Download Geochemistry and petrogenesis of extrusive rocks, dykes and high

Geochemistry and petrogenesis of extrusive rocks,
dykes and high-level plutonic rocks on the island of
Oldra, Solund-Stavfjord Ophiolite Complex, western
Norway
HANNE LONE RYTIV AD, HARALD FURNES, KJELL PETIER SKJERLIE & RINN ROLFSEN
Ryttvad, H. L., Furnes, H., Skjeriie, K. P. & Rolfsen, R.: Geochemistry and petrogenesis of extrusive rocks, dykes and high-leve!
plutonic rocks on the island of Oldra, Solund-Stavfjord Ophiolite Complex, western Norway. Norsk Geologisk Tidsskrift, Vol. 80,
pp. 97-110, Oslo 2000. ISSN 0029-196X.
The island of Oldra, part of the Late Ordovician Solund-Stavfjord Ophiolite Complex of the western Norwegian Caledonides,
comprises extrusive rocks, a sheeted dyke complex and high-leve! gabbros. The metabasalts are of N-MORB affinity, and their Nd
isotopic composition (eNd=+7.8 to +8.4) indicates generation from a relatively homogeneous, bul strongly depleted mantle.Eighty
percent of the metabasalts classify as FeTi-basalts (FeO'/MgO >l. 75, Ti02 >2 wt% ), but pseudostratigraphically their distribution
varies significantly (51% of the extrusive rocks and 92% of the sheeted dykes). The contents of FeO', MgO, AI203, Cr, Ni and Zr
indicate that differences in depth of melting (30 to 12 kbar), crystal fractionation of olivine, plagioclase and clinopyroxene at
different levels in the mantle and crust ( <8 kbar) and magma mixing affected the composition of the rocks. The MgO-Zr relations
of the established chemostratigraphy of the extrusive rocks indicate that magma mixing caused significant scatter among magma
compositions.
Hanne Lone Ryttvad, Norsk Hydro ASA, Sandslivn.
90,
N0-5049 Bergen, Norway; Harald Furnes, Geologisk Institutt, A/legt. 4/,
N0-5007 Bergen, Norway (e-mail. [email protected]); Kjell Petter Skjerlie, Institutt for Geologi, N0-9037 Tromsø,
Norway; Rinn Rolfsen, Esso Norge AS, Grensevn. 6, N0-4313 Sandnes, Norway
Introduction
Field relationships
The Solund-Stavfjord Ophiolite Complex (SSOC) occurs
in the Upper Allochthon of the Norwegian Caledonides in
westernNorway (Furnes et al. 1990), where it crops out on
a number of islands and skerries in the Solund-Stavfjord
area (Fig. 1). A quartz diorite from within a high-level
gabbro provided a U-Ph zircon age of 443 ± 3 Ma
(Dunning & Pedersen 1988).
The part of the SSOC presented here, from the island of
Oldra, represents crust that may have been generated in a
propagating rift setting (Skjerlie et al. 1989; Dilek et al.
1997), and its geochemical evolution is dominated by
FeTi-basalts, a typical feature displayed at short distances
(0-100 km) behind active propagating rift tips (Furnes et
al. 1998). The ophiolite pseudostratigraphic units on Oldra
are, from bottom to top: vari-textured and fine-to-medium­
grained gabbro, a sheeted dyke complex and extrusive
rocks. The rocks have suffered lower greenschist-facies
metarnorphism of ocean-floor and subsequent late Cale­
donian origin (Fonneland 1997).
The aims of this study were to establish the stratigraphy
of the extrusive rocks in order to study the magmatic
evolution with time, and to propose a petrogenetic model
based on field observations and the geochemical evolution
of the metabasalts.
The distribution of extrusive rocks, sheeted dykes and
gabbros, each separated by steeply dipping shear zones
striking at approximately 040°, is shown in Fig. l.
Extrusive rocks
The extrusive rocks comprise pillow lavas, massive lava
flows and hyaloclastite breccias, described in detail by
Furnes (1972, 1974). The shape of the pillow lavas, convex
roofs and cusps at the bases, in addition to well-defined
drain-out structures, shows the way up in the volcanic
sequence. Dykes appear in close association with extrusive
rocks. A mixed zone of dykes and screens of extrusive
rocks represents a transition zone between the sheeted
dyke complex and the volcanic sequence. The pillow lavas
are aphyric to moderately phyric (<l% and 2-10%,
respectively).
Sheeted dykes
The sheeted dyke complex is dominated by parallel to
subparallel, regular dykes. Most of the dykes (65%) have
not been split and range in width from 1.5 to 190 cm, with
an average thickness of 71 cm (Ryttvad 1997). This
98
H. L. Ryttvad et al.
NORSK GEOLOGISK TIDSSKRIFT 80
(2000)
A
10km
� Western Gneiss Region
[=:J
}
Undiff. Precambrian
� Caledonian nappes
bd
Oslo Graben
Devonian basinal strata
Domain
Domain
11
Domain Il l
Undifferent1ated
Ill
D
Q
J+ +l
�
�
�
Sognefjorden
Solund-Stavfjord
ophiolite Complex
and 1ts sed1mentary
cover ( Slavenes
Group )
Sunnfjord Melange
Herland Group
( HG )
Høyvik Group
Dalsfjord suite
Undifferentiated mylonties and
gneisses of the western Gneiss Region.
Eclogite-bearing rocks ( e ) are indicated.
Nordfjord-Sogn Detachment Fault
Other normal faults
Extrusive rocks (pillow lavas, massive flows,
hyaloclastite breccia)
Transition zone
Sheeted dyke complex
,4.
A
�
A'-,8
\
��
Vari-textured and
fine- to medium-grained gabbro
Strike and dip of dyke
Strike and dip of foliation
Younging direction of pillow lava
Profile line
Shearzone
Sense of shear
500m
Fig. l. A. Simplified geological map of part of western Norway, showing the occurrence of the Solund-Stavfjord Ophiolite Complex (SSOC) within the Norwegian
Caledonides (modified from Dilek et al. 1997). B. Simplified geological map of Oldra.
suggests that pre-existing dykes may have offered struc­
tural guidance to succeeding dykes, favouring adjoining
intrusions (Cann 1974). Where younger dykes split older
dykes, they provide a way to study the evolution of magma
through time (e.g. Robson & Cann 1982). At least six
generations of magma intrusions, revealed by a series of
dyke-splittings, occur within the sheeted dyke complex.
The dykes are invariably aphyric.
High-leve/ plutonic rocks
On the basis of textural variations, two types of plutonic
Fine to medium
FMG16
FMG28
FMG29
FMG18
FMG17
14.10
13.36
13.68
13.38
13.84
5.16
14.14
4.38
4.12
4.86
14.70
4.67
4.61
4.66
4.92
4.34
4.58
14.50
4.19
4.44
3.96
Dykes from the sheeted dyke complex
12.33
47.94
3.05
0171
13.21
2.51
49.07
DS
2.78
12.66
48.53
0169
2.86
12.58
48.13
0173
2.71
12.73
48.45
0172
13.82
2.57
47.71
013
12.79
48.29
2.69
0181
12.87
2.62
48.37
0162
12.87
2.75
48.40
0157
2.71
13.06
48.04
D44
13.01
48.97
2.55
065
13.14
2.57
48.18
055
2.76
13.53
48.34
09
13.91
2.55
47.05
070
13.87
2.44
47.99
054
2.32
13.68
47.94
050
grained gabbro
2.31
49.20
2.26
48.87
2.10
48.77
2.18
47.31
2.31
47.38
3.59
4.21
3.55
3.56
3.15
12.37
13.26
12.93
13.28
14.21
Dykes from the transition zone
2.87
47.53
TZ!02
TZ97
48.16
2.36
47.38
2.56
TZ108
2.22
TZI07
47.93
1.86
47.52
TZ95
3.12
4.80
4.41
4.23
4.09
4.04
14.33
4.20
13.52
3.83
13.13
14.67
13.40
3.36
3.68
12.51
13.48
13.06
14.34
13.08
14.06
15.30
14.04
13.49
13.22
Fe203
Pillow Javas and massive flows
47.98
EK140
2.73
48.63
EK16
2.58
2.51
EK146
48.02
2.41
45.26
EK1
2.37
EK132
47.46
2.30
EKS
47.31
2.83
43.91
EK3
2.24
EK12
47.90
2.48
49.05
EK145
48.00
2.30
EKI25
Sample
Ai203
Si02
8.85
8.35
8.17
9.32
9.72
10.04
n.a.
9.85
10.62
9.21
n.a.
9.34
8.73
9.02
8.83
8.51
8.92
n.a.
9.96
8.48
8.49
10.72
8.56
10.22
9.00
8.10
10.31
n.a.
8.92
n.a.
9.30
n.a.
n.a.
n.a.
8.38
8.06
FeO
0.14
0.18
0.17
0.19
0.19
0.26
0.25
0.32
0.32
0.28
0.26
0.27
0.23
0.23
0.21
0.25
0.25
0.27
0.25
0.21
0.24
0.23
0.22
0.24
0.22
0.20
0.25
0.20
0.22
0.20
0.21
0.20
0.21
0.22
0.20
0.20
MnO
6.84
6.87
6.89
7.01
7.02
5.54
6.13
6.20
6.30
6.33
6.41
6.44
6.48
6.58
6.61
6.67
6.69
6.78
6.98
7.09
7.11
6.42
6.56
7.09
7.15
7.80
6.12
6.21
6.37
6.61
6.73
6.76
6.77
6.77
7.14
7.17
MgO
9.07
10.47
10.46
9.66
8.96
9.38
8.93
9.28
8.49
9.65
9.21
9.01
9.87
9.48
9.88
9.23
10.21
8.93
8.30
9.75
10.00
10.03
9.37
9.70
10.69
11.89
10.13
10.57
10.78
10.67
11.39
10.90
9.83
11.51
7.96
12.02
CaO
3.57
2.85
3.15
3.08
3.17
3.17
3.03
3.18
3.40
3.14
2.88
3.09
3.33
3.47
3.18
3.56
3.03
3.26
3.18
3.13
3.07
2.48
2.79
2.75
2.55
2.14
2.48
3.32
2.82
2.53
2.37
2.66
2.29
2.20
3.84
2.48
Na20
Representative major and trace element analyses of volcanic rocks, dykes and gabbros from Oldra.
Ti02
Table l.
0.12
0.12
0.13
0.11
0.12
0.05
0.08
0.05
0.04
0.05
0.02
0.03
0.03
0.04
0.05
0.04
0.05
0.06
0.03
0.04
0.05
0.03
0.03
0.05
0.04
0.03
0.17
0.18
0.09
0.27
0.15
0.12
0.18
0.09
0.07
0.06
K20
0.20
0.22
0.19
0.20
0.22
0.30
0.23
0.24
0.26
0.24
0.24
0.25
0.25
0.25
0.24
0.23
0.25
0.25
0.22
0.21
0.21
0.24
0.17
0.21
0.15
0.13
0.24
0.22
0.22
0.22
0.20
0.20
0.27
0.19
0.21
0.19
P205
1.38
1.01
1.00
1.56
2.04
1.77
2.31
1.54
1.79
1.65
1.99
2.02
1.80
1.52
1.44
1.62
1.55
1.95
2.39
1.93
1.88
2.50
2.81
2.24
2.28
2.04
1.74
1.91
1.88
2.95
1.72
1.73
2.88
1.88
2.41
1.71
LOi
98.90
99.36
99.12
98.23
99.06
98.99
99.89
99.01
98.91
99.30
99.81
98.89
99.19
99.27
99.17
98.98
99.42
100.63
99.01
99.58
98.95
99.01
98.50
98.92
99.07
99.07
98.70
101.63
99.09
98.98
98.81
99.37
99.14
100.44
98.59
99.09
Sum
396
372
336
375
388
457
389
433
445
406
392
427
407
431
429
403
414
431
418
396
397
456
391
420
387
348
412
403
407
400
404
366
448
364
438
388
V
241
145
238
205
174
81
!59
134
151
150
213
161
256
249
191
301
248
186
321
299
299
142
271
221
215
319
95
223
211
328
258
265
238
259
256
297
Cr
67
59
60
73
59
44
51
58
55
53
65
58
66
68
58
65
70
61
89
88
77
57
73
62
50
95
51
67
64
78
74
75
91
83
71
79
Ni
121
157
150
140
121
128
133
121
95
108
147
104
157
119
114
114
126
100
110
125
127
127
147
142
147
119
140
159
146
173
168
130
174
135
130
166
Sr
59
57
55
59
57
84
71
74
79
70
74
72
70
76
72
68
70
77
71
66
62
76
63
69
55
48
77
67
66
69
64
62
79
61
70
59
y
166
155
142
166
156
228
185
201
213
191
186
196
189
199
193
184
180
201
187
173
163
203
167
178
152
114
196
181
176
170
165
153
202
145
173
159
Zr
9
9
7
9
9
Il
8
li
Il
9
8
Il
10
12
lO
!O
10
8
lO
10
8
10
9
9
9
7
lO
8
9
7
lO
7
9
8
10
lO
Nb
8
\0
\0
Q
.§
�
�
-·
(5•
;:::-
�
�
�
�
�
?V:!
;::s
�
�
§
-.::>
00
o
�
�
;<l
Cll
Cll
...,
o
Cl
i'ii
�
�
t""
z
o
;<l
Cll
�
Cl
100
H. L. Ryttvad et al.
NORSK GEOLOGISK TJDSSKRIFf 80
(2000)
Table 2. Representative rare earth elements and Hf analyses of volcanic rocks (EK), dykes (TZ and D) and gabbro (FMG) from Oldra. The numbers in brackets are
extrapolated.
Sample
La
Ce
Nd
Sm
Eu
Gd
Th
Ho
Yb
Lu
Hf
EK 3
EK 140
TZ 9S
TZ 108
D 70
FMG 18
21
12.6
8.7
16.6
13.1
9
[31]
21.8
[16]
[26]
18.S
23.7
l S.l
14.8
8.S
10.7
10.2
[13]
S.S
S.4
2.9
4.9
3.7
4.4
1.09
0.83
0.74
0.6
0.79
1.08
8.4
8.7
[4.5)
[7)
[4.2]
[S.S]
[1.4]
[l.S]
0.9
[1.3]
0.8
[l]
2
2.2
[1.4]
1.8
[1.3]
l.S
4.8
S.2
3.8
4.3
[3.4]
3.6
0.9
l
0.7
0.9
0.6
0.7
2.9
4.3
2.7
3.7
4.S
3.4
rocks can be distinguished: fine- to medium-grained
gabbro (FMG) and vari-textured gabbro (VTG). The
aphyric, FMG occurs as irregular bodies intruded by
regular, 10 to 40 cm-wide, and irregular, 0.5 to 15 cm­
wide, mafic dykes. Some of the dykes are rooted in
irregular gabbro bodies. This close, rooted association of
dykes and irregular magma bodies has been interpreted as
the root zone of a sheeted dyke complex (Pedersen 1986;
Nicolas & Boudier 1991).
The VTG varies in grain size from medium grained to
pegmatitic. Primary plagioclase, clinopyroxene and mag­
netite are partly preserved, and secondary minerals are
actinolite, epidote, chlorite and leucoxene. Plagioclase­
rich layers within the VTG indicate that it partly represents
a cumulate rock. The geochernistry of the VTG will
therefore not be considered here.
Samples were dissolved in a rnixture of HF and HN03•
REE were separated by specific extraction chromatogra­
phy using the method described by Pin et al. (1994). Sm
and Nd were subsequently separated using a low-pressure
ion-exchange chromatographic set-up, with HDEHP­
coated Tefion powder as the ion-exchange resin (Richard
et al. 1976). Sm and Nd were loaded on a double filament
and analysed in static mode and their concentrations were
5
deterrnined using a mixed 1 CNd/149Sm spike. Nd isotopic
ratios were corrected for mass fractionation using a
146Nd/144Nd ratio of 0.7219. Repeated measurements of
3
the JM Nd-standard yielded an average 14 Nd/144Nd ratio
of 0.511113 ± 15 (2a) (n = 62). The typical Nd blank leve!
in the laboratory is 5 pg.
Geochemistry
Analytical techniques
Major- and trace-element analyses were performed on an
X-ray fiuorescence spectrometer (XRF) at the University
of Bergen. The glass-bead technique of Padfield & Gray
(1971) was used for the major elements and pressed­
powder pellets for the trace elements, using international
basalt standards with recommended or certified values
from Govindaraju (1994) for calibration. The rare earth
elements (REE) and Hf were analysed by instrumental
neutron activation analysis (INAA) using a coaxial HP Ge
detector and the analytical methods described by Brunfelt
& Steinnes (1969).
Sm and Nd isotopes were analysed on a Finnegan 262
mass-spectrometer at the University of Bergen. All
chemical processing was carried out in a clean-room
environment with HEPA filtered air supply and positive
pressure. The reagents were either purified in two-bottle
Tefion stills or passed through ion-exchange columns.
Table 3.
The geochemical study is based on major- and trace­
element analyses of the extrusive rocks, the transition zone
dykes, the sheeted dykes and the FMG (collectively
referred to as metabasalts). A total of 193 samples (54,
18, 102 and 19 from the volcanic zone, the transition zone,
the dyke complex and the FMG, respectively) were
analysed. Of these, six representative samples were
analysed for REE, Hf, and Sm and Nd isotopes. A
selection of representative major- and trace-element
analyses are shown in Table l. The REE and Hf analyses
are presented in Table 2, and Sm-Nd isotopes in Table 3.
Alteration effects
For co-magmatic metabasaltic rocks, Coish (1977) pro­
posed that elements which systematically co-vary with Zr
have not been mobilized to a great extent. For the Oldra
metabasalts, Ti02 displays nearly linear, and Fe2031 and
MgO relatively good co-variation with Zr. Si02, A}z03,
Representative Nd isotope analyses of volcanic rocks (EK), dykes (TZ and D) and gabbro (FMG) from Oldra.
Sample
Sm ppm
Nd ppm
147Sm/144Nd
EK 3
EK 140
TZ 9S
TZ 108
D 70
FMG 18
6.92
6.Sl
4.02
S.82
6.0S
S.l8
20.40
19.11
11.13
17.14
17.78
1S.62
0.21
0.21
0.22
0.21
0.21
0.20
143Nd/144Nd
O.S l 3086 +10.51307S +10.513132 +10.513103 +10.513088 +1O.S13063 +1-
s
6
8
9
s
6
eNd (1=440)
8.09
7.84
8.2S
8.42
8.11
7.92
NORSK GEOLOGISK TIDSSKRIFT 80
(2000)
100 r-------�
3
..
o-r----;::====::::=:--�
o Extrusive rocks
Transition zone dykes
•
Gl
.:=
..
'O
5
2,5
c3 10
....
.li:
u
a_
--o-EK3
--Å-070
1
Ce
Nd
o Sheeted dykes
o
• Fine- to mediurn-grained gabbro (FMG)
Ferrobasalt
��==�==��==+=�--��-
La
o
-o-EK140
�TZ108
-FMG18
-+-TZ95
Sm
Eu
Gd
Tb
Ho
Yb
Lu
o
--------
•
�o
� ----
10 r-------�
B
o
m
a:
N-MORB
o
:::E
....
.li:
g
--o-EK3
a:
--Å-070
0,1
Sr
Nb
Ce
P.Os
Zr
Hf
o
•
o
o
1,0+-----+----+--l
7,5
8
6,5
7
5,5
6
MgOwt%
-D-EK140
�TZ108
-FMG18
-+-TZ95
101
Solund-Stavjjord Ophiolite Complex
Sm
Ti02
Y
Yb
Cr
Fig. 2. Chondrite-normalized (A) and MORB-normalized (B) multi-element dia­
grams of representative samples from the extrusive rocks, dykes and gabbros.
Chondrite-normalization values after Haskin et al. (1968), and MORB-normali­
zation values after Pearce (1980) (in ppm, unless otherwise stated: Sr=120,
Nb=4, Ce=10, P205=0.12 (wt%), Zr=90, Hf=2.4, Sm=3.3, Ti02=1.5
(wt% ), Y=30, Yb=3.4, Cr=250). Normalized values indicated for Ce, Nd,
Gd, Tb, Ho and Yb, for which no data are given in Table 2, are extrapolated.
CaO, Cr andNi co-vary poorly with Zr, a phenomenon that
may be due to alteration and metamorphism, as well as to
petrogenetic processes (as will be discussed below).
Experimental studies of water-basalt interactions over a
range of different pressure and temperature conditions
show that Fe, Ca and Si may be removed from basalts, and
Na and Mg may be added, whereas Al, Ti, Zr, Cr and Ni
are relatively immobile (e.g. Thompson 1973; Coish 1977;
Staudigel & Hart 1983). Thus, we assume that the
composition of the rocks reftects that of the original
magma.
Classification
Chondrite-normalized REE diagrams and MORB-normal­
ized multi-element diagrams for the metabasalts of Oldra
are presented in Fig. 2. Except for a slight to moderate
enrichment of La, the REE define a flat pattem, mostly 15
to 35 times chondritic values. The samples also show small
to moderate, negative Eu anomalies, which suggest
plagioclase fractionation. The MORB-normalized multi­
element diagram defines relatively horizontal pattems with
ratios from l to 3. This suggests that the rocks represent
melts with compositions similar to moderately fractionated
N-MORBs.
Eighty per cent of the metabasalts classify as FeTi­
basalts (Fig. 3) by using the criteria Fe01/Mg0 > 1.75,
which separate ferrobasalts (Fe01/Mg0 > 1.75, Ti02 > 2
Fig. 3. MgO-FeO'/MgO for the metabasalts of Oldra. The horizontal, dotted line
FeO'!MgO=1.75 separates ferrobasalts (above) from N-MORBs (below) (from
Sinton et al. 1983).
wt%) from N-MORBs (FeOt/MgO < 1.75) (Melson et al.
1976; Sinton et al. 1983). The remaining 20% are of
N-MORB composition. Of the extrusive rocks, 51% are
FeTi-basalts, whereas 83% of the transition zone dykes,
92% of the sheeted dykes and 79% of the FMG are of FeTi­
basalt composition. The rocks of FeTi-basalt composition
have FeOt/MgO ratios from 1.75 to 2.65 and Ti02 contents
ranging from 2.1 to 3.1 wt%. The metabasalts ofN-MORB
composition have Fe01/Mg0 ratios from 1.39 to 1.74, and
Ti02 contents of 1.9 to 2.6 wt%.
Bulk rock composition
The MgO content of the metabasalts ranges from 5.6 to 7.9
wt%, with the majority (93.8%) in the 6.1 to 7.3 wt%
interval (Fig. 4). The concentrations of other elements vary
significantly at a given MgO content. The regression lines
(least squares linear) of the major and trace elements, all
definin � poor fits, are shown in Fig. 4. The best regression
lines (r between 0.225 and 0.383) are obtained for Ti02,
Fe203\ P205 , Y and Zr, whose concentrations increase
with decreasing MgO content (Fig. 4). The concentrations
of A1z03, Cr and Ni decrease with decreasing MgO
content, and the elements show less good � results (�
between 0.194 and 0.283). Plots of Si02 and CaO versus
decreasing MgO content show a relative scatter, but the
regression lines indicate an overall slight increase in the
Si02 concentration and decrease in the CaO concentration
with decreasing MgO content.
Chemical variations in bulk rock composition with time
The chemical data for the extrusive rocks versus strati-
102
H. L. Ryttvad et al.
NORSK GEOLOGISK TIDSSKR!Fr 80
52 T-------,
4 -r-------.
no. wt"J.
32._:>�� 0
•
o
2
r"= 0.03 5
16
0
r' = 0.225
9��
r" = 0.361
100
90
80
7
-rP-��
Crppm
a>
i
--
0
--
::, otP
ro
::.e�
o
o
�
80
50
....
40
5,5
�
r" = 0.026
%
o
o
o
•
o
•
o
Nippm
120
.
6,5
Yppm
o
250
200
o
o�-JI•
=0.354
6
o
.... =0. 222
•
:2-.
Å
o
o
�o��._
11
0,2
o
o•o
CaO wt"k
13
200
Å
•
11
0,3
300
o
;
15 f?._"_t,�
� --
r" =0.194
400
•
-�-
Fe,o.• wt"l.
13
0, 1
•
r' = 0.383
17
o
(2000)
7
MgO wt«'.tO
7,5
150
Zrppm
o_�l&�
o
o
() fl%5
100
8
o
o
•
.:> •
-�-
.... =0.351
5,5
6
6,5
7
MgO wt«'.tO
7,5
8
Fig.
4. Bowen variation diagrams showing MgO versus selected major and trace elements for extrusive rocks (white diamonds), transition zone dykes (lilled trian­
gles), sheeted dykes (white circles), and FMG (lilled squares). The dotted lines are regression lines and il indicates the lit of the regression lines to the data points.
Best lit is il l, while il O indicates no correlation.
=
=
graphic height (decreasing age) are shown in Fig. 5.
Several elements show co-variation throughout the profile.
One group that co-varies is that of Cr and Ni, and to some
extent Ah03. A second group is Ti02o Fe203\ P205, Y and
Zr. The two groups display concentration variations that
plot in zigzag pattems which are relatively good mirror
images of each other. Diagrams demonstrating the
correlation between Ah03 and Ni, Cr and Ni, Zr and
Ti02, and Zr and Y for the extrusive rocks and the dykes
are shown in Fig. 6. The Si02 concentration is nearly
constant throughout the profile, whereas the MgO and CaO
contents display distinct trends toward increase and
decrease, without good correlation, as also demonstrated
in Fig. 4.
Sm and Nd isotope systematics
Six samples that span the Cr and Zr range of the
metabasalts were analysed for Sm and Nd isotopes. The
NORSK GEOLOGISK TIDSSKRIFT 80
Profile
(2000)
Solund-Stavjjord Ophiolite Complex
103
0�-+-r�--���T-��--r-�-+����_L-+--���J_��L---�-r��
12 14
45 47 49 2,0
2,5 3,0 13 14 15
9
11
0,20 0,25 150 250
6,5 7,0 7,5
65 85 60 70 150170190
SiO.
Al203
Fe•O.'
MgO
CaO
p,o,
Cr
Ni
y
Zr
Fig. 5. Selected major and trace elements versus stratigraphic height (decreasing age) for the extrusive rocks. The profile shows the abundance and distribution of
pillow Javas (open rectangles), massive tlows (filled rectangles) and hyaloclastite breccias (dotted rectangles).
Sm/Nd ratios show a narrow range, from 0.3316 to 0.3612,
and eNd va1ues from +7.8 to +8.4 (Table 3).
Discussion
Source region
The similarities observed in the multi-element diagrams
for the Oldra metabasalts (Fig. 2) indicate that these rocks
represent melts that derived from a mantle source region
which, compositionally, was relatively homogeneous. The
relative flat pattems without sign of negativeNb anomalies
argue for a typical MORB source, and the slightly negative
Eu anomalies and generally lower Cr values than average
N-MORB show that the melts are moderately fractionated.
The Nd isotopic data (Table 3) show only minor variations
in the eNd (t=440 Ma) values (+7.8 to +8.4) and a
common source is thus strongly suggested. In Fig. 7 the eNd
data of the SSOC metabasalts are shown in relation to the
depleted mantle growth curve of Nelson & De Paolo
(1985). They all plot above the growth curve, showing that
the rocks represent melts that were generated from a
depleted mantle.
Petrogenetic processes
Langmuir et
al. (1992) showed that differences in the extent and depth
of fractional melting cause variations in the geochemical
compositions of the primitive melt. Magmas generated by
fractional melting at progressively higher pressures
become increasingly enriched in FeO and MgO and
Partial metting and crystal fractionation.
-
depleted in Na20, and crystal fractionation at different
levels in the mantle causes further variations in the
chemistry of the melts.
The melts that gave rise to the metabasalts of Oldra were
modified by crystal fractionation (Fig. 4). The co-variation
of MgO and Ni reflects fractional crystallization of olivine
and the decrease in A}z03 concentration with decreasing
MgO content may indicate crystallization and fractiona­
tion of plagioclase. This is supported by the Ni versus
A}z03 relationships shown in Fig. 6a. The decrease in Cr
concentration with decreasing MgO content may indicate
fractional crystallization of clinopyroxene. However, with
decreasing MgO content, the Cr content decreases sig­
nificantly, while CaO shows a minor decrease (Fig. 4),
which is not compatible with clinopyroxene fractionation
only. The MgO-CaO relations show poor correlation;
hence, only parts of the data are likely to be attributed to
clinopyroxene fractionation. It is thus 1ike1y that the
decrease in the Cr content with fractionation (e.g. Fig.
6b) could partly be related to fractional crystallization of
Cr-spinel. We thus conclude that the primary liquids were
dominantly modified by fractional crystallization of
olivine, plagioclase and Cr-spinel, i.e. typical mineral
phases on the N-MORB liquidus at low pressures, and
olivine, plagioclase and clinopyroxene at higher pressures.
To obtain information on the pressures at which the
Oldra metabasalts were generated, and at which pressures
fractional crystallization took place, the compositions of
pooled melts generated at 12, 20, 30 and 40 kbar were
considered (Langmuir et al. 1992). The Alz03 and Fe01
wt% concentrations for the Oldra metabasalts in the 6-8
wt% MgO range are shown in Fig. 8. The chemical data in
Fig. 8a, b are displayed in diagrams showing the liquid
104
H. L. Ryttvad et al.
100
NORSK GEOLOGISK TIDSSKRIFT 80
90
100
<>o<>
A
80
80
z
70
60
z
o
40
30
60
14
13
12
11
AI203
15
30
16
•
150
50
90
c
3
.�
40
•
3,5
N
70
50
50
o
i=
B
90
o
(2000)
250
Cr
350
o
80
70
2,5
> 60
2
50
o
1,5
<)
50
o
40
150
100
Zr
Fig.
6.
200
30
250
eDykes
�
50
o Extrusives
100
150
Zr
200
250
Selected element-element diagrams for Al203-Ni, Cr-Ni, Zr-Ti02 and Zr-Y.
lines of descent (LLD) for a melt generated at rv20 kbar.
The Fe01 versus MgO data from the Oldra plot within the
LLDs are defined by fractional crystallization at l atm and
8 kbar (Fig. 8b). It should be made clear that the mineral
proportions are markedly different at different pressures.
The proportions of plagioclase:olivine:clinopyroxene used
to construct the LLDs at MgO contents comparable with
the metabasalts on Oldra, are hence 1:0.27:0.56,
1:0.23:0.7, and 1:0.25:5.25 at l atm, 4 kbar and 8 kbar,
respectively (Langmuir et al. 1992). This indicates that
melts generated at rv20 kbar and modified by crystal
fractionation at pressures between 8 kbar and l atm
reproduce the Fe01 versus MgO data from Oldra. How­
ever, crystal fractionation of magma generated at rv20 kbar
cannot explain the scatter in the Al203 data at a given MgO
content, because crystal fractionation at different pressures
of magma generated at rv20 kbar results in coinciding
LLDs for Ah03 at MgO < 8.5 wt% (Fig. 8a).
The Ah03 versus MgO data indicate that magmas were
generated by fractional metting over a range of pressures.
Melts generated at 12 kbar reproduce the Fe01 versus MgO
data when modified by crystal fractionation in the pressure
range rv4 to rv8 kbar (Fig. 8c). A melt generated at 30 kbar
must fractionate in the pressure range rv4 kbar to l atm to
reproduce parts of the data (Fig. 8d), and a melt generated
at 40 kbar only touches the Fe01 versus MgO data when
fractionating at rv l atm (Fig. 8e). This shows that the
metabasalts of Oldra probably represent melts generated at
a range of pressures in the mantle, that were subsequently
modified by crystal fractionation at various levels in the
mantle and crust.
Mixing of magmas. - To illustrate that the rocks, ranging
from N-MORB to FeTi-basalt compositions, may repre8,5
•
8
......
o
..,
..,
--
...
�
7,5
7
Depleted l
I mantle
•
•
l
�------�,�C�H�U�Rr-L--�
6,5
6
400
420
440
460
480
500
Age (Ma)
Fig.
7. eNd and model depleted mantle growth curve of Nelson & DePaolo
(1985), onto which the metabasalts from Oldra are plotted.
NORSK GEOLOGISK TIDSSKRIFT 80
17
16
� 15
�
o"'14
;i13
12
(2000)
Solund-Stavfjord Ophiolite Complex
a
,..,.
melts from 20 kbar
17
15
�
,..,.� 13
� 11
r-e
b
.._ Fe0'1Mg0=1.75
melts from 20 kbar
9
7
17
15
�
i13
�11
r-e
c
�
9
melts from 12 kbar
17
15
�
i13
�11
d
�
�
9
melts from 30 kbar
17
15
�
i13
�11
e
�
�
9
7
melts from 40 kbar
6
7
8
9
MgOwt%
Fig.
10
11
12
105
sent mixed magmas, the concentrations of the compatible
elements Cr and Ni are plotted versus the concentration of
the incompatible element Zr (Fig. 9). The two fractionation
curves are calculated based on data from Tviberg (Skjerlie
1988), which represent the most primitive high-MgO
basalts and the most fractionated FeTi-basalts (Fig. 9).
The fractionation curves have been calculated from an
assumed primary magma, and the distribution
coefficients used for the modelling are shown in Table 4
(for further details, see text Fig. 9).
Compared to the Tviberg metabasalts, whose Cr and Zr
contents range from approximately 30--500 ppm to 50-300 ppm, respectively, those on Oldra show relatively
limited Cr, Ni and Zr concentrations. They plot above the
calculated fractionation curves, and display scatter that is
not consistent with fractional crystallization alone. The
element concentrations of the rocks can be explained by
mixing of different magmas, creating hybrid compositions
(Rhodes et al. 1979). Samples lying above the mixing line
can be explained by mixing of magmas whose composi­
tions extend beyond the chosen maximum values of Cr, Ni
and Zr (Fig. 9). It should be noted, however, that mainly
samples in the Ni-Zr diagram plot above the mixing line.
This feature can be explained by the presence of small
amounts of olivine phenocrysts in the rocks, observed as
pseudomorphs in thin sections, that have increased the Ni
content considerably (see Table 4).
As discussed above, the scatter in the Al203 and Fe01
data can, to a significant extent, be attributed to melt
generation at different pressures. If the total degree of
partial melting differs between various batches of compo­
site melts, this will have an effect on the content of
incompatible elements. Thus, the smaller the degree of
partial metting, the higher the content of an incompatible
element, such as for example Zr (e.g. Cox et al. 1979). If
we, on the other hand, make the assumption that the total
degree of partial melting represented by each batch of
composite melt is comparable, then the scatter displayed
by the Cr-Zr and Ni-Zr diagrams can largely be explained
in terms of mixing of liquids.
Sparks et al. (1980) showed that magmas with approxi­
mately 9-10 wt% MgO occupy a density minimum
compared with FeO-rich, evolved magmas like FeTi­
basalts. Thus, primitive magmas are more buoyant and can
physically mix with more fractionated magma. However,
the ability of two magma batches to mix is controlled by
the density gaps between the melts; a large gap makes
mixing difficult. Thus, if a magma batch is relatively
primitive, substantial mixing with a more evolved liquid
can be minimal (Huppert et al. 1986). Consequently, a
relatively primitive magma batch may ascend rapidly and
may be the first to erupt, followed by denser, more evolved
magma.
8. The MgO versus Ah03 (A) and FeO' (B toE) data for the Oldra metabasalts (193 samples) displayed in diagrams showing the calculated liquid lines of des­
cent (LLD) for melts generated by fractional melting at 20 kbar (a and b), 12 kbar (c), 30 kbar (d) and 40 kbar (e), and subsequently modified by fractional crystal­
lization of olivine, plagioclase and clinopyroxene at l atm, 4 and 8 kbar (moditied from Langmuir et al. 1992). The dotted lines in (a) represent calculated LLDs
for melts generated by fractional melting at pressures higher and lower than 20 kbar, which are subsequently modified by crystal fractionation of clinopyroxene at l
atm, 4 and 8 kbar. The diagonal line FeO'/MgO 1.75 separates ferrobasalts (above, FeO'!MgO > 1.75) from N-MORBs (below, FeO'!MgO < 1.75).
=
H. L. Ryttvad et al.
106
b
2,335
t',, ....
l
....
'' ........
\
3
300
\
....
......
....
. � �·.
•-=-
.
Mixing line
.
.
'....
2
•
_
Fractionation curve
.... ....
....... .... ....
- - --
.('"'
e
"'
........
....
....
�
.�·
..
....
- -----
1
":.� a
Crppm
b �....
l
1
3\
\
e
Cl.
Cl.
..
N
200
100
. .........
\
''
.... ....
'
'--
B
Mixing line
.
........ �
•
-
�
........
�
--•
Fractionatio
2
',
. l
.
•
� ...........
�urve
_
.
.
....
....� ....
..........
--.,------'-a
Cr versus Z:t (A) and Ni versus Zr (B) for the Oldra metabasalts. The
fractionation curves are calculated from a primitive magma containing 900 ppm
Cr, 350 ppm Ni and 70 ppm Z:t. Between the assumed primary magma and 1:
16% fractionation of olivine and cbromite; between l and 2: 40% fractionation
of olivine (0.16) + plagioclase (0.84) + cbromite (trace); between 2 and 3: 45%
fractionation of olivine (0.16) + plagioclase (0.38) + clinopyroxene (0.48). a
and b represent primitive high-MgO basalt and FeTi-basalt, respectively, from
Tviberg. Modified from Skjerlie (1988). Partition coefficients used in calcula­
tions are from Cox et al. (1979) and Pearce & Norry (1979).
9.
o o
o
2,315
oo
o
o
�
2,305
"
A
A
2,300
1,30
,
,
••
00
2,20
1,90
1,60
line FeO'!MgO
=
1.75 separates ferrobasalts (right) from N-MORBs (left).
El
250
�
-7
Modell
250
•
c.
c.
..
N
inpatmagma
----.
�
�
input magma
250
• •
150
Model3
• l
intrudedl
erupted magma
•
•
-
El
c.
c.
..
N
Model4
250
"
150
intrudtd/
eruptedmagma
El
c.
c.
..
N
250
150
"
o
<>
.
e
cbamber magma
l
;-
Olivine
Clinopyroxene
Plagioclase
Z:t
o.o1•
0.1•
om•
• From Pearce & Norry (1979).
From Cox et al. (1979).
b
Cr
Ni
5,5
6.5
MgOwt%
7,5
l
MgOwt%
l
�
\
increase
Zrppm
l
t-----1
t-----1
MgOwt%
Zrppm
\
MgOwt%
i
,.-_.
rncrease
MgOwt%
Input magma
4,5
Distribution coefficients used in fractional crystallization.
Zrppm
t-----1
-�.
'
MgOwt%
.
rncrease
Mode15
't(-.1
,,
t-----1
,.-_.
rncrease
0e
l
t-----1
.
1ncrease
?
intruded/erupted
magma
El
o
•
.... .....
150
\
1ncrease
chamber m•gm•
c.
c.
�
•
•
150
magma
El
Modell
"
c.
c.
50
Mineral
2,80
2,50
FeO'/MgO
chamber magma
Iron-rich, differentiated
basaltic liquids are rare arnong erupted rocks but are found
in small quantities at divergent plate margins (Brooks et al.
1991). Brooks et al. (1991) suggested that erupted N­
MORB may represent a mixture of primitive liquid and
differentiated, iron-rich liquid which exists at depth, but
normally does not reach the surface because of its high
density.
Pseudostratigraphically, there is a significant difference
in the distribution of the FeTi-basalts on Oldra. Fifty-one
percent of the extrusive rocks are FeTi-basalts, whereas
Table 4.
A
o
2,320
Nippm
Emplacement of FeTi-basalts.
6>
83% of the transition zone dykes and 92% of the sheeted
dykes are of FeTi-basalt composition. Liquid densities at
1300°C, using partial molar volumes (Bottinga et al. 1983;
Lange & Carmichael 1987) have been calculated (anhy-
•
0 +-----+-----�--�--�
80
40
60
100
20
120
o
Fig.
o
Fig. JO. The FeO'/MgO ratios versus density for the extrusive rocks (0), transi­
tion zone dykes (A), sheeted dykes (0) and FMG ( . ) . The vertical, dotted
/
....
2,325
� 2,310
---+---�--�
....
0 +--300
200
100
400
o
500
300
o
=
..
....
(2000)
o
2,330
A
\:���
�
....
.....
100
NORSK GEOLOGISK TIDSSKRIFr 80
,.-_.
1ncrease
\
t-----1
increase
Zrppm
i
.
rncrease
Zrppm
8.5
Fig. Il. The MgO-Z:t relations (l to 5) that occur in ascending order from one
sample to another in the extrusive rock profile are shown by solid arrows on the
right side of the diagram. To the left, the respective processes that cause varia­
tions (replenishment of magma, crystal fractionation and/or mixing) are illu­
strated. Curved, dotted arrows crystal fractionation; dotted lines chords
connecting two mixing magmas. The hybrid magma produced by magma mix­
ing is situated on the chord. The location of the hybrid on the chord depends on
the quantity of the two magmas that mix.
=
=
NORSK GEOLOGISK TIDSSKR!Ff 80
(2000)
Solund-Stavjjord Ophiolite Complex
A
,�-+-----....
- - - - -
�
-1 atm
B
o
ExtJUsivO
�
107
rockS
---
-1
-1
-2
30
10 ������ -3
-1 .5
1 00
-30
-o
Distance from spreading centre (km)
-o
-
15
.
Distance from spreading centre (km)
5
-
Fig.
12. Cartoon depicting a petrogenetic model for the Oldra metabasalts. A. Solid circles are melts generated by fractional metting at pressures from 30 to 12 kbar.
Solid arrows indicate upward transport and crystal fractionation of olivine, plagioclase, clinopyroxene and minor Cr-spinel from melts at pressures <8 kbar. During
ascent, individual batches of magma mix, as indicated by the ovals. B. Enlargement of part of A. Melts of FeTi-basalt (dotted arrows) and N-MORB (solid arrows)
composition mix. Ovals indicate areas of magma mixing and crystal fractionation. Low-density N-MORBs (solid diapir in oval) may ascend rapidly, without sub­
stantial mixing with high-density FeTi-basalts.
drous) for the Oldra metabasalts, and show good correla­
tion with the Fe01/Mg0 ratios (Fig. 10). The higher density
of the FeTi-basalts compared with the N-MORBs affects
the ability of the magma to erupt, and may explain the
lower abundance of FeTi-basalts in the extrusive rock
sequence compared to the sheeted dyke complex.
Petrogenetic processes and time
The stratigraphically sampled extrusive rocks (Fig. 5)
provide a good opportunity to study the geochemical
evolution of the magmas with time. In this account, the
concentrations of MgO and Zr are considered, and the
presence of magma chambers is assumed. Based on the
behaviour of these two elements, five models will be
shown below. The petrogenetic processes considered are
crystal fractionation of olivine, plagioclase and clino­
pyroxene, and magma mixing.
In Fig. 11 we show the five models that may account for
the different MgO-Zr relationships. In model l, MgO
decreases and Zr increases owing to crystal fractionation of
the magma. In model 2, MgO increases and Zr decreases
owing to mixing of the chamber magma with a more
primitive magma. In model 3, both MgO and Zr increase
because of magma mixing. In model 4, both elements
decrease owing to magma mixing, and in model 5, the
MgO and Zr contents are relatively constant. The latter is
caused by crystal fractionation of magma l to magma 2.
Prior to intrusion/eruption, chamber magma 2 mixes with a
more primitive magma, and the hybrid magma that
intrudes/erupts has a composition relatively sim ilar to
that of chamber magma l. Thus, the MgO-Zr relations
shown in Fig. 11 indicate that crystal fractionation was an
important process modifying the chamber magmas. How­
ever, magma mixing, in which the compositions of the
chamber magmas and the input magmas may have been
considerably different, seems to have been the most
important modifying process affecting the chemistry of
the intruded and erupted magmas.
Petrogenetic model
Grove et al. ( 1992) discussed three petrogenetic models for
MORB genesis. Model l - Partial melting and later
fractional crystallizationlmixing events are separated by an
aggregationlmixing step; Model 2 - Fractional melts are
moved to a shallower, cooler mantle, where they are
modified by differentiation befare aggregationlmixing
beneath the spreading centre; and Model 3 - Fractional
melting, aggregationlmixing and differentiation occur
throughout the upper mantle, and the pressure ranges of
aggregationlmixing and differentiation overlap. Aggrega­
tion of melts may, thus, occur at many levels during the
melt production and modification process, i.e. in the region
of melt production and in the region of melt modification.
The separation of melt production and melt modification
processes, as suggested in Model l, may represent an
oversimplification of the more complex Model 3. Further­
more, Model 2 may be viewed as an end member case.
Grove et al. ( 1 992) concluded that Model 3 is the most
likely to occur because it closely approximates the way in
108
H. L. Ryttvad et al.
NORSK GEOLOGISK TIDSSKRIFT 80
which melt modification by fractional crystallization fits
into the overall evolution of MORB magmas.
We suggest a petrogenetic model for the rocks of Oldra
(Fig. 12) that is fairly similar to Model 3 of Grove et al.
(1992). Accumulated fractional melting occurred at
pressures from ""'30 to ""'12 kbar from a strongly depleted
mantle (Fig. 12A). Furthermore, on their way to the surface
the melts were modified by fractional crystallization of
olivine, plagioclase, clinopyroxene and minor Cr-spinel
and by mixing, the extent of mixing depending on the
densities of the magmas. Low-density N-MORBs erupted
more commonly than high-density FeTi-basalts, and
therefore constitute "'50% of the extrusive rocks, whereas
the FeTi-basalts constitute the majority of the transition
zone dykes and the sheeted dykes (Fig. 12B). In the crust,
fractional crystallization of plagioclase and clinopyroxene
generated cumulates, as represented by the VTG (Fig.
12A). The gabbro-dyke transitions observed in the FMG
are interpreted to be the root zone of the sheeted dyke
complex and the roof zone of the inferred chamber(s).
S ummary
The island of Oldra, which exposes part of the Late
Ordovician Solund-Stavfjord Ophiolite Complex (SSOC),
contains the following units: vari-textured and fine- to
medium-grained gabbro, metabasalts defining a sheeted
dyke complex, and extrusive rocks. On the basis of
detailed field and geochemical studies, the following
conclusive remarks can be made:
l. The rocks are of N-MORB affinity. The majority
classify as FeTi-basalts (FeOt/MgO > 1.75, Ti02 > 2 0
wt% ), resulting partly from crystal fractionation of
olivine, plagioclase, clinopyroxene, and minor Cr­
spinel from N-MORB type melts.
2. Pseudostratigraphically, there is a significant difference
in the distribution of the FeTi-basalts. Approximately
50% of the extrusive rocks are FeTi-basalts, whereas ca.
90% of the sheeted dykes are of FeTi-basalt composi­
tion. This difference is explained by the higher density
of the FeTi-basalts compared with the less fractionated
N-MORBs.
3. The Nd isotopic data show minor variations in the eNd
(t = 440 Ma) values (+7.8 to +8.4), and a homogeneous,
strongly depleted source is suggested.
4. The FeO\ MgO and Ah03 data suggest that the
metabasalts were generated by partial melting at
pressures from ""'30 to ""'12 kbar. The melts were
subsequently modified by fractional crystallization at a
range of pressures (probably <8 kbar) in the mantle and
crust.
5. The Cr-Zr and Ni-Zr relationships of the metabasalts
suggest mixing of magma batches that ranged from N­
MORB to FeTi-basalt compositions.
6. In addition to crystal fractionation, magma mixing was
.
(2000)
apparently an important modifying process affecting the
chemistry of the magmas.
- Financial support to carry out this study was provided by the
University of Bergen and the Research Council of Norway. We !hank Dr Y. Dilek
for constructive discussions during fieldwork. We also !hank the reviewers C.
Bames, E.-R. Neumann and D. Roberts for constructive criticism.
Acknowledgements.
Manuscript received January 1999
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