DTD 5
ARTICLE IN PRESS
Lithos xx (2004) xxx – xxx
www.elsevier.com/locate/lithos
Mantle wedge involvement in the petrogenesis of Archaean grey
gneisses in West Greenland
Agnete Steenfelta,*, Adam A. Gardea, Jean-François Moyenb,1
a
Geological Survey of Denmark and Greenland, aster Voldgade 10, DK-1350 Copenhagen K, Denmark
b
Université Claude Bernard Lyon-I, 2 Rue Raphael Dubois, 69622 Villeurbanne Cedex, France
Received 16 October 2003; accepted 2 September 2004
Abstract
The Archaean crust in West Greenland is dominated by grey orthogneiss complexes formed in periods of crustal
accretion at around 3.8, 3.6, 3.2, 3.0–2.9 and 2.8–2.7 Ga. The majority of the gneisses have tonalite–trondhjemite–
granodiorite (TTG) compositions, while subordinate quartz–dioritic and dioritic gneisses have calc-alkaline
compositions. The major and trace element chemistry of gneiss samples has been compiled from three large regions
representing different terranes and ages in southern and central West Greenland, the Godth3bsfjord, Fiskefjord and
Disko Bugt regions. The TTG gneisses are typical for their kind and show little variation, except marked Sr
enrichment in the Fiskefjord area and slight Cr enrichment in a unit within the Disko Bugt region. Thus, while most
of the crust has probably formed from magmas derived by slab melting, local involvement of mantle-derived
components is suggested.
Most of the diorites have geochemical signatures compatible with mantle-derived parental magmas, i.e., elevated
Mg, Cr and flat chondrite-normalised REE patterns. A group of quartz–diorite and diorite samples from the Fiskefjord
region exhibits marked enrichment in Sr, Ba, P, K and REE, combined with steep REE patterns. A similar but much
more pronounced enrichment in the same elements characterises Palaeoproterozoic subduction-related monzodiorites
within the Nagssugtoqidian orogen, as well as carbonatites and carbonatitic lamprophyres within the same part of
West Greenland. We argue that the parental magmas of the enriched diorites are derived by partial melting from
regions within the mantle that have been metasomatised by carbonatite-related material, e.g., in the form of
carbonate–apatite–phlogopite veins. Alternatively, ascending slab melts may have reacted with carbonatite-metasomatised mantle.
Carbonatitic carbonates have high Sr and Ba, and carbonatitic apatite has high P2O5 and very steep REE spectra.
Adding such a component to a peridotite-derived magma produces geochemical features similar to those of sanukitoids,
except that high phosphorus is not described as typical of sanukitoids. We observe that the enriched diorites from
* Corresponding author. Tel.: +45 38 14 22 16; fax: +45 38 14 22 20.
E-mail address: [email protected] (A. Steenfelt).
1
Now at: Department of Geology, University of Stellenbosch, Private Bag X1, 7602 Matieland, South Africa.
0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.lithos.2004.04.054
LITHOS-01184; No of Pages 22
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2
A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
Greenland are sanukitoid-like, although they are not sanukitoids by the original definition, and their genesis requires a
twist to the current models for sanukitoid petrogenesis.
D 2004 Elsevier B.V. All rights reserved.
Keywords: TTG gneiss; Sanukitoids; Mantle carbonatite; Mantle metasomatism; Archaean crust; West Greenland
1. Introduction
The bulk of Archaean continental crust consists of
grey gneiss complexes (e.g., Windley, 1995), predominantly sodium-rich granitoid rocks belonging to the
tonalite–trondhjemite–granodiorite (TTG) suite, as
opposed to the more potassium-rich calc-alkaline
granitic rocks that predominate in more recent
continental crust. The major and trace element
characteristics of the TTG suite have been described,
e.g., by Arth and Hanson (1975), Barker (1979),
Drummond and Defant (1990) and Martin (1994).
Archaean TTG complexes are generally polyphase and
record a complex succession of intrusion, deformation
and partial melting events and are commonly associated with dioritic orthogneisses of calc-alkaline
affinity. They often also contain amphibolite inclusions, which may both represent disrupted basic dykes
(e.g., Martin, 1994) and remnants of older mafic crust
that was magmatically or tectonically intercalated with
the grey gneiss precursors (e.g., Garde, 1997).
It is now well established by experimental data (e.g.,
Wolf and Wyllie, 1994; Rapp and Watson, 1995;
Zamora, 2000) compared with natural rock compositions (e.g.. Barker and Arth, 1976; Martin, 1987;
Drummond and Defant, 1990; Martin, 1994) that TTG
melts are generated by partial melting of hydrous basalt
in the garnet stability field, although the geodynamic
setting of their petrogenesis remains controversial.
Two end-member hypotheses persist (along with
hybrid or intermediate scenarios): (1) Archaean TTGs
were formed in hot plate-tectonic subduction zones, by
partial melting of the subducting slab (Martin, 1986;
Peacock et al., 1994; Martin and Moyen, 2002), and (2)
TTGs were formed by partial melting of underplated
hydrous basalt, either at the base of the continental
crust or in overthickened oceanic crust (basaltic
plateaux; Rudnick et al., 1993; Albarède, 1998).
Regardless which of the two main scenarios is
preferred, it is generally accepted that syn- to post-
kinematic potassium-rich granitic rocks that are commonly associated with TTG suites are probably derived
from the latter by local or regional partial melting
(Querré, 1985; Sylvester, 1994; Windley, 1995; Berger
and Rollinson, 1997; Moyen et al., 2003).
It is also well known that Archaean grey gneisses are
more complex than simply TTGs and their melt
products. Recent work has shown that, besides being
derived from partial melting of hydrated basalt, they
may also contain signs of geochemical interaction with
the mantle (e.g., Rudnick et al., 1993; Martin and
Moyen, 2002). It has also been suggested that a
significant part of the late Archaean K-rich components
within grey gneiss complexes may not be related to
crustal recycling, but represent slab melts altered by
geochemical interaction with normal or metasomatised
mantle (Shirey and Hanson, 1984, 1986; Stern et al.,
1989; Stern and Hanson, 1991; Rapp et al., 1999;
Moyen et al., 2001; Martin et al., 2005). Furthermore,
the degree of mantle interaction may have increased
over time—perhaps related to the progressive cooling
of the Earth and an inferred increasing depth of slab
melting (Martin and Moyen, 2002; Smithies and
Champion, 2003; Martin et al., 2005).
One of the volumetrically minor but important
component of the Archaean gneisses is known as
sanukitoids, which generally occur as syn- to latetectonic intrusions, occasionally as bdark gneissesQ,
interlayered with classical TTGs. In the original
definition of Shirey and Hanson (1984), sanukitoids
are diorites to granodiorites with high Mg# (N0.6),
high Ni (N100 ppm) and high Cr contents (200–500
ppm), together with relatively high K, Sr, Zr and Nb.
They have strong LREE enrichment (CeN N100 and
[Ce/Yb]N =10–50). The term bsanukitoidQ has since
been used for a range of dark diorites to granodiorites
found within the Archaean crust, and ranging from
btrueQ sanukitoids exactly matching the initial definition to mafic or intermediate rocks associated with
syenites or other alkali intrusives (Lobach-Zhuchenko
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
et al., 2003; 2005). The term has also been used by
Moyen et al. (2003) for intermediate rocks with high
LREE and [Ce/Yb]N , but with lower Mg# (~0.45) and
Cr (~120 ppm), corresponding to the broader
bsanukitoid suiteQ of Shirey and Hanson (1984).
There is no good agreement on the petrogenesis of
sanukitoids; part of the problem, maybe, arises from
the variety of rocks that have been called bsanukitoidQ.
However, two main models have been proposed for
the rocks matching the original definition, which both
call for the involvement of two components, a slab
melt (i.e., TTG-like magma) and a peridotitic mantle
wedge. In the first model (e.g., Balakrishnan and
Rajamani, 1987; Stern and Hanson, 1991; Krogstad et
al., 1995; Smithies and Champion, 2000), sanukitoids
are the product of partial melting of a mantle that has
been metasomatised by slab melts. In the second (e.g.,
Rapp et al., 1999; Moyen et al., 2003), sanukitoids are
formed from hybridised slab melts that have assimilated olivine during their ascent through the mantle.
Both models can account for the high Mg# combined
with a bTTG-likeQ incompatible element signature.
However, despite attempts by, e.g., Martin et al.
(2005), it proves difficult to convincingly decide
between the two models on petrological or geochemical grounds, and they probably represent two
end members of a whole range of processes, depending on the effective melt/rock ratio (Rapp et al., 1999;
Martin and Moyen, 2002). It should nevertheless be
stated that the first model allows sanukitoids to be
generated at any time after a subduction event and not
necessarily during the subduction itself.
In summary, at least four main components are
likely to have contributed to the petrogenesis of
Archaean grey gneiss complexes: (1) pure slab melts,
i.e., partial melt products of hydrous basalt in the
garnet stability field producing rocks of the TTG
family; (2) partial melts of a peridotitic mantle wedge,
where melting is triggered by fluids derived from
dehydration of the subducting slab producing calcalkaline rock suites; (3) melts derived from a
metasomatised mantle wedge or from slab melts
interacting with metasomatised mantle producing
sanukitoids; and (4) partial melts from preexisting
continental crust, such as older TTGs or sedimentary
rocks producing K-rich granodiorites and granites. All
four components are recognised within the generally
well-preserved and excellently exposed Archaean
3
grey gneiss complexes in Greenland, which have
hitherto largely been overlooked in the debate about
Archaean continental crustal evolution.
In this paper, we present chemical data on grey
gneisses from three large regions in southern and
central West Greenland representing three important
crust forming events between 3.8 and ca. 2.8 Ga. Our
aim is to compare subduction-related gneiss complexes over space and time and discuss mantle
involvement in the genesis of such complexes. Our
data do not support Martin and Moyen’s (2002)
observation that the depth of slab melting and, hence,
the degree of mantle interaction have increased over
Archaean time. Conversely, certain rock associations
in our data have geochemical signatures resembling,
although not matching, those of sanukitoids. They are
spatially associated with 3.0- to 0.16-Ga-old carbonatites (see below), and we suggest that their parental
magmas were derived from, or interacted with, a
mantle wedge metasomatised by carbonatite-related
components. Additional data from predominantly
calc-alkaline orthogneisses of Palaeoproterozoic age
in central West Greenland corroborate this suggestion.
2. Geological setting of the study areas
Archaean crust underlies all of West Greenland
(Fig. 1). The Archaean craton, a part of the North
Atlantic craton also comprising areas of Labrador,
East Greenland and Scotland, has escaped postArchaean deformation. The exposed Archaean crust
comprises an estimated 85% of orthogneisses dominated by TTG compositions, and 15% supracrustal
associations dominated by mafic metavolcanic rocks.
They have all been metamorphosed at amphibolite to
granulite facies conditions. Previous zircon geochronology of major TTG suites in various parts of West
Greenland documents continental crustal accretion at
around 3.8, 3.6, 3.2, 3.0–2.9 and 2.8–2.7 Ga.
Several tectono-stratigraphic terranes have been
recognised in the Archaean craton. The different
terranes appear to have been assembled by continent–continent collision around 2.7 Ga (Friend et al.,
1988). To the north, the Archaean craton was variably
reworked during Palaeoproterozoic orogenesis, and to
the south, the slightly younger Ketilidian orogen
(Garde et al., 2002) was accreted to the southern
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4
A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
Fig. 1. Index map and geological overview of Greenland with study areas. The Palaeoproterozoic metaigneous complexes in the
Nagssugtoqidian orogen are the Arfersiorfik quartz–diorite (AQD) and the Sisimiut intrusive complex (SIS).
ARTICLE IN PRESS
A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
margin of the Archaean block at 1.8 Ga. This orogen
largely consists of a major juvenile continental arc and
its deformed and metamorphosed erosion products.
Palaeoproterozoic orogeny in central and northern
West Greenland has traditionally been regarded as
comprising two distinct belts, the Nagssugtoqidian
orogen in the south and the Rinkian fold belt in the
north, but recent geochronological and structural
evidence suggests that the two belts may be different
domains within the same large-scale continental
collision zone (Garde et al., 2003; Thrane et al.,
2003). The suture between the two continents has not
been identified with certainty but is probably located
in the central part of the Nagssugtoqidian belt.
Besides intensely reworked Archaean basement, this
region hosts Palaeoproterozoic lithologies likely to be
associated with the opening and closure of an ocean,
including subduction-related calc-alkaline orthogneisses (Kalsbeek et al., 1987; van Gool et al., 2001).
2.1. Early Archaean orthogneiss complexes at
Godthåbsfjord, Akulleq terrane
The earliest continental crust in Greenland is the
ca. 3.8- to 3.6-Ga-old Itsaq Gneiss Complex within
the Akulleq terrane (Nutman et al., 1996). The Itsaq
gneiss complex is dominated by tonalites and trondhjemites, but also comprises dioritic rocks, besides
rare granodiorites and granites, and was metamorphosed under middle to upper amphibolite facies and
locally granulite facies conditions during several early
and late Archaean episodes. The chemistry and Nd
isotopic signature of the 3.8 Ga TTG suite agrees with
an origin as slab melts (Nutman et al., 1996), whereas
late, 3.6-Ga sheets of biotite granite are interpreted as
partial melts of the TTG gneisses. A 3.6-Ga augen
gneiss suite that includes ferrodiorites and ferrogabbros was regarded by Nutman et al. (1984, p. 25) as
incomplete mixtures of melted deep sialic crust and
fractionated basic magma ponded at the crust–mantle
interface.
The Akulleq terrane also hosts the 2.8-Ga-old
Ikkattoq gneiss complex (McGregor et al., 1991) of
granodioritic composition, which is related to another
major, late Archaean event of subduction, continental
crustal accretion and regional thrusting. Widespread
anatexis and granite veining at 2.7 Ga in the Akulleq
and neighbouring Akia and Tasiusarsuaq terranes are
5
related to the assembly of the three terranes and represent the first common event that has been recognised in all of them. The youngest Archaean event in
the Akulleq terrane is the intrusion at 2.55 Ga of the
Qôrqut granite complex considered to have originated
by partial melting of older TTG gneisses (Friend et al.,
1985). Here, we present chemical data from the
tonalitic Itsaq gneiss complex (13 samples) and of
the augen gneiss suite (12 samples, Table 1, Fig. 2).
2.2. Mid-Archaean orthogneiss complexes at
Fiskefjord, Akia terrane
The high-grade gneiss–amphibolite terrain of the
Fiskefjord area in the central Akia terrane was
studied in detail by Garde (1997) and comprises a
high proportion of grey gneisses with TTG and
dioritic compositions that are intercalated with
amphibolites. The grey gneisses are interpreted as
arc-related magmas generated and accreted during
two major crust-forming events at around 3.2 and 3.0
Ga ago. The dioritic gneisses comprise the 3.2 Ga
Nordlandet diorites with calc-alkaline chemical characteristics and the 3.04 Ga Qeqertaussaq diorite with
distinctly different chemistry, such as high P, Sr, and
Ba and fractionated REE patterns (La/YbN N20). The
latter rock unit contains abundant accessory apatite
besides equant calcite grains (0.1–0.5 mm in size),
which are apparently in equilibrium with the highgrade metamorphic silicate assemblage. According to
Garde (1997), the chemistry of the TTG gneisses
agrees with an origin as slab melts, while a
considerable mantle component was probably
involved in the generation of the diorites. Garde
(1997) further suggested that the Qeqertaussaq diorite
has sanukitoid affinity, and that metasomatised
mantle was involved in the genesis of the precursor
magmas. Large homogeneous, late-kinematic complexes with TTG compositions were interpreted as
variably fractionated slab melts, whereas several
bodies of late-kinematic granodiorite and granite are
probably the results of crustal remelting from local
grey gneiss sources.
Here, we include chemical data from 121 samples
of TTG gneisses in amphibolite, granulite and
retrogressed granulite facies, besides 20 samples of
Nordlandet diorites and 22 samples of the Qeqertaussaq diorite (Table 1, Fig. 2).
Godth3bsfjord
Fiskefjord
Disko Bugt
Nagssugtoqidian orogen
Ilulissat
Carbonatitic components
Qaqarssuk
Kangaatsiaq
Itsaq Ferrodiorite Amph. Granulite Retro
gneiss
facies facies
from
gran.
fac.
Nordlandet Qeqertaussaq Grey Atâ
Ilulissat CalcSanukitoid AQD
gneiss tonalite diorite alkaline diorite
diorite
AQD
SIS
SIS
Monzo- Shondiorite kinite
SiO2
N64%
b64%
N64%
N64%
b64%
b64%
N64%
b64%
N64%
b64%
b64%
N
SiO2
TiO2
Al2O3
FeOa
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Mg#
Ba
Co
Cr
Cu
Ga
Hf
Nb
Ni
Pb
Rb
Sc
Sr
Th
U
V
Y
Zn
Zr
La
Ce
Nd
Sm
Eu
Tb
Yb
Lu
Sr/Y
(La/Yb)N
13
69.79
0.31
16.23
2.12
0.03
0.94
3.57
5.06
1.08
0.08
0.64
0.46
136
36
14
3
22
3.1
1.5
17
10
37
5
416
0.7
0.3
20
3.5
50
122
5
11
6
1.3
0.5
0.2
0.3
0.0
134.0
13.1
12
54.37
1.23
15.35
8.14
0.13
3.06
6.05
3.33
2.26
0.40
0.79
0.27
202
36
18
50
25
1.8
5.6
38
18
35
15
259
1.3
1.1
115
38.7
85
147
18
54
37
7.4
1.2
0.6
2.5
0.3
6.0
6.0
16
19
71.58 70.56
0.27
0.29
15.57 15.67
1.84
3.34
0.04
0.06
0.58
0.95
2.95
3.80
4.90
4.38
1.75
0.73
0.09
0.11
0.17
0.33
0.38
0.40
508
364
21
72
10
11
5
10
18
18
2.8
2.0
3.6
3.0
6
9
20
7
77
2
3
8
376
300
3.5
b0.5
0.7
b0.5
18
31
5.5
5.0
45
53
94
126
16
14
26
22
10
10
1.7
1.5
0.5
0.9
0.1
0.3
0.4
0.6
0.1
0.1
74.8
61.8
26.6
13.0
86
70.74
0.26
16.02
1.69
0.03
0.72
3.08
5.08
1.24
0.08
0.36
0.43
676
62
7
8
18
2.0
2.1
5
12
11
5
681
b0.5
b0.5
24
2.0
41
104
13
19
8
1.3
0.7
0.2
0.4
0.1
357.5
54.1
20
57.08
0.73
17.00
7.26
0.13
3.67
7.69
4.06
0.64
0.17
0.52
0.46
212
55
75
14
20
2.7
5.5
52
7
3
18
223
b0.5
b0.5
141
19.0
86
114
12
22
12
2.8
1.0
0.5
1.6
0.2
12.2
4.8
22
59.23
0.61
17.33
5.49
0.11
3.03
5.64
5.30
1.30
0.36
0.83
0.49
1375
40
29
21
22
2.0
5.4
24
18
10
14
1185
0.7
b0.5
105
16.0
92
113
45
75
34
6.2
1.7
0.7
1.4
0.2
76.2
23.3
63
88
69.74 69.33
0.29
0.38
15.58 15.42
2.35
2.80
0.03
0.04
0.83
1.08
3.06
3.39
5.00
4.60
1.73
1.45
0.10
0.11
0.50
0.70
0.39
0.40
457
415
6
60
18
41
7
8
20
19
4.0
4.3
4.1
9
8
13
10
63
47
4
4
387
422
6.7
6.1
b0.5
33
11
5.8
8.8
66
65
114
122
21
22
40
42
12
20
2
3.7
0.6
1.0
0.0
0.4
0.4
0.8
0.1
0.1
62.0
50.0
32.7
26.9
N64%
N64%
5
57.89
0.73
16.12
7.83
0.14
4.11
7.28
3.88
1.23
0.15
0.54
0.48
150
102
49
b64%
2
55.04
1.82
16.45
7.88
0.11
4.11
5.76
3.82
2.79
0.69
1.11
0.48
953
72
31
45
26.5
57
29
44
59
15
238
4.0
0.9
105
26.5a
93
175
17
34
12
2.8
1.0
0.0
2.3
0.4
9.2
5.8
14
114
230
1138
4.5
121
21.0
94
109
17.0
112
13
35
24
6.4
1.6
1.0
3.0
0.4
58.4
54.0
88
176
81
12.2
2.9
0.9
1.1
0.1
14.9
3.1
N64%
b64%
4
7
4
10
6
8
1
1
66.99
57.59 70.12
59.62
49.37
38.46
3.34
8.24
0.54
0.86
0.43
1.13
1.64
1.36
0.15
0.18
15.57
16.93 14.23
15.88
16.03
10.56
0.03
2.06
3.30
6.09
3.50
7.73
9.53
9.52
8.76
2.94
0.07
0.13
0.05
0.10
0.12
0.16
0.35
0.24
2.06
3.15
1.26
2.72
4.53
7.58
14.97
4.69
4.07
6.48
3.78
5.74
6.72
13.23
30.28
42.36
3.65
4.01
3.72
3.65
2.89
2.22
0.53
0.22
2.06
2.03
1.40
1.68
4.73
5.10
0.10
1.77
0.16
0.42
0.12
0.31
1.60
3.73
4.40
6.79
0.20
0.31
0.39
0.27
1.14
5.16
35.85
28.29
0.47
0.48
0.39
0.44
0.47
0.61
0.80
0.79
1695
1053
938
1697
7771
6233
53
1185
52
45
24
35
20
11
32
28
16
31
39
163
0
0
26
26
14
20
22
70
21
68
21
19
19
19
20
18
2
9
3.6
4.7
9.0
10.0
6.0
10.0
3.5
10.2
8.9
23.0
8.0
57.0
14
18
10
15
19
123
0
13
9
8
35
73
99
6
47
60
16
27
94
109
0
33
5
15
10
17
21
12
38
12
598
952
382
583
3182
3372
3381
8295
3.0
5.0
1.5
1.2
8.4
36.5
0.0
3.0
0.5
0.5
0.0
61
121
48
123
170
117
103
14
8
19
11
22
38
48
28
76
68
87
41
96
115
154
51
102
125
132
110
248
210
517
12
56
35
32
14
34
300
644
168
542
64
70
30
68
532
1430
663
997
26
42
15
42
230
731
186
497
3.5
7
33
88
29
60
1.5
1.9
7.0
19.4
6.7
14.3
0.3
0.7
2.3
4.2
1.6
4.0
1.2
1.9
1.0a
1.9a
2.6
2.5
0.9
3.6
0.2
0.3
0.4
0.3
0.5
111.0
43.7
36.7
26.5
83.5
67.7
121
109
30.3
9.3
11.9
15.1
83.1
211
129
104
N: number of samples.
AQD: Arfersiorfik Quartz Diorite (Kalsbeek, 2001). SIS: Sisimiut Intrusive Suite.
Godth3bsfjord data from Nutman et al. (1984, 1996). Qaqarssuk data from Knudsen (1991). Composition of mantle apatite from Belousova et al. (2002).
a
Calculated concentration of Y or Yb using Y=11.6Yb; the equation is based on the regression of all TTG gneiss samples for which both Y and Yb have been determined.
5560
5560
13175
170
102
125
1535
1167
3005
2497
1312
1185
219
201
51.6
49.5
14.6
14.8
5.5
5.1
0.8
0.6
193
158
9
1761
4620
1036
104
32
60
6.2
0.9
197
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b64%
4
61.98
0.70
16.02
6.15
0.10
2.76
5.79
3.41
1.44
0.17
0.96
0.44
357
320411 320511 320411 320511 Mantle
Rock
Rock
Apatite Apatite apatite
A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
b64%
6
Table 1
Compositions of orthogneiss complexes in West Greenland (expressed as median element concentrations), compositions of two whole rocks and apatite from the Qaqarssuk carbonatite (north of the Fiskefjord area) and composition of apatite
from carbonatised mantle
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2.3. Late Archaean TTG complexes in the Disko
Bugt region
The study area comprises the Kangaatsiaq and
Ilulissat areas, collectively termed the Disko Bugt
region (Garde and Steenfelt, 1999). The Archaean
basement of the presumed northern continent, in
relation to the inferred Nagssugtoqidian suture zone,
is dominated by TTG orthogneiss complexes with
protolith ages of around 2.8 Ga. The most common
lithology is deformed, polyphase, biotite orthogneiss
in amphibolite facies, except for a few patches in the
southern part of the study area, where granulite facies
conditions were attained. Other lithologies in this
region comprise Archaean, as well as Palaeoproterozoic supracrustal sequences. In the Ilulissat area, two
homogeneous granitoid complexes have been distinguished during geological mapping, namely, the Atâ
tonalite and the Rodebay granodiorite (Garde and
Steenfelt, 1999; Kalsbeek and Skjernaa, 1999). The
former has TTG chemical characteristics, whereas the
latter has more potassium than typical TTG gneiss and
is not included in this study. Quartz–dioritic to dioritic
orthogneisses are subordinate in the entire region,
making up less than 5% of the outcrop area. Their
field relations show that they are generally older than
the TTG gneisses. Two kinds of dioritic enclaves have
been distinguished: one with calc-alkaline chemistry
and poorly fractionated REE patterns (La/YbN b5 )
and one with fractionated REE (La/YbN N40) and
elevated K and Sr. The latter has tentatively been
considered to be of sanukitoid affinity (Steenfelt et al.,
2003).
The entire region appears to have experienced
crustal melting at ca. 2.7 Ga, whereby small granite
bodies, pegmatite sheets and widespread migmatites
were formed. Palaeoproterozoic heating, on the other
hand, has only rarely resulted in melting.
The chemical data presented here (Table 1, Fig. 3)
comprise 87 samples of grey gneiss with TTG
composition, 88 samples of the Atâ pluton and 13
samples of diorite, including 2 with sanukitoid affinity.
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
Fig. 3. Simple Precambrian geology and sample locations in the Disko Bugt region with the towns Kangaatsiaq and Ilulissat.
2.4. Palaeoproterozoic calc-alkaline metaigneous
complexes of the Nagssugtoqidian orogen
The Nagssugtoqidian orogen contains the two
juvenile Palaeoproterozoic, calc-alkaline Arfersiorfik
and Sisimiut magmatic complexes. Both were
emplaced between 1.92 and 1.87 Ga, presumably
during the subduction of oceanic crust preceding
continent–continent collision (Kalsbeek and Nutman,
1996; Connelly et al., 2000; van Gool et al., 2001).
The Arfersiorfik complex, which embraces both
intermediate metavolcanic rocks and an intrusive
quartz–diorite, is interpreted as the extrusive and
intrusive members of a volcanic arc. The Sisimiut
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
9
Fig. 4. Simple geology and sample locations in the central Nagssugtoqidian orogen (Arfersiorfik and Sisimiut complexes). The sample locations
to the west of the main Arfersiorfik complex are from folded sill-like intrusions that are not shown at this scale.
complex is a suite of gabbroic, dioritic to granodioritic, and monzodioritic to syenitic rocks interpreted as
continental arc rocks. The extent of the Sisimiut
complex is not known in detail, and Fig. 4 only shows
an approximate outline.
Chemical data from the two Palaeoproterozoic
complexes, for which a mantle wedge origin is most
likely, are included here for comparison with similar
Archaean lithologies, for which the involvement of
mantle in their origin is suspected. The chemical
data set included in this paper (Table 1, Fig. 4)
comprises 11 samples from the Arfersiorfik quartz–
diorite (Kalsbeek, 2001) and 20 samples from the
Sisimiut complex (Kalsbeek and Nutman, 1996),
including six monzodiorite samples (Steenfelt, 1994,
1996).
3. Geochemical signatures of grey gneisses in West
Greenland
The gneiss samples from each of the regions
outlined in the previous section are divided into
groups and are plotted on Figs. 6–12 according to
their chemistry and field unit. Following Moyen et
al. (2003) and Martin et al. (2005), samples with
N64% SiO2, Na2O between 3% and 7%, and K2O/
Na2O b0.5 are designated TTG. They make up the
largest number by far of the sample collections from
the Archaean regions. Samples with quartz–dioritic,
monzodioritic and dioritic compositions have SiO2
between 50% and 64%. In the following, quartz–
diorites and diorites are collectively termed diorites.
3.1. TTG gneisses
The diagrams in Figs. 5–7 illustrate that the
Archaean TTG gneiss samples exhibit the general
TTG characteristics of low MgO and Cr, and high Sr/Y,
but there are also some regional differences. Among the
three Archaean TTG groups, those from Godth3bsfjord
have the lowest MgO, Sr/Yand Cr. The range of Sr/Y in
the Fiskefjord TTGs reaches much higher values than
in the other regions, while the TTGs of the Atâ pluton
have higher Cr concentrations than the remaining TTG
suites, both from within and outside the Disko Bugt
region. The high-SiO2 members of the calc-alkaline
Palaeoproterozoic plutonic rocks from the Nagssugtoqidian orogen are only slightly more enriched in MgO
and Cr than the Archean TTG suites are, but their Y
concentrations reach higher values.
The Atâ tonalite is associated in space and time
with bimodal acid-mafic volcanic rocks in a
volcanic arc setting (Garde and Steenfelt, 1999).
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
Fig. 5. Variations in SiO2–MgO for orthogneisses and diorites within one Palaeoproterozoic and three Archaean regions and in southern and
central West Greenland. Scales show concentrations in percent. Average Archaean TTG from Martin (1994). C-A: calc-alkaline. Nag orogen:
Nagssugtoqidian orogen. AQD: Arfersiorfik quartz–diorite. SIS: Sisimiut intrusive suite.
The major-element characteristics classify the Atâ
tonalite as a TTG suite, but it has a higher CaO/
Na2O ratio than the other grey gneisses of the
Ilulissat area, and the elevated Cr may reflect
involvement of mantle wedge material in its
genesis. Alternatively, the Atâ protolith, which was
emplaced into a very high crustal level (Garde and
Steenfelt, 1999; Kalsbeek and Skjernaa, 1999),
could have been contaminated during its ascent
through the mafic volcanic complex within which it
resides.
In summary, the Archaean TTG gneisses from each
of the three areas are similar with regard to the elements
depicted in Figs. 5–7, and except for the 2.8 Ga Atâ
tonalite, they may be assumed to have derived from
slab melting without significant contribution from a
mantle wedge.
3.2. Diorites
The dioritic rocks vary widely in the components
depicted in Figs. 5–7. With regard to Mg and Cr, the
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
11
Fig. 6. Variations in SiO2–Cr for orthogneisses and diorites within one Palaeoproterozoic and three Archaean regions and in southern and central
West Greenland. Reference line indicates upper limit of TTG field and separates the Atâ tonalite from the other TTG gneisses. SiO2 in percent,
Cr in ppm. Average Archaean TTG from Martin (1994). C-A: calc-alkaline. Nag orogen: Nagssugtoqidian orogen. AQD: Arfersiorfik quartz–
diorite. SIS: Sisimiut intrusive suite.
highest values relative to SiO2 are displayed by the
Nordlandet diorites of the Fiskefjord region. The
ferrodiorites of Godth3bsfjord have the lowest
concentrations of MgO and Cr, and the remaining
diorites have intermediate and mutually similar
variations. In Fig. 7, the diorites show important
differences; most of the dioritic units have low Sr/Y
combined with high Y, i.e., the characteristics of
calc-alkaline diorites. The exceptions are the Qeqertaussaq diorites, the high-K diorites of Kangaatsiaq
and the Sisimiut monzodiorites, in particular, which
are displaced towards higher Sr/Y values at the same
Y values. In Fig. 8, the same three diorite units are
seen to have elevated (La/Yb)N relative to the other
diorites, which have a low (La/Yb)N typical of calcalkaline rocks.
In summary, the most common dioritic rocks in this
study display moderately high Sr contents (up to 500
ppm), their REE patterns are poorly fractionated (La/
YbN V10), and in La/Yb vs. Yb or Sr/Y vs. Y diagrams,
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
Fig. 7. Variations in Y–Sr/Y for orthogneisses and diorites within one Palaeoproterozoic and three Archaean regions and in southern and central
West Greenland. Y (x-axis) in ppm. C-A: calc-alkaline. Nag orogen: Nagssugtoqidian orogen. AQD: Arfersiorfik quartz–diorite. SIS: Sisimiut
intrusive suite.
they plot in the bmodern calc-alkalineQ field of Martin
(1994). They have variable but generally high Cr and
Mg contents; they also have high Mg#. By contrast, the
Qeqertaussaq diorite of Fiskefjord area, the
dsanukitoidT diorite of Disko Bugt and the Nagssugtoqidian monzodiorites display very high Sr contents
above 1000 ppm, strongly fractionated REE patterns
(La/YbN N20) and high Sr/Y; that is, they plot in the
TTG or sanukitoid field (Martin, 1994; Moyen et al.,
2003). Generally, their Cr and MgO contents are equal
to, or lower than, those of the calc-alkaline diorites
within the same area.
4. Discussion
4.1. Sr enrichment in TTG gneisses
Martin and Moyen (2002) investigated the Sr
concentrations of Archaean TTGs and found evidence
that Sr increases relative to CaO+Na2O with decreasing age. The corresponding data for Greenland, shown
in Fig. 9, do not sustain an inverse correlation
between age and Sr content. The highest Sr values
in the Greenlandic TTGs are found within the 3.2–3.0
Ga Fiskefjord region, where also many Qeqertaussaq
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
13
Fig. 8. Variations in YbN –(La/Yb)N for orthogneisses and diorites within one Palaeoproterozoic and three Archaean regions and in southern and
central West Greenland. C-A: calc-alkaline. Nag orogen: Nagssugtoqidian orogen. AQD: Arfersiorfik quartz–diorite. SIS: Sisimiut intrusive suite.
diorite samples are enriched in Sr. The much older
TTGs from the Godth3bsfjord region are very low in
Sr, and also the TTGs of the 2.8 Ga Disko Bugt region
are lower in Sr, although a few diorite samples from
the latter region are enriched in Sr.
This observation is apparently in contradiction to
the conclusion of Martin and Moyen (2002), who
proposed that an increasing Sr content during the
Archaean was related to an increasing depth of melting.
Instead, we observe that Sr enrichment is related to
particular sections of the West Greenland crust, i.e., the
Akia terrane and the Nagssugtoqidian orogen.
The Sr enrichment in the TTG and dioritic gneisses
from the Fiskefjord region requires a closer look.
Garde (1997) subdivided the samples from this region
according to their metamorphic grade into amphibolite
facies, granulite facies and those retrogressed from
granulite to amphibolite facies and also identified the
distinctive chemistry of the Qeqertaussaq diorite. Fig.
9 shows that Sr is correlated with Na2O in all the three
metamorphic TTG groups, but that the trend is steeper
for the granulite facies and retrogressed granulite
facies samples (Fig. 9a). Based on petrography, whole
rock and mineral chemistry, Garde (1997) argued that
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
4.2. Sr enrichments in diorites
The diorites with sanukitoid-like signatures are
distinguished by unusually high Sr (Table 1, Figs. 7
and 10). In Fig. 11, we show that the Sr enrichment
is correlated with enrichment in P, Ba, La, La/Yb
and K. The concentrations of these elements in the
most enriched diorites are much higher than those
obtained in experimental slab melts that have
assimilated peridotitic mantle (Rapp et al., 1999).
The enriched diorites contain carbonates, and a high
Sr–Ba–P–REE signature with highly fractionated
REE spectra is also characteristic of carbonatites,
as illustrated in Fig. 11 by data for the Qaqarssuk
carbonatite and apatite (Knudsen, 1991), as well as
from carbonatite-metasomatised lherzolite (O’Reilly
and Griffin, 1988, 2000).
4.3. Evidence for a relationship between Sr-enriched
diorites and carbonatites in West Greenland
Fig. 9. Correlation between Na2O (%) and Sr (ppm) in TTG
gneisses in granulite (upper diagram) and amphibolite facies (lower
diagram).
the elevated Sr contents in the retrogressed gneisses
were due to the migration of Sr during retrogression,
along with K, Rb and Na. The same author showed
that the Sr enrichment in the Qeqertaussaq diorite is
accompanied by enrichment in P2O5, Ba, La, La/Yb
and K2O (see later). A similar pattern is observed in
some of the TTG gneisses within the outcrop area of
the Qeqertaussaq diorite, and we therefore suggest that
these TTG gneisses are genetically related to the
Qeqertaussaq diorite; accordingly, their high Sr concentrations may be primary and not due to metamorphic migration.
The Archaean craton in southern West Greenland
has been the site of recurrent carbonatitic magmatism
since the Archaean (Larsen and Rex, 1992). Two
major carbonatite complexes, Qaqarssuk (ca. 0.17 Ga;
Knudsen, 1991) and Sarfartoq (ca. 0.6 Ga; Secher and
Larsen, 1980), and a minor one, Tupertalik (3.0 Ga;
Larsen and Pedersen, 1982; Bizzarro et al., 2002), lie
between the Fiskefjord area and the Palaeoproterozoic
rocks of the Nagssugtoqidian orogen. In addition,
potassic lamprophyres with a high carbonate content
(termed shonkinites by Larsen and Rex, 1992) were
intruded into the southern foreland of the Nagssugtoqidian orogen, close to the Sarfartoq carbonatite
complex (Fig. 1). Their chemistry strongly resembles
that of Palaeoproterozoic monzodiorites within the
central part of the Nagssugtoqidian orogen, and they
are probably coeval with the latter rocks, having
yielded an imprecise Palaeoproterozoic age (Larsen
and Rex, 1992). It therefore appears that the lithospheric mantle in this part of Greenland has been
prone to produce carbonate-rich melts since, at least,
3.0 Ga ago; by contrast, carbonatites have not been
recorded in either of the adjacent Godth3bsfjord and
Disko Bugt regions.
The carbonatites of the Akia terrane and northwards comprise calcio- and magnesiocarbonatites
with subordinate ferrocarbonatites. They are rich in
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
15
Fig. 10. Variations in CaO+Na2O–Sr for orthogneisses and diorites within one Palaeoproterozoic and three Archaean regions and in southern
and central West Greenland. CaO+Na2O in percent, Sr in ppm. Notice shift of the Sr scale for the Nag orogen. C-A: calc-alkaline. Nag orogen:
Nagssugtoqidian orogen. AQD: Arfersiorfik quartz–diorite. SIS: Sisimiut intrusive suite.
apatite, phlogopite and magnetite and have high
concentrations of REE (Larsen and Rex, 1992). These
features are shared by the shonkinites, although the
latter have much more phlogopite and feldspar than
the former does. The bulk chemistry of the carbonatites and shonkinites reflects their mineralogy;
consequently, very high concentrations of P, Sr, Ba
and REE are recorded (Table 1, Fig. 12).
Although the origin of carbonatitic magmas is not
fully understood, it is generally assumed that they
form by melting of a modified (enriched or meta-
somatised) mantle source. Studies of mantle xenoliths
suggest that carbonatite-related metasomatism is
widespread, and chemical analyses of mineral phases
in the metasomatic products, such as carbonates and
apatite, confirm that such metasomatism is accompanied by a marked enrichment in LREE, Sr, Ba and
Rb (O’Reilly and Griffin, 1988; Ionov et al., 1993;
Rudnick et al., 1993; Kogarko et al., 1995). Likewise,
apatite residing in metasomatised mantle is very rich
in Sr, U, Th and LREE (Table 1, Fig. 12; O’Reilly and
Griffin, 2000; Xu et al., 2003).
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
Fig. 11. Correlation between Sr (ppm) and La (ppm), REE fractionation, Ba (ppm), K2O (%) and P2O5 (%) in normal calc-alkaline (Nordlandet
diorite) and Sr-enriched diorites. The enrichment trends towards the position of carbonatite, apatite in carbonatite and apatite in lherzolite.
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
17
Fig. 12. Chondrite-normalised REE spectra. Upper diagram: Archaean Sr-enriched diorites, Qeqertaussaq diorite and Kangaatsiaq dsanukitoidT
diorite have intermediate positions between TTG and Nordlandet diorite and carbonatite and carbonatite-related apatites. Lower diagram: The
monzodiorite from the Nagssugtoqidian (Nag) orogen has REE spectra similar to shonkinite (Palaeoproterozoic lamprophyre in the
Nagssugtoqidian foreland) and carbonatite and carbonatite-related apatites.
The almost linear trends in the variation diagrams of Fig. 11, from a normal calc-alkaline
diorite composition (exemplified by the Nordlandet
diorites) towards the compositions of carbonatite
and apatite in carbonatised mantle, suggest that the
parental magmas of the Sr- and P-rich diorites and
shonkinites in West Greenland have incorporated
variable to high amounts of carbonatised mantle.
Because the Palaeoproterozoic arc magmas of the
Nagssugtoqidian orogen are probably mantle
derived, their carbonatitic signature is assumed to
reflect the presence of carbonatitic components in
the melt source area of the mantle. The carbonatitic
component is probably unevenly distributed because
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18
A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
only some of the arc magmas, namely, the
monzodiorites and shonkinites, are enriched in this
component.
Fig. 12 demonstrates that the shape of the REE
patterns in the Sr-enriched diorites resembles that of
carbonatite-related apatite; apatite is probably the
main REE host within these diorites. The very
strongly fractionated REE signature distinguishes
carbonatitic and mantle-derived apatites from apatites
of other lithologies (Belousova et al., 2002). Although
the Archaean Sr-enriched diorites are less enriched
than their Palaeoproterozoic counterparts are, they
occupy similar positions in Fig. 11 and possess
similarly shaped REE-spectra (Fig. 12), which suggests that they have a similar petrogenesis.
Evidence for carbonatite emplacement in a subduction setting is provided from a study of the 2.7-Gaold alkaline Skjoldungen igneous province in southern
East Greenland, some 500 km southeast of Fiskefjord
(Blichert-Toft et al., 1995). Still farther to the east, on
the eastern Lewisian part of the North Atlantic craton,
subduction-related mela-syenites have a similar high
Sr–Ba–P–REE signature with extremely high concentrations of these elements. The mela-syenites have been
interpreted by Tarney and Jones (1984) as resulting
from partial melting of an apatite–phlogopite–carbonate–veined mantle wedge.
We therefore propose that subduction-related
diorites that carry a carbonatite-related geochemical
signature are derived from melting within a mantle
wedge containing domains or patches of apatiteand carbonate-rich materials or from the interaction
of slab-derived magma with such metasomatised
mantle.
in the enriched diorites from West Greenland, as well
as the extremely high concentrations of Sr, Ba and
REE (i.e., not only LREE), as well as carbonate
contents observed in some of the enriched diorites
(Table 1), if the involved mantle was different from
normal peridotite. Even if it is accepted that the least
enriched of the sanukitoid-like diorites, namely, the
Qeqertaussaq diorite, could be generated in the same
way as current models proposed for other sanukitoids
(despite its low MgO, Ni and Cr contents), a different
petrogenetic model would still have to be adopted for
the Palaeoproterozoic enriched diorites. Our preferred
model, which essentially ascribes the high Sr–Ba–P–
REE signature to incorporation of mantle-derived
apatite, phlogopite and carbonate, can be applied to all
rocks with this signature that are presented in this
study.
In the diagrams of Fig. 13, it can be observed that
sanukitoids of the Superior province (Shirey and
Hanson, 1984; Shirey and Hanson, 1986; Stern et al.,
1989) share their high Sr and P and the correlation
between Sr and REE fractionation with the enriched
diorites in West Greenland (Fig. 13a), while sanukitoids from other parts of the world, such as the
Dharwar and Pilbara cratons, do not (Fig. 13b; Reddy,
1991; Krogstad et al., 1995; Smithies and Champion,
2000). It therefore remains possible that the mantle
involved in the generation of the Sr- and P-rich
sanukitoids of the Superior Province was also
carbonatite metasomatised, as suggested here for
several parts of the North Atlantic craton. It may also
be speculated that the chemical differences among the
abovementioned sanukitoids reflect craton-scale differences in the chemical character of the underlying
lithospheric mantle.
4.4. Greenlandic carbonatite-enriched diorites and
sanukitoids
5. Conclusions and implications
The Greenlandic diorites (both normal and
enriched) examined in this study have Mg# less than
0.6 and generally moderate Cr contents; that is, they
are not high-Mg diorites or sanukitoids in the sense
proposed by Shirey and Hanson (1984). Models for
the genesis of sanukitoids implicate interaction
between slab melts and mantle to explain their
combined elevated LILE and Mg–Cr–Ni concentrations and steep REE patterns. However, in our view,
such models can only explain the high P2O5 observed
(1)
Archaean TTGs in West Greenland have uniform compositions, close to the average Archaean TTGs of Martin (1994). The TTG
complexes range in age from 3.8 to 2.8 Ga.
Chemical differences, e.g., in Sr, Mg or Cr
concentrations, are observed between terranes,
but the variations cannot be related to emplacement ages. The chemistry of the suites is
compatible with more or less pure slab melting,
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A. Steenfelt et al. / Lithos xx (2004) xxx–xxx
19
Fig. 13. Comparison of Qeqertaussaq diorite with sanukitoids; all samples shown have SiO2 less than 64%. Upper diagrams: Sanukitoids from
Superior craton (Shirey and Hanson, 1984; Stern, 1989) show same order of Sr and P enrichment, as well as REE fractionation, as the
Qeqertaussaq diorite from Fiskefjord does. Lower diagrams: Sanukitoids from Dharwar and Pilbara cratons have less Sr and P2O5.
although elevated Cr in the Atâ complex might
reflect mantle involvement. Slab melting apparently prevailed throughout Archaean times.
(2) Diorites of calc-alkaline composition with moderate to high Mg and Cr contents and flat
chondrite-normalised REE spectra are also similar irrespective of age and location, implying
that mantle melting has been active since 3.6 Ga.
(3) Some subduction-related diorites, quartz–diorites and monzodiorites of various ages are
enriched in Sr, Ba, P and REE, have fractionated
REE patterns, and occur in a certain section of
the Archaean crust, where carbonatites and
carbonatitic lamprophyres have been generated
in both subduction and cratonic environments
since at least 3.0 Ga. The REE spectra of the
enriched diorites are governed by apatite, and we
propose that their parental magmas resulted from
partial melting of carbonatite-veined parts of a
mantle wedge.
(4)
(5)
The abundance of Sr in magmas is not tied
exclusively to plagioclase; in this study, we have
shown that when, e.g., apatites and carbonates
are involved in the magma genesis, they strongly
influence Sr concentrations. This implies that
care must be taken when Sr is used as indicator
of plagioclase stability.
Some care should also be exercised when using
the term bsanukitoidQ: There are Mg-rich diorites
in the Archaean which are not sanukitoids. In
West Greenland, the carbonatite-related diorites
exhibit compositions which, in some respect,
resemble that of sanukitoids, but their genesis
requires a phosphorus-bearing component that is
not accounted for in current genetic models for
sanukitoids. However, in the light of the present
investigation, it is possible that the geochemical
signature of some of the rocks classified as
sanukitoids in the literature also reflects involvement of variably carbonatised mantle.
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Acknowledgements
This investigation has made use of partly
unpublished chemical analyses derived from databases of the Geological Survey of Denmark and
Greenland. In addition to data acquired by the
authors, samples and analyses have been acquired
by Feiko Kalsbeek (Atâ tonalite, Arfersiorfik quartzdiorite, Sisimiut quartz-diorite), Hanne Tv&rmose
Nielsen and Lotte M. Larsen (shonkinites), Jeroen
van Gool, Sandra Piazzolo and Kristine Thrane
(orthogneisses from the Kangaatsiaq area). The
authors are grateful for constructive criticism by
W. L. Griffin and R. P. Rapp. The Geological
Survey of Denmark and Greenland authorised the
publication of this manuscript.
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