451
4.6
7.85
0.92
3.5
0.74
0.31
0.84
0.14
0.94
0.20
0.59
0.09
0.59
0.09
0.65
0.13
0.63
2.23
1.13
0.31
56.7
0.34
0.20
70
3.5
1.91
0.22
3.87
1.02
1.21
5.17
13.4
30
101
118
5.0
15
1.01
0.13
0.04
0.23
8.1
896
2.90
6.77
0.77
3.1
0.73
0.17
0.81
0.14
0.93
0.19
0.57
0.08
0.56
0.08
0.50
0.10
0.44
1.03
0.73
0.13
60.4
0.20
0.15
74
3.0
1.30
0.17
2.49
1.07
0.66
3.48
10.9
33
136
78
9.8
35
2.30
0.24
0.07
0.24
2.66
745
5.6
12
1.40
5.5
1.33
0.39
1.49
0.26
1.69
0.36
1.04
0.15
1.04
0.16
1.10
0.21
0.48
3.12
2.09
0.43
43.4
0.19
0.21
119
3.6
2.02
0.20
2.64
1.06
0.85
3.62
6.2
40
134
129
8.7
30
2.25
0.75
0.07
0.14
1.47
505
5.6
12
1.31
5.0
1.14
0.34
1.29
0.22
1.42
0.31
0.89
0.13
0.87
0.14
0.99
0.19
0.51
2.62
1.97
0.42
55.7
0.40
0.23
63
3.5
2.27
0.22
3.08
1.06
0.87
4.33
12.3
11
14
345
51
337
37
1.32
0.19
1.24
0.60
2232
31
68
8.3
35
8.6
1.98
8.8
1.51
9.7
2.04
5.9
0.87
5.7
0.85
7.9
2.17
0.07
9.8
5.4
1.63
31.7
0.33
0.11
15
6.6
0.95
0.38
2.30
1.10
0.69
3.67
16.4
26
35
80
19
91
9.6
0.31
0.07
1.35
1.84
84
15
36
4.84
20
4.4
0.92
3.9
0.59
3.5
0.72
2.04
0.29
1.95
0.29
2.35
0.51
0.19
14.9
4.3
0.98
26.1
0.56
0.23
76
4.9
2.20
0.26
2.11
1.18
0.67
5.14
Major element concentrations were determined by XRF analysis, trace element concentrations by ICP-MS. LC, Lockett Conglomerate; DGC, Dead Goat Conglomerate.
mixed 149 Sm– 150 Nd tracer. All isotope ratios for samples were calculated
back to an age of 500 Ma. Uncertainties concerning the age of the
granitic rocks (500 15 Ma) have a negligible effect on the initial
isotopic ratios and are within the analytical uncertainty. Neodymium
isotope results obtained by other workers and used here for correlation
were corrected to a La Jolla value of 0.51186.
A U–Pb zircon age was obtained for a metagranitic clast from the
Dead Goat Conglomerate by sensitive high-resolution ion microprobe
L O C K E T T A N D D E A D G OAT C O N G L O M E R AT E S T U DY, N Z
1001
Table 3. Whole-rock Sm and Nd isotope data for igneous and sandstone clasts of the Lockett and the Dead Goat Conglomerate
Sample
Rock type
Dead Goat Conglomerate
DG100
Metagranite
DG107
Metagranite
DG146
Metagranite
DG147
Tuffaceous
breccia
DG150
Tuffaceous
breccia
DG97
Lithic arenite
DG98
Lithic arenite
DG148
Greywacke
Lockett Conglomerate
HL48
hbmetagranodiorite
HL50
Metagranite
HL66
Metagranite
HL84
Metagranite
HL138
Metagranite
HL58
Lithic arenite
HL120
Lithic arenite
HL140
Lithic arenite
143
Nd/144 Ndmeas
147
Sm/144 Nd
ENd500
TDM (Ga)
Nd (ppm)
Sm (ppm)
45.7
25.2
42.7
15.7
9.36
5.45
8.42
3.24
0.51270
0.51260
0.51264
0.51251
0.124
0.131
0.119
0.125
5.9
3.5
4.9
2.1
0.78
1.02
0.84
1.11
24.5
5.53
0.51255
0.136
2.1
1.20
33.5
28.5
28.5
6.17
5.18
5.18
0.51163
0.51161
0.51162
0.111
0.110
0.110
14.2
14.5
14.3
2.25
2.25
2.24
1.41
0.51236
0.123
0.7
1.33
2.50
2.54
4.12
3.73
4.36
5.45
3.66
0.51214
0.51203
0.51251
0.51253
0.51155
0.51155
0.51161
0.129
0.122
0.139
0.154
0.111
0.107
0.108
5.4
7.1
1.1
0.7
15.7
15.4
14.3
1.82
1.86
1.33
1.60
2.36
2.27
2.21
6.95
11.8
12.6
17.9
14.6
23.8
30.9
20.6
ENd500 was calculated using ºSm ¼ 6:543 3 1012 a1 with respect to a chondrite uniform reservoir with a present-day 143 Nd/144 Nd of 0.512638 and 147 Sm/144 Nd of 0.1966.
143
Nd/144 Nd (LaJolla) ¼ 0:511859 23 (2, n ¼ 28) Depleted mantle model ages TDM were calculated after Goldstein et al. (1984). Classification of the sandstone clasts after
Pettijohn (1963). Over a period of 9 months, the La Jolla standard analysed at the University of Münster gave values of 0.511860 23 for 143 Nd/144 Nd (2, n ¼ 28). Sm/Nd
concentrations were determined using a mixed 149 Sm– 150 Nd tracer to a precision of 1% for the respective concentrations and 0.1% for the elemental ratio. One analysed blank
for Sm and Nd was 55 pg and 320 pg, respectively, and is negligible.
(SHRIMP) using SHRIMP II at the Australian National University,
Canberra. Methods have been described in detail by Muir et al. (1996).
Results and a description of the detailed analytical procedure can be
found in the Supplementary Publication (see p. 999).
The Cambrian time scale used in this paper follows that of Davidek et
al. (1998) and Encarnación et al. (1999). Absolute ages in the simplified
stratigraphic column in Figure 1b are taken from a recent compilation by
Cooper (2004) for the Cambrian stages in New Zealand. Detailed
geochemical data for the igneous clasts are given in Table 2 and the
Supplementary Publication. The classification of high-Mg rocks is after
Le Bas (2000). To avoid misinterpretations for the ultramafic igneous
clasts two samples in excess of 1000 ppm Cr and 200 ppm Ni were
dismissed as ultramafic cumulates and excluded from the geochemical
provenance analysis.
Results
Clast populations
Besides quartzose sandstone clasts, which contribute about a third
of the clast population in the Dead Goat Conglomerate, abundant
chert and clasts of diverse igneous origin are present (count 9 in
Table 1). The Dead Goat Conglomerate is dominantly poorly
sorted and only sandstone and chert clasts exceed cobble size.
Accessory amounts of clasts of laminated sandstone–siltstone
occur, which resemble sediments of the Tasman Formation.
The Lockett Conglomerate is dominated by igneous clasts,
which make up almost three-quarters of the clast population
(Table 1). In contrast to the Dead Goat Conglomerate, sandstone
clasts are only a minor contributor (3%). Sorting is very variable
in the Lockett Conglomerate, which Pound (1993) grouped into
eight depositional facies. Several blocks of mafic to intermediate
composition in the Lockett Conglomerate exceed 1 m in diameter, whereas only few granitic clasts reach more than 50 cm.
The clast distribution in the Lockett Conglomerate is remarkably
uniform in different parts of the conglomerate (Table 1).
Petrology of the igneous and sandstone clasts
Igneous clasts in the Dead Goat Conglomerate. Of the various
types of igneous clasts that are present in the Dead Goat
Conglomerate, only two types listed in Table 1 were sufficiently
large to be analysed. The tuffaceous breccia clasts of intermediate to felsic compositions consist of dark red scoria and brown to
white volcaniclastic fragments with rare plagioclase phenocrysts.
Plutonic clasts are mainly pink metagranitic rocks with the
exception of one fresh dioritic sample. Most metagranitic
samples contain zones of granophyric intergrowth of quartz and
partly metasomatically replaced alkali feldspar, indicative of
either rapid cooling or chilling of the melts (Shelley 1993).
Almost all analysed samples contain secondary calcite, quartz
and chlorite. Comparison of the metamorphic mineral assemblage of the matrix and igneous clasts indicates post-depositional
metamorphism of the lower greenschist facies.
Igneous clasts in the Lockett Conglomerate. The majority of
volcanic clasts in the Lockett Conglomerate are effusive rocks of
mafic to intermediate composition. Cryptocrystalline hornblende
and feldspar needles dominate the matrix of the mafic to
intermediate samples, whereas the matrix of the rhyolitic samples
contains mainly cryptocrystalline quartz, an opaque mineral
(presumably magnetite), albitized feldspar and fibrous chlorite.
Phenocryst phases, when present, are hornblende, augite, uralitized pyroxene (possibly opx), albitized and/or sericitized plagioclase in the mafic to intermediate samples, and quartz in the
rhyolitic samples. The compositions of the plutonic clasts range
from ultramafic to highly differentiated. Original ultramafic
mineral assemblages are fully replaced by sericite pseudomorphs
replacing pyroxene. Mafic to intermediate samples have a
mineral assemblage of hornblende, sericite pseudomorphs and
fibrous secondary actinolite. Intermediate samples contain increasing amounts of quartz, decreasing amounts of hornblende
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M . G U T JA H R E T A L .
and actinolite, magnetite, calcite, chlorite and accessory epidote.
Virtually all volcanic and plutonic clasts show evidence of
lower greenschist facies metamorphism with the occurrence of
albitized, sericitized, saussuritized and calcitized feldspars, uralitized pyroxenes (if present), abundant chlorite and calcite in most
specimens, and epidote in a few samples. The common presence
of sericitized and/or albitized feldspar and uralite suggests a high
availability of water-rich fluids (Shelley 1993), contemporaneous
with only minor strain, as indicated by the lack of preferred
mineral orientations. The well-preserved state of alkali feldspars
in the matrix, compared with the highly altered condition of
alkali feldspars within igneous clasts, as well as the lack of
secondary actinolite in the matrix, suggests that the greenschist
metamorphism was pre-depositional.
Sandstone clast petrography. The sandstone clasts in the Dead
Goat Conglomerate and the Lockett Conglomerate are strikingly
similar in terms of petrography, geochemistry and Nd isotope
composition (see below). They are quartzose, medium-grained,
dark greyish green, well-indurated sandstone clasts that resemble
turbiditic sandstones. Biogenic material is absent. Monocrystalline, often undulose quartz grains and alkali felspar dominate the
sandstone clasts. Plagioclase is of minor importance and accessory minerals include detrital muscovite, polycrystalline quartz,
rare micritic limestone fragments, and an opaque phase in
varying proportions. Secondary calcite and chlorite occur
throughout. In the classification scheme of Dickinson et al.
(1983), the sandstone clasts of the two conglomerates suggest a
common, purely continental origin (Fig. 2).
Geochemical compositions of clasts
The igneous clasts have undergone greenschist-facies metamorphism during which the large ion lithophile elements (LILE:
Na, K, Rb, Sr, Ba, Cs, U) were potentially mobilized. The
primary total alkali contents, for example, were severely dis-
Fig. 2. QFL diagram for two sandstone clasts of the Lockett
Conglomerate (r) and the Dead Goat Conglomerate (d), respectively,
compared with the modal composition of the Benson–Tasman Mélange
sandstones and the Junction Formation examined by Pound (1993).
Inferred tectonic settings are from Dickinson et al. (1983). Junction
Formation sandstones contain a significant proportion of lithic fragments,
indicating relative proximity to the Devil River arc during deposition.
turbed during the metamorphic event (Fig. 3b). Therefore the
geochemical discussion focuses on the REE, the HFSE (Zr, Nb,
Hf, Ta), and the elements Th, Sc and Y because these are
essentially immobile up to granulite-facies conditions (e.g. Wood
et al. 1976; Humphris & Thompson 1978).
Igneous clasts in the Dead Goat Conglomerate. The tuffaceous
volcanic clasts have an andesitic composition, covering a relatively narrow range in SiO2 contents from 56 to 63 wt% at MgO
contents between 1.5 to 3.9 wt%. Relative to the Lockett
Conglomerate clasts they are enriched in HFSE and REE (Fig.
3). The analysed plutonic clasts have SiO2 contents above
65 wt% except for one diorite (53.3 wt% SiO2, 4.8 wt% MgO).
A prominent feature of the plutonic clasts in the Dead Goat
Conglomerate is the extreme enrichment in HFSE and REE (Figs
3 and 4a, b). Neodymium isotope ratios for the two volcanic and
three metagranitic clasts are presented in Table 3. The two
volcanic clasts have identical ENd500 of +2.1; the metagranitic
clasts have higher ENd500 values of +3.5, +4.9 and +5.9.
Uranium–lead SHRIMP age determinations were carried out on
a set of 15 zircons from one metagranitic clast of the Dead Goat
Conglomerate. The results obtained give a homogeneous age
population of 496 6 Ma (2 SE) (Fig. 5 and Supplementary
Publication).
Igneous clasts in the Lockett Conglomerate. Plutonic clasts of
the Lockett Conglomerate cover the entire spectrum from ultramafic to highly felsic compositions, whereas most volcanic clasts
have an intermediate composition (Fig. 3). Incompatible trace
element concentrations for both volcanic and plutonic clasts in
the Lockett Conglomerate are much lower than those for the
igneous clasts in the Dead Goat Conglomerate (Fig. 3c and d).
The majority of the volcanic clasts have SiO2 contents between
53 and 63 wt% (Table 2 and Supplementary Publication).
Because of their high MgO contents (from 11.3 to 5.6 wt%,
respectively) most of the less evolved volcanic clasts can be
classified as boninites and boninitic andesites. Volcanic clasts
intermediate between dacitic and highly felsic compositions are
notably absent in the Lockett Conglomerate (Fig. 3). Three
pebble-sized rhyolitic samples have SiO2 contents between 73
and 75 wt%, but are depleted in incompatible trace elements to
the same extent as the mafic to intermediate volcanic clasts (Fig.
3). The plutonic clasts in the Lockett Conglomerate range in
SiO2 contents from 50 to 78 wt%.
For the ultramafic to intermediate samples, the volcanic and
plutonic clasts have similar MgO and trace element contents
(Fig. 3). Relative to normal mid-ocean ridge basalt (N-MORB)
both the volcanic and plutonic (ultra)mafic to intermediate clasts
are depleted in incompatible elements (Fig. 4c–e). In the
chondrite-normalized multi-element diagram the samples show
either a slight light REE (LREE)-enriched pattern characteristic
of subduction-related melts, or a V-shaped, middle REE
(MREE)-depleted pattern typical of boninitic melts (Fig. 4f–h)
(Crawford 1989). No clear covariation between REE concentrations and LaN /SmN , LaN /YbN or Eu/Eu* with increasing SiO2
concentration can be observed (Table 2), therefore the geochemical variation observed cannot simply reflect different degrees of
melt evolution.
Because of their relatively low Fe-number (i.e. Fe2 O3T/
(Fe2 O3T + MgO)) for a given SiO2 content the felsic clasts in
the Lockett Conglomerate can be classified as magnesian
metagranitoid rocks (Frost et al. 2001). Samarium–neodymium
results obtained for five metagranitic samples of the Lockett
Conglomerate gave ENd500 of 7.1 to +1.1 (Table 3). In recent
L O C K E T T A N D D E A D G OAT C O N G L O M E R AT E S T U DY, N Z
1003
Fig. 3. Selected Harker variation diagrams
for clasts of the Dead Goat Conglomerate
compared with samples from the Lockett
Conglomerate. Clasts in the Dead Goat
Conglomerate are significantly enriched in
HFSE and REE (as inferred from the Y
concentration) compared with igneous
clasts in the Lockett Conglomerate.
Samples DG102 and DG150 from the Dead
Goat Conglomerate, which were also
analysed for REE and HFSE, are
highlighted in (c). Most volcanic clasts in
the Lockett Conglomerate have
intermediate compositions, whereas
plutonic clasts range in composition from
ultramafic to highly felsic.
intra-oceanic arcs, the range of ENd is generally confined to
values between +2 and +9 (White & Patchett 1984). Whereas the
ENd500 values of +1.1 to 0.7 for three of the measured clasts
could appear realistic for an intra-oceanic setting, values of 5.4
and 7.1 for the remaining two clasts clearly imply a significant
older crustal component.
Sandstone clasts in the Dead Goat Conglomerate and the Lockett
Conglomerate. Following the classification scheme of Bhatia &
Crook (1986), trace element ratios such as Th/Sc and Ti/Zr in the
quartzose sandstone clasts of both conglomerate units indicate an
active continental margin tectonic setting of deposition (Th/Sc
between 1.8 and 2.4; Ti/Zr between 11 and 15) (Bhatia & Crook
1986). ENd500 and depleted mantle model ages (TDM ) for Lockett
Conglomerate sandstone clasts range from 15.7 to 14.3 and
from 14.5 to 14.2 for sandstone clasts of the Dead Goat
Conglomerate, corresponding to similar TDM of c. 2.2–2.3 Ga.
Discussion
Petrological, geochemical and isotopic lines of evidence point to
an intra-Devil River arc source for the igneous clasts in the Dead
Goat Conglomerate, as well as for the ultramafic to intermediate
igneous clasts in the Lockett Conglomerate. Conversely, the
metagranitic clasts in the Lockett Conglomerate and the sandstone clasts in the two conglomerates are exotic to the Devil
River arc and derived from a different, continental source.
A valuable feature for the identification of clast sources within
the Devil River arc is the previously reported progressive change
of igneous rock compositions through the evolution of the arc
from low-K to high-K compositions (Münker 2000).
Origin of the igneous clasts in the Dead Goat
Conglomerate
In Figure 6, the compositions of the igneous clasts in the Dead
Goat Conglomerate are compared with volcanic samples from
the Devil River arc (intermediate medium-K and high-K rocks
(Benson Volcanic Suite) and felsic high-K rocks (Snowden
Volcanic Suite) (Münker 2000). For the volcanic clasts, there is a
close compositional similarity to the intermediate medium-K and
high-K Devil River arc lavas, notably the enrichments in HFSE
and REE, particularly LREE (see also Fig. 4b). The Nd isotope
composition of the metavolcanic clasts (ENd500 of 2.1) matches
that reported from medium-K to high-K mineral separates from
within the Devil River arc (ENd500 of 2.1–4.1, Münker 2000).
For the metagranitic clasts in the Dead Goat Conglomerate the
agreement is not as good, as highly differentiated plutonic suites
have not yet been reported from the Devil River arc. The high
ENd500 values (3.5–5.9) of the metagranitic clasts in the Dead
Goat Conglomerate strongly suggest an intra-Devil River arc
origin, and the closest equivalents reported so far are felsic highK volcanic rocks emplaced in the latest stages of arc activity (i.e.
Snowden Volcanic Suite, Münker 2000). The extreme enrichment
in incompatible trace elements of the metagranitic clasts (Figs 4
and 6) points to a most feasible origin within the Devil River
arc, produced either contemporary with or subsequent to the
emplacement of the high-K Snowden Volcanic Suite. In comparison with the other plutonic clasts analysed from the Dead Goat
Conglomerate, the dioritic sample plotted in Figure 4 (sample
DG102) displays very high incompatible trace element contents.
This clast could possibly be derived from a different within-arc
source from the remaining more felsic plutonic clasts in the
Dead Goat Conglomerate.
Origin of the ultramafic to intermediate clasts in the
Lockett Conglomerate
The geochemical similarities of the volcanic and plutonic (ultra)mafic to intermediate clasts suggest a common petrogenetic
history. Because a direct assessment of the alkalinity of the
igneous clasts in the Lockett Conglomerate cannot be made, Th/
Yb v. Ta/Yb plots are employed because these elemental ratios
mimic the compositional differences observed between low-K
and high-K igneous suites (Fig. 7a; Pearce 1983). The compositional spread of the ultramafic to intermediate clasts in the
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M . G U T JA H R E T A L .
Fig. 4. N-MORB- and C1-normalized multi-element plots for two clasts of the Dead Goat Conglomerate (a, b) and the Lockett Conglomerate (c–h),
compared with several medium-K and high-K volcanic suites of the Devil River Volcanics (Peel, Cobb Flat, Circular Bush, Snowden Volcanic
Conglomerate and Snowden Volcanic Suite formations). (c–e) N-MORB-normalized and (f–h) C1-normalized multi-element plots of mafic to
intermediate clasts of the Lockett Conglomerate (LC) compared with samples of the ‘pre-Devil River’ source, the Cobb Igneous Complex and the low-K
Benson Volcanic Suite. Literature data are from Münker (2000). Average N-MORB composition is from Hofmann (1988); chondrite composition is taken
from Boynton (1984).
L O C K E T T A N D D E A D G OAT C O N G L O M E R AT E S T U DY, N Z
1005
Lockett Conglomerate observed in Figure 7 corroborates the
variability illustrated in Figure 4c–h of a mixture of boninitic
and low-K subduction-related signatures. Within the Devil River
arc the low-K arc-related suites, the boninitic ‘pre-Devil River’
rocks, and suites of the boninitic Cobb Igneous Complex
presented by Münker (2000) are geochemically virtually identical
to the ultramafic to intermediate clasts in the Lockett Conglomerate (Figs 4 and 7). On the other hand, the geochemical
composition of the medium-K and high-K Devil River arcrelated rocks deviates substantially from that of the Lockett
Fig. 5. Tera–Wasserburg plot for SHRIMP U–Pb zircon data for the
cobble-sized metagranitic clast DG141 of the Dead Goat Conglomerate.
Fig. 6. Trace element variation plots for the plutonic and volcanic clasts
of the Dead Goat Conglomerate (DG), correlated with various medium-K
and high-K rocks of the Benson Volcanic Suite (BV), and five samples of
the high-K Snowden Volcanic Suite (SVS). Samples DG150 and DG102
were also analysed for REE and HFSE contents (see Fig. 4). Correlation
data are from McLean (1994), Funk (1996) and Münker (2000).
Fig. 7. Trace element variation plots for mafic to intermediate volcanic
and plutonic clasts of the Lockett Conglomerate, compared with possible
equivalents and sources. Clasts of the Lockett Conglomerate correlate
best with boninitic samples of the ‘pre-Devil River’ source and the Cobb
Igneous Complex (CIC), and moderately well with low-K Devil River
arc-related samples. Medium-K and high-K Devil River arc-related
volcanic rocks are generally more enriched in these elements.
Comparative data are from Münker (2000). r, Most boninitic sample
analysed by ICP-MS in the Lockett Conglomerate (LC94); m, sample of
the Lockett Conglomerate showing the most typical low-K arc-related TE
pattern (LC123); d, Lockett Conglomerate samples geochemically
intermediate between the two end-member compositions.
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M . G U T JA H R E T A L .
Conglomerate clasts. For this reason we propose a boninitic to
low-K Devil River arc-related source as the origin of these
clasts.
(Fig. 9). On the other hand, data for comparison are scarce and
the source might be either not exposed or no longer preserved.
Origin of the sandstone clasts in the conglomerates
Origin of the metagranitic clasts in the Lockett
Conglomerate
The metagranitic clasts in the Lockett Conglomerate are exotic
to the intra-oceanic Devil River arc and require a nearby
continental source area to supply these clasts. The palaeogeographical position of the Devil River island arc in the Mid- to Late
Cambrian was outboard from the palaeo-Pacific margin of SE
Gondwana. The Australian–Tasmanian–Antarctic section of the
Cambrian palaeo-Pacific margin of Gondwana is the most
proximal continent to the Devil River arc and the metagranitic
clasts are most likely to have been derived from this craton.
Despite the good Nd isotopic agreement between metagranitic
clasts and igneous suites in different sections along this continental margin (Fig. 8) the observed wide range of Nd isotope
compositions alone does not allow an unambiguous identification
of the source area of these clasts. Moreover, only poor agreement
is observed when using Zr and Y as additional proxies to ENd500
Combining the petrological, trace element and Nd isotope data,
the sandstone clasts in the conglomerates can be described as
being derived from a turbiditic quartzo-feldspathic sandstone
suite deposited adjacent to an old cratonic province in which
early Proterozoic rocks were extensively eroded. The very negative ENd500 values preclude a significant input of juvenile arcrelated material.
In an earlier study of the Lockett Conglomerate, Pound (1993)
inferred the sandstone clasts to be derived from the Junction
Formation, a petrologically very similar quartzo-feldspathic
sandstone suite exposed in a separate tectonic slice in the Takaka
terrane (Münker & Cooper 1999). The results of the present
study argue against this conclusion. The sandstone clasts
analysed from the Dead Goat Conglomerate and the Lockett
Conglomerate are petrologically more mature than those analysed by Pound (1993) (Fig. 2). Additionally, the Nd isotope
composition of the sandstone clasts in the Dead Goat Conglomerate and the Lockett Conglomerate is less radiogenic (ENd500 in
Fig. 8. Range in ENd500 of felsic clasts examined from the Lockett and Dead Goat conglomerates compared with published data for felsic igneous suites
along the Australian–Tasmanian–Antarctic segment of the Cambrian palaeo-Pacific margin of Gondwana. d, individual results. All correlation samples
depicted here are derived from igneous suites that were emplaced into continental crust. Data from Antarctica are geographically grouped. Literature
sources: (1) Turner et al. (1993); (2) Foden et al. (2002); (3) Whitford et al. (1990); (4) Borg et al. (1987); (5) Armienti et al. (1990); (6) Cox et al.
(2000); (7) Borg et al. (1990); (8) Wareham et al. (2001).
L O C K E T T A N D D E A D G OAT C O N G L O M E R AT E S T U DY, N Z
1007
detritus from continental sandstones along the Cambrian palaeoPacific margin of Gondwana, following the approach of Wombacher & Münker (2000). Compared with a variety of sandstone
suites from different sections along the palaeo-Pacific continental
margin of Gondwana the sandstone clasts in the Dead Goat and
Lockett conglomerates are the least radiogenic in Nd isotope
composition, reflected in lowest ENd500 (Fig. 10). Although
sandstone sequences of the Kanmantoo Group and the Adelaide
Fold Belt in Australia geochemically and in Nd isotope composition correlate with sandstones analysed from the Junction Formation presented by Wombacher & Münker (2000), this agreement
cannot be observed for sandstone clasts of the Dead Goat and
Lockett conglomerates. The sandstone suites of the Wilson and
Bowers terranes in Antarctica (Henjes-Kunst & Schüssler 2003)
represent the closest equivalent. However, the sedimentary
sequences in the Bowers terrane are unlikely to represent a
possible source but rather are a probable lateral continuation of
the Devil River arc sequence in present-day New Zealand (see
Weaver et al. 1984; Bradshaw et al. 1985).
Quartzo-feldspathic sandstone suites of Cambrian or older age
are widely exposed along the Antarctic Cambrian palaeo-Pacific
margin (Rowell et al. 2001; Goodge et al. 2002), but published
Nd isotope data are scarce (Borg et al. 1990; Turner et al. 1993;
Cox et al. 2000; Henjes-Kunst & Schüssler 2003). The sandstone
clasts in the Dead Goat and Lockett conglomerates were fed
from the Cambrian Gondwana palaeo-Pacific continental margin
but an unequivocal source area cannot be nominated.
Tectonic implications
Fig. 9. Nd isotope ratios of the clasts examined from the Dead Goat and
Lockett conglomerates plotted against Zr and Y concentrations,
compared with several igneous suites from the Cambrian Australian–
Tasmanian–Antarctic palaeo-Pacific margin of Gondwana. No
satisfactory correlation between felsic clasts of the Lockett Conglomerate
and other igneous suites can be observed. The intra-oceanic signature of
igneous clasts deposited in the Dead Goat Conglomerate is reflected in
the offset of this field from those of the other igneous suites. KGSA,
Granites, Kanmantoo Group, South Australia (Turner et al. 1993); NVL,
Granite Harbour Intrusives, Northern Victoria Land (Borg et al. 1987;
Armienti et al. 1990); SVL, Dv1a and Dv1b suites, Southern Victoria
Land (Cox et al. 2000); BGA, granitoids of the Beardmore Glacier Area,
Central Transantarctic Mountains (Borg et al. 1990); LVG, Liv Volcanic
Group, Central Transantarctic Mountains (Wareham et al. 2001).
the range of 14.2 to 15.7) than that of the Junction Formation
(ENd500 in the range of 9.7 to 11.2) (Fig. 10; Wombacher &
Münker 2000). No other sandstone suite of appropriate age and
similar source composition has been reported in New Zealand
(see Cooper (1989) and Roser et al. (1996) for an overview).
Hence, the source is either not exposed in present-day New
Zealand, located in the Antarctic–Tasmanian section of the
Cambrian palaeo-Pacific margin of Gondwana, or has been
eroded and is no longer preserved.
In Figure 10, a plot of ENd500 v. Th/Sc is used to discriminate
Devil River arc-related sedimentary suites fed with volcaniclastic
From consideration of tectonic models proposed in earlier studies
and incorporating the data presented above, our results allow for
a tighter reconstruction of the structural evolution of the
Cambrian palaeo-Pacific margin of SE Gondwana. This region
underwent profound deformation in the course of the Cambrian
Ross–Delamerian orogenic event. Although still under debate,
the favoured structural setting along the Cambrian palaeo-Pacific
margin of Gondwana is that of eastward-dipping subduction in
the SE Australian and Tasmanian segment (e.g. Flöttmann et al.
1998; Meffre et al. 2000; Foster et al. 2005) and westwarddipping subduction in the Antarctic segment of this continental
margin (Kleinschmidt & Tessensohn 1987; Encarnación &
Grunow 1996; Münker & Crawford 2000). Only a small fraction
of the original island-arc sequence is preserved in New Zealand,
but sufficient information is available to conclude that the Devil
River intra-oceanic island arc was formed outboard of the
Antarctic palaeo-Pacific margin.
The Devil River arc-related sediments of the Haupiri Group
are intruded by dykes of the back-arc tholeiitic Mataki Volcanics
Formation (Fig. 11a) (Münker & Cooper 1999), providing strong
support for the back-arc character of the Haupiri Group sediments. The Tasman Formation, which hosts the Dead Goat
Conglomerate, is part of the Haupiri Group. Furthermore, at a
few locations the Lockett Conglomerate rests conformably on
sediments of the Tasman Formation (Fig. 1b), indicating that the
sediments of the Haupiri Group and the Lockett Conglomerate
were deposited in the same sedimentary basin. Both the Dead
Goat Conglomerate and the Lockett Conglomerate contain clasts
that are in part derived from the Devil River arc and the
Gondwana continental margin (Fig. 11b and c), therefore the
back-arc basin of the Devil River arc was situated between the
Gondwana continental margin and the Devil River arc. For this
reason subduction underneath the Devil River arc must have been
westward-dipping. To accrete the Devil River intra-oceanic arc to
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M . G U T JA H R E T A L .
Fig. 10. ENd500 v. Th/Sc for the sandstone clasts of the Lockett Conglomerate and the Dead Goat Conglomerate compared with the compositions of
sandstones and shales from various sources within the Devil River arc and the Junction Formation (Wombacher & Münker 2000), Sandstone and shale
sequences of the Kanmantoo Group and the Adelaide Fold Belt in SE Australia are shown for comparison (Turner et al. 1993), as well as compositions of
various sandstone sequences in the Bowers terrane (BT), Robertson Bay terrane (RBT) and Wilson terrane in Antarctica (Henjes-Kunst & Schüssler
2003). The composition of the sandstone clasts of the Lockett Conglomerate (r) and the Dead Goat Conglomerate (d) are distinct from all other
sedimentary suites shown here. The closest equivalent can be found in a few locations in the Bowers and Wilson terranes (see Henjes-Kunst & Schüssler
(2003) for details).
Fig. 11. Simplified schematic sketch map
illustrating the late-stage tectonic evolution
of the Devil River Arc complex and the
final accretion to the Gondwana continental
margin. Münker & Crawford (2000) have
provided a detailed schematic model of the
early stages in the evolution of the Devil
River Arc. (See text for discussion.)
L O C K E T T A N D D E A D G OAT C O N G L O M E R AT E S T U DY, N Z
the Gondwana continental margin the back-arc basin between the
arc and the continent had to be closed, either by strike-slip
movement or by initiation of a second subduction zone between
the arc and the continent. Subduction-related magmatism has
been reported for the entire Cambrian Antarctic palaeo-Pacific
margin (see references in Fig. 8 caption), hence the initiation of a
second subduction zone is more feasible (Fig. 11b and c).
The U–Pb SHRIMP age of 496 6 Ma for the metagranitic
clast of the Dead Goat Conglomerate (Fig. 5) requires that highK igneous suites of the Devil River arc were uplifted and eroded
immediately after emplacement. In addition, the Lockett Conglomerate contains boulder-sized ultramafic to intermediate
clasts derived from older sequences of the Devil River arc (see
above). To supply these clasts to the Lockett Conglomerate
significant uplift and erosion was necessary to expose the
boninitic to low-K igneous sequences of the Devil River arc to
erosion (Fig. 11c), whereas at the same time the Antarctic
continental margin was close enough to supply boulder-sized
Andean-type metagranitic clasts. Accretion ended with the
emplacement of the diapiric Balloon Mélange in the Latest
Cambrian (Fig. 11d; see Jongens et al. 2003).
We cannot pinpoint an exact location for the accretion of the
Devil River arc to the Antarctic continental margin. However, the
polarity of subduction of the different segments along the SE
Gondwana Cambrian palaeo-Pacific margin (see above), the
timing of events recorded by the Granite Harbour Intrusives in
Antarctica (e.g. Encarnación & Grunow 1996), and the spatial
and temporal distribution of deformational phases along the
Transantarctic Mountains (e.g. Rowell et al. 2001) make the area
of Southern Victoria Land or the northern Central Transantarctic
Mountains the most likely location of accretion of the Devil
River arc to the Cambrian palaeo-Pacific Gondwana margin.
Conclusions
Conglomerate clasts from Cambrian conglomerates in New
Zealand provide important information relevant to tectonic events
in SE Gondwana close to the Mid-Cambrian–Late Cambrian
boundary. A back-arc basin (documented by the tholeiitic Mataki
Volcanics Formation) separated the intra-oceanic Devil River arc
from the Gondwana margin until the Mid-Cambrian. This backarc basin was subsequently closed by the onset of a second
subduction system along the Gondwana margin, allowing Gondwana-derived material to be deposited in the intra-arc basins by
the end of the Mid-Cambrian. The overlying conglomerates of
late Mid-Cambrian age contain granitoids that were probably
derived from the Ross–Delamerian continental margin and were
deposited in a marginal marine to fluvial setting. The coarse
sizes of this debris (.50 cm) imply a continuous subaerial slope
of moderate gradient from continental margin to arc basin. The
most likely position of accretion of the Devil River intra-oceanic
arc to the Cambrian palaeo-Pacific margin was in the area of
Southern Victoria Land or the northern Central Transantarctic
Mountains.
The Brian Mason Trust of the University of Canterbury is thanked for
providing financial support for the fieldwork, the geochemical analysis and
the SHRIMP dating. K. Mezger (ZLG Münster) generously provided access
to the ZLG research facilities for the Sm–Nd isotope analysis. D. GarbeSchönberg in Kiel (Germany) facilitated REE and HFSE analyses. SHRIMP
measurements were conducted at the Australian National University in
Canberra. The research of M.G. was supported by a University of Canterbury’s Master’s Scholarship. The Department of Conservation, New
Zealand, provided free accommodation facilities during the fieldwork in
1009
Cobb Valley, NW Nelson. We are grateful to T. Waight and J. Foden for their
constructive reviews of the manuscript, and D. Peate is thanked for editorial
handling and additional constructive reviews.
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Received 27 January 2005; revised typescript accepted 23 March 2006.
Scientific editing by David Peate