JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 6
PAGES 909–933
1999
Petrology and Geochemistry of the Tromøy
Gneiss Complex, South Norway, an Alleged
Example of Proterozoic Depleted Lower
Continental Crust
T.-L. KNUDSEN∗ AND T. ANDERSEN
MINERALOGICAL–GEOLOGICAL MUSEUM, SARS GATE 1, N-0562 OSLO, NORWAY
RECEIVED MAY 1, 1998; REVISED TYPESCRIPT ACCEPTED DECEMBER 8, 1998
A granulite-facies Precambrian meta-igneous gneiss complex at
Tromøy, South Norway, which was previously assumed to represent
a fragment of strongly large ion lithophile element (LILE)-depleted
lower continental crust, has been reinvestigated using major and
trace element data, radiogenic isotopes and secondary ion mass
spectrometry (SIMS) U–Pb geochronology. The Tromøy gneiss
complex consists of mafic and tonalitic gneisses (SiO2 = 60–70
wt %) that are intruded by trondhjemitic dykes (SiO2 > 70 wt
%). The mafic and tonalitic members are metaluminous, low-K
rocks that have characteristic negative spikes for niobium and
positive spikes in lead, are moderately enriched in middle rare earth
elements–light rare earth elements (MREE–LREE) and have
relatively flat MREE–heavy REE (HREE) patterns. Their compositions resemble evolved magmas in modern oceanic island arcs.
The trondhjemites have major element compositions close to minimum
melts in either mafic or tonalitic systems. They display low LILE
and LREE contents, with high K/Rb (up to >13 000), and their
REE patterns are concave in the MREE to HREE and have a
positive Eu anomaly. SIMS U–Pb analyses of zircons from the
mafic gneiss, tonalite and one trondhjemite suggest three different
episodes of zircon growth: (1) oscillatory zoned magmatic cores at
1198 ± 13 Ma (2r); (2) metamorphic overgrowths at 1125 ±
23 Ma (2r); (3) later fluid-controlled embayments and paths of
zircon reworking. The mafic gneisses and tonalites have indistinguishable magmatic ages. The trondhjemites originated as
anatectic melts in the mafic–tonalitic rock complex during highgrade metamorphism at 1100 Ma; their most likely source was a
leucogabbroic or dioritic facies within the igneous complex. Nd, Sr
and Pb isotope data suggest involvement of mantle- and crustalderived source components in the petrogenesis of the gneiss protolith,
probably in a subduction-zone setting. The present data show that
∗Corresponding author. Telephone: +47 22 85 17 89. Fax: +47 22
85 18 00. e-mail: [email protected]
the Tromøy gneiss complex is not a typical example of ‘depleted
lower continental crust’, nor has it been highly metasomatized or
severely depleted by metamorphic fluids.
crustal differentiation; Proterozoic juvenile magmatism;
Sveconorwegian high-grade metamorphism; U–Pb zircon SIMS data
KEY WORDS:
INTRODUCTION
Tonalite–trondhjemite–dacite suites are generally formed
at destructive plate boundaries (Ringwood, 1974; Smith
et al., 1997). This tectonic setting gives rise to a geochemically complex spectrum of magmas as a result of
a variety of potential source components (mantle wedge,
subducted sediments, subducted oceanic crust) and processes (i.e. degree of melting, slab dehydration) in the
source region. Ascending, high-Mg magmas are trapped
beneath the relatively low-density island arc or continental margin crust, where they may fractionate and
evolve until they become sufficiently buoyant to rise
through the crust (Smith et al., 1997). Assimilation of
continental crust in a continental margin setting or remobilization of pelagic sediments brought to depth by
the subducted slab generally gives calc-alkaline magmas
with elevated LILE/HFSE (large ion lithophile elements/
high field strength elements) ratios (Perfit et al., 1980;
Whalen, 1985; Hess, 1989; Janser, 1994; Smith et al.,
 Oxford University Press 1999
JOURNAL OF PETROLOGY
VOLUME 40
1997). In an island arc setting, silicic magmas with
low K2O may be generated in the mantle wedge by
fractionation of high-Mg, low-K basaltic parent magmas
with elevated Cr and Ni concentrations.
Juvenile crust generated at a destructive plate margin is
modified into a compositionally differentiated continental
crust by processes including high-grade metamorphism
and anatexis ( Johannes & Holtz, 1996). Experiments
show that high-grade partial melting of both mafic rocks
and tonalites can give tonalite–trondhjemite–granodiorite
(TTG) melt compositions (i.e. Johnston & Wyllie, 1988;
Beard & Lofgren, 1991; Winther & Newton, 1991; Wolf
& Wyllie, 1994; Singh & Johannes, 1996), leaving a
pyroxene- or amphibole-rich granulitic restite behind
(Beard & Lofgren, 1991). The present work is a restudy of
the Precambrian Tromøy mafic–tonalitic–trondhjemitic
gneiss complex, southern Norway, which gives a rare
opportunity to study both of these processes within a
single rock complex.
The Tromøy complex is a classical example of low-K
rocks metamorphosed at granulite facies (Field et al.,
1980). The hitherto accepted petrogenetic model for the
precursor of the gneisses is that they represent a suite of
plagioclase–quartz dominated cumulates and trapped
melts that were formed by fractional crystallization of a
dacitic magma within the deep continental crust at 1·54
Ga (Rb–Sr whole-rock age; Field & Råheim, 1979; Field
et al., 1980, 1985). This has been taken as a classical
example of processes generating an LILE-depleted lower
continental crust (Touret, 1996). More recent Sm–Nd
mineral ages and U–Pb zircon ages (Kullerud & Machado, 1991; Kullerud & Dahlgren, 1993) have, however,
demonstrated that the high-grade metamorphism in the
area occurred at 1100 Ma. These findings question a
correlation between the emplacement of the Tromøy
rocks and the high-grade metamorphism, and leave the
genesis and metamorphic evolution of the Tromøy gneiss
complex as open questions. The present study integrates
field observations, geochemical data, isotopic data (Sr,
Nd, Pb) and secondary ion mass spectrometry (SIMS)
U–Pb zircon geochronology, and the results demonstrate
that a more complex sequence of events is needed to
account for the geochemical features of the Tromøy
gneiss complex, where both crust-forming and crustmodifying processes play central parts.
Geological setting
The Proterozoic part of the Baltic Shield (Fig. 1a, b) is
built up by several crustal segments (traditionally named
‘Sectors’ or ‘Belts’), separated by major shear zones that
were active in Sveconorwegian times and possibly earlier
(Hageskov, 1980; Park et al., 1991). Little is, however,
known about the pre-Sveconorwegian evolution of the
NUMBER 6
JUNE 1999
Kongsberg and Bamble Sectors: Rb–Sr whole-rock ages
of 1536 Ma (Field & Råheim, 1979) for the tonalitic
gneiss at Tromøy (Fig. 1c) and 1520 ± 50 to 1580 ±
50 Ma for dioritic gneiss and ‘enderbitic granulite’ of the
Kongsberg Sector ( Jacobsen & Heier, 1978) indicate the
possible influence of a Gothian (1750–1500 Ma) event
that pre-dates the deposition of the sediments in the
Bamble Sector and that occurred at 1370–1500 Ma
(U–Pb detrital zircon ages; Knudsen et al., 1997a; Åhäll
et al., 1998; de Haas et al., in review). Numerous gabbros
intruded the southwestern part of the Baltic Shield at
1230–1110 Ma (Rb–Sr whole-rock ages, U–Pb and
Sm–Nd mineral ages, Jacobsen & Heier, 1978; Munz &
Morvik, 1991; Dahlgren et al., 1990; de Haas et al., 1992,
1993) during an extension-related magmatic event that is
generally regarded as the first stage of the Sveconorwegian
(1230–900 Ma) orogenic period (Starmer, 1990). A compressive, early Sveconorwegian tectonometamorphic
event at 1100 Ma apparently affected the Bamble Sector
only, whereas the main phase of the Sveconorwegian
orogeny at 1000–900 Ma made no recognizable metamorphic imprint on the rocks of the Kongsberg–Bamble
Sectors, but caused greenschist- to granulite-facies metamorphism in adjacent parts of the southern Baltic Shield
(Fig. 1b; Johansson et al., 1991; Dahlgren, 1996; Bingen
& Van Breemen, 1998). Post-orogenic magmatism in
South Norway at 930 Ma (Rogaland) to 925 Ma (Bamble,
Kongsberg and Østfold Sectors) (K–Ar, Rb–Sr, Pb–Pb
and U–Pb datings; i.e. Pedersen & Falkum, 1975; Pedersen & Måløe, 1990; Schärer et al., 1996; Andersen,
1997) define a minimum age limit for Sveconorwegian
orogenic processes in this area.
The Tromøy gneiss complex crops out within the
area of most intense Sveconorwegian metamorphism and
deformation in the Baltic Shield (e.g. Field & Clough,
1976; Knudsen, 1996; Starmer, 1996). It is characterized
by a granulite-facies mineralogy and low LILE and REE
(rare earth elements) concentration levels (Moine et al.,
1972; Field et al., 1980), and carries abundant evidence
of the presence of syn-metamorphic carbonic fluids of
possible mantle origin (Touret, 1971; Hoefs & Touret,
1975; Van den Kerkhof et al., 1994; Knudsen & Lidwin,
1996). The petrogenesis of these gneisses has been controversial ever since their first description by Bugge
(1940). Early petrogenetic interpretations ascribe the geochemical characteristics of the Tromøy complex to metasomatism (Moine et al., 1972; Cooper & Field, 1977)
or to loss of mobile elements during high-grade metamorphism (Field & Clough, 1976; Cooper & Field, 1977;
Cameron, 1989; Touret, 1996). An alternative model
suggesting that the gneisses originated as cumulates with
varying fractions of trapped andesitic–dacitic melt, emplaced at high-grade P–T conditions in the deep crust at
~1540 Ma (Field & Råheim, 1979; Field et al., 1980),
has been widely accepted during the last couple of
910
KNUDSEN AND ANDERSEN
TROMØY GNEISS COMPLEX, NORWAY
tonalitic gneiss consists of Pl + Qtz + Opx + Cpx +
Hbl + Bt + Grt lithologies [mineral abbreviations from
Kretz (1983)] with occasional anhydrous Pl + Qtz +
Cpx + Opx domains, and grades into the associated
mafic gneiss, consisting of Hbl + Pl + Qtz + Cpx +
Opx + Bt + Grt. As the rocks are generally devoid
of K-feldspar but are hornblende bearing, the terms
charnockitic gneiss (Cooper & Field, 1977; Field et al.,
1980, 1985) and enderbitic gneiss (Van den Kerkhof et
al., 1994; Knudsen & Lidwin, 1996) are formally incorrect. The gneisses are crosscut by decimetre- to metrewide, relatively fine-grained mafic dykes (Fig. 3b), which
have been metamorphosed with their country rocks and
are partly tectonically broken up into separate lenses
(Fig. 3d). Decimetre-wide, anhydrous veins and dykes
of trondhjemite to leucotonalite (Fig. 3a, c, d;
Pl + Qtz + Opx + Hbl + Grt; here referred to as ‘ordinary trondhjemite’ for simplicity) are common within
the tonalitic–mafic gneiss complex, but are also found in
the metapelites of the islands in the nearby Tromøy–
Hisøy–Torungen area, and as dykes up to 1 m wide
crosscutting a Sveconorwegian coronitic gabbro in the
Hisøy–Torungen area (Knudsen & Lidwin, 1996). Most
trondhjemite intrusions are Opx bearing, with minor
hornblende and minor to accessory garnet (i.e. enderbite
sensu stricto). Decimetre-wide, coarse-grained dykes and
veins with the primary assemblages of Pl + Qtz + Grt,
Pl + Qtz + Hbl, or similar veins where hornblende is
overgrown by orthopyroxene (Kullerud & Dahlgren,
1993) are found locally. At Tybakken (Fig. 2), pegmatitic
hornblendite forms decimetre-wide veins and pods in the
tonalite, spatially associated with veins of coarse-grained,
garnet-rich trondhjemite. The intrusive orthopyroxeneand hornblende-bearing trondhjemitic veins have induced dehydration zones of 4–5 mm width in surrounding
hornblende-bearing lithologies (arrow in Fig. 3c), and the
trondhjemites carry abundant magmatic CO2 inclusions
(Knudsen & Lidwin, 1996) with a carbon isotope mantle
signature (Hoefs & Touret, 1975; Van den Kerkhof
et al., 1994). Field observations demonstrate that the
trondhjemite is intrusive into the other lithologies, and
is thus unlikely to have formed as plagioclase-dominated
cumulates, as was suggested by Field et al. (1980). An
Sm–Nd mineral age of 1073 ± 28 Ma (Kullerud &
Dahlgren, 1993) on trondhjemitic veins from Hove
(Fig. 2), indicates that the intrusion of the trondhjemites
at Tromøy overlaps with the regional granulite-facies
metamorphism (M2 at 7·5 kbar, 840°C; Knudsen, 1996)
at 1100 Ma (U–Pb zircon age; Kullerud & Machado,
1991). Trondhjemite forming diffuse, millimetre-wide
zones or pods in the tonalite (arrow in Fig. 3a), represents
a similar in situ incipient melt unable to segregate.
The inhomogeneous shear deformation of the highgrade Bamble rocks is particularly well expressed on
certain wave-washed beach localities (i.e. at Hove and
Fig. 1. (a) The main chronological division of the Baltic Shield. 1,
Archaean; 2, Svecofennian; 3, Trans Scandinavian Igneous Belt; 4,
Gothian and Sveconorwegian; 5, Caledonian. (b) The geological division
of the southern part of the Baltic Shield (modified from Starmer, 1996).
Major shear zones: FB, ‘Friction breccia’; PKF, Porsgrunn–Kristiansand
fault; MU, Mandal–Ustaoset Lineament; PZ, Protogine Zone; MZ,
Mylonite Zone; DB, Dalsland Boundary Thrust; GÄ, Göta Älv Shear
Zone. Crustal segments: WG, Western Gneiss Region; RV, Rogaland–
Vest Agder Sector; TS, Telemark supracrustal suite; TB, Telemark
intrusive gneiss complex; B, Bamble Sector; K, Kongsberg Sector; ØM,
Østfold–Marstrand Belt; Å-H, Åmål–Horred Belt. (c) The central
Bamble Sector including Tromøy, showing amphibolite-facies rocks in
NW and granulite-facies rocks in SE.
decades. According to this hypothesis, the low LILE
and REE concentrations are due to primary magmatic
processes acting in the deep crust at ambient granulitefacies P–T conditions.
Field relations and petrography
The rocks along the present sampling traverses (A to C,
Fig. 2), which reproduce those of Cooper & Field (1977),
show gradational variations between the lithologies both
on a 10 m and hand-specimen scale (Table 1) as a result
of strong shear deformation and isoclinal folding postdating the Sveconorwegain high-grade metamorphism
(Knudsen, 1996; Knudsen & Lidwin, 1996). The green
911
912
48·9
18·6
0·0
1·6
19·6
0·0
1·5
3·8
0·5
ab
an
ne
di
hs
ol
ilm
mt
ap
2·73
97·9
0·3
3·9
1·3
0·0
19·1
0·0
0·0
9·0
55·4
3·7
2·2
3·0
99·45
0·6
0·14
0·63
6·63
2·01
4·43
0·16
6·15
1·36
96·1
0·6
4·2
2·6
0·0
19·6
8·1
0·0
17·8
34·6
5·6
0·0
3·1
98·58
1·51
0·28
0·95
4·13
6·00
4·87
0·19
8·71
2·90
14·42
93·2
0·4
3·3
2·3
0·0
22·4
7·7
0·0
18·8
30·9
4·0
0·0
3·4
98·17
4·01
0·20
0·68
3·70
5·97
4·34
0·24
10·43
2·32
13·79
1·22
51·28
0·2
15.95
97·9
0·5
3·8
1·7
0·0
14·8
7·8
0·0
18·8
31·4
1·6
0·0
17·4
99·13
0·26
0·24
0·27
3·76
6·07
3·86
0·25
6·75
2·63
13·44
0·92
60·68
0·35
6.952
97·1
0·2
1·9
0·8
0·0
8·8
3·2
0·0
19·7
38·9
2·2
0·0
21·4
99·37
1·31
0·08
0·38
4·65
4·92
2·68
0·07
2·95
1·31
15·35
0·42
65·24
0·4
10.953
97·4
0·1
2·4
0·7
0·0
8·6
0·0
0·0
7·3
46·7
7·0
0·2
24·4
98·88
0·52
0·04
1·19
5·58
1·54
1·45
0·07
3·82
1·70
13·37
0·38
69·23
0·4
16.953
98·3
0·2
1·7
0·7
0·0
4·7
3·1
0·0
13·0
46·5
1·1
0·0
27·3
99·87
0·6
0·09
0·18
5·56
3·51
1·11
0·06
2·72
1·21
14·16
0·36
70·31
0·4
19.95
98·1
0·3
2·8
1·5
0·0
8·8
6·3
0·0
19·7
35·7
3·1
0·0
19·9
99·38
0·24
0·16
0·54
4·26
5·77
2·21
0·11
5·02
1·95
14·91
0·78
63·44
0·35
20.953
97·9
0·1
3·4
0·9
0·0
13·6
5·8
0·0
25·7
24·3
1·5
0·0
22·6
98·96
0·09
0·05
0·25
2·90
6·72
3·15
0·16
6·15
2·39
14·56
0·47
62·06
0·35
His
98·1
0·1
1·7
0·5
0·0
5·2
0·0
0·0
9·3
46·5
0·8
0·1
34·0
99·32
0·21
0·03
0·13
5·56
1·94
1·09
0·05
2·09
1·16
12·84
0·28
73·94
0·5
12.951
97·5
0·0
1·3
0·3
0·0
1·6
1·7
0·0
13·9
34·8
3·2
0·0
40·7
99·40
0·92
0·02
0·55
4·16
3·24
0·20
0·06
1·59
0·88
12·58
0·17
75·02
0·5
18.95
Trondhjemites
98·0
0·2
2·0
1·0
0·0
7·5
1·3
0·0
14·5
36·0
4·5
0·0
31·1
99·20
0·2
0·07
0·77
4·30
3·36
1·73
0·07
3·13
1·39
13·30
0·52
70·36
0·4
21.951
97·7
0·2
1·6
0·7
0·0
3·3
0·9
0·0
18·2
33·0
1·3
0·0
38·6
99·65
0·77
0·07
0·22
3·94
4·02
0·58
0·07
0·00
3·37
13·47
0·35
72·78
0·4
3b.96
97·9
0·1
1·9
0·5
0·0
6·1
0·7
0·0
17·2
34·7
4·4
0·0
32·4
99·57
0·45
0·04
0·75
4·14
3·72
1·37
0·09
0·00
3·99
14·00
0·27
70·75
0·4
4b.96
98·4
0·1
2·4
0·7
0·0
5·9
0·1
0·0
15·4
27·8
1·7
0·0
44·4
99·50
0·07
0·06
0·28
3·32
3·23
0·95
0·05
3·01
1·67
11·48
0·37
75·00
0·5
11.951
97·9
0·1
1·0
0·5
0·0
1·3
1·8
0·0
16·5
30·3
1·6
0·0
44·9
99·46
0·55
0·04
0·27
3·63
3·84
0·33
0·03
1·25
0·69
12·38
0·27
76·17
0·5
14.95
NUMBER 6
97·5
0·9
or
0·68
17·14
53·25
0·3
9.95
Tonalite gneiss
VOLUME 40
Total4
0·0
c
98·54
Total
2·2
0·03
qz
0·22
CaO
LOI
4·46
MgO
P2 O 5
3·65
MnO
5·84
0·19
FeO
0·15
7·98
Fe2O3
Na2O
2·66
Al2O3
K2 O
0·78
16·65
TiO2
0·4
58·16
0·3
55·92
SiO2
8.951
FeO∗
Fe2O3/
Sample: 7.951
Mafic gneiss
Table 1: The main element compositions and normative calculations of tonalitic gneiss, mafic gneiss, mafic dykes and trondhjemitic gneisses from
Tromøy
JOURNAL OF PETROLOGY
JUNE 1999
KNUDSEN AND ANDERSEN
TROMØY GNEISS COMPLEX, NORWAY
Table 1: continued
Sample:
Fe2O3/FeO∗
SiO2
Grt rich
Hornblendite
7.96
5b.96
Mafic dykes
6.96
8.96
3.95
5.95
13.95
22.951
3a.96
31.96
34.96
0·2
0·15
0·2
0·2
0·2
0·2
0·4
0·2
0·2
0·2
0·2
43·62
44·39
48·90
46·57
49·22
46·34
47·34
48·84
48·05
46·39
46·39
TiO2
0·04
0·22
0·27
0·29
0·85
0·93
1·42
1·55
1·34
3·26
3·26
Al2O3
30·27
16·42
10·85
12·05
16·12
16·41
14·74
13·50
14·66
12·27
12·27
Fe2O3
6·49
11·55
13·77
11·50
2·14
2·43
4·23
2·75
13·99
19·31
19·31
FeO
0·00
0·00
0·00
0·00
9·61
10·91
9·51
12·38
0·00
0·00
0·00
MnO
0·30
0·16
0·23
0·20
0·23
0·25
0·20
0·33
0·23
0·34
0·34
MgO
3·29
12·79
14·78
14·43
8·55
7·86
7·56
5·86
6·54
4·85
4·85
CaO
11·65
11·19
9·30
11·24
7·10
8·21
8·74
7·89
9·91
9·29
9·29
Na2O
0·94
1·59
1·46
1·48
3·40
3·24
3·46
3·86
3·45
3·36
3·36
K 2O
2·19
0·31
0·33
0·41
0·46
0·41
0·17
0·72
0·51
0·60
0·60
P 2 O5
<0·01
<0·01
0·00
0·00
0·09
0·11
0·12
0·21
0·13
0·68
0·68
LOI
1·63
0·95
0·01
1·32
0·9
1·29
1·55
0·45
1·29
0·44
0·44
100·43
99·58
99·91
99·51
98·68
98·40
99·05
98·34
100·10
100·79
100·79
qz
0·0
0·0
0·0
0·0
0·0
0·0
0·0
0·0
0·0
0·0
0·0
c
5·1
0·0
0·0
0·0
0·0
0·0
0·0
0·0
0·0
0·0
0·0
or
12·8
1·8
2·0
2·4
2·7
2·4
1·0
4·2
3·0
4·3
3·5
ab
7·9
11·6
12·3
12·4
28·5
27·1
29·0
32·3
27·8
27·5
28·1
an
57·3
36·4
21·8
24·7
27·0
28·7
23·9
17·2
22·7
18·4
16·4
ne
0·0
0·9
0·0
0·0
0·0
0·0
0·0
0·0
0·6
0·0
0·0
di
0·0
15·0
19·3
24·5
5·9
9·0
14·9
16·8
20·7
23·0
20·9
Total
hs
1·4
0·0
24·5
5·6
13·6
0·4
6·5
3·8
0·0
1·4
5·0
ol
11·2
27·8
14·2
23·3
14·2
23·1
12·3
15·3
15·7
15·2
11·5
ilm
0·1
0·4
0·5
0·6
1·6
1·7
2·7
2·9
2·5
3·9
6·1
mt
1·6
2·8
3·3
2·8
3·1
3·5
6·1
4·0
3·3
4·1
4·6
ap
0·0
0·0
0·0
0·0
0·2
0·2
0·3
0·5
0·3
0·5
1·5
97·3
96·7
97·8
96·3
96·8
96·1
96·5
96·9
96·7
98·2
97·7
Total
1
Mixed in with tonalitic gneiss.
With abundant retrograde chlorite.
3
With in situ melt pods.
4
lc, ac, sph, ru, pv and hm = 0.
∗From Rollinson (1993).
2
Sandum, Figs 2 and 3a, d). Here, trondhjemite veins
have intruded the tonalitic gneiss in a NW–SE direction,
and the white weathering colour of plagioclase makes
the veins especially prominent. The trondhjemites form
alternating less deformed, open folded bands (Fig. 3d)
with NE–SW directed fold axes and strongly deformed,
NE–SW directed remobilized bands parallel to the main
foliation of the area. This pattern of low-strain zones
alternating with strongly NE–SW deformed rocks is repeated at a larger scale. Several metre-wide, low-strain
‘pockets’ (for example, in an old quarry at Tybakken,
Fig. 2) reveal that the interbanded tonalitic to mafic
gneisses are composed of a suite of medium-grained
tonalitic gneiss and hornblende-bearing mafic granulite,
crosscut by fine-grained mafic dykes and later by
trondhjemitic veins.
GEOCHEMISTRY AND
GEOCHRONOLOGY
Analytical procedures
Major elements were analysed by X-ray fluorescence
(XRF) on fused lithium borate glass pellets, and trace
elements by XRF on pressed powder pellets. The analyses
913
JOURNAL OF PETROLOGY
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NUMBER 6
JUNE 1999
Fig. 2. Geological map of Tromøy island, modified from Cooper & Field (1977) but with the same sampling traverses, which are marked
A to C.
were performed on a Philip 2400 instrument with X47
software at the Department of Geology, University of
Oslo. The XRF major element precision is within ±1%,
whereas the trace elements have a detection limit below
1 ppm, except Rb (5 ppm), and U, Th and Nb (all 1
ppm). The REE were analysed by instrumental thermal
neutron activation (Brunfelt & Steinnes, 1969) at the
Institute of Energy Technology (IFE) at Kjeller, Norway.
Accuracy and precision are better than 3% for La and
Sm, 5% for Ce and Nd, 6% for Tb, 7% for Tb and Yb,
and 13% for Lu. In the samples where a given trace
element concentration is lower than its detection limit
(Table 2), the detection limit is used in the normalized
multi-element plots, and serves as an upper concentration
limit for the element in the given sample.
Zircons from the non-magnetic fraction of the samples
were mounted in epoxy, and 29 of these were selected for
SIMS analysis. The U–Pb zircon dating was performed
in the NORDSIM laboratory located at the Swedish
Museum of Natural History in Stockholm, using a CAMECA IMS1270 ion microprobe. Technical details regarding sample preparation have been given by Knudsen
et al. (1997a) and the analytical conditions have been
described by Whitehouse et al. (1997). The procedures
for the Rb-, Sr-, Sm-, Nd- and Pb-isotope analysis are
identical to those of Knudsen et al. (1997b). Nd isotopic
ratios are normalized to 146Nd/144Nd = 0·7219. During
the period when the present analyses were made, the
Johnson and Matthey Batch S819093A Nd2O3 gave
143
Nd/144Nd = 0·511101 ± 0·000013 (2r), whereas the
NBS 987 Sr standard yielded 87Sr/86Sr = 0·710190 ±
0·000050 (2r). The 2r error of the 147Sm/144Nd ratio
was 0·025%. Lead isotope analyses were corrected for
mass fractionation off-line, using correction factors derived from multiple runs of the NBS SRM 981 common
lead standard and the standard composition determined
by Todt et al. (1984). The instrumental fractionation
amounted to 0·095%/a.m.u.; a 2r external precision of
0·2% (counting statistics + fractionation) is assumed in
the lead isotope ratios.
Major and trace elements
The major and trace element data reflect the difficulty
of sampling pure end members. The terms ‘mafic gneiss’,
‘tonalitic gneiss’ and ‘trondhjemite’ of Tables 1 and 2
indicate the dominant rock type present in the samples,
which are often heterogeneous on a small scale. The
classification of tonalite (SiO2 = 60–70 wt %) and
trondhjemite (SiO2 > 70 wt %) is based on CIPW norms
(Table 1). Figure 4a shows that the normative Ab/Or
ratios are similar for the mafic gneisses, the tonalites and
trondhjemites, and the rocks are generally low in Al (Al2O3
< 15 wt % for most samples, Table 1), metaluminous [Al/
(Na + K + Ca) < 1] and show decreasing Al2O3 with
increasing SiO2 content. They are characterized by normative quartz + orthoclase + albite + anorthite +
diopside + hypersthene, except the four peraluminous
samples, which are corundum normative. Additional
normative minerals are ilmenite, magnetite and apatite.
The mafic and tonalitic gneisses define two restricted
fields within the Pl–Opx–Qtz triangle of the Pl–Ol–Qtz
diagram (Fig. 4b). The linear trend defined by the trondhjemites is mainly due to the difficulty of separating
trondhjemite mechanically from its host, and the best
914
KNUDSEN AND ANDERSEN
TROMØY GNEISS COMPLEX, NORWAY
Fig. 3. (a) A tonalitic gneiss with centimetre-wide trondhjemitic veins, which appear as alternating open folds and strongly deformed NE–SW
directed remobilized bands parallel to the regional foliation. The white arrow points to blebs of incipient anatectic melt in the tonalite. (b) A
late mafic dyke crosscutting the tonalite, both pre-dating the regional foliation. (c) Close-up of an Opx- and Hbl-bearing trondhjemitic vein,
which has induced a dehydration zone of 4 mm width in the surrounding tonalite (i.e. at the arrow). (d) Tonalite crosscut by a boudinaged,
mafic dyke and numerous later trondhjemitic veins.
estimate of the trondhjemite end-member composition
is given by the most silica-rich compositions. The trondhjemite end-member composition overlaps with minimum melts formed by partial melting of either a mafic
or tonalitic source at moderate pressures (SJ1, SJ2, BL1
and BL2 in Fig. 4b). On the other hand, high-pressure
melting within the stability field of garnet in tonalite
compositions (of the order of 15 kbar), gives far more
plagioclase-normative melts (CW in Fig. 4b).
The Tromøy rocks define a moderate Fe-enrichment
trend in the AFM diagram (Fig. 4c), straddling the
tholeiitic to calc-alkaline division line. The MgO concentrations and mg-numbers are within the range commonly observed in evolved, low-magnesium lavas from
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Table 2: The trace element compositions of tonalitic gneiss, mafic gneiss, mafic dykes and trondhjemitic dykes and
veins from Tromøy
Mafic gneisses
Sample: 7.95
Rb3
U
8.95
0·7
<1
Tonalitic gneisses
9.95
15.95
6.95
10.95
Trondhjemites
16.95
19.95
0·5
20.95
His1
21.951
12.952
18.952
6
30
16
5
5
16
<5
<5
11
<1
<1
<1
<1
<1
<1
<1
<1
<1
2
<1
0·2
<1
10
01E
1·3
<1
Th
1
1
1
1
1
1
3
1
1
1
4
3
2
<1
Pb3
32
28
7
8
8
6
8
4
5
6
4
3
7
4.1
Nb
Sr
3
2
1
2
2
<1
1
1
<1
2
1
<1
1
3
209
162
176
108
195
321
42
80
214
162
145
95
91
217
251
Zr
50
72
84
84
58
72
117
123
137
45
226
151
106
Y
17
13
29
33
21
5
29
34
31
19
27
40
11
5
163
41
84
30
110
5
346
5
40
116
16
27
2
10
Zn
734
492
118
195
182
53
38
26
84
93
42
20
30
25
V
192
115
313
310
243
88
41
45
139
164
75
28
19
34
Co
21
20
38
43
22
16
11
5
20
24
11
9
3
7
Ni
11
6
24
23
14
41
9
6
14
22
11
8
6
17
1779
858
260
348
448
631
612
2989
574
5396
448
1022
Cu
K/Rb
Table 2: continued
Trondhjemites
Sample:
Rb3
02E
Mafic dykes
03E
1·2
3.95
5.95
13.95
22.95
2.97
1.97
<5
9
4
10
9
18
10
U
<1
<1
<1
<1
<1
1
1
4
2
Th
1
<1
<1
1
1
1
2
2
2
Pb3
8·5
6
7
6
6
6
5
Nb
0·3
05E
Mafic body
3·3
4·7
1
1
4
3
3
2
3
1
3
266
189
182
190
256
193
132
209
208
Zr
2
25
7
60
66
91
84
112
115
Y
2
2
4
16
20
22
67
33
41
Cu
5
2
10
15
36
43
111
95
66
Zn
19
6
41
164
170
105
253
104
108
V
300
Sr
13
7
35
238
330
330
381
295
Co
5
0
13
41
60
60
55
52
55
Ni
10
9
22
187
88
88
26
92
113
13282
1453
424
851
141
655
K/Rb
1
Trondhjemitic veins present.
Some tonalite present.
3
Elements given with one decimal place are from isotope dilution. The trace elements are given in
ppm.
2
916
KNUDSEN AND ANDERSEN
TROMØY GNEISS COMPLEX, NORWAY
Fig. 4. (a) The CIPW normative compositions of rocks of the Tromøy complex, plotted in the Ab–Or–An diagram, showing that all samples
except for the garnet-rich trondhjemite plot with similar Ab/(Or + Ab) ratios of 2–14. (b) The CIPW normative compositions of rocks of the
Tromøy complex, plotted in the Ol–Pl–Qtz diagram, with symbols as in (a). The figure illustrates that the pure trondhjemite composition (which
is close to the most quartz-rich end member) overlaps with minimum melts formed by partial melting of either a mafic (•) or a tonalitic source
(Β) at moderate pressures. High-pressure melting (in the stability field of garnet) gives far more plagioclase-normative melts, as shown by the
points marked CW. SJ1–2, Singh & Johannes (1996); BL1–3, Beard & Lofgren (1991); CW, Carroll & Wyllie (1990). (c) The CIPW normative
compositions of the rocks of the Tromøy complex, plotted in the AFM diagram, showing that the samples straddle the tholeiitic to calc-alkaline
division line (unbroken line). The broken lines a to c give increasing arc maturity and are taken from Janser (1994). Line d is constructed from
a calculation of Grove & Kinzler (1986), assuming the high amount of 60 wt % assimilation of silicic crust, and represents a continental arc
setting. Symbols are as in (a) and (b). (d) K2O–SiO2 variation in the Tromøy gneisses; symbols as in (a). The shaded field represents a typical
low-K, immature oceanic arc trend (Kermadec arc), the ruled field a high-K trend of evolved arc or continental margin affinity [Papua, data
from Smith et al. (1997)]. The boxes define low-K, medium-K and high-K magmatic series (Smith et al., 1997).
modern oceanic arcs (Smith et al., 1997). Compared with
recent analogues, most samples from Tromøy plot in the
field of relatively low arc maturity (a to b in the figure;
Janser, 1994), which is different from low-Fe trends
formed in continental margin arc settings where significant assimilation of continental crust is involved (d in
the figure; Grove & Kinzler, 1986). The mafic gneiss
ranges from low-K basaltic to trachyandesitic compositions, and most samples of the mafic gneisses, tonalites
and trondhjemites plot in the field of low-K magmatic
rocks, typical of an immature oceanic island arc setting
(Fig. 4d; Smith et al., 1997).
The tonalitic gneiss and the trondhjemite show a clear
‘volcanic arc granite’ signature in terms of Rb–Y–Nb.
The mafic and tonalitic gneisses are enriched in LILE
(Rb, Th, K, Pb) and depleted in Nb, Ti, Zr [i.e. high field
strength elements (HFSE)], Y and heavy REE (HREE)
relative to N-MORB (normal mid-ocean ridge basalt)
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potassium, and high to extreme K/Rb ratios of >13 000
[Table 2 and Knudsen et al. (1997b)].
The mafic dykes plot solely in the tholeiitic field of
the AFM diagram. Their lithophile element distribution
patterns overlap with the tonalitic gneiss, except for
slightly lower incompatible element enrichment, and a
flat trend in the compatible end of the pattern. The Ni
content of the mafic dykes (26–187 ppm) exceeds that of
the mafic gneiss.
The REE analyses of 13 carefully selected samples of
practically pure end members of the different rock types
of the Tromøy complex are given in Table 3. Chondritenormalized REE distribution patterns of the tonalitic and
mafic gneisses are overlapping, showing slight light REE
(LREE) enrichment, no or a shallow negative Eu anomaly
and flat HREE patterns (Nd/Sm ratios in the range
2·9–5·0 and La/Yb = 2·9–12·9), resembling the patterns
of modern calc-alkaline magmatic complexes (Gill, 1981).
The ordinary trondhjemite has lower REE contents, Nd/
Sm = 4·9 and 6·3, its REE patterns are distinctly concave
upwards in HREE, with La/Yb ranging from 3·6 to 6·7,
and show positive Eu anomalies. The hornblendites and
Grt-bearing trondhjemite have low REE levels compared
with the tonalitic gneiss. REE data on the Tromøy rocks
by Field et al. (1980), which were interpreted to represent
LILE-deficient, low total-REE cumulates from an andesitic–dacitic magma, overlap with the present REE data
on the tonalites and trondhjemites. The two late mafic
dykes analysed have nearly flat REE patterns at 30 to
50 times chondritic concentration level with La/Yb =
2·6 and 3·4 and weak negative Eu anomalies.
Fig. 5. Element distribution patterns for rocks of the Tromøy complex,
normalized to N-MORB values from Pearce & Parkinson (1993). The
grey line in the upper part of the figure gives the analytical detection
limit of Rb, Th, U and Nb.
[Fig. 5, normalized to N-MORB data of Pearce &
Parkinson (1993)] and show the low HFSE/REE and
low HREE/LILE values typical of subduction-related
magmatism (McCulloch & Gamble, 1991; Hawkesworth
et al., 1994). The range of Ni concentrations observed in
the mafic gneiss (9–41 ppm) is similar to the level found
in evolved magmas in modern oceanic arcs (e.g. Smith
et al., 1997). Uranium concentrations below the 1 ppm
detection limit are observed in a majority of samples,
but it should be noted that this is not necessarily a
magmatic feature, as the entire region has suffered uranium loss during high-grade metamorphism post-dating
emplacement of the gneiss protolith (Knudsen et al.,
1997b). The trondhjemite has an LILE and HFSE distribution similar to the tonalitic and mafic gneisses, but
some practically pure trondhjemite samples have low
U–Pb geochronology
Zircon morphology and zoning
Two samples of mafic gneiss mixed with tonalite and
one tonalitic gneiss were selected for single zircon SIMS
analyses. The intrusion of the trondhjemitic veins is well
dated to 1073 ± 28 Ma from a previous Sm–Nd mineral
study (Kullerud & Dahlgren, 1993), and therefore only
one zircon from a trondhjemitic vein was included in
the present SIMS analyses. All zircons were investigated
by scanning electron microscopy (SEM) cathodoluminescence imaging before analysis, and by detailed
backscatter electron (BSE) imaging after analysis. The
images reveal zircons with complex internal structures
that are unsuitable for conventional thermal IR multispectral scanning (TIMS) U–Pb dating. Three different
episodes of zircon growth can be identified: (1) oscillatory
zoned, magmatic cores (Fig. 6a–d); (2) overgrowths of
zircon of up to 20 lm width, with homogeneous, moderately intense BSE brightness (Fig. 6c, d); (3) BSE bright
zircon occurring as embayments or ~5 lm wide domains
parallel to the oscillatory zoning (Fig. 6c), or also as
918
KNUDSEN AND ANDERSEN
TROMØY GNEISS COMPLEX, NORWAY
Table 3: REE concentrations of the Tromøy tonalitic complex
Rock:
Mafic gneiss
Sample: 8/95
La
8·50
15/95
7·50
Tonalitic gneiss
6/95
7·70
1
10/95
8·10
Trondhjemites
1
16/95
1
20/95
3b/96
4b/96
2
7/96
Hornblendites
Mafic dykes
6/96
31/96
8/96
9·50
34/96
21·6
11·6
2·20
4·90
0·43
2·30
2·40
Ce
12·0
15·0
18·0
13·0
39·0
23·0
5·00
8·00
2·00
7·00
5·00
21·0
32·0
15·6
Nd
14·0
11·0
12·0
5·0
28·0
14·0
17·0
27·0
2·00
6·00
2·00
5·00
5·00
Sm
2·90
3·60
3·70
1·70
5·60
4·50
0·41
0·96
0·09
1·40
1·50
5·70
8·90
Eu
0·87
0·86
0·94
0·47
1·20
0·82
0·33
0·54
0·10
0·34
0·19
1·40
1·60
Gd∗
4·81
4·78
6·27
4·75
3·03
3·70
3·33
3·22
0·00
5·23
0·00
0·00
0·00
Tb
0·34
0·61
0·34
0·12
0·56
0·45
0·06
0·10
0·06
0·12
0·14
0·79
1·10
Yb
1·60
2·60
1·70
0·63
2·60
2·70
0·61
0·73
0·34
0·71
0·63
3·60
4·60
Lu
0·41
0·65
0·47
0·14
0·69
0·69
0·18
0·19
0·10
0·25
0·22
0·93
1·20
Nd/Sm
4·83
3·06
3·24
2·94
5·00
3·11
4·88
6·25
23·26
3·57
3·33
2·98
3·03
La/Yb
5·31
2·88
4·53
12·86
8·31
4·30
3·61
6·71
1·26
3·24
3·81
2·64
3·39
All REE data are from neutron activation analyses for consistency. Gd∗ = Tb + (Sm – Tb)/4.
1
Tonalite with in situ melt.
2
Abundant garnet.
micrometre-wide channels crosscutting the magmatic
zoning (Fig. 6b). These textures suggest that the BSE
bright areas are related to fluid-induced zones of zircon
reworking. The following relative ages are indicated:
(1) oscillatory zoned zircon (oldest), (2) metamorphic
overgrowth and (3) reworked channels or domains
(youngest). A contrasting type of apparently unzoned and
homogeneous, metamorphic zircon has been identified
in the mafic and tonalitic gneisses.
SIMS data
Six U–Th–Pb SIMS zircon analyses have been performed
on the tonalitic gneiss, 22 on the mafic gneiss and one
on the trondhjemitic vein, with altogether 20 and nine
analyses of magmatic and metamorphic zircons, respectively. The maximum 204Pb/206Pb ratio observed is
0·00111 (Table 4), which gives a 206Pbnon-radiogenic/206Pbtotal
ratio of 1·03%. Of the analysed spots 81% give 204Pb/
206
Pb ratios below 0·0002, which gives a maximum
206
Pbnon-radiogenic/206Pbtotal ratio of 0·19%, and only minor
corrections for common lead. The complex internal
textures of most zircons suggest that special care must
be taken in interpreting the results. Careful BSE investigations of all spots analysed have shown that the
~30 lm ion beam is commonly too large to resolve the
complex internal zonation pattern of most zircons, and
thereby gives mixed ages. Also, apparently simple, metamorphic zircons display a spread in ages. Many spots
are inversely discordant, suggesting U loss (or radiogenic
Pb gain), and 45% of all zircons analysed are concordant
within the 1r error (Fig. 7a, b). The data set is not suitable
for an exact age determination, but it demonstrates clearly
that the intrusive tonalite–mafic complex is Sveconorwegian rather than Gothian as was assumed previously (e.g.
Field & Råheim, 1979). It is suggested that the maximum
207
Pb/206Pb ages obtained for the magmatic and metamorphic zircons are the best estimates of the ages of
these events, giving the following ages for the three
zircon-forming processes: (1) oscillatory zoned, magmatic
zircons formed at 1198 ± 13 Ma or slightly earlier
(analysis 23 B, Table 4)—this dating is referred to as
1200 Ma in the following text; (2) metamorphic zircon
growth at 1125 ± 23 Ma (analysis 02A, Table 4), which
overlaps with the metamorphic zircon ages of 1122 and
1133 Ma from metasediments in the Hisøy–Torungen
area (U–Pb SIMS zircon ages; Knudsen et al., 1997a);
(3) later zircon reworking and U loss. The intrusion age
of the mafic and tonalitic gneisses cannot be separated
by the present U–Pb data.
Whole-rock radiogenic isotope systematics
Rb–Sr
Regression of the whole-rock Rb–Sr isotope data for the
tonalites and mafic gneisses gives a very poorly defined
correlation line with (87Sr/86Sr)i = 0·7031 and an apparent age of 1480 Ma. This is comparable with the
Rb–Sr whole-rock isochron age of 1536 Ma obtained by
Field & Råheim (1979). Both ages pre-date the present
U–Pb SIMS zircon data by ~300 my, suggesting that
any Rb–Sr whole-rock isochron age calculated for the
Tromøy rocks is geologically meaningless. Statistically
valid Rb–Sr correlation lines without age significance
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Fig. 6. SEM backscatter images of zircons from the Tromøy complex, taken after SIMS analyses. The halo around the beam spot is created
by the SIMS sputtering process, and the bright domains in the mounting medium and along some of the cracks in the zircon b are remnants
of the gold coating. (a) An oscillatory zoned, magmatic zircon from the tonalitic gneiss. (b) An oscillatory zoned zircon from the trondhjemitic
vein showing micrometre-wide paths and domains of fluid reworking, which are particularly prominent in the lower parts of the grain. (c) Bright
domains of fluid reworking which parallel the magmatic zoning and appear as an embayment on the outer left-hand side of the grain. From
the mafic gneiss sample 8.95. (d) An oscillatory zoned, magmatic grain rimmed by a homogeneous zone of up to 15 lm width representing
metamorphic zircon growth (from the mafic gneiss sample 8.95). A bright zone of fluid reworking parallels the magmatic zoning and overlaps
with the elliptic beam spot.
may form by two-component mixing (e.g. Faure, 1986)
and in the present case, potential end members include
a juvenile, mantle-derived and a crustal component. The
calculated intercept and slope of such a mixing line
is dependent on the isotopic compositions of the end
members, and not on their proportions (Faure, 1986).
The mantle-derived component ( J) can be approximated
by the least radiogenic mafic gneiss sample (7.95; Table 5),
whereas a strongly LILE-enriched upper-crust component (U) can be represented by the average of nine
metasediment samples from the region analysed by Knudsen et al. (1997b). The Rb–Sr systematics of the local
deep continental crust (L) is constrained by data on
granitoid intrusions (Andersen, 1997; Simonsen, 1997);
it is distinctly different from the metasediments by a
less extreme LILE-enriched composition, but does not
represent a typical LILE-depleted rock from the lower
continental crust. Mixing of the mantle-derived component with either of the two crustal end members results
in present-day Rb–Sr correlation lines (Fig. 8) with slopes
and intercepts indistinguishable from the 1480 Ma correlation line based on the present data and the 1·54 Ga
isochron of Field & Råheim (1979). The present data
suggest that the precursor of the Tromøy gneisses formed
at ~1200 Ma by mixing of a juvenile, mantle-derived
component with material having a prolonged prehistory
in an LILE-enriched crustal reservoir, but the Rb–Sr
data can neither characterize this crustal end member
920
Texture1
U
hom.
hom.
29A
30A
50
66
110
921
ov+re
co+ov+re
co+ov+re
23A
23B
116
177
169
75
156
181
complex
109
29
39
124
35
11
20
29
25
41
39
18
37
38
28
60
26
34
37
17
14
45
38
30
11
14
14
27
12
Pb
39
31
94
21
9
22
37
5
2
2
2
4
5
4
1
3
2
1
26
6
31
18
3
4
7
3
23
11
Th
0·36
0·17
0·18
0·14
0·17
0·24
0·28
0·04
0·01
0·01
0·03
0·03
0·03
0·03
0·00
0·03
0·02
0·01
0·34
0·10
0·15
0·10
0·02
0·09
0·13
0·05
0·21
0·21
Th/U
0·0001
0·0002
0·0000
0·0000
0·0002
0·0000
0·0000
0·0001
0·0000
0·0002
0·0001
0·0000
0·0008
0·0002
0·0011
0·0000
0·0000
0·0002
0·0002
0·0000
0·0000
0·0000
0·0000
0·0000
0·0000
0·0002
0·0003
0·0003
Pb/206Pb
204
0·0779
0·0752
0·0785
0·0789
0·0746
0·0771
0·0762
0·0768
0·0800
0·0774
0·0745
0·0757
0·0735
0·0747
0·0700
0·0781
0·0792
0·0735
0·0743
0·0745
0·0775
0·0771
0·0771
0·0736
0·0746
0·0728
0·0729
0·0765
1·17
1·65
0·65
0·95
1·67
1·13
1·32
1·17
0·64
1·44
1·88
1·17
3·29
1·13
4·52
0·69
0·61
1·46
1·83
0·88
0·51
0·39
1·10
1·81
1·85
1·62
2·01
2·59
Pb∗/206Pb∗ ±1r
207
Pb∗/235U
2·3628
1·9669
2·2558
2·3025
1·9459
2·0652
1·9741
2·0931
2·3337
2·2930
2·2531
2·2898
2·1157
2·1854
1·7909
2·3067
2·2616
1·9992
2·1309
2·0456
2·1749
2·1073
2·2108
2·1454
2·1553
1·9957
2·1324
2·1229
207
3·95
3·05
2·83
4·23
3·44
3·62
4·52
3·47
2·15
6·23
3·79
2·69
2·52
5·77
3·85
2·53
2·86
2·67
2·85
2·27
2·58
1·83
2·42
4·77
3·47
4·55
3·44
4·32
±1r
Pb∗/238U
0·2200
0·1896
0·2083
0·2117
0·1892
0·1942
0·1879
0·1978
0·2115
0·2149
0·2193
0·2192
0·2088
0·2121
0·1857
0·2143
0·2072
0·1973
0·2080
0·1990
0·2035
0·1982
0·2080
0·2113
0·2095
0·1990
0·2121
0·2012
206
3·81
2·90
2·76
4·12
3·30
3·44
4·34
3·36
2·06
6·16
3·42
2·44
2·25
5·71
3·06
2·44
2·81
2·44
2·71
2·16
2·54
1·79
2·17
4·42
3·02
4·44
3·30
3·85
±1r
Pb/206Pb
1144
1074
1160
1169
1057
1125
1100
1115
1198
1131
1055
1088
1028
1061
927
1148
1177
1027
1050
1056
1134
1124
1124
1031
1059
1007
1011
1108
207
23
33
13
19
34
23
26
23
13
29
38
23
67
23
93
14
12
30
37
18
10
8
22
37
37
33
41
52
±1r
Pb/235U
1231
1104
1199
1213
1097
1137
1107
1147
1223
1210
1198
1209
1154
1176
1042
1214
1200
1115
1159
1131
1173
1151
1184
1164
1167
1114
1159
1156
207
Apparent ages ( Ma)
28
21
20
30
23
25
30
24
15
44
27
19
17
40
25
18
20
18
20
15
18
13
17
33
24
31
24
30
±1r
Pb/238U
1282
1119
1220
1238
1117
1144
1110
1163
1237
1255
1278
1278
1222
1240
1098
1252
1214
1161
1218
1170
1194
1165
1218
1236
1226
1170
1240
1182
206
44
30
31
46
34
36
44
36
23
70
40
28
25
64
31
28
31
26
30
23
28
19
24
50
34
48
37
42
±1r
hom., apparently homogeneous and metamorphic zircon; co, oscillatory zoned core; ov, metamorphic overgrowth; re, zones of zircon reworking.
1
By BSE investigation.
∗Radiogenic Pb.
31A
Trondhjemite, sample 11.95
526
27A
53
25A
hom.
03A
89
132
145
hom.
02A
24A
hom.
01A
Tonalitic gneiss, sample 6.95
23C
co+re
22A
165
21A
149
175
123
hom.
15A
co+ov+re
hom.
14A
76
co+ov+re
ov
13A
63
199
19A
co
11B
18A
co+ov
11A
174
111
co+re
10A
134
294
co+ov
08A
45
58
17A
hom.
07A
16A
hom.
05A
Mafic gneiss, sample 8.95
complex
28A
Mafic gneiss, sample 7.95
No.
Table 4: Single zircon ion-probe data
5
15
13
15
5
13
5
9
11
7
4
11
10
7
17
Discordance
KNUDSEN AND ANDERSEN
TROMØY GNEISS COMPLEX, NORWAY
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 6
JUNE 1999
Fig. 7. SIMS analyses of the magmatic and metamorphic zircons plotted in the concordia diagram and illustrating that most analyses are
inversely discordant. The best estimates of the magmatic and metamorphic events are given by the highest 207Pb vs 206Pb zircon ages (insets).
in more detail nor prove that it originated from the Baltic
Shield. As the whole-rock system of the Tromøy complex
was not initially homogeneous in strontium isotopic composition, any linear correlation reported is likely to be
devoid of chronological significance, and ages around
1500 Ma reflect the composition and history of the
end members rather than the emplacement age of the
magmatic protolith.
Sm–Nd
The Tromøy gneiss complex shows a wide range of
present-day Nd isotopic compositions (143Nd/144Nd from
0·51207 to 0·51286 and 147Sm/144Nd from 0·0981 to
0·2789; Table 6). However, three samples of trondhjemite
and mafic gneiss with 147Sm/144Nd > CHUR (Chondrite
Uniform Reservoir) have experienced Sm–Nd differentiation because of garnet crystallization at 1100 Ma,
and are not further considered. Furthermore, tonalite
samples 19.95Tro and 10Ma show excessively high
143
Nd/144Nd at 1·2 Ga at low but reasonable 147Sm/144Nd
ratios. This is due to the presence of minor metamorphic
(1100 Ma) garnet in these samples, which failed to dissolve
completely during analysis. The high eNd at 1200 Ma of
these samples is thus an analytical artefact, and Nd data
for samples 10Ma and 19.95 are not considered further.
Most mafic and tonalitic gneiss samples have eNd(1200)
values in the range –2 to 6 and a negative evolution of
eNd with time to the present (Fig. 9), reflecting their
LREE-enriched REE patterns. eNd(1200) values close to
the depleted mantle curve indicate the presence of a
depleted mantle derived component in the Tromøy complex. The trondhjemites have eNd(1100) values that overlap with the values of the mafic and tonalitic gneisses,
and this suggests that the rocks may be genetically related.
Despite the low La/Yb ratio of the trondhjemite, the
distinct concave-upwards curvature of the REE patterns
causes low Sm/Nd ratios, and thus a steep trend of eNd
with time (Fig. 9).
In a plot of eNd vs initial 87Sr/86Sr at 1200 Ma (Fig. 10a),
most samples of mafic and tonalitic gneiss plot at or close
to a binary mixing curve between a mafic component
derived from a depleted mantle reservoir (DePaolo, 1981)
and a crustal component, at ~10–50% mantle contribution. Only one sample of tonalitic gneiss indicates a
much higher crustal contribution. The crustal component
is poorly defined from the Sr–Nd data. The component
shown in the figure represents the deep crust in the SW
Baltic Shield (Andersen, 1997; Simonsen, 1997), but
serves as an example only, as other, more strongly Rbenriched reservoirs would account equally well for the
variation in the present data. At 1100 Ma, the majority
of trondhjemite samples and the hornblendite plot well
within the range of variation of mafic and tonalitic
gneisses, strongly suggesting that the older lithologies of
the Tromøy complex were involved in the genesis of the
trondhjemitic melts.
Lead
Lead isotope data (Table 5) for 19 samples of tonalite,
mafic gneiss, trondhjemite and late mafic dykes are
plotted in Fig. 11, together with data for relevant global
reservoirs at 1200 Ma. Both mafic and tonalitic gneisses
span a considerable range of lead isotope compositions
(206Pb/204Pb from 16·26 to 20·33), with two samples of
mafic gneiss defining the unradiogenic end of the array.
Six out of seven of the trondhjemite samples analysed
show limited variation of Pb composition, with 206Pb/
204
Pb in the range 17·60–19·95. The late mafic dykes
overlap with the tonalitic gneiss, indicating that their
922
3·9
14·6
mafic gneiss
tonalitic gneiss
tonalitic gneiss5
tonalitic gneiss
tonalitic gneiss
15/95
6.95
16/95
19.95
His2
0·7
1·7
0·2
3·4
8·3
trondhjemite5
trondhjemite5
trondhjemite5
trondhjemite7
12.95
14.95
18.95
923
0·3
1·2
trondhjemite
trondhjemite
trondhjemite6
trondhjemite
trondhjemite
trondhjemite
hornblendite
hornblendite
maf. dykes
maf. dykes
maf. dykes
maf. dyke
maf. dyke
01E
02E
03E
06E
09E
4Bb.96
5b.96
6.96
3.95
5.95
13.95
3a.96
31.96
The error is 0·5% of the calculated ratio.
2r = 3 × 10–5.
The error is 0·025% of the calculated ratio.
DePaolo (1981).
Dominant rock type.
Grt-bearing.
Chl-bearing.
× 10–6.
1
2
3
4
5
6
7
8
3·0
3·7
0·1
4·9
6·0
1·5
0·7
1·3
trondhjemite5
21.95
9·4
3·4
tonalitic gneiss6
11.95
121·4
166·6
18·2
248·5
143·5
184·9
67·1
92·6
64·3
76·6
142·2
89·4
135·8
91·3
77·5
315·8
0·0703
0·0647
0·0158
0·0573
0·1200
0·0228
0·0293
0·0367
0·0121
0·0497
0·1913
0·2678
0·0736
0·0063
0·0645
0·0309
0·7054
0·7047
0·7037
0·7064
0·7046
0·7046
0·7036
0·7050
0·7046
0·7052
0·7078
0·7155
0·7051
0·7047
0·7099
0·7037
0·7044
10Ma
0·1110
tonalitic gneiss
61·6
2·4
tonalitic gneiss
5·56
1·29
3·46
1·91
2·11
1·69
0·99
0·84
1·67
0·04
0·13
0·18
0·29
4·06
3·81
9·54
3·41
3·06
1·93
1·53
1·66
06Ma
0·7040
05Ma
0·0760
tonalitic gneiss
02Ma
2·54
1·38
1·99
6·30
70·2
2·86
7·46
2·62
0·7045
0·7050
0·7259
0·7057
0·7098
0·7108
Sm (ppm)
tonalitic gneiss
1·8
Sr/86Sr2
0·7032
87
tonalitic gneiss6,7
0·0388
0·0179
1·0221
0·0579
0·3151
0·4670
0·0102
Rb/86Sr1
87
17.95
156·9
76·9
41·4
194·9
101·0
170·1
211·7
Sr (ppm)
His1
2·1
0·5
11·0
27·4
mafic gneiss5
mafic gneiss
7.95
Rb (ppm)
9.95
Rock
Sample
18·35
8·00
9·61
8·58
7·13
8·42
5·03
4·14
8·59
0·27
0·81
0·81
1·24
6·65
15·18
11·06
17·05
5·01
13·75
5·07
5·56
8·60
27·84
7·28
8·05
9·46
34·38
11·28
Nd (ppm)
Sm/144Nd3
0·1847
0·0982
0·2191
0·1195
0·1806
0·1223
0·1198
0·1229
0·1185
0·0981
0·0998
0·1347
0·1412
0·1887
0·1527
0·2286
0·1219
0·2789
0·1195
0·1838
0·1819
0·1850
0·1855
0·1379
0·2123
0·1048
0·1277
0·1322
0·1542
147
Nd/144Nd
0·51276
0·51246
0·51279
0·51268
0·51272
0·51228
0·51208
0·51228
0·51238
0·51207
0·51276
0·51230
0·51239
0·51273
0·51238
0·51266
0·51228
0·51260
0·51285
0·51284
0·51275
0·51271
0·51277
0·51223
0·51274
0·51244
0·51218
0·51227
0·51222
143
10
10
10
16
10
10
10
24
10
10
10
10
10
10
10
10
10
10
19
10
10
10
10
10
12
10
10
7
26
2r
1·4
1·3
1·4
1·4
1·9
1·6
1·3
1·0
1·4
1·4
1·6
1·5
t(DM) (Ga)4
Pb/204Pb
18·823
19·739
17·607
17·632
17·600
18·005
17·431
19·952
23·094
18·118
20·328
19·058
18·537
16·910
18·238
19·471
18·036
19·643
16·262
206
0·032
0·027
0·020
0·018
0·018
0·018
0·017
0·040
0·023
0·018
0·020
0·019
0·015
0·015
0·016
0·017
0·018
0·020
0·016
2r
Pb/204Pb
15·681
15·744
15·503
15·477
15·524
15·491
15·483
15·867
16·109
15·536
15·685
15·689
15·528
15·470
15·599
15·692
15·649
15·730
15·425
207
Table 5: Isotopic characteristics of calc-alkaline tonalitic gneiss, enderbitic intrusive veins and mafic granulites from Tromøy
0·028
0·023
0·016
0·015
0·016
0·015
0·015
0·034
0·021
0·016
0·016
0·016
0·016
0·015
0·021
0·021
0·018
0·016
0·015
2r2
Pb/204Pb
37·869
37·451
36·578
36·755
36·596
36·597
36·437
38·713
43·902
37·600
37·956
37·802
37·261
36·645
37·392
39·017
37·064
38·162
36·015
208
0·066
0·054
0·037
0·037
0·037
0·037
0·036
0·081
0·079
0·038
0·038
0·038
0·037
0·066
0·067
0·070
0·047
0·038
0·036
2r
KNUDSEN AND ANDERSEN
TROMØY GNEISS COMPLEX, NORWAY
JOURNAL OF PETROLOGY
VOLUME 40
NUMBER 6
JUNE 1999
U–Th–Pb systematics are completely controlled by contamination with their wallrocks. 207Pb/204Pb is moderately
well correlated with 206Pb/204Pb, giving rise to a positively
inclined array in the 207Pb/204Pb vs 206Pb/204Pb diagram
(Fig. 11). Regression of all lithologies together yields
a poorly fitted regression line [mean square weighted
deviation (MSWD) = 13] with a spurious age of 1703
± 290 Ma. Such a distribution of present-day lead
compositions suggests a special case of two-component
mixing, in which the lead isotopic compositions and U/
Pb ratios of rock volumes intermediate between the two
end members are positively correlated at the time of
mixing (Whitehouse, 1989; Romer & Bridgewater, 1997).
Accumulation of radiogenic lead since closure of the
system at ~1200 Ma has led to a clockwise rotation of
the mixing line, but also to increased scatter around this
line.
Fig. 8. The 1·48 Ga Rb–Sr whole-rock correlation line calculated for
the Tromøy rocks, overlapping with two-component mixing lines
calculated for juvenile magma mixed with the upper ( JU) the lower
( JL) continental crust of the SW part of the Baltic Shield. Symbols as
in Fig. 4a.
Table 6: Isotope characteristics of the endmembers of the two-component Rb–Sr isotope mixing
calculations
Rb (ppm)
Sr (ppm)
87
Rb/86Sr
87
Sr/86Sr
Component
Rock
Comments
Juvenile crust, J
7.95 mafic gneiss
1
212
0·0102
0·7032
the least radiogenic sample
Upper crust, U
metapelites1
99
109
2·9047
0·7662
average Nd model age of 1·7 Ga
‘Lower’ crust, L
charnockite2
60
242
0·5277
0·7185
U–Pb zircon age of 1152 ± 2 Ma3
1
The average of nine metapelites from the Hisøy–Torungen area, from Knudsen et al. (1997b).
The average of four samples; data from Simonsen (1997).
3
From Kullerud & Machado (1991).
2
10
De Paolo 19
81
Epsilon Nd
5
0
0.2
0.4
0.6
0.8
1
Time (Ga)
1.2
1.4
1.6
–5
–10
trondhjemite
tonalite
hornblendite
late mafic dyke
field of tonalite and mixed tonalite-trondhjemite
Fig. 9. Nd evolution of the Tromøy rocks shows that the mafic and tonalitic gneiss plot with relatively parallel eNd vs time trends reflecting
their moderately LREE-enriched patterns. The trondhjemites have eNd(1·1 Ga) values overlapping with the gneisses. (See text for further
explanation.)
924
KNUDSEN AND ANDERSEN
TROMØY GNEISS COMPLEX, NORWAY
Fig. 10. Time-corrected Nd and Sr isotopic compositions of the
Tromøy gneiss complex. (a) Mafic (Β) and tonalitic (Φ) gneisses
calculated to the time of primary emplacement at 1200 Ma. The stars
represent potential end members in a binary mixing scenario: global
depleted mantle (DePaolo, 1981, filled) and Baltic Shield deep crust
(Andersen, 1997, open). The two-component mixing curve has been
constructed for Sr concentrations of 190 ppm in the mantle-derived
end member and 50 ppm in the crustal component and corresponding
Nd concentrations of 15 and 25 ppm (Andersen, 1997). Tick-marks
are shown at 10% intervals. (b) The situation at 1100 Ma, including
the composition of trondhjemites (Α) and hornblendite (Ο). The shaded
field represents the overall variation of the mafic and tonalitic rocks at
1100 Ma.
The U–Th–Pb systematics of the mantle beneath the
southwestern part of the Baltic Shield can be described
by a single-stage 238U/204Pb ratio in the range 7·90–7·96
(Andersen et al., 1994; Andersen, 1997). The resulting
mantle composition at 1200 Ma is slightly less radiogenic
than the theoretical mantle composition of Zartman &
Doe (1981), but as the two models are nearly collinear
along a 1200 Ma isochron, the difference between them is
insignificant for the present discussion; 1200 Ma mantlederived rocks would today plot on the line marked ‘100%
mantle’ in Fig. 11.
The lead isotope evolution of the upper continental
crust in South Norway is comparatively well known
from studies on metasediments and their protoliths, and
common to most metasediments in the area are high
present-day 238U/204Pb ratios and a pre-Sveconorwegian
crustal history in a reservoir with elevated 238U/204Pb
(Andersen & Munz, 1995; Andersen et al., 1995; Knudsen
et al., 1997b). Neodymium isotope systematics on metasediments and SIMS U–Pb dating of clastic zircon grains
indicate that the crustal protolith formed at 1750–1900
Ma (Andersen et al., 1995; Knudsen et al., 1997a, 1997b);
its lead isotopic composition at 1200 Ma is similar to the
Fig. 11. Lead correlation diagram showing rocks from the Tromøy
complex; symbols as in Fig. 4. Growth curves are shown for a hypothetical mantle reservoir with a time-integrated 238U/204Pb ratio of
7·9 (Andersen et al., 1994) and for the second stage of the global twostage lead model of Stacey & Kramers (1975) (SK). The open stars are
upper and lower continental crust compositions at 1200 Ma from
Zartman & Doe (1981) (ZD); the filled stars are mantle compositions
from Zartman & Doe (1981) and Andersen et al. (1994). The reference
isochron marked ‘100% mantle’ represents the present-day locus of
systems derived from a mantle source at 1200 Ma, without addition
of upper-crustal material. The line marked ‘100% deep crust’ is a 1200
Ma isochron for systems derived from local SW Baltic Shield deep
crust at 1200 Ma, based on data of Andersen et al. (1994) and Andersen
(1997). The bold line (1·2 Ga mixing line) joining the end-member
components defines the initial lead of intermediate members in a
mixing series between mantle and upper continental crust at 1200 Ma.
1200 Ma theoretical upper continental crust end member
of Zartman & Doe (1981).
The lead isotope characteristics of the deep continental
crust in southern Norway are constrained by data on
Sveconorwegian granites (Andersen et al., 1994; Andersen, 1997; Simonsen, 1997). Although the continental
crust in South Norway is compositionally stratified, the
deep crust is not depleted in LILE (Andersen, 1997),
differing significantly from the much less radiogenic global
‘depleted lower crust’ end member of Zartman & Doe
(1981), indicated in Fig. 11. At present, rocks formed by
remobilization of SW Baltic Shield deep crust would plot
on the line marked ‘100% deep crust’ in Fig. 11.
All but two of the samples of mafic and tonalitic gneiss
as well as all of the trondhjemite samples plot well above
the ‘100% deep crust’ line in Fig. 11, showing that binary
mixing of a mantle-derived component and the deep
crust of the SW Baltic Shield cannot account for the
variation in 1200 Ma initial lead composition in the
Tromøy complex. To account for the elevated 207Pb/
204
Pb of these rocks, significant amounts of a component
similar to the global ‘upper continental crust’ or to SW
Baltic Shield sediments are required.
DISCUSSION
925
The widely accepted petrogenetic model of Field et al.
(1980), which implies a single-stage evolution of the
entire Tromøy gneiss complex during a Gothian orogeny
JOURNAL OF PETROLOGY
VOLUME 40
(~1600 Ma), is contradicted by the field observations,
U–Pb geochronology and geochemical data of the present
study. Although the tonalitic and mafic gneisses are
depleted in LILE relative to average values of the upper
continental crust (Taylor & McLennan, 1985), the rocks
are enriched relative to N-MORB, giving the LILE/
HFSE and LILE/REE patterns typical of subductionrelated magmatism (McCulloch & Gamble, 1991;
Hawkesworth et al., 1994). Furthermore, the mafic and
tonalitic members are metaluminous, low-K rocks which
have characteristic negative spikes in niobium and positive spikes in lead, are moderately enriched in middle
REE (MREE)–LREE and have relatively flat MREE–
HREE patterns that resemble evolved magmas in modern
oceanic island arcs.
The data presented suggest that the protoliths of the
mafic–tonalitic gneiss association formed by differentiation of a subduction-zone related magma at ~1200
Ma. Fractionation and emplacement of the parent
magma took place at pressure conditions where garnet
was unstable, and with a reduced water activity (restricted
amounts of hornblende), in accordance with earlier interpretations of abundant mantle-derived magmatic CO2
inclusions in the complex (Hoefs & Touret, 1975; Van
den Kerkhof et al., 1994). The rocks were affected by the
regional metamorphism ~100 my later, at P–T conditions
of the order of 7·5 kbar, 840°C [the M2 event of Knudsen
(1996)], causing anatexis and emplacement of trondhjemite dykes and veins with accessory, or more rarely,
major amounts of garnet.
Magmatic differentiation and anatexis in
the Tromøy complex
The major and trace element characteristics of different
petrogenetic scenarios in the Tromøy complex have
been quantified from data in Table 1, using built-in
multivariate linear regression tools of Microsoft Excel.
Trace element distributions have been estimated for
different models using standard equations of fractional
crystallization and non-modal batch melting, as given for
example by Rollinson (1993). Partition coefficents for
‘tonalitic’ systems, as compiled by Martin (1987), have
been used, supplemented by data on rhyolitic systems
from Rollinson (1993) (Table 7).
Tonalite and mafic gneiss
The present-day mineralogy of the mafic gneiss, characterized by orthopyroxene and hornblende, reflects
high-grade metamorphism rather than igneous crystallization. The mafic gneiss and tonalite have overlapping REE concentration levels and near-parallel
NUMBER 6
JUNE 1999
distribution patterns, which is inconsistent with fractionation or accumulation of a mineral assemblage dominated by hornblende, or with garnet as a major phase,
as this would modify the HREE levels beyond what is
observed. The tonalite and mafic gneiss undoubtedly
represent igneous precursors that are closely genetically
related to each other, but as any original structural
relationship between the two has been obliterated by
later deformation, field observations cannot help identify
the actual process involved. The mafic gneiss may represent a magma that was parental to tonalitic magmas,
it may be a mafic cumulate from a tonalitic liquid, or
the two lithologies may both be cumulates from a common parent magma (dominated by pyroxene with minor
Fe–Ti oxides and hornblende, and plagioclase, respectively). Although it is possible to generate a liquid
similar to average tonalite from a magma with average
mafic gneiss composition by 25–30% fractionation of
plagioclase, mafic silicates, apatite and iron–titanium
oxides, in proportions depending on mineral compositions and on constraints on the crystallizing mineral
assemblage, this does not exclude the other possible
mechanisms. To preserve the parallel REE distribution
patterns, hornblende and garnet, respectively, cannot
exceed 10–15% and 1% of the accumulated or fractionated solid assemblage, suggesting that the rocks crystallized at pressures lower than the limit of the stability
field of garnet.
Ordinary trondhjemite
The trondhjemite is close to minimum melt compositions
in mafic–tonalitic experimental systems at the pressure
conditions of the M2 granulite-facies event at Tromøy
(Fig. 4b), suggesting that the older, less silicic lithologies
in the complex may have acted as the source rock for
trondhjemitic partial melts. The distinct positive europium anomaly and the rising HREE distribution pattern
of the ordinary trondhjemite (Fig. 12a) indicate that
neither plagioclase nor garnet remained in the residue
after extraction of trondhjemite melt. Otherwise, these
minerals would have retained enough Eu, Yb and Lu to
give a negative Eu anomaly and a declining HREE
distribution pattern.
Simple mass-balance estimates on average compositions (derived from Table 1), indicate that the maximum yield of trondhjemitic liquid is of the order of
45–50% from a mafic gneiss protolith, and as high as 80%
from an average tonalite, assuming that all plagioclase is
consumed. The restite consists of 95–100% hornblende,
with minor clinopyroxene and iron–titanium oxides.
However, even with plagioclase completely removed from
the residue, none of the lithologies observed at the present
section through the Tromøy complex would be able to
produce partial melts with positive europium anomalies
and the overall low REE level observed (Fig. 12b).
926
KNUDSEN AND ANDERSEN
TROMØY GNEISS COMPLEX, NORWAY
Table 7: Partition coefficients used in modelling of tonalitic–trondhjemitic magmas
Ce
Nd
Sm
Eu
Gd
Yb
Lu
Qtz
0·014
0·016
0·014
0·056
0·017
0·017
0·014
Pl
0·27
0·21
0·13
2·15
0·097
0·049
0·046
Kfs
0·037
0·035
0·025
4·45
0·025
0·3
0·33
Opx
0·93
1·25
1·6
0·825
1·9
2·2
2·25
Cpx
0·5
1·11
1·67
1·56
1·85
1·58
1·54
Hbl
1·52
4·26
7·77
5·14
8·4
6
Grt
Ap
0·69
34·7
0·603
57·1
2·035
0·515
62·8
30·409
10
6·975
56·3
43·475
39·775
23·9
20·2
Bt
0·037
0·044
0·058
0·145
0·082
0·179
0·185
Mt
1
1
1
1
1
1
1
Ilm
0·006
0·0075
0·01
0·007
0·0017
0·075
0·1
Values in italics are partition coefficients for tonalitic melts from Martin (1987); the others are taken
from the compilation of partition coefficients for rhyolitic–dacitic melts by Rollinson (1993).
Generation of a low-REE liquid with a positive Eu
anomaly requires partial melting of a protolith that
itself has a moderate positive Eu anomaly, such as a
plagioclase-rich cumulate. A model cumulate consisting
of 50% plagioclase, 30% clinopyroxene and 20% orthopyroxene would be able to produce 35–45% of trondhjemitic liquid, depending on its plagioclase composition,
and a 100% hornblende restite by partial melting. A
leucogabbroic to dioritic cumulate of this character would
naturally form by low-pressure fractional crystallization
of a tonalitic magma. It differs only slightly from the
mafic gneiss, mainly in having higher CaO + Na2O.
The REE pattern of a leucogabbroic–dioritic model
cumulate would mimic the mafic gneiss, but for a slight
positive Eu anomaly and marginally lower LREE
(Fig. 12a). Ten to 40% partial melting of a leucogabbroic–
dioritic cumulate would in turn produce a liquid with a
positive Eu anomaly and an LREE distribution mimicking that of the trondhjemite dykes. However, its HREE
distribution would be flat, significantly underestimating
the observed Yb and Lu concentrations (Fig. 12a). Accumulation of 1–3% of garnet in the trondhjemite, in
agreement with the minor modal abundance of garnet,
adequately reproduces the observed increase in Yb and
Lu concentrations of ordinary trondhjemite.
Hornblendite and garnet-rich trondhjemite
The field relations and the coarse-grained texture of the
hornblendite and garnet-rich trondhjemite suggest that
these rocks are genetically related to the intrusive trondhjemite dykes. In silica-rich liquids, hornblende has partition coefficients for REE well above 1·0 (Martin, 1987;
Rollinson, 1993). Nevertheless, the pegmatitic hornblendite in the Tromøy complex has REE concentration
levels comparable with the normal trondhjemite dykes
(Table 3). However, the garnet-rich trondhjemite (7/96)
spatially associated with hornblendite (8/96) has distinctly
lower REE concentrations than any of the other samples
analysed. The extreme REE distribution patterns of
these rocks are thus most probably due to late-stage
differentiation processes in anatectic melts that otherwise
form the ordinary trondhjemite. The coarse-grained
hornblendite does not represent the (unexposed)
hornblendite residue after trondhjemite formation, as this
is likely to show substantially higher REE concentrations
in accordance with the relatively high REE partition
coefficients for hornblende (Martin, 1987; Rollinson,
1993).
Element mobility during 1100 Ma highgrade metamorphism
The present data strongly suggest that most of the geochemical characteristics of the Tromøy complex can
be explained by magmatic fractionation processes and
subsequent anatexis. The tonalites and mafic gneisses at
the present level of exposure have generally retained
their primary whole-rock REE characteristics through
the ~100 my high-grade metamorphism.
REE fractionation took place at mineral scale only,
during growth of metamorphic garnet. These findings
disagree with earlier interpretations of the Tromøy rocks
as severely metasomatized or as depleted in LILE and
REE by syn-metamorphic fluids during the high-grade
metamorphism (Moine et al., 1972; Touret, 1985, 1996;
Cameron, 1989). The present findings are also in conflict
with earlier ideas about severe element mobility, which
927
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VOLUME 40
NUMBER 6
JUNE 1999
Fig. 12. (a) Rare earth element distribution patterns for rocks of the Tromøy complex, normalized to average CI chondrite values of Boynton
(1984). (b) Modelling of the REE distribution of ordinary trondhjemite. The observed range of ordinary trondhjemite is shown by shading. The
bold, broken lines are average tonalitic and mafic gneiss compositions, respectively. The bold, grey line represents a leucogabbroic–dioritic
cumulate from tonalitic magma (composition as given), which is the most likely source of trondhjemitic anatectic melts. The continuous lines
(10–40) are partial melts of this source rock (numbers indicate percent of melting), leaving a 100% hornblende residual. The light broken lines
branching off from these at Eu are the distributions resulting from 3% of garnet accumulation in these anatectic liquids.
would have caused resetting of the Rb–Sr system (Field
& Råheim, 1981, 1983; Weis & Demaiffe, 1983; Field et
al., 1985), which inevitably has been related to highgrade metamorphism.
The high-grade event affecting the coastal part of the
Bamble Sector at ~1100 Ma reached P–T conditions of
7·5 ± 0·5 kbar, 840 ± 40°C (Knudsen, 1996), which
corresponds to a level well within the lower continental
crust. At the thermal maximum, the Tromøy complex was
in a state of partial melting, generating a trondhjemitic
anatectic melt. This process did not, however, involve
extraction and upwards migration of hydrous, potassic
and LILE-enriched granitic magmas, as envisaged by
Frost et al. (1989), and did not generate a ‘classical’
compositional stratification in the continental crust with
a ‘depleted’ and dehydrated lower crust, as suggested by
Field et al. (1980) and Cameron (1989). In fact, the
opposite evolution took place, as trondhjemitic melts
forming below the present erosional section through the
tonalite complex had lower concentrations of LILE and
REE than their source rocks (Figs 5a and 11). As hornblende was an important restite phase, water was kept
back in the solid residue, whereas CO2 was dissolved in
the anatectic melts, to be released during crystallization
of the trondhjemite intrusions.
The free fluid phase in the Tromøy gneisses during
the high-grade event is well characterized, consisting of
carbon dioxide with a mantle d13C signature (Hoefs &
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KNUDSEN AND ANDERSEN
TROMØY GNEISS COMPLEX, NORWAY
Touret, 1975; Van den Kerhof et al., 1994). This fluid
phase is abundant also in the 1100 Ma trondhjemite dykes
and veins (Knudsen & Lidwin, 1996), which suggests that
the carbonic fluid was contained within the rock complex
since its primary crystallization at 1200 Ma, without
exchange with the country rocks or dilution by externally
derived fluids. The stabilization of hornblende in the
restite after generation of trondhjemite melt indicates
some introduction of water-bearing fluids to deeper levels
of the complex after its primary crystallization, but the
mafic and tonalitic gneisses at the present level of exposure
have stayed remarkably closed to fluid-induced element
exchange with their surroundings during Sveconorwegian
metamorphism. The reason may be found in the rheologic properties of the rather massive mafic and tonalitic
gneisses and that lithologies affected by late external
fluids have been systematically avoided during sampling.
The tonalites and metapelites exposed at the present
section through the Tromøy–Hisøy–Torungen area experienced high-grade incipient melting only [Fig. 3a and
Knudsen (1996)], producing millimetre-wide melt pods
or veinlets unable to segregate (Knudsen, 1996). This
process was locally controlled by mineral reactions giving
a strongly reduced water activity. A fluid composition of
XH2O = 0·3 can be estimated for nearby metapelites
(Knudsen, 1996), again with CO2 as an important free
fluid (Van den Kerkhof et al., 1994; Knudsen & Andersen,
1997).
There is textural evidence of zircon reworking in the
Tromøy complex (Fig. 6), and a high number of inversely
discordant zircons giving 206Pb/207Pb ages younger than
1100 Ma suggest a separate episode of U loss after 1100
Ma. This relatively late process might also have affected
whole-rock U concentrations, but as calc-alkaline rocks
generally have low LILE concentrations of a few parts per
million (i.e. McCulloch & Gamble, 1991; Hawkesworth et
al., 1994), and a majority of the samples have concentrations at or below the XRF detection limit, this
cannot be evaluated from the present data. A study
of element mobility in the metasediments and mafic
granulites of the Bamble Sector (Knudsen et al., 1997b)
has demonstrated that the surrounding area experienced
a large-scale U loss, which may be a parallel to the
process observed in the Tromøy complex.
Tectonic setting of the 1200 Ma Tromøy
magmatism
Only geochemical parameters can give any indications
of the 1200 Ma tectonic setting of the Tromøy gneisses,
as the present-day mineralogy and field relations reflect
the ~1100 Ma metamorphic overprint. The major and
trace element compositions of the mafic and tonalitic
gneisses suggest that the protoliths were similar in composition to present-day magmatic rocks formed at destructive plate margins. The observed low-K series is
most commonly associated with immature island arcs
and is much less frequent in mature arcs or continental
margin settings (e.g. Hess, 1989; Smith et al., 1997). At
modern destructive plate margins, magmas are generated
by partial melting within a hydrated and metasomatized
mantle wedge above a subducted slab (Barker, 1979) and
primitive melts may undergo fractional crystallization in
sub-arc magma chambers (e.g. Smith et al., 1997). A
crustal signature in oceanic island arc magmas is commonly interpreted as the result of metasomatic enrichment of the mantle wedge in LILE-enriched
components with a crustal prehistory, derived from subducted sediments (e.g. McCulloch & Gamble, 1991;
Pearce & Parkinson, 1993; Hawkesworth et al., 1994). In
a magmatic arc underlain by continental crust, mantlederived magmas can easily be contaminated by assimilation during ascent (Hildreth & Moorbath, 1988;
Barnes et al., 1995, 1996; Galán et al., 1996; Mason et
al., 1996).
The magma(s) forming the Tromøy igneous protolith
contained components derived from a depleted mantle
and an evolved, LILE-enriched upper-crustal component,
but the present lead isotope data indicate that the deep
continental crust of the SW part of the Baltic Shield was
not involved in the petrogenesis of these rocks. Metapelitic
lithologies make up an important constituent of the crust
surronding the Tromøy complex and represent a possible
continental crust source. However, the effects of contaminating a mantle-derived magma with (1) high-K
granitic melts derived from deep-seated metapelites, (2)
aqueous fluids or (3) the metapelites are not observed.
These processes generally produce negative eNdi values
(Hildreth & Moorbath, 1988; Barnes et al., 1995, 1996;
Galán et al., 1996; Mason et al., 1996) and medium- to
high-K rocks, which are not observed. The high to
extreme K/Rb ratios observed in the trondhjemites do
not favour a massive influx of aqueous fluids or brines
derived by dehydration of metapelitic lithologies at this
stage, as such fluids would be expected to carry significant
amounts of Rb into the Tromøy complex, lowering the
K/Rb ratio of the anatectic melts. The conspicuous
absence of sediment-derived, pre-1360 Ma inherited zircons from the samples of tonalitic and mafic gneiss
analysed in the present study adds to the evidence against
direct interaction of Tromøy magmas with their present
country-rocks during emplacement at 1200 Ma. The
absence of a discernible local crustal input to the magmas
suggests that the protoliths of the mafic and tonalitic
gneisses were emplaced as part of an island arc system,
somewhere off the margin of the Baltic Shield.
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JUNE 1999
accordance with earlier observations of abundant mantlederived magmatic CO2 inclusions in the complex.
Accretion of the arc fragment onto the Baltic Shield
continental margin took place at medium- to high-grade
metamorphic conditions, during the early phase of
Sveconorwegian metamorphism at 1100 Ma. In this
process, trondhjemitic melts formed by anatexis of a
plagioclase-rich facies of the complex, some of which
aggregated and intruded as dykes in the older lithologies.
The trondhjemites represent 10–45% melting of plagioclase-rich cumulates, leaving a hornblende residue
behind. Late-stage differentiation processes of the trondhjemite melt caused formation of rare garnet-rich trondhjemite and hornblendite with extremely low REE
concentrations.
Rb–Sr and Pb–Pb correlation lines with apparent ‘ages’
of 1703 ± 290 Ma and 1480 Ma pre-date SIMS U–Pb
ages of magmatic zircons by several hundred million
years. Sr, Nd and Pb isotope data indicate mixing between
a mantle-derived component and an LILE-enriched,
upper-crustal component. The Pb-isotopic signature of
the crustal component is distinctly different from the
lower continental crust present in the southwestern part
of the Baltic Shield prior to 1100 Ma, and the data
suggest that the LILE-enriched crustal component was
introduced from subducted sediments into the source of
magmas in a mantle wedge.
The present LILE, HFSE and REE concentrations of
the tonalites and mafic gneisses are connected to the
primary crystallization conditions and not the results of
element mobility during the high-grade metamorphism.
However, an event of U loss occurred after the highgrade metamorphism at 1100 Ma and caused zircon
reworking along fluid-induced channels and produced
negatively discordant magmatic and metamorphic zircon
grains.
The present geochemical evidence indicates that the
Tromøy gneisses represent a metamorphosed and deformed fragment of an ~1200 Ma low-K igneous complex, which probably formed in an island arc setting. If
this is the case, there must be a significant tectonic break
between the Tromøy area and adjacent parts of the
Bamble Sector. Tonalitic rocks metamorphosed in the
amphibolite facies form a discontinuous belt along most
of the south coast of Norway southwest of Tromøy
(Starmer, 1987), but unlike at Tromøy, trondhjemitic
intrusions are not observed in these medium-grade
gneisses (authors’ unpublished field observations). In the
granulite-facies area immediately southwest of Tromøy
island, no tonalitic gneisses are exposed at the present
surface, but the metasediments and an early
Sveconorwegian gabbro are crosscut by trondhjemitic
dyke intrusions indistinguishable from those at Tromøy
(Knudsen & Lidwin, 1996), which indicates the presence
of Tromøy-like gneisses at depth at ~1100 Ma in this
area as well. These observations suggest that a substantial
amount of Tromøy-type juvenile crust may have formed
south of the present coast of Norway at ~1200 Ma. The
1100 Ma Sveconorwegian medium- to high-grade event
recognized in the Bamble Sector including Tromøy (Kullerud & Machado, 1991; Kullerud & Dahlgren, 1993)
appears to be specific for this area. It pre-dates the main
Sveconorwegian orogeny recognized in the rest of South
Norway and Southwest Sweden by ~100 my ( Johansson
et al., 1991; Dahlgren, 1996; Bingen & Van Breemen,
1998), but this has remained unexplained. We suggest
that the 1100 Ma collisional event represents a likely
time for the accretion of the Tromøy island arc system
onto the southwestern margin of the Baltic Shield.
CONCLUSIONS
The Tromøy calc-alkaline gneisses have retained many
features of their magmatic precursors despite the fact
that the rocks have suffered a granulite-facies metamorphism. A suite of tonalites, mafic rocks (of basaltic–
trachyandesitic compositions) and unpreserved or
unexposed plagioclase-rich, pyroxene-bearing cumulates
(leucogabbro–diorite) represent remnants of an island arc
formed south of the present coast of southernmost Norway at ~1200 Ma. The tonalites probably formed by
fractional crystallization of a mafic parent magma similar
in composition to the mafic gneiss, with plagioclase,
pyroxene, magnetite, biotite and apatite as crystallizing
phases; alternatively, both rock types are differentiates
from a common mafic parent magma. Fractionation and
emplacement of the parent magma took place at pressure
conditions where garnet was unstable, and with a reduced
water activity (restricted amounts of hornblende), in
ACKNOWLEDGEMENTS
The thorough and constructive reviews of Ken Johnson
and an anonymous reviewer are gratefully acknowledged.
We are greatly indebted to several persons in the NORDSIM laboratory: Martin Whitehouse for assistance with
the SIMS data reduction, Torbjörn Sunde for assistance
during zircon analyses and Jessica Vestin for zircon
mounting. Turid Winje kindly assisted with the BSE
imaging of the zircons, and Gunnborg Bye-Fjeld and
Toril Enger assisted during sample preparation. The
advice and comments of Else-Ragnhild Neumann are
appreciated. The partners of the NORDSIM consortium,
in particular the Norwegian Reseach Council, are
thanked for making this work possible. The analytical
930
KNUDSEN AND ANDERSEN
TROMØY GNEISS COMPLEX, NORWAY
work was supported by grants from the Nansen Foundation to T.-L. Knudsen and from the Norwegian Reseach
Council to T. Andersen.
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